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Rabu, 28 Mei 2008

Network Planning

Kali ini bahasan kita adalah tentang Network Planning. Digambarkan seperti sbb...
Proses Network Planning kita bagi menjadi langkah"berikut ini :
1. PRE PLANNING;
- Dimensioning = memberikan konfigurasi awal jaringan sbg hasilnya. Targetnya adl coverage dan quality. Basic input dimensioning yaitu coverage requirement, quality requirement (drop call rate, call blocking), Frequency Spectrum (number of channel including information about possible needed guard bands, subscriber information), Trafik tiap user, busy hour value, services.

2. PLANNING;
Memberikan ide awal ttg lokasi dan juga jarak antar sites. Nominal plan adalah titik awal utk survey dan menggunakan planning tool untuk coverage planning.
- Coverage planning
- Capacity planning
- Site Survey

3. DETAILED PLANNING;
Mencakup frekuensi, adjacency, dan parameter planning. Planning tool mempunyai algoritma utk frek planning dan merupakan fase kritikal di network planning. Jumlah frekuensi yg bs digunakan selalu terbatas dan oleh sebab itu tugas disini adalah menemukan kemungkinan solusi terbaik. Neighbour plan dengan melihat 2 site pertama di lingkungan BTS". Untuk Radio Planning tanggung jawabnya adalah mengalokasikan parameter" spt handover control, power control dan menentukan location area lalu set parameternya.
- Frekuensi Planning
- Neighbour Planning
- Parameter Planning

4. VERIVICATION & ACCEPTANCE
Coverage,capacity, dan quality yg hrs memenuhi KPI yang ditetapkan sebelum network acceptance. Drive testing juga diperlukan utk verifikasi network functionality.

5. OPTIMIZATION
Setelah jaringan sudah beroperasi, planning dan optimization blm berakhir karena network optimization adalah proses yang berkelanjutan. Utk optimisasi, input yang dibutuhkan adalah semua informasi ttg jaringan dan statusnya. Statistik jaringan, alarms, dan trafik itu sendiri dimonitor secara baik. proses optimisasi meliputi baik network level measurements dan field test measurements utk menganalisa masalah dan juga mengindikasikan masalah yg berpotensial terjadi.

Kamis, 22 Mei 2008

3G Extension Band


Introduction
In order to continue realising these benefits and to accommodate future growth, an issue of
importance to all mobile operators is that of secure and predictable access to spectrum. The
objective of the GSM Association, representing over 650 mobile operators in more than 210
countries and territories, is to guarantee the use of 2.5GHz as an extension band for 3GSM and
to ensure its global availability.
Governments and Regulators are under increasing pressure to liberalise the allocation of
wireless service licences and allocate spectrum, previously identified with a particular type of
service, to new services that are being introduced on a fragmented basis. If followed without
international coordination, such policies will destroy the major benefits of global mobile
interoperability, including roaming and scale economies. In particular, Regulators may
permit the 3G extension bands to be used in a manner inconsistent with a harmonised and
structured band plan.
GSMA Policy Positions
Mobile services have transformed economies in every part of the world,
with global penetration of mobile subscribers now exceeding that of fixed
lines in most regions. The contribution of the mobile industry to national
GDP, employment and government revenue is substantial in the vast
majority of countries around the globe.
3G WCDMA is a proven standard with over 20 million users across 70 networks in 32 countries.
Governments should, in parallel with operators, invest in the long-term future of 3GSM
networks and ensure that capacity for growth is secured, via the allocated 2500-2690 MHz
"3G Extension Band."
To achieve their full potential, 3GSM networks will need spectrum, in addition to the ITU "Core
Band", in order to provide capacity for:
Increasing numbers of subscribers
Ongoing development of new 3GSM broadband applications
Delivering ubiquitous mobile broadband
This is the same situation as was experienced in the successful growth of 2G networks
To maximize the value to all parties, the GSMA believes that governments should follow the
harmonized and structured channeling arrangements, for the band 2500-2690 MHz,
recommended by the ITU.
The ITU has recommended three options for the 3G extension bands (see illustration below).
The first two options provide structured channeling arrangements. They provide separation of
FDD and TDD blocks and have the advantage of enabling 3GSM to be deployed with minimum
risk of interference. The GSMA supports these first two options.
Structuring the band in this way, within individual countries and/or regions, will encourage
investment in networks and services, through minimizing the risks and costs associated with
interference management.
Harmonising the band plan globally will increase still further the investment incentives and
ensure that economies of scale are maximized.
A harmonized and structured band plan will facilitate seamless roaming and network
interoperability, at a global level.
GSMA Policy Positions continued

The GSMA believes that the adoption of the ITU’s third option and, therefore, the co-existence
of FDD and TDD, across the same band, would have a significant, negative impact on the
efficiency of that band's use. In turn, this would have a negative impact on the future benefits
to consumers from 3GSM mobile broadband.
The co-existence of FDD and TDD, across the band, would permit technologies to be deployed
within the band that could create interference.
Within individual countries, an unstructured band plan would reduce incentives to invest and
increase industry costs.
More broadly, a lack of global commitment to a harmonized and structured band plan would
reduce economies of scale, for both operators and manufacturers. It would increase costs and
result in slower times to market for innovative mobile data services.
In Europe, the ECC has reached the conclusion that the first two options are the most
appropriate. The GSMA supports this conclusion.
The GSMA believes that a harmonized and structured band plan will bring benefits to
consumers, governments and industry, in the form of faster times to market for innovative
services and lower prices due to scale economies. A harmonized plan will also facilitate
seamless roaming and network interoperability, at a global level, and extend the significant
benefits of global 2G roaming to 3G.
The GSMA believes that interference from satellite services could severely affect 3GSM systems
and, in some areas of the world, satellite services are already interfering with the 3G extension
band. It is possible for satellites of one country to interfere with the spectrum of many
neighbouring nations, due to the size of their footprints.
The GSMA recommends that governments introduce policies that only allow satellite systems to
operate, provided they do not cause harmful interference to 3G in the 2.5 to 2.69 GHz band. In
this regard, the GSMA supports the ITU's WRC-07 agenda item 1.9, aimed at reducing the
power of satellite systems in the 3GSM band.
For further information, please contact:
Tom Phillips
Government and Regulatory Affairs Officer, GSM Association
Mobile: +44 (0) 7917 046595
Email: tphillips@gsm.org
GSMA Policy Positions continued

Spectrum Harmonisation

Source : GSM world

Introduction
An important contributor to GSM’s unrivalled success is the global harmonisation of
standards and spectrum bands, resulting in unparalleled co-operation and support between all
those supplying, operating and using the technology. This has enabled manufacturers and networks
to realise significant economies of scale and to offer seamless global roaming
services, to the benefit of consumers worldwide. Harmonisation must be preserved to enable
the global mobile industry to continue delivering the benefits that mobile users have come to
expect and that are valued so highly. The continued economic success of mobile services creates
the climate for future investment in 3GSM technologies, which will deliver still greater
benefits to consumers.
The GSM Association, consisting of over 650 mobile operators and more than 150
manufacturers and suppliers, supports harmonised, open standards and globally coordinated
spectrum bands, in a world of competition among standards.
Today's GSM platform is a uniquely successful wireless technology and
an unprecedented story of global achievement. In less than ten years since
the first GSM network was commercially launched, it has become the
world's leading and fastest growing mobile standard, spanning over 200
countries. Today, GSM technology is used by more than one in six of the
world's population and it is estimated that there are well over 1.25 billion
GSM subscribers today. Beyond GSM, WCDMA and its 3G evolutionary
path (3GSM*) is already in use by more than 70 live networks with over
20 million subscribers in 32 countries and provides a clear evolutionary
path towards full ubiquitous mobile broadband use.
* 3GSM is the collective term for 3G WCDMA technology evolution that includes 3G WCDMA,
‘evolved 3G’ and ‘super 3G.’

The GSMA believes that the harmonisation of standards and frequency bands brings benefits to
consumers and has been a significant factor in the success of the mobile industry. In particular:
Global spectrum planning, led by the International Telecommunication Union (ITU), is
of crucial importance and is the best way to secure the cooperation of national
governments and industry in developing harmonised frequency bands.
Industry has proven to be the most effective party for delivering standards
harmonisation.
The GSMA actively supports industry-led standards development bodies such as
3GPP.
The GSMA believes that a level regulatory playing field for licensees who chose the GSM
and/or 3GSM* family of standards, vis a vis licensees that chose other technologies, is
essential in creating a predictable and stable environment that maximises long-term investments
and benefits to consumers. In particular:
The GSMA opposes policies that, in the name of "technology neutrality", discriminate
against GSM and 3GSM*.
Governments and Regulators should be aware of the variety of ways in which
technology bias can be introduced, for example in specifying minimum data rates or
allocating frequencies in amounts of bandwidth and channel definitions that exclude
certain technologies.
The GSMA calls upon Regulators to examine closely their policies regarding technology
neutrality and ensure that opportunities for bias are eliminated and harmonisation
achieved.
GSMA Policy Positions

Kamis, 15 Mei 2008

Bridging the optimization gap

sumber: ericsson
TEMS Visualization is revolutionizing the way network operators troubleshoot their Ericsson networks. Ericsson's infrastructure customers now can find problems that were impossible to pinpoint before. TEMS Visualization gives them this ability by analyzing the event-based data captured directly from the infrastructure.
By analyzing specific network data from the switch, TEMS Visualization bridges the gap between traditional performance management solutions and drive test tools.Use TEMS Visualization to:
Trap, track, reproduce, and eliminate problems that were previously virtually impossible to find
Understand problems by viewing full details of all calls made on an Ericsson network - all calls, all subscribers, all phones
Drill-down into subscriber issues easily using powerful filtering capabilities and fast browsing functions

Tems cell planner

TEMS™ CellPlanner is Ericsson's advanced tool for design, rollout, and optimization of mobile radio networks. Developed by Ericsson, TEMS CellPlanner provides superior planning and optimization capabilities to save time and money during network deployment of 2G, 2.5G, and 3G networks.

TEMS CellPlanner is built on the latest Java platform and is designed for class-leading accuracy and speed in planning and optimization tasks. It is a graphical and easy-to-use PC-based tool.

TEMS CellPlanner assists the user in performing complex tasks, including network dimensioning, traffic planning, site configuration, frequency, code planning and Automatic Cell Planning . Features such as accurate network modeling and advanced algorithms make TEMS CellPlanner key to competing successfully in the mobile communications marketplace.

