GCSE OCR 3 A451 Computing Client-server and peer-to-peer networks 2.1.6 Computer communications and networking 3

Objectives Explain the role of computers in a client-server network Explain the role of computers in a peer-to-peer network Explain the advantages and disadvantages of client- server and peer-to-peer networks

A client-server network The server is a powerful computer which provides services required by any of the clients Client computers are those which people use to log on to and use the services provided by the server Client Query Response Search Data Server

Servers on LANs and WANs One or more servers may be part of a local area network A server may also be part of a wide area network Google has massive servers in several different countries

A Google server room in Council Bluffs, Iowa Photo: Google/Connie Zhou See more images of Google data centres

A server can be part of a LAN

Serving all your needs In a school network, there may be dozens or even hundreds of clients and several servers A file server holds all the data files and databases and manages backups A print server may organise printing on different printers An email server may receive emails, detect and block spam, distribute emails to users A web server may host the school’s external website

Role of the server The server waits for requests from the clients It may need to pass the request (e.g. for a web page) to another server – in which case, it becomes the “client”

Role of the client The client sends requests to the server Waits for a reply Receives the reply

The “handshaking” process This is the process by which two devices initiate communication One device sends a message to another indicating it wants to communicate If two devices try to transmit simultaneously, both wait a random time and then try again The “handshaking” negotiation then takes place to establish what protocol to use Computer A: “I am ready to receive data” Computer B: “I am ready to send data” Computer A: “OK, send it!”

Types of server Servers are often dedicated computers, meaning each one performs no other tasks beside its server tasks However, on a multiprocessing operating system, a single computer can perform the tasks of several servers

Advantages of client server networking All data can be backed up centrally rather than on each individual computer Security is better because data is held in one location rather than all over the organisation Users can log in from any computer on the network and access their data

Peer-to-Peer (P2P) Networks An alternative to a client-server network There is no central server. Each computer holds its own applications and data All computers have equal status

Advantages of a peer-to-peer network Easier to set up – computers can simply be cabled together No special software is required to run the network Individual computers can share a printer, router, modem and other hardware Users can communicate directly with each other and share each others’ files Where would you use a peer-to-peer network? What are the disadvantages?

Drawbacks of a peer-to-peer network Viruses and malware are easily transferred over this type of network Data recovery and backup is not done centrally, so each computer has to have its own backup system Files are not centrally organised, but stored on individual computers and may be hard to locate if the computer’s owner does not have a logical filing and naming system

Peer-to-peer on a WAN The P2P configuration can also be used for file- sharing websites BBC iPlayer, for example, uses P2P networking This means that with thousands of people downloading, data can be passed between computers rather than just from the server Peer-to-peer networks are often used for the illegal distribution of copyright material such as music and films, as they are very difficult to close down

Cloud computing Software as a Service (SaaS) Servers host software or services that you can access

Online storage Dropbox, OneDrive and Google Drive How do they work? Are these P2P or Client-Server?

Worksheet 3 Test your understanding with the questions on the worksheet

Plenary What are the relative advantages of client server and peer-to-peer networks?

Client server vs Peer-to-Peer A central backing store is available to all computers Storage facilities are distributed across all computers Security is controlled by the central computer Security is not centrally controlled Backup is done centrally on the server Backup must be done separately for each computer All users are reliant on the central server No central server Can support hundreds or even thousands of users and grow with an organisation Easy to set up but most suited to homes and small businesses