OCR GCSE Computer Networks

Types of Network

Network: Two or more computers connected together to share information and resources.

  • LAN - Local Area Network

A connection of computers and devices in a small geographical area. Often used by organisations as the owner of a LAN is able to control connections and cabling. Each device on the network is known as a node.Examples: In a school or office

  • WAN - Wide Area Network

A connection of computers and devices in a large geographical area. Devices like phone lines and satellites may be provided by telecoms companies instead of the owner of the network. WANs can be made up of LANS.Example: The Internet

Advantages of LANs

  • Enables digital communication.

  • Enables sharing of digital information.

  • Enables sharing of peripheral devices (E.g. printers or scanners).

  • Enables computers to be updated centrally.

  • Enables distributed processing - ability for a single program to run on different devices simultaneously.

  • Require less cabling and so are less expensive than WANs.

Disadvantages of LANs

  • Requires expertise to install and maintain.

  • Security issues may arise from unauthorised access to data.

    • LANs can be more secure by adding measures such as passwords, encryption, user control (E.g. not allowing users to install software) and changing passwords frequently.

Advantages of WANs

  • Cover a large area so able to connect different areas.

  • Enables digital communication

  • Enables sharing of digital information

Disadvantages of WANs

  • Increased vulnerability to viruses and data breaches.

  • Expensive to maintain and higher cable costs.

  • May have slower data transmission speeds due to the longer distance it needs to travel.

LANs VS WANs

LANs may be connected via Ethernet cables or wirelessly due to their small sizes. However, WANs cover a much larger area so may need to use other forms of connection like: Dedicated lines, Leased lines, Phone lines or satellites. These may be owned by other companies and can be very expensive.

LANs are generally safer in terms of security due to their small size but may still experience risks like: Viruses, Email attachments, Trojan horses and clicking on unsafe links.

WANs are more likely to be targeted due to the large amounts of people that may be connected to them. Security risks may include: Gaps in security, Increased vulnerability to viruses and data breaches.

Hardware and factors affecting performance

Data Packet: Parts of files that have been split up to be sent over the internet. They are reassembled to form files at the other end.

Hardware components

  • Wireless Access Point (WAP)

    • Allow for wireless devices to connect to a network. Include connections through Wi-Fi and Bluetooth.

  • Router

    • Used to connect two or more devices on networks. They contain a routing table with a list of IP addresses and routes for packets to take. They can identify the network of a packet and determine which network the packet should be forwarded to, or which host if it stays on the same network. Packets from different network types can be exchanged.

    • Performs a variety of tasks and handles traffic from the internet.

  • Switch

    • Connects two or more devices together and helps stop data collision. Switches create direct connections to the device a packet has as its destination address.

    • Learns which devices are on which connection.

  • Hub

    • Device replaced by the switch as they would pass on anything received on one connection to all other connections.

  • Network Interface Card (NIC)

    • Allows a device to connect to a wired network, allowing Ethernet cables to be plugged in. It contains a MAC address (see later in notes) and allows packets to travel to and from a device.

  • Transmission media

    • There are three main types: Ethernet, Fibre Optic and Coaxial Cable. The Ethernet cable is the current networking standard while the Coaxial cable is an older standard. Fibre Optic cables are fast but expensive.

Factors affecting network performance

  • Data collisions

    • Quieter networks experience very little data collisions but busier networks may appear slow due to packets having to be resent multiple times to make it though.

  • Bandwidth

    • Transmission capacity of a network connection in a given amount of time. A higher bandwidth means more can be transmitted in a certain amount of time.

  • Latency

    • The time it takes for one packet to arrive. It’s often called ping and lower latency networks usually have better performance.

  • Users on the network

    • Themore users are on the network, the more data is being transferred over the network. This could reduce performance as data collisions are more likely to occur.

Client-Server and Peer-to-Peer networks

Client: Device able to request information from a server.

Server: Manages access to centralised resources.