TEMS CellPlanner provides full support for voice and data services in GSM, WCDMA and WiMAX . It provides support for GPRS and EDGE implemented in GSM systems, HSDPA/EUL(HSUPA) including the latest release of the HSDPA (phase2) implemented in WCDMA systems, and the PUSC (Partial Usage of Subcarriers) feature in WiMAX.
Use TEMS CellPlanner to:
Plan, design, and optimize networks
Analyze coverage and interference
Plan and optimize network with live interference and traffic data
Analyze TEMS logfile data and autotune propagation models
Utilize drive test data with the Automatic Measurement Integrator (AMI), which keeps pathloss prediction constantly updated and drive test measurements effectively reused
Optimize inter- and intra-technology neighbor cell relationships
Review network capacity planning and optimization for voice and data services
Create scripts and improve work efficiency
Perform Automatic Frequency Planning (AFP)
Optimize WCDMA networks with Automatic Cell Planning (ACP)
Connect Planning tool and live system using OSS RC WCDMA Interface

Kamis, 08 Mei 2008

Frekuensi Hopping di GSM

Frekuensi Hopping (FH) adalah teknik dimana frekuensi yang digunakan oleh sepasang base station dan mobile station diubah pada interval waktu yang teratur. Setiap burst dalam physical channel akan ditransmisikan pada frekuensi carrier yang berbeda dalam tiap frame TDMA, dan tiap frame TDMA akan ditransmisikan pada frekuensi yang berbeda.
Namun perlu diingat ada logical channel yang tidak dihopping yaitu kanal BCCH (Broadcast Control Channel), FCH (Frequency Correction Channel), dan SCH (Synchronization Channel) karena kanal ini harus selalu dipancarkan dengan daya maksimum disebabkan merupakan signalling utama.

Parameter FH :

1. MA (Mobile Allocation) = merupakan sekumpulan daftar frekuensi tertentu yang digunakan dalam hopping sequence. Sekelompok MS ditentukan oleh sebuah MA.

2. MAI (Mobile Allocation Index) = index pertukaran yang diperoleh dari algoritma FH yang menunjukkan frekuensiaktif hopping dari daftar MA. MAI dikalkulasi oleh BTS dan MS menggunakan HSN, MAIO, dan frame number yang ada.

3. MAIO (Mobile Allocation Index Offset) = suatu frekuensi offset dalam MA dan berfungsi untuk menunjukkan pada frekuensi mana dalam MA FH tu dimulai. MAIO digunakan untuk menjamin bahwa tiap TCH menggunakan frekuensi berbeda selama proses hopping.

4 Hopping Group = sekumpulan timeslot (RSTL) yang menggunakan MA dan HSN yang sama dalam suatu sel.

5. HSN (Hopping Sequence Number) = berfungsi untuk menentukan bagaimana sistem pseudorandom akan mulai hopping. Tiap TCH yang berpindah ke frekuensi baru dalam MA, berdasarkan nomor HSNnya. HSN memberikan nomor algoritma untuk mengkalkulasi frekuensi yang akan digunakan untuk mentransmisikan TCH berikutnya dalam MA. Algoritma yang ada sampai dengan 63 algoritma HSN (jenis cyclic = 0 dan random = 1 s.d 63).

6. MAIO Step = parameter pilihan untuk meningkatkan kualitas pembagian daftar MA yang diperoleh dengan MAIO offset.

METODE HOPPING :

FH pada BSS dapat diimplementasikan menggunakan teknik Baseband Hopping (BB-FH) dan Synthesized Hopping (SFH).

1.
BB-FH , pada baseband semua transceiver beroperasi pada frekuensi yang tetap. FH dihasilkan dengan menswitch burst data baseband secara terus menerus di antara bagian radio TRX/TRU baik uplink maupun downlink. Kekurangannya dalam BB-FH adalah jumlah frekuensi hop yang terbatas pada jumlah TRX/TRU.

2.
SFH , dengan SFH semua transceiver terkecuali BCCH TRX-1, diset untuk mengubah frekuensinya frame per frame sesuai hopping sequence. Jumlah loncatan frekuensi yang melalui channel radio tidak terbatas oleh hardware yang digunakan, namun jumlahnya terbatas pada MA yang dialokasian berapa tergantung jatah dari tiap operator. Oleh karena itu hopping sequence dapat mengikutsertakan lebih banyak frekuensi pada TRX dalam suatu sel.

Quality service

Quality of Service dalam Data Komunikasi

Arif Hamdani Gunawan

Komunikasi data merupakan salah satu teknologi telekomunikasi yang berkembang sangat pesat, khususnya pada implementasi IP. Layanan-layanan yang berbasiskan IP juga ikut merasakan dampaknya dengan adanya standard-standard yang terus berkembang pada network layer ini, oleh karena itu komunikasi data juga mengalami akselerasi.

Banyak sekali aplikasi yang berbasiskan komunikasi data dan saat ini tidak hanya beroperasi di LAN (Local Area Network), tetapi juga di WAN (Wide Area Network). Aplikasi-aplikasi terebut membutuhkan suatu tingkat jaminan layanan (Quality of Service/QoS) untuk dapat beroperasi. Oleh karena itu, QoS sudah sepatutnya diketahui oleh banyak pihak, seperti penyedia infrastruktur, LAN administrator, WAN administrator, service provider, yang memang berhubungan dengan komunikasi data.

Tulisan ini akan mendiskusikan mengenai konsep dasar dari Quality of Service (QoS), mengapa kita membutuhkannya dan akan dipaparkan juga mengenai tipe-tipe mekanisme QoS secara sederhana.

Pengertian

QoS merupakan terminologi yang digunakan untuk mendefinisikan kemampuan suatu jaringan untuk menyediakan tingkat jaminan layanan yang berbeda-beda. Melalui Q0S, seorang network administrator dapat memberikan prioritas trafik tertentu. Suatu jaringan, mungkin saja terdiri dari satu atau beberapa teknologi data link layer yang mampu diimplementasikan QoS, misalnya; Frame Relay, Ethernet, Token Ring, Point-to-Point Protocol (PPP), HDLC, X.25, ATM, SONET. Setiap teknologi mempunyai karakteristik yang berbeda-beda yang harus dipertimbangkan ketika mengimplementasikan QoS. QoS dapat diimplementasikan pada situasi congestion management atau congestion avoidance. Teknik-teknik congestion management digunakan untuk mengatur dan memberikan prioritas trafik pada jaringan di mana aplikasi meminta lebih banyak lagi bandwidth daripada yang mampu disediakan oleh jaringan. Dengan menerapkan prioritas pada berbagai kelas dari trafik, teknik congestion management akan mengoptimalkan aplikasi bisnis yang kritis atau delay sensitive untuk dapat beroperasi sebagai mana mestinya pada lingkungan jaringan yang memiliki kongesti. Adapun teknik collision avoidance akan membuat mekanisme teknologi tersebut menghindari situasi kongesti. Melalui implementasi QoS di jaringan ini, network administrator akan memiliki fleksibilitas yang tinggi untuk mengontrol aliran dan kejadian-kejadian yang ada di trafik pada jaringan.

QoS merupakan peralatan-peralatan yang tersedia untuk menerapkan berbagai jaminan, dimana tingkat minimum layanan dapat disediakan. Banyak protokol dan aplikasi yang tidak begitu sensitif terhadap network congestion. File Transfer Protocol (FTP) contohnya, mempunyai toleransi yang besar untuk network delay dan terbatasnya bandwidth. Di sisi user, kejadian tersebut akan menyebabkan proses transfer file seperti download atau upload yang lambat, walaupun mengganggu user, namun kelambatan ini tidak akan menggagalkan operasi dari aplikasi tersebut. Lain halnya dengan aplikasi-aplikasi baru sepertiVoice dan Video, yang pada umumnya sensitif terhadap delay. Jika paket dari voice mengalami proses yang lama untuk sampai ke tujuan, maka akan dapat merusak Voice yang didengarkan. Dalam hal ini QoS dapat digunakan untuk menyediakan jaminan layanan untuk aplikasi-aplikasi tersebut. SNA merupakan salah satu contoh protokol yang sangat sensitif dengan menggunakan protokol handshake dan biasanya akan melakukan terminasi dari session jika tidak memperoleh suatu acknowledgement, lain halnya dengan TCP/IP. Sehingga dalam kasus ini, memberikan prioritas pada trafik SNA di atas protokol lainnya akan memberikan QoS yang lebih baik.

Ada beberapa alasan mengapa kita memerlukan QoS, yaitu:

· Untuk memberikan prioritas untuk aplikasi-aplikasi yang kritis pada jaringan.

· Untuk memaksimalkan penggunaan investasi jaringan yang sudah ada.

· Untuk meningkatkan performansi untuk aplikasi-aplikasi yang sensitif terhadap delay, seperti Voice dan Video.

· Untuk merespon terhadap adanya perubahan-perubahan pada aliran trafik di jaringan.

Point terakhir nampaknya terasa tidak penting, benarkah demikian? Naptser, PointCast, World-Wide-Web adalah contoh aplikasi-aplikasi “self-deployed” yang dapat menyebabkan mimpi buruk bagi network administrator. Tidak seorang pun pernah merencanakan jalannya Web browsing seperti sekarang ini, hampir seluruh trafik di Internet membawa prefix “http”. Dengan adanya perubahan permintaan bandwidth, QoS dapat digunakan untuk menjamin kualitas layanan, jika beberapa user dalam suatu perusahaan sedang mendengarkan siaran radio lewat Internet, maka tidak akan memperlambat trafik penting yang ada ke perusahaan tersebut.

Metode paling sederhana yang sering digunakan untuk memperoleh performansi yang lebih baik pada jaringan adalah dengan meminta lebih banyak bandwidth. Saat ini Gigabit Ethernet dan Optical Networking sudah tersedia. Peningkatan bandwidth dapat menjadi solusi sementara untuk meningkatkan kualitas layanan, namun tidak akan dapat untuk menjamin kualitas layanan seterusnya. Aplikasi-aplikasi yang didukung oleh protokol-protokol yang ada akan terus meminta bandwidth lagi. Langkah tepat untuk kondisi demikian adalah menganalisa trafik yang lewat, mengidentifikasi urutan kepentingan dari protokol dan aplikasi di jaringan, dan menentukan strategi untuk memberikan prioritas untuk mengakses bandwidth yang tersedia. QoS akan membuat seorang network administator mengawasi bandwidth, latency dan jitter, serta memiinimisasi paket yang hilang pada suatu newtwork, dengan memberikan prioritas pada protokol. Bandwidth adalah ukuran kapasitas pada suatu jaringan atau link. Latency adalah delay dari suatu paket untuk melewati jaringan. Jitter adalah perubahan latency pada suatu periode waktu. Melalui penerapan teknik-teknik QoS, maka akan dapat dilakukan pengaturan dari ketiga parameter di atas.

