# 电子工程代写|计算机网络概论代写Introduction to Computer Networking代考|CS6250

## 电子工程代写|计算机网络概论代写Introduction to Computer Networking代考|Frame Relay

In the previous section, we learnt that $\mathrm{X} .25$ was designed for analogue networks. With the advent of digital networks $\mathrm{X} .25$ was found to be too slow, so a more modern approach to packet-switching networks, Frame Relay, had to be devised. Since digital networks were more reliable than analogue networks, there was no need for Frame Relay to include link-by-link error checking. Any error checks could be done by a higher layer protocol at the end stations. Frame Relay functions only at OSI layers 1 and 2 , while X. 25 provides layer 3 services as well.

As the name would suggest, data is broken up into frames before being sent out over the Frame-Relay network. Just as in the case of X.25, the data is sent over virtual circuits (either SVCs or PVCs). PVCs are commonly used to construct virtual private networks (VPNs), which provide the equivalent of a private network but run over a public network. Before Frame Relay, if an organisation needed a WAN, private lines or circuit switching over a leased line were necessary. Single, dedicated lines are not needed for WAN-to-WAN connections with Frame Relay. This reduces costs.

Frame Relay is so much faster than X.25 that it can be used to connect LANs together. Frame Relay is ideally suited to carrying a high volume of traffic, which occurs unpredictably in bursts. Such traffic is typical of LANs. Even voice traffic can be sent using Frame Relay.

An important feature of Frame Relay is that frame-relay service providers are able to offer their customers tightly defined service agreements. If the framerelay service fails to perform to the level that has been agreed, then the user can be compensated. There is a committed information rate (CIR) at which data is transmitted. But if the traffic and the service agreement allow this, data can burst above the committed rate for short periods.

The Frame-Relay data frame, Link Access Procedure for Frame Mode Services (LAPF), is based on HDLC (HDLC is described in Section 6.3). The frame includes a field for the data-link connection identifier (DLCI). The DLCI, a number, represents the virtual circuit to which the frame belongs. In other words, it is the destination address of the frame.

## 电子工程代写|计算机网络概论代写Introduction to Computer Networking代考|Leased Lines

A leased line is used when an organisation needs a permanent, dedicated, pointto-point link. (A point-to-point link is a link from one place to another place.) The leased line provides a communications path that has been set up in advance from one site to another using the facilities of a telecommunications carrier company.

This system allows tight budget control because the cost does not depend on the amount of usage. For a monthly fee, the organisation is free to send as much data as it likes down the line, up to its maximum capacity. Usually, the cost of the rental depends on the data rate and the distance between the two sites that are being connected. Leased lines are usually more expensive than Frame Relay, particularly when many sites are connected. Two advantages of leased lines are that there is negligible latency and jitter and that the line is always available. Such features may be essential for certain kinds of application. The disadvantages include the cost compared with most of the alternative ways of setting up a WAN. Leased lines are not very flexible either. The data rate offered by a leased line seldom corresponds to the exact traffic requirements. (We have already seen in the previous section how it is often necessary to top up the carrying capacity of a leased line using ISDN.) If the capacity of the leased line needs to be changed, this usually entails an employee of the carrier visiting the site.

## 电子工程代写|计算机网络概论代写Introduction to Computer Networking代考|Leased Lines

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