Introduction to NetworkingCT043-3-1

Data Communication Model and Architectures

Topics and Structure of the lesson

• Standards bodies: IEEE, ITU, ISO, IETF, IANA

• Open System Interconnection • Standard Internet Protocols• Implication of protocols and standards on

telecommunication systems

Learning Outcomes

• At the end of this lecture YOU should be able to:

• Explain and differentiate between analog and digital transmissions

• Identify 2 different techniques used for error detection

• List and describe the functionality of Data Communication components

• Understand the need for protocols

Key Terms you must be able to use:

If you have mastered this topic, you should be able to use the following terms correctly in your assignments and exams:

– Data Communication– Source– Medium– Sink– Protocol– Analog Signal– Digital Signal

Data Communication

• Data communication are the exchange of data between two devices via some form of transmission medium such as a wire cable.

• Data can be transmitted through a data communication system to a printer, a disk drive, a remotely located terminal or computer.

• The data communication system is actually a part of the telecommunication system that links together the capabilities of the computer.

Data Communication System Components

Main Teaching Points

Protocols

• A set of rules governing the data communication process

• What, how and when communication happens

• Can be described as the “language” of data communications

How Does E-Mail Reach It's Destination?

To demonstrate the components driving computer networks in general, and specifically - the Internet, let us assume that you are at home, writing an electronic mail message to your uncle, who works on the other side of the world. You know that once you send your mail, it will somehow make itself to your uncle's computer.

How exactly does this happen?

Step 1: Sending (uploading) mail

To connect to the Internet, you must have an account with an Internet Service Provider (ISP), a company that connects people to the Internet.

Connection from your computer to the ISP's computers will probably be made through the phone, using a device called a modem. The modem enables your computer to transmit digital data over the phone lines, which are analog.

The data passed between the two computers is decoded using computer protocols.  In this case, various modem specific protocols (such as X.25) are used, as well as protocols from the Internet protocol suite (such as PPP and SLIP). 

Your mail program sends your mail through the use of yet another protocol - the Simple Mail Transfer Protocol (SMTP)

Step 2: At the ISP's

The ISP's modem converts data from the phone line back to digital pulses, and it's server interprets the information. The server figures out the nature of you data, by decoding the SLIP/PPP data (in this case, the data is your e-mail). The ISP has a local area network (LAN) (or specfically, an Ethernet LAN). The LAN connects computer in the ISP's office space. Once the data is interpreted, the server first role is to figure out if the data you had just sent is destined for a computer in the ISP's LAN. In your particular, this isn't true, since your uncle's computer isn't even in the same country.

Data not destined for a computer connected to the ISP's LAN, is forwarded to the Wide Area Network (WAN) - the Internet. For security reasons, the ISP had installed a firewall, a device that keeps unwanted people from tampering with data on it's LAN. The firewall job is to receive all data going to/from the Internet, and filtering out it's "bad" part. The firewall, in this case, also serves as a router.

Notice that the firewall is connected at a great distance from the network bus.  Under normal circumstances, electrical signals won't be able to travel such a long distance.   But here we see that the ISP installed a repeater, a device that enhances the signals, thus allowing them to travel a longer distance.

Step 3: The WAN

Wide Area Network (WAN) is a general term that refers to a network that exceeds local boundaries. 

A WAN usually includes many LANs in it, which together form a huge network (as represented by the cloud). 

The world's most famous WAN is the Internet.

Step 4: At your uncle's office

Your uncle's office has a local area network that's connected directly to the internet through a router.  A router is a device that serves as a kind of electronic "yellow-pages" of computers.  Every computer on the internet has an "address", and routers are the devices that know how to get data to each computer using this address.

The LAN is also has a special connection to the Integrated Services Digital Network (ISDN - digital phone lines).  The is done by using a gateway - a device that "translates" between the "languages" of the LAN and ISDN.

