Frame Relay Tutorial
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. Sangoma supports Frame Relay mostly through its WANPIPE® routing packages, and also through user level APIs for OEM developers. 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. As line speeds have increased from speeds below 64kbps to T1/E1 and beyond, the delays inherent in the store-and-forward mechanisms of X.25 become intolerable. At the same time, improvements in digital transmission techniques have reduced line errors to the extent that node-to-node error correction throughout the network is no longer necessary. The vast majority of Frame Relay traffic consists of TCP/IP or other protocols that provide their own flow control and error correction mechanisms. Much of this traffic is fed into the Internet, another packet switched network without any built-in error control. Because Frame Relay does not ‘care’ whether the frame it is switching is error-free or not, a Frame Relay node can start switching traffic out onto a new line as soon as it has read the first two bytes of addressing information at the beginning of the frame. Thus a frame of data can travel end to end, passing through several switches, and still arrive at its destination with only a few bytes delay. These delays are small enough that network latency under Frame Relay is not noticeably different from direct leased line connections. As a result, the performance of a Frame Relay network is virtually identical to that of a leased line, but because most of the network is shared, costs are lower.
Frame Relay uses the synchronous HDLC frame format (see Synchronous and Asynchronous Communications) 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.
Packets are routed through one or more Virtual Circuits known as Data Link Connection Identifiers (DLCIs). Most Virtual Circuits are Permanent Virtual Circuits or PVCs, which means that the network provider sets up all DLCI connections at subscription time. Switched Virtual Circuits (SVCs) are also part of the Frame Relay specification. They provide a link that lasts only only as long as the session. By having a system with several DLCIs configured, you can communicate simultaneously with several different sites. Sangoma’s WANPIPE® and APIs support up to 250 DLCIs per physical link, allowing servers to be used as substantial network hubs.
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 usual to run an upper layer protocol above Frame Relay that is capable of recovering from errors, such as TCP/IP, X.25 or IPX. In practice, however, the network delivers data quite reliably. Unlike the analog communication lines that were used in the past, 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 true flow control with Frame Relay. The network simply discards frames it cannot deliver. However, the protocol does include features designed to control and minimize frame loss at the user level. 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. For example, your Frame Relay access may be a full T1 (1.54Mbps), but you may have subscribed to a CIR of only 512kbps. What happens is that the Access Node measures your average throughput over a time period, usually one second. If the average throughput is over 512kbps, then the ‘extra’ frames are marked with a DE bit, and will be discarded first. Note that all your data transmissions always occur at the line speed, in this case 1.54Mbps. Because this is synchronous communications the data is clocked out at a constant speed. The frames you are transmitting have idle gaps between them so that in one second, the total number of bits sent is a number smaller than 1.54Mbits, and that number is the Information Rate. 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. It is a good idea never to subscribe to high CIR until you are absolutely sure that your data is being discarded. There are significant savings to be made in choosing a low or zero CIR, unless there is evidence of packet loss. Sangoma’s WANPIPE® and APIs 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.
The Frame Relay Customer Premises Equipment (CPE) polls the Access Node (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. The official ANSI T1.617 Annex D (known as ANSI or Annex D) standard is currently the one most used. The newer Q.933 standard also supports Switched Virtual Circuits. Sangoma’s WANPIPE® and APIs support all three standards of Status Polling.
Uses of Frame Relay
For companies with numerous distributed offices, Frame Relay provides a cost-effective way of providing a secure private IP-based network. While some companies use VPNs over the Internet for intra-company communications, that option does expose the organization to some serious security issues, not the least of which is keeping viruses and hackers out of perhaps hundreds of individual Internet connections at the offices. In contrast, Frame Relay privacy is guaranteed by the nature of the network, backed up by legislation. Frame Relay is also used as a low cost carrier to replace the networks of leased lines previously used to connect ATM machines, POS terminals and other legacy devices to head office mainframes. These applications involve some kind of protocol conversion to spoof the equipment at both ends. Many Frame Relay connections are used for high-end Internet connections.
Frame Relay is a mature technology that is well-implemented worldwide. You can expect reliability as good as or exceeding that of point-to-point leased digital lines.