Topic 9 - Communication and networking

Question 1

What is the internet?

Answer 1

A global network of interconnected networks that communicate using a common set of standards and protocols

Question 2

What is the internet backbone?

Answer 2

A mesh of super-fast fibre optic cables and industrial routers that move data extremely quickly, most of which is provided by telecommunication companies

Question 3

How do most individuals and organisations connect to the internet?

Answer 3

Most connect through an Internet Service Provider (ISP - a company that gives consumers access to the internet), which could be through an ADSL connection (telephone line) or a dedicated fibre optic cable. If there is no physical cable, it could be via 3G, 4G or satellite

Question 4

How does an ISP give consumers access to the internet?
Describe its features

Answer 4

Provides a combined device providing an outgoing connection from your home network to the ISP.
Features:
- Usually has an ethernet switch with a few ports for wired devices
- Wireless Access Point (WAP) for wireless connections
- A router to manage the outgoing connection
- A modem (sometimes a separate box) converts the signal to a suitable type for outgoing media

Question 5

What is a WAN?
What is a LAN?

Answer 5

Wide Area Network - Several Local Area Networks (LANs) connected together by routers. Covers a large geographical area

A network covering a small geographical area, such as a home or small business

Question 6

How do most companies connect to the internet? Why?

Answer 6

Most connect using a Virtual Private Network (VPN) to provide a secure connection between sites. Allows users to send and receive data as if they were directly connected to the network, reducing costs

Question 7

What is a router?
What are the two types?

What is a gateway?

Answer 7

A networking device that forwards data packets between computer networks
Edge routers and Core routers

A piece of hardware connecting two networks with different transmission protocols

Question 8

What is an edge router?
What are the two types of edge router?

Answer 8

Links networks together
- Subscriber routers - small scale, low cost routers with 2 interfaces given to homes + small businesses. Can link network segments
- Enterprise routers - connect large businesses + ISP networks to the internet, more powerful, expensive + handle high volumes of data

Question 9

What is a core router?

Answer 9

Part of the internet backbone, with multiple interfaces that work at the highest speed simultaneously

Question 10

Describe data packets

Answer 10

Traffic on the internet is transported as packets
Internet packets are made up of the payload (the data being transported) and a header, containing information about the header, such as the protocol (type of data), source + destination addresses, packet length and time to live

Question 11

What is packet switching?
How does it ensure all packets reach their destination?

Answer 11

The method of moving data packets around the internet, where different packets from the same ‘conversation’ can be sent over different routes
The internet has an ‘end-to-end’ principle meaning the end points are responsible for checking that everything that has been sent is received. This type of communication is connectionless, so the internet is a connectionless network

Question 12

Describe the role of routers in packet switching

Answer 12

The key device - they examine incoming packets + forward them on to one of their interfaces until they reach their destination
Each router has a routing table - a set of rules that it uses to decide where to send an incoming packet. Each packet is treated individually with a separate routing decision
Each router-router link is a hope - router decides the next best hop for the packet
Routers can be configured to share data with each other so they learn best routes + are updated with any news of network traffic + infrastructure failure

Question 13

Packet switching - what happens if routers are experiencing high network traffic?

Answer 13

If they are receiving packets faster than they can route them on, packets are buffered in memory, causing delays. If buffering is severe, router can run out of memory and the packets are discarded.
IP packets contain a Type Of Service field in the header, letting them be marked with a priority level and request special treatment, but the router may choose to ignore it

Question 14

Why are the time to live counters in the headers of IP packets important?

Answer 14

If packets are sent to an unreachable destination address, routers may send them to a default device, which could also pass them on, causing an infinite loop.
The time to live counters prevent this, as each arrival at a router reduces it by one. If it reaches 0, the packet is discarded

Question 15

What is a protocol?
How are internet protocols arranged?

Answer 15

A set of rules that specify how two devices can communicate with each other
Internet protocols are arranged in a protocol stack - four layers sat on top of each other

Question 16

What handles most traffic on a
LAN?
WAN?

Answer 16

LAN - switches
WAN -routers

Question 17

How do protocols enable devices to communicate over a network?

Answer 17

Appropriate protocols are used to format the data from the application, turning into bits encoded as a suitable optical/radio/electrical frequency signal.
It is transmitted onto the media and the reverse happens at the receiving end
This is achieved by organising the process into 4 functional layers that data flows through at each end - The TCP/IP stack

Question 18

What is the TCP/IP stack?

Answer 18

Transmission Control Protocol / Internet Protocol
A set of internet protocols allowing users to communicate
Consists of 4 layers:
- Application layer
- Transport layer
- Internet layer
- Link layer

Question 19

What is encapsulation?
How does encapsulation work?

