1.3.3 Networks

What you need to know

(a) Characteristics of networks and the importance of protocols and standards.

(b) The internet structure:

• The TCP/IP Stack.

• DNS

• Protocol layering.

• LANs and WANs.

• Packet and circuit switching.

(d) Network hardware.

(e) Client-server and peer to peer.

Networks

Networks are a medium over which any number or type of device can share data.

Networks: - “A number of devices (‘nodes’), linked together in order to share resources: hardware (e.g. printers), software and data”

Most large networks have at least one server: a powerful computer to manage the network, including perhaps providing services such as file storage or email

Benefits of networking:

Sharing devices and resources (e.g. printers, internet access) saves equipment and thus money
File sharing between users/devices allows for easier collaborative working
Allows sharing and work over much larger geographical regions
Users on a network can communicate via email or Instant Messenger
One ‘volume license’ can be bought for all devices on a network, saving money compared to having to buy licences for each individual device
Centralised management: all nodes can be managed from one central location (e.g. to install software on all at once)
Joint security for all users (including firewalls and Internet restrictions to prevent accidental viruses, entering the network)
Restrictions (viewable folders, Internet access, runnable programs, etc.) to maintain user and system safety
File storage; access files/accounts from different devices connected to the network

Limitations of networking:

Difficult to implement and manage
Expensive to implement and manage
Specific expertise is required, hence costing money
Viruses can spread quickly between devices on a network
Hacking is possible, and hence security features are required to prevent this – a large quantity of files must be protected
Cables can be difficult and tedious to replace if broken, and breaks may affect large numbers of devices at once.

Task

Using the Google Slide create a mindmap to show what a network is and the advantages and disadvantages of networks (you will need to make a copy if your teacher has not provided you a copy in Google Classroom) (CLICK HERE)

Protocols & Standards

Protocols

Protocols are a set of rules necessary to govern communications between devices on a network. Protocols are needed because...

Different manufacturers create different systems with different architectures
These architectures cannot communicate unless they agree on the same set of communication ‘rules’
Hence, protocols are these ‘rules’

Examples of commonly used protocols are:

HTTP (Hypertext Transfer Protocol) – Used for web page rendering, an encrypted version, HTTPS (Secure), is becoming more common
TCP/IP (Transmission Control Protocol / Internet Protocol) – This is a networking protocol used in the routing of packets through networks
POP3 (Post Office Protocol) and IMAP (Internet Message Access Protocol) – Mailing protocols, used for email access.
FTP (File Transfer Protocol) – Used for the transmission of files over networks

IP addressing

IP addressing doe the following:
It Identifies a device on a network
It has Internal IP addresses identify a device on a local (e.g. home) network
It has External IP addresses that identify a router connected to the internet
IPV4 addresses are made up of 4 octet (8 bit) values, each separated by a full stop, to create a unique address however, due to the increasing number of internet users we now use more commonly IPV6 has been created to extend the possible number of unique addresses.

Domain Name System (DNS) Explained.

DNS is a data exchange protocol it is used to resolve domain names (e.g. www.google.com) to IP addresses (e.g. 139.130.4.5)

DNS servers exist around the world and contain databases of pairs of domain names and IP addresses, over which that individual server has the relevant information stored such as webpage files.

Domain names contain a hierarchy:

1. Top level

2. Second level

3. Domain name

4. Subdomain name

5. Host name - ROOT DOMAIN

BBC games developer Simon Lumb explains how DNS is a large database of web addresses (CLICK HERE) to listen

Task

Create a poster explaining about DNS and Domain Name Hierarchy strructure for the following webaddress:

www.gosford-hill-oxon.sch.uk

The Internet Structure

TCP/IP

TCP/IP has become the global standard suite of networking protocols, these operate in a four layer stack
The TCP/IP stack is a set of rules used in turn, to format a message so it can be sent over a network
Each layer provides a specific function within the transmission of the message

The 4 layers can be remembered by using a mnemonic such as:

Angry Terrorists Irritate Llamas

Application, Transport, Internet/network, Link

APPLICATION

Ensures the data is formatted in a way appropriate to the type of application/data being sent, so that it can be understood by the recipient
Ensures the correct application protocols are used (e.g. http)
Thus allows the end user to interact with the application once it’s received
DNS (Domain Name System) resolves the email/website address to an IP address if required

TRANSPORT

When sending:

A virtual connection between the two communicating devices is established using a handshake signal
The appropriate port numbers (used to identify the protocols required and hence the types of application) are added
Defines the level of service and the connection status

When receiving:

Packets are reassembled in the right order
High-level error checking takes place
Packets with errors/Missing packets are re-requested (TCP only)

INTERNET / NETWORK

When sending:

The data is broken down into packets, which are numbered based on their order
Adds the IP address to port numbers, creating a socket address – one for the sender’s information, another for the recipient’s
This is added to each packet’s source and destination addresses
This ensures the recipient knows where to send a confirmation
The packets are now datagrams

When receiving:

The IP address is removed from the packets

LINK

When sending:

Adds MAC addresses, identifying the sender’s and recipient’s unique network interface cards, to packets in a frame header and footer
Packets are moved over the communication medium via connected routers

When receiving:

Headers/footers removed from packets
Low-level error checking takes place

Sending: Application -> Transport -> Network/Internet -> Link/Network interface

Receiving: Link/Network interface -> Network/Internet -> Transport -> Application

Task

Read through the teacher presentation about TCP/IP (CLICK HERE)

2. Watch the 11min video from a teacher called Mr Clarkson who explains about the TCP/IP stacks (below)

IP Stack_360P.mp4

LANs and WANs

Local Area Network

A client – server network based in a single location is known as a Local Area Network (LAN). LAN networks are usually located in a single building or small cluster of buildings.

