Avaya a500 Betriebsanweisung
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.