CS33 : It's a connected world!


In the days before networks were commonly available, the world of computing was a little dull. Nowadays, computers are fastened together by default. In this section, we will learn some of the basic methods by which computers are fastened together and the characteristics of how they operate.

We are learning ...
  • About the ways in which computers are connected together.
So that we can ...
  • Describe networking topologies and standards
    - Star, bus
    - Handshaking
    - Carrier Sense Multiple Access with Collision Detection (CSMA/CD(A))
    - Request To Send / Clear To Send (RTS / CTS)
  • Explain the operation of the WiFi protocol
    - SSID
    - Wireless security
  • Describe communication methods
    - Serial and parallel
    - Simplex, half-duplex and full duplex
    - Multiplexing and switching
    - Synchronous and asynchronous transmission / starts and stop bits
  • State and explain the characteristics of communication
    - Bit rate / Baud rate
    - Bandwidth
    - Latency
    - Protocols
    Calculate data transfer rates on a network
  • Explain the difference between peer-to-peer and client-server networks
    - Explain the terms 'peer-to-peer' and 'client-server'
  • Discuss thick and thin client systems



Activity 1 Introductory fun

Before we start, it's time to delve into the past and watch a pivotal video - "A Communications Primer". Produced by Charles and Ray Eames in 1953, it was intended to introduce the concepts of communication to designers and architects of the time to encourage them to incorporate ideas of communication into their work. Definitely worth watching as it introduces and explains lots of the concepts that we are learning about here.

22 minutes of retro joy :)

Networked devices can be arranged in a Local Area Network (LAN) where all the devices are in one geographical area or in a Wide Area Network (WAN) where the devices are spread out over a wider geographical area. The advantages of networking (sharing hardware, software and other resources, communication, backup, user administration) far outweigh the disadvantages (dependency, bottlenecks, security) in most cases. Networking enable interoperability but requires tight control over communications protocols.


Task 1.1
 Fundamental FUN!

Visit the following websites and have a go at the tasks they offers. They *should* work OK - the first one uses Java and might give trouble if your Java version is out of date and the site is not in the exceptions list. Nevertheless ...
Connecting computers together in a LAN or a WAN (see above) has advantages and disadvantages. Discuss the advantages and disadvantages with a partner and write down your ideas.

OUTCOME : A better understanding of networking basics and the advantages and disadvantages of networking.



Activity 2 Connecting computers together

First, some definitions ...
  • Data communication is the transfer of data from one place to another
  • Computers can only communicated data in binary format
  • Transmission media can be guided or unguided - physically different but logically the same.

Task 2.1
 Copying a diagram

Copy the following diagram into your notebooks.


Use Google Images to find diagrams / images of the different connectivity methods and perform a traditional cut and stick activity in your notebooks.

Extension : You may wish to take a look at this website which compares fibre optics to copper, it's nice.

OUTCOME : A nice diagram in your notes which includes images which represent the different connectivity methods.



Data is transmitted in binary form, though not like water in a pipe

We can't physically send '1's and '0's down a wire, so how is data transmitted? Logically, there has to be some way of representing '1's and '0's on a wire - voltage levels on a DC line or signal amplitudes on an AC line are often used for this. In so called 'uni-polar encoding' ...
  • A 'high' voltage (normally 5V) or a high signal amplitude is used to represent a '1'
  • A 'low' voltage (normally 0V) or a low signal amplitude is used to represent a '0'
The voltage 'travels' from the sender to the receiver at around 230,000,000m/s; waves travel at the speed of light - so essentially, if I apply a high voltage to or an oscillation on a wire, it is sensed immediately at the other end. We need to get away from the idea that data 'travels' down wires like water through a pipe; more correctly, it is applied by the sender and sensed by the receiver. The 'pressure' is felt immediately it is applied :

https://drive.google.com/file/d/0B83yXMOilskaakFGZUVwb2FoVnM/view?usp=drive_web

However, since efficient transmission of data over moderate distances using DC voltages is not practical (and certainly not possible in unguided media or optical fibre), digital transmission usually requires the use of frequency or a combination of amplitude and phase modulation techniques and the presence of a MODEM (modulator / demodulator) for transmission along an analogue carrier.

Task 2.2
 Describing 'transmission'

In your notebooks : Describe 'transmission' in your own words.

OUTCOME : Your own description of 'transmission'.



Networking topologies

In the context of networking the term topology means "the logical shape, layout, configuration or structure of the connections that join devices to a network." There are two common network topologies - bus and star.

