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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|CS6250

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

在上一节中,我们了解到X.25专为模拟网络而设计。随着数字网络的出现X.25被发现太慢了,因此必须设计一种更现代的分组交换网络方法,即帧中继。由于数字网络比模拟网络更可靠,因此帧中继不需要包括逐个链路的错误检查。任何错误检查都可以由终端站的更高层协议完成。帧中继仅在 OSI 第 1 层和第 2 层起作用,而 X.25 也提供第 3 层服务。

顾名思义,数据在通过帧中继网络发送之前被分解为帧。与 X.25 的情况一样,数据通过虚拟电路(SVC 或 PVC)发送。PVC 通常用于构建虚拟专用网络 (VPN),它提供与专用网络相同的功能,但在公共网络上运行。在帧中继之前,如果组织需要 WAN,则需要专用线路或租用线路上的电路交换。使用帧中继的 WAN 到 WAN 连接不需要单条专用线路。这降低了成本。

帧中继比 X.25 快得多,它可以用来将 LAN 连接在一起。帧中继非常适合承载大量流量,这些流量以突发的方式发生不可预测。这种流量是典型的局域网。甚至语音流量也可以使用帧中继发送。

帧中继的一个重要特性是帧中继服务提供商能够为其客户提供严格定义的服务协议。如果帧中继服务未能达到约定的水平,则可以对用户进行补偿。存在传输数据的承诺信息速率 (CIR)。但是,如果流量和服务协议允许这样做,数据可能会在短时间内突然超过承诺的速率。

帧中继数据帧,帧模式服务的链路访问过程 (LAPF),基于 HDLC(HDLC 在第 6.3 节中描述)。该帧包括数据链路连接标识符 (DLCI) 字段。DLCI,一个数字,代表该帧所属的虚电路。换句话说,它是帧的目的地址。

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

当组织需要永久的、专用的点对点链路时,使用租用线路。(点对点链路是从一个地方到另一个地方的链路。)租用线路提供了一条使用电信运营商的设施预先建立的从一个站点到另一个站点的通信路径。

该系统允许严格的预算控制,因为成本不取决于使用量。按月收费,该组织可以免费发送尽可能多的数据,直至其最大容量。通常,租赁成本取决于数据速率和正在连接的两个站点之间的距离。租用线路通常比帧中继更昂贵,特别是在连接许多站点时。租用线路的两个优点是延迟和抖动可以忽略不计,并且线路始终可用。这些特征对于某些类型的应用可能是必不可少的。与大多数设置 WAN 的替代方法相比,缺点包括成本。租用线路也不是很灵活。租用线路提供的数据速率很少符合确切的流量要求。

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

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