06.523 Data Comms 1

1. 06.523Data Comms 1 Steve Corich Lecture 1

2. Aim • To provide the students with a knowledge of the concepts of data communications and networks.

3. Learning Outcomes The student will be able to: • Identify the basic components of communication networks and describe their application. • Discuss the different types of physical transmission media and identify the issues relating to them. • Discuss the types of data transmission and the outline the issues relating to them. • Describe the functions and applications of character-oriented and bit-oriented protocols. • Describe the features of LANs and the functions of a selection of network-dependent protocols. • Discuss the facilities and identify the issues related to PSTN. • Explain the current and future trends of transmission media.

4. Assessment • Assignment (elements 1,2,3)Due date: 17 August 2001 • Final Exam (elements 1,2,3,4,5,6,7)

5. Early Communication • Dispatch riders • Carrier pigeons • Smoke signals • Semaphore • Signal lamps • Drums All these methods were insecure

6. Communication Today • Today the human race uses the electron as the carrier of information. • It is reliable, invisible and intangible and they exist in excessive numbers. • It is because we have mastered the control of the behaviour of these electrons that we have a modern communication system. • Communication by electronic networks is here to stay for the foreseeable future, until or unless some form of thought transfer becomes possible. • In this course you will be introduced to some of the technologies, techniques, devices and facilities found in a modern communication system

7. Communications Components • The basic hardware usually needed for a modern communication network includes: • Mainframes/Mini computer/Microcomputer. (Host) • Front end processors (generally with Mainframes) • Modems • Terminals and microcomputers. (client) •  A circuit. (cable/media)

8. Mainframe • In many large organisations a mainframe (sometimes loosely called a host computer) provides the organisation’s computing resource and plays a vital role in the distribution of information around the organisation. • Historically, a mainframe was selected for its ability to do fast number crunching, and its ability to process, store and retrieve data/information for many concurrent users. • The suitability of “big” machines to provide on-line real-time data communication networks depend upon the ability of the devices attached to them. • In our modern world the demands on host computers are so great that they have outpaced the mainframe’s ability to handle both its own tasks as well as those required for network communication. • Thus front end processors are attached to mainframes to share the tasks, usually in a specifically defined way.

9. Front End Processor • A front end processor is just a computer. It acts as a servant to a larger computer to which it is attached. Its function is to do what the main computer requires it to do. • The front end processor can take two forms: • A non-programmable, specifically designed product, whose function is to take over specified tasks from the mainframe • A programmable device designed to handle all input. The trend today is to remove all communication tasks from the mainframe and allocate them to other processors down the line.

10. FEP Components • The components of an FEP are: • Channel interface - a device for connecting the FEP to the mainframe. • Software, specially written to provide the necessary protocols (some software is now hard coded into chips built into the FEPs). They can act as protocol converters (changing one type of code into another) so that incompatible systems can communicate. • Ports. These are hardware devices used to link front end processors to modems that terminate each circuit. A specific FEP may have 10 to 200 ports or circuits.

11. Uses of FEP • Uses for Front End Processors include: • Protocol converters • Character/message assembly • data and message editing • message queuing and buffering • error control • message recording • multiplexing • task managing • auto switching to a backup host in the event of a catastrophe. • Network control - whereby control is exercised as from a central control mode where polling is done centrally or interrupt mode, where the central processor is interrupted when it needs assistance.

12. Analogue Transmission • The information waveform is encoded onto a carrier signal by varying (modulating) the amplitude of the carrier, proportional to the information waveform [Amplitude Modulation (AM)]. • The information waveform is encoded onto a carrier signal by varying (modulating) the frequency of the carrier, proportional to the information waveform [Frequency Modulation (FM)]. • The information waveform is encoded onto a carrier signal by varying (modulating) the phase of the carrier, proportional to the information waveform [Phase Modulation (PM)].

13. Modems • Modems get their name from the primary function they perform; ie they Modulate an outgoing digital signal from a computer into an analogue transmission over (say) a telephone line, and they Demodulate incoming analogue transmissions into the digital signal required by the computer. They always work in pairs - one at each end of the communications line. • Currently, telephone systems used in New Zealand use an analogue transmission system between the subscriber (customer) and the telephone exchange of the Common Carrier (the telephone service provider - normally Telecom New Zealand).

