FIT1005 FIT – Monash University

Topic 5 - Digital Data Communication Techniques

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FIT1005FIT – Monash University


Reference:Chapter 6 - Stallings


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Introduction

• For two devices linked by a transmission medium to exchange data, a high degree of cooperation is required

• Typically data are transmitted one bit at a time over the medium

• The timing (rate, duration, spacing) of these bits must be the same for transmitter and receiver

• Two common techniques used for controlling this timing are known as synchronous and asynchronous transmission


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Introduction

Serial transmission

• signalling elements are sent down the line one at a time• data are transferred over a single signal line

Parallel transmission

• data are transferred over multiple signal lines• I/O devices, system bus• Inside computer


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Asynchronous Transmission

• The strategy with this scheme is to avoid the timing problem by not sending long, uninterrupted streams of bits

– Instead, data are transmitted one character at a time, where each character is 7 bits in length

– Timing or synchronisation must only be maintained within each character

– The receiver has the opportunity to resynchronise at the beginning of each new character


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• When no character is being transmitted, the line between the transmitter and receiver is in an idle state

– The definition of idle is equivalent to the signalling element for binary 1

• The beginning of a character is signalled by a start bit with a value of binary 0

• This is followed by the 7 bits that actually make up the ASCII character


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• Parity bit– The data bits are usually followed by a parity bit– This set by the transmitter such that the total number of ones in the

character, including the parity bit, is even (even parity) or odd (odd parity) depending on the convention used

– This bit is used by the receiver for error detection

• The final element is a stop element, which is a binary 1

– A minimum length for the stop element is specified (usually 1, 1.5, or 2 time the duration of an ordinary bit)

– No maximum value is specified as the stop bit the same as the idle state


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• In (c) the data rate is 10,000bps, therefore each bit time is 1/10000secs or 0.1ms or 100microsecs.

• Receiver’s clock is fast by 6% or 6microsecs• The receiver samples the incoming character every 94microsecs

• This results in two errors– the last sampled bit is incorrectly received– the bit count may now be out of alignment

• If bit 7 is 1 and bit 8 is a 0, bit 8 could be mistaken for a start bit

• This condition is termed as a framing error, as the characters plus start bit and stop element are sometimes referred to as a frame


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• Is simple but requires an overhead of two (20%) to three bits per character

• % of overhead could be reduced by sending larger blocks of bits between the start bit and stop element

• However, the larger the block of bits, the greater the cumulative timing error

• To achieve greater efficiency, a different form of synchronisation, known as synchronous transmission is used


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Synchronous Transmission

• A block of bits is transmitted in a steady stream without a start bit and stop element

– The block may be many bits in length

• To prevent timing drift between transmitter and receiver, their clocks must somehow be synchronised:– Separate clock line– Clocking information in the data signal


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Separate Clock Line

• One side (transmitter or receiver) pulses the line regularly with one short pulse per bit time

• The other side uses these regular pulses as a clock

• This technique works well over short distances, but over longer distances the clock pulses are subject to impairments


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Clocking information in the data signal

• Embed the clocking information in the data signal

• For digital signals, this can be achieved with Manchester encoding

• For analog signals, the carrier frequency itself can be used for synchronisation


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Block Level Synchronisation

• Another level of synchronisation is required

• To allow the receiver to determine the beginning and end of a block of data

• To achieve this, each block begins with a preamble bit pattern and generally ends with a postamble bit pattern


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Block Level Synchronisation

• In addition, other bits are added to the block that convey control information used in the data link control procedures

• The data plus preamble (flag), postamble (flag), and control information are called a frame

• The exact format of the frame depends on which data link control procedure is being used


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Efficiency

• For sizeable blocks of data, synchronous transmission is far more efficient than asynchronous transmission

• Asynchronous transmission requires 20% or more overhead

• The control information, preamble, and postamble in synchronous transmission are typically less than 100 bits

• For example, – HDLC contains 48 bits of control, preamble, and postamble– Thus for 1000-character block, the % of overhead is only 0.6%


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Types of Errors

• In digital transmission, an error occurs when a bit is altered between transmission and reception

Single-bit errors

• An isolated error condition that alters one bit but does not affect near by bits

• Can occur in the presence of thermal noise, a slight random deterioration of the signal-to-noise ratio is sufficient to confuse the receiver’s decision of a single bit


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Types of ErrorsBurst errors

• A burst error of length B is a contiguous sequence of B bits in which the first and last bits and any number of intermediate bits are received in error

• Burst errors can be caused by impulse noise and fading in a mobile wireless environment

• The effect of burst errors are greater at higher data rates


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Error Detection Techniques

• For a given frame of bits, additional bits that constitute an error-detecting code (also known as check bits) are added by the transmitter

This code is calculated as a function of the other transmitted bits

• The receiver performs the same error-detection calculation on the data bits and compares this value with the value of the incoming error-detection code

A detected error occurs, if and only if there is a mismatch


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Error Detection Techniques


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Parity Check

• The simplest error detection scheme is to append a parity bit to the end of a block of data

– A typical example is a character transmission, in which a parity bit is attached to each 7-bit IRA character

