DQDB
Network Security
What is DQDB?
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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
Network Security
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
Network Security
Few technical facts of DQDB
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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
Network Security
DQDB Architecture
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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
Network Security
DQDB Architecture (Cont…)
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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.
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The buses run in parallel in such a fashion as to allow the
cells generated to travel across the stations in opposite
directions.
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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).
Network Security
Upstream & Downstream
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As Bus A is configured
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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
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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
Network Security
DQDB working
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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
Network Security
DQDB Cell Format
 Segment Type (ST): Identify the cell as one of the following:
Single Segment
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First Segment
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Intermediate Segment
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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
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Network Security
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.
Network Security