Frame Relay is a standardized WAN technology meant to interconnect LANs over packet-switching networks, usually consisted of high quality digital-grade error-free transmission lines. It meant to simplify X.25 by removing excessive error processing. It defines only physical and layer 2 components. As X.25, it mimics a continuous dedicated connection between end nodes, which is called permanent virtual circuit (PVC) but is cheaper than dedicated leased lines. Data is transmitted in variable size units called frames. Error correction is left to end nodes. The bandwidth is dynamically allocated using congestion notifications. It can operate over fractional T1, T1 or E1 lines.
| Flag Field (01111110) | Address Field (2-5 octets) | Information Field | FCS |
Flag is served to delimit the beginning of a frame.
Address field includes DLCI (Data Link Connection Identifier) and ECN bits (Forward, Backward and Discard Eligibility bits) for explicit congestion notification. Congestion control is based on Admission Control, which decides whether to accept a new connection request defined by requested traffic descriptor based on network capacity. The traffic descriptor consists of a set of parameters communicated to the switching nodes during call time or at the service subscription time. These parameters are:
DLCI is locally significant. A physical link may have multiple logical (virtual) circuits, thus multiple DLCIs, each on its own subinterface. Local Management Interface (LMI) extension makes DLCI values global. DLCI values become DTE addresses that are unique in Frame Relay WAN.
Information field sets the max number of data bytes in the frame.
FCS is CRC, the same as in Ethernet frames. When an error is detected, the frame is dropped.