Saat ini di kebanyakan jaringan di perkantoran tidak begitu memperhatikan QoS. Namun, dengan berkembangnya aplikasi-aplikasi, misalnya mulicast, streaming multimedia, dan Voice over IP (VoIP) kebutuhan akan QoS akan semakin terasa. Terlebih lagi aplikasi-aplikasi tersebut terhadap jitter dan delay dan performansi yang buruk akan sangat terasa pada end user. Dalam hal ini seorang network administrator dapat melakukan tindakan manajemen proaktif untuk aplikasi-aplikasi sensitif yang baru dengan mengaplikasikan teknik-teknik QoS pada jaringan. Penting untuk diketahui, bahwa QoS bukanlah solusi yang ajaib untuk setiap masalah kongesti, karena dapat saja solusi terbaik untuk mengatasi congested network memang adalah melakukan upgrade pada bandwidth.

Tingkatan QoS

Terdapat 3 tingkat QoS yang umum dipakai, yaitu best-effort service, integrated service dan differentiated service. Ketiga level tersebut akan diuraikan lebih detail dibawah ini.

Best-Effort Service

Best-effort service digunakan untuk melakukan semua usaha agar dapat mengirimkan sebuah paket ke suatu tujuan. Penggunakan best-effort service tidak akan memberikan jaminan agar paket dapat sampai ke tujuan yang dikehendaki. Sebuah aplikasi dapat mengirimkan data dengan besar yang bebas kapan saja tanpa harus meminta ijin atau mengirimkan pemberitahuan ke jaringan. Beberapa aplikasi dapat menggunakan best-effort service, sebagai contohnya FTP dan HTTP yang dapat mendukung best-effort service tanpa mengalami permasalahan. Untuk aplikasi-aplikasi yang sensitif terhadap network delay, fluktuasi bandwidth, dan perubahan kondisi jaringan, penerapan best-effort service bukanlah suatu tindakan yang bijaksana. Sebagai contohnya aplikasi telephony pada jaringan yang membutuhkan besar bandwidth yang tetap, 0agar dapat berfungsi dengan baik; dalam hal ini penerapan best-effort akan mengakibatkan panggilan telephone gagal atau terputus.

Integrated Service

Model integrated service menyediakan aplikasi dengan tingkat jaminan layanan melalui negosiasi parameter-parameter jaringan secara end-to-end. Aplikasi-aplikasi akan meminta tingkat layanan yang dibutuhkan untuk dapat beroperasi dan bergantung pada mekanisme QoS untuk menyediakan sumber daya jaringan yang dimulai sejak permulaan transmisi dari aplikasi-aplikasi tersebut. Aplikasi tidak akan mengirimkan trafik, sebelum menerima tanda bahwa jaringan mampu menerima beban yang akan dikirimkan aplikasi dan juga mampu menyediakan QoS yang diminta secara end-to-end. Untuk itulah suatu jaringan akan melakukan suatu proses yang disebut admission control. Admission control adalah suatu mekanisme yang mencegah jaringan mengalami over-loaded. Jika QoS yang diminta tidak dapat disediakan, maka jaringan tidak akan mengirimkan tanda ke aplikasi agar dapat memulai untuk mengirimkan data. Jika aplikasi telah memulai pengiriman data, maka sumber daya pada jaringan yang sudah dipesan aplikasi tersebut akan terus dikelola secara end-to-end sampai aplikasi tersebut selesai.

Cisco Internetwork Operating System (IOS) mempunyai dua fitur untuk menyediakan layanan terintegrasi untuk mengawasi beban yang ditanggung di jaringan, yaitu Resource Reservation Protocol (RSVP) dan Intelligent Queuing. Saat ini RSVP sudah menjadi salah satu standard yang dikeluarkkan oleh salah satu working group dari Internet Engineering Task Force (IETF). Intelligent queueing yang serting digunakan adalah Weighted fair queuing (WFQ) dan Weighted Random Early Detection (WRED). Penting untuk diketahui bahwa baik RSVP maupun Intelligent Queuing bukanlah merupakan routing protocol. RSVP akan bekerja sama dengan routing protocol untuk menentukan jalur yang terbaik di jaringan untuk dapat diberikan QoS.

Differentiated Service

Model terakhir dari QoS adalah model differentiated service. Differentiated service menyediakan suatu set perangkat klasifikasi dan mekanisme antrian terhadap protokol-protokol atau aplikasi-aplikasi dengan prioritas tertentu di atas jaringan yang berbeda. Differentiated service bergantung pada kemampuan edge router untuk memberikan klasifikasi dari paket-paket yang berbeda tipenya yang melewati jaringan. Trafik jaringan dapat diklasifikasikan berdasarkan alamat jaringan, protocol dan port, ingress interface, atau klasifikasi lainnya selama masih didukung oleh standard access list atau extended access list.

Congestion Management

Congestion management merupakan terminologi umum yang mencakup penggunaan strategi antrian untuk mengatur situasi di mana permintaan akan bandwidth lebih besar daripada bandwidth yang dapat disediakan oleh jaringan. Beberapa teknik congestion management yang sering digunakan adalah:

· First In First Out Queuing (FIFO)

· Priority Queuing

· Custom Queuing

· Weighted Fair Queuing (WFQ)

Priority Queuing dan Custom Queuing memerlukan perencanaan dasar untuk implementasi dan konfigurasi secara benar di router. Perencanaan yang salah bahkan dapat mengakibatkan terjadinya kongesti. Sedangkan FIFO dan WFQ memerlukan sedikit konfigurasi, Pada Cisco IOS, secara default WFQ berfungsi pada link dengan kecepatan E1 (2,048 Mbps) atau di bawahnya; dan secara default FIFO berfungsi pada link dengan kecepatan di atas kecepatan E1.

Konsep Antrian

Konsep antrian diterapkan di router dan akan menahan paket di dalam router, sampai dengan sumber daya yang ada mencukupi untuk mengirimkan paket tersebut. Jika tidak terdapat kongesti pada router, maka paket akan segera dikirimkan. Antrian di jaringan dapat dianalogikan dengan sistem pengantrian pembelian karcis film di bioskop, jika tidak ada orang yang sedang mengantri untuk membeli tiket, maka kita dapat langsung ke depan untuk membeli tiket tersebut tanpa harus mengantri, hal ini berarti tidak terjadi kongesti di jaringan.

Pada router yang mempunyai FastEthernet LAN dan E1 WAN sangat memungkinkan pada suatu waktu untuk terjadi antrian, hal ini disebabkan karena kecepatan interface FastEthernet LAN dengan E1 WAN tidak sama, sering dikenal dengan istilah speed-match. Speed-match ini bukanlah sesuatu yang mutlak untuk dihindari pada kondisi ini, kita harus melihat frekuensi dan potensi speed-match yang menyebabkan terjadinya kongesti pada router. Namun demikian, pada teknologi switch di data link layer, sedapat mungkin kita memang menghindari adanya speed-match.

Leaky Bucket

Leaky bucket atau ember yang bocor adalah konsep dasar untuk dapat mengetahui mengenai teori antrian. Kucuran air yang menetes atau mengalir ke ember secara acak dapat dianalogikan sebagai trafik kedatangan yang random. Sedangkan tetesan air dari ember yang bocor dapat dianalogikan dengan trafik yang keluar. Jika kucuran air ke dalam ember sangat deras, maka ember akan tergenang air dianalogikan dengan antrian yang ada.

Bagaimana jika kucuran air yang ke ember jauh lebih besar daripada tetesan air yang keluar dari ember? Tentu saja ember akan terisi oleh air. Lantas, apa yang terjadi jika ember penuh dengan air dan air terus mengucur ke ember? Tentu saja air akan melimpah. Hal ini juga terjadi di komunikasi data, jika antrian paket sudah tidak dapat ditangani lagi oleh memori di router, sedangkan kedatangan paket yang ada masih tetap tinggi, maka akan ada paket yang ‘melimpah’, karena tidak tertangani.

Network administrator dapat melakukan konfigurasi dari besarnya antrian yang dibutuhkan. Pada Cisco IOS, sudah terdapat suatu default untuk besar antrian tersebut. Jika paket yang datang lebih besar daripada kapasitas yang dapat ditangani oleh router, maka router akan melakukan drop pada paket yang sudah tidak dapat ditanganinya lagi. Protokol-protokol yang berada di layer atas mendukung pemberitahuan dan proses re-transmisi untuk mengidentifikasi adanya paket yang drop lalu melakukan retransmisi. Drop tidak selalu berarti adanya kesalahan pada jaringan.Sebagai contoh, pada interface FastEthernet dengan kecepatan 100 Mbps yang akan mengirmkan banyak informasi secara cepat ke interface E1 dengan kecepatan 2,048 Mbps. Drop dari paket-paket yang mungkin terjadi dapat digunakan oleh protokol-protokol yang berada di layer atas untuk mengurangi kecepatan pengiriman ke router. Beberapa mekanisme QoS seperti Random Early Detection (RED) dan Weighted Random Early Detection (WRED) mengunakan prinsip-prinsip tersebut untuk mengontrol tingkat kongesti yang terjadi di jaringan.

Drop paket berarti menuntut adanya retransmisi, dan ini akan menimbulkan suatu fenomena baru yang dikenal dengan global synchronization. Global synchronization terjadi, karena interaksi dari mekanisme di layer atas dari TCP/IP, yang disebut dengan sliding window. Jika blok-blok data berhasil dikirimkan tanpa adanya error, maka window atau jendela akan maju ke blok berikutnya, untuk kemudian mengirimkan blok data selanjutnya, sehingga hal ini dinamakan sliding window. Jika error terjadi saat pengiriman, maka window akan bergerak mundur untuk mengirimkan kembali blok yang mengalami error. Komunikasi ini akan menggunakan semua bandwidth yang tersedia, di mana dapat menyebabkan antrian paket menjadi drop. Paket-paket yang mengalami drop diintepretasikan sebagai transmission error, yang secara simultan akan menyebabkan berkurangnya ukuran window untuk pengiriman paket selanjutnya pada setiap interval. Global synchronization ini menyebabkan fluktuasi pada penggunaan jaringan, seperti dapat dilihat pada gambar berikut ini.


Penutup

Implementasi QoS sebenarnya merupakan suatu hal yang sederhana pada jaringan komunikasi data, namun pada kenyataanya QoS merupakan salah satu hal yang paling tidak diperhatikan. Permasalahan kongesti seringkali dianalogikan kepada permasalahan bandwidth, dan dijawab dengan peningkatan bandwidth. Dengan penerapan QoS, maka akan dapat diberikan jaminan layanan kepada aplikasi yang dijalankan oleh end user. Melalui QoS ini, nantinya juga dapat dilakukan kontrol dan fungsi manajemen pada jaringan.

Advantages of CDMA

CDMA2000 benefited from the extensive experience acquired through several years of operation of cdmaOne systems. As a result, CDMA2000 is a very efficient and robust technology. Supporting both voice and data, the standard was devised and tested in various spectrum bands, including the new IMT-2000 allocations.

