EDEXCEL GCSE Computer Science, Topic 4- Networks

INTRO TO NETWORKS

Reasons for Connecting Computers On a Network:

1. Sharing data and software

2. Sharing peripherals

3. Sharing internet connections and services

4. Providing centralised support and backup

5. Enabling rapid deployment of new software and updates

6. Allowing people to communicate

7. Supporting collaborative working

Local Area Networks (LANs):

1. Small geographical area

2. Single site

3. Usually managed by a local manager or team (distributed ownership)

4. Managed on site

Wide Area Networks (WANs):

1. Network of networks- connect separate lans

2. Large geographical area

3. Spread over different sites

4. Managed by several different people (collective ownership)

THE INTERNET

Connection:

1. Super-fast fibre-optic cable

2. Satelites

IP Addressing:

1. Every internet-connected device has a unique IP address

2. IPv4 /32-bit addresses/from ‘70s/sufficient for 4 bil. devices to be uniquely identified

3. IPv6 /128-bit/340 tril. tril. devices with unique IDs

4. Most devices use dynamic IP addresses

5. Dynamic IP addresses = allocated from a communal pool when they connect to a network & returned when they leave. Next time they connect, their IP address will probably be different

6. Static IP addresses = never change, permanently connected to the internet. Used by devices such as routers and web servers.

7. Domain name = human friendly IP address eg- www.google.com is a lot easier to use than a string of numbers

URLs (Uniform Resource Locators):

Uniform Resource Locators

1. Complete web address of any resource on the internet

What Happens When a Web Page is requested?

1. When a URL is typed into a browser, the browser sends it to a DNS server

2. The DNS server returns its corresponding IP address to the browser

3. Now it has the IP address, the browser will send an HTTP request to the web server that holds the page data

4. The server finds the correct files and sends them to the browser

5. The browser renders the page and displays it on the screen

PACKET SWITCHING

Sending Data:

1. The sender’s large file is broken into small packets

2. Routers direct packets to their destination by inspecting them and finding the cheapest (remember- this means quickest) route to the next router

3. Packets take different routes and may arrive out of order. A packet has to be forwarded to several routers before reaching its destination

4. The receiving computer reassembles the packets into the correct order using the packet headers. Any lost or corrupted packets will be requested to be re-sent

Routers:

1. Routers form a physical connection between networks and forward data from one to another

2. When a router receives a packet it finds the packet’s destination address then uses its routing table to determine the cheapest route for the next part of the packets journey

3. Routers inform each other on traffic conditions so if a route is congested a packet can be sent another way

Data Packets:

1. A data packet consists of a header, payload and footer

2. A header contains the source & destination IP addresses, the sequence number. The total number of packets and a checksum

3. The payload contains part of the data of the file that has been split and sent

4. The footer contains the end of packet flag

WIRED VERSUS WIRELESS

Wired Connections:

1. Devices can be physically connected using cables

2. The most common cable is made of thin copper wire and it transmits data as electrical pulses

3. The modern alternative is a fibre-optic cable which is made of very thin glass strands and transmits data as light pulses

4. Copper wire is susceptible to electrical interference while fibre optic is immune

5. Copper wire is typically used to connect devices in a LAN and fibre-optic for long distance data traffic

6. Fibre-optic is more efficient however it is a lot more expensive and fragile

Wireless Connections:

1. Radio waves transmit data through air

2. Wi-Fi is the most well-known and the oldest but there are others that have different uses, ranges and power requirements- Bluetooth, Zigbee, RFID & NFC

3. These all consume less power and all but Zigbee have a smaller range, RFID has a particularly small range and NFC’s is even smaller

4. Uses - Wi-Fi = networking devices in LANs, Bluetooth = pairing devices, Zigbee = home automation, RFID = security tags, passports and implants & NFC = contactless payment

CONNECTIVITY ON A LAN:

Pros of a Wired LAN:

1. Big range- up to 100m and larger if signal booster is installed

2. High bandwidth- up to 10 Gbps

3. Low latency- cables have layers of protective covering making them less susceptible to interference

4. Good security- impossible to tap physical cables without being in the same location

Cons of a Wired LAN:

1. Difficult installation- each device connected must have its own cable

2. Limited flexibility- the number and location of cable connections is fixed. Making changes to the network once its already running is time-consuming and disruptive

Pros of a Wireless LAN:

1. Easy installation- does not take long as only the wireless access points need to be connected by cable

2. Good flexibility- devices can be moved around without losing their connection (provided they stay within the range) & new users can easily be added with the network ID and its password

Cons of a Wireless LAN:

1. Shorter range- large physical objects like walls can obstruct the signal

2. Lower bandwidth- up to 3.2 Gbps, all active devices on the network must share this

3. Higher latency- susceptible to interference from other wireless devices and networks

4. Poor security- anyone within range can intercept transmissions

How are Wired and Wireless Connections Used in a LAN?

