ELEKTRO INDONESIA                      Edisi Perdana, Maret 1996

• Kontrol Kongesti pada Jaringan Frame Relay
• The MSS Spectrum Requirements for The Asia-Pacific

Pendahuluan

1. Panggilan yang baru di blok,dan
   
2. Menyesuaikan dengan situasi jaringan (membuat sumber- sumber baru atau dengan mengurangi perintah di dalam jaringan atau dengan mengurangi tambahan servis).

Kongesti terjadi ketika sumber jaringan kelebihan beban, sumber akan menjadi individual transmission link, kelompok buffer penuh pada node-intermediate atau pada sistem tujuan atau proses dalam salah satu dari sistem-sistem ini. Kongesti mungkin juga terjadi karena adanya gangguan.

Bahwa kontrol kongesti tidak dapat tercapai dengan menambahkan sumber-sumber dalam jaringan dalam formasi kapasitas buffer atau menambah kecepatan link lebih tinggi. Kedua-duanya tidak dapat dikontrol dengan konfigurasi balans; sebab kejadian trafik dapat diramalkan, kemacetan masih dapat terjadi. Kongesti mungkin "inherent" terjadi dalam beberapa jaringan packet dan jaringan Frame Relay tanpa kecuali. Sebab itu hal ini penting untuk memiliki "strategi" kontrol kongesti untuk jaringan Frame Relay.

Jika beban trafik terus meningkat, kongesti akan menjadi semakin serius (parah) dan beban prosesor sistem akan semakin berat, serta dapat mengakibatkan kegagalan sistem. Untuk mencegah terjadinya kegagalan sistem ini, diperlukan suatu kontrol kongesti yang dapat mengurangi beban sistem.

Indikator-indikator dan cara mengatasi kongesti

Dengan menetapkan daerah kinerja jaringan suatu pendekatan skema kongesti dapat diimplementasikan untuk mengatasi kongesti yang mana akan mengarahkan bentuk kurva dengan 2 (dua) daerah pertama dan mencegahnya melewati daerah cliff. Rencana penanggulangan kongesti memiliki banyak obyek angka masukan frame discard yang diminimumkan, mempertahankan QOS, mencegah suatu single-user dari monopoli penggunaan jaringan dan membatasi timbulnya kongesti pada jaringan tersebut atau pada jaringan yang lain. Suatu rencana pencegahan yang baik akan jadi mudah untuk dilaksanakan dan akan menghasilkan penambahan jaringan trafik minimal.

Pendekatan untuk kontrol kongesti dalam jaringan Frame Relay

1. User mendeteksi secara implisit daerah kongesti pada jaringan,
   
2. User dapat mengindikasi jaringan yang mana dari frame-frame mungkin dibuang pada saat terjadi kongesti, dan
   
3. Pemberitahuan diberikan pada user pada saat jaringan itu sendiri mendeteksi adanya kongesti.

User dapat menetapkan kongesti yang implisit didasarkan pada suatu jumlah parameter-parameter sebagai contoh jumlah REJ yang diterima, rate kehilangan frame yang terjadi atau throughput dari call. Kontrol biasanya digunakan dengan memvariasikan ukuran window. Untuk contoh di dalam sistem prediksi-dasar yang digambarkan belakangan ini, perbedaan window meningkatkan/menurunkan "algoritma" yang digunakan tergantung daerah kongesti mana yang diantisipasi.

Di dalam rencana (scheme) Bandwidth Management (BMW), end-user menegosiasi dengan jaringan selama tahap pembentukan call bagi suatu jaminan throughput (Committed Information Rate = CIR). Selama tahap pentransferan data, jaringan memonitor rate dari frame yang mana yang dikirim oleh end-user. Jika rate kedatangan berlebihan sesuai throughput yang disetujui, beberapa frame dikirim di atas level yang diberi label (menggunakan indikator Discard Eligibility = DE) sesuai kelebihan frame. Frame-frame yang dikirim di bawah level diberi label committed frames. Pada saat jaringan terjadi kongesti, kelebihan frame dibuang. Ketika tidak terjadi kongesti, end-user mungkin meningkatkan rate pengiriman di atas negosiasi yang disetujui. Penerapan BMW untuk call admittance dan kontrol kongesti digunakan dalam jaringan ATM.

Secara eksplisit skema dasar pada jaringan mengetahui antrian buffer occupancy dan tingkat yang mana dari link individu yang mungkin overload (kelebihan beban). Informasi Explicit Binary Feedback (EBF) dapat digunakan langsung pada kondisi kongesti, dan dalam rekomendasi I.370 menjelaskan daerah kongesti. Untuk transfer kontrol-tujuan (tempat untuk rate-control yang mana biasanya berfungsi pada layer-transport) untuk meneruskanpemberitahuan kongesti secara eksplisit. Dalam keadaaan transfer kontrol-sumber (dimana informasi menyusun window dengan baik) mengirimkan informasi langsung pada backward memberitahu kongesti secara eksplisit. Pemberitahuan yang mungkin dapat mengidikasikan letak dalam frame yang menormalkan travelling langsung tepat pada sisa overhead (di atas) dengan teknik pendekatan.