There is tremendous demand for new services and operators are looking to provide these to many more subscribers at reasonable prices.

The unique features, benefits, and performance of CDMA2000 make it an excellent technology for high-voice capacity and high-speed packet data. The fact that CDMA2000 1X has the ability to support both voice and data services on the same carrier makes it cost effective for wireless operators.

Due to its optimized radio technology, CDMA2000 enables operators to invest in fewer cell sites and deploy them faster, ultimately allowing the service providers to increase their revenues with faster Return On Investment (ROI). Increased revenues, along with a wider array of services, make CDMA2000 the technology of choice for service providers.

Increased Voice Capacity
Voice is the major source of traffic and revenue for wireless operators, but packet data will emerge in coming years as animportant source of incremental revenue. CDMA2000 delivers the highest voice capacity and packet data throughput using the least amount of spectrum for the lowest cost.

CDMA2000 1X supports 35 traffic channels per sector per RF (26 Erlangs/sector/RF) using the EVRC vocoder, which became commercial in 1999.

Voice capacity improvement in the forward link is attributed to faster power control, lower code rates (1/4 rate), and transmit diversity (for single path Rayleigh fading). In the reverse link, capacity improvement is primarily due to coherent reverse link.

Click here for more information on CDMA capacity

Higher Data Throughput
Today's commercial CDMA2000 1X networks (phase 1) support a peak data rate of 153.6 kbps. CDMA2000 1xEV-DO, commercial in Korea, enables peak rates of up to 2.4 Mbps and CDMA2000 1xEV-DV will be capable of delivering data of 3.09 Mbps.

Frequency Band Flexibility
CDMA2000 can be deployed in all cellular and PCS spectrum. CDMA2000 networks have already been deployed in the 450 MHz, 800 MHz, 1700 MHz, and 1900 MHz bands; deployments in 2100 MHz and other bands are expected in 2004. CDMA2000 can also be implemented in other frequencies such as 900 MHz, 1800 MHz and 2100 MHz. The high spectral efficiency of CDMA2000 permits high traffic deployments in any 1.25 MHz channel of spectrum.

Increased Battery Life
CDMA2000 significantly enhances battery performance. Benefits include:

  • Quick paging channel operation
  • Improved reverse link performance
  • New common channel structure and operation
  • Reverse link gated transmission
  • New MAC states for efficient and ubiquitous idle time operation

Synchronization
CDMA2000 is synchronized with the Universal Coordinated Time (UCT). The forward link transmission timing of all CDMA2000 base stations worldwide is synchronized within a few microseconds. Base station synchronization can be achieved through several techniques including self-synchronization, radio beep, or through satellite-based systems such as GPS, Galileo, or GLONASS. Reverse link timing is based on the received timing derived from the first multipath component used by the terminal.

There are several benefits to having all base stations in a network synchronized:

  • The common time reference improves acquisition of channels and hand-off procedures since there is no time ambiguity when looking for and adding a new cell in the active set.
  • It also enables the system to operate some of the common channels in soft hand-off, which improves the efficiency of the common channel operation.
  • Common network time reference allows implementation of very efficient "position location" techniques.

Power Control
The basic frame length is 20 ms divided into 16 equal power control groups. In addition, CDMA2000 defines a 5 ms frame structure, essentially to support signaling bursts, as well as 40 and 80 ms frames, which offer additional interleaving depth and diversity gains for data services. Unlike IS-95 where Fast Closed Loop Power Control was applied only to the reverse link, CDMA2000 channels can be power controlled at up to 800 Hz in both the reverse and forward links. The reverse link power control command bits are punctured into the F-FCH or the F-DCCH (explained in later sections) depending on the service configuration. The forward link power control command bits are punctured in the last quarter of the R-PICH power control slot.

In the reverse link, during gated transmission, the power control rate is reduced to 400 or 200 Hz on both links. The reverse link power control sub-channel may also be divided into two independent power control streams, either both at 400 bps, or one at 200 bps and the other at 600 bps. This allows for independent power control of forward link channels.

In addition to the closed loop power control, the power on the reverse link of CDMA2000 is also controlled through an Open Loop Power Control mechanism. This mechanism inverses the slow fading effect due to path loss and shadowing. It also acts as a safety fuse when the fast power control fails. When the forward link is lost, the closed loop reverse link power control is "freewheeling" and the terminal disruptively interferes with neighboring. In such a case, the open loop reduces the terminal output power and limits the impact to the system. Finally the Outer Loop Power drives the closed loop power control to the desired set point based on error statistics that it collects from the forward link or reverse link. Due to the expanded data rate range and various QoS requirements, different users will have different outer loop thresholds; thus, different users will receive different power levels at the base station. In the reverse link, CDMA2000 defines some nominal gain offsets based on various channel frame format and coding schemes. The remaining differences will be corrected by the outer loop itself.

Soft Hand-off
Even with dedicated channel operation, the terminal keeps searching for new cells as it moves across the network. In addition to the active set, neighbor set, and remaining set, the terminal also maintains a candidate set.

When a terminal is traveling in a network, the pilot from a new BTS (P2) strength exceeds the minimum threshold TADD for addition in the active set. However, initially its relative contribution to the total received signal strength is not sufficient and the terminal moves P2 to the candidate set. The decision threshold for adding a new pilot to the active set is defined by a linear function of signal strength of the total active set. The network defines the slope and cross point of the function. When strength of P2 is detected to be above the dynamic threshold, the terminal signals this event to the network. The terminal then receives a hand-off direction message from the network requesting the addition of P2 in the active set. The terminal now operates in soft hand-off.

The strength of serving BTS (P1) drops below the active set threshold, meaning P1 contribution to the total received signal strength does not justify the cost of transmitting P1. The terminal starts a hand-off drop timer. The timer expires and the terminal notifies the network that P1 dropped below the threshold. The terminal receives a hand-off message from the network moving P1 from the active set to the candidate set. Then P1 strength drops below TDROP and the terminal starts a hand-off drop timer, which expires after a set time. P1 is then moved from candidate set to neighbor set. This step-by-step procedure with multiple thresholds and timers ensures that the resource is only used when beneficial to the link and pilots are not constantly added and removed from the various lists, therefore limiting the associated signaling.

In addition to intrasystem, intrafrequency monitoring, the network may direct the terminal to look for base stations on a different frequency or a different system. CDMA2000 provides a framework to the terminal in support of the inter- frequency handover measurements consisting of identity and system parameters to be measured. The terminal performs required measurements as allowed by its hardware capability.

In case of a terminal with dual receiver structure, the measurement can be done in parallel. When a terminal has a single receiver, the channel reception will be interrupted when performing the measurement. In this instance, during the measurement, a certain portion of a frame will be lost. To improve the chance of successful decoding, the terminal is allowed to bias the FL power control loop and boost the RL transmit power before performing the measurement. This method increases the energy per information bit and reduces the risk of losing the link in the interval. Based on measurement reports provided by the terminal, the network then decides whether or not to hand-off a given terminal to a different frequency system. It does not release the resource until it receives confirmation that hand-off was successful or the timer expires. This enables the terminal to come back in case it could not acquire the new frequency or the new system.

Transmit Diversity
Transmit diversity consists of de-multiplexing and modulating data into two orthogonal signals, each of them transmitted from a different antenna at the same frequency. The two orthogonal signals are generated using either Orthogonal Transmit Diversity (OTD) or Space-Time Spreading (STS). The receiver reconstructs the original signal using the diversity signals, thus taking advantage of the additional space and/or frequency diversity.

Another transmission option is directive transmission. The base station directs a beam towards a single user or a group of users in a specific location, thus providing space separation in addition to code separation. Depending on the radio environment, transmit diversity techniques may improve the link performance by up to 5 dB.

Voice and Data Channels
The CDMA2000 forward traffic channel structure may include several physical channels:

  • The Fundamental Channel (F-FCH) is equivalent to functionality Traffic Channel (TCH) for IS-95. It can support data, voice, or signaling multiplexed with one another at any rate from 750 bps to 14.4 kbps.
  • The Supplemental Channel (F-SCH) supports high rate data services. The network may schedule transmission on the F-SCH on a frame-by- frame basis, if desired.
  • The Dedicated Control Channel (F-DCCH) is used for signaling or bursty data sessions. This channel allows for sending the signaling information without any impact on the parallel data stream.

The reverse traffic channel structure is similar to the forward traffic channel. It may include R-PICH, a Fundamental Channel (R-FCH), and/or a Dedicated Control Channel (R-DCCH), and one or several Supplemental Channels (R-SCH). Their functionality and encoding structure is the same as for the forward link with data rates ranging from 1 kbps to 1 Mbps (It is important to note that while the standard supports a maximum data rate of 1 Mbps, existing products are supporting a peak data rate of 307 kbps).

Traffic Channel
The traffic channel structure and frame format is very flexible. In order to limit the signaling load that would be associated with a full frame format parameter negotiation, CDMA2000 specifies a set of channel configurations. It defines a spreading rate and an associated set of frames for each configuration.

The forward traffic channel always includes either a fundamental channel or a dedicated control channel. The main benefit of this multichannel forward traffic structure is the flexibility to independently set up and tear down new services without any complicated multiplexing reconfiguration or code channel juggling. The structure also allows different hand-off configurations for different channels. For example, the F-DCCH, which carries critical signaling information, may be in soft hand-off, while the associated F-SCH operation could be based on a best cell strategy.

Supplemental Channels
One key CDMA2000 1X feature is the ability to support both voice and data services on the same carrier. CDMA2000 operates at up to 16 or 32 times the FCH rate-also referred to as 16x or 32x in Release 0 and A, respectively. In contrast to voice calls, the traffic generated by packet data calls is bursty, with small durations of high traffic separated by larger durations of no traffic. It is very inefficient to dedicate a permanent traffic channel to a packet data call. This burstiness impacts the amount of available power to the voice calls, possibly degrading their quality if the system is not engineered correctly. Hence, a key CDMA2000 design issue is assuring that a CDMA channel carrying voice and data calls simultaneously do so with negligible impact to the QoS of both.

Supplemental Channels (SCHs) can be assigned and deassigned at any time by the base station. The SCH has the additional benefit of improved modulation, coding, and power control schemes. This allows a single SCH to provide a data rate of up to 16 FCH in CDMA2000 Release 0 (or 153.6 kbps for Rate Set 1 rates), and up to 32 FCH in CDMA2000 Release A (or 307.2 kbps for Rate Set 1 rates). Note that each sector of a base station may transmit multiple SCHs simultaneously if it has sufficient transmit power and Walsh codes. The CDMA2000 standard limits the number of SCHs a mobile station can support simultaneously to two. This is in addition to the FCH or DCCH, which are set up for the entire duration of the call since they are used to carry signaling and control frames as well as data. Two approaches are possible: individually assigned SCHs, with either finite or infinite assignments, or shared SCHs with infinite assignments.

