Introducing Networks
"A network is a system or group of interconnected elements. A computer network is a group of computers and peripherals connected together to communicate with each other and to share information and resources."
http://www.micro2000.co.uk/network_glossary.htm
Client Server Architecture
Client-Server architecture is a system of network structure which many of the applications we use every day depend upon. It is based around the principle that every computer cannot perform every task entirely by itself, and so different systems will be allocated to remote computers - known as servers. Users will then connect, as ‘clients', to these servers in order to access the resources they require and potentially update data or systems on the servers.
- A "Fat Client" is one where only data processing is carried out by the server, and the client performs application logic and presentation by itself.
- A "Thin Client" is one where the client only worries about presenting data, the application logic and data processing are carried out on the server.
- With a three-tier system, data processing, application logic and presentation can all be carried out on separate servers, this is a much more scalable environment.
Transport Methods
Circuit Switching
This is a method of data transfer where a (single) physical pathway of cabling is established between the sender and receiver for the duration of a data transfer. This is very much like the plain old telephone system (POTS). A complex series of electric switches at telephone exchanges ensures that the senders phone is physically connected to the receivers phone – and so a physical pathway is formed. To have a dedicated cable for each line can become a little tedious so methods of transmitting multiple signals over one line by multiplexing have been developed (Frequency Division Multiplexing and Time Division Multiplexing)
Packet Switching
In this method of data transmission, messages are broken down into packets (or datagrams). These are fixed length blocks of information which are then all transmitted individually through the network. In a system such as TCP/IP, a large computer network directs packets around until they reach their destination. They might not take the same route to reach their destination (although in a virtual circuit, there is one dedicated “virtual” pathway formed between source and destination).
The path a packet should take may be predetermined, as with a virtual circuit system, the two computers arrange a route before beginning data transfer. However, it may be that nodes in the network simply redirect the packet around until it reaches the destination it is addressed to, this would be the case in a datagram system.
Packet switching is used in the Internet, and it has many advantages over circuit switching. Firstly it is a more efficient use of lines, and also it is less likely to be affected by things like line failure and faults, since the packets are simply able to use another route instead – with a multitude of different routes possible. Finally security can be seen to be better in a PSS since the packets are sent via different routes and interleaved with other information, which makes intercepting a whole data block very difficult (unlike a circuit switching system where you would only need to tap one line to listen in on all communications between sender and receiver). There are risks however of packets getting lost, and there are additional time lags as each node stores and processes each packet before passing it on.
The Internet Is...
A network of networks inside a network? It is comprised of Internet service Providers, connected together and offering services for end-user clients. ISPs fall into three categories:
- Tier 1 - "Backbones" - A relatively small group of companies which directly connect to each other on very high speed links with international coverage.
- Tier 2 - Often have regional or national coverage, and connect to 'local' Tier 1 ISPS, and to other Tier 2 ISPs.
- Tier 3 - Residential Access ISPs, allow users to connect with dial up modems, ADSL, cable modems etc.
Points of Presence (POP) are the collections of routers in an ISPs network which other ISP's connect to. Network Access Points (NAPs) are complex high speed networks which relay and switch high volumes of network traffic.
Layered Network Architecture
This allows the functions of a network to be abstracted into layers, each layer having different protocols and services. In the Internet Protocol, there are 5 key layers:
- Application (Many different protocols, such as HTTP, FTP..)
- Transportation (TCP and UDP protocols)
- Network (IP and other routing protocols)
- Link (Delivers frames from one node to the next, Ethernet)
- Physical (Bits along the physical medium)
References:
- Janet Lavery - Durham University Computer Systems, Introduction to Networks Lecture 1, 2007
- Janet Lavery - Durham University Computer Systems, Introduction to Networks Lecture 2, 2007
- A2 Computing Revision - Paul Nicholls - http://resources.r9paul.org/ASA2/Computing/A2ComputingRevision.pdf