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DQDB header

The DQDB header is the header of the Distributed Queue Dual Bus (DQDB) protocoll. The header length and structure correspond to the ATM cell. Like this, the DQDB header is 5 bytes long and has a 1- byte access control field( ACF) and an extended data field for channel identification( VCI) compared to ATM, this comprises 2.5 octets. In contrast, the DQDB cell has no data field for path identification( VPI) because there is no multiplexed link.

The Access Control Field (ACF) is a control field used to control access authorization to the DQDB bus. The ACF field contains a busy bit, which marks the time slot as free or busy, and a request bit. Both are used for the QPSX access procedure, Queued Packet and Synchronous Exchange (QPSX).

DQDB data frame

DQDB data frame

In addition, the ACF field has a Slot Type (SLT) indicating the time slot type. The SLT field is a one bit long data field that identifies the Pre-Arbitrated Slot( PA). The Previous Slot Received( PSR) data field is a 1-bit long data field reserved for the transmission of an acknowledgement for the previous segment ( payload). This is followed by the RSVD data field, which consists of two bits and is reserved for control purposes. Finally, the three request bits are used to account for priority classes in media access.

The Session Layer Header is a 4-byte data field that contains the Virtual Channel Identifier (VCI), Payload Type( PT), Segment Priority( SP), and Checksum Field ( HCS). The Virtual Channel Identifier (VCI) data field contains an identification number for the logical connection. In the DQDB header, the VCI data field is 20 bits long. The VCI field is followed by the Payload Type (PT). There is a 3 bit long data field in the that is used for payload identification. The following data field Segment Priority (SP) indicates the priority of the different cells and corresponds to the Cell Loss Priority(CLP) data field in an ATM cell. The last data field of the DQDB header forms the checksum field for the header, Header Checksum (HCS).

Englisch: DQDB header
Updated at: 20.04.2009
#Words: 339
Links: distributed queue dual bus (MAN) (DQDB), header (HDR), ATM cell, byte (B), field
Translations: DE

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