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.