ATM layer is responsible for establishing connections and passing cells through the ATM network. The ATM
Adaptation layer translates the different types of network traffic. Four AALs are defined, but only three are
actively in use:
AAL1—Used to transport timing−dependent traffic such as voice• AAL3/4—Used by network service providers in Switched Multimegabit Data Service (SMDS)
networks
•
AA5—The primary AAL used for non−SMDS traffic that doesn’t require the pacing AAL1 would
provide
•
Figure 8.4 shows the mapping of the ATM Reference Model compared to the OSI Reference Model.
Figure 8.4: The ATM Reference Model layers compared to OSI Reference Model layers.
The Physical Layer
The ATM Physical layer controls transmission and receipt of bits on the physical media. This layer also tracks
the ATM cell boundaries and packages cells into the appropriate frame type. This layer is divided into two
sublayers: the physical medium dependent (PMD) sublayer and the transmission convergence (TC) sublayer.
The PMD sublayer is responsible for sending and receiving a continuous flow of bits with the timing
information to synchronize the transmission and reception of data. ATM does not care about the physical
media being used, and all widely used physical topologies are capable of supporting ATM cells. Existing
high−speed topologies capable of supporting ATM cells include Synchronous Optical Network (SONET),
DS3/E3, Fiber Distributed Data Interface (FDDI), and unshielded twisted pair (UTP).
The TC sublayer is used to maintain the ATM cell boundaries, verify the validity of data, maintain data
synchronization, create and check header error control to ensure valid data, and put the cells into a format that
the physical media can use. This sublayer also extracts and inserts ATM cells within either a Plesiochronous
Digital Hierarchy (PDH) or a Synchronous Digital Hierarchy (SDH) Time Division Multiplexed (TDM)
frame and passes this frame to and from the ATM layer.
The ATM Layer
The ATM layer is responsible for establishing connections, passing cells to and from the AAL, inserting the
ATM header, and extracting the ATM header. This layer is also responsible for multiplexing and
demultiplexing data through the ATM network. To do this, ATM uses information contained in the header of
each ATM cell. ATM switches use a VPI and a VCI field inside the ATM cell header to identify the next
network segment a cell needs to transit on its way to its final destination.
A VCI is also known as a virtual channel. It is an identifier for the physical connection between the two ends
that form a logical connection. A VPI is the identifier for a group of VCIs that allows an ATM switch to
perform operations on a group of virtual connections (VCs).
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