System information

Troubleshooting ATM Switching Environments 21-433
Placing User Data into ATM Cells
Figure 21-3 The OSI Reference Model
The data link layer is concerned with data transmission between two network switches. This layer is
not concerned with the transmission of an entire message between a source and a destination
switch—this responsibility belongs to Layer 3 (the network layer). Rather, the data link layer
transports portions of messages (cells, in the case of ATM) between two points in the network. These
points may be the source and the destination of the message, or they may be only intermediate hops
between the source and the destination.
The data link layer may divide higher-level data into smaller units (cells, in this case), whose sizes
are compatible with overall network requirements. Layer 2 data units contain a cell header, an
information field, and some method of checking for transmission errors.
Placing User Data into ATM Cells
Before frames can be transported across an ATM network, they must be divided into ATM cells. The
processes that divide the frames into cells occur at Layer 2. Layer 2 is divided into two parts: the
ATM adaptation layer (AAL) and the ATM layer. After frames are divided into ATM cells, the cells
can be transferred to Layer 1 (see Figure 21-4).
Figure 21-4 Layer 2—The Data Link Layer
Physical media
Layer 6: Presentation layer
Layer 5: Session layer
Layer 4: Transport layer
Layer 3: Network layer
Layer 2: Data link layer
Layer 1: Physical layer
Layer 6: Presentation layer
Layer 5: Session layer
Layer 4: Transport layer
Layer 3: Network layer
Layer 2: Data link layer
Layer 1: Physical layer
User application
(ATM endpoint)
Higher-layer
protocols
ATM adaptation
layer
ATM layer
Physical layer
Physical media
OSI Layer 1
OSI Layer 2
(data link layer)
Network equipment
(ATM switch)
ATM layer
Physical layer
ATM adaptation
layer
ATM layer
Physical layer
User application
(ATM endpoint)
Higher-layer
protocols
Physical media