DIGITAL TRANSMISSION

An IntroductionDIGITAL TRANSMISSIONDigital CodesThe proliferation of applications that send digital data over communication channels has resulted in the need for efficient methods of transmission, conversion, and reception of digital data.

Modern Binary CodesFor modern data communication, information is transmitted using a system in which the numbers and letters to be represented are coded, usually by way of a keyboard, and the binary word representing each character is stored in a computer memory

Modern Binary Codes : Hex ValuesBinary codes are often expressed using their hexadecimal, rather than decimal values.

To convert a binary code to its hexadecimal equivalent, first divide the code into 4-bit groups.

Start at the least significant bit on the right and work to the left. (Assume a leading zero on each of the codes.)

A. American Standard Code for Information InterchangeThe most widely used data communication code is the 7-bit binary code known as the American Standard Code for Information Interchange (ASCII).ASCII code can represent 128 numbers, letters, punctuation marks, and other symbols.ASCII code combinations are available to represent both uppercase and lowercase letters of the alphabet.Several ASCII codes have two- and three-letter designations which initiate operations or provide responses for inquiries.

4B. Extended Binary Coded Decimal Interchange CodeThe Extended Binary Coded Decimal Interchange Code (EBCDIC) was developed by IBM.The EBDIC is an 8-bit code allowing a maximum of 256 characters to be represented.The EBCDIC is used primarily in IBM and IBM-compatible computing systems and is not widely used as ASCIIPrinciples of Digital TransmissionPrinciples of Digital TransmissionData can be transmitted in two ways:ParallelSerial

Serial Transmission

Data transfers in long-distance communication systems are made serially.In a serial transmission, each bit of a word is transmitted one after another.Parallel data transmission is not practical for long-distance communication8Serial Transmission : Expressing the Serial Data RateThe speed of data transfer is usually indicated as number of bits per second (bps or b/s).Another term used to express the data speed in digital communication systems is baud rate.Baud rate is the number of signaling elements or symbols that occur in a given unit of time.A signaling element is simply some change in the binary signal transmittedSerial Transmission of the ASCII letter M

DIGITAL ENCODINGFor digital signalling, the data source can be either analog or digital, which is encoded into digital signal, using different encoding techniques

All the four possibilities

LINE CODING (the DC Component)DC component is also called the disparity, the bias, or the DC coefficient a result after line coding, wherein the signal may have zero frequency component in the spectrum of the signalnot desirable because the DC component does not pass through some components of a communication system such as a transformer leading to distortion or errorresults in unwanted energy loss on the lineLINE CODING (the DC Component)There are two ways of eliminating the DC component:Use a constant-weight code. -design each transmitted code word such that every code word contains some positive or negative levels also contains enough of the opposite levels, such that the average level over each code word is zero.

Use a paired disparity code. - design the receiver such that every code word that averages to a negative level is paired with another code word that averages to a positive level. -design the receiver so that either code word of the pair decodes to the same data bits. -Design the transmitter to keep track of the running DC build up, and always pick the code word that pushes the DC level back towards zero.LINE CODING converts digital data to digital signalconsists of representing the digital signal to be transported by an amplitude- and time-discrete signal that is optimally tuned for the specific properties of the physical channel (and of the receiving equipment). Line coding should make it possible for the receiver to synchronize itself to the phase of the received signal.

LINE CODING After line coding, the signal is put through a "physical channel", either a "transmission medium" or "data storage medium

The most common physical channels are:the line-coded signal can directly be put on a transmission line, in the form of variations of the voltage or current (often using differential signaling).the line-coded signal (the "base-band signal") undergoes further pulse shaping (to reduce its frequency bandwidth) and then modulated (to shift its frequency bandwidth) to create the "RF signal" that can be sent through free space.the line-coded signal can be used to turn on and off a light in Free Space Optics, most commonly infrared remote control. the line-coded signal can be printed on paper to create a bar code.the line-coded signal can be converted to magnetized spots on a hard drive or tape drive.the line-coded signal can be converted to pits on an optical disc LINE CODING TECHNIQUES

LINE CODING TECHNIQUESUNIPOLAR- two voltage levels are used and only one polarity of voltage leveluses only one polarity of voltage levelbit rate is same as data rate DC component present in the encoded signal and there is loss of synchronization for long sequences of 0s and 1s.

17LINE CODING TECHNIQUESb. POLAR- uses two voltage levels one positive and the other one negative.

LINE CODING TECHNIQUESb.1. Non Return to zero (NRZ): -The most common and easiest way to transmit digital signals is to use two different voltage levels for the two binary digits-data is encoded as the presence or absence of a signal transition at the beginning of the bit time

LINE CODING TECHNIQUESThe advantages of NRZ coding are: Detecting a transition in presence of noise is more reliable than to compare a value to a threshold.

NRZ codes are easy to engineer and it makes efficient use of bandwidth.

Main limitations are:the presence of a dc component lack of synchronization capability.

LINE CODING TECHNIQUESb.2. Return to Zero RZTo ensure synchronization, there must be a signal transition in each bit. Key characteristics of the RZ coding are: Three levels Bit rate is double than that of data rate No dc component Good synchronization Main limitation is the increase in bandwidth

b.3. BIPHASE (MANCHESTER and DIFFERENTIAL MANCHESTER CODING)To overcome the limitations of NRZ encodingmid-bit transition serves as a clocking mechanism and also as datathere is a transition at the middle of each bit period. A binary 1 corresponds to a low-to-high transition and a binary 0 to a high-to-low transition in the middle For DIFFERENTIAL MANCHESTER; *encoding of a 0 -transition both at the beginning and at the middle *encoding of 1 - transition only in the middle of the bit period.Key characteristics are: Two levels No DC component Good synchronization Higher bandwidth due to doubling of bit rate with respect to data rate

C. Bipolar EncodingBipolar AMI uses three voltage levels.the zero level used to represent a 0binary 1s are represented by alternating positive and negative voltages,

c.1. Pseudoternary:same as AMI, but alternating positive and negative pulses occur for binary 0 instead of binary 1. Key characteristics are: Three levels No DC componentLoss of synchronization for long sequence or zeroLesser bandwidth Modulation Rate Data rate is expressed in bits per second.Modulation rate is expressed in bauds

Where:D = modulation rate in bauds, R = data rate in bps, L = is the number of different signal elements b = is the number of bits per signal element.Modulation Rate for Different Encoding Techniques

Frequency Spectrum of Different Encoding Schemes