Frame Relay

Frame Relay is a high-performance WAN protocol that operates at the physical and data link layers of the OSI reference model.
Eric Scace, an engineer at Sprint International, invented Frame Relay as a simpler version of the X.25 protocol to use across Integrated Services Digital Network (ISDN) interfaces. Today, it is used over a variety of other network interfaces as well. When Sprint first implemented Frame Relay in its public network, they used StrataCom switches. Cisco's acquisition of StrataCom in 1996 marked their entry into the carrier market.
Network providers commonly implement Frame Relay for voice and data as an encapsulation technique, used between LANs over a WAN. Each end user gets a private line (or leased line) to a Frame Relay node. The Frame Relay network handles the transmission over a frequently changing path transparent to all end users.
Frame Relay has become one of the most extensively used WAN protocols, primarily because it is inexpensive compared to dedicated lines. In addition, configuring user equipment in a Frame Relay network is very simple. Frame Relay connections are created by configuring CPE routers or other devices to communicate with a service provider Frame Relay switch. The service provider configures the Frame Relay switch, which helps keep end-user configuration tasks to a minimum.
This chapter describes Frame Relay and explains how to configure Frame Relay on a Cisco router.
Frame Relay: An Efficient and Flexible WAN Technology
Frame Relay has become the most widely used WAN technology in the world. Large enterprises, governments, ISPs, and small businesses use Frame Relay, primarily because of its price and flexibility. As organizations grow and depend more and more on reliable data transport, traditional leased-line solutions are prohibitively expensive. The pace of technological change, and mergers and acquisitions in the networking industry, demand and require more flexibility.
Frame Relay reduces network costs by using less equipment, less complexity, and an easier implementation. Moreover, Frame Relay provides greater bandwidth, reliability, and resiliency than private or leased lines. With increasing globalization and the growth of one-to-many branch office topologies, Frame Relay offers simpler network architecture and lower cost of ownership.
Using an example of a large enterprise network helps illustrate the benefits of using a Frame Relay WAN. In the example shown in the figure, Span Engineering has five campuses across North America. Like most organizations, Span's bandwidth requirements do not fit "a one size fits all" solution.
The first thing to consider is the bandwidth requirement of each site. Working out from the headquarters, the Chicago to New York connection requires a maximum speed of 256 kb/s. Three other sites need a maximum speed of 48 kb/s connecting to the headquarters, while the connection between the New York and Dallas branch offices requires only 12 kb/s.
Before Frame Relay became available, Span leased dedicated lines.
Using leased lines, each Span site is connected through a switch at the local telephone company's central office (CO) through the local loop, and then across the entire network. The Chicago and New York sites each use a dedicated T1 line (equivalent to 24 DS0 channels) to connect to the switch, while other sites use ISDN connections (56 kb/s). Because the Dallas site connects with both New York and Chicago, it has two locally leased lines. The network providers have provided Span with one DS0 between the respective COs, except for the larger pipe connecting Chicago to New York, which has four DS0s. DS0s are priced differently from region to region, and usually are offered at a fixed price. These lines are truly dedicated in that the network provider reserves that line for Span's own use. There is no sharing, and Span is paying for the end-to-end circuit regardless of how much bandwidth it uses.
A dedicated line provides little practical opportunity for a one-to-many connection without getting more lines from the network provider. In the example, almost all communication must flow through the corporate headquarters, simply to reduce the cost of additional lines.
If you examine what each site requires in terms of bandwidth, you notice a lack of efficiency:
Of the 24 DSO channels available in the T1 connection, the Chicago site only uses seven. Some carriers offer fractional T1 connections in increments of 64 kb/s, but this requires a specialized multiplexer at the customer end to channelize the signals. In this case, Span has opted for the full T1 service.
Similarly, the New York site only uses five of its available 24 DSOs.
Because Dallas needs to connect to Chicago and New York, there are two lines connecting through the CO to each site.
The leased-line design also limits flexibility. Unless circuits are already installed, connecting new sites typically requires new circuit installations and takes considerable time to implement. From a network reliability point of view, imagine the additional costs in money and complexity of adding spare and redundant circuits.
Span's Frame Relay network uses permanent virtual circuits (PVCs). A PVC is the logical path along an originating Frame Relay link, through the network, and along a terminating Frame Relay link to its ultimate destination. Compare this to the physical path used by a dedicated connection. In a network with Frame Relay access, a PVC uniquely defines the path between two endpoints. The concept of virtual circuits is discussed in more detail later in this section.
Span's Frame Relay solution provides both cost effectiveness and flexibility.
Cost Effectiveness of Frame Relay
Frame Relay is a more cost-effective option for two reasons. First, with dedicated lines, customers pay for an end-to-end connection. That includes the local loop and the network link. With Frame Relay, customers only pay for the local loop, and for the bandwidth they purchase from the network provider. Distance between nodes is not important. While in a dedicated-line model, customers use dedicated lines provided in increments of 64 kb/s, Frame Relay customers can define their virtual circuit needs in far greater granularity, often in increments as small as 4 kb/s.
The second reason for Frame Relay's cost effectiveness is that it shares bandwidth across a larger base of customers. Typically, a network provider can service 40 or more 56 kb/s customers over one T1 circuit. Using dedicated lines would require more DSU/CSUs (one for each line) and more complicated routing and switching. Network providers save because there is less equipment to purchase and maintain.
The Flexibility of Frame Relay
A virtual circuit provides considerable flexibility in network design. Looking at the figure, you can see that Span's offices all connect to the Frame Relay cloud over their respective local loops. What happens in the cloud is really of no concern at this time. All that matters is that when any Span office wants to communicate with any other Span office, all it needs to do is connect to a virtual circuit leading to the other office. In Frame Relay, the end of each connection has a number to identify it called a Data Link Connection Identifier (DLCI). Any station can connect with any other simply by stating the address of that station and DLCI number of the line it needs to use. In a later section, you will learn that when Frame Relay is configured, all the data from all the configured DLCIs flows through the same port of the router. Try to picture the same flexibility using dedicated lines. Not only is it complicated, but it also requires considerably more equipment.
The table shows a representative cost comparison for comparable ISDN and Frame Relay connections. While initial costs for Frame Relay are higher than for ISDN, the monthly cost is considerably lower. Frame Relay is easier to manage and configure than ISDN. In addition, customers can increase their bandwidth as their needs grow in the future. Frame Relay customers pay only for the bandwidth they need. With Frame Relay, there are no hourly charges, while ISDN calls are metered and can result in unexpectedly high monthly charges from the telephone company if a full-time connection is maintained.
The next few topics will expand your understanding of Frame Relay by defining the key concepts introduced in the example.


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