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 provider’s Frame Relay network is shared amongst many subscribers. This sharing of resources leads to significantly lower costs than traditional leased lines.
Many people tend to be confused by packet-switching technologies like Frame Relay. Mostly this is a result of trying to understand 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 over the shared physical links of the network. As an example, consider the figure below. It shows two different companies, each connecting two offices over a provider’s 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 stays within 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 would travel across the path defined by the SVC. However, if the circuit was terminated and a new SVC established at a later time, 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 Relay environment. You’ll look at how broadcast traffic can be handled on Frame Relay networks later in this section.
