Text C. Networks

Defining a network. A network is the interconnection of a set of devices capable of communication. In this definition, a device can be a host (or an end system as it is sometimes called) such as a large computer, desktop, laptop, workstation, cellular phone, or security system. A device in this definition can also be a connecting device such as a router, which connects the network to other networks, a switch, which connects devices together, a modem (modulator-demodulator), which changes the form of data, and so on. These devices in a network are connected using wired or wireless transmission media such as cable or air.

Networks may be classified by various characteristics, such as the media used to transmit signals, the communications protocols used to organize network traffic, network scale, network topology and organizational scope. The best-known computer network is the Internet.

Communication protocols define the rules and data formats for exchanging information in a computer network. Well-known communications protocols include Ethernet, a hardware and link layer standard that is widely used for local area networks, and the Internet protocol suite (TCP/IP), which defines a set of protocols for communication between multiple networks, for host-to-host data transfer, and for application-specific data transmission formats. Protocols provide the basis for network programming.

Network Criteria. A network must be able to meet a certain number of criteria. The most important of these are performance, reliability, and security.

Performance. Performance can be measured in many ways, including transit time and response time. Transit time is the amount of time required for a message to travel from one device to another. Response time is the elapsed time between an inquiry and a response. The performance of a network depends on a number of factors, including the number of users, the type of transmission medium, the capabilities of the connected hardware, and the efficiency of the software.

Performance is often evaluated by two networking metrics: throughput and delay. We often need more throughput and less delay. However, these two criteria are often contradictory. If we try to send more data to the network, we may increase throughput but we increase the delay because of traffic congestion in the network.

Reliability. In addition to accuracy of delivery, network reliability is measured by the frequency of failure, the time it takes a link to recover from a failure, and the network's robustness in a catastrophe.

Security. Network security issues include protecting data from unauthorized access, protectingdata from damage and development, and implementing policies and procedures for recovery from breaches and data losses.

3.7 Answer the questions.

1. What devices are connected in the network?

2. By what characteristics can networks be classified?

3. Why are protocols needed?

4. What are the most important network criteria?

5. Why are throughput and delay called contradictory criteria?

6. What do you understand by the terms “security” and “reliability”?

3.8 Match the terms in column A and their definitions (descriptions) in column B:

A B

1. Router a) software used to find and look at information on

the web. Each web page has its own unique address

known as a Universal Resource Locator (URL).

2. Browser b) allows two different types of network to

communicate with each other.

3. Bridge c) hardware used to change analogue signals from the

phone line into digital signals needed by the

computer, also used to change the signals from

digital to analogue.

4. Modem d) define the rules and data formats

specifying the individual address.

5. Hub e) sends the signal to all the devices on the network

and only one individual PC will accept the signal.

6. Switch f) Hardware allowing computer to access the Internet

using a phone line.

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