6-22
Configuring the Data Link Layer Protocol for E1, T1, and Serial Interfaces
Configuring the Logical Interface
Figure 6-7. Components in a Frame Relay Network
DTE. The DTE receives data from the LAN in the form of multiple protocol
packets and encapsulates each packet into a Frame Relay frame. The header
of such a frame is called the Data Link Connection Identifier (DLCI) and
contains the frame’s ultimate destination.
You can configure the DTE to manage congestion and maintain quality of
service. For example, the DTE can manipulate the actual size of each frame
sent through the network. It also can buffer and fragment packets to reserve
bandwidth for particular circuits and ensure quality of service for time-
sensitive packets such as voice applications.
DCE. in a Frame Relay network, the DCE is the Frame Relay switch, which
establishes and maintains the Frame Relay connection. After receiving frames
from the DTE, the DCE converts these frames into signals supported by the
physical media of the network. The DCE also reads the DLCI on incoming
packets, checks its switch lookup table, and then forwards data to the appro-
priate outgoing port—which leads to the correct virtual endpoint.
UNI. UNIs connect the DTE to the DCE and provide access to the Frame
Relay network.
NNI. NNIs connect a DCE to a DCE, using bidirectional signaling. That is,
NNIs connect one Frame Relay switch to another.
DLCI
As mentioned earlier, the DTE marks each outgoing frame with a DLCI, a 10-
bit field in the Address Field of the Frame Relay header. The switch reads the
DLCI to determine the appropriate PVC endpoint to which to send the frame.
DLCIs are locally, not globally, significant. (See Figure 6-8.)
UNI: DTE to DCE
NNI: DCE to DCE
Router (DTE)
Frame Relay
Switch (DCE)
Frame Relay
Switch (DCE)
Frame Relay
Switch (DCE)
Router (DTE)
Router (DTE)
UNI
NNI
UNI
UNI
NNI
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