CSN200 Introduction to Telecommunications, Winter 2000 Lecture_15 Data, Encoding and Transmission Data: What is Data? Any meaningful information is data. Otherwise it is noise or nonterrestrial data. Data in Electronic form: Any meaningful information can be represented into Electrical, Electromagnetic or Optical form for the purpose processing, storing and transmission. Electronic Data can be of two kind: Analogue and Digital Analogue Data - take on continuous values as time changes. E.g., Voice - uses microphone to convert to electrical form Video - uses video camera Temperature - uses sensor, like thermocouples Pressure - uses piezoelectric crystals Digital Data - take on discrete values, switching ON/OFF, text, integers, etc. Can be grouped into Bits - either a voltage or no voltage, 0 or 1 Bytes - group of 8 bits, like 0100 0001 is A in ASCII code What is Data (2nd time)? Normally we mean digital data processed by a computer. Signals: In a communication system, data are propagated from one point to another by means of electromagnetic signals. A signal is a series of analogue or digital data. That is a varying voltage (either continuous or discrete) with time in an electronic communication system. Definitions of Data and Signals: Analog Data - Data represented by a continuous physical quantity whose magnitude is proportional to a suitable function of the data (information). It can have more than two discrete values in contrast to digital data. Digital Data - Data represented by discrete values or conditions. Analogue Signals - A continuously varying electromagnetic wave that may be propagated over a variety of media. A series of Analog data. Digital Signal - A discrete or discontinuous signal, such as a sequence of voltage pulses. A series of digital data or bits. (Ref: Fig 5.1 ,4.2, 4.7, 4.10 Slyke) Lecture15.doc Page 1 (7) CSN200 Introduction to Telecommunications, Winter 2000 Lecture_15 Data, Encoding and Transmission Encoding: The simplest end-to-end data communications session between two PCs: PCs are digital electronic systems. PCs processes data in digital format (discrete voltages of electricity). To process or transmit data by the machines, it must be transformed from its humanly understandable form (letters, numbers, voice, images) to an electronically (digital) based machine-understandable form. Character Encoding: The process of transforming humanly readable characters into machine-readable code is known as character encoding. In this process, characters are turned into a series of 1s and 0s. Why 1s and 0s? Digital electronic system processes information (operates) by discrete voltages of electricity. These discrete voltages of electricity (2.8v and 0v) representing coded characters can then be processed and transmitted. The 1s and 0s are used as symbols to represent two discrete states. Characters can be encoded according to variety standards: ASCII (American Standard Code for Information Interchange) code uses 7 bits; used in PCs. EBDIC (Extended Binary Coded Decimal Interchange Code) uses 8 bits; used IBM mainframes. Unicode and ISO10646 use 16bits; Windows NT supports Unicode. Digital Encoding of Digital Data: (p. 108) Unipolar (the voltage always positive or negative, 0v for 0 and 5v for 1) Bipolar (fewer errors than unipolar signaling as changing the polarity of a current/voltage is more difficult than changing its magnitude, -5v for 0, +5v for 1) RZ (return to zero) NRZ (non return to zero) NRZ-L (NRZ, level) NRZ-I (NRZ, invert on ones) Biphase (all of the biphase techniques require at least one transition per bit time and may have as many as two transitions. Manchester (used in Ethernet LANs) - The midbit transition serves as a clocking mechanism and also as data. 0 - a high-to-low represents a 0, and 1 - A low-to-high represents a 1. Differential Manchester (used in Token-ring LANs) - The midbit transition is used only to provide clocking. 0 - A transition at the beginning of the bit period represents 0, and 1 - an absence of a transition at the beginning of the bit period represents 1. (Figure 4-3, p-111, Dennis; Figure 5-5, p-105, Slyke) Lecture15.doc Page 2 (7) CSN200 Introduction to Telecommunications, Winter 2000 Lecture_15 Data, Encoding and Transmission Data Transmission: Data transmission is the transfer of data between two electronic system on a suitable media. Both analog and digital data can be represented and propagated by either analog or digital signals. Both analog and digital signals can be transmitted on suitable media. Analog transmission - is the transmission of analog signals. Analog signals may represent either analog (voice) or digital (text) data. Digital transmission - is the transmission of digital signals. Digital signals may represent either analog (voice) or digital (text) data. (Ref: Table 5-1 Slyke) Four possible combination of data and transmission: Digital data, digital signal. In general, the equipment for encoding digital data into a digital signal is less complex than digital-to-analog equipment. Analog data, digital signal. Conversion of analog data to digital form permits the use of modern digital transmission and switching equipment. Digital data, analog signal. Some transmission media, such as optical fiber and