Frame relay systems are a simple bearer (transport only) technology and do not offer advanced error protection or retransmission. This reduces the time for packet switching (reduced transmission delay time). It is up to the sender and receiver of frame relay data to ensure the integrity of the data. When used in systems that have good digital communication systems, frame relay provides reliable data communication service.
The first frame relay standard I.122 was defined in 1988 by the International Telecommunications Union (ITU). The current frame relay specification standards include the ITU I.233 and American National Standards Institute (ANSI) T1.606.
The key components of a frame relay system include frame relay access device (FRAD) frame relay network devices (FRND) and frame relay switches. The FRAD converts end user data into protocol data unit (PDU) variable length packets. The FRAD communicates to the FRND over an access line (e.g., fractional T1/E1 or ISDN line).
The FRND is a packet switch that also operates as a gateway to the frame relay network. The FRND passes frames it receives from the FRAD to other frame relay switches that forward packets toward their destination network. Frame relay switches have buffer memory that allows them to hold and prioritize packets before they are retransmitted. Packet switches can selectively discard packets if network congestion occurs. The FRAD and FRND provide information about the priority of the frames (e.g., non-essential discard eligibility) and status of the system (e.g., network congestion notification).
The frame relay system uses a discard eligibility (DE) flag system to indicate the essential nature of the packet’s data. The DE flag(s) allow systems to selectively discard data packets or frames that are non-essential. This process allows some data transmission systems to send more data than is agreed to (dynamic bandwidth). If the network is not congested, it may allow the extra packets of data to reach their destination.
Congestion notification is a control flag signaling system that is used to indicate status of network congestion in a data network. Congestion notification allows data communication devices that are connected to the data network to send or delay the sending of data dependent on the status of the network.
The frame relay system uses both forward and backward congestion notification. Forward explicit congestion notification (FECN) indicates to upstream switching devices that data that is being transmitted through congested switches and it is likely that some of the remaining packets may be discarded. The upstream switch can then change the data discard priority level accordingly. Backward explicit congestion notification (BECN) indicates to the sending (downstream) switching devices that congestion is occurring and packets that are received may be discarded. The sending switch can then change the priority of packet discarding and send and indication to other switches indicating network congestion. This should eventually reduce the amount of data end-users are sending into the network.
The frame service provider usually agrees to provide the frame relay service at certain data transmission rate (service level). The frame relay system may provide a committed information rate (CIR) and a maximum burst information rate (BIR).
Figure 1 shows a frame relay system. This diagram shows a local area network (LAN) in San Francisco is connected to a LAN in New York. A virtual path is created through the frame relay network so data can rapidly pass through each frame relay switch as its path is previously established. When data is to be transferred through the LAN (e.g., a large image file), the data file passes through a FRAD that is the gateway to the frame relay network. The FRAD divides the data file from the LAN into variable length data frames. The FRAD sends and receives control commands to the frame relay network that allows the FRAD to know when and if additional data frames can be sent.
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