Cabletron Systems CSX200 manual Frame Relay Protocol

Chapter 2: About the CSX200

This is done to replace the hidden local IP addresses from the sub network with one or more public InterNic assigned IP addresses that can be sent over the outside network on the WAN interfaces. Once the CSX200 is assigned at least one public IP address, over 250 IP clients can share this address simultaneously using NAT. This public IP address is assigned statically by the Internet Service Provider (ISP).

Frame Relay Protocol

Frame Relay can be defined as a “packet mode” service, organizing data into individually addressed units known as “frames”. Frame Relay eliminates Layer 3 processing. Only a few Layer 2 functions are used, such as checking for a valid, error free frame, but not requesting retransmission if an error is found. Frame Relay uses a variable length framing structure, which, depending on user data, can range from a few to more than a thousand characters.

A Frame Relay Network will often be depicted as a cloud, because the Frame Relay Network is not a single physical connection between one endpoint and another. Frame Relay Protocol is based on the concept of “virtual circuits” (VCs). VCs are two-way, software defined data paths between two ports that take the place of private lines in the network. There are two types of Frame Relay connections; Switched Virtual Circuits (SVCs), and Permanent Virtual Circuits (PVCs).

Permanent Virtual Circuits, or PVCs, are set up via a network management system, and initially defined as a connection between two sites, or endpoints. PVCs may be added as the demand arises for more bandwidth, alternate routing, or more sites, or endpoints. PVCs are fixed paths, not available on demand, or on a call-by-call basis. Although the actual path through the network may change from time to time, such as when automatic rerouting takes place, the beginning and end of the circuit will not change.

Switched Virtual Circuits, or SVCs, are available on a call-by-call basis using the SVC signaling protocol (Q.933). The network must quickly establish the connection, and allocate bandwidth based on the user’s request.

In a Frame Relay frame, user data packets are not changed in any way. A two-byte header is appended to the frame. Contained in this header is a 10-bit number called the Data Link Connection Identifier (DLCI). The DLCI is the “virtual circuit” number which corresponds to a particular destination. The DLCI allows data coming into a Frame Relay switch to be sent across the network using a three-step process: check the integrity of the frame and discard it if it is in error, look up the DLCI in a table and if not intended for this link, discard the frame. If the frame passes the previous tests, relay the frame toward its destination out the port specified in the table. If the frame passes the previous tests, relay the frame toward its destination out the port specified in the table.

2-6 CSX200 Installation Guide