Avaya Configuring ATM Half-Bridge Services Uživatelský manuál stáhnout pdf (Strana 83)

Understanding ATM Concepts

300017-A Rev. 00

B-3

The fields in each ATM cell header provide all the information necessary for

networking. These fields include the following:

• Generic Flow Control (GFC): The first 4 bits of the cell header contain the

GFC. The GFC controls traffic flow onto the ATM network by controlling the

user-network interface (UNI).

• Virtual Path Identifier (VPI): The next 8 bits of the cell header (that is, the

last half of byte 1 and the first half of byte 2) contain the VPI. The VPI

specifies a virtual path on the physical ATM link. See the next section, “AT M

Data Transmission,” for additional information on virtual paths.

• 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. See the next section, “ATM Data Transmission

,” for

additional information on virtual channels.

• Payload Type (PT): The next 3 bits (that is, bits 5 to 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 that 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.

ATM Data Transmission

Data transmission (also called cell switching) through the ATM network relies on

the establishment of logical connections between ATM entities. ATM is a

connection-oriented service. This means that an ATM entity cannot transmit

information until it establishes a connection with a receiving entity. These

connections consist of virtual channels, virtual paths, and transmission paths.

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Avaya Configuring ATM Half-Bridge Services Uživatelský manuál Strana 83