2.9 Network Environments

1. 2.9 Network Environments In this section you must be able to: • Describe the characteristics and relative advantages of network and standalone environments • Describe the difference between a Local area and a Wide area network • Describe the elements of network environments

2. Standalone Systems • Everyone has their own PC – processing and data are dispersed throughout the organisation • Lack of centralisation leads to problems: • No standardisation – e.g. shared templates • Data redundancy/integrity problems – multiple copies of data could be changed separately • Communication between departments is not possible

3. What is a network? A network is two or more computers with a common electrical connection! Local Area Network Examples A network in one building or on one site Company, school or home networks Wide Area Network Examples A network covering a larger area than just one site ATM networks, lottery machines, the internet!

4. Advantages and Disadvantages • Shared hardware, software and data – leading to savings? • Easier to install applications, etc. • Improved communications • Centralised management of users, backups, software, etc. • Dependency - server or cable failure affects everyone • Security issues – can be difficult to make secure • Performance - network slows down under load • Difficult to manage – may require a network manager

5. Networking Terminology • A client-server network is one that has one or more file servers storing data and applications and print servers - this is the most common type for anything but very small networks • A peer-to-peer server has no server: • files and peripherals (e.g. printers) are shared between workstations on the network • all workstations may need to be switched on • suitable for small networks due to reduced cost • A gateway is used to link two different networks together, e.g. to connect a LAN to the internet • A bridge links together two similar networks

6. Client-Server or Peer-to-Peer? • Choose client-server if: • you have a large network that can absorb the cost of the server • you want centralised management of users and resources • users are mobile and/or don’t always have their PCs turned on • Choose peer-to-peer if: • the network is small and you can’t justify the expense of a dedicated server • you have a workforce that generally uses the same PC and works the same hours so that all workstations are on at the same time

7. In a bus network, a single cable running the length of the network connects all of the nodes. Bus Star Each node in a star network is connected to a central computer, usually the file server. Topologies The topology is how the computers are connected – there are 3 main ways: In a ring network, the nodes are connected in a circle. In a token ring network, the information travels around the network in one direction only. A “token” travels around the network, like a train, carrying the information. Ring

8. 1 3 P 2 S Bus Networks The bus is free Nothing coming! • Any node can try to send a message at any time, but the bus can only carry one at a time Boom! • Before a node sends a message, it must check that the bus isn’t being used – it senses the data carrier • If the bus is not being used, the message can be sent – but two nodes could start at the same time, resulting in a collision of the two messages – the nodes need to detect when this happens and re-send the message • This protocol is called carrier sense multiple access with collision detection – or CSMA/CD

9. S 2 3 1 P Token Ring Network • So-called because a token passes around the ring to carry messages between the nodes • Nodes can only send when the token passes and is empty • There will be no collisions, but a node may have to wait for the empty token to arrive, so may be slower than a bus network under light loads but will slow down less when it gets busier • Any breaks in the cable will affect all users

10. 1 P S 2 3 Star Networks • All nodes are connected directly to the server by a dedicated server – this means there are no collisions • This means that if a cable is cut, only one of the nodes is affected • Messages might be send by the server polling each node in turn to see whether they have a message to send, and if they have, delivering it. • Logically, this is the same as a token ring!

11. Slower Faster Cabling/Connection The type of cable used can also affect the speed at which data travels around the network. There are three main types of cabling used: Twisted pair – this is cable with two wires next to each other, like bell or speaker wire. Co-axial cable – this has one cable in the form of a braid around the outside of the other, with a (usually plastic) spacer in between, like television aerial cable. The wire around the outside shields the inside one from interference, and allows higher transmission rates. Fibre-optic – information is sent down a glass fibre in the form of light. This is obviously faster, as light travels at the speed of light! Light is also less prone to interference and can travel further without amplification.

12. Glossary

13. Security What security measures do networks use? • Training and user awareness • Usernames and passwords • Firewalls • Proxy servers • Auditing / audit logs • Monitoring performance • Access rights for users • Encryption • Physical methods – lock the server away

14. Wide Area Networks • Some large organisations are national or multi-national, and data might need to be exchanged globally • Large wide area networks (WANs) therefore exist to make this data transfer: • possible • secure • Examples of WANs include: • airline booking systems • credit card payment systems • national lottery machines • Police National Computer

15. The Internet and the World Wide Web • The internet and the world-wide web are not the same! The internet also supports: • e-mail • newsgroups • IRC chat • FTP - file transfer • telnet - for logging into other computers remotely • Through a variety of protocols, e.g.: • TCP/IP - transfer control protocol/internet protocol • POP3 (post office protocol) and SMTP (simple mail transfer protocol) • HTTP - hyper-text transfer protocol • FTP - file transfer protocol

16. Consumers ISP NSP NSP NSP NSP ISP ISP Other ISPs Internet Infrastructure • Consumers connect to Internet Service Providers (ISPs), e.g. Freeserve, AOL, Tiscali • ISPs connect to Network Service Providers (NSPs), e.g. BT • The links between the NSPs is the internet backbone Internet backbone

17. Connecting to the Internet • PSTN (public service telephone network) • connect using a modem • allows speeds up to 56kbps • ISDN (integrated services digital network) • digital phone line - dialling of number required • 64kbps per channel - usually 2 channels = 128 kbps. • Connect using an ISDN or terminal adapter • ADSL (asymmetric digital subscriber line) • connect using a terminal/network adapter • “always on” - no need to dial connection • faster to download than to upload (asymmetric!) • not charged per minute (unlike PSTN and ISDN)

18. Synchronous and Asynchronous • With synchronous transmission, the bits are sent at regular intervals governed by a clock in the computer. • This is the method usually employed by mainframe computers • The rate of transmission is governed by the speed of the clock • Asynchronous transmission is used by PCs • Messages are preceded by a start message, e.g. “ready to send” • Messages have a stop message at the end • Parity bits or check digits can be added to detect errors in the communication

19. Network Protocol • In general, a protocol is a set of rules governing communication – this term doesn’t only apply to computers, but also to diplomatic communication • If both communicating parties are using the same protocol, communication is much easier • An agreed protocol could include: • The transmission mode – synchronous or asynchronous • How characters are represent, e.g. ASCII or EBCDIC • Any encryption and keys used • Error detection and correction methods used, e.g. parity, check digits

20. Factors Affecting Transmission Rate Transmission rate will be affected by: • The hardware used – e.g. how fast your network card, modem, etc., is • The type of connection you’re using – e.g. PSTN, ADSL, wireless • The type of cable you’re using, e.g. twisted pair, coaxial or fibre-optic • The transmission mode – synchronous or asynchronous • You can also make your transmission rate appear to be higher by using compression techniques to compress the data before you send it