Theory of data communication

BTEC IT Practitioners Unit 8

Communication Technologies

This assessment provides the opportunity to meet the grading criteria for the unit as indicated in the grid below.

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If you copy from someone else or allow another candidate to copy from you, or if you cheat in any way you may be disqualified from at least the assignment concerned.

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I have read and understood the above statements. I have produced the work without help except for help from my subject lecturer and the help which I have declared in the work itself. I have acknowledged all source material in the work itself.

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__________________________(Learner) Date ____________________

To be read in conjunction with the Grading Criteria Grid for the unit and the Assignment General Instructions in the Student Handbook.–

The theory of data communications

WHAT YOU WILL LEARN IN THIS ASSIGNMENT

types of communication devices, signal theory, error reduction techniques, protocols, OSI and TCP/IP models

ASSIGNMENT OBJECTIVES

In this assignment you will need to show that you:

know the main elements of data communications
understand transmission protocols and models

WHAT YOU WILL DO IN THIS ASSIGNMENT

In this assignment you will:

explain the theory that lies behind data communications, including the basic types of communication devices, signal theory, protocols and error reduction techniques
describe protocols in common use and explain their importance
compare the OSI and TCP/IP protocol models

TASK INTRODUCTION

You have taken a job as a junior technician at NetCo, a company that makes networking hardware (switches, routers, etc.). They are planning to set up a networking academy (rather like the Cisco Network Academy) and create related qualifications. The project manager has asked you to prepare some learning materials for ‘Chapter 1’ of the online course. Chapter 1 is entitled ‘The Theory of Data Communications’. The chapter is subdivided into a number of sections. The materials can be produced in the form of a Word document or a series of linked web pages.

TASK 1 AND GRADING CRITERIA COVERED

Task 1a

P1 Identify and explain types of communication devices

P2 Explain the principles of signal theory

Write suitable materials for Chapter 1, section 1 which explains the purpose and workings of basic communication devices and the principles of signal theory.

Task 1b

M1 Explain techniques that can be used to reduce errors in transmissions

Add a section 2 to your guide (entitled ‘Error detection and correction’) which explains the techniques that can be used to reduce data transmission errors.

TASK 2 AND GRADING CRITERIA COVERED

Task 2

P3 Describe communication protocols used and explain why they are important

D1 Critically compare the OSI seven layer model and the TCP/IP model

Write section 3 (entitled ‘Communication protocols’), which should include a description of the communication protocols in common use and explain why protocols are important. Write a detailed comparison of the OSI seven layer model and the TCP/IP model, commenting on the approach taken and the suitability of each of the models.

GUIDANCE NOTES

This assignment is very much about explaining the basic theory that lies behind communication technology. There are lots of sources for this information in books and on the internet, but remember what you write must be in your own words; you cannot copy material directly from these sources.

For Task 1a you will need to list the various types of communication devices and explain how they are used. Note that the unit content makes a distinction between ‘general’ communication devices, network components (servers, workstations, NICs, etc.) and interconnection devices (hubs, routers, switches, etc.). For this task (covering P1) you should be describing general communication devices, not specific components which are covered in Assignment 3. In practice, it might be quite difficult to make this distinction, but don’t spend too much time describing network components in this assignment.

In terms of communication devices, the unit content mentions wired devices, such as DTE devices (e.g., a computer) and DCE devices (such as a modem), and wireless devices (such as mobile phones, PDAs and laptops, etc.). You need to explain in outline the way these devices can be connected to a network.

To cover P2 an explanation of digital signal theory is required. Refer again to the unit content to see the issues you must cover.

For Task 1b you need to write an explanation of the techniques used for error reduction. The only technique mentioned in the unit ...

This is a preview of the whole essay

To cover P2 an explanation of digital signal theory is required. Refer again to the unit content to see the issues you must cover.

For Task 1b you need to write an explanation of the techniques used for error reduction. The only technique mentioned in the unit content is checksums, but other simple techniques such as parity might also be covered.

For Task 2 an explanation of what protocols are, why they are important and examples of ones in common use are required.

To achieve D1 you will need to write a detailed comparison of the OSI and the TCP/IP models. This will need to point out the similarities and differences between the two models.

