Communication model


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Communication model

  1. 1. Concept and Model of Communications Concept and Model of CommunicationsGeneral Communications: face-to-face conversation, write a letter, etc.Electronic Communications: telephone, wireless phone, TV, radar, etc. Our Focus  Computer CommunicationGeneral Communication Model S(t) T(t) Transmission Tr(t) Sd(t)Source Transmitter Receiver Destination SystemMicrophone Transformer Line/Cable Transformer SpeakerTelephone Encoder Fiber/Air Decoder EarphoneComputer Compress Satellite Uncompress ComputerScanner Modulator Network Demodulator PrinterBasic Communication Criteria: Performance, Reliability, Security
  2. 2. Simplified Communications Model (2)Source generates data to be transmittedTransmitter converts data into transmittable signalsTransmission System carries dataReceiver converts received signal into dataDestination takes incoming data from the receiver
  3. 3. Components of a data communications system l.Message The message is the information (data) to be communicated. Popular forms of information include text, numbers, pictures, audio, and video. 2. Sender The sender is the device that sends the data message. It can be a computer, workstation, telephone handset, video camera. 3. Receiver The receiver is the device that receives the message. It can be a computer, workstation, telephone handset, television. 4. Transmission medium The transmission medium is the physical path by which a message travels from sender to receiver. Some examples of transmission media include twisted-pair wire, coaxial cable, fiber-optic cable, and radio waves. 5. Protocol A protocol is a set of rules that govern data communications. It represents an agreement between the communicating devices. Without a protocol, two devices may be connected but not communicating, just as a person speaking French cannot be understood by a person who speaks only Japanese
  4. 4. Transmission Media Transmission Media A transmission medium: - a connection between a sender and a receiver - a signal can pass but with attenuation/distortion - a special system with a transmission bandwidthGuided (Wired) Media Unguided (Wireless) Media (lines) (air, vacuum, water, etc.)- Twisted pair (0~10MHz) - LF (30~300KHz, Navigation)- Coaxial cable (100K~500MHz) - MF/HF (300~3000KHz, AM/SW radio)- Optical fiber (180~370THz) - VHF (30~300MHz, TV & FM radio) - UHF (0.3~3GHz, TV, mobile phone) - SHF (3~30GHz, satellite, microwave) - EHF (30~300GHz, experimental com) - Infrared (no frequency allocation)
  5. 5. What is Data Communications?Exchange of digital information between two digital devices is data communication
  6. 6. Data TransmissionData transmission is the transfer of data from point-to-point often represented as an electromagnetic signal over a physical point-to- point or point-to-multipoint communication channelA communication channel refers to the medium used to convey information from a sender (or transmitter) to a receiver, and it can use fully or partially the medium.Examples of channels: copper wires, optical fibbers or wireless communication channels.
  7. 7. Requirements of Data Communications  At least Two Devices ready to communicate  A Transmission Medium  A set of Rules & Procedure for proper communication (Protocol)  Standard Data Representation  Transmission of bits either Serial or Parallel
  8. 8. case of Asynchronous TransmissionBit synchronisation using Start/stop bits inIn Synchronous Transmission the agreed pattern of FlagSignal encoding rules viz. NRZ or RZAnd other higher layer protocol
  9. 9. Data RepresentationsA group of bits are used to represent a character/number/special symbol/Control Characters5-bit code can represent 32 symbols (25=32)7-bit code can represent 128 symbols (27=128)8-bit code can represent 256 symbols (28=256)
  10. 10. Code Set A code set is the set of codes representing the symbols Very common code sets are : – ASCII : this is ANSI’s 7-bit American Standard Code for Information Interchange ASCII code(7-bit) is often used with an 8th bit known as parity bit used for detecting errors during Data Transmission
  11. 11. Parity bit is added to the Most Significant bit (MSB) – EBCDIC : this is IBM’s 8-bit Extended Binary Coded Decimal Interchange Code
  12. 12. ASCII Code ASCII is defined in ANSI X3.4 – Corresponding CCITT recommendation is IA5 (International Alphabet No.5) – ISO specification is ISO 646 Total 128 codes – 96 codes are graphic symbols (in row. 2~7 in code chart).  94 codes are printable
  13. 13.  And 2 codes viz. SPACE & DEL characters are non printable– 32 codes control symbols (row. 0 & 1 in code chart)  All are non printable
  14. 14. Parallel Transmission and Serial TransmissionParallel Transmission and Serial Transmission …011000110111010111… Segment the 0/1 ? stream into Sender Receiver N bits groups N N N N… 01…00 01…10 11…10 10…11 … Parallel Transmission Serial Transmission 0 0 0 1 1 1 1 1 0110001 1 0 Sender 0 0 Receiver Sender Receiver 0 0 0 0 0 0 1 1 1 P/S converter S/P converter 7 (N) bits are sent together 7 (N) bits are sent one after another 7 (N) lines are needed Only 1 line is needed
  15. 15. Parallel Transmission  Parallel transmission allows transfers of multiple data bits at the same time over separate media  In general, parallel transmission is used with a wired medium that uses multiple, independent wires  Furthermore, the signals on all wires are synchronized so that a bit travels across each of the wires at precisely the same time  Engineers use the term parallel to characterize the wiring 17