Notice that the LAN in this office is arranged in a "star" topology (unlike the network we saw in the ISP).  This is a Token Ring Network 

Elements of Data Communication Network

• The three basic elements are source, medium and sink.– Source is the equipment sends data. It will encode the

information and transfer the encoded information into a form that can be carried by the medium.

– Medium can carry the data sent by the source to the sink.

– Sink is the equipment that receives the data carried by the medium. It will decode the information and transfer the information to its original or any other useful form.

Digital Information

• Digital information is stored in bytes.• Bytes can be used to represent anything in

the computer sense.• 256 different byte combinations can be used

to represent a variety of data.• Should a computer be equipped with a

program to translate these bytes into something meaningful, any number of variance is possible.

Signals

• In computer information systems, data is represented and processed in the forms of bits.

• To transmit this binary signal to another location, computers utilizes a system of signals to relay their information.

• These signals are usually a form of energy traveling along a physical medium.

Signal Formats

• Data and signals can be represented in either digital or analog formats.

• Digital is something that’s discrete in nature and has very identifiable points in its transformations.

• Analog represents those which are continuous. A stream of points with points in between.

Analog Signal

• A continuous wave form that changes by time.

• Has an infinite amount of point between point A and point B.

• It’s transformation between states is smooth in comparison to digital signals.

Digital Signal

• Has a limited number of values.• Often used to represent only 1 or 0.• There are no transitory points in digital

signals, only a finite number is agreed upon.

Interface Control

• A set of rules must be adhered to by both communicating parties to ensure that information being exchanged across a link is received and interpreted correctly.

• An interface standard is needed for both party to have same format, type and order of the message exchanged.

Flow Control

• If the amount of data to be transmitted is small, it is possible for the sending device to transmit all the data immediately because the receiving device will have sufficient space to hold the data.

• If the space is not sufficient, we have to adopt a method to control the flow of data.

• It is to make sure the receiver will not lose any of the transmitted data.

Error Control

• If data has to be transmitted for a long distance, it is likely the transmitted signal is corrupted.

• That is the signal level corresponding to a binary 0 is interpreted by the receiver as the level for a binary 1.

• Error control is a way to make sure that the receiver can identify the error data and can inform the transmitter to send the corrupted data again.

Error Detection Techniques

• Redundancy is a method of introducing extra bits to verify the integrity of the stream.

• These bits are appended to the stream, and once integrity has been verified, they are discarded.

• Error detection methods include:– Cyclic Redundancy Check– Checksum

Error Detection Techniques

• Any system which uses the addition of redundancy bits to the stream would use the same model.

Cyclic Redundancy Check

• CRC is the most effective of the redundancy checks, which also makes it the most complex.

• CRC does not work on addition, but on binary division.

• The logic of the system is that a data stream is divided through a predetermined divisor, and the CRC bits are appended to the data stream.

• On the receiving device, the same division occurs.

Checksum

• At the sending device, the checksum generator subdivides the data unit into equal segments of n bits (usually 16 bits).

• The segments are added using ones complement arithmetic.• The total is then complemented and appended to the end of the original data

unit as redundancy bits, called the checksum.

• Suppose the following block of 16 bits is to be sent using a checksum of 8 bits:10101001 00111001

• The number are added using ones complement:101010010011100111100010 sum00011101 checksum

• The pattern being sent is 10101001 00111001 00011101

Checksum

• At the sending device, the segments and the checksum are added using ones complement to get the sum.

• The sum is then complemented. If the result is zero, the data are accepted; otherwise they are rejected.

• Now suppose the receiver receives the pattern sent in previous example. When the receiver adds the three sections, it will get all 1s, which after complementing, is all 0s and shows that there is no error.10101001001110010001110111111111 sum00000000 means that there is no error

Summary

• Data Communication• Elements of Data Communication Network• Protocols• Flow Control• Error Control• Interface Control• Signals - Analog vs Digital• Digital Information

Question and Answer Session

Q & A