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Chapter 13 Metropolitan Area Networks. IEEE 802.6 DQDB (Distributed Queues, Dual Bus) SMDS(Switched Multimegabit Data Services). IEEE 802.6 DQDB. Can be used in LANs or MANs Uses a dual bus configuration Access to the buses by distributed queues
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Chapter 13Metropolitan Area Networks • IEEE 802.6 • DQDB (Distributed Queues, Dual Bus) • SMDS(Switched Multimegabit Data Services)
IEEE 802.6 DQDB • Can be used in LANs or MANs • Uses a dual bus configuration • Access to the buses by distributed queues • Buses are unidirectional with each bus carrying traffic on the opposite direction of the other. • Each bus has a head and an end. • The source station must choose a bus for which the destination is considered downstream.
Transmission Slots • Data travel on each bus as a stream of 53-byte slots. • Heads are responsible for generating slots. • Date rate depends on slots rate. • Stations are required to make reservations for the slots they want on the other bus. • Stations must respect the reservation and send only on their designated slots.
Distributed Queues • Each station has two queues, one for each bus. • A station adds a token to its Queue A each time a slot passes on bus B with a reservation bit set. • If a station wants to send data, it makes reservation in a slot and adds a token to the queue. • For each empty slot passes, the station removes one token from the queue, when it sees its own token, it inserts its data in the slot and remove its token from the queue. • Queues can be implemented using counters.
DQDB Layers • IEEE defines both the MAC and the physical layers for DQDB. • The MAC sublayer splits the data stream coming from upper sublayers into 48-byte segments and adds a 5-byte header (53-bytes total compatible with ATM).
Access field (1-byte): controls access to the bus: - Busy (B): indicate whether or not the slot is carrying data. - Slot Type (ST) packet or isochronous transmission. - Reserved (R) - Previous Slot Read (PSR) 2-bit is set to 0 by the addressed station once it has read the contents. - Request (RQ) 3-bit for reservation of slots, 8-levels of priority are allowed.
Address field: 20-bit virtual channel identifier (VCI) to be used for MANs and WANs. In LANs, this field is all 1s and an additional header is added to carry the MAC address. • Type field: 2-bit: identifies the type of the payload: user data, management data, etc. • Priority: the priority of the slot in networks that use priority. • CRC: x8+x2+x+1.
Connecting LANs using T-Lines T-1: 1.544 Mbps ($54,000/year) T-3: 44.736 Mbps ($540,000/year)
Switched Multimegabit Data Services • SMDS packet-switched datagram service used for high speed MAN traffic. • Developed by Bell Communication Research. • Subscriber LANs link to an SMDS network through routers. • SMDS I coordinated using the SMDS Interface Protocol (SIP) which is based on DQDB.
