Frame Relay in Brief

Frame-Relay is typically less expensive than point to point circuits because you only pay for the connectivity between your site and the carrier's point of presence (POP). This connection between your site and the carrier's POP is known as the local loop. However, in point to point circuits, you pay for local loops on each end and the mileage between the carrier's POP's. Therefore frame-relay is less expensive over longer distances.

There are several components to a Frame-Relay network. First you must determine the port speed you need at each location. Port speeds start out at 56K or 64K and go up in 64K increments to T1 (1.544 Mbps) or T3 (45 Mbps) speeds. You must then determine how to route your PVC's (Permanent Virtual Circuits) and the CIR (Committed Information Rate) for each PVC. The CIR is the amount of guaranteed throoughput you need. Some carriers will allow you to specify a 0 Kbps CIR, where others require you to specify at least a 2, 4 or 8 Kbps CIR. The CIR can be specified from 0 Kbps all the way up to the port speed. If you have applications such as voice or critical data that need guaranteed bandwidth you will want to specify a high enough CIR to allow these applications the throughput they need to operate properly.

Frame-Relay Network Terminology :
Port Speed - This is the speed of the frame-relay port that you connect to at the carrier's POP. This is also the spped that you transmit and receive data between your router and the Frame-Relay network.

PVC - PVC stands for Permanent Virtual Circuit. This is a predefined path through the carrier's frame-relay network from one location to another. A majority of frame-relay networks are designed with one cetral site with a single PVC going to each remote site as shown in the diagram below. Other frame-relay networks have PVC's in a mesh configuration to connect all of the sites. Each PVC has a commited information rate (CIR) that guarantees the data throughput on the PVC.

Local Loop - The connection between your site and the carriers POP (Point Of Presence)

CIR - CIR stands for Committed Information Rate. This is the data rate or throughput that is guaranteed by the carrier on each PVC. The CIR on each PVC should be set based on your network throughput requirements. The CIR is ussually some fraction of the port speed but can be as high as the port speed itself.
Note : Some carriers may not allow a 0kbps CIR. The sum of the CIR's for each PVC on a port should not exceed the port speed.

Bursting - Bursting is when you send data over and above the CIR. This data is data that is flagged by the network as discard eligible and will have to be resent if discarded by the network. Your frame-relay carrier can discard these packets if their network is busy or congested. you can only burst or transmit data at speeds up to the port speed. Most frame-relay carriers will provide a guarantee on the percentage of burst traffic that will get through their network. This guarantee can sometimes be 98% or higher.

Carrier POP - The carrier OPO or Point of Presence is where you connect into the communication carriers network. Each carrier usually has one or two POP's in each major city. Many carrier's have what are called remote POP's or virtual POP's in smaller cities that clients can connect into. This connection between your site and the carrier's POP is called the local loop.

Over Subscription - This is when sum of the speed of the PVC's or the CIR's coming into a site exceed the port speed for that site. For example, you have four site A,B,C,D. Site A is the main site, it has a port speed of 256Kbps. You also have sites B,C, adn D with all with port speed of 128 Kbps with PVC's back to site A. You have the potential to send 384kbps to a site that can only accept 256 Kbps of data, this is known as over subscription. Site A will only be able to receive 256 Kbps of data, the rest of the packets will be discarded adn have to be resent. It is common to design Frame-Relay networks with some over-subscription of bandwidth.
Note : Some carriers may not allow over subscription based on the sum of PVC or CIR speeds on a particular port.