The service was once widely available and implemented. If the FCS fails, the frame is discarded.Frame relay is a packet-switching telecommunications service designed for cost-efficient data transmission for intermittent traffic between local area networks ( LANs) and between endpoints in wide area networks ( WANs). Frame Check Sequence is used to verify the data integrity. It has variable length up to 16,000 octets.įCS (Frame Check Sequence) or CRC (Cyclic Redundancy Code): It is either 16 bits, or 32 bits wide. If the EA value is 1, then the current byte is determined to be the last octet of the DLCI.ĭata: This field contains encapsulated upper-layer protocol data. Note that all FECN, BECN, and DE enable FR network congestion control by regulating the communication, and prioritizing traffic.Įxtended Address (EA): The eighth bit of each byte of the Address field (header) is used to indicate the EA. Discard Eligible frames are discarded first before removing frames that do not have DE bit set, in the event of network congestion. A value of 1 indicates that the FR network has experienced congestion in the direction of destination to source.īy using FECN and BECN, upper layer protocols can control the communication for efficient utilization of FR network.ĭiscard Eligibility (DE): This is set by the DTE device to indicate that the marked frame may be discarded in the event of network congestion. A value of 1 indicates network congestion in the direction of source to destination, known as Forward Explicit Congestion Notification.īackward Explicit Congestion Notification (BECN): This is a single-bit field that can be set to either 0 or 1 by a switch in the FR network. DLCI identifies the virtual connection between the end node (a DTE device) and the switch (a DCE device).Ĭ/R: The C/R bit says whether the frame is a command or response.įorward Explicit Congestion Notification (FECN): This is a single-bit field that can be set to either 0 or 1 by a switch. The different fields are explained below with respect to Frame Relay:įlag (both opening and closing flags): 8 bits (01111110 or 7E hex)Īddress (Also known as Frame Relay Header): It is a 16-bit field as given below.ĭata Link Connection Identifier (DLCI): The DLCI is 10-bit wide. They primarily differ in how the address and control bits in HDLC frame are used. Other protocols that use HDLC frames include SDLC, Frame Relay, and X.25. We have discussed HDLC in earlier sections, and the HDLC frame is given below. This results in simple link management procedures, and more efficient data transfers.įrame Relay Protocol: FR is an HDLC protocol based network. Unlike SVC, there is no call setup, and call termination procedures in PVC. SVCs are advantageous when you have burst traffic, and you don't want to block the network bandwidth for a given virtual circuit 24hours a day. If there is some more data to be transmitted at a later time, an SVC is negotiated again. Call termination: The virtual circuit between end nodes is terminated.An SVC call is terminated after a certain period of idle time Idle: The connection between end nodes is still active, but no data is transferred.
Data transfer: Data is transmitted between the end nodes over the virtual circuit.Call setup: The virtual circuit between two Frame Relay end nodes is established.An SVC communication session has four states: Switched Virtual Circuit(SVC): A switched virtual circuits (SVC) provide temporary connection between end nodes (DTEs) across a Frame Relay network. Note that PVC does not terminate the virtual circuit even when there is no data being transferred between the end nodes. Idle state: No data is transferred between the end nodes.Data transfer state: Data is transmitted between the end nodes over the virtual circuit.A PVC can have two operational states as given below: This type of connection (PVC) is used when it is required to consistently transfer data between the end nodes. The virtual circuit is always available irrespective of whether any data is being transmitted or not. Permanent Virtual Circuit (PVC): PVC is a permanent connection between the end nodes (DTEs) within a Frame Relay network. A virtual circuit provides a logical connection between end nodes for the flow of information. In data circuits, the term virtual circuit is also used in similar sense. Remember that a circuit provides connection between end nodes by means of an electrical connection. Before going ahead with Frame Relay protocol, and its operation, we discuss virtual circuits.
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