GCSE OCR Computer Science: Networks

What is a network?

A collection of computers or electronic devices, that are connected together for the purpose of communication.

Give six advantages of using a LAN.

- File sharing: network users can access the same files, work collaboratively on them and copy files between machines.
- Hardware sharing: users can share hardware, such as a printer.
- Internet connection: the internet connection can be shared between every device connected to the LAN.
- Software installing, software can be installed and updated on all computers, rather than one by one.
- Communication: LAN users can communicate cheaply and easily.
- User accounts can be stored centrally, so users can log in from any device on the network.

Give four disadvantages of using networks.

- Cost: digital equipment is needed to allow computers to communicate.
- Management: networks require management by technical staff, such as a network manager.
- Spread of malware: viruses and other forms of malware can easily spread across an improperly secure network.
- Hacking, once a device is connected another device, it is possible that data may be accessed without the device owner's permission.

What is a LAN?

A local area network; a network that is spread over a small geographical area, where all the hardware for the network is owned by the organisation that uses it.

What is a WAN?

A wide area network; a network that that consists of multiple LANs connected together, is spread over a large geographical area, and the infrastructure is hired not owned outright.

What is a NIC?

A network interface card; an internal piece of hardware that allows a device to connect to a network.

What is a Switch?

A switch is a networking device, that connects devices on a LAN. They receive data in units called frames from one device and transmit this data to the device on the network with the correct MAC address.

What is a hub?

A hub is a networking device, that allows devices to connect to a network. Hubs broadcast the signals they receive to all connected computers, unlike a switch, which only sends data to the intended recipient. This greatly decreases network performance due to the amount of transmissions.

What is a router?

A router is a networking device that transmits data between networks in packets. Alternatively a router can mean a device which contains a hub, modem and WAP, to allow devices to connect to the internet.

What are coaxial cables.

Transmission media, that consist of a single coper wire surrounded by a plastic layer for insulation and a metallic mesh which provides shielding from outside interference.

What are fibre optic cables?

Transmission media that use light to transmit data, through cables. They are high performance, therefore very expensive, they can transmit data over very large distances without loss of signal quality.

What is a wireless network?

A network that uses radio waves to transmit data.

What is a WAP?

A wireless access point, a device that uses a radio transceiver to allow wireless connections to a network, in effect creating its own network.

What is needed to set up a wireless network?

You need a WAP(wireless access point), and the devices need wireless capability.

What is WiFi?

Wireless Fidelity; a standard for connecting devices on a wireless network using radio waves.

What is a client-server network?

A network managed by a server, where the devices connected to it are the clients. The clients send requests to the server, and the server processes the requests and responds.

Give 5 advantages of a client-server network.

- They make it easier to keep track of files, as they're stored centrally.
- Easier to perform back-ups.
- Easier to install and update software.
- Easier to manage network security(e.g anti-malware software and user access levels).
- Servers are very reliable and always on.

Give 3 disadvantages of a client-server network.

- They are expensive to set up and need IT specialists to maintain the network and server.
- Server dependence - if the server goes down, all clients lose access to the network.
- The server may become overloaded if too many different clients are accessing it at once.

What is a peer-to-peer network?

A network, where all devices are equal, connecting directly to each other without a server. Files are stored on individual devices, and are shared with others.

What are 2 advantages of a peer-to-peer network?

- They are easy to maintain: they don't need any expertise or expensive hardware.
- No dependence on server: if one device fails, the whole network isn't lost.

Give 4 disadvantages of a peer-to-peer network.

- No centralised management: devices need their updates and security installed individually. Backups are more complicated.
- Copying files between devices creates duplicate files - its easy to lose track of whats stored where and which files are up to date.
- Peer machines are less reliable and data may be lost if one fails.
- Machines are prone to slow down when other devices access them.

What is a star topology?

A type of network, where all the devices are connected to a central node, which is usually a switch or server.

What are 3 advantages of a star topology?

- If a device fails, the rest of the network is unaffected.
- Its simple to add more devices to the network.
- It has better performance than alternatives, as data goes straight to the central node, so devices can all transmit data at once, meaning there are very few data collisions.

What are 2 disadvantages of a start topology?

- In wired start topologies, every devices needs a cable to connect to the central node, this can be expensive.
- If there is a problem with the central node, then the whole network is affected.

What is a mesh toplogy?

A network arrangement, where the devices are either directly or indirectly connected to every other one without the need for one central server or switch. Data is sent along the fastest route from one device to another.

What is an advantage of a mesh network?

There is no single point of failure in the network, if a device fails, then data is sent along a different route.

What is a disadvantage with a mesh network?

Wired mesh networks require lots of cabling to connect all the devices together, this can be expensive.

What is meant by a "full" or "partial" mesh?

A full mesh is where every device is connected to every other device. In a partial mesh, not all devices are fully connected.

What is a protocol?

A set of rules for how devices communicate and how data is transmitted across a network

What is a MAC address?

MAC addresses are unique identifiers so a device can be found on a network, they are made up of 48 or 64 bit binary numbers.

What is an IP address?

A unique address for each computer device on a network that uses TCP/IP.