Answer 19

Preparing and passing the data by any upper layer in the protocol stack to the one below it

When sending data, each layer of the TCP/IP stack accepts data from the layer above, adds additional information (a header) and sometimes processes the data (e.g encryption) and passes it down to the layer below
The reverse happens at the receiving end, called decapsulation

Question 20

Describe the application layer of the TCP/IP stack

Answer 20

Uses protocols designed to work with different types of applications
Many specify the use of control messages to the applications at the other end, and the data is formatted as specified by the protocol - headers are added providing information about the data. At the end a termination message is sent

Question 21

Describe the transport layer of the TCP/IP stack

Answer 21

Breaks application data down into segments (TCP) or datagrams (UDP). Allocates sequence numbers + source / destination port numbers to the header
Can use TCP for reliable transmission or UDP for unreliable transmission
At the receiving end, transport layer must pass data to the correct application layer service, so source/destination port numbers are added to guide this

Question 22

What is a socket?

Answer 22

A TCP end point - a combination of an IP address and port number
Facilitates asynchronous communication - only 1 device can communicate at once

Question 23

Describe the use of TCP in the transport layer of the TCP/IP stack

Answer 23

Reliable transmission - used for applications that can’t lose data, e.g online banking.
Assigns sequence numbers to each packet, at the receiving end acknowledgements are sent for every successful packet
If the device doesn’t receive an acknowledgement it re-sends the packet

Question 24

Describe the use of UDP in the transport layer of the TCP/IP stack

Answer 24

User Datagram Protocol
Unreliable transmission - used when speed is more important than reliability, e.g live streaming

Question 25

Describe the internet layer of the TCP/IP stack

Answer 25

Uses the Internet Protocol (IP) to encapsulate segments from the transport layer into IP packets. Each packet is given a header with the source and destination IP addresses. The destination address is found using the DNS (Domain Name Service)

Question 26

Describe the link layer of the TCP/IP stack

Answer 26

The device may have a choice of many media to transmit on (e.g 4G, WiFi, ethernet etc) so either the OS of the device or the application chooses the most appropriate one, and the corresponding protocol is used.
Most common protocol on LANs is Ethernet protocol, which adds source and destination MAC addresses (unique addresses to each NIC) to the header.
This layer of encapsulation only remains for one hop - at each router the source/destination MAC addresses are updated

Question 27

What is a NIC?

Answer 27

Network Interface Card- a hardware component that connects a computer to a network

Question 28

What are the 7 application layer protocols and their port numbers?

Answer 28

• HTTP, Hypertext Transfer Protocol, port 80
• HTTPS, Hypertext Transfer Protocol Secure, port 443
• FTP, File Transfer Protocol, port 20 (control) / port 21 (data)
• SSH, Secure Shell, port 22
• POP3, Post Office protocol v3, port 110
• IMAP, Internet Message Access Protocol, port 143
• SMTP, Simple Mail Transfer Protocol, port 25

Question 29

Describe the HTTP protocol

Answer 29

Hypertext Markup Language (HTML) is used to mark up web pages
Runs on a client-server architecture - the web browser is the client that requests web pages and linked resources from the web server
- Server waits + listens for requests
- Client sends server a HTTP request for a web page
- Server receives it + sends a HTTP response which encapsulates the requested page
- Client receives response + displays web page in the web browser

For dynamic pages there are extra steps before the page is returned. Processing can also happen on the client side

Question 30

Describe the HTTPS protocol

Answer 30

Encrypts HTTP data exchanged between client + server by providing a digital certificate containing a public encryption key. The authenticity of the certificate is checked automatically in your browser. Your browser uses the public encryption key to encrypt the data and only the authorised server has the corresponding private key to decrypt it

Question 31

Describe the FTP protocol

Answer 31

Used to move files between 2 devices.
A client-server application - 1 device (client) runs FTP client software + is responsible for initiating connections. Another device runs the FTP server + has access to disk space to store files.
Once logged in, user can access directories in the server + download / upload files.
Traditionally it has a command-line interface but now there are GUI-based applications
Can also use anonymous FTP.

Question 32

Describe the SSH protocol

Answer 32

Provides an encrypted connection for remote access to a computer.
Encrypts messages between the SSH client and SSH server, so the network can’t see the destination address or nature of the data (protocol), only SSH data headed to a SSH server.
Commonly used by network administrators who need to access remote devices.
To connect to another computer using SSH you must have an account on the computer you’re trying to connect to + have sufficient administrative rights to carry out some operations
Can be accessed through command-line or a GUI-based SSH client

Question 33

Describe how emails are handled on the internet

Answer 33

Based on a client-server architecture
A user has access to their emails through an email application client on their device
Traditionally, when the user opens their email client it connects to their email server and downloads any new messages. Now most use ‘push’ technology where new emails are pushed to the client as soon as they arrive on the server

Question 34

Describe the POP3 protocol

Answer 34

Retrieves emails from the email server, downloading them onto the client device and then deleting them from the server

Question 35

Describe the IMAP protocol

Answer 35

Retrieves emails from the email server, creating copies on the local client device and synchronising any deletion on both client and server.
The preferred protocol because it lets you manage emails on multiple devices

Question 36

Describe the SMTP protcol

Answer 36

Used to send emails. Transfers the email from the client device to the linked mail server. If the recipient is on a different domain (email provider, SMTP is used to forward the email to the recipient’s mail server