Wide-Area Networks - CCIE Routing and Switching Exam Certification Guide - page 222

Traffic that is time-sensitive, connection-oriented, and arrives at a variable rate, such as
compressed video, uses AAL2.
Traffic that is connection-oriented, not time-sensitive, and arrives at a variable rate, such as
Frame Relay or LAN data, uses AAL5. Compressed video and voice also can use AAL5, with
the correct QoS parameters.
AAL 3/4 is defined for connectionless or connection-oriented, non-time-sensitive data. However,
it is not often implemented.
The ATM layer is responsible for creating cells. The ATM layer takes the 48-byte payload from
the AAL and appends a 5-byte cell header. The ATM layer multiplexes cells from different
AALs onto the same outgoing interface and demultiplexes cells coming in from the interface.
The physical layer is responsible for encoding the 53-byte cell on to the physical media. The
physical layer is composed of two sublayers. The transmission convergence (TC) sublayer is
responsible for implementing the physical line protocol, such as DS3 or SONET. The TC
sublayer is also responsible for header error control (HEC) functions. The physical medium
dependent (PMD) sublayer implements the electrical or optical media.
Refer to the ATM model in Figure 5-14 while studying this chapter to see the relationships
among ATM components.
ATM Cell Format
ATM transports voice, data, or video information in fixed length units called cells. Each cell
is 53 bytes long and contains cell header and payload fields. The 5-byte cell header contains
addressing, cell type, cell loss priority, and error checking information. The 48-byte cell payload
contains a portion of the data, voice, or video information being transported.
The format of the ATM cell header varies depending upon whether the interface being traversed
is a UNI or an NNI. The UNI connects the customer premise equipment to the network switch.
The NNI is the interface between switches or between networks.
Figure 5-15 shows the format of an ATM cell on a UNI and on an NNI, and Table 5-4 identifies
the fields of the cell headers.