DQDB Network Security

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 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 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 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…) 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). Network Security

Upstream & Downstream As Bus A is configured As Bus B 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 Network Security

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 Network Security

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 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