For bursty and delay-tolerant traffic, assigning a few scheduled fat pipes is preferable to dedicating many thin or slow pipes. The fat-pipe approach exploits variations in the channel conditions of different users to maximize sector throughput. The more sensitive the traffic becomes to delay, such as voice, the more appropriate the dedicated traffic channel approach becomes.

Turbo Coding
CDMA2000 provides the option of using either turbo coding or convolutional coding on the forward and reverse SCHs. Both coding schemes are optional for the base station and the mobile station, and the capability of each is communicated through signaling messages prior to the set up of the call. In addition to peak rate increase and improved rate granularity, the major improvement to the traffic channel coding in CDMA2000 is the support of turbo coding at rate 1/2, 1/3, or 1/4. The turbo code is based on 1/8 state parallel structure and can only be used for supplemental channels and frames with more than 360 bits. Turbo coding provides a very efficient scheme for data transmission and leads to better link performance and system capacity improvements. In general, turbo coding provides a performance gain in terms of power savings over convolutional coding. This gain is a function of the data rate, with higher data rates generally providing more turbo coding gain.

Bluetooth

BLUETOOTH


"Bluetooth" adalah sebuah standart baru yang diluncurkan oleh The Bluetooth SIG (Special Interest Group) pada bulan Mei 1998. The Bluetooth SIG ini terbentuk dari lima perusahaan besar.

Standart baru dalam wireless networking yang mereka luncurkan pada dasarnya adalah menggunakan hubungan radio jarak dekat atau short-range radio link untuk pertukaran informasi, sehingga hubungan antar hp, mobile PC, PDA, dan lainnya dapat dilakukan tanpa gangguan kabel atau wireless.

Tujuan dari peluncuran bluetooth ini adalah untuk mengganti spesifikasi IrDA dari InfraRed pada hp dan peralatan mobile lainnya.


Ericsson memberikan sumbangan mereka pada teknologi radio,Toshiba dan IBM mengembangkan spesifikasi untuk mengintegrasi teknologi "Bluetooth" kedalam peralatan mobile.Intel menyumbangkan keahlian mereka dalam chip dan software sedangkan Nokia menyumbangkan keahlian mereka dalam teknologi radio dan mobile handset software.

Banyak perusahaan lain juga diundang untuk mendukung teknologi intinya sehingga diharapkan teknologi ini dapat dipakai dalam banyak peralatan.

Radio ini akan beroperasi pada 2.45 GHz ISM 'free band', yang memungkinkan pengguna internasional dengan peralatan yang dilengkapi dengan"Bluetooth" dapat menggunakan peralatan mereka dimana saja diseluh dunia.


Sistem Bluetooth:

· Beroperasi pada 2.4 GHz Industrial-Scientific-Medical (ISM) band.

· Rangenya antara 10m s/d 100m

· Menggunakan Frequence Hop (FH) spread spectrum, yang membagi frequency band ke beberapa hop channels.

· Dalam sebuah koneksi, radio transceivers hop dari satu channel ke channel lainnya

· Mendukung sampai dengan 8 peralatan dalam suatu piconet (dua atau lebih unit Bluetooth dalam channel bersamaan).

· Keamanan built in..

· Omni-directional.

· Isochronous and asynchronous , integrasi mudah dengan TCP/IP bagi networking.


Penggunaan:

akan menghubungkan ......:

· Printers

· Mobile Phones

· Handsfree Headsets

· LCD projectors

· Modems

· Wireless LAN devices

· Notebooks

· Desktop PCs

· PDAs

· dll

....satu sama lainnya via Bluetooth short-range radio modules yang ada pada setiap peratan tersebut.

GPRS

GPRS (General Packet Radio Services)


Teknologi transmisi data GSM berupa GPRS(General Packet Radio Services) adalah sebuah teknologi yang dipergunakan untuk pelayanan data wireless seperti pada wireless internet atau intranet serta pelayanan multimedia.

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Alat komunikasi dengan GSM GPRS mempunyai fasilitas pertukaran data sampai dengan kecepatan 170kbps "

Juga biasanya disebut sebagai GSM-IP(internet Protocol), karena akan menghubungkan pengguna dengan ISP (Internet Service Provider).

Salah satu keuntungan dari teknologi baru ini adalah, pengguna akan selalu terhubung atau connected. Selalu online, tetapi akan dikenai biaya hanya dari besarnya data yang ditransmisi. Dengan teknologi ini panggilan secara voice dapat dilakukan secara bersamaan dengan transmisi data. Tetapi ini tergantung dari jenis hp yang digunakan.

Banyak pembuat hp telah menggunakan teknologi ini pada hpnya. Seperti Erricson, Nokia, Siemens, Sagem, Alcatel, Samsung, Motorola dan sebagainya.

GPRS merupakan suatu tambahan pelayanan baru pada network yang telah ada. Dengan GPRS , para operator network GSM dapat memberikan pelayanan kompetitif untuk pertukaran data, sehingga melengkapi jasa pelayanan yang mereka berikan.

Teknologi ini sedang berkembang, dan tentu saja seperti teknologi pada dunia wireless lainnya, teknologi ini akan berkembang sangat cepat. Seperti GPRS roaming yang merupakan kebutuhan dasar untuk terwujudnya global mobile internet bagi pelanggan GPRS pada berbagai operator GPRS network pada masa yang akan datang. Ini diterapkan pada GRX, yang merupakan suatu IP routing network tersentralisasi untuk interkoneksi antar GPRS network. Sistem GRX yang berdasarkan GPRS roaming telah berhasil dilakukan antara Sistem roaming Sonera dengan network dari Nokia.

Tentu saja banyak perusahaan besar lainnya terus mengembangkan sistem ini, yang pada akhirnya mudah2an dapat menguntungkan konsumen.

Fitur-fitur/features penting di GPRS

General Packet Radio Service (GPRS) adalah sebuah jasa pelayanan tambah berupa nonvoice yang memungkinkan informasi dapat dikirim dan diterima pada sebuah mobile telephone network. Sistem ini menambahkan fasilitas baru dari apa yang ada sekarang yaitu Circuit Switched Data and Short Message Service. GPRS tidak ada hubungannya dengan GPS (the Global Positioning System). Beberapa features unik GPRS adalah sebagai berikut :

KECEPATAN

Secara teoritis kecepatan maksimum sistem ini 171.2 kilobytes per second (kbps). Ini adalah adalah 3 kali lebih cepat dari transmisi data dari sistem network telekomunikasi fixed yang ada dan 10 kali lebih cepat dari Circuit Switched Data pada sistem network telekomunikasi wireless GSM saat ini.Dengan kecepatan pengiriman data, maka sistem ini tentu saja akan memberikan pelayanan yang lebih murah dibanding dengan SMS atau Circuit Switched Data.

SELALU TERSEDIA

GPRS adalah sebuah sistem yang terhubung terus, dimana informasi dapat segera dikirim atau diterima saat diperlukan. Tidak perlu adanya dial-up modem. Karena itu seperti telah diterangkan diatas sebelumnya, GPRS adalah sebuah sistem yang "always connected".

APLIKASI BARU YANG LEBIH BAIK

GPRS memberikan fasilitas baru untuk beberapa aplikasi yang sebelumnya tidak ada pada network GSM, karena adanya keterbatasan pada kecepatan di Circuit Switched Data (9.6 kbps) dan panjang message/berita pada Short Message Service (160 characters). GPRS akan dapat secara penuh mengaplikasi internet seperti yang biasa kita dapatkan di desktop komputer, dari web browsing sampai dengan chat. Selain itu GPRS akan memberikan kemudahan transfer file, dan home automation yaitu dengan dapatnya kita mengontrol alat2 dirumah kita secara remote dari daerah diluar rumah kita sendiri.

MENGGUNAKAN GPRS

Untuk dapat menggunakan GPRS, diperlukan beberapa hal yaitu:

  • Hp atau terminal dengan sistem GPRS (banyak telepon baru yang telah menyediakan sistem ini)
  • Mendaftar pada operator network dengan sistem GPRS
  • Penggunaan GPRS harus telah diizinkan oleh operator network bersangkutan.
  • Pengetahuan tentang bagaimana mengirim dan menerima informasi GPRS dengan hp yang dimiliki.
  • Sebuah tujuan untuk mengirim atau menerima informasi melalui GPRS. Kalau pada SMS biasanya tujuan ini adalah hp orang lain, maka pada GPRS kemungkinan tujuan yang paling banyak adalah alamat pada internet, karena GPRS didisain agar internet dapat digunakan oleh pemegang hp.

GSM Specifications

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Global System for Mobile Communication (GSM)

GSM Specifications

Before looking at the GSM specifications, it is important to understand the following basic terms:

  • bandwidth—the range of a channel's limits; the broader the bandwidth, the faster data can be sent
  • bits per second (bps)—a single on-off pulse of data; eight bits are equivalent to one byte
  • frequency—the number of cycles per unit of time; frequency is measured in hertz (Hz)
  • kilo (k)—kilo is the designation for 1,000; the abbreviation kbps represents 1,000 bits per second
  • megahertz (MHz)—1,000,000 hertz (cycles per second)
  • milliseconds (ms)—one-thousandth of a second
  • watt (W)—a measure of power of a transmitter

Specifications for different personal communication services (PCS) systems vary among the different PCS networks. Listed below is a description of the specifications and characteristics for GSM.

  • frequency band—The frequency range specified for GSM is 1,850 to 1,990 MHz (mobile station to base station).
  • duplex distance—The duplex distance is 80 MHz. Duplex distance is the distance between the uplink and downlink frequencies. A channel has two frequencies, 80 MHz apart.
  • channel separation—The separation between adjacent carrier frequencies. In GSM, this is 200 kHz.
  • modulation—Modulation is the process of sending a signal by changing the characteristics of a carrier frequency. This is done in GSM via Gaussian minimum shift keying (GMSK).
  • transmission rate—GSM is a digital system with an over-the-air bit rate of 270 kbps.
  • access method—GSM utilizes the time division multiple access (TDMA) concept. TDMA is a technique in which several different calls may share the same carrier. Each call is assigned a particular time slot.
  • speech coder—GSM uses linear predictive coding (LPC). The purpose of LPC is to reduce the bit rate. The LPC provides parameters for a filter that mimics the vocal tract. The signal passes through this filter, leaving behind a residual signal. Speech is encoded at 13 kbps.

