Transcript Topic 5 - Digital Data Communication Techniques

FIT1005
FIT – Monash University
Topic 5 - Digital Data Communication
Techniques
Reference:
Chapter 6 - Stallings
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Topic 5 - Digital Data Communication Techniques
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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Topic 5 - Digital Data Communication Techniques
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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Topic 5 - Digital Data Communication Techniques
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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Topic 5 - Digital Data Communication Techniques
Asynchronous Transmission
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Topic 5 - Digital Data Communication Techniques
Asynchronous Transmission
• 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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Topic 5 - Digital Data Communication Techniques
Asynchronous Transmission
•
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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Topic 5 - Digital Data Communication Techniques
Asynchronous Transmission
• 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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Topic 5 - Digital Data Communication Techniques
Asynchronous Transmission
• 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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Topic 5 - Digital Data Communication Techniques
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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Topic 5 - Digital Data Communication Techniques
Synchronous Transmission
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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Topic 5 - Digital Data Communication Techniques
Synchronous Transmission
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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Topic 5 - Digital Data Communication Techniques
Synchronous Transmission
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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Topic 5 - Digital Data Communication Techniques
Synchronous Transmission
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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Topic 5 - Digital Data Communication Techniques
Synchronous Transmission
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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Topic 5 - Digital Data Communication Techniques
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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Topic 5 - Digital Data Communication Techniques
Types of Errors
Burst 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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Topic 5 - Digital Data Communication Techniques
Error Detection Techniques
• For a given frame of bits, additional bits that constitute an errordetecting 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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Topic 5 - Digital Data Communication Techniques
Error Detection Techniques
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Topic 5 - Digital Data Communication Techniques
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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Topic 5 - Digital Data Communication Techniques
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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Topic 5 - Digital Data Communication Techniques
Feedback Error Correction
• Correction of errors using an error-detection code requires
that block of data to be retransmitted
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Topic 5 - Digital Data Communication Techniques
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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Topic 5 - Digital Data Communication Techniques
Forward Error Correction
• 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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Topic 5 - Digital Data Communication Techniques
Forward Error Correction
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Topic 5 - Digital Data Communication Techniques
Forward Error Correction
• 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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Topic 5 - Digital Data Communication Techniques
Forward Error Correction
• 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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Topic 5 - Digital Data Communication Techniques
Forward Error Correction
• 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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Topic 5 - Digital Data Communication Techniques
Forward Error Correction
• 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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Topic 5 - Digital Data Communication Techniques
Interfacing
Data Terminal Equipment (DTE)
• The devices such as terminals and computers
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Topic 5 - Digital Data Communication Techniques
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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Topic 5 - Digital Data Communication Techniques
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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Topic 5 - Digital Data Communication Techniques
Interfacing
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Topic 5 - Digital Data Communication Techniques
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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Topic 5 - Digital Data Communication Techniques
Mechanical and Functional Characteristics
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Topic 5 - Digital Data Communication Techniques
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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Topic 5 - Digital Data Communication Techniques
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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Topic 5 - Digital Data Communication Techniques
Slides after this are for your interest only
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Topic 5 - Digital Data Communication Techniques
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
rate (BER)
– P1: Probability that a frame arrives with no bit error
– P2:Probability that, with an error-detection algorithm in use, a
arrives with one or more undetected errors
error
frame
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Topic 5 - Digital Data Communication Techniques
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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Topic 5 - Digital Data Communication Techniques
Interfacing
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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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Topic 5 - Digital Data Communication Techniques