Frame Relay

                Network communications have moved toward digital and fiber – optic environments.  There is less need for the error checking found in the X.25 protocol.  As the result, many large corporations use Frame Relay provide fast, variable-length packet-packet-switching over digital networks. Frame Relay includes a cyclic redundancy check (CRC) algorithm that can detect whether a packet is corrupted and can discard it.  It does not; however ask for retransmission of the data.  It leaves that up to higher levels that up to higher levels of the protocol.

                Frame Relay uses permanent virtual circuits (PVCs) so the entire path between two hosts is known from end to end.  This creates an optimal network environment in which the path between two hosts is predetermined.   Instead of always having to calculate the best path to remote host, the PVC has predetermined that route.  In addition, because the hosts are connected using a common frame relay network, packets do not have to be fragmented due to differing Maximum Transmission units (MTUs) The MTU is the largest packet size that canbe used on a network segment.  Frame relay networks all have a same MTU, removing the issues with differing MTUs.

                Frame Relay also includes the following local management interface (LMI) extensions:

·         Virtual circuit status messages provide information about PVC integrity.  They report the addition of any new PVCs and the deletion of existing PVCs.  These status messages prevent hosts from sending messages to a PVC that has ceased to exist.

·         Multicasting is an optional LMI extension that enables a host to send a single frame destined for multiple recipients.  This reduces overall network traffic because a single frame can be sent to multiple hosts instead of one message per host.

·         Global addressing provides globally significant connection identifiers.  Frame Relay uses data link connection identifiers (DLCIs) to identify a circuit ID.  When global addressing is implemented, each connection has a globally unique ID.  This ID is known to all other connections.

If Winnipeg must send a frame to Minneapolis, Winnipeg places a value of 40 in the DLCI field and sends the frame into the Frame Relay network.  When the frame arrives in Minneapolis, the network changes the DLCI field contents to 10.  This shows that the frame came from the Winnipeg network.  This addressing scheme enables the Wan to function using the same methods as a LAN.

·         Simple flow control provided an XON/XOFF flow-control mechanism.  Frame Relay includes simple congestion-notification messages that enable the network to inform user devices when network resources are approaching a congested state.  The simple flow control LMI extension is provided for devices that cannot use these notification messages and that need some level of flow control.