Global System for Mobile Communication (GSM)

GSM Subscriber Services

There are two basic types of services offered through GSM: telephony (also referred to as teleservices) and data (also referred to as bearer services). Telephony services are mainly voice services that provide subscribers with the complete capability (including necessary terminal equipment) to communicate with other subscribers. Data services provide the capacity necessary to transmit appropriate data signals between two access points creating an interface to the network. In addition to normal telephony and emergency calling, the following subscriber services are supported by GSM:

  • dual-tone multifrequency (DTMF)—DTMF is a tone signaling scheme often used for various control purposes via the telephone network, such as remote control of an answering machine. GSM supports full-originating DTMF.
  • facsimile group III—GSM supports CCITT Group 3 facsimile. As standard fax machines are designed to be connected to a telephone using analog signals, a special fax converter connected to the exchange is used in the GSM system. This enables a GSM–connected fax to communicate with any analog fax in the network.
  • short message services—A convenient facility of the GSM network is the short message service. A message consisting of a maximum of 160 alphanumeric characters can be sent to or from a mobile station. This service can be viewed as an advanced form of alphanumeric paging with a number of advantages. If the subscriber's mobile unit is powered off or has left the coverage area, the message is stored and offered back to the subscriber when the mobile is powered on or has reentered the coverage area of the network. This function ensures that the message will be received.
  • cell broadcast—A variation of the short message service is the cell broadcast facility. A message of a maximum of 93 characters can be broadcast to all mobile subscribers in a certain geographic area. Typical applications include traffic congestion warnings and reports on accidents.
  • voice mail—This service is actually an answering machine within the network, which is controlled by the subscriber. Calls can be forwarded to the subscriber's voice-mail box and the subscriber checks for messages via a personal security code.
  • fax mail—With this service, the subscriber can receive fax messages at any fax machine. The messages are stored in a service center from which they can be retrieved by the subscriber via a personal security code to the desired fax number.

Supplementary Services

GSM supports a comprehensive set of supplementary services that can complement and support both telephony and data services. Supplementary services are defined by GSM and are characterized as revenue-generating features. A partial listing of supplementary services follows.

  • call forwarding—This service gives the subscriber the ability to forward incoming calls to another number if the called mobile unit is not reachable, if it is busy, if there is no reply, or if call forwarding is allowed unconditionally.
  • barring of outgoing calls—This service makes it possible for a mobile subscriber to prevent all outgoing calls.
  • barring of incoming calls—This function allows the subscriber to prevent incoming calls. The following two conditions for incoming call barring exist: baring of all incoming calls and barring of incoming calls when roaming outside the home PLMN.
  • advice of charge (AoC)—The AoC service provides the mobile subscriber with an estimate of the call charges. There are two types of AoC information: one that provides the subscriber with an estimate of the bill and one that can be used for immediate charging purposes. AoC for data calls is provided on the basis of time measurements.
  • call hold—This service enables the subscriber to interrupt an ongoing call and then subsequently reestablish the call. The call hold service is only applicable to normal telephony.
  • call waiting—This service enables the mobile subscriber to be notified of an incoming call during a conversation. The subscriber can answer, reject, or ignore the incoming call. Call waiting is applicable to all GSM telecommunications services using a circuit-switched connection.
  • multiparty service—The multiparty service enables a mobile subscriber to establish a multiparty conversation—that is, a simultaneous conversation between three and six subscribers. This service is only applicable to normal telephony.
  • calling line identification presentation/restriction—These services supply the called party with the integrated services digital network (ISDN) number of the calling party. The restriction service enables the calling party to restrict the presentation. The restriction overrides the presentation.
  • closed user groups (CUGs)—CUGs are generally comparable to a PBX. They are a group of subscribers who are capable of only calling themselves and certain numbers.

GSM Network Areas

The GSM network is made up of geographic areas. As shown in , these areas include cells, location areas (LAs), MSC/VLR service areas, and public land mobile network (PLMN) areas.
Figure 3

The cell is the area given radio coverage by one base transceiver station. The GSM network identifies each cell via the cell global identity (CGI) number assigned to each cell. The location area is a group of cells. It is the area in which the subscriber is paged. Each LA is served by one or more base station controllers, yet only by a single MSC. Each LA is assigned a location area identity (LAI) number.Figure 4

An MSC/VLR service area represents the part of the GSM network that is covered by one MSC and which is reachable, as it is registered in the VLR of the MSC .

Figure 5

The PLMN service area is an area served by one network operator .


GSM Overview

GSM 06.10 A Brief Overview of GSM, by John Scourias


A Brief Overview of GSM, by John Scourias, U of Waterloo

See also, John Scourias' extended abstract of this paper, and his recently finished updated version of this text.

1 History of GSM

2 Services provided by GSM

3 Architecture of the GSM network

3.1 Mobile Station
3.2
Base Station Subsystem
3.3
Network Subsystem

4 Radio link aspects

4.1 Channel structure
4.2
Speech coding
4.3
Channel coding and modulation
4.4
Multipath equalization
4.5
Frequency hopping
4.6
Discontinuous transmission
4.7
Discontinuous reception
4.8
Power control

5 Network aspects

5.1 Handover
5.2
Location updating and call
5.3
Authentication and security

6 Conclusion and comments


1 History of GSM

References: [Che91, Bal91, Hau88, Mal88, Bal93, DS93, FR93]

During the early 1980s, analog cellular telephone systems were experiencing rapid growth in Europe, particularly in Scandinavia and the United Kingdom, but also in France and Germany. Each country developed its own system, which was incompatible with everyone else's in equipment and operation. This was an undesirable situation, because not only was the mobile equipment limited to operation within national boundaries, which in a unified Europe were increasingly unimportant, but there was a very limited market for each type of equipment, so economies of scale, and the subsequent savings, could not be realized.

The Europeans realized this early on, and in 1982 the Conference of European Posts and Telegraphs (CEPT) formed a study group called the Groupe Sp├ęcial Mobile (GSM) to study and develop a pan­European public land mobile system. The proposed system had to meet certain criteria:

  • good subjective speech quality,
  • low terminal and service cost,
  • support for international roaming,
  • ability to support handhald terminals,
  • support for range of new services and facilities,
  • spectral efficiency, and
  • ISDN compatibility.

In 1989, GSM responsibility was transferred to the European Telecommunication Standards Institute (ETSI), and phase I of the GSM specifications were published in 1990. Commercial service was started in mid­1991, and by 1993 there were 36 GSM networks in 22 countries, with 25 additional countries having already selected or considering GSM [DS93]. This is not only a European standard - South Africa, Australia, and many Middle and Far East countries have chosen GSM. By the beginning of 1994, there were 1.3 million subscribers worldwide [Nil]. The acronym GSM now (aptly) stands for Global System for Mobile telecommunications.

The developers of GSM chose an unproven (at the time) digital system, as opposed to the then­standard analog cellular systems like AMPS in the United States and TACS in the United Kingdom. They had faith that advancements in compression algorithms and digital signal processors would allow the fulfillment of the original criteria and the continual improvement of the system in terms of quality and cost. The 8000 pages of the GSM recommendations try to allow flexibility and competitive innovation among suppliers, but provide enough guidelines to guarantee the proper interworking between the components of the system. This is done in part by providing descriptions of the interfaces and functions of each of the functional entities defined in the system.

2 Services provided by GSM

References: [Har93a, Har93b, DS93, FR93, LM92, Hub92]

From the beginning, the planners of GSM wanted ISDN compatibility in services offered and control signalling used. The radio link imposed some limitations, however, since the standard ISDN bit rate of 64 kbps could not be practically achieved.

Using the ITU­T definitions, telecommunication services can be divided into bearer services, teleservices, and supplementary services. The digital nature of GSM allows data, both synchronous and asynchronous, to be transported as a bearer service to or from an ISDN terminal. Data can use either the transparent service, which has a fixed delay but no guarantee of data integrity, or a non­transparent service, which guarantees data integrity through an Automatic Repeat Request (ARQ) mechanism, but with a variable delay. The data rates supported by GSM are 300 bps, 600 bps, 1200 bps, 2400 bps, and 9600 bps [Har93a].

The most basic teleservice supported by GSM is telephony. There is an emergency service, where the nearest emergency­service provider is notified by dialling three digits (similar to 911). Group 3 fax, an analog method described in ITU­T recommendation T.30 [Har93b], is also supported by use of an appropriate fax adaptor. A unique feature of GSM compared to older analog systems is the Short Message Service (SMS). SMS is a bidirectional service for sending short alphanumeric (up to 160 bytes) messages in a store­and­forward fashion. For point­to­point SMS, a message can be sent to another subscriber to the service, and an acknowledgement of receipt is provided to the sender. SMS can also be used in a cell­broadcast mode, for sending messages such as traffic updates or news updates. Messages can be stored in the SIM card for later retrieval [Bal93].

Supplementary services are provided on top of teleservices or bearer services, and include features such as caller identification, call forwarding, call waiting, multi­party conversations, and barring of outgoing (international) calls, among others.

3 Architecture of the GSM network

References: [DS93, FR93, B+93, LM92, Hub92, Rah93, SK93]

A GSM network is composed of several functional entities, whose functions and interfaces are defined. Figure 1 shows the layout of a generic GSM network. The GSM network can be divided into three broad parts. The Mobile Station is carried by the subscriber, the Base Station Subsystem controls the radio link with the Mobile Station. The Network Subsystem, the main part of which is the Mobile services Switching Center, performs the switching of calls between the mobile and other fixed or mobile network users, as well as management of mobile services, such as authentication. Not shown is the Operations and Maintenance center, which oversees the proper operation and setup of the network. The Mobile Station and the Base Station Subsystem communicate across the Um interface, also known as the air interface or radio link. The Base Station Subsystem communicates with the Mobile service Switching Center across the A interface.

3.1 Mobile Station

The mobile station (MS) consists of the physical equipment, such as the radio transceiver, display and digital signal processors, and a smart card called the Subscriber Identity Module (SIM). The SIM provides personal mobility, so that the user can have access to all subscribed services irrespective of both the location of the terminal and the use of a specific terminal. By inserting the SIM card into another GSM cellular phone, the user is able to receive calls at that phone, make calls from that phone, or receive other subscribed services.

The mobile equipment is uniquely identified by the International Mobile Equipment Identity (IMEI). The SIM card contains the International Mobile Subscriber Identity (IMSI), identifying the subscriber, a secret key for authentication, and other user information. The IMEI and the IMSI are independent, thereby providing personal mobility. The SIM card may be protected against unauthorized use by a password or personal identity number.

3.2 Base Station Subsystem

The Base Station Subsystem is composed of two parts, the Base Transceiver Station (BTS) and the Base Station Controller (BSC). These communicate across the specified A­bis interface, allowing (as in the rest of the system) operation between components made by different suppliers.