1. Many networks use a combination of wired and wireless connections (a mixed network)

2. Devices such as TVs or printers and other peripherals would likely be connected by cables

3. Devices such as laptops and tablets would likely be connected wirelessly

4. The router assigns an internal IP address to each device (connected by cable or wirelessly) so they can all be part of the same network

NETWORK SPEEDS

What is Network Speed?

1. Network speed is the data transfer rate of a network (the number of bits that can be transferred from one device to another in a specified period of time)

2. Data transfer rate is measure in bits per second (bps)

3. As modern technology is able to transfer more data more quickly we usually measure this in Megabits per second (Mbps)

Units of Measurement:

1. Data transfer rate (speed) is measured in base-10 units (bps ,Bps, Kbps ,Mbps, Gbps)

2. File size is measured in base-2 units (bit, byte, KiB, MiB, GiB, TiB)

Formula:

Time (in seconds) = File Size (in bits) Data Transfer rate (in bps)

NETWORK PROTOCOLS

Protocols:

1. Define rules about how data must be formatted, transmitted and received on a network

2. Without them, network devices would not be able to understand the electronic signals sent to one another

3. Protocols need data formats (to make sure data can be exchanged consistently and correctly), address formats (to identify senders and recipients & ensure data is sent to the right places) & routing (to provide the right information so that data can flow through networks correctly)

Protocol Layers:

1. Protocols in the top layer are the ones users see

2. Those at the bottom handle the converting of binary data into electrical, light or radio signals for transmission

3. The TCP/IP (Transmission Control Protocol/Internet Protocol) model is a collection of protocols known as a stack that decides how data is transmitted

4. The TCP/IP model has four layers, the top is the application layer, applications such as file transfer, web browsers and email operate at this level

Application Layer Protocols:

1. FTP (File Transfer Protocol) = rules for transferring files between computers, often used for transferring those that are too large for email attachment

2. HTTP (HyperText Transfer Protocol) = rules followed by a web browser and web server when requesting and supplying information

3. HTTPS (HyperText Transfer protocol secure) = makes sure communications between client and server are secure using encryption

4. SMTP (Simple Mail Transfer Protocol) = rules for sending emails from client to server and server to server until it reaches its destination

5. POP (Post Office Protocol) = used by a client to retrieve emails from a mail server, all emails are downloaded when there is a connection between them and are deleted from the server once done so

6. IMAP (Internet Message Access Protocol) = unlike POP, messages do not need downloading, they can be read and stored on the message server. This is better for users with multiple devices as they can read emails from them all rather than just the one that they are downloaded on

THE TCP/IP MODEL

The Four Layer Protocol Stack:

1. A hierarchical set of protocols working together to allow devices to communicate, each layer is responsible for a different aspect

2. (Sending computer) → application layer → transport layer → internet/ network layer → link layer → link layer → internet/ network layer → transport layer → application layer → (receiving computer)

3. Outgoing data is passed down the stack and incoming data is passed upwards

4. The individual layers do not know how the others function, only how to pass data to them

5. Each layer may contain several protocols that all provide a suitable service to the layer’s function

6. Each protocol has a role to play at both the sending and receiving ends

Purposes of the Layers for Sending Data:

1. Application layer = providing the interfaces and protocols needed by the user

2. Transmission layer = splitting outgoing data into packets and numbering them, a header containing a sequence number and a checksum is added to each

3. Internet/network layer = adding source and destination IP addresses to packet headers so routers can guide them

4. Link layer = using network specific protocols to convert binary data into light, electrical or radio signals for transmission

Purposes of the Layers for Receiving Data:

