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DQDB notes.pdf

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DQDB is a distributed queue dual bus protocol for metropolitan area networks. It uses two unidirectional logical busses and the queued-packet distributed switch algorithm. Stations can transmit data, voice, and video. DQDB operates at the data link layer and provides connection-oriented and connectionless services. It can extend up to 30 miles at speeds between 34-155 Mbps using fiber optic or copper links.

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DQDB DQDB
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
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
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
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
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).
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
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
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
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.

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DQDB notes.pdf

  • 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.