The Base Transceiver Station houses the radio tranceivers that define a cell and handles the radio­link protocols with the Mobile Station. In a large urban area, there will potentially be a large number of BTSs deployed. The requirements for a BTS are ruggedness, reliability, portability, and minimum cost.

The Base Station Controller manages the radio resources for one or more BTSs. It handles radio­channel setup, frequency hopping, and handovers, as described below. The BSC is the connection between the mobile and the Mobile service Switching Center (MSC). The BSC also translates the 13 kbps voice channel used over the radio link to the standard 64 kbps channel used by the Public Switched Telephone Network or ISDN.

3.3 Network Subsystem

The central component of the Network Subsystem is the Mobile services Switching Center (MSC). It acts like a normal switching node of the PSTN or ISDN, and in addition provides all the functionality needed to handle a mobile subscriber, such as registration, authentication, location updating, handovers, and call routing to a roaming subscriber. These services are provided in conjuction with several functional entities, which together form the Network Subsystem. The MSC provides the connection to the public fixed network (PSTN or ISDN), and signalling between functional entities uses the ITU­T Signalling System Number 7 (SS7), used in ISDN and widely used in current public networks.

The Home Location Register (HLR) and Visitor Location Register (VLR), together with the MSC, provide the call­routing and (possibly international) roaming capabilities of GSM. The HLR contains all the administrative information of each subscriber registered in the corresponding GSM network, along with the current location of the mobile. The current location of the mobile is in the form of a Mobile Station Roaming Number (MSRN) which is a regular ISDN number used to route a call to the MSC where the mobile is currently located. There is logically one HLR per GSM network, although it may be implemented as a distributed database.

The Visitor Location Register contains selected administrative information from the HLR, necessary for call control and provision of the subscribed services, for each mobile currently located in the geographical area controlled by the VLR. Although each functional entity can be implemented as an independent unit, most manufacturers of switching equipment implement one VLR together with one MSC, so that the geographical area controlled by the MSC corresponds to that controlled by the VLR, simplifying the signalling required. Note that the MSC contains no information about particular mobile stations - this information is stored in the location registers.

The other two registers are used for authentication and security purposes. The Equipment Identity Register (EIR) is a database that contains a list of all valid mobile equipment on the network, where each mobile station is identified by its International Mobile Equipment Identity (IMEI). An IMEI is marked as invalid if it has been reported stolen or is not type approved. The Authentication Center is a protected database that stores a copy of the secret key stored in each subscriber's SIM card, which is used for authentication and ciphering of the radio channel.

4 Radio link aspects

The International Telecommunication Union (ITU), which manages the international allocation of radio spectrum (among other functions) allocated the bands 890-915 MHz for the uplink (mobile station to base station) and 935-960 MHz for the downlink (base station to mobile station) for mobile networks in Europe. Since this range was already being used in the early 1980s by the analog systems of the day, the CEPT had the foresight to reserve the top 10 MHz of each band for the GSM network that was still being developed. Eventually, GSM will be allocated the entire 2x25 MHz bandwidth.

Since radio spectrum is a limited resource shared by all users, a method must be devised to divide up the bandwidth among as many users as possible. The method chosen by GSM is a combination of Time­ and Frequency­Division Multiple Access (TDMA/FDMA). The FDMA part involves the division by frequency of the total 25 MHz bandwidth into 124 carrier frequencies of 200 kHz bandwidth. One or more carrier frequencies are then assigned to each base station. Each of these carrier frequencies is then divided in time, using a TDMA scheme, into eight time slots. One time slot is used for transmission by the mobile and one for reception. They are separated in time so that the mobile unit does not receive and transmit at the same time, a fact that simplifies the electronics.

In the rest of this section, the procedure involved in digitally transmitting a voice signal in a GSM network is examined, along with some of the features, such as discontinuous transmission and reception, used to improve voice quality, reduce the mobile unit's power consumption, and increase the overall capacity of the network.

4.1 Channel structure

The structure of the most common time­slot burst is shown in Figure 2. A total of 156.25 bits is transmitted in 0.577 milliseconds, giving a gross bit rate of 270.833 kbps. There are three other types of burst structure for frame and carrier synchronization and frequency correction. The 26­bit training sequence is used for equalization, as described below. The 8.25 bit guard time allows for some propagation time delay in the arrival of bursts.

Each group of eight time slots is called a TDMA frame, which is transmitted every 4.615 ms. TDMA frames are further grouped into multiframes to carry control signals. There are two types of multiframe, containing 26 or 51 TDMA frames. The 26­frame multiframe contains 24 Traffic Channels (TCH) and two Slow Associated Control Channels (SACCH) which supervise each call in progress. The SACCH in frame 12 contains eight channels, one for each of the eight connections carried by the TCHs. The SACCH in frame 25 is not currently used, but will carry eight additional SACCH channels when half­rate traffic is implemented. A Fast Associated Control Channel (FACCH) works by stealing slots from a traffic channel to transmit power control and handover­signalling messages. The channel stealing is done by setting one of the control bits in the time slot burst.

In addition to the Associated Control Channels, there are several other control channels which (except for the Stand­alone Dedicated Control Channel) are implemented in time slot 0 of specified TDMA frames in a 51­frame multiframe, implemented on a non­hopping carrier frequency in each cell. The control channels include:

  • Broadcast Control Channel (BCCH): Continually broadcasts, on the downlink, information including base station identity, frequency allocations, and frequency­hopping sequences.
  • Stand­alone Dedicated Control Channel (SDCCH): Used for registration, authentication, call setup, and location updating. Implemented on a time slot, together with its SACCH, selected by the system operator.
  • Common Control Channel (CCCH): Comprised of three control channels used during call origination and call paging.
    • Random Access Channel (RACH): A slotted Aloha channel to request access to the network
    • Paging Channel (PCH): Used to alert the mobile station of incoming call.
    • Access Grant Channel (AGCH): Used to allocate an SDCCH to a mobile for signalling, following a request on the RACH.

4.2 Speech coding

GSM is a digital system, so speech signals, inherently analog, have to be digitized. The method employed by ISDN, and by current telephone systems for multiplexing voice lines over high speed trunks and optical fiber lines, is Pulse Coded Modulation (PCM). The output stream from PCM is 64 kbps, too high a rate to be feasible over a radio link. The 64 kbps signal contains much redundancy, although it is simple to implement. The GSM group studied several voice coding algorithms on the basis of subjective speech quality and complexity (which is related to cost, processing delay, and power consumption once implemented) before arriving at the choice of a Regular Pulse Excited - Linear Predictive Coder (RPE­LPC) with a Long Term Predictor loop. Basically, information from previous samples, which does not change very quickly, is used to predict the current sample. The coefficients of the linear combination of the previous samples, plus an encoded form of the residual, the difference between the predicted and actual sample, represent the signal. Speech is divided into 20 millisecond samples, each of which is encoded as 260 bits, giving a total bit rate of 13 kbps.

4.3 Channel coding and modulation

Due to natural or man­made electromagnetic interference, the encoded speech or data transmitted over the radio interface must be protected as much as is practical. The GSM system uses convolutional encoding and block interleaving to achieve this protection. The exact algorithms used differ for speech and for different data rates. The method used for speech blocks will be described below.

Recall that the speech codec produces a 260 bit block for every 20 ms speech sample. From subjective testing, it was found that some bits of this block were more important for perceived speech quality than others. The bits are thus divided into three classes:

Class Ia 50 bits - most sensitive to bit errors
Class Ib
132 bits - moderately sensitive to bit errors
Class II
78 bits - least sensitive to bit errors

Class Ia bits have a 3 bit Cyclic Redundancy Code added for error detection. If an error is detected, the frame is judged too damaged to be comprehensible and it is discarded. It is replaced by a slightly attenuated version of the previous correctly received frame. These 53 bits, together with the 132 Class Ib bits and a 4 bit tail sequence (a total of 189 bits), are input into a 1/2 rate convolutional encoder of constraint length 4. Each input bit is encoded as two output bits, based on a combination of the previous 4 input bits. The convolutional encoder thus outputs 378 bits, to which are added the 78 remaining Class II bits, which are unprotected. Thus every 20 ms speech sample is encoded as 456 bits, giving a bit rate of 22.8 kbps.

To further protect against the burst errors common to the radio interface, each sample is diagonally interleaved. The 456 bits output by the convolutional encoder are divided into 8 blocks of 57 bits, and these blocks are transmitted in eight consecutive time­slot bursts. Since each time­slot burst can carry two 57 bit blocks, each burst carries traffic from two different speech samples.

Recall that each time­slot burst is transmitted at a gross bit rate of 270.833 kbps. This digital signal is modulated onto the analog carrier frequency, which has a bandwidth of 200 kHz, using Gaussian­filtered Minimum Shift Keying (GMSK). GMSK was selected over other modulation schemes as a compromise between spectral efficiency, complexity of the transmitter, and limited spurious emissions. The complexity of the transmitter is related to power consumption, which should be minimized for the mobile station. The spurious radio emissions, outside of the allotted bandwidth, must be strictly controlled so as to limit adjacent channel interference, and allow for the co­existence of GSM and the older analog systems (at least for the time being).

4.4 Multipath equalization

At the 900 MHz range, radio waves bounce off everything - buildings, hills, cars, airplanes, etc. Thus many reflected signals, each with a different phase, can reach an antenna. Equalization is used to extract the desired signal from the unwanted reflections. Equalization works by finding out how a known transmitted signal is modified by multipath fading, and constructing an inverse filter to extract the rest of the desired signal. This known signal is the 26­bit training sequence transmitted in the middle of every time slot burst. The actual implementation of the equalizer is not specified in the GSM specifications.

4.5 Frequency hopping

The mobile station already has to be frequency agile, meaning it can move between a transmit, receive, and monitor time slot within one TDMA frame, which may be on different frequencies. GSM makes use of this inherent frequency agility to implement slow frequency hopping, where the mobile and BTS transmit each TDMA frame on a different carrier frequency. The frequency hopping algorithm is broadcast on the Broadcast Control Channel. Since multipath fading is (mildly) dependent on carrier frequency, slow frequency hopping helps alleviate the problem. In addition, co­channel interference is in effect randomized.

4.6 Discontinuous transmission

Minimizing co­channel interference is a goal of any cellular system, since it allows better service for a given cell size, or the use of smaller cells, thus increasing the overall capacity of the system. Discontinuous transmission (DTX) is a method that takes advantage of the fact that a person speaks less that 40 percent of the time in normal conversation [S+89], by turning the transmitter off during silence periods. An added benefit of DTX is that power is conserved at the mobile unit.