  16. 16. Parallel TransmissionThe figure omits two important details: (1) In addition to the parallel wires that each carry data, a parallel interface usually contains other wires that allow the sender and receiver to coordinate (2) To make installation and troubleshooting easy, the wires for a parallel transmission system are placed in a single physical cableA parallel mode of transmission has two chief advantages: (1) High speed: it can send N bits at the same time  a parallel interface can operate N times faster than an equivalent serial interface (2) Match to underlying hardware: Internally, computer and communication hardware uses parallel circuitry  a parallel interface matches the internal hardware well
  17. 17. Serial Transmission Serial transmission  sends one bit at a timeIt may seem that anyone would choose parallel transmission for high speeds  However, most communication systems use serial modeThere are two main reasons (1)serial networks can be extended over long distances at less cost (2)using only one physical wire means that there is never a timing problem caused by one wire being slightly longer than anotherSender and receiver must contain a hardware that converts data from the parallel form used in the device to the serial form used on the wire
  18. 18. Serial TransmissionThe hardware needed to convert data between an internal parallel form and a serial form can be straightforward or complexIn the simplest case, a single chip that is known as a Universal Asynchronous Receiver and Transmitter (UART) performs the conversionA related chip, Universal Synchronous-Asynchronous Receiver and Transmitter (USART) handles conversion for synchronous networks
  19. 19. Timing of Serial TransmissionSerial transmission mechanisms can be divided into three broad categories (depending on how transmissions are spaced in time): Asynchronous transmission can occur at any time with an arbitrary delay between the transmission of two data items Synchronous transmission occurs continuously with no gap between the transmission of two data items Isochronous transmission occurs at regular intervals with a fixed gap between the transmission of two data items 21
  20. 20. Asynchronous Transmission It is asynchronous if the system allows the physical medium to be idle for an arbitrary time between two transmissions The asynchronous style of communication is well-suited to applications that generate data at random  (e.g., a user typing on a keyboard or a user that clicks on a link) The disadvantage of asynchrony arises from the lack of coordination between sender and receiver  While the medium is idle, a receiver cannot know how long the medium will remain idle before more data arrives Asynchronous technologies usually arrange for a sender to transmit a few extra bits before each data item  to inform the receiver that a data transfer is starting  extra bits allow the receiver to synchronize with the incoming signal  the extra bits are known as a preamble or start bits
  21. 21. Synchronous TransmissionA synchronous mechanism transmits bits of data continually  with no idle time between bits  after transmitting the final bit of one data byte, the sender transmits a bit of the next data byteThe sender and receiver constantly remain synchronized  which means less synchronization overheadOn a synchronous system  each character is sent without start or stop bits Synchronous transmission:  A bit stream is segmented into relative large groups/blocks many characters or bytes  Add control bits at the beginning and end of each block  Frame = H_control_bits + characters (data_bits) + T_control_bits  No gap between two characters in a data block 25
  22. 22. Asynchronous and Synchronous TransmissionAsynchronous and Synchronous Transmission 1 0110001 0 1 1001100 0 1 0011101 0 1 1011100 0 Sender Receiver independent Stopwtch.ani Stopwtch.ani Con_bits 0110001 ... 0110001 1001100 0011101 1011100 Con_bits Sender Receiver synchronized Stopwtch.ani
  23. 23. Asynchronous Serial Transmission (RS232 Example) Because no signal lines are used to convey clock (timing) information, this method groups data together into a sequence of bits (five to eight), then prefixes them with a start bit and a stop bit. This is the method most widely used for PC or simple terminal serial communications. In asynchronous serial communication, the electrical interface is held in the mark position between characters. The start of transmission of a character is signaled by a drop in signal level to the space level. At this point, the receiver starts its clock. After one bit time (the start bit) come 8 bits of true data followed by one or more stop bits at the mark level. The receiver tries to sample the signal in the middle of each bit time. The byte will be read correctly if the line is still in the intended state when the last stop bit is read. Thus the transmitter and receiver only have to have approximately the same clock rate. A little arithmetic will show that for a 10 bit sequence, the last bit will be interpreted correctly even if the sender and receiver clocks differ by as much as 5%. It is relatively simple, and therefore inexpensive. However, it has a high overhead, in that each byte carries at least two extra bits: a 20% loss of line bandwidth.
