Asynchronous transfer mode

Asynchronous transfer mode

Asynchronous transfer mode (ATM), also known as cell relay, is in some ways similar
to packet switching. Like packet switching, ATM involves the transfer of data in
discrete chunks. Also, like packet switching,ATM allows multiple logical connections
to be multiplexed over a single physical interface. In the case of ATM, the information
flow on each logical connection is organized into fixed-size packets, called cells.
ATM is a streamlined protocol with minimal error and flow control capabilities.
This reduces the overhead of processing ATM cells and reduces the number of
overhead bits required with each cell, thus enabling ATM to operate at high data
rates. Further, the use of fixed-size cells simplifies the processing required at each
ATM node, again supporting the use ofATM at high data rates.
ATM Logical Connections
Logical connections in ATM are referred to as virtual channel connections (VCCs).A
VCC is analogous to a virtual circuit in a packet-switching network; it is the basic unit
of switching in an ATM network. A VCC is set up between two end users through the
network, and a variable-rate, full-duplex flow of fixed-size cells is exchanged over the
connection. VCCs are also used for user-network exchange (control signaling) and
network-network exchange (network management and routing).
For ATM, a second sublayer of processing has been introduced that deals
with the concept of virtual path (Figure 3.10). A virtual path connection (VPC) is a
bundle ofVCCs that have the same endpoints. Thus all of the cells flowing over all
of the VCCs in a single VPC are switched together.
The virtual path concept was developed in response to a trend in high-speed networking
in which the control cost of the network is becoming an increasingly higher

proportion of the overall network cost. The virtual path technique helps contain the
control cost by grouping connections sharing common paths through the network into
a single unit. Network management actions can then be applied to a small number of
groups of connections instead of a large number of individual connections.