LEARNER INSTRUCTIO

Table of Content

Assignment 8.1: The theory of data communications

I recently just got a job as a junior technician at NetCo; NetCo is a company that manufactures networking hardware such as switches, routers, etc... They’re planning to start up a networking academy (rather like the Cisco Network Academy) and create related qualifications. The project manager has approached me so as to prepare some learning materials for ‘Chapter 1’ of the online course. Chapter 1 is entitled ‘The Theory of Data Communications’. The chapter will be subdivided into a number of sections. And I’ve decided that the materials shall be produced in the form of a Word document.

Chapter 1

Section1

Identify and explain types of communication devices

There are different types of communication devices; Communication devices are electronic hardware that interacts (communicates) with each other to perform specific tasks. The technologies that are used to communicate are data terminal equipment (DTE) and data circuit-terminating equipment (DCE). Another type of communication devices would be Wireless devices. I plan to explain all the above in detail.

Data Terminal Equipment (DTE)

DTE is a form of expression for a device that is at the end of the line. Examples of this could be a modem, network interface card (NIC), mobile phone or Bluetooth. DTEs differ depending on its purpose, for example, with Bluetooth, size and range are an important feature and with mobile phones, the quality and bandwidth are more important. Network cards and modems exist in a range of formats that are designed for a wide range of speeds i.e. bandwidth depending on the type of network being used.

Data Circuit-terminating Equipment (DCE)

DCE and DTE work together by the DTE connecting to the Internet or network service offered by the DCE. This is network equipment which controls the communication. Some examples of these are:

A Bluetooth dongle in a PC, which is used to synchronise a mobile phone or PDA.
A switch in a communications/server room at school, college or work, which will connect all the computers to the Local Area Network (LAN) and the Internet

Wireless Devices

Wireless devices are devices which transmit their data over the air. Types of mobile communication devices would be third (3G) and (2G) group of mobile phones, wireless laptops and wireless PDAs. Wireless networks use the 802.11x standard. This is the IEEE standard (Institute of Electrical and Electronics Engineers) which defines the speed of the network and its range. The mobile phone network is a large connection of stations throughout the nation and internationally which allows subscribers to the service to communicate through low bandwidth voice system.

The principles of signal theory

Data

The smallest portion of data is a bit. The word bit comes from Binary Digit which is either a 0 or 1. 0 or 1 means Off or On. This controls hardware within communication devices to perform operations. When bits are grouped together the can become more useful. For example, 8 bits of data will form 1 Byte.

Denary to Binary

Denary means 10 numbers. In our standard number system we have 10 numbers. These are:

0,1,2,3,4,5,6,7,8,9.

Now to convert Denary to Binary, you must know the base 2 rules. These are similar to units, tens, hundreds that primary school children are taught. So for example say…

I want to convert the denary number 37 into binary. I would do this by taking the number and dividing it by 2 each time:

Now what I do is take the remainders from the bottom of the table going up:

100101.

With this I will now do 2 to the power of X, X being 0,1,2,3,4,5,6 etc.

25 24 23 22 21 20 - This is the 2x

1 0 0 1 0 1 - This is the binary for 37

32 16 8 4 2 1 - This is the base 2 rule

Now to check that the binary is correct I will take the base 2 numbers where the binary is a 1 and add them together. I should get the denary number:

32+4+1=37

This is how denary is converted to binary.

Data Packets

In networking, data must be formed in a package to be able to be transported over a network. Package formats vary for different types of networks. However the term data packet is always used to describe a package.

A packet will contain a minimum of the foundation address, the destination address, data and error control. The foundation address is sent so the destination knows who sent the data. The destination address is used to find the right destination. Data is the actual information being sent. Error control is sent to help identify problems with the data once it has arrived at its destination (see chapter 1b for error detection techniques).

Asynchronous and Synchronous Transmissions

Asynchronous transmission means that the receiving device must accept the receipt of data before the source device will send more data.

In synchronous transmission, both devices will synchronise with each other before any data is sent.

Bandwidth (Analogue)

Analogue signals are sine waves which look like the waveform in image 1.

Sine waves are known as analogue signals is because they are an analogy of sound waves which travel in the same way.

