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2-8 C
HAPTER
2: N
ETWORK
L
AYER
C
ONCEPTS
ATM Adaptation Layer (AAL)
Ethernet frames can be between 64 and 1514 bytes
in length. ATM transmits data in fixed length
cells
.
Each cell contains 48 bytes of user data. The
ATM
Adaptation Layer (AAL)
converts data between the
Ethernet and ATM formats.
The AAL has a
Segmentation and Reassembly (SAR)
sub-layer that does the conversion.
In the sending device the LEC passes the Ethernet
frames to the SAR. The SAR converts the user data
into fixed length cells, and passes these cells to the
ATM Layer for transmission across the ATM network.
In the receiving device, the SAR converts the ATM
cells back into the appropriate user data again, and
passes this data to the LEC.
As ATM can carry different traffic types (for example,
voice, video, and other data), several Adaptation
Layer protocols have been defined. These protocols
operate simultaneously within the Adaptation Layer,
and allow the ATM Layer to support different
applications and traffic types.
The IBM 8271 Nways Ethernet LAN Switch ATM
OC-3c Module uses the AAL5 ATM Adaptation Layer
protocol, which is a data-oriented protocol. The ATM
Module will only work with other AAL5 devices.
Asynchronous Transfer Mode (ATM) Layer
Asynchronous Transfer Mode
(ATM)
is a connection-
oriented transmission protocol that has the following
features:
■
ATM uses the
Signalling Protocol (Q.2931)
to
dynamically create, maintain and clear ATM
connections between end-systems.
■
ATM uses fixed length packets known as
cells
, and
each cell identifies the connection to be used.
■
ATM is transparent to the multiple services it
supports and can carry cells from different
applications over the same physical connection.
■
ATM has well-defined user and network interfaces.
ATM is Cell-based
ATM uses fixed length packets called
cells
. The first
five bytes of the cell is the
cell header
. The cell header
contains the information necessary to deliver the cell
to the correct destination.
Fixed-length cells offer smaller and more predictable
switching delays, because cell switching is less
complex than variable-length packet switching.
Having all the data in the same cell format also
dramatically increases the speed of transmission, by
eliminating the need for protocol recognition and
decoding. A good analogy is containerized shipping,
where uniform shape and weight containers with
standardized labelling, ease and speed up processing.