Unit 5

WAN - collection of computers spread over a wide geographical area. Communication may be via microwave, satellite link or telephone line (typically transitory)

Communication links

Types of coaxial cable

Synchronous data transmission e.g. LAN

Multiplexor - combines more than one input signal into a stream of data that can be transmitted over a communication channel

Demultiplexor - separates the single stream of data into separate components at the receiving end

Time-division multiplexing - transmission time split up into tiny time-slices

Circuit switching e.g. public telephone system

Packet switching e.g. Internet

Virtual circuit - temporary 'dedicated' pathway between two points on a PSS -> bandwidth allocated for a specific transmission pathway

Advantages of packet switching:-

Asynchronous Transfer Mode (ATM)

Standard (network) protocols - in order for two computers on a network to communicate successfully, they must share a common set of rules about how to communicate

Protocol stack - provides software that enables computers to communicate across a network

e.g. Transmission Control Protocol/Internet Protocol (TCP/IP)

Open Systems Interconnection (OSI) Model

Local Area Network

Network topology

Bus network

Ring network

Token passing

Star topology

Ethernet - network architecture with several different implementations using different types of cable

Segmentation - networks can be segmented to reduce traffic on each segment by inserting a bridge or router between each network segment -> better network performance

Server-based network e.g. Novell's NetWare

Peer-to-peer network e.g. Microsoft Windows 98

Wide Area Network

Dial-up networking

Dedicated/leased line

Value-added network (VAN)

Electronic data interchange (EDI)


ISDN line - requires a network adapter and a network termination device (no modem required)

Cable modems - employ broadband transmission across regular cable television wires

Asymmetric Digital Subscriber Line (ASDL) - offers Internet connection up to 30 times faster than dial-up modems still using traditional copper wires but allocating more bandwidth to the data flow from the ISP to the PC than is allocated from the PC to the ISP





The Internet

Internet backbone - major Internet communications links maintained by network service providers (NSPs)

World Wide Web

Internet Service Provider (ISP)

On-line Service Provider - provides users with access to the Internet and its own private network of services

Usenet newsgroups - different newsgroup devoted to specific areas or interest


Telnet - software program that allows users to connect to a remote computer via the Internet and use the programs on that computer

Disadvantages of email:-

Web browser - software used to view and download Web pages

Facilities include:-

Search engine - enables users to search the contents of millions of Web pages simultaneously

Java applets - small Java programs with the extension .class, which can be executed though Web browsers

Java script - allows fast creation of Web page events

- creates interaction between the user and the Web page

HTML (Hypertext Markup Language) - programming language behind every Web page, controlling how the text and graphics are displayed

FTP (File Transfer Protocol) - used to download files from the Web or upload pages to a Web site

On-line banking e.g. Smile (on-line bank with no branch network to support customers)

Online shopping

e.g. Tesco for weekly shopping

Internet registry - to register a domain name (e.g. Nominet)

Cybersquatting - speculating domain names in order to try to earn a profit by selling the names they have registered

Digital certificate

Digital signature


Decryption - unscrambling ciphertext back to the original plaintext

Strong encryption - implies it would effectively be impossible to find the key within the effective lifetime of the secret

Weak encryption - implies that the key could be found with a realistic amount of processing capacity and a reasonable amount of time

Factoring - attempting to find the two prime factors of a product (used in encryption)

Firewall - mechanism for protecting a corporate network from external communications systems e.g. Internet

Viruses e.g. Melissa (1999 macro virus) - mailed itself as attachment to first 50 addresses in Outlook Express Address Book

Blocking software e.g. Net Nanny and Surf Watch

- software censors unsuitable material to combat the dangers to children