The most important component of DTX is, of course, Voice Activity Detection. It must distinguish between voice and noise inputs, a task that is not as trivial as it appears, considering background noise. If a voice signal is misinterpreted as noise, the transmitter is turned off and a very annoying effect called clipping is heard at the receiving end. If, on the other hand, noise is misinterpreted as a voice signal too often, the efficiency of DTX is dramatically decreased. Another factor to consider is that when the transmitter is turned off, there is a very silent silence heard at the receiving end, due to the digital nature of GSM. To assure the receiver that the connection is not dead, comfort noise is created at the receiving end by trying to match the characteristics of the transmitting end's background noise.

4.7 Discontinuous reception

Another method used to conserve power at the mobile station is discontinuous reception. The paging channel, used by the base station to signal an incoming call, is structured so that the mobile station knows when it needs to check for a paging signal. In the time between paging signals, the mobile can go into sleep mode, when almost no power is used.

4.8 Power control

There are five classes of mobile stations defined, according to their peak transmitter power, rated at 20, 8, 5, 2, and 0.8 watts. To minimize co­channel interference and to conserve power, both the mobiles and the Base Transceiver Stations operate at the lowest power level that will maintain an acceptable signal quality. Power levels can be stepped up or down in steps of 2 dB from the peak power for the class down to a minimum of 13 dBm (20 milliwatts).

The mobile station measures the signal strength or signal quality (based on the Bit Error Ratio), and passes the information to the Base Station Controller, which ultimately decides if and when the power level should be changed. Power control should be handled carefully, since there is the possibility of instability. This arises from having mobiles in co­channel cells alternatingly increase their power in response to increased co­channel interference caused by the other mobile increasing its power. This in unlikely to occur in practice but it is (or was as of 1991) under study.

5 Network aspects

References: [Aud88, Rah93, Che91, Bal91, Bal93]

Ensuring the transmission of voice or data of a given quality over the radio link is only half the problem in a cellular mobile network. The fact that the geographical area covered by the network is divided into cells necessitates the implementation of a handover mechanism. Also, the fact that the mobile can roam nationally and internationally in GSM requires that registration, authentication, call routing and location updating functions exist in the GSM network.


The signalling protocol in GSM is structured in three layers [Rah93, Aud88], shown in Figure 3.
Layer 1 is the physical layer, which uses the channel structures discussed above. Layer 2 is the data link layer. Across the Um interface, the data link layer uses a slight modification of the LAPD protocol used in ISDN, called LAPDm. Across the A interface, the lower parts of Signalling System Number 7 are used. Layer 3 is subdivided into 3 sublayers.

Radio Resources Management

controls the setup, maintenance, and termination of radio channels

Mobility Management

manages the location updating, handovers, and registration procedures, discussed below

Connection Management

handles general call control, similar to CCITT Recommendation Q.931, and provides supplementary services.

Signalling between the different entities in the network, such as between the HLR and VLR, is accomplished throught the Mobile Application Part (MAP). Application parts are the top layer of Signalling System Number 7. The specification of the MAP is complex. It is one of the longest documents in the GSM recommendations, said to be over 600 pages in length [Che91].

Described below are the main functions of the Mobility Management sublayer.

5.1 Handover

Handover, or handoff as it is called in North America, is the switching of an on­going call to a different channel or cell. There are four different types of handover in the GSM system, which involve transferring a call between

  • channels (time slots) in the same cell,
  • cells (Base Transceiver Stations) under the control of the same Base Station Controller (BSC),
  • cells under the control of different BSCs, but belonging to the same Mobile services Switching Center (MSC), and
  • cells under the control of different MSCs.

The first two types of handover, called internal handovers, involve only one Base Station Controller (BSC). To save signalling bandwidth, they are managed by the BSC without involving the Mobile service Switching Center (MSC), except to notify it at the completion of the handover. The last two types of handover, called external handovers, are handled by the MSCs involved. Note that call control, such as provision of supplementary services and requests for further handoffs, is handled by the original MSC.

Handovers can be initiated by either the mobile or the MSC (as a means of traffic load balancing). During its idle time slots, the mobile scans the Broadcast Control Channel of up to 16 neighboring cells, and forms a list of the six best candidates for possible handover, based on the received signal strength. This information is passed to the BSC and MSC, and is used by the handover algorithm.

The algorithm for when a handover decision should be taken is not specified in the GSM recommendations. There are two basic algorithms used, both closely tied in with power control. This is because the BSC usually does not know whether the poor signal quality is due to multipath fading or to the mobile having moved to another cell. This is especially true in small urban cells.

The 'minimum acceptable performance' algorithm [Bal91] gives precedence to power control over handover, so that when the signal degrades beyond a certain point, the power level of the mobile is increased. If further power increases do not improve the signal, then a handover is considered. This is the simpler and more common method, but it creates 'smeared' cell boundaries when a mobile transmitting at peak power goes some distance beyond its original cell boundaries into another cell.

The 'power budget' method [Bal91] uses handover to try to maintain or improve a certain level of signal quality at the same or lower power level. It thus gives precedence to handover over power control. It avoids the 'smeared' cell boundary problem and reduces co­channel interference, but it is quite complicated.

5.2 Location updating and call routing

References: [MJ94, Rah93, DS93]

The MSC provides the interface between the GSM mobile network and the public fixed network. From the fixed network's point of view, the MSC is just another switching node. However, switching is a little more complicated in a mobile network since the MSC has to know where the mobile is currently roaming - and in GSM it could even be roaming in another country. The way GSM accomplishes location updating and call routing to the mobile is by using two location registers: the Home Location Register (HLR) and the Visitor Location Register (VLR).

Location updating is initiated by the mobile when, by monitoring the Broadcast Control Channel, it notices that the location­area broadcast is not the same as the one previously stored in the mobile's memory. An update request and the IMSI or previous TMSI is sent to the new VLR via the new MSC. A Mobile Station Roaming Number (MSRN) is allocated and sent to the mobile's HLR (which always keeps the most current location) by the new VLR. The MSRN is a regular telephone number that routes the call to the new VLR and is subsequently translated to the TMSI of the mobile. The HLR sends back the necessary call­control parameters, and also sends a cancel message to the old VLR, so that the previous MSRN can be reallocated. Finally, a new TMSI is allocated and sent to the mobile, to identify it in future paging or call initiation requests.


With the above location­updating procedure, call routing to a roaming mobile is easily performed.
The most general case is shown in Figure 4 [Aud88], where a call from a fixed network (Public Switched Telecommunications Network or Integrated Services Digital Network) is placed to a mobile subscriber. Using the Mobile Subscriber's telephone number (MSISDN, the ISDN numbering plan specified in the ITU­T E.164 recommendation), the call is routed through the fixed land network to a gateway MSC for the GSM network (an MSC that interfaces with the fixed land network, thus requiring an echo canceller). The gateway MSC uses the MSISDN to query the Home Location Register, which returns the current roaming number (MSRN). The MSRN is used by the gateway MSC to route the call to the current MSC (which is usually coupled with the VLR). The VLR then converts the roaming number to the mobile's TMSI, and a paging call is broadcast by the cells under the control of the current BSC to inform the mobile.

5.3 Authentication and security

References: [DS93, FR93, LM92]

Since the radio medium can be accessed by anyone, authentication of users to prove that they are who they claim to be, is a very important element of a mobile network. Authentication involves two functional entities, the SIM card in the mobile, and the Authentication Center (AC). Each subscriber is given a secret key, one copy of which is stored in the SIM card and the other in the Authentication Center. During authentication, the AC generates a random number that it sends to the mobile. Both the mobile and the AC then use the random number, in conjuction with the subscriber's secret key and a ciphering algorithm called A3, to generate a number that is sent back to the AC. If the number sent by the mobile is the same as the one calculated by the AC, the subscriber is authenticated.

The above calculated number is also used, together with a TDMA frame number and another ciphering algorithm called A5, to encipher the data sent over the radio link, preventing others from listening in. Enciphering is an option for the very paranoid, since the signal is already coded, interleaved, and transmitted in a TDMA manner, thus providing protection from all but the most persistent and dedicated eavesdroppers.

Another level of security is performed on the mobile equipment, as opposed to the mobile subscriber. As mentioned earlier, each GSM terminal is identified by a unique International Mobile Equipment Identity (IMEI) number. A list of IMEIs in the network is stored in the Equipment Identity Register (EIR). The status returned in response to an IMEI query to the EIR is one of the following:

white­listed

The terminal is allowed to connect to the network

grey­listed

Under observation from the network, possible problems

black­listed

The terminal has either been reported as stolen, or it is not type approved (the correct type of terminal for a GSM network). The terminal is not allowed to connect to the network.

6 Conclusion and comments

References: [Mal88]

In this paper I have tried to give an overview of the GSM system. As with any overview, and especially one covering a standard 8000 pages long, there are many details missing. I believe, however, that I gave the general flavor of GSM and the philosophy behind its design. It was a monumental task that the original GSM committee undertook, and one that has proven a success, showing that international cooperation on such projects between academia, industry, and government can succeed. It is a standard that ensures interoperability without stifling competition and innovation among suppliers, to the benefit of the public both in terms of cost and service quality. For example, by using Very Large Scale Integration (VLSI) microprocessor technology, many of functions of the mobile station can be built in one chipset, resulting in lighter, smaller, and more energy­efficient terminals.

Telecommunications are evolving towards personal communication networks, whose objective can be stated as the availability of all communication services anytime, anywhere, to anyone, by a single identity number and a pocketable communication terminal [Win93]. Having a multitude of incompatible systems throughout the world moves us farther away from, not closer to, this ideal. The economies of scale created by a unified system are enough to justify its implementation, not to mention the convenience to people of carrying just one communication terminal anywhere they go, regardless of national boundaries.

The GSM system, and its twin system operating at 1800 MHz, called DCS1800, are a first approach at a true personal communication system. The SIM card is a novel approach that implements personal mobility in addition to terminal mobility. Together with international roaming, and support for many other services such as data transfer, fax, Short Message Service, and supplementary services, in addition to telephony, GSM comes close to fulfilling the requirements for a personal communication system: close enough that it is being used as a basis for the next generation of communication technology in Europe.

Another point where GSM has shown its commitment to openness, standards and interoperability is the compatibility with the Integrated Services Digital Network (ISDN) that is evolving in most industrialized countries, and Europe in particular (the so­called Euro­ISDN). GSM is the first system to make extensive use of the Intelligent Networking concept in ISDN, in which services like 800 numbers are concentrated and handled from a few centralized service centers, instead of being distributed over every switching center in the country. This is the concept behind the use of the various registers such as the HLR. In addition, the signalling between these functional entities uses Signalling System Number 7, an international standard already used in many countries and specified for ISDN.

GSM is a very complex standard, but that is probably the price that must be paid to achieve the level of integrated service and quality offered while subject to the fairly severe restrictions imposed by the radio environment.


Copyright 1994 by John Scourias, jscourias@barrow.uwaterloo.ca.

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