Frame Relay Concepts

Frame Relay remains the most popular WAN technology used today.  However its popularity is waning.  It is being replaced mainly by the virtual private network technology of two main types: Internet VPNs, which use the Internet to transport packets, and Multiprotocol Label Switching (MPLS) VPNs, which follow the same basic service model as Frame Relay, typically offered by the same Frame Relay providers, but with significant technical advantages.

Frame Relay most closely compares to the OSI data link layer (Layer 2).  If you remember that the word “frame” describes the data link layer protocol data unit, it will be easy to remember that Frame Relay relates to OSI Layer 2.  Like other data-link protocols, Frame Relay can be used to deliver packets (Layer 3 PDUs) between routers.  Frame Relay protocol headers and trailers are simply used to let a packet traverse the Frame Relay network, just like Ethernet headers and trailers are used to help a packet traverse an Ethernet segment.

Frame Relay is a set of WAN standards that create a more efficient WAN service as compared to point-to-point links, while still allowing pairs of routers to send data directly to each other.  With leased line, each leased line requires a serial interface on each router and a separate physical circuit built by the telco.  Frame Relay supports the ability to send data to multiple remote routers over a single physical WAN circuit.

Frame Relay networks have more benefits and features but are also more complicated.  Frame Relay networks are multiaccess networks, which means that more than two devices can attach to the network, similar to LANs.  Unlike LANs, you cannot send a data link layer broadcast over Frame Relay.  Frame Relay networks are called nonbroadcast multiaccess (NBMA) networks.  Because Frame Relay is multiaccess, it requires the use of an address that identifies to which remote router each frame is addressed.

Frame Relay requires a leased line installed between the router and a nearby Frame Relay switch; this link is called the access link.  To ensure that the link is working, the device outside the Frame Relay network, called the data terminal equipment (DTE), exchange regular messages with the Frame Relay switch.  These keepalive messages are defined by the Frame Relay Local Management Interface (LMI) Protocol.  The routers are considered DTE, and the Frame Relay switches are data communications equipment (DCE).

The logical communication path between each pair of DTEs is a virtual circuit (VC).  Typically, the service provider preconfigures all the required details of a VC; predefined VCs are called permanent virtual circuits (PVC).

Routers use the data-link connection identifier (DLCI) as the Frame Relay address; it identifies the VC over which the frame should travel.  When a router needs to forward a packet to another router, it encapsulates the Layer 3 packet into a Frame Relay header and trailer and sends the frame.  The Frame Relay header includes the correct DLCI so that the provider’s Frame Relay switches correctly forward the frame to the other router.

Frame Relay Terms and Concepts

Term Description
Virtual circuit (VC) A logical concept that represents the path that frames travel between DTEs. VCs are particularly useful when you compare Frame Relay to leased physical circuits.
Permanent virtual circuit (PVC) A predefined VC. A PVC can be equated to a leased line in concept.
Switched virtual circuit (SVC) A VC is set up dynamically when needed. An SVC can be equated to a dial connection in concept.
Data terminal equipment (DTE) DTEs are connected to a Frame Relay service from a telecommunications company. They typically reside at sites used by the company buying the Frame Relay service.
Data communications equipment (DCE) Frame Relay switches are DCE devices. DCEs are also known as data circuit-terminating equipment. DCEs are typically in the service provider’s network.
Access link The leased line between the DTE and DCE.
Access rate (AR) The speed at which the access link is clocked. This choice affects the connection’s price.
Committed Information Rate (CIR) The speed at which bits can be sent over a VC, according to the business contract between the customer and provider.
Data-link connection identifier (DLCI) A Frame Relay address used in Frame Relay headers to identify the VC.
Nonbroadcast multiaccess A network in which broadcasts are not supported, but more than two devices can be connected.
Local Management Interface (LMI) The protocol used between a DCE and DTE to manage the connection. Signaling messages for SVCs, PVC status messages, and keepalives are all LMI messages.


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