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Communication Networks WJEC

Communication Networks and Switching

There are many choices of route for a particular message through a Wide Area Network. Message switching prevents all data being sent to all parts of the network, and makes efficient use of the data lines. Two methods of switching will be discussed here: circuit and packet switching.

Circuit Switching

Here a path is set up from sender to receiver immediately before the start of transmission and kept open until the transmission is completed, and all the data travels along this path in order. The path cannot be used by any other traffic during that time. The data is sent in order, i.e. synchronously. After the transmission is complete, all parts of the path are released and can be used for other transmissions.

Because switches are used to connect and disconnect the circuits, electrical interference is produced, and although this is not a serious problem for speech, it may produce corrupt or lost data if the path is being used to transmit data. If this is likely to be a serious problem, a leased line may be used instead.

Packet Switching

Messages are divided into packets – fixed length blocks of data – which are sent individually through the network. Each packet can take a completely different route from sender to receiver. Each node in the network directs the packet to the next, most suitable node, When all the packets have arrived at the destination, the data is re-assembled. From time to time, some channels may become busy, but the nodes are intelligent enough to direct the packets around these busy routes. This means that the packets can sometimes arrive out of sequence, and may need to be sorted and then re-assembled by the receiver.

A packet contains the following information: • The sender and destination address.

• A packet sequence number (used to re-assemble the message).

• The data.

• Error control signals.

• Control signals.

The internet is an example of packet switching.

Advantage of Circuit Switching over Packet Switching

Message always arrives in the correct sequence.

Advantages of Packet Switching over Circuit Switching

More efficient use of lines (channels) as there is no waiting between gaps.

Less likely to be affected by network failure, as multiple routes used.

Better security; data is less likely to be intercepted as packets take separate routes.

Less prone to electrical interference.

Protocols

A protocol is a standard set of rules for transmitting data across a network. In order for two computers on a network to communicate successfully, they must share a common set of rules about how to communicate. At a minimum, such rules must include how to initiate and end networked communications, and how to manage information exchange across the network medium. The most common protocol is TCP/IP, which is used for internet users.

Collision Detection

This is a method of managing data traffic on a network. With the token ring networks, nodes have to wait for the token before they can transmit, and therefore collisions do not occur. However, on a bus network, individual computers are responsible for waiting for the network to become free before sending a message. It is likely that several computers are waiting to transmit messages and that they will do so at the same time. If two messages are sent at the same time, then a collision occurs, and message corruption will result. The network interfaces detect this corruption, and the two messages must then be re-transmitted when the network is free. In some networks, each node would be made to wait for a different amount (random) of time before re-transmitting. This staggers the load on the network.

Routers

A router is a communications device that receives data and forwards it to the correct location via an appropriate route. It is able to do this as it holds information about the addresses of computers or devices in the network, or other networks. A router can also acts as a gateway to a large network.

Internet

The internet is probably the most well-known network system. Every computer that is connected to the internet is part of this network. Routers act as gateways to the internet, and all of the machines on the internet are either servers or clients. The machines that provide services to other machines are servers, and the machines that are used to connect to those services are clients. Servers can be categorised into the following: • Web servers.

• File transfer protocol (FTP) servers.

• E-mail servers.

• Newsgroup servers.

When you connect to a website to read a page, you are accessing that site’s web server, using a web browser, e.g. Internet Explorer. The server machine finds the page you requested and sends it to you. The web server can also filter the content and restrict access to certain users.

All web pages are written using a special language called HTML (HyperText Markup Language). Pages can include text, graphics, sounds, video clips. There are two main sections on a web page: • the head - which contains information about the page. • the body - which contains the information to be displayed.

You can use tags to set - • the colours of background and text .

• the font and size of text.

• alignment of text.

• lists and bullets.

• tables - used for arranging blocks of text and graphics on a page.

• details of graphics to be displayed.

• links to other pages.

There are many programs which allow users to create web pages without knowing about the tags. Eg Microsoft Frontpage.

FTP(File Transfer Protocol)

Links on a web page may be to files, which can be downloaded. These use FTP (File Transfer Protocol). When you create your own web page you will need an FTP program to upload your pages to a web site.

computer science


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