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SMDS

SMDS. Nirmala Shenoy Information technology Department Rochester Institute of Technology. SMDS. Scope DQDB operation SMDS Architecture. DQDB. Uses MAN protocol – hence first briefly into DQDB DQDB – Distributed Queue Dual Bus 2 communication channels Each station connects to both bus

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SMDS

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  1. SMDS Nirmala Shenoy Information technology Department Rochester Institute of Technology updated 1/2002

  2. SMDS • Scope • DQDB operation • SMDS Architecture updated 1/2002

  3. DQDB • Uses MAN protocol – hence first briefly into DQDB • DQDB – Distributed Queue Dual Bus • 2 communication channels • Each station connects to both bus • Queues to use each bus • Queues carry information about other stations updated 1/2002

  4. DQDB • Architecture updated 1/2002

  5. DQDB • Architecture • Bus B • With respect to Station 3, Stn 1, and 2 are downstream • Stn 4, 5 are upstream • Bus A • With respect to Station 3, stn 4, 5 are down stream • Stn 1, 2 are upstream updated 1/2002

  6. DQDB • Operation • On Bus A, head of bus A generates slots of 53 bytes which goes down bus A • On Bus B, head of Bus B generates slots which go down bus B • Empty slot travels down the bus, till a station wanting to transmit, sends its 53 bytes updated 1/2002

  7. DQDB • Operation • Bus A: Stn 3 can send to 1 and 2 • Bus B, stn 3 can send to stn 4 and 5 • Upstream Node monopolization • Station to reserve for slots if they want to communicate to downstream nodes • Reservation goes in direction opposite to the direction you want to send updated 1/2002

  8. DQDB • Operation • To book a downstream slot in Bus A, use the slots passing by in Bus B to the head of bus A • All upstream nodes in Bus A see the reservation going by • Keep an account of the number of reservations that go by • On empty slots going downstream A, the upstream station let that many slots go by updated 1/2002

  9. DQDB • Operation • Distributed Queues • Queue A for Bus A • Queue B for Bus B • Queues hold the booking for slots • Each station puts a token in the queue when it sees a request going by • When it wants to send, it will put its token in the queue updated 1/2002

  10. DQDB • Operation • As empty slots go by • Tokens are removed for each empty slot • Till – this station token is at HoQ • Empty slot comes by and is taken by this station • Ring configuration possible updated 1/2002

  11. DQDB • DQDB layers • Physical • MAC layer • 48 byte payload • 5 byte header updated 1/2002

  12. DQDB • DQDB MAC PDU • 5 byte Header • Access • Address • Type • Priority • CRC updated 1/2002

  13. DQDB • Access field – 8 bits • Busy bit – slot is carrying data • Slot type – pkt transmission, isochronous transmission • Reserved bit • PSR – (Previous Slot Read) 2 bits – set to 0 once the destination stn has read • RQ – 3 bits – 8 levels of priority on request updated 1/2002

  14. DQDB • Address Field • 20 bit VCI • Type field – 2 bits –payload type, user data, management data • Priority field – Priority of the slot • DQDB – self healing • Determination of upstream and downstream stations updated 1/2002

  15. SMDS • SMDS • Packet switched datagram service for high speed MAN traffic • Switched service – pay for time of usage • Networks use Router to connect to SMDS switches using DQDB • SMDS Interface protocol - SIP updated 1/2002

  16. SMDS • SMDS topology updated 1/2002

  17. SMDS • SMDS - SIP Level 3 • Adds header and trailer • Header has sender and receiver address • Packet is segmented into 44 bytes • Each 44 byte is given 2 bytes header, 2 bytes trailer • 2 byte header - updated 1/2002

  18. SMDS • SMDS - SIP Level 3 • header • ST – 2 bits – segment type • 4 bits sequence count • Message ID – 10 bits • Trailer • 6 bit length indicator • 10 bit crc updated 1/2002

  19. SMDS • SMDS - SIP Level 2 – DQDB • SIP level 1 – physical layer updated 1/2002

  20. SMDS • Summary • Ease on geographic limits • Offers Switched services • Addresses high speed interconnectivity • End Systems are routers • Real-time? • Bulk data updated 1/2002

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