satellite, will only propagate analog signals. Analog data, analog signal. Analogue data are easily converted to an analogue signals. Advantages and disadvantages of analog and digital transmission: • Digital signal suffer more from attenuation than do analog signals. • Digital signal can be propagated only a limited distance before attenuation endangers the integrity of data. • Analog signal can be transmitted to longer distances. • Analog transmission is affected by noise. • Digital signal can only be transmitted through Copper media. • Analog signal can be transmitted through Copper, Fiber and Satellite. Lecture15.doc Page 3 (7) CSN200 Introduction to Telecommunications, Winter 2000 Lecture_15 Data, Encoding and Transmission Parallel vs. Serial Transmission: Parallel: In parallel transmission all bits in a single character are transmitted simultaneously. Data travels in parallel (simultaneously) within a PC over the data bus (bundle of data lines). Similarly we can transmit data between two PCs in parallel. That is all data lines (normally 8) from one PC are connected to the other PC. Parallel transmission is primarily limited to transmission of data within a computer, between computers and between a computer and a printer. It is fast compared to serial transmission but limited to shorter distances. Serial: In serial transmission bits are transmitted in a linear fashion, one after the other. It is slower but can travel longer distances and is widely used. Serial and Parallel Transmission (p. 109) Serial Transmission is the predominant method of transferring information in data communications. Data bits are sent serially or sequentially bit-by-bit over a single wire or communications circuit. Sending bits one after another is slower than sending them in parallel but most long-distance circuits only allow serial transmission. It is the mode of communication used on a PC’s serial port as well as on local area networks. Parallel Transmission sends data bits in parallel (typically 8 bits at a time) using separate wires or circuits for each bit. This “byte-wide” transmission is faster than serial transmission but requires more wires in the cable and can not be used over long distances. It is not used very often in data communications but is used extensively for computer busses and for printer connections. (Figure 4-1, 4-2, p-110) Reception of Digital data: The reception of digital data involves sampling the incoming signal once per bit time to determine the binary value (0 or 1). To get the data correctly the receiver must know the arrival time and duration of each bit that it receives. One of the reasons is that all the digital system operates in steps with a stepper (also called a clock, which creates a stream of pulses). If the receiver's clock drifts by 1% and the transmission rate are 10 kbps, reception of a long stream of bits will go out of step and cause erroneous reception. Effect of noise on a digital signal: The erroneous reception is also caused by the transmission impairments. Within any communication system, the signal that is received will differ from the signal that is transmitted, due to various transmission impairments (attenuation, delay, noise, etc). Lecture15.doc Page 4 (7) CSN200 Introduction to Telecommunications, Winter 2000 Lecture_15 Data, Encoding and Transmission Asynchronous and Synchronous Transmission: (p. 148) The timing difficulties in data transmission can be avoided as follows: arrival time of a bit or packet - avoided by sending start/stop bits or start/stop preambles, and duration of each bit- avoided by synchronizing the clocks at both end, i.e., sampling the data at the right time. Two approaches are common for achieving the correct data. The first is called the asynchronous transmission and the second one is the synchronous transmission. Asynchronous transmission: The strategy with this scheme is to avoid the timing problem (associated with the drifts of the clock speeds at two ends) by not sending long, uninterrupted streams of bits. Instead, data are transmitted one character at a time, that way timing or synchronization are maintained within each character. That is done by synchronizing at the beginning of each new character, using start and stop bits. Though both the transmitter and the receiver agree upon one transmission speed, their bit time may vary slightly. But this slight variation will not affect the reception of a few individual bits. Asynchronous transmission is achieved by stoping the transmission before the error occurs because of timing drifts. In Asynchronous transmission a maximum of 9 bits are transmitted between each pair of start and stop signals. Asynchronous transmission requires 20% or more overhead (a waste of 2 bits for every 10 bits of transmission (1 start + 8 data + 1 stop). The solution is the synchronous transmission. (Figure 5-8, p-148) • Asynchronous Transmission involves sending each character (or byte) with its own start and stop bits. • Asynchronous means not synchronous or unsynchronized. The start bit tells the receiver to expect a new character. • Keyboard input is asynchronous as is all transmission from a serial port. • It is also called “character-framed” data. • It is inexpensive and often used for low speed connections of PCs or terminals to mainframe computers or other full duplex point-to-point circuits. • When the sender is waiting for the next character to be sent, the line is idle and remains in its default state which is the same as the stop bit state (binary 1 or -12v in case of serial port transmission). Lecture15.doc Page 5 (7) CSN200 Introduction to Telecommunications, Winter 2000 Lecture_15 Data, Encoding and Transmission Synchronous transmission: With synchronous transmission, a block of bits is transmitted in a steady stream without start and stop codes for each character. To prevent timing drift between transmitter and receiver, their clocks must somehow be synchronized. 