14. Modem Future • Modems, although a very common communication device, may be the first to die where Digital Subscriber Line (DSL) orIntegrated Services Digital Network (ISDN) is introduced. • With a digital telephone network there will be no need for modems. The signals will be sent digitally from node to node via the proposed digital ‘phone system . Expect this to take place early in the 2000s. Until then, modems can operate at great speeds sending data quickly from transmitting device to receiving device, but modems of speeds greater than 25,000 bits per second are of little use with our present analogue telephone system.

15. Shannon’s Law Shannon’s Law states that :- • Maximum bits/sec =Available Bandwidth x Log 2 ( 1 + Signal-to-noise ratio). •  Formula:C= B log 2 (1 +S/N) Signal-to-noise ratio (S/N) = 30dB (1000:1) Useable bandwidth of telephone= 2600 Hz B = Bandwidth of carrier Since C=B Log 2 (1+S/N)=2600 x Log 2 (1 + 1000)=25,900 bits/second.

16. Modem Limitations • The maximum speed at which the modem can operate is limited by: • the UART chip on the serial interface of the computer’s COM board. • the analogue telephone system. • the maximum speed of the modem.

17. Modem Types • Dumb Modems • This type of modem is not able to respond to a command language and everything has to be manually set on the modem before transmission/reception can be obtained. • Smart Modems • Are commanded to perform their functions by using a command syntax language, entered from the keyboard of the micro computer to which they are attached. • Short Haul Modems • It consist of an electronic black box and a pair of wires connecting the two modems. No modulation or demodulation takes place, but the black box obtains power from its connection to the RS232 circuit.

18. Modem Types • Digital Modems • This modem shapes the digital pulses and performs all auxilliary functions. Its function is to take a digital signal from a computer and convert it to a more precise digital shape. These modems are more properly referred to as channel service units(CSUs) or as data service units(DSUs). These are very high speed modems and are classified as T-1 to T-4. T-1 operates at 1,544,000 bits/sec. While T-3 operates at 44,376,000 bits/sec. • Wireless Modems • These modems transmit the signals through the air by radio means rather than through a cable. • Optical Modems • Transmitting on an optical fibre requires the use of an optical modem, in which the electrical signals are converted to pulses of light.

19. Personal computers • Personal Computers are devices which are self dependent and provide their own processing and storing facilities. The personal computer comprises of • Central Processing Unit.(CPU) • display device (Screen). • keyboard and mouse for direct user entry. • port for input/output (egs RS232, Printer port, USB, FIREWIRE) • Memory • operating system. • storage system (floppy and/or hard disk).

20. Workstations • Workstations are devices, which are similar to personal computers but are customised to do special tasks. • Workstations may or may not depend on an attached host computer to perform their function remotely. • Workstations are specialised in that they may be optimised for mathematical modelling eg Computer Aided Design (CAD). They may be specially designed to perform tasks in a factory eg Computer Aided Manufacturing (CAM) in the car industry. These Workstations are programmed and modified by the operator to do some specific type of manufacturing. Eg. welding, cutting etc. Most workstations are capable of multi-tasking.

21. Terminals • Video Terminals or Dumb Terminals • devices consisting of a screen and a keyboard, attached to a host computer. This host is usually at some remote site. On the video screen is a marker, known as the cursor, which indicates the position of the next information. The keyboard is for entry of data. They are common at airline terminals and flight reservation centres. All processing for video terminals is performed on the remote host computer to which they are attached. • Teleprinter Terminals • like video terminals but have no display device. Teleprinters have typewriter type keyboards, and are not capable of being programmed. Some teleprinters have self diagnostic features to check for problems within the device. They print at very high speeds and are usually dot matrix devices.

22. Facsimile • Facsimile devices send and receive graphic copies of documentation over the telephone network. They have alpha numeric keyboards allowing the user to key in a destination number for transmitting the data and other features for storing, duplicating, and printing the received/sent information. • They are divided into four  groups or types. Group 1-3 are analogue devices, each an improvement on the last, with Group 4 (G4) being a digital device. • Facsimile devices are also built on expansion cards to be fitted within personal computers. In this situation they are usually incorporated within the modem where they are known as fax modems.