Satu lagi bentuk secara eksplisit yang betul-betul dipertimbangkan untuk panggilan Stop Duration. Dalam skema, jaringan secara terus-menerus memonitor kedatangan frame-frame di dalam buffer. Jika penempatan frame yang lain dapat menekan dalam buffer- occupancy melebihi beberapa threshold, pengiriman akan "stop message" oleh jaringan ke pengirim end-user menjawab dan seluruhnya berhenti pada trafik layer-3. Selama parameter dibawa dalam menentukan "stop message" setelah periode yang mana boleh mengirim lagi ke trafik.

Teknik prediksi untuk mengatasi kongesti

Skema ini memerlukan 4 (empat) tahap :

1. Observasi throughput.
   
2. Membuat prediksi level kongesti.
   
3. Keputusan dibuat sesuai untuk menambah atau mengurangi beban trafik.
   
4. Window ditambahkan atau dikurangi dalam line dengan tegas.
   

Kesimpulan

Teknik dasar prediksi kita seringkali menggunakan pendekatan untuk penanggulangan kongesti dan akan bekerja ketika jaringan tidak memberikan pemberitahuan adanya kongesti pada user. Hal ini tidak memerlukan kerja lebih dengan jaringan dan frame-frame untuk implementasi. Ini seperti halnya suatu kombinasi dari skema ini dengan beberapa bentuk pemberitahuan yang eksplisit akan menjadi suatu kekuatan penuh untuk mencegah terjadinya kongesti di dalam jaringan Frame Relay.

Daftar pustaka

1. A.G. Waters and K.Ab. Hamid, "Congestion Control for Frame Relay Networks", CDROM BPPT Jakarta, 1995.
   
2. Darren L.Spohn, "Data Network Design", McGraw-Hill, Inc., 1993.
   
3. Frank J.Derfler, Jr., "PC Magazine Guide to Linkins LAN'S", Ziff. Davis Press, 1992.
   
4. Kim-Joan Chen and Kiran M.Rege, "A Comparative Performance Study of Various Congestion for ISDN Frame Relay Networks", IEEE 1989.
   
5. Lynn A. Neir & David W.Petr, "Time-to-delivery queuing : A Multi Purpose Resource Allocation and Congestion Control Technique", IEEE 1993.

Ir. Syafruddin Syarif tinggal di Bandung

Introduction

Figure 1 depicts a severe overlapping situation that the future MSS allocated spectrums will be in conflict with those of the present terrestrial fixed services.
The coming WRC-95 in Geneva November 1995 shall attempt to shed a light of relevant problems in particular to pursuing the measures to this very spectrum coordination equation. Judgement from various operators will seemingly be predominated by the oblique idea of heavy investment being made in the past in the realm of fixed services. That a large amount of existing backbone terrestrial system in place should prevent the use of MSS system despite its potential to relieve great many problems in developing very basic telecommunications infrastructure in most developing countries.

The November 95 conference shall urge some future CPM to perform a more thorough and realistic study in view of the terrestrial fixed service-MSS tradeoffs. We have thought that compromise measures based upon more accurate result of the MSS bandwidth requirement prediction and existing data on the present and future investments on the Non-MSS systems, shall be exercised. This paper present some preliminary analysis to the said equation.

The MSS Spectrum Requirements

• Ensemble of telecommunications-related parameters of various countries [see Table 1]
• Prediction of mobile, rural and remote communications users which call for MSS capacity.

MSS Frequency Coordinations Problems

In spite of the various WARC92 bands allocated for the MSS service, however the CPM95 has put a focus on the bands that will be of immediate interest for the implementations of some MSS. It seems that there is still a long way to arrive to a state where significant agreement on the sharing possibilities based upon coexistence between MSS and the rest of the services can be achieved. This in part is due to the high investments already in place for different service, in the developing countries.

Although some of the problems are under study, it is clear that the coming WRC will make use of some results and recommendations of the CPM95, so it is worthwhile if we could discuss those results at present. The CPM95 finding are as follows:

1. Sharing of Frequency Bands in the 1-3 GHz between transmitting Stations in FS and E-S MSS (non- GSO/MSS) (The studies have focussed on sharing in the 1610- 1626.5 MHz and 1970-2010 MHz) [see Table 3]
   In view of the above results, regulatory options as regard to the 1970-2010 MHz bands will eventually be needed for introduction of the MSS system.
   