Wide Area Network

Organisations which are based across a wider site are usually made up of a number of individual Local Area Networks , but all these LANs would be joined together using a bridge, router or modem, in order to form a large Wide Area Network.

This allows multiple LANs to:

Share access to devices such as printers and scanners
Share file access across multiple LANs
Share access to a single internet service.

The internet is an example of a very, very large WAN.

Task

Answer the following question:

Why is a computer network like a family? What are the two types of networks?

Using complete sentences, give at least seven sentences to answer these questions:

1.What is a computer network?

2.Explain what the two types of networks are, giving examples.

Extension: give 3 advantages and 3 disadvantages of computer networks...

Data Packets

When sending across a network, data is broken into chunks (units) called data packets which are assembled
again at the receiving end
This increases efficiency and reliability
Latency refers to the time taken for packets to travel from one location to another
A packet from London to Sydney may take 30ms to be received while a packet from London to Paris may only take 5ms
Packets include additional information called headers and trailers

Header

Sender and recipient IP addresses

The sender and the recipient’s IP addresses

Acts like a postcode, allowing the packet to be delivered to the correct destination and enabling the recipient device to trace where the packet came from.

Protocol being used

The protocol allows the recipient computer to understand how to interpret the packet.

Order of the packets

Upon arriving at the recipient device, packets are reconstructed in the appropriate order as specified in the header.

Time To Live

The Time To Live (TTL), tells the packet when to expire so that it does not travel forever.

Payload

Raw data to be transmitted

Trailer

Checksum or cyclic redundancy check The trailer contains a code used to detect whether any errors have occurred during transmission.

Circuit Switching

Circuit Switching is a dedicated circuit connection that is established between the two nodes. This connection is reserved (it cannot be used by anyone else) and remains open until the communication is complete.

Packets flowing between the nodes will always follow this circuit; hence all packets are sent and received chronologically

The recipient sends a response every time a packet is received
If the circuit is broken, the connection is lost completely unless it can be rerouted
Used for time-critical transmissions (where data must arrive immediately)

Packet Switching

All packets are sent individually and hence may take any route to the destination, including via other routers that redirect them. Since packets may not arrive chronologically, they’re labeled with their order so they can be reassembled when received. Packets that do not arrive in time will be re-requested (only with TCP protocols)

BBC Bitesize 'Packet Switching'.mp4

BBC Bitesize Packet Switching explained

Circuit Switching vs. Packet Switching_720P HD.mp4

Task

Watch the video circuit vs packet switching

Read the presentation about circuit Vs packet switching

(CLICK HERE)

Complete the advantages and disadvantages worksheet (if your teacher has not provided you a copy in Google Classroom- please make a copy)

Worksheet (CLICK HERE)

Network Security

Firewalls

A firewall is a device designed to prevent unauthorised access to a network. A firewall consists of two network interface cards (NICs) between the user and the Internet. The firewall passes the packets between these two NICs and compares them against a set of rules set by the firewall software. The preconfigured rules are called packet filters

Packet filtering / static filtering limits network access in accordance with administrator rules and policies. It works by examining the source IP, destination IP and the protocols being used as well as the ports being requested.

When access is denied by a firewall, two things can occur. The packet can either be dropped or rejected. A rejected packet sends an alert to the sender to notify them of the error whereas a dropped packet will not.(if your names not down you're not coming in)

Encryption

Encryption is a way of keeping data secure when transmitting it over the Internet. Encryption makes data unreadable if it is intercepted. Data is encrypted and decrypted using a set of keys.

Encryption is a way of mapping data so its unreadable if intercepted

More details about encryption is covered in 1.3.1 Compression, Encryption and Hashing

Network Hardware

Network interface cards (NIC) A network interface card is the card required to connect a device to a network. This is usually built into the device and assigns a unique media access control (MAC) address to each device. The MAC address is a 48-bit value coded into the device and is usually written as a twelve-digit hexadecimal number.

Switch A switch is a device used to direct the flow of data across a network. Switches are most commonly used in networks using a star topology.

Wireless Access Point (WAP) This is a device that allows a device to connect to a network. It is more commonly combined with a router to enable internet access. These are used in mesh networks

Client-Server and Peer-to-Peer

Client-server networks consist of terminals known as clients connected to a server. The server is a powerful, central computer. The server holds all of the important information and resources and has greater processing power than the terminals. Clients can request to use the server

Advantages of Client-server

Disadvantages of Client-server

More secure as data is stored in one location
Central backups are carried out so there is no need for individual backups
Data and resources can be shared between clients

Relatively expensive to set up
The functionality of terminals depends on the server; if this fails, performance falls
Trained staff are required to maintain the server

Peer-to-Peer A network in which computers are connected to each other so that they can share files. Each device effectively acts as both a server and client, as it can both provide and request resources. Peer-to-peer networks are used in piracy, since it’s almost impossible to trace the origin of files.

Advantages of Peer-to-peer

Disadvantages of Peer-to-peer

Cheaper to set up
Allows users to share resources
Easy to maintain
Not dependent on a central server
Specialist staff are not required

Impossible to trace the origin of files
Backups must be performed separately
Poorer security
May be difficult to locate resources

Task

Read the presentation about client server Vs Peer to Peer networks - (CLICK HERE)

PAST PAPER QUESTIONS

Try and answer the past paper exam questions -You can write your answers on paper or print out the exam paper - Mark Scheme is provided at the end of the paper questions. (try not to look at the answers before attempting all questions)

Exam_Questions_-1.3.3_Networks.pdf

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