Frame Relay provides statistical time division multiplexing (Stat-TDM). Time division multiplexing
(TDM) is like going to Disneyland. It’s true. Remember how you have to stand in line to get
into Space Mountain? Well, after you get to the loading area, you’re placed in a section with rails
that separate you from the other passengers. You can then get into only the one car that is in front
of you and only when it is empty. Think of the holding area as the interface buffers of a router;
the cars are the time slots on the circuit. When a time slot drives up, you can get in, but not before
that, and not if someone is already in that slot.
Now Stat-TDM is an improvement over straight TDM. Stat-TDM enables you to jump into
a different line if it is not in use and to get into any car. This is a first-come, first-served technology.
Stat-TDM is used with Frame Relay to allow multiple logical data connections (virtual
circuits) over a single physical link. Basically, these circuits give time slots to first-come, firstserved
and priority-based frames over the physical link. Going back to our analogy, you can
think of Frame Relay as the capability to send multiple cars through space on one train, each
car holding a different person.
So, how is each person (data) identified in the car (time slot)? How does the frame switch
know where to send each frame? The answer to this is a
data link connection identifier (DLCI)
.
Because Frame Relay is based on virtual circuits instead of physical ones, DLCIs are used to
identify a virtual circuit and tie it to a physical circuit. This means each frame can be identified
as it traverses the Frame Relay switch and is then sent to the routers at the remote ends.
DLCIs are considered only locally significant, which means that they see the entire virtual circuit
but only up to the point of the Frame Relay switch. The provider is responsible for assigning
DLCIs and their significance to the network.
DLCIs identify the logical virtual circuit between the customer premises equipment (CPE)
and the Frame Relay switch. The switch then maps the DLCIs between each pair of routers in
order to create the PVC. The Frame Relay switch keeps a mapping table of DLCI numbers to
outgoing ports; it uses this table to forward frames out ports on the switch. (More information
about mapping follows in the next section, “DLCI Mapping.”)
When configuring your Cisco router to participate in a Frame Relay network, you must configure
a DLCI number for each connection. The Frame Relay provider supplies the DLCI numbers
for your router. If a DLCI is not defined on the link, the switch will discard the frame.
Figure 29.2 shows an example of how DLCIs are assigned to offices in Chicago and Miami.
The Chicago office will communicate through the Frame Relay switch to Miami by using
DLCI 17. Miami will communicate to Chicago by using DLCI 16. Remember that the valid
range of DLCIs is from 16 to 991.
FIGURE 2 9 . 2
Frame Relay PVC configuration
PVC
DLCI = 16 PVC
DLCI = 17
Chicago
Miami
NOTE:
Some providers assign a DLCI in such a way that it appears that the DLCI is
globally significant. For example, all circuits that terminate in Miami could be
assigned the local DLCI 17 at each site. But remember that even though all of
these DLCIs have the same number, they are not the same because DLCIs are
typically only locally significant.