1. Link layer = converting incoming signals into binary data

2. Internet/network layer = stripping addresses information from incoming packet headers

3. Transport layer = checking incoming packets and sending a resend request for any that are damaged or lost. the sender is notified when all have arrived and the packets are reassembled then passed to the application layer

4. Displaying received information from the user

NETWORK TOPOLOGIES

Star Topology:

1. Each computer is connected individually to a central point/node which can be a hubor a switch

2. Data is only sent to the intended destination

3. If one link fails the other devices can still work

4. Easy to add new devices without disrupting the network

5. If the central point fails, the entire network will fail

6. Requires a lot of cable because each computer has its own connection to the node

Mesh Topology:

1. Each computer is connected to at least one other

2. As well as sending its own signals, each computer also relays data from the others

3. Commonly used on wireless networks where there is high demand

4. Data can be transmitted from different devices simultaneously

5. Always an alternative route if one component fails

6. Can handle high data traffic

7. Adding more devices does not slow transmission on the network

8. Overall cost is high if it is not wireless as it would require a lot of cable

9. Difficult to manage and requires expert supervision

Bus Topology:

1. Each node is connected to the main cable called a bus

2. There are terminators at each end to absorb signals when they reach the end to prevent interference

3. Easy to set up and add extra devices

4. Relatively cheap as only one cable is needed

5. Lots of data collisions when multiple devices try to transmit data at the same time. Devices detect collisions and rewend the data which slows the network

6. The whole network will fil if a terminator is removed

NETWORK SECURITY

Reasons for Network Security:

1. To protect a network from internal and external threats

2. Ensures only authorised users have access to the network and its resources

3. Making sure users only access data relevant to them

4. Preventing misuse

5. Preventing damage to hardware

The Importance of Network Security:

1. Business success = data on an organisation’s network is vital and the business may fail if it is compromised

2. Privacy = data stored on a network may be sensitive and must be stored securely to comply with the law

3. Finance = data can be financially valuable (eg. details of new products and marketing campaigns)

Network Vulnerabilities:

1. Hackers = people who exploit security vulnerabilities to gain unauthorised access to a network

2. Insiders = people who work for the organisation and have authorised access to the network but create problems for it, often this isn't malicious and is just careless human error

3. Malware = software engineered to cause damage and disruption to a network

Penetration Testing:

1. Used to check a network for any security vulnerabilities that a hacker could exploit

2. Testers attempt to access a network in the same way that hackers do (gaining unauthorised access)

3. Testing assesses security awareness of users and efficiency of security policies

4. Black box penetration testing = the tester is given absolutely no information about the network and must try and breach its security

5. White box penetration testing = the tester is given relevant information about the network and looks for security loopholes that could be exploited by insiders or outsiders.

Ethical Hacking:

1. An ethical hacker is a computer and networking expert that systematically tries to penetrate a computer system or network to identify security vulnerabilities

2. Often referred to as “white-hat” to differentiate between criminal hacking known as “black-hat”

PROTECTING NETWORKS

Access Control:

1. Limiting who can log into a network and what they are permitted to do

2. Authentication is determining whether a person on a network is who they claim to be

3. Many organisations will use multi-factor authentication, as well as having a valid ID and password people must have a swipe card and/or some sort of biometric information about themselves

4. Permissions are the levels of access to files. Some users may be given “read-only” files where they can only view the file, others may have “read and write” where they can also make edits. Those at a higher level can delete files and change other users’ permissions

5. When assigning permissions, network managers often apply the “principle of least privilege” where users are given only the amount of access they need to do their job and no more

Physical Security:

1. The first line of defence is to prevent unauthorised people from entering the premises where the network equipment is located

2. Access doors should be kept locked and fitted with security recognition measures

3. Biometric security recognition can be used on mobile devices to authenticate users

4. Swipe cards containing a users’ details can be used for entry

5. Closed-circuit television (CCTV) can be used to monitor the premises

6. RFID chips can be fitted to all equipment so that an alert will be triggered if it is removed from the premises

Firewalls:

1. Protect networks connected to WANs

2. Can be software or hardware

3. Can be configured to prevent communications from entering the network and prevent programs and users from accessing the internet inside the network

4. Most individual computers have a software firewall installed that has some default rules

5. Businesses are likely to have a PC-like hardware-based firewall as they have greater flexibility in the rules that can be applied and can allow faster throughput of data