Pfa-35

Frame Relay and Testing with the PFA-35

Application Note:

PFA-35

 Summary
This application note describes the essentials of Frame Relay, and how to perform three important tests for installing and maintaining Frame Relay services using the PFA-35 with Frame Relay option.

The Key Benefits of Frame Relay:
Frame Relay brings two key benefits: q It uses a packet-switching, not a circuit-switching, technology. As a result multiple data communication paths can be configured over one physical link so the high cost of multiple point-to-point circuits is eliminated. q The switching performed by Frame Relay nodes is a very efficient process with minimal end-to-end delays. This low delay, high bandwidth combination equals increased capacity and throughput and so is ideal for data intensive LAN/WAN internetworking applications.

Frame Relay Frame Structure
To transmit data between one user and another across the Frame Relay network, the data is inserted into frames. These can be of variable length and the structure employed is based on HDLC framing. All frames start and end with a flag. The start flag is followed by an address field, the exact format of which is dependant on the Link Management Protocol in use and may be two, three, or four bytes long. In the example below a two byte, sixteen bit header is employed. 10 bits are used for the Data Link Connection Identifier (DLCI) - this identifies the address to which data being carried in the frame is going to, or coming from. The Extended Address bit (EA) is set to 1 in the last address byte. The Command/Response indication bit (CR) is not used by the Frame Relay protocol. The Forward Explicit Congestion Notification bit (FECN); Backward Explicit Congestion Notification bit (BECN); and Discard Eligibility bit (DE) are used to notify the user that congestion is occurring in the network. The information field is a variable length field and contains the user data being passed across the network. This is