Peer: Something of the same status/ability.

Client-Server

All clients request resources from the server, it relies on the central device. Additional hardware would be needed.

  • Files and other resources are stored centrally and clients can access them from any device on the network. All devices are able to update the central databases.

  • Backups are done centrally from the file server so each device updates at the same time in one go.

  • They have centralised security so an antivirus and firewall can be installed from the server.

  • Client-Servers are useful for high traffic environments.

Peer-to-Peer

All devices have equal status and can each act as a client or server. The can request and provide network services and don’t usually require addition hardware or software.

  • Files are stored on each device individually and the user has to log into the same device each time.

  • Backups have to be done by each device individually.

  • Individual security can be installed on each device (less susceptible to viruses).

  • Peer-to-Peer is more suitable for low traffic environments.

DNS, Hosting and The Cloud

The Internet: Collection of computers worldwide that are connected together.

The Internet is aWANmade up of smaller networks.It’s connected using undersea cables and underground fibre optic cables managed by multiple different companies.

The Internet works on a Client-Server model. Web servers stores web content which can be downloaded to client devices upon request. Mail servers and media servers act similarly with emails and streaming music and/or videos.

IP - Internet Protocol

IPis aset of rulesdevices use to communicate with each other. Each device is given a unique IP address which tells other devices (like a switch or router) where to send files to and from.

  • It can be static (always keeps the same address) or dynamic (changes every time a device reconnects to the network). Dynamic IP addresses are used more often to allow more devices to be able to connect to a network. Static addresses may be used so that a network manager knows which devices are on a network.

  • It can also be IPv4 or IPv6 as due to the large increase in devices IPv4 addresses were not enough to provide for each device. There are trillions of addresses in IPv6.

ISP - Internet Service Provider

A business that provides a connection to the internet as well as other services like email addresses or web spaces.

Hosting and URL

The URL (uniform resource locator) is the host name. It has three levels with the top being generic, the second denoting the type of organisation and the third is the organisation name.

Hosting is the storing of files and data on a web server. The web server is seen as the host.

For a website to be seen, it needs to provide a suitable domain name and register with a domain registrar to ensure the domain name is not already registered. It would then gain an associated IP address which would be registered on the DNS.

DNS - Domain Name System

Every website on the Internet has a network address. These addresses are hard to remember so domain names are used as an easier alternative for the user.

When a user visits a website they enter the domain name as a URL, the computer then queries the DNS server. The DNS searches for its equivalent IP/network address in its database. The users request is then forwarded to the server on that IP/network address.

  1. User types address into web browser

  2. Web browser sends web address to a DNS server

  3. DNS server sends back the corresponding IP address

  4. Web browser sends request to the IP address for the website

Web servers and clients

Webs servers hold data needed for websites. When a user wants to view a website, the web browser sends a request to the web server. The web server processes the request and prepares the data requested before sending it back. The web browser receives it and displays the website to the user.

The web server/host acts as a server.

The web browser acts as a client.

The Cloud

The cloud is remotely accessed storage, which is accessed through the Internet.

Advantages of The Cloud

  • Access files from any location or device.

  • Access can be granted to others.

  • Backups are done by the cloud.

Disadvantages of The Cloud

  • Some else may be accessing your data

  • Data may not get backed up

  • Access only possible with an internet connection

Cloud software is good for low-powered devices or for travelers as it just stores your files and allows you to use web based software.

It doesn’t require any secondary storage usage.

Network Topologies

There are two main types of topology (in the case of the OCR specification) which are the Star Topology and the Mesh Topology.

Star Topology

All devices connect to a central device like a router or switch. Every message gets sent through the central device before reaching the clients. Each device has its own cable connecting to the centre.

It can be reliable if one cable fails as the others will still continue and it has good performance as no data collisions can occur. However, if the central device fails, the whole network goes down and the extra hardware to install it is expensive.

