1 DISTRIBUTED QUEUE DUAL BUSDistrib

1 DISTRIBUTED QUEUE DUAL BUS

Distributed Queue Dual Bus (DQDB) is a Data-link layer communication protocol for Metropolitan Area Networks (MANs), specified in the IEEE 802.6 standard and designed for use in MANs. DQDB is designed for data as well as voice and video transmission and is based on cell switching technology (similar to ATM). DQDB, which permits multiple systems to interconnect using two unidirectional logical buses, is an open standard that is designed for compatibility with carrier transmission standards such as SMDS.

For a MAN to be effective it requires a system that can function across long, “city-wide” distances of several miles, have a low susceptibility to error, adapt to the number of nodes attached and have variable bandwidth distribution. Using DQDB, networks can be thirty miles long and function in the range of 34 Mbps to 155 Mbps. The data rate fluctuates due to many hosts sharing a dual bus, as well as to the location of a single host in relation to the frame generator, but there are schemes to compensate for this problem making DQDB function reliably and fairly for all hosts.

The DQDB is composed of two bus lines with stations attached to both and a frame generator at the end of each bus. The buses run in parallel in such a fashion as to allow the frames generated to travel across the stations in opposite directions. Below is a picture of the basic DQDB architecture.

Figure: DQDB Architecture

DQDB Architecture

Each bus supports traffic in only one direction
Beginning of bus is denoted by a square and end by a triangle
Bus B traffic moves from right to left and Bus A traffic from left to right
Each bus connects to stations directly through input and output ports
The DQDB is composed of a two bus lines with stations attached to both and a cell (Empty slots) generator at the start of each bus.
The buses run in parallel in such a fashion as to allow the cells generated to travel across the stations in opposite directions.
The cell generator (head-end) is constantly producing empty cells consisting of fifty-three bytes (a five byte header and a forty-eight byte payload).
Upstream & Downstream

As Bus A is configured
Stations 2 & 3 are considered to be upstream w.r.t station 1
Stations 1 & 2 are considered to be downstream w.r.t. station 3
As Bus B is configured
Station 2 & 3 are considered to be downstream w.r.t. station 1
Stations 1 & 2 are considered to be upstream w.r.t. station 3
DQDB Working

Head-ends generate fixed size cells in both directions (cell generators)
To transmit, a host must know whether the destination is to its right or its left
If right, the host must send on left bus
If left, the host must send on the right bus
A “Distributed Queue” is used to make sure that cells are transmitted on a first-come first-serve basis
Technical Facts Of DQDB

Distance up to 200 KM
Medium: Copper or Fiber
At distance up to 160 KM approx speed is 44.73 Mbps (Copper)
At distance up to 100 KM approx speed is 150 Mbps (Fiber)
Transmission Rate: 34 Mbps to 150 Mbps
DQDB Features

DQDB is a DLL communication protocol for MAN
Unlike FDDI, DQDB is an IEEE standard: 802.6
Designed for both voice & video
Topology used: Dual Bus - uses 2 unidirectional logical buses
Extend up to 30 miles at 34-55 Mbps
Uses optical fibre links
Queued-packet distributed switch (QPSX) algorithm
Works on Data-link layer (especially in MAC sub-layer)
Used in data, voice and video transmissions
Used in data over cable services
Based on Cell Relay Technology (like ATM)
Provides connection-oriented, connection less services & asynchronous services










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1 DISTRIBUTED QUEUE DUAL BUS
Campus address: MVN University,
74 km stone, NH-2,
Delhi-Agra Highway (NCR),
Haryana-121105

