12-2
C
HAPTER
12: ATM, LAN E
MULATION
,
AND
V
IRTUAL
LAN
S
ATM Basics
Cell Switching
ATM uses short, fixed length packets called
cells.
The first five bytes of
each cell, the header, contain the information necessary to deliver the cell
to its destination.
Fixed-length cells offer the following advantages:
Q
Network and switching queueing delays are more predictable with
fixed data cells than for variable-length packets.
Q
Cell switching is less complex and more reliable. ATM hardware can be
implemented more efficiently because control structures, buffers, and
buffer management schemes can be designed to known size criteria.
Q
Cell-relay switches can process cells in parallel, achieving speeds that
far exceed the limitations of packet switch architectures.
Q
Having all data in the same cell format speeds transmission
dramatically by eliminating the need for protocol recognition and
decoding. A good analogy is containerised shipping where uniform
shape and weight of containers and standardized labelling
considerably ease and quicken processing.
Q
The cell format also allows for multi-protocol transmissions. Since ATM
is protocol transparent, the various protocols can be transferred at the
same time. With ATM, one line can carry phone, fax, video, data and
other information simultaneously. This multiprotocol advantage also
offers scalability, greatly reducing the configuration changes necessary
for adding a new traffic type to your network.
Network interfaces
ATM establishes the
User Network Interface
(UNI) which is typically used
to interconnect an ATM user with an ATM switch that is managed as part
of the same network, as well as the
Network to Network Interface
(NNI)
which is typically used to interconnect two ATM switches managed as
part of the same network.
Layered architecture
ATM is based on a layered architecture. In the protocol stack, the ATM
layer sits directly above the physical layer. Many
physical layers
can be
specified, including several for 100 to 155 Mbps. The ATM adaptation
layer (AAL) sits above the ATM layer. LAN Emulation (LE) sits above the
AAL5 in the protocol hierarchy.
abtthgde.book Page 2 Tuesday, June 23, 1998 10:29 AM
