Avaya Configuring ATM Services Bedienungsanleitung Seite 29

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ATM Overview
1-3
Virtual Channel Identifier (VCI)
: The next 16 bits of the cell header (that is,
the last half of byte 2, all of byte 3, and the first half of byte 4) contain the
VCI. The VCI specifies a virtual channel within the virtual path on the
physical ATM link. Refer to the “Data Transmission” section, later in this
chapter, for additional information on virtual channels.
Payload Type (PT)
: The next 3 bits (that is, bits 5 through 7 of byte 4)
indicate the type of information the cell is carrying (for example, user data or
management information).
Cell Loss Priority (CLP)
: The last bit of byte 4 indicates the priority of the
cell and whether the network can discard it under heavy traffic conditions.
Setting the bit to 1 indicates the network may discard the cell if necessary.
Header Error Control (HEC)
: The last byte of the header field contains the
HEC. Its primary function is to guard against misdelivery of cells due to
header or single bit errors. However, the HEC does not gauge the quality of
the data in the information field.
Cell Information Field
Following the 5-byte cell header is a 48-byte information field containing user
data. The ATM Adaptation Layer (AAL) organizes the data in this field. Refer to
the following section, “ATM Layers,” for additional information about the AAL.
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