2. What is DQDB?
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 34-55 Mbps
• Uses optical fibre links
• Queued-packet distributed switch (QPSX) algorithm
3. About DQDB
About DQDB
• Works on Data-link layer (specially 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
4. Few technical facts of DQDB
Few 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
5. 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
6. DQDB Architecture (Cont…)
DQDB Architecture (Cont…)
• 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).
7. Upstream & Downstream
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
8. DQDB working
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
9. DQDB Cell Format
DQDB Cell Format
Segment Type (ST): Identify the cell as one of the
following:
• Single Segment
• First Segment
• Intermediate Segment
• Last Segment
Message Identifier (MID): MID is the same for all
DQDB cells from the same MAC frame. This allows the
identification of intermediate segments.
Information: Actual Data
Length (LEN): The length of the information field.
CRC: For error correction
10. DQDB cell header
DQDB cell header
Access Control Field (ACF):
This contains the BUSY and REQUEST bits that are used in the
operation of the DQDB. The BUSY bit indicates the cell is in use. The
REQUEST bit is set in a cell by a node that is waiting to transmit.
Virtual Channel Identifier (VCI):
This is used to identify a virtual channel address.
Payload type (PT): 1st bit Data or Control ; 2nd
bit congestion.
Cell loss priority (CLP): Retain / Discard on
congestion.
Header Error Control (HEC): CRC for the
header.