• The value of this bit is selected so that the character has an even number of 1s (even parity) or an odd number 1s (odd parity)

• However, if two (or any even number) of bits are inverted due to error, an undetected error occurs

• The use of the parity bit is not foolproof, as noise impulses are often long enough to destroy more than one bit


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Cyclic Redundancy Check (CRC)

• CRC is one of the most common and powerful error-detecting codes

– Given a k-bit block of bits, the transmitter generates an (n-k) bit sequence, known as a frame check sequence (FCS), such that the resulting frame , consisting of n bits, is exactly divisible by some predetermined number

– The receiver then divides the incoming frame by that number, and if there is no remainder, assumes there was no error


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Feedback Error Correction

• Correction of errors using an error-detection code requires that block of data to be retransmitted


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Forward Error Correction

For wireless applications this approach is in adequate:

– The bit error rate on a wireless link can be quite high, which would result in a large number of retransmissions

– In some cases (satellite links), the propagation delay is very long compared to transmission time of a single frame

– Results in a very inefficient system


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• It desirable to enable the receiver to correct errors in an incoming transmission on the basis of the bits in that transmission

• Usually this is done by mapping each k-bit block of data into an n-bit block called a codeword using an FEC (forward error correction) encoder


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• The codeword is then transmitted

• During transmission, the signal is subjected to impairments, which may produce bit errors

• At the receiver, the incoming signal is demodulated to produce a bit string that is similar to the original codeword but may contain errors


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• The received codeword is passed through an FEC decoder with possible outcomes:

– Decoder detects no bit errors

– For certain error patterns, it is possible for the decoder to detect an correct those errors

Thus, even though the incoming data block differs from the

transmitted codeword, FEC decoder is able to recreate the original data block


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• The received codeword is passed through an FEC decoder with possible outcomes:

– For certain error patterns, the decoder can detect but not correct them

The decoder simply reports an uncorrectable error

– For certain, typically rare, error patterns, the decoder does not detect that any errors have occurred


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• Works by adding redundancy to the transmitted message

• The redundancy makes it possible for the receiver to deduce what the original message was, even in the face of certain level of error rate


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Interfacing

Data Terminal Equipment (DTE)

• The devices such as terminals and computers


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Interfacing

Data Circuit-terminating Equipment (DCE)

• A DTE makes use of the transmission system through the mediation of the DCE - modem

• Is responsible for transmitting and receiving bits, one at a time, over a transmission medium or network

• It must interact with the DTE


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Interfacing

• Requires both data and control information to be exchanged

• It is done over a set of wires referred to as interchange circuits

• Two DCEs that exchange signals over a transmission medium must use the same encoding scheme (e.g., Manchester) and data rate

• DCE-DTE pairs must be designed to interact cooperatively

– To ease the data processing equipment manufactures and users, standards have been developed that specify the exact nature of the interface between the DTE and DCE


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Interfacing


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Interfacing

Interface characteristics

• Mechanical

– Pertain to the actual physical connections of the DTE to DCE

– Typically, the signal and control interchange circuits are bundled into a cable with terminator connector, male or female, at each end – 9 Pin, 25 Pin, RJ45 connectors

• The DTE and DCE must present connectors of opposite genders at one end of the cable


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Mechanical and Functional Characteristics


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Interfacing Interface characteristics

• Electrical

– Deal with the voltage levels and timing of voltage changes

– Both DTE and DCE must use the same code (e.g., NRZ-L), must use the same voltage levels to mean the same things and must use the same duration of signal elements

• These characteristics determine the data rates and distances that can be achieved

• Functional

– Specifies the functions that are performed by assigning meanings to each of the interchange circuits


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Interfacing

Interface characteristics

• Procedural

– Specify the sequence of events for transmitting data, based on functional characteristics of the interface

• A variety of standards exist for interfacing

– Example, the V.24/EIA-232-F and ISDN physical interfaces

– The former is used to connect DTE devices to 56K modems for use on PSTN


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Slides after this are for your interest only


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Error Detection• In the following discussion, we assume that data are transmitted as one or

more contiguous sequences of bits, called frames

• The following probabilities with respect to errors in transmitted frames are defined

– Pb: Probability that a bit is received in error; also known as bit error rate (BER)

– P1: Probability that a frame arrives with no bit error

– P2:Probability that, with an error-detection algorithm in use, a frame arrives with one or more undetected errors


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Error Detection Contd.

– P3: probability that, with an error-detection algorithm in use, a frame arrives with one or more detected bit errors but no undetected bit errors

• Consider the case in which no means are taken to detect errors– Then P3 = 0

• Assuming that the probability that any bit is in error (Pb) is constant, we have

• P1 = (1-Pb)F where F is the number of bits per frame • P2 = 1 – P1


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Interfacing

• Most digital data processing devices have limited data transmission capability

• Typically, they generate a simple digital signal, and the distance across which they can transmit data is limited

– As a result, it is rare for such a device (terminal , computer) to attach directly to a transmission or networking facility

• The devices such as terminals and computers are generally referred to as data terminal equipment (DTE)

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