The Star Topology requires fewer cables but that can cause bottlenecks on a busy network. It’s also easier to add or remove devices as it’s simple to understand and troubleshoot.

Mesh Topology

Devices are all connected to other devices with no central device. Messages being sent to devices can choose to travel through multiple different routes. It can be done as a full mesh where all devices connect to each other or a partial mesh where at least two of the computers are connected to multiple other computers.

This topology is less expensive in terms of hardware but still requires more cabling. Packets are able to be routed around bottlenecks but it’s more difficult to understand and troubleshoot. A single failure won’t stop the rest of the network so it’s quite reliable but it’s more complicated to add or remove devices.

IP, MAC and Encryption

IP recap

IP as stated previously is a set of rules devices use to communicate and it’s used to identify computers within a network. They are mostly dynamic and when you move network the IP address will change which is known as a logical address.

IPv4 is usually 4 sets of digits between 0 and 255 while IPv6 is usually 8 blocks of 4 hexadecimal digits. They are used to identify devices on that network. They are dynamic and ideal for use on WANs.

MAC - Media Access Control

The MAC address is the device’s physical address and it cannot change - it’s static. Every MAC address is unique to its NIC. It’s usually represented using 12 hexadecimal digits.

Encryption

Encryption forces anyone trying to access packets that’s not theirs to try and decrypt the data being sent across the network. It requires a key that only the users device would know. Encryption is necessary on public networks to avoid data being intercepted.

Standards and Protocols

Standards: Rules that make sure different manufacturers and providers create products that work together.

Protocol: Standard used to define a method of exchanging data over a computer network.

When two devices send messages to each other it’s called handshaking.

Main Protocols

(For the OCR Specification)

  • TCP/IP - Transmission Control Protocol/Internet Protocol

    • Ensures data is split into data packets in the same way, are put back together correctly when received and the data received is the same as was sent.

    • IPensures data packets are directed in the most appropriate way.

  • HTTP/HTTPS - Hyper Text Transfer Protocol (Secure)

    • Governs howwebsitesandweb serversare accessed byweb browsers.

    • Secure makes sure data is encrypted.

  • FTP - File Transfer Protocol

    • Ensures all clients on a network access files from a server in the same way. It can upload, download, delete, rename, move and copy files.

Email Protocols

  • POP - Post Office Protocol

    • Retrieves emails from a mail server and deletes it from the server once received. It can then only be accessed from the device it was received on.

  • IMAP - Internet Message Access Protocol

    • Similar toPOPbut email remains on the server once downloaded/received. This means the email can be accessed by other devices that can be synchronised.

  • SMTP - Simple Mail Transfer Protocol

    • Used for sending an email from anemail clientto anemail server*. May also be used to forward an email from one* email serverto another.

Layers

Packet switching: Splitting data into packets to be sent along different routes to its destination. The packets are then reassembled to reform the data.

Each packet is given a header which contains the IP address of the network it’s on, going to and the IP address of the network it’s being sent to.

The header also contains the packet number and the total number of packets so that it can be used to reassemble the data once sent. Packet loss can sometimes occur so the device may request for the packet to be sent again.

If the packet never arrives it’s deleted by the router.

TCP/IP Stack

(For the OCR Specification)

The TCP/IP Stack is a 4 layer concept where there is an Application layer, Transport/Transmission Layer, Internet Layer and Data Link/Network Layer.

4) Application Layer

Provides networking services where it prepares data using different protocols.

Example Protocols: FTP, HTTP, SMTP, IMAP, and POP

  1. Transport Layer

Sets up communications between devices so data can be transported and splits data into packets ensuring they’ve been sent and received correctly.

Example Protocol: TCP

  1. Internet Layer

Responsible for handing IP addresses for data packets - routing data across. It directs them between devices and controls the traffic.

Example Protocol: IP

  1. Data Link Layer

Controls how data is sent as electrical signals over cables, wireless or other hardware. It provides the physical transport of data.

Example Protocols: Wi-Fi and Ethernet