Data Link Connection Identifier (DLCI)

Before understanding what a Data Link Connection Identifier (DLCI) is, we need to first get our hands on what a frame relay is. According to IBM, a Frame relay is a protocol that defines how frames are routed through a fast-packet network based on the address field in the frame. Frame relay behaves as Wide Area Network (WAN) using the packet switching methodology. Prior to packet switching, Point to Point(P2P) technology was used. In a P2P technology, there is a physical connection between the routers. Obviously, it is going to be very costly (lots of circuits and equipment required)  when we have a large number of locations/ cities to connect, hence, the need for frame relay. Using the packet switching technology we can connect one router to many routers. Frame relay is cheaper and more scalable because the connections are made logically and not physically which saves a lot of equipment overhead.

Point to Point Connection

Frame Relay

The above figure shows frame relay, we can see many routers are connected to the cloud, and via the cloud, it is able to connect with other routers without any separate physical connection. The circuits are referred to as “Access Links”. Inside the frame relay cloud (that is owned by the service provider) each circuit is terminated by a frame relay switch, known as DCE or Data Communication Equipment, this switch is not only responsible for terminating the circuit but also for providing the frame relay “services”. 

When the router or the Data Terminating Equipment (DTE) creates a packet and sends it to the frame relay switch it is examined and looks for a “number” known as the Data Link Connection Identifier (DLCI).

Data Link Connection Identifier (DLCI)

A DLCI is a number that is required by the frame relay switch (DCE) in order to identify the destination router of the incoming packet. DLCI values typically are assigned by the Frame Relay service provider. Every DCE knows the DLCI of every other DCE in the frame relay cloud so that the DCE receiving the packet knows where to send the package next. DLCI is locally significant to each endpoint and this allows us to have duplicate DLCIs, but a DCE cannot have the same DLCI for two different destinations(because it will not be able to decide the destination router as they have the same DLCI). Frame relay also helps in saving the bandwidth as it is shared among all the DCEs. A minimum bandwidth as per the service you opted for is ensured by the Committed Information Rate (CIR).