FRAME RELAY:

In many wan connections Bandwidth is not used 100%, all the time. In frame relay if somebody is not using the bandwidth, chances are somebody else is.

Frame relay is a part of packet switch class of network of which X.25 was the first one.  X.25 became frame relay and frame relay became ATM and ATM became MPLS.

If a server provider has 2 Gbps bandwidth available, he can sell 3 Gbps bandwidth to its customer, as not all the users would be using the bandwidth at the same time.

FRAME RELAY TERMINOLOGY:

Committed Information Rate (CIR):  is the minimum bandwidth the Service Provide guarantees you.
Lots of time you are using the bandwidth above the CIR, if the bandwidth is available. If you are always using more bandwidth the ISP will monitor this and tell you that you need to pay more money to up your CIR as you are continuously using more bandwidth than what you are paying for.

Local Access Rate (LAR): is physically, how fast that circuit can go. (like CAT 5 cable can go 100 mbps, and data can travel at this speed if required) you might have LIR of 2 Mbps but CIR of 500kbps. Even though the physical cable can handle 2 Mbps, you are only paying for 500 kbps and you should be configuring your router to work at that  rate (500 Kbps) unless you have some sort of bursting-agreement (if you require to use more bandwidth then you can) setup with your service provider.

Local Management Interface (LMI) is the language that you use between your router and the service provider. It is a signaling protocol that service provider can use to send you statistics on the line. It is also used to send DLCI information.

Data Link Connection Identifier (DLCI): Ethernet uses Mac address. In frame relay you use DLCI. Every single site is identified by a DLCI that allow you to communicate.

For addressing the frame relay is using is DLCI. If you have two devices in Ethernet you have a source MAC address and destination MAC address. DLCI is locally significance; I might have Mizoram DLCI 200, Texas DLCI 300, California 400, Arizona 100, 500, 900. DLCI range 16 – 1024.

When Mizoram send its data to Arizona it is going to send it to the DLCI 200 (that's the destination), and then ISP will take that data through the could and when it comes out in the Arizona, it comes out on DLCI 100.

As DLCI is locally significance. California, Texas and Arizona can have same DLCI number. But you can't have the same DLCI number in one place like, In Arizona down below, you can't have same DLCI 300 at both interface.

Permanent Virtual Circuit (PVC): PVC dictates where service provider can take you to. (Dotted line is PVC)