Avaya Configuring ATM and MPLS Services Bedienungsanleitung PDF herunterladen (Seite 38)

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Configuring ATM and MPLS Services

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Although Bay Networks uses raw bandwidth to describe line rates, inherent

overhead within the media limits maximum bandwidth for the line. Use Table 1-1

to determine the maximum bandwidth for the media you are using.

ATM Layer

The ATM layer defines how two nodes transmit information between them. It is

concerned with the format and size of the cells and the contents of the headers.

The addresses of the cells are meaningful only to the two adjacent local nodes

(that is, usually not to the end nodes).

ATM Adaptation Layer

The ATM adaptation layer (AAL) converts upper-layer protocol data into formats

that are compatible with the requirements of the ATM layer, enabling ATM to

handle different types of information within the same format.

The AAL is divided into two sublayers: the convergence sublayer (CS) and the

segmentation and reassembly (SAR) sublayer. These two sublayers convert

variable-length messages into 48-byte segments, while ensuring the integrity of

the data.

The CCITT (now ITU-T) has defined different types of AALs to handle different

kinds of traffic. Bay Networks ATM routers support a CS function compatible

with AAL 5, as defined in Section 1 of the CCITT (now ITU-T) B-ISDN Protocol

Reference Model (PRM).

In AAL 5, only a trailer attaches to the data from the upper-layer protocols to

create a CS PDU. AAL 5 divides the protocol data unit (PDU) into a 48-octet SAR

PDU; however, it does not add an SAR header and trailer. This 48-octet SAR PDU

becomes the payload of the ATM cell (Figure 1-5

Table 1-1. Maximum Bandwidth by Media Type

Media

Raw

Bandwidth (Mb/s)

Maximum

Bandwidth (Mb/s) Maximum Cells/s

OC-3 SONET/SDH 155 149.76 353207

DS-3 (Cbit and M23

framing)

44.736 40.704 96000

E-3 (G.832 framing) 34.368 33.920 80000

E-3 (G.751 framing) 34.368 30.528 72000

1 2 ... 33 34 35 36 37 38 39 40 41 42 43 ... 528 529

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