  24. 24. Synchronous Serial Transmission (PS2 Example) The PS/2 mouse and keyboard implement a bidirectional synchronous serial protocol. The bus is "idle" when both lines are high (open-collector). This is the only state where the keyboard/mouse is allowed begin transmitting data. The host has ultimate control over the bus and may inhibit communication at any time by pulling the Clock line low. The device (slave) always generates the clock signal. If the host wants to send data, it must first inhibit communication from the device by pulling Clock low. The host then pulls Data low and releases Clock. This is the "Request-to-Send" state and signals the device to start generating clock pulses.  Summary: Bus States Data = high, Clock = high: Idle state. Data is transmited 1 byte at a time: Data = high, Clock = low: Communication Inhibited. •1 start bit. This is always 0. Data = low, Clock = high: Host Request-to-Send •8 data bits, least significant bit first. •1 parity bit (odd parity - The number of 1s in the data bits plus the parity bit always add up to an odd number. This is used for error detection.). •1 stop bit. This is always 1. •1 acknowledge bit (host-to-device communication only)
  25. 25. Simplex Transmission and Duplex Simplex Transmission and Duplex Transmission Transmission Direction of dataSimplex Device A Device BTransmission One can send and the other can receive Direction of data at time 1Half Duplex Device A Device BTransmission Direction of data at time 2 Both can send and receive but in different time Direction of data all the timeFull Duplex Device A Device BTransmission Both can send and receive simultaneously
  26. 26. SimplexIn simplex mode, the communication is unidirectional.Only one of the two devices on a link can transmit; the other can onlyreceiveKeyboards and traditional monitors are examples of simplex deviceskey-board can only introduce input; the monitor can only accept output.The simplex mode can use the entire capacity of the channel to send datain one direction
  27. 27. Half-duplexIn half-duplex mode, each station can both transmit and receive, butnot at the same time.When one device is sending, the other can only receive, and vice versaln a half-duplex transmission, the entire capacity of a channel is takenover by whichever of the two devices is transmitting at the time.In half-duplex, the entire capacity of the channel is taken over by thetransmitting (sending).Walkie-talkies and CB (citizens band) radios are both half-duplexsystems
  28. 28. Full-duplex In full-duplex mode both stations can transmit and receive simultaneouslyIn full-duplex mode, signals going in one direction share the capacity ofthe link with signals going in the other direction.This sharing can occur in two ways: either the link must contain twophysically separate transmission paths, one for sending and the otherfor receiving; or the capacity of the channel is divided between signalstraveling in both directions.One common example of full-duplex communication is the telephonenetwork. When two people are communicating by a telephone line, bothcan talk and listen at the same time
  29. 29. Communication Standards and Related OrganizationsCommunication Standards and Related Organizations Communications need standards for inter-operations of different devicesStandard Organizations:- ISO (International Standards Organization): ISO number- ITU (International Telecommunication Union): V.num & X.num- EIA (Electronic Industries Association): EIA-num- IEEE (Institute of Electronics Engineers): IEEE.num- ANSI (American National Standards Institute): ASCII, etc.- IETF (Internet Society and Internet Engineering Task Force): RFC num- W3C (World Wide Web Consortium): HTTP, HTML, XML, …- WAP Forum (Wireless Application Protocol): WAP-num