Frame Relay Concepts notes

Frame Relay is a layer 2 protocol running at the data-link layer.

Frame Relay Components




As seen in the picture Frame Relay is made up of a number of switches owned by the service provider. The concept is very similar to a point to point, but the frame relay cloud is always the DCE side of a PPP and the router is always the DTE side of the PPP.

The link between the frame relay cloud and the router is called the Access Link

LMI local management interface messages are the messages sent over frame relay connections to transfer information pertaining to the link.

VC virtual circuit - is a virtual connection through the frame relay cloud to a set of routers. Creating a direct link between the two to exchange information.

Virtual Circuit
Frame Relay Terms

Virtual Circuit VC - a logical conecpt that represent a path between two end points through a frame relay cloud.
Permanent Virtual Circuit PVC - a predefined VC, very similiar to a leased line as the ID number of the VC is already statically defined.
Switched Virtual Circuit - a VC that is set up to dynamically be used when needed. Most commonly used for dial up connections.
Data Terminal Equipment DTE - End of the frame relay connection that received information from the cloud, typically a router.
Data Communications Equipment DCE - The sending end of the frame relay could that provides a connection point to other networks. Switches within the frame relay cloud.
Access Link - a leased line between DTE and DCE ends.
Access Rate - the speed at which the access link is clocked.
Committed Information Rate CIR - the speed of bits that are sent over a VC, according to the business contract between the customer and service provider.
Data-link connection identifier DLCI - an address used to identify VCs located in the header.
Nonbroadcast multiaccess - a network in which broadcasts are not supported, but more than two devices can communicate.
Local Management interface LMI - a protocol used between DCE and DTE to manage connections.

LMI

  • perform a keep alvine between DTE and DCE. If the access link as a problem the absence of the keep alive messages implies the link is down.
  • signal whether a PVC is active or inactive. Informs routers which PVCs are down, informing it if routes are down.
Frame Relay Encapsulation types 
  • cisco
  • ansi
  • itu 
Frame Relay local addressing

DLCI are numbers assigend to identify a router from the frame relay cloud. The DLCI numbers are local significant only meaning that only the router assigned the DLCI and the frame relay cloud are concerned with that number. Local addressing requires that one unique DLCI be assigned per side of a VC. In a hub and spoke with routers B and C connected to A. All VCs connected to router A would need to be different.

Frame Relay Global Addressing

Unlike local addressing, global assigns only one DLCI to each router. Making it easier to document and configure. This allows only one DLCI number to be used for each router in the network regardless of which end of the VC the router is on.

Key point
The local DLCI on a single access link must be unique among all PVCs that use one physical frame relay access link

IP addressing of Frame Relay

Full mesh networks can use only one subnet to provide IP addresses

Partial mesh networks require a subnet per end of the VC