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28.8 REPRESENTATIVE FRAME RELAY NETWORKS
Frame Relay networks can be used not only for data services, but for voice and video as well. Because committed information rate (CIR) can be specified by the subscriber, a desired quality of service (QoS) can be maintained.
Figure 28.6 shows a Frame Relay network that supports both voice and data services. The FRAD can interface with the PBX through an E1 trunk (2.048Mbps), and the LAN can be interfaced through a router. The FRAD will do the necessary framing for both data and voice. However, for voice applications, instead of coding the voice signals at 64kbps, low bit rate codecs are used that are an integral part of the FRAD. Many commercial systems use G.729-based voice coding technique to reduce the voice data rate to 8kbps.
Figure 28.6: Voice/data services over Frame Relay.
Figure 28.7 shows how Frame Relay and ISDN networks can be used by an organization. FRAD with ISDN interfaces are available to facilitate connecting the ISDN BRI or PRI equipment to the FRAD. Organizations can use the ISDN network for their regular voice and data traffic; whenever bulk data transfers are involved or when video conferencing has to be set up, they can use the Frame Relay network with switched virtual connections.
Figure 28.7: Integration of Frame Relay and ISDN.
A Frame Relay network supports a variety of access speeds from 64kbps to 34.368Mbps (E3). Frame Relay access devices are available with 64kbps interfaces, E1 interface, ISDN basic rate interface, ISDN primary rate interface, and others.
Frame Relay networks have to maintain a committed information rate and are well suited for multimedia communication. Many corporate networks are based on Frame Relay.
Frame Relay networks are gaining popularity in recent years. However, there is stiff competition from Asynchronous Transfer Mode (ATM) networks. We will study ATM in the next chapter.
Summary
This chapter presented the details of Frame Relay networks. Frame Relay offers very fast packet switching by eliminating the protocol overhead, as compared to X.25. Frame Relay provides virtual connection service. Two types of virtual connections can be set up. Permanent virtual connection (PVC) is set up between end points to provide permanent connectivity, such as leased line. Switched virtual connection (SVC) is established on a call-by-call basis. In Frame Relay, the packet is called a frame and can contain up to 4096 Bytes of user information. The datalink layer protocol is called link access protocol for Frame Relay (LAPF), derived from LAPB. Two bytes of the frame Relay header contain information about the virtual connection identification called data link connection identifier (DLCI). There are two fields to notify about congestion in the switch and one bit to indicate whether the packet can be discarded. If the subscriber exceeds the committed information rate, then the frame can be discarded. Frame Relay networks are gaining popularity due to fast switching and support for multimedia services.
References
R. Horak. Communications Systems and Networks. Wiley-Dreamtech India Pvt. Ltd., 2002. Chapter 10 covers broadband networks including Frame Relay.
http://www.frforum.com Web site of the Frame Relay Forum.
http://www.ansi.org Web site of the American National Standards Institute.
http://www.cisco.com Cisco is a leading supplier of Frame Relay equipment. You can obtain product information from this site.
Questions
What are the advantages of Frame Relay as compared to X.25? Compare the protocol stacks of X.25 and Frame Relay networks.
Explain how fast switching is achieved in Frame Relay.
Describe the LAPF frame format.
Explain the procedure for congestion control in Frame Relay.
What is committed information rate? What does the Frame Relay switch do if this rate is exceeded?
Exercises
| Study the features of Frame Relay commercial products and prepare a technical report. | |
| Study the various interfaces provided by a commercial FRAD (for instance, that of Cisco). Study the voice compression techniques used for voice over Frame Relay. | |
| Compare the number of packets/frames exchanged in an X.25 network and a Frame Relay network. | |
| As compared to X.25, Frame Relay protocols are a better choice when optical fiber is the transmission medium. Justify this statement. | |
| Is it necessary to use low bit rate coding of voice/video in Frame Relay networks? If so, why? |
Answers
| Information about Frame Relay commercial products can be obtained from http://www.lucent.com and http://www.cisco.com. |
| You can study the FRAD interfaces provided by Cisco equipment at the Web site http://www.cisco.com. |
| In X.25, packets are exchanged for flow control and error control. In Frame Relay, there is no layer 3 protocol. |
| When optical fiber is the transmission medium, the error rate is very low. Hence, the Frame Relay protocols enable fast transmission of data without the need for flow control and error control. |
| Because low bit rate coding also gives good quality voice, the bandwidth can be better utilized. Using low bit rate coding allows fast packet transmission, so the delay will be less. |
Projects
Develop a software package that simulates the LAPF frame format.
Carry out a paper design to develop a Frame Relay network for an organization that has five offices spread over the country. Each office has a LAN, a PBX with E1 interface for trunking, and basic rate interface (BRI) ISDN equipment. Identify the commercial products that have these interfaces.
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