DATA AND PULSE COMMUNICATION UNIT-III DATA AND PULSE COMMUNICATION

History of Data Communication 1837- Telegraph was invented by Samuel Morse 1849- First slow speed telegraph printer 1876- Telephone was invented 1949- All electronic diode based computer 1969- Internet began to evolve

Network Standards

Why Standards? Standards provide a fixed way for hardware and/or software systems to communicate. For example, USB enables two pieces of equipment to interface even though they are manufactured by different companies. By allowing hardware and software from different companies to interconnect, standards help promote competition.

Types of Standards There are two main types of standards: Formal: a standard developed by an industry or government standards-making body De facto: standards that emerge in the marketplace and are widely used, but lack official backing by a standards-making body

The Standardization Processes Three Steps Specification: developing the nomenclature and identifying the problems to be addressed. Identification of choices: identify solutions to the problems and choose the “optimum” solution. Acceptance: defining the solution, getting it recognized by industry so that a uniform solution is accepted.

Some Major Standards Making Bodies ISO: International Organization for Standardization (www.iso.ch) ITU-T: International Telecommunications Union –Telecom Group (www.itu.int) ANSI: American National Standards Institute (www.ansi.org) IEEE: Institute of Electrical and Electronic Engineers (see standards.ieee.org) IETF: Internet Engineering Task Force (www.ietf.org)

Digital Data Transmission The transmission of binary data across a link can be accomplished either in parallel mode or serial mode In parallel mode, multiple bits are sent with each clock pulse In serial mode, one bit is sent with each clock pulse There are 2 subclasses of serial transmission: synchronous and asynchronous

Parallel Transmission Binary data may be organized into groups of n bits each By grouping, we can send data n bits at a time instead of one. This is called parallel transmission The advantage of parallel transmission is speed but its advantage is cost

FIGURE 3-8: PARALLEL DATA TRANSMISSION Used most often for communication with local devices

Serial Transmission In serial transmission, one bit follows another, so we need only one communicating channel to transmit data between 2 communicating devices The advantage of serial transmission is the reduction of the cost of transmission over the parallel transmission Serial transmission occurs in one of 2 ways: asynchronous or synchronous

FIGURE 3-9: SERIAL DATA TRANSMISSION Used most often for data communication

Asynchronous Transmission In asynchronous transmission, the timing of a signal is unimportant Information is received and translated by agreed-upon patterns Patterns are based on grouping the bit stream into bytes The sending system handles each group independently, relaying it to the link whenever ready, without regard to a timer

Asynchronous Transmission (cont.) To alert the receiver to the arrival of a new group, an extra bit called start bit is added to the beginning of each byte To let the receiver know that the byte is finished, one or more additional bits called stop bits are appended to the end of the byte This mechanism is called asynchronous because at the byte level, sender and receiver do not have to be synchronized But within each byte, the receiver must still be synchronized with the incoming bit stream

Asynchronous Transmission (cont.) When the receiver detects a start bit, it sets a timer and begins counting bits as they come in After n bits, the receiver looks for a stop bit and after the stop bit is detected, it ignores any received pulses until the next start bit

Synchronous Transmission In synchronous transmission, the bit stream is combined into longer frames which may contains multiple bytes Each byte is introduced onto the transmission link without a gap between it and the next one It is the responsibility of the receiver to reconstruct the information

Synchronous Transmission (cont.) Without gaps and start/stop bits, timing becomes very important therefore the accuracy of the received information is completely dependent on the ability of the receiver to keep an accurate count of the bits as they come in

Simplex, Half-Duplex, and Full-Duplex Transmission A communications channel is classified as one of three types: (depending on the direction of transfer) Simplex Full-Duplex Half-Duplex Simplex: a simplex mechanism can only transfer data in a single direction It is analogous to broadcast radio or television Figure 9.8a illustrates simplex communication Full-Duplex: allows transmission in two directions simultaneously It is analogous to a voice telephone conversation in which a participant can speak even if they are able to hear background music at the other end Figure 9.8b illustrates the concept

DTE-DCE Interface There are usually four basic functional units involved in the communication of data: a DTE and DCE on both end of transmission The DTE generates the data and pass them to a DCE. The DCE converts the signal to a format appropriate to the transmission medium When the signal arrives at the receiving end, this process is reversed

Data Terminal Equipment (DTE) DTE includes any unit that functions either as a source of or as a destination for binary digital data It can be a terminal, microcomputer, printer, fax machine and etc.

Data Circuit-Terminating Equipment (DCE) DCE includes any functional unit that transmits or receives data in the form of an analog or digital signal through a network Commonly used DCEs include modems

DTE-DCE Interface Standard Many standards have been developed to define the connection between a DTE and a DCE Each standard provides a model for the mechanical, electrical, and functional characteristics of the connection The most active organizations defining the interface standard are the Electronic Industries Association (EIA) and the International Telecommunication Union-Telecommunication Standards Committee (ITU-T)

DTE-DCE Interface Standard (cont.) The EIA standards are called EIA-232, EIA-422, EIA-449, and so on The ITU-T standards are called the V series and the X series

EIA-232 Interface Originally issued in 1962 as the RS-232 standard (recommended standard) The most recent version, EIA-232-D, defines not only the type of connectors to be used but also the specific cable and plugs and the functionality of each pin

