networks

computer science unit 5

the internet

a group of interconnected networks

the world wide web

one of the services that makes use of the internet

Wide Area Networks

• Any network in which the computer communicate using resources supplied by a ‘third party carrier’.

• Uses cables, telephone lines, satellites or radio waves to connect two or more remote locations.

• The internet is the largest one of these in the world.

Organisations that use WANs

• organisations that have a large number of locations, such as universities or schools
  → each one will have their own network, forming a WAN together

• used by organisations with many offices such as banks
  governments
  research organisations

IP addresses

• four 8-bit numbers separated by dots

• each computer will have one of these

circuit switching network

• a dedicated connection set up between you and the recipient for for the duration that you need it (for example a phone call)
  → works fine for phone calls, but too many lines would be needed if this is how data was to be sent across the internet (billions of users)

packet switching

• file broken up into ‘packets’ of a few bytes

• each packet is given a header with the IP address it is going to, the IP address it has come from, the sequence number of the packet, the number of packets in the whole communication, and error checking data

data packets

• each contains the same number of bytes

• each may flow through the internet via different routes

• each is marked with a number so they could be put in the correct order by the reciever

Network Interface Cards

• built into every networked device

• may be wireless with an antenna or wired with a network cable socket

• mostly incorporated into the motherboard

MAC addressing

assigned to each NIC by the manufacturer

role of MAC addressing

when a web page is requested, each router will use the MAC address of the next router to send the data packet onto the next part of its journey

routers

• looks at the destination of packets and sends they to the network closest to their destination

• a home router will route packets between the LAN and the internet

switches

• connect each computer in a network

• know the MAC addresses of all the connected computers and devices and can send packets to the correct computer when they arrive

wireless access points

• a device which allows devices to connect to a wired network using wifi

• usually connects to a router using a wired network

advantages of wireless

• no need to lay cables

• easy to connect new devices

• devices portable within range of access point

advantages of wired

• less likely to have slow connections as access point is not needed

• connection speed is not reduces with distance

• more secure

packet

one unit of data sent through an IP network

IP address

internet protocol address

network topology

a way in which parts of a system are connected

star topology

all devices and computer connected to a central switch

star topology advantages

• fast data transfer to hub as each wire isnt shared with other computers

• if one cable fails, only one computer is affected

star topology disadvantages

• requires additional hardware such s the central switch and network cables

• if the central switch fails, whole network goes down

bus topology

computers and other devices are connected to a central coaxial cable

bus topology advantages

• less cables so cheaper

• easier to add ore devices as they only need to connect to the central cable

bus topology disadvantages

• the single cable is shared by many devices so there are many collisions of data

• if the cable is damaged, the whole network fails

• any device can view all data on the cable creating a security risk

ethernet

family of standard local networking protocols

• describes how devices should format data ready for transmission between computers on the same network

ethernet transmission

• data divided into frames (similar to packets)

• each frame contains source and destination addresses and error checking data

pairing Bluetooth devices

• persistent link is made between the two devices

• a device can ask for a passkey to pair with it (gives a layer of security)

name 3 types of cable used in wired networks

• copper cable

• coaxial cable

• fibre optic cable

authentication

checking that the computer and its user are allowed to access content (most commonly done using a username and password)

symmetric encryption

a single key is used to encrypt and decrypt a message and must be given to the recipient

asymmetric encryption

two keys are used - a public key to encrypt data and a private key to decrypt data
this means you wont have to reveal your decryption key

HTTP

hypertext transfer protocol used fir accessing and receiving web pages via the internet

HTTPS

secure HTTP. encrypts information so that it cannot be accessed by an eavesdropper

• used for banks, online shops and social networks

FTP

file transfer protocol - used or sending and retrieving files to or from a FTP server

SMTP

simple mail transfer protocol

emails

• when an email is sent, it will first be sent to a mail server using the SMTP protocol

• then forwarded on by other SMTP servers

• stored when it reaches the destination mail server

• the user’s computer uses POP or IMAP to access the email

POP

post office protocol - downloads every new message to your device and it is no longer available on the server

IMAP

internet message access protocol - message is left on the server and can be accessed on multiple devices until the user deletes them

TCP

transmission control protocol - breaks up messages into packets and reassembles them at the other end. detects errors and resends lost packets.

IP

internet protocol - routes the individual packets from one IP address to another

layers of TCP/IP protocol stack

• application layer

• transport layer

• internet layer

• link layer

application layer

applications such as email clients and web browsers create data to send

SMTP, FTP and HTTPS operate on this layer

transport layer

creates connection between 2 computers or hosts

uses TCP (transmission control protocol) and UDP (user datagram protocol)

internet layer

responsible for routing packets

IP (internet protocol) operates on this layer

link layer

where the physical hardware is located
includes NICs, cables and wifi, OS device drivers

advantages of layers

self contained, so the functionality of one layer can be altered without affecting the others

UDP

user datagram protocol maintains an open data connection to send a stream of packets without making sure that they are received correctly
used in online gaming and video streaming