SMDS Nirmala Shenoy Information technology Department Rochester Institute

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

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

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 3

DQDB • Architecture updated 1/2002 4

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 5

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 6

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 7

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 8

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 9

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

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

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

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 13

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 14

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 15

SMDS • SMDS topology updated 1/2002 16

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 17

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 18

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

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