Frame Relay Fast Packet Switching

Frame Relay is a simplified form of Packet Switching similar in principle to X.25 in which synchronous frames of data are routed to different destinations depending on header information.

The biggest difference between Frame Relay and X.25 is that X.25 guarantees data integrity and network managed flow control at the cost of some network delays. Frame Relay switches packets end to end much faster, but there is no guarantee of data integrity at all.

Frame Relay is cost effective, partly due to the fact that the network buffering requirements are carefully optimized. Compared to X.25, with its store and forward mechanism and full error correction, network buffering is minimal. Frame Relay is also much faster than X.25: the frames are switched to their destination with only a few byte times delay, as opposed to several hundred milliseconds delay on X.25.

Frame Relay uses the synchronous HDLC frame format up to 4kbytes in length. Each frame starts and ends with a Flag character (7E Hex). 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. 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. This means that all DLCI connections are set up by the network provider at subscription time.

Data Integrity

There is none. The network delivers frames, whether the CRC check matches or not. It does not even necessarily deliver all frames, discarding frames whenever there is network congestion. Thus it is imperative to run an upper layer protocol above Frame Relay that is capable of recovering from errors, such as HDLC, IPX or TCP/IP.

In practice, however, the network delivers data quite reliably. Unlike the analog communication lines that were originally used for X.25, modern digital lines have very low error rates. Very few frames are discarded by the network, particularly at this time when the networks are operating at well below design capacity.

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 specify the line speed (e.g. 56kbps or T1) 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 the CIR 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 first if there is congestion. 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 any frame away when it needs to.

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) bits in data frames. These are used to tell the application to slow down, hopefully before packets start to be discarded.

Our advice is to always accept the highest CIR that your provider will give you for free, but never to pay for a higher CIR until you are absolutely sure that your data is being discarded. There is evidence that some public networks cannot even measure your Information Rate, although that does not stop them for charging for a CIR.

Our products always include a mechanism for accessing detailed statistics on the network performance and operation. These can give you an indication of whether to pay for a higher CIR or not.

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 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.

There exist various standards for the Status Polling function. The oldest, the Link Management Interface (LMI), was a temporary standard adopted by manufacturers prior to the international standards bodies getting their standards out. It is supposed to have disappeared when the official ANSI T1.617 Annex D (known as ANSI or Annex D) standard came out, but it has acquired a life of it's own. A newer standard, Q.933 has also been approved, largely to accommodate Switched Virtual Circuits, when these become available.

Our products, support all three standards of Status Polling.

Usage

Frame Relay is used mostly to route Local Area Network protocols such as IPX or TCP/IP. It can also be used to carry asynchronous traffic, SNA or even voice data. Its primary competitive feature is its low cost. In North America it is fast taking on the role that X.25 has had in Europe: the most cost effective way to hook up multiple stations with high speed digital links.

Frame Relay networks do not yet have the reliability of X.25 networks. Expect problems with new installations. You cannot take any features for granted. At the time of writing, some public networks do not even support Status Polling properly. This makes it difficult to find out whether remote links are up or not.

In general, the newer the network, the better the implementation. For instance, one of the best Frame Relay networks around is in Poland, while some of the pioneer US networks are still struggling with older equipment.

JBM Electronics Frame Relay Support

Frame Relay networks do not yet have the reliability of X.25 networks. Expect problems with new installations. You cannot take any features for granted. At the time of writing, some public networks do not even support Status Polling properly. This makes it difficult to find out whether remote links are up or not.

In general, the newer the network, the better the implementation. For instance, one of the best Frame Relay networks around is in Poland, while some of the pioneer US networks are still struggling with older equipment.

Our units offer the following Frame Relay support:

If you need further information on Frame Relay or our implementation and support for this protocol, please e-mail us.

Information in this document is provided by Sangoma Technologies.