Click to enlarge


Bus and star networks have certain advantages and disadvantages :

Click to enlarge

Click to enlarge

Task 2.3
Notes on network topologies

Write yourself some notes on bus and star network topologies only. Try to present your work in an individual manner. Don't just copy what I've given you :) You may wish to use some of the following web resources ...
OUTCOME : Notes on network topologies.



Networking hardware

In order to connect to any network, a device needs a Network Interface Card (NIC). The NIC contains the electronic circuitry required to connect the device to a wired (e.g. ethernet) or wireless network. A network interface card is a parallel to serial convertor.

Click to enlarge more

Each network interface adapter is identified by a MAC or Media Access Control address. The MAC address is unique amongst all NICs ever manufactured. Crazy. They look like this - 00:15:E9:2B:99:3C (48 bit).


In a bus network, the devices are connected to a common, high capacity wire called a bus. Devices on the bus are connected using a specialist device called a tap or, sometimes, a vampire tap.

Click to enlarge

In a star network, the devices are connected to a central device like a hub or a switch using a twisted pair cable.

Can you see why it's called 'Twisted Pair Cable'?

Task 2.4
 Investigating NIC and Twisted Pair

First, your teacher might show you an EtherLink XL PCI network card. Look at it. Buy one.

Next, have a go at wiring up an RJ45 Twisted Pair cable to a terminator plug. There are two styles of wiring - 'straight through' and 'crossover' ...


OUTCOME : Appreciation of the three common network connections and also your own RJ45 cable.

Hubs and switches

First, a bit of news from 2015 ...

Town Crier announces birth of Royal baby girl (0:48)
What has this got to do with networking?

Star networks always have a device at the centre of the star with multiple network ports. The choice is either a network hub or a network switch. Very few networks use hubs these days, for a very special reason ...

hub based star network
We don't like hubs 'cause they shout too much.

A switch based star network
We do like switches 'cause they only talk to the right node.

https://drive.google.com/file/d/0B83yXMOilskaN1Q3cldTdUFaY0E/view?usp=drive_web
Click to enlarge

Click to enlarge

Task 2.5
 Analogies

Often, using analogies help us to remember the function of devices. Match the following concept to the networking device they most closely resemble. They are not in the correct order!


You will have to discuss your ideas with a shoulder partner but do not use the World Wide Web. You should be able to describe their functions based on these analogies alone.
  • Tram station in Manchester
  • American traffic cop
  • Conscierge in a hotel
  • Chinese whispers
  • An old telephone exchange
  • Connection between an island and the mainland
  • Town Crier
In your notebooks : Write down this list of these analogies and briefly justify your decisions.

OUTCOME : A vague awareness of the function of networking devices, plus some notes.



Wireless networks (WLAN and WWAN)

There are two commonly used wireless network types - WLAN or Wireless Local Area Network and WWAN or Wireless Wide Area Network. WLANs are generally used in domestic or business settings whereas WWAN is usually used to connect to the internet from a mobile device - a so called 'data connection'.

Watch & Learn - Introduction to Wireless Networking (4:30)

Tip 1 : The Wireless LAN uses a unique 32-bit code (name) called a Service Set Identifier or SSID to uniquely identify the network to the clients. The client network card is authorised to access the WLAN with a particular SSID.

Tip 2 : Wireless security helps to prevent unauthorised access to your network and encrypts network traffic

Task 2.6
 WiFi How-To Guide

Create a How-To guide to describe how to connect to a wireless network.

OUTCOME : How-To Guide


Bluetooth

Bluetooth is a short range communications protocol intended to allow the connection of peripheral devices to computer systems. It works in a similar frequency range to WiFi (2.4GHz) but only operates over a maximum of 30ft (10m). Development of the protocol began in 1998 with the first devices being demonstrated in 2000. 

Task 2.7
 Harald Bluetooth

Read the following extract from the Wikipedia article on Bluetooth which explains where the name 'Bluetooth' and the logo comes from.

Click to enlarge

Now perform some of your own research into Harald - find out who he was and what he did. Why was Bluetooth named after him?

OUTCOME : Research into Harald Bluetooth and the connection with short range communication protocols.



Physical (and software) networking protocols

First, just to get a grip on exactly how many different networking protocols there are (and this is probably not a complete list), have a look at the document called Network Communication Protocols in the lesson resources or click on the image below (which opens the same document). You will probably have to download the PDF to view it correctly. How many protocols do you recognise? This list covers physical and software protocols.

https://drive.google.com/file/d/0B83yXMOilskabm1wSUc4cGw0N3c/view
Click to open document - you might have to download
it to view it properly because the writing is so small ...