Whats the difference between an IP and a MAC address?

MAC addresses are linked to hardware, whereas IP addresses aren't, they're either assigned manually(static) or automatically(dynamic).

How is data sent between networks?

- Data sent on networks is split into equal sized packets.
- Every packet of data has a header - which contains the control information, the control information is like the envelope of a letter, and includes the destination address, the source address, and the packet number.
- The packets payload is like the letter inside an envelope, it is the whole reason the packet was sent and contains its contents.
- Packets may also have a checksum number, a checksum number is a form of validation used to check that the payload data has not been corrupted during transit. The sending and receiving devices both calculate a checksum value by performing a function on the payload data, if the values match, then the data has been received correctly.

What is packet switching?

Packet switching is how routers direct data packets on the internet and other IP networks.

What are the steps involved in packet switching?

- The sending device splits the data into packets to be sent across the network. Each packet is given a packet number to show the order of the data.
- The routers in the network read the packet header and decide which way to send the packet next, according to the IP rules.
- The way the data is sent changes depending on network traffic so the packets can take different routes. If a router receives too many packets at once it may prioritise some over others.
- As the packets take different routes, they can arrive in the wrong order. The receiving device uses the packet numbers to reassemble them in the right order.
- Some packets go missing in transit, so the receiving device checks periodically that all the packets have been received. If it hasn't received them within a certain time, then it sends a timeout message back to the sending device.
- If all the data is received and the checksums match, a receipt confirmation is sent to the sending device.

What is TCP/IP?

A protocol which dictates how data is sent between networks. It is made up of two protocols - TCP and IP.

What is TCP?

Transmission control protocol; sets the rules for how devices connect on a network. It's in charge of splitting the data into packets and reassembling the packets back into the original data once they reach the receiving device. It's also responsible for checking the data is sent correctly and delivered.

What is IP?

A protocol responsible for packet switching.

List 6 other protocols, and explain their functions.

HTTP - Hyper text transfer protocol: used by web browsers to access websites and communicate with web servers.
HTTPS - HTTP secure: a more secure version of HTTP, encrypts all information sent and received.
FTP - File transfer protocol: used to access, edit and move files between devices on a network, e.g. to access files on a server from a client computer.
POP 3 - Post office protocol v3: used to retrieve emails from a server. The server holds the email until you download it, at which point it is deleted from the server.
IMAP - Internet message access protocol: used to retrieve emails from a server. The server holds the email until you actually delete it - you only download a copy. Used by most web-based clients.
SMTP - Simple mail transfer protocol: used to send emails. Also used to transfer emails between servers.

What are network protocols divided into?

Layers.

What is meant by a layer?

A layer is a group of protocols which have similar functions. Layers are self-contained, so protocols in each layer can do their job without needing to know what's happening in other layers. In the order of layers, each layer serves the layer above it - it does the hidden work needed for an action on the layer above.

What are the four layers and protocol examples?

Layer 4 - Application layer: Turning data into websites and other applications and visa versa. E.g. HTTP, FTP, SMTP.
Layer 3 - Transport layer: Controlling the flow of data. E.g. packet splitting. E.g. TCP.
Layer 2 - Internet Layer: Making connections between networks, directing data packets and handling traffic. Used by routers. E.g. IP.
Layer 1 - Link layer: Passing data over the physical network. Responsible for how bits are send as electric signals over cables, wireless and other hardware. E.g. Ethernet.

What are 3 advantages of using layers?

- It breaks network communication into manageable pieces. This helps developers concentrate on only one area of the network without having to worry about the others.
- As layers are self contained, they can be changed without the other layers being effected.
- Having a set of rules for each layer, forces companies to make compatible, universal hardware and software, so different brands will work with each other and always work in basically the same way.

What is the internet?

The internet is a WAN which connects devices from all over the world, using the TCP/IP protocol.

What is the difference between the world wide web and the internet?

The internet is the physical infrastructure that allows devices to communicate, whereas the world wide web is a collection of websites that are hosted on web servers.

What are URLs?

Addresses used to access web servers and resources on them.

What is a DNS?

A domain name server; a computer that translates a websites domain name into its IP address. The internet has a network of domain name servers, meaning you don't need to remember IP addresses to access websites - you can use domain names instead.

What is hosting?

When a business uses its servers to stores the files of another organisation.

What is the cloud?

General storage of user files accessed through online software on the internet.

Give six advantages of using the cloud.

- Users can access files from any connected device.
- It's very easy to increase the amount of storage available for users.
- No expensive hardware is needed by the user to store data.
- No need to pay IT staff to manage hardware.
- Cloud host may provide security and backups for you.
- Cloud software is usually updated automatically.

Give five disadvantages of using the cloud.

- A connection to the internet is needed to access files.
- Dependent on the host for security and backups.
- Data in the cloud can be vulnerable to hackers.
- Unclear who has ownership over cloud data.
- Subscription fees for using cloud storage and software may be expensive.

What is a virtual network?

A virtual network is a network that is entirely software based, they are created by partitioning off some of a physical networks bandwidth to form a separate network.

What is a VPN?

A virtual private network; a type of virtual network that can be used to send data securely over a large network.