1. One possibility is to provide a separate clock line. But for a long distances transmission impairments can cause timing errors. 2. The other alternative is to embed the clocking information in the data signal. This can be accomplished with Manchester data encoding. The block synchronization is maintained with a start/stop preamble bit patterns, just like the character synchronization achieved with start/stop bits in asynchronous transmission. As the blocks has replaced the characters the overhead reduces to 1% or less, depending on the block size. Synchronous Transmission involves sending data in frames (or packets) where many individual characters or bytes are grouped and transmitted together as a block. The start and end of the entire block of data must be marked with a header at the beginning and a trailer at the end which may include an error-checking code and an ending flag but individual characters do not need start and stop bits. • It is more efficient than asynchronous transmission because more information can be sent in fewer bits. You can think of Synchronous transmission as sending large blocks of data in an envelope as one unit and Asynchronous transmission as sending each character in its own envelope. Data Transmission Modes: There are three ways in which data can flow in a circuit: one way only, one way at a time or both directions simultaneously. Simplex Data can flow in only one direction. Examples are radio, TV, computer to printers, public address systems or any other unidirectional transmission. Half Duplex Data flows in only one direction at a time. It is sometimes called two-way alternate. Communication occurs in one direction and then the line is “turned around” and information flows in the other direction. CB radio is half-duplex. Full Duplex Data flows in both directions at the same time. Most modem connections today transmit full duplex increasing efficiency with data flowing on the same pair of wires in both directions simultaneously. Lecture15.doc Page 6 (7) CSN200 Introduction to Telecommunications, Winter 2000 Lecture_15 Data, Encoding and Transmission Interfacing: Most digital data processing devices generate simple digital signals (e.g., NRZL) are limited of data transmission capability. Consequently, it is rare for such a device (terminal, computer) to attach directly to a transmission or networking facility. (Figure 5-8, p-112, Slyke) The devices we are discussing, which include terminals and computers, are referred to generically as Data Terminal Equipment (DTE). Rather, a DTE makes use of the transmission system through the mediation of Data Circuit- Terminating Equipment (DCE). Data Terminal Equipment (DTE) - converts user information into data signals for transmission or reconvert the received data signals into user information. e.g., computers, printers, etc. Data Circuit-Terminating Equipment (DCE) - provides the signal conversion and coding between the data terminal equipment (DTE) and the line. e.g., modems, multiplexers, etc. Each DTE-DCE pair must be designed to interact cooperatively and that requires both data and control information to be exchanged over a set of wires referred to as interchange circuits. To ease the process for the manufacturers and the users standards have been developed that specify the exact nature of the interface between the DTE and the DCE. Such an interface has four important characteristics: 1. Mechanical - effecting the physical connection 2. Electrical - voltage levels and timing 3. Functional - data, control, timing, ground, etc. 4. Procedural - sequence of events for transmitting data EIA232 was first started in 1962 as RS232: The ITU-T standard corresponds to it is as follows: 5. Mechanical - ISO 2110 6. Electrical - V.28 7. Functional - V.24 8. Procedural - V.24 (Figure A-2, pp.433-437) For Pin configuration of RS232 connectors: http://cirilla.unibase.cz/mirrors/pub/hwb/index.html maintained by Joakim Ögren. Biphase - having two phase. In NRZ-L NRZ-I coding the output is constant voltage over a long period of time for 0's. A timing drift between the clocks will cause error in data. To avoid this problem is the biphase. In biphase for a series of 0's the phase changes every bit. Lecture15.doc Page 7 (7) . 2000 Lecture_ 15 Data, Encoding and Transmission Data: What is Data? Any meaningful information is data. Otherwise it is noise or nonterrestrial data. Data. Slyke) Lecture1 5.doc Page 2 (7) CSN200 Introduction to Telecommunications, Winter 2000 Lecture_ 15 Data, Encoding and Transmission Data Transmission: Data