Frame Relay

By daxm

… What is Frame Relay …

Devices that “speak” Frame Relay fall into 1 of 2 categories:  data terminal equipment (DTE) or data circuit-terminating equipment (DCE).

  • DTE equipment is at the customer premise and are usually the “user” end of the connection.  Think of terminals, PCs, routers.
  • DCE equipment is the service provider end of the connection and provide the clocking and switching services in the FR network.

Frame Relay is a connection-oriented protocol. This means that before data is sent across the FR network an end-to-end (DTE to DTE) path must be established.  This end-to-end path is known as a virtual circuit (VC). NOTE:  Multiple VCs can travel down the same physical wire(s).  Through the use of TDM (time division multiplexing) each VC can get its share of the wire’s available bandwidth.

Again, each DTE-to-DTE connection through a FR network requires a VC.  Each end of each VC has a data-link connection identifier (DLCI) to unique identify that end of the connection. (More on DLCIs later.)  Virtual Circuits are broken into 2 categories: Switched Virtual Circuits and Permanent Virtual Circuits.

  • Switched Virtual Circuit (SVC) is a temporary VC that is setup “on demand”. (A good example of an SVC is a telephone call.  The path between the caller and the callee does not exist until the caller dials the number.  At that point a dedicated path (VC) is created through the PSTN to the destination phone number.  Through the duration of the call that path through the PSTN doesn’t change.  Once the call is disconnected that VC is erased.  Should the caller redial the callee a completely new VC would be established and may or may not follow the same path through the PSTN.)
  • Permanent Virtual Circuit (PVC) is a dedicated VC between to end points through the FR network. (An example of a PVC would be the “2 cans and a string” phones that kids make.  They are permanently connected and can send data (kids’ voices) between ONLY those 2 endpoints.  Should a 3rd endpoint be needed then additional PVCs would have to be built and that would mean additional cans and strings for the kids.)
  • NOTE:  Since no VC setup or tear down phases are needed, data transfer can begin immediately.  Thus PVCs are “quicker” than SVCs.  However, due to the setup and tear down phases, SVCs are “slower” than PVCs but do offer a cost savings since the connection need not be up all the time.

Data-Link Connection Identifier (DLCI) is a locally significant value assigned to each VC on a physical wire. The important thing to remember here is that a DLCI of 16 on one end of a VC can connect to a DLCI of 25 on the other end of the VC.  The end-to-end DLCIs don’t have to match.

Frame Relay uses 2 bits in the FR frame header to notify the DTE devices of congestion within the FR network.  These two bits are know as the forward-explicit congestion notification (FECN) and the backward-explicit congestion notification (BECN).  The key thing to know is that these bits are set by the DCE equipment and is the FR network’s way of informing the DTE devices that the path that this VC takes through the network is experiencing congestion.  What the DTE equipment does with this information is handled by higher layer protocols. (Examples of what the higher layer protocol may choose to do is to take a different path to the destination, or shrink the window size of the number of packets sent, or set the discard eligible (DE) bit in the FR header.

During times of network congestion the DTE equipment has 1 mechanism to assist the DCE equipment’s choice on what to drop.  That mechanism is the discard eligible (DE) bit in the FR header.  If the DE bit is set it indicates to the DCE equipment that it should drop this frame first instead of a frame that doesn’t have the DE bit set.

FR does error-checking, via the CRC value, but NOT error correction.

Local management interface (LMI) is a set of enhancements that was developed in 1990 by consortium of vendors (Cisco being one of them).  LMI adds extensions to the basic frame relay specification. (LMI is to FR was CDP is to Ethernet, kind of.)

  • LMI’s global addressing extension give FR’s DLCI values the ability to have global significance.  (Rarely used in service provider networks but has more applicability in private enterprise FR networks.)
  • LMI virtual circuit status messages provide communication and synchronization between the local DTE and DEC devices.
  • LMI multicasting extension adds multicast abilities to the FR network.

An LMI frame is identified by the DLCI address of 1023.

In order to configure FR on a router 2 things MUST be setup before data transmission.  First, the interface must be set for frame relay encapsulation.  Second, an mapping must be made (whether dynamic or static)

… References …

These notes are from studying and


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