Verilink 9000 Series (34-00271) Product Manual - Manual (Page 227)

12
F
RAME
R
ELAY
P
ROTOCOL
Frame Relay
Basics Frame relay is a bit-oriented transport protocol that functions similar to X.25. It is
used to transport a variety of data such as legacy protocols, video, and LAN traffic.
One of the main differences between frame relay and X.25 is that frame relay
connections are of a permanent nature. A PVC (permanent virtual circuit) is set up
at the beginning of service between two points in the network and does not
change, whereas X.25 allows the user to connect to a destination, communicate,
disconnect, and then reconnect to a different destination. It is possible, though, to
have multiple, logical connections to unique destination on one physical link.
Frame relay is what is called a Layer 2 protocol. TXPORT places Layer 3 of X.25
above frame relay to maintain its station level routing scheme. This allows the
platform to place individual call requests for each device the same way it does
when using X.25. This combination results in a best-of-both-worlds environment.
The user maintains the ability to connect each device to a unique destination while
taking advantage of a network with higher throughput.
Frame relay, unlike X.25, does not place a high priority on data integrity. When a
frame relay network receives data from a user, it does not acknowledge receipt of
the information in any form. It is also allowed to throw away user data if the
network is congested at the time. What’s more, the network does not have to
inform the sender that the information was discarded. It is the responsibility of the
end points to determine receipt and integrity of the message.
This type of reasoning is based on the fact that frame relay utilizes high-speed
digital links that are almost error free. It is assumed that only rarely would a
message not survive transition through the network intact and therefore there is no
reason to constantly check its integrity at every intermediate node. This greatly
reduces the transition time through the network, allowing more data to actually be
passed on the same circuit when compared to X.25.
The PVCs in a frame relay network consist of three parts. The first two parts are
the DLCIs (Data Link Connection Identifiers) on each link to the network and the
third part is the routing tables within the switches that comprise the network. A
DLCI is nothing more than a logical channel number that is used to separate traffic
destined for different endpoints. It is important to remember that a DLCI exists
only between the network and a particular endpoint (user). Data may be sent on
one DLCI to the network and come out of the network on a different DLCI at the
destination point. The routing tables within the switches take care of mapping a
DLCI at one point to a DLCI at another.