Frame Relay is a #packet-switched Wide Area Network (WAN) service that operate at the Physical Layer# (EIA/TIA-232 specification is the standard) and Data Link Layer#. Its speed usually ranges from 56 kbps to 2Mbps, and it provides connection-oriented services using virtual circuits. Recently, it’s been viewed as obsolete because of the wide adoption of Dense Leased Line (DLL).
Virtual circuit# is basically a logical connection between two data terminal equipment (DTE) devices, such as terminals, PCs, routers and bridges, across a Frame Relay packet-switched network which can be uniquely identified by data-link connection identifier (DLCI) at the network endpoints. Since it is logical, within a single physical circuit there can exist multiple virtual circuits. Within this virtual circuit, there could be multiple data communications equipments (DCE), usually a Switch#, which provides clocking and switching services in a network.
Frame Relay protocol will utilise the DLCI to map the packet to the service provider’s switch’s outbound ports by referring to the lookup table stored. The path to the destination is determined before the frame is sent. Local Management Interface (LMI) is used to manage a connection between a router and a Frame Relay switch, where it tracks and manage keep-alive mechanisms multicast messages and status.
To deal with network congestion#, Frame Relay switching equipment can mark a Frame Relay packet with forward explicit congestion notification (FECN) or backward explicit congestion notification (BECN). The former will be sent to the destination device whereas the latter will be sent to the source router. The respective DTE equipment will notify higher layer about the congestion based on such mark, and decide whether act accordingly or ignore such messages. Other than FECN and BECN, the packet could be marked as discard eligible (DE), which enable the switching equipment to drop it whenever there is a congestion.