Analogue bandwidth is the number is cycles that occurs with the sine wave over a period of time, for example in image 1, the wave is 1Hz and occurred within 20ms so the bandwidth is 1Hzpms (1 Hertz per millisecond).

So, finally, the bandwidth of an analogue signal is the difference in frequency between the highest and lowest frequencies contained in the signal.

Bandwidth (Digital)

Digital bandwidth is different to analogue bandwidth because digital signals are either 0 (off) or 1 (on) and look like squares (see image 1.2).

Digital bandwidth is the quantity of data that can be sent through a transmission over a specified period of time. These are generally measured in Bits per second (bps), Kilobits per second (kbps), Megabits per second (mbps) and Giga bits per second (gbps).

Section2 Error detection and correction

Parity (Odd and Even)

Even - Parity checking is usually used for Bytes of data. A parity bit is added to every Byte of data transmitted, whether the ninth bit is ‘0’ or ‘1’ depends on the combination of the other eight bits. If there is an uneven number of ‘1’s then the parity bit will be a ‘1’, if there is an even number of ‘1’s the parity bit will be a ‘0’. There should always be an even number of ‘1’s in the data. For example:

1011001 Parity bit = 0

Odd – This works the same way as even parity checking, however, instead of the ‘1’s equalling an even number, they should equal an odd number. For example: 1011001 Parity bit = 1

Parity checking is easy to apply and can detect errors, however, it cannot correct errors, therefore, if an error is detected the data would need to be transmitted again. This method does not always work, this is because only an odd number of bits in error can be detected, if there is an even number of bits but there is still an error, this will not be detected. For example:

1011001 could be transmitted as 1100011. This is not the way that the data should have been transmitted but the error would not be detected because the data has an even number of bits in error.

Cyclic Redundancy Check (CRC)

Cyclic Redundancy Check is an error-detecting system. It performs a long division equation where the answer is thrown away and the remainder becomes the result. The data is then transferred and the same equation is done at the other end, if the remainder of the equation isn’t the same as the receipt, the data is corrupt and therefore rejected.

Chapter 2

Section3 Communications Protocols

Hyper Text Transfer Protocol (HTTP)

Hyper Text Transfer Protocol is a set of rules that order the distribution of information over the internet. Its use to retrieve hypertext documents, which are interlinked text documents, led to the development of the World Wide Web.

HTTP is a request and response made by the client and server, the client being the end user and the server being the web site. A client that makes a HTTP request using a web browser is known as a User Agent. The server, which stores resources such as HTML files or images, is called the Origin Server. Between the User Agent and the Origin Server may be intermediaries i.e. a negotiator who acts as a link between parties and examples are proxies, tunnels and gateways. HTTP is not controlled by the TCP/IP model and is the most popular application on the internet.

Transmission Control Protocol/Internet Protocol (TCP/IP)

The Internet Protocol Suite (or TCP/IP) is a set of protocols for communication used for the internet and similar networks. Its name derives from the two most important protocols in it, Transmission Control Protocol (TCP) and Internet Protocol (IP), which are the first protocols in the Internet Protocol Suite standard.

The Internet Protocol Suite may be viewed as a set of layers that work together in the transportation of data from one place to another. The TCP/IP model consists of four layers, from lowest to highest they are the:

Link Layer
Internet Layer
Transport Layer
Application Layer

File Transfer Protocol (FTP)

File Transfer Protocol is a protocol that is used to transfer data from one computer to another computer or device. FTP is a protocol for file transfer which allows the transfer and change of files over a TCP network. A FTP client connects to an FTP server and the client can then make changes, add or delete files on the FTP server. An example of an FTP client would be the webpage upload feature on Microsoft FrontPage. FTP has been designed so files can be transferred to any FTP server regardless of operating system type or version.

Open Systems Interconnection (OSI)

The Open Systems Interconnections Basic Reference Model (OSI Reference Model or OSI Model) is a description for layered communications and network protocol design. This was developed as part of the Open Systems Interconnection intelligence. Basically, network architecture has been divided into seven layers. From top to bottom, these layers are:

Application
Presentation
Session
Transport
Network
Data Link
Physical

This is known as the OSI Seven Layer Model.

