Avaya Configuring ATM and MPLS Services Manual do Utilizador descarregar pdf (Página 34)

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Configuring ATM and MPLS Services

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117374-C Rev 00

• Virtual channel identifier (VCI): The next 16 bits of the cell header (that is,

the last half of byte 2, 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, “Data Transmission,” for additional information

about 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 the cell 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. See “ATM

Layers” on page 1-6 for additional information about the AAL.

Data Transmission

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

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

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

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

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

A virtual channel is a logical connection between two communicating ATM

devices. Each virtual channel can carry a different protocol or traffic type. The

virtual channel transports cells that have a common identifier, the VCI, that is part

of the cell header. You can establish virtual channels permanently or set them up

dynamically, allowing the network to adjust itself to the traffic demand.

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