2. Sharing of frequency bands in the 1-3 GHz range between transmitting stations in the FS and the Earth-to-Space GSO/MSS:
   
   The CPM95 has concluded that co-channel sharing is not feasible due to severe constraints to be exerted to the FS transmitting stations. Provision shall be made for the avoidance of co-channel sharing between the MSS and the FS.3.
   
3. Sharing of frequency bands in the 1-3 GHz range between stations of non- GSO/MSS Space-to-Earth service and the FS receiving stations:
   
   Based on the studies undertaken in the 2483.5 - 2500 MHz and 2160 - 2200 MHz bands, sharing between non- GSO/MSS (space-to-earth) systems and the FS should be feasible. However, in the long term, sharing difficulties could arise between non-GSO/MSS systems and the FS.
   Accordingly, considerations should be given to a possible gradual transition plan in order to enable the FS to migrate to other frequency bands with no overlap with the MSS allocations between 1-3 GHz.
   
4. Sharing of frequency bands in the 1-3 GHz range between stations of GSO/MSS Space-to-Earth service and the FS receiving stations :
   
   It is generally possible to share the frequencies in the range of 1-3 GHz between the MSS and FS, as addressed in the draft Recommendations ITU-RIS (Doc 2/7). Recommendations provides coordination threshold power-flux density levels that are consistent with the RR 2566 for all the downlink bands allocated by WARC-92, with the exception of the 2520-2535 MHz band where a more stringent threshold value is recommended.
   
5. Sharing of frequency bands in the range below 1 GHz between non GSO/MSS service and the FS.
   Frequency Bands allocated for the MSS below 1 GHz (Table 4) will be shared by various other existing services such as Space Operations, Meteorological Satellites, Meteorological Aids, Space Research, Radio Navigation-Satellite, Radio Astronomy, Broadcasting, Fixed and Mobile Service. Whilst the sharing studies between MSS and Meteorological Aids are not yet accomplished, the present studies have shown many ways of alleviating the obstacles in achieving sharing-possible conditions. Among other methods are : that the MSS systems are to operate in either a narrow-band, frequency agile fashion to coexist with terrestrial services, or with wideband, low-power density, spread spectrum transmissions (that conditions apply to MSS uplink directions).
   The MSS downlink operations shall also exploit various techniques in view of achieving sharing possible conditions. These include spread spectrum, orthogonal- polarizations, and co-channel avoidance procedures.

6. Sharing of frequency spectrums in the C, Ku and Ka bands between MSS Feeder Links, FS and FSS services :
   
   The sharing problems in this matter lie mostly between non-GSO/MSS Feeder Links and GSO/FSS networks. There will be outage times experienced by both systems, (for a codirectional sharing to be exercised) when an earth station, a non-GSO satellite and a geostationary satellite will be in line, or nearly in line with each other. This makes the codirectional sharing solution will be apparently not feasible.
   
   Method of bidirectional sharing, on the other hand, seems to be promising albeit certain constraints. Table 3 outlines the CPM95 study result on the sharing problems between MSS Feeder links and other services.

Matters Relating to The Date of Entry into Force of Allocations in the Bands 1980-2010 MHz, and 2170-2200 MHz in Regions 1 and 3

The sharing problems in the MSS feeder links, as stated earlier, can be solved partly by bidirectional scheme. But this means that redesign is to be made in the MSS feeder links, to ensure bidirectional sharing with the existing FSS networks. Only if the MSS planner be ready to modify the specifications timely, then there will be no significant delay affecting the original schedule.

Possible Arrangement of Mitigating The Interference Problem with the MSS frequency band

1. Use of narrow-band, frequency-agile fashion, or
   
2. Wideband, low-power density spread spectrum transmission uplinks MSS transmission systems.MSS and FS planners have to exploit at maximum the following techniques.
   
3. Orthogonally-polarized, low-power density, spread-spectrum downlinks.
   
4. Co-channel avoidance.MSS planner shall make its best efforts to
   
5. Use of higer frequency spectrum, i.e. : Upper Ku and Ka bands, for their feeder links, in view of heavy investment in the C and lower Ku bands.This will retain at some longer periods the use of the C and some lower Ku bands by the FSS and FS, which will be benefited particularly by the developing countries.
   FS planner should endeavour and do a diligence in exercising the
   
6. Liberalization of some bands < 1 GHz and between 1 to 3 GHz is to be exercised.MSS planner shall continue to do a
   
7. Detailed study about the evolution of the MSS bandwidth requirement.
   
8. Every country is obliged to incorporate in their long term national plan, as a part of their network modernization framework, to systematically phase out the FS networks and replace them with fibre networks.

By Dr. Ir. Arifin Nugroho