Frame Relay

• Frame Relay is packet-switching WAN technology that exists at the Data Link layer of the OSI model
• Typically more cost-effective than leased lines
• Communication occurs over virtual circuits
• Permanent virtual circuits (PVCs) are always available
• Switched virtual circuits (SVCs) are created and then terminated as required

Frame relay is a packet switching technology that exists at the data link layer of the OSI model and is one that has become increasingly common as a WAN solution since the early 1990s. Unlike with leased lines and circuit – switched networks, the available bandwidth on a providers frame relay network is shared amongst many subscribers. This sharing of resources leads to significantly lower costs tan traditional leased lines.

Many people tend to be confused by packet switching technologies like frame relay. Mostly this is a result of trying to under stand how data actually gets from one location to another. On packet switched networks (like frame relay), data streams are separated through the use of “virtual” rather than dedicated hardware circuits. In other words, a logical path is defined between endpoints, through a provider’s packet switched network. Many virtual circuits will be defined for different customers, and will be multiplexed overt the shared physical links of the network. An example, consider the figure below. It shows two different companies, each connecting two offices over a providers frame relay network. Notice that between frame relay switches X and Y, both of their virtual circuits traverse a common physical link. The data that one company passes between their own offices is completely separate from the data of the other company all data stay witching each company’s dedicated virtual circuit only.

Two main types of virtual circuits can be defined on a frame relay networks permanent virtual circuits (PVCs) and switched virtual circuits (SVCs) a PVC functions somewhat similar to a leased line, in that a service provider defines a path through the packet switched network to each customer location, in cases where companies wish to have “always-on” connectivity between locations using frame relay, PVCs are usually defined.

A switched virtual circuit (SVC) functions somewhat differently, almost like a circuit switched connection. SVCs are not permanent, and can instead be created as required across a packet –switched network. For example, an SVC could be created between a company’s head office and a remote location. For the duration of the connection, data might travel over a completely different path.  

Frame relay networks are referred to as being non-broadcast multi access (NBMA). What this means is that, by default, broadcast traffic will normally not be passed over a virtual circuit without explicit configuration. This is an important consideration when dealing with the use of broadcast-based routing protocols like RIP or IGRP in a frame ready environment. You will look at how broadcast traffic can be handled on frame relay networks later in this section.

Frame relay equipment

• connections to a frame relay network require both DTE and DCE equipment
• the DTE equipment is a Cisco equipment
• the DCE equipment is a CSU/DSU

In order to connect to a frame relay network, both DTE and DCE equipment needs to be located at the customer premises. This DTE equipment is usually a router whose serial interface connects to a DCE device. In the past, customers required a completely separate DTE device known as a frame relay access device (FRAD) to connect to a frame relay network. However, almost all routers sold today (with an appropriate serial interface) are capable of handling frame relay encapsulation and communication. The DCE device is usually a CSU/DSU that provides clocking functions and the connection to the provider’s physical circuit. Ultimately m the physical ink from the customer, premises connects to the frame relay switching equipment of the service provider. This switching equipment is not the responsibility of the customer. The figure blow illustrates the interconnections of equipment on a frame relay network.