Communication protocols cover such diverse areas as ...
  • How to map data values onto electrical signals
  • The purpose of different wires within a cable
  • The format of messages
  • Special control messages
  • Error detection and correction
  • How communications will be kept secure
  • Mode of transmission (synchronous, asynchronous, simplex, duplex)
  • Control of flow (handshaking)

One of the most common networking protocols (that we've briefly met before) is Ethernet. The following three videos introduce Ethernet and how it works. Whilst you do not need to know this for the examination, it gives you an idea of how data communication occurs on a shared medium - important for the next section on collision detection. Don't worry about references to the TCP/IP protocol stack - we'll meet this again at A Level.

Problems of communication over a shared medium (3:30)

How to build an Ethernet frame (5:21)

Minimal length of an Ethernet frame (4:22)

Essential goodies, yeah! The ethernet frame is composed of ...
  • Preamble : Synchronises the clocks on the machines in the LAN
  • MAC Header : Media Access Control address of the source and destination machines
  • EtherType : Indicates either type of ethernet or length of payload
  • Payload : Data to send between 46 and 1500 bytes
  • CRC Checksum : Checks integrity of data / can correct some errors.

Task 2.8
Day in the life of an Ethernet packet

Write a story entitled 'A day in the life of an Ethernet packet'. Your story should read like a fairytale. Use this website to inspire you.

OUTCOME : A fairytale staring 'Ethernet Packet'



Networking collisions and CSMA/CD and CSMA/CA

A collision on a bus network. How about playing the 'number shouting game' to help you understand!

During this section, we will learn about ...
  • Carrier Sense Multiple Access with Collision Detection (CSMA/CD)
  • Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)


When transmitting over a shared medium (bus networks and WiFi), there is always a possibility of interference of the signals which are transmitted by different devices. In order to remove the chances of these 'collisions' occurring, devices employ algorithms to (D)etect or (A)void them. First, watch the video about CSMA/CD.

Ethernet collision detection algorithm (7:51)

CSMA/CD is only used on wired networks. On a wireless shared medium, there is no way that the transmitting station can 'hear' interference and detect a collision. The wireless adapter can only monitor the wireless channel for traffic, waiting for a 'gap' before transmitting.

Task 2.9
Spot the difference?
  1. Visit the following website and read about the difference between CSMA/CD and CSMA/CA
  2. Watch the following Professor Messer video ...

    Understanding CSMA/CD and CSMA/CA (5:32)

  3. Download the worksheet CSMA(CD).docx and CSMA(CA).docx from the lesson resources and complete the activities contained therein.
  4. Ask your teacher for the solutions to the worksheet and assess your answer.
  5. The video discusses handshaking - RTS / CTS. Write some notes about handshaking.

OUTCOME : Completed flowcharts which describe CSMA/CD and CSMA/CA and some notes about handshaking.




Network segmentation

Sometimes, bus networks are segmented in order to reduce collisions and increase performance.

Segmenting a bus network using a network bridge
 
Networks are segmented based on IP Address Ranges (we'll not meet this concept until next year). The network bridge acts as a repeater and connects one part of the network to another. The bridge 'knows' which nodes are connected to which port (through their IP address or MAC address) and examines the data packets and routes them to the correct segment of the network. You can also connect network segments through a router. 

Networks are often segmented to aid administration.

Task 2.10 Segmentation

The following question is similar to ones found on past examination questions. The segmentation is achieved in these situations through the use of IP Address Ranges using a Router rather than a Network bridge, but the principle is the same.



Quick Facts : IP Addresses are unique addresses within a network composed of 4 dotted decimal digits in the range 0 to 255, for instance 192.168.5.100 or 214.56.2.44 which you will probably have seen before. There is a connection between the four values used in the IP address, but, for the purposes of questions like this, the only rule you need to obey is that the right most value is used to identify the machine on the network (the 'node') and its value can be anything other than 0 or 255 (because these values are reserved).

Click to enlarge and print!

In your notebooks : I have missed out four of the 'node' IDs (shown in black) and labelled them A, B, C and D. Your job is to suggest suitable node IDs to use. Print out the diagram and label it with the correct node IDs, then stick it in your notebook.

OUTCOME : Correctly allocated node ID to aid segmentation of a network.


Data Transfer Rate
(A Level Only)

The first thing to realise is that network data transfer rates are measured in multiples of bits per second whereas data storage is (normally) measured in multiples of bytes.


