Introduction to Tandem X.25 Capabilities

Interfacing with a Private Frame-Relay Network
X25AM Implementations
3–14 065307 Tandem Computers Incorporated
Interfacing with a
Private Frame-Relay
Network
A large transportation company based in the United States has installed a reservations
system on a Tandem Cyclone/R system, interfacing with a private frame-relay
network.
Frame-relay is a relatively new protocol providing faster, more efficient capabilities for
packet-switching data communications. Frame-relay does not include explicit
flow-control procedures per virtual circuit; instead, the flow-control is handled by the
higher layers. Frame-relay includes only a basic congestion-notification mechanism
that allows a network to inform a user device when the network resources are close to
a congested state.
In this transportation reservations implementation, transactions are initiated at
personal computers, which are running PC6530 emulation software, and are connected
to a PAD. The PADs at various locations are connected to a frame-relay router at each
site. The frame-relay router selects the optimum path to the Tandem host.
Frame-relay provides a means for statistically multiplexing many switched virtual
circuits over a single physical transmission link. Thus, a PAD does not require a
dedicated port on the Tandem host. Instead, the frame-relay router that is local to the
Tandem host dynamically selects a port based on port availability. If multiple calls
come in from the same PAD at the same time, the frame-relay router directs the calls to
multiple different ports. So there is no time delay.
The Tandem host is connected to its local frame-relay router via 12 X25AM lines, each
running at 56Kb, as shown in Figure 3-7. The router does all the load-balancing and
switching.