Chapter14.Frame Relay Technology Background


Chapter 14. Frame Relay Technology Background

Frame Relay technology includes a combination of hardware, software, standards, and architectures to provide a variety of services. They include data, Voice over Frame Relay (VoFR), Frame Relay multicast, Internet Protocol (IP) multicast over Frame Relay, Frame Relay compression, and others. It is beyond the scope of this book to include all available features of Frame Relay, so the content of Part IV, "Frame Relay," only includes the technology background, common design and configuration solutions for enterprise remote access, and proven troubleshooting techniques and scenarios.

This chapter also provides a more detailed background of Frame Relay technology, including the following:

  • Frame Relay standards

  • Frame Relay service architecture

  • Frame Relay protocols, including Link Access Procedures for Frame Mode Bearer Services (LAPF) core and control protocol

One of the fundamental reasons for creating Frame Relay technology was the need for higher speeds. The other driving factor was the industry change from mainly text exchange, to graphical exchanges, which requires peak bandwidth or dynamic bandwidth and lower response time. Widespread digital facilities demand more reliability and less overhead, and the ability to handle bursty traffic. This combination of factors drove the development of new technology at the end of the 1980s.

Frame Relay combines the statistical multiplexing and port sharing of X.25 with the high-speed and low-delay characteristics of time-division multiplexing (TDM) circuit switching. Conceived as a derivative from X.25, Frame Relay eliminates the Layer 3 protocols of X.25 and concentrates the addressing and multiplexing in Layer 2. The architecture model is more compliant with Open System Interconnection (OSI), where the second layer deals with frames and not with packets. Only a few Layer 2 functions are used in permanent virtual circuit (PVC) solutions, and they are known as the core aspects such as checking for valid error-free frames but not requesting retransmission if an error is found. Therefore, many high-level protocol functions such as sequencing, windowing, acknowledgments, and supervisory frames are not duplicated within Frame Relay.

What Is Statistical Multiplexing?

The term multiplexing refers to sharing a single resource among many users. Analog communications typically use frequency-division multiplexing (FDM) to carry multiple conversations. A good example is the 6-MHz TV bandwidth, or simultaneous voice and video. FDM divides the available bandwidth between a narrow voice channel (about 4 kHz) and a wider band for video, and allocates a 900 Hz guard band between them. Digital signaling is mostly based on TDM. TDM allocates a small slot of time for every member sharing the channel. On a round-robin basis, every member gets a dedicated slot.

For more bursty applications (bursty means that the peak data rate is much greater than the average rate), statistical multiplexing is more commonly used because individual applications are supplied with bandwidth on an as needed basis. Thus, the channel is idle if all applications have nothing to send, but if at least one application has data to send, the channel is active. This provides optimal use of available bandwidth, but unlike TDM, cannot guarantee bandwidth for individual applications.


The lack of certain protocol functions in Frame Relay dramatically increases throughput because each frame requires much less processing time. Table 14-1 summarizes the characteristics of these protocols, comparing X.25 switching and Frame Relay1.

Table 14-1. X.25 and Frame Relay Comparison[1]

Feature

X.25 Packet Switching

Frame Relay

Time-slot multiplexing

No

No

Statistical (virtual circuit) multiplexing

Yes

Yes

Port sharing

Yes

Yes

Packet sequencing

Yes

No

Error checking

Yes

No

Perform flow control

Yes

No, drops frames; on congestion

Network access

300 bps-64 kbps

56-2048 kbps; the technology provides speeds up to 45 Mbps; recent advancements include speeds up to 155 Mbps

Switch delay

10-40 msec

2-6 msec

One-way delay

200-500 msec

40-150 msec


Frame Relay uses a variable length framing structure, which ranges from a few to thousands of bits. This feature affects delay-sensitive user data because the delay is a function of the packet size. Although this is a feature in Frame Relay compression, it is a disadvantage when carrying voice traffic. However, Frame Relay has been adapted to handle the voice traffic, as defined in FRF11.1 (see Table 14-3).

The two factors that make Frame Relay a desirable choice for data transmission are the following:

  • Frame Relay virtual circuits consume bandwidth only when they transport data, thus many virtual circuits can exist simultaneously across a given transmission line. Also, each device can use more of the bandwidth as needed, and thus operate at higher speeds.

  • The improved reliability of communication lines and increased error-handling sophistication at end stations allows the Frame Relay protocol to discard erroneous frames, and thus eliminates time-consuming error-handling processes.




Troubleshooting Remote Access Networks CCIE Professional Development
Troubleshooting Remote Access Networks (CCIE Professional Development)
ISBN: 1587050765
EAN: 2147483647
Year: 2002
Pages: 235

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