EIA-232 Mechanical specification The EIA-232 defines the interfaces as a 25-wire cable with a male and a female DB-25 pin connection attached to either end. The length of the cable may not exceed 15 meters A DB-25 connector is a plug with 25 pins, each of which is attached to a single wire with a specific function. However, fewer are actually used in current practice Another implementation of EIA-232 uses a 9-wire cable with a mail and a female DB-9 pin connector attached to either end 9-pin connector is more commonly found in PCs but it covers signals for asynchronous serial communication only Male connector is used on DTE and female connector is used on DCE

Electrical Specification EIA-232 states that all data must be transmitted as logical 1s and 0s (called mark and space) using NRZ-L encoding, with 0 defined as a positive voltage and 1 defined as a negative voltage EIA-232 defines 2 distinct ranges, one for positive voltages and one for negative To be recognized as data, the amplitude of a signal must fall between 3 and 15 volts or between -3 and -15 volts EIA-232 allows for a maximum bit rate of 20 kbps, although in practice this often is exceeded

Functional Specification DB-25 Implementation DB-9 Implementation

Functioning of EIA-232 in Synchronous Full-Duplex Transmission

Flow Control Means to ask the transmitter to stop/resume sending in data Required when: DTE to DCE speed > DCE to DCE speed (e.g. terminal speed = 115.2kbps and line speed = 33.6kbps, in order to benefit from modem’s data compression protocol) without flow control, the buffer within modem will overflow – sooner or later the receiving end takes time to process the data and thus cannot be always ready to receive

Modem The most familiar type of DCE is a modem Modem is derived from the words Modulator and Demodulator It is an electronics device used to transmit information of a computer to the destination through the telephone channel

Telephone Bandwidth Traditional telephone line can carry frequencies between 300 Hz and 3300 Hz (note: some say that 3400 Hz) All of this range is used for transmitting voice, where a great deal of interference and distortion can be accepted without loss of intelligibility However, the edges of this range cannot support the transmission of base-band digital data

Bell Modems The first commercial modems were produced by the Bell Telephone Company in the early 1970 ITU-T Bell Baud Rate Bit Rate Modulation V.21 V.22 V.23 V.26 V.27 V.29 103 212 202 201 208 209 300 600 1200 1600 2400 4800 9600 FSK 4-PSK 8-PSK 16-QAM

DATA COMMUNICATION CODES

MORSE CODE First character code developed For transmitting data over telegraph wires telegrams (remember Western Union) Used dots (short beep) and dashes (long beeps) instead of 1’s and 0’s More frequent the character, the fewer the beeps Problems: variable “length” character representation required pauses between letters no lower case, few punctuation or special characters no error detection mechanism

FIGURE 3-2: MORSE CODE

BAUDOT CODE One of first codes developed for machine to machine communication Uses 1’s and 0’s instead of dots and dashes For transmitting telex messages (punch tape) Fixed character length (5-bits) 32 different codes increased capacity by using two codes for shifting 11111 (32) Shift to Lower (letters) 11011 (27) Shift to Upper (digits, punctuation) 4 special codes for SP, CR, LF & blank Total = 26 + 26 + 4 = 56 different characters

BAUDOT CODE (cont.) Problems: International Baudot required shift code to switch between character sets no lower case, few special characters no error detection mechanism characters not ordered by binary value designed for transmitting data, not for data processing International Baudot Added a 6th bit for parity Used to detect errors within a single character

FIGURE 3-3: BAUDOT CODE

EBCDIC Extended Binary Coded Decimal Interchange Code 8-bit character code developed by IBM used for data communication, processing and storage extended earlier proprietary 6-bit BCD code designed for backward compatibility or marketing? still in use today on some mainframes and legacy systems. Allows for 256 different character representations (28) includes upper and lower case lots of special characters (non-printable) lots of blank (non-used codes) assigned to international characters in various versions used with/without parity (block transmissions)

ASCII CODE American Standard Code for Information Interchange 7-bit code developed by the American National Standards Institute (ANSI) most popular data communication character code today Allows for 128 different character representations (27) includes upper and lower case lots of special characters (non-printable) generally used with an added parity bit better binary ordering of characters than EBCDIC Extended ASCII uses 8 data bits and no parity Used for processing and storage of data Allows for international characters 8th bit stripped of for transmission of standard character set

FIGURE 3-5: 7-BIT ASCII CODE

Error detection Error detection means to decide whether the received data is correct or not without having a copy of the original message. Error detection uses the concept of redundancy, which means adding extra bits for detecting errors at the destination. Make sense of message.

Types of Errors

Single-bit error

Burst error

Redundancy

Four types of redundancy checks are used in data communications

Vertical Redundancy Check VRC

Longitudinal Redundancy Check LRC

VRC and LRC

Checksum

Cyclic Redundancy Check CRC

Cyclic Redundancy Check Given a k-bit frame or message, the transmitter generates an n-bit sequence, known as a frame check sequence (FCS), so that the resulting frame, consisting of (k+n) bits, is exactly divisible by some predetermined number. The receiver then divides the incoming frame by the same number and, if there is no remainder, assumes that there was no error.

Binary Division

Error Correction It can be handled in two ways: receiver can have the sender retransmit the entire data unit. The receiver can use an error-correcting code, which automatically corrects certain errors.

Single-bit error correction To correct an error, the receiver reverses the value of the altered bit. To do so, it must know which bit is in error. Number of redundancy bits needed Let data bits = m Redundancy bits = r Total message sent = m+r The value of r must satisfy the following relation: 2r ≥ m+r+1

Error Correction

Hamming Code

Hamming Code