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1 tarqatilgan QUEUE DUAL BUS taqsimlangan navbat Dual avtobus (DQDB) IEEE 802.6 standart va Mans foydalanish uchun mo'ljallangan ko'rsatilgan Metropolitan Uchastka maydoni tarmoqlari (Mans), bir Data-link qatlam aloqa protokoli hisoblanadi. DQDB ma'lumotlar, shuningdek, ovoz va video uzatish uchun mo'ljallangan va (ATM o'xshash) hujayra kommutatsiya texnologiyasi asoslangan. Ikki tomonlama mantiqiy avtobuslar yordamida biriga ulash uchun, bir necha tizimlari ruxsat beradi DQDB, bunday SMDS sifatida tashuvchisi uzatish standartlari bilan muvofiqligi uchun mo'ljallangan ochiq standart hisoblanadi. A MAN samarali bo'lishi uchun u uzoq bo'ylab faoliyat mumkin tizimini talab qiladi, "city- bir necha kilometr keng "masofalar, xato biriktirilgan tugunlari soni moslashish va o'zgaruvchan bandwidth taqsimlash ega bo'lishi past sezuvchanlik bor. DQDB foydalanib, tarmoqlari 155 Mbit 34 Mbps oralig'ida o'ttiz kilometr uzoq va vazifasi bo'lishi mumkin. Ma'lumotlar tezligi, bir dual avtobusga almashish ko'p xostlar tufayli, shuningdek ramka generator nisbatan bir uy egasi o'rnini o'zgarib turadi, lekin barcha xostlar uchun ishonchli va adolatli DQDB vazifasini qilish, bu muammo bartaraf qilish uchun sxemalar bor. The DQDB har bir avtobus oxirida har ikki va bir ramka generator biriktirilgan stantsiyalar bilan ikki avtobus liniyalari tashkil topgan. Kabi bir moda parallel ishlaydigan avtobuslar qarama-qarshi yo'nalishlarda stantsiyalari bo'ylab sayohat qilish hosil rom ruxsat berish. Quyida asosiy DQDB me'moriy rasm bo'lgan. Shakl: DQDB Arxitektura DQDB Arxitektura Har avtobus faqat bir yo'nalishda trafikni qo'llab-quvvatlaydi bir uchburchak tomonidan maydonida va oxirida bilan belgilanadi avtobus boshlanishi A trafikni dan o'ngdan chapga avtobus B transport harakat va avtobus chapdan o'ngga har avtobus to'g'ridan-kirish va chiqish portlari orqali stantsiyalari ulanadi DQDB ham ilova stantsiyalari va har bir avtobus boshida bir hujayraning (Empty Slot) generator bilan ikki avtobus liniyalari iborat. avtobuslar kabi parallel ravishda ishga tushirish sifatida moda. qarama-qarshi yo'nalishlarda stantsiyalari bo'ylab sayohat qilish hosil hujayralarni ruxsat berish uchun hujayra generator (bosh-end) doimiy bo'sh ellik uch bayt iborat hujayralarni (besh bayt boshi va qirq sakkiz bayt yuk) ishlab chiqaradi. upstream va Downstream avtobus sifatida konfiguratsiya qilingan stantsiyalar 2 & 3 1 yuqorida turgan wrt stantsiyasi hisoblanadi 1 & 2 mansap wrt stantsiyasi 3 hisoblanadi stantsiyalar konfiguratsiya qilingan avtobus B kabi pastga tomon wrt stantsiyasi 1 hisoblanadi Stantsiyani 2 & 3 punktlari 1 & 2 3 upstream wrt stantsiyasi hisoblanadi DQDB ishchi boshlig'i-uchlari har ikki yo'nalishda (hujayra generatorlar) o'lchamli hujayralarini sobit hosil uzatish uchun, bir ota-joy, uning o'ng yoki chap yoki yo'qligini bilish kerak , mezbon kerak bo'lsa, o'ng chap avtobusga yuborish qoldirgan bo'lsa, o'ng avtobus ustida yuborish kerak mezbon "Navbatsiz taqsimlangan" a hujayralar birinchi kelgan asosi birinchi xizmat haqida uzatiladi ishonch hosil qilish uchun ishlatiladi DQDB Of Texnik ma'lumotlar 200 km gacha masofa mis: o'rta yoki Fiber 160 km taxminan tezligi masofa qadar o'zida 44,73 Mbps (mis) deb 100 kM taxminan tezligi masofa qadar o'zida 150 Mbps (Fiber) bo'ladi 150 Mbps 34 Mbps: Transmission Rate DQDB xususiyatlari DQDB MAN uchun DLL aloqa protokoli hisoblanadi FDDI farqli o'laroq 802,6: DQDB IEEE standart bo'lib , ham ovoz & video uchun mo'ljallangan ishlatiladigan topologiyasiga: Dual Bus - 2 tomonlama mantiqiy avtobuslar foydalanadi 34-55 Mbit 30 milya qadar uzaytirish optik tolali ishoratlar foydalanadi Kutmoqda-paketi tarqatiladi switch (QPSX) algoritm ishlari Data-link qatlam (ayniqsa, MAC sub-qatlamida) ma'lumotlar, ovoz va video uzatish ishlatiladi kabel xizmatlari orqali ma'lumotlarni ishlatiladi Cell (ATM kabi) Relay texnologiyalari asosida ulanish-yo'naltirilgan ta'minlaydi, aloqa kam xizmatlar va asinxron xizmatlar Exit kitob jadval Siz identifikatsiyadan o'tmadingiz mazmuni Mundarija 1 tarqatilgan QUEUE DUAL BUS MVN universiteti,: Campus manzili 74 km tosh, NH-2, Dehli-Agra avtomobil yo'lini (NCR), Haryana-121105