Of course, when the number of bit gets larger, we have to use prefixes for the data transfer ...
  • kb/s or kbps : Kilobits per second
  • Mb/s or Mbps : Megabits per second
  • Gb/s or Gbps : Gigabits per second

It's a very common mistake to quote networks speeds in multiples of BYTES per second but this is WRONG!

Typical data transfer rates which you might be familiar with include ...
  • Old style modems : 28kbps / 56kbps / 128kbps (if you were lucky!)
  • ADSL / Slow ethernet : Up to 10Mbps (a so called '10 meg connection')
  • Cable / FTTC : Up to 100Mbps, but typically 30-80Mbps
  • Fast ethernet : 100Mbps
  • Gigabit ethernet : 1000Mbps / 1Gbps
However, these are theoretical limits and in practice, very hard to achieve due attenuation / distortion (see below).

Task 2.11
 Measuring your internet speed

Visit a website which is designed to test your network speed like speedtest.net and use it to measure your download and your upload speed. Take a screenshot of the results and paste this into your notebooks.

Underneath, explain the meaning of 'data transfer rate' and specifically the units it is measure in.

OUTCOME : Evidence of your download and upload speed and an explanation of data transfer rates.


Logical and physical network topology

It's often easier to represent complex networks in a logical topology rather than a physical one. Logical topology removes the networking hardware from the network and merely shows the logical connections between the workstations and servers in a network.


Task 2.12
 Network topology challenge

Draw your own physical network on a piece of paper and give it to your shoulder partner for them to convert it from a physical topology to a logical topology.

OUTCOME : Physical to logical topology



Activity 3 Peer-to-Peer and Client-Server networks

In a peer-to-peer network, all networked devices exist at the same hierarchy. They can exist as both client (consuming data) and server (providing data) depending on the task. In a client-server network, there is one networked devices which acts above the other devices - acting as a controller and a server of data. The client devices merely consume.


Task 3.1
 Comparison of peer-to-peer and client / server networks.

These two network management models are inherently different. Compare and contrast peer-to-peer and client / server networks in the following areas ...
  • Type of user using the network
  • Size of organisation using the network
  • Administrator of the network
  • Who is responsible for security
  • Network traffic requirements
  • Cost of setting up and maintaining network
  • Scalability of the network
OUTCOME : Table of comparisons of these two network architectures




Activity 4 Thick and Thin Clients A Level Only


When you are setting up a client / server network, attention must be paid to where the bulk of the processing will be carried out. If the client is expected to handle many different operations and only rely on the server for user administration and centralised storage, then a thick client system might be appropriate. If the clients will only be expected to carry out simple, narrow operations, with the bulk of the processing occurring on the server, then a thin client system might be appropriate. Thin clients can be physical machines with limited or no secondary storage or can run as virtual machines / software machines.



Task 4.1
 Linux Terminal Server Project

This is an extended project idea which may take you a little / long time - maybe you could work in a team? You will need access to a number of redundant PCs / Raspberry Pi's / old laptops plus one slightly beefier machine which can act as the server. There are plenty of free, thin client software systems available, but one of the most popular is the Linux Terminal Server Project.

http://www.ltsp.org/
Click to visit the Linux Terminal Server Project

There are plenty of instructions out there on the web on how to get this thin client system up and running, including those on the LTSP website. However, I would refer you to a little known 'cookbook' written by Jean Jordaan called 'tuXlab' where he shows you how to set up not only the software required to implement an educational thin client system but also the hardware and the room it lives in (also available to download from the lesson resources courtesy of the Wayback Machine).

OUTCOME : Your own Linux Terminal Server Project network


Activity 5 Communication methods

Serial data transmission

Serial transmission methods have been around since the the early days of the telegraph. This video (from the 1930s) shows how a photograph is transmitted through a telephone line (from archive.org).


Serial transmission is analogous to a single track road where the 'data' is sent and received in sequence.

https://drive.google.com/file/d/0B83yXMOilskaRFJkMGl2eldDMTQ/view?usp=drive_web
Click to enlarge.

Simplex, half and full duplex

These terms refer to the direction that data communication occurs in.


Parallel data transmission

Another arty video - this time a proper art installation called "A Parallel Image" by Gebhard Sengmüller. If you are interested in finding out more, click on the link or look at the brochure in the lesson resources.

A Parallel Image (3:13)

In parallel data transmission, data is sent simultaneously along parallel wires. Parallel communication is only used for short range communication for a variety of reasons. In fact, it's not used often these days, even inside computers .(PATA > SATA).

https://drive.google.com/file/d/0B83yXMOilskaMXVkalp2SGRaVE0/view?usp=drive_web
Click to enlarge

Task 5.1
 Which is which?