Each layer is characterized by their formation. These provide services to the layer above and receive services from the layer below it. An example of this would be a layer that can provide error free communications across a network provides the path needed by applications above it. It then calls the next layer below it to send and receive packets that make up the contents.

Comparison between the OSI seven layer model and TCP/IP model

OSI Seven Layer Model

The OSI Seven Layer Model is a model for data transfer through layered communications allowing data from one PC to be able to be read by another PC anywhere in the world. This model packets the data into many layers, an example would be the layers of an onion, so it then transfers itself to another computer where it goes through the same seven layers but in the opposite direction to unwrap those layers.

See image 6 for the structure of an OSI Seven Layer Model.

These layers don’t communicate across to their ‘doppelganger’ layer on the other computer. If they wish to communicate, they have to go through all the other layers that stand between them and their ‘doppelganger’ layer.

This table to the right is basically explaining what each layer’s job is. (Table obtained from www.humanreffence.com under the download section)

TCP/IP Four Layer Model

The TCP/IP model captures data to provide idea of protocols and services. Data is captured in the same way as data is within the OSI Seven Layer Model, so data has to be captured on each level in the same way.

The OSI Seven Layer model and the TCP/IP model are both similar in that they both use layers to perform tasks. They also have 2 layers that are very similar, the Transport and Network layers.

The TCP/IP internet protocol suite has a wide range of protocols working at each layer of the OSI and TCP/IP models.

Critical Comparison

OSI and TCP/IP are both similar in a few ways, these being that they both work with layers to communicate, they are both a set of rules and they were also developed alongside.

The Session Layer (OSI)

In the OSI model, the Session layer allows two devices to hold on going communications called a session across a network. This is not found in TCP/IP. In TCP/IP, these characteristics are found in the Transport layer.

The Presentation Layer (OSI)

In the OSI model, the Presentation layer handles data format information for communications across a network. This is done by converting the information into a common format that both sides can understand. This is provided by the Application layer in the TCP/IP model.

The Application Layer (OSI)

In the OSI model, the Application layer is the top most layer of the model. It provides a set of interfaces for applications to gain access to services within a network as well as access to network services that support applications directly e.g:

OSI – FTAM, VT, MHS, DS, CMIP

TCP/IP – FTP, SMTP, DNS, SNMP

Although the idea of an application process is common within both, their approach to the task of constructing application entities is very different.

How OSI builds up applications

OSI demands that distributed applications operate over a strict hierarchy of layers and are constructed from a tool kit of standardised application service elements, making them compatible.

How TCP/IP builds up applications

In TCP/IP, each application is created on whatever set of functions it needs beyond end to end transport to support a distributed communications service.

Most of these processes build upon what it needs and assumes that only an underlying transport method (such as a datagram or connection) will be provided.

Transport Layer (OSI)

In OSI, the Transport layer takes the data that is going to be sent and breaks it down into individual packets that are sent and then reassembled by the Transport layer at the destination. It also provides a signalling service which tells the sender that the destination had received the data has been successfully received.

Transport Layer (TCP/IP)

In TCP/IP, the Transport layer introduces two transport protocols, TCP and UDP. TCP implements reliable transportation of data whereas UDP doesn’t.

The reason this is, is because TCP required acknowledgement to ensure that data is sent correctly and successfully. If an error is detected, the packet can be resent. UDP just broadcasts the data without any need for a reply from the destination therefore data transportation will be faster but will be less reliable as errors can occur.

Comparing Transport for both Models

Transport for both models is very similar as they both contain transfer protocols that required acknowledgement that data has been received by the destination. While this would be slower, this is more effective as data will always end up being sent correctly.

Network Vs. Internet.

Both OSI and TCP/IP support a connectionless network service. OSI’s CLNP is practically identical to the Internet’s IP. Both are best-effort-delivery protocols. They are virtually identical but the one major difference that sets them apart is that CLNP supports variable length addresses whereas IP supports fixed, 32-bit addresses.

Data Link/Physical Vs. Subnet

In OSI, the Data Link and Physical layers resemble directly to the Subnet layer of TCP/IP. Most of the time, the lower layers below the Network layer of TCP/IP is rarely discussed. During the creation of both OSI and TCP/IP, it was decided that the Network layer for TCP/IP would match the Data Link layer of OSI.