1. DQDB
Network Security

2. What is DQDB?
 Distributed Queue Dual Bus
 DQDB is a DLL comm. 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 to 55 Mbps
Network Security

3. DQDB Architecture
 Each bus supports traffic in only one direction
 Beginning of bus is denoted by q square and end by a triangle
 Bus A traffic moves from right to left and Bus B traffic from left to right
 Each bus connects to stations directly through input and output ports
Network Security

4. Upstream & Downstream
 As Bus A is configured
 stations 1 & 2 are considered to be upstream w.r.t station 3
 Stations 4 & 5 are considered to be downstream w.r.t. station 3
 Station 1 has no upstream stations, but it has 4 downstream
stations
 Station 5 has no downstream stations, but it has 4 upstream
stations
 As Bus B is configured
 Station 1 & 2 are considered to be downstream w.r.t. station 3
 Stations 4 & 5 are considered to be upstream w.r.t. station 3
 Station 1 has no downstream stations, but it has 4 upstream
stations
 Station 5 has no upstream stations, but it has 4 downstream
stations
Network Security

5. DQDB Working
 Head-ends generate fixed size cells in both
directions
 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
Network Security

6. DQDB Working (continued)
 Each node is aware of relative position of all
other nodes
 Correct bus must be chosen to transmit data
 DQ is independent of physical size of network
 To Transmit Data:
 A Node acquires slot
 Sets header
 Copies data into slot
 Cells propagate to end of bus
 Copied by intended destination on the way
Network Security

7. DQDB Data Transmission
Network Security

8. Queues
Network Security

9. Distributed Queues
 53-Byte transmission slot is used
 Slots are continuous streams of bits
 The heads of Bus A & B generate empty slots for use on
buses
 Each station maintains two different queues: A & B
Network Security

10. Things to do
 DQDB layers and frame format
 DQDB cell & cell header
 Compare DQDB with SMDS
Network Security