Consider the following diagram of a set of (fairly old) motherboard connections. Your job is to identify which are serial and which are parallel and explain your decision You may wish to use the World Wide Web to help you.


OUTCOME : A copy of the image with an explanation of which connections are serial and which are parallel and why.



Synchronous and asynchronous transmission

https://drive.google.com/file/d/0B83yXMOilskacThXTTNjQm5EeTA/view?usp=drive_web
Click to enlarge

In synchronous mode, sending and receiving computers synchronise a clock during transmission using a timing signal. No demarcation in the data packets are required.

In asynchronous transmission, the transmitter and receiver are not kept synchronised. They are synchronised temporarily for the duration of each transmission. Asynchronous transmission is commonly used on low speed serial links which transmit individual characters separately rather than in groups as packets / frames of data. For example, it can be used on the serial link between a keyboard and a computer. The transmitter and receiver have separate clocks that run at (approximately) the same speed. Each character that is transmitted is sent with a start bit, stop bit(s) and parity bit which are used to ensure that the data is received correctly.

https://drive.google.com/file/d/0B83yXMOilskaeTE0Z09JR3lJSGM/view?usp=drive_web
Click to enlarge

Task 5.2
 Questions

In your notebook : Answer the following questions ...
  1. In synchronous transmission, how do the sender and receiver know when each bit start and stops?
  2. Why is a start and stop bit not required in synchronous transmission?
  3. Explain the use of the following in asynchronous transmission :
    - Start bit
    - Parity bit
    - Stop bit
  4. Can you explain why data transfer is more efficient in synchronous transmission that in asynchronous transmission?
  5. Follow the following link. Create a table of comparison of these two transmission methods.

OUTCOME : Answers to the questions, a comparison of synchronous and asynchronous transmission.


Activity 6 Characteristics of communication

Baud rate

Baud rate (unit Hz) is the rate at which signalling events occur in a communications channel. One baud is equivalent to one signalling event per second. For a modem, this could correspond to a change in frequency or phase whereas for a line code it could be a voltage pulse.


Bit rate

Bit rate (unit bits per second or bps) is the rate of bit transmission along a transmission media. It must be at least the same as the baud rate but can be higher ...

https://drive.google.com/file/d/0B83yXMOilskaa0R5Zy1vNV9BUjQ/view?usp=drive_web
Click to enlarge

The effects of attenuation and distortion make it difficult for multiple voltage signals like the ones above to be received reliably so techniques such as frequency and amplitude / phase modulation are often used instead.

Bandwidth

In electronics, bandwidth is the range of signal frequencies that a particular medium can transmit without significant reduction in signal strength. Essentially, this range of frequencies places a limit on the maximum bit rate (number of bits of data that can be transmitted per second) - a high bitrate signal can only be effectively transmitted over a high bandwidth channel.

60 Symbols Video - great explanation of bandwidth vs data rate (11:38)


Click to enlarge

Nyquist also determined that "the maximum theoretical bit rate for a transmission medium with bandwidth B Hertz is 2B bits per second, when binary data is being transmitted". So, for instance, an 8kbps bitrate signal (8000bps) can be be effectively transmitted over a channel with a 4kHz bandwidth.

Think back to the work we did on sound sampling. You have to take a binary sample of the height of a wave at at least twice the frequency of the wave in order to faithfully reproduce it. This is the opposite - one wavelength of the carrier signal holds two binary samples, so the wave carries data at twice it's frequency. See?

Nyquist theorem (20:11)

In computing, however, the term bandwidth is synonymous with bit rate.

Latency

Latency is the time delay between some event occurring and it's effects being felt. In communications terms, you only really notice this when you are watching satellite broadcasts from wartorn countries or in the old days when you were speaking to your friend in Australia using analogue phone technology.

Click to enlarge

Attenuation and distortion

Signals can be ...
  • Attenuated (made smaller) by distance travelled
  • Distorted (misshapen) by interference

Repeaters can be used to boost signal amplitude and error checking techniques like parity and Hamming codes can be used to detect and correct errors.

Task 6.1
 Mindmap

Create an informative mindmap of the information in this section.
  • Baudrate
  • Bitrate
  • Bandwidth
  • Latency
  • Attenuation / distortion
OUTCOME : Colourful mindmap



Extension Activities 

How about these?

What's next?

Before you hand your book in for checking, make sure you have completed all the work required and that your book is tidy and organised. Your book will be checked to make sure it is complete and you will be given a spicy grade for effort.

END OF TOPIC ASSESSMENT