Avaya a500 Guia Do Utilizador

Cajun A500 ATM Switch User Guide

Gl-1

Glossary

ACM - ATM Connection

Manager

A software entity within each device that is responsible for setting up

and maintaining ATM virtual channel connections.

Address Resolution

Protocol (ARP)

A protocol used to map 32-bit IP addresses into 48-bit Ethernet

addresses. Conforms to RFC 826.

Asynchronous Transfer

Mode (ATM)

A high-speed, connection-oriented switching and multiplexing

technology that uses 53-byte cells (5-byte header, 48-byte payload) to

transmit different types of traffic simultaneously, including voice,

video, and data. It is asynchronous in that information streams can be

sent independently without a common clock.

ATM Adaptation Layer

(AAL)

A set of four standard protocols that translate user traffic from the

higher layers of the protocol stack into a size and format that can be

contained in the payload of an ATM cell and return it to its original

form at the destination. Each AAL consists of two sublayers: the

Segmentation And Reassembly (SAR) sublayer and the convergence

sublayer. Each is geared to a particular class of traffic, with specific

characteristics concerning delay and cell loss. All AAL functions occur

at the ATM endstation rather than at the switch.

AAL 1 addresses CBR (Constant Bit Rate) traffic, such as digital voice

and video and is used for applications that are sensitive to both cell

loss and delay and to emulate conventional leased lines. It requires an

additional byte of header information for sequence numbering,

leaving 47 bytes for payload.

AAL 2 is used with time-sensitive VBR (Variable Bit Rate) traffic, such

as packetized voice. It allows ATM cells to be transmitted before the

payload is full to accommodate an applications timing requirements.

The AAL 2 specification has not been completed by the ATM Forum.

AAL 3/4 handles bursty connection-oriented traffic, like error

messages, or variable rate connectionless traffic, such as LAN file

transfers. It is intended for traffic that can tolerate delay but not cell

loss. To ensure that cell loss is kept to a minimum, AAL 3/4 performs

error detection on each cell and uses a sophisticated error-checking

mechanism that consumes four (4) bytes of each 48-byte payload.

AAL 3/4 allows ATM cells to be multiplexed.