The ATM Protocol
The ATM protocol reference model is
consistent with the telecommunications protocol approach. Basically
it consists of three planes: The user plane, control plane and the
management plane. The user and control planes include a physical
layer, the ATM layer and the AAL.
The ATM layer
provides cell-transfer capabilities in a per-virtual-circuit fashion
by multiplexing/demultiplexing different virtual connections in/from
a single flow of ATM cells. It is responsible, at the transmitting
side, for assigning the correct VPI/VCI values to the cells
belonging to a virtual connection and for delivering incoming cells
to the related logical connection.
The AAL performs
service-dependent functions: four different AAL types have been
defined in order to adapt the application requirements to the ATM
cell transport. The control plane has its own AAL, conventionally
called (signalling) SAAL. The AAL types take into account the
application needs in terms of timing relation between source and
destination, traffic emission profile (CBR or VBR) and connection
mode (connection oriented or connectionless). Each AAL type is
subdivided into two layers: SAR (segmentation and reassembly)
sublayer performs the higher layer information framing/
reconstruction in/from a sequence of small units fitting into the
ATM cell payload. The CS (convergence sublayer) provides service
specific functions (timing, frame error detection, and frame
sequence checking and recovery).
Out of the four
AAL types, AAL5 is favored for connection-oriented variable-rate
data (VBR) services. AAL5 offers to the majority of data
applications the best tradeoff between the reliability of data
delivery, supported by error-detection mechanisms, and the
associated overhead. (ATM and AAL protocolsí overhead is high: it
spans up to 17%, that is, up to 9 bytes over the 53-byte-long cell).
AAL1 supports connection-oriented services that require CBRís and
have specific timing and delay requirements, while AAL2 supports
connection-oriented services that do not require constant bit rates.
AAL3/4 is intended for both connectionless and connection-oriented