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Appendix A. Frame Relay Basics

Frame Relay is basically a simplified form of Packet Switching in which synchronous frames of data are routed to different destinations depending on header information.

Framing

Frame Relay uses synchronous HDLC frames up to 4 kbytes in length. Each frame starts and ends with a Flag character (7E). The first 2 bytes of each frame following the flag contain the information required for multiplexing across the link. The last 2 bytes of the frame are always generated by a Cyclic Redundancy Check (CRC) of the rest of the bytes between the flags. This allows easy checking of the data integrity. The rest of the frame contains the user data.

Virtual Circuits

Packets are routed through one or more Virtual Circuits known as Data Link Connection Identifiers (DLCIs). Each DLCI has a permanently configured switching path to a certain destination. Thus, by having a system with several DLCIs configured, you can communicate simultaneously with several different sites. Currently, only permanent virtual circuit connections are supported.

Flow Control and Information Rates

There is no flow control on Frame Relay. The network simply discards frames it cannot deliver.

When you subscribe, you will set the line speed (e.g. 56 kbps) and also, typically, you will be asked to specify a Committed Information Rate (CIR) for each DLCI. This value specifies the maximum average data rate that the network undertakes to deliver under "normal conditions". If you send faster than that on a given DLCI, the network will flag some frames with a Discard Eligibility (DE) bit. The network will do its best to deliver all packets but will discard any DE packets if there is congestion.

Frame Relay provides indications that the network is becoming congested by means of the Forward Explicit Congestion Notification (FECN) and Backward Explicit Congestion Notification (BECN). These are used to tell the application to slow down, hopefully before packets start to be discarded.

Frame Relay is cost effective, partly due to the fact that the network buffering requirements are carefully optimized. Many inexpensive Frame Relay services are based on a CIR of zero. This means that every frame is a DE frame, and the network will throw them away when it needs to.

Status Polling

The Frame Relay Customer Premises Equipment (CPE) polls the switch at set intervals to find out the status of the network and DLCI connections. A Link Integrity Verification (LIV) packet exchange takes place about every 10 seconds, which basically verifies that the connection is still good. It also provides information to the network that the CPE is active, and this status is reported at the other end.

About every minute, a Full Status (FS) exchange occurs, which passes information on which DLCIs are configured and active. Until the first FS exchange has occurred, the CPE does not know which DLCIs are active, and so no data transfer can take place.

Note: You may have to wait for up to 2 minutes after first boot up before you try any kind of data communication. Such communication may fail if the first FS exchange has not yet occurred.

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