BTEC Unit 8: The Theory of Data Communications

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 sued 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 commutation devices would be third (3G) and fourth (4G) generation cellular 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.

Explain the principles of signal theory

Data

The smallest element 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. Let’s do an example:

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 source address, the destination address, data and error control. The source 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).

Asynchronous and Synchronous Transmissions

Asynchronous transmission means that the receiving device must acknowledge 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 1.1.

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 1.1, the wave is 1Hz and occurred within 20ms so the bandwidth is 1Hzpms (1 Hertz per millisecond).

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), Kilo bits per second (kbps), Mega bits per second (mbps) and Giga bits per second (gbps).

Chapter 1b

Explain techniques that can be used to reduce errors in transmissions

Parity Checking

Parity Checking is where for every byte of data, an extra bit is added that the receipt will check for when data has been transmitted. Here is an example of how this works:

11011 (The bold 1 is the parity bit)

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     /  ← Lightning

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    V

11011 (The bold and struck through 1 means that bit was lost due to thunder)

Because the bit was lost, the receipt machine will see that there are an odd number of bits. The machine was told to look for an even number of bits and because there is an odd number, the data is corrupt and therefore rejected. There is a fatal flaw to this system, if 2 bits are lost within transmission, then the recipient will not recognise it as an error and will accept the corrupt data. While this is very rare, it is still a flaw in the system.

Cyclic Redundancy Check (CRC)

Cyclic Redundancy Check is an error-detecting system. It performs a long division equation where the answer is discarded 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 at the receipt, the data is corrupt and therefore rejected.

Chapter 2

Describe communication protocols used and explain why they are important

Hyper Text Transfer Protocol (HTTP)

Hyper Text Transfer Protocol is a set of rules that dictate 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 such as proxies, tunnels and gateways. HTTP is not constrained to 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. It’s 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 ‘Filezilla’. 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 initiative. 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 a collection of conceptually similar functions. 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.

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

OSI Seven Layer Model

The OSI Seven Layer Model is a conceptual 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 concept packets the data into many layers, a literal example being like the layers of an onion, and 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 2.0 for a diagram of the OSI Seven Layer Model.

These layers don’t communicate across to their ‘sister’ 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 ‘sister’ layer.

To the right is a table explaining basically what each layer’s job is. (Table obtained from www.wikipedia.org under the GNU Free Documentation License)

TCP/IP Four Layer Model

The TCP/IP model encapsulates data to provide abstraction of protocols and services. Data is encapsulated in the same way as data is within the OSI Seven Layer Model, so data has to be encapsulated 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 concurrently.

The Session Layer (OSI)

In the OSI model, the Session layer allows two devices to hold ongoing 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 generic 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, TELNET, DNS, SNMP

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

OSI Approach in Constructing Application Entities

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.

TCP/IP Approach in Constructing Application Entities

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 ACK (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 correspond directly to the Subnet layer of TCP/IP. Most of the time, the lower layers below the Network layer of TCP/IP are 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.