Written by Christopher E. Strangio
Copyright © 1993-2006 by CAMI Research Inc., Lexington, MassachusettsSend Us Your Comments . . . Contents
What is Data Communications?
Communications Channels
Serial Communications
Asynchronous vs. Synchronous Transmission
The ASCII Character Set
Parity and Checksums
Data Compression
Data Encryption
Data Storage Technology
Data Transfer in Digital Circuits
Transmission over Short Distances (< 2 feet)
Noise and Electrical Distortion
Transmission over Medium Distances (< 20 feet)
Transmission over Long Distances (< 4000 feet)
Transmission over Very Long Distances (> 4000 feet)
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What is Data Communications?
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The distance over which data moves within a computer may vary
from a few thousandths of an inch, as is the case within a single
IC chip, to as much as several feet along the backplane of the
main circuit board. Over such small distances, digital data may
be transmitted as direct, two-level electrical signals over simple
copper conductors. Except for the fastest computers, circuit designers
are not very concerned about the shape of the conductor or the
analog characteristics of signal transmission.
Frequently, however, data must be sent beyond the local circuitry
that constitutes a computer. In many cases, the distances involved
may be enormous. Unfortunately, as the distance between the source
of a message and its destination increases, accurate transmission
becomes increasingly difficult. This results from the electrical
distortion of signals traveling through long conductors, and from
noise added to the signal as it propagates through a transmission
medium. Although some precautions must be taken for data exchange
within a computer, the biggest problems occur when data is transferred
to devices outside the computer's circuitry. In this case, distortion
and noise can become so severe that information is lost.
Data Communications concerns the transmission of digital messages
to devices external to the message source. "External"
devices are generally thought of as being independently powered
circuitry that exists beyond the chassis of a computer or other
digital message source. As a rule, the maximum permissible transmission
rate of a message is directly proportional to signal power, and
inversely proportional to channel noise. It is the aim of any
communications system to provide the highest possible transmission
rate at the lowest possible power and with the least possible
noise.
Communications Channels
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A communications channel is a pathway over which information can
be conveyed. It may be defined by a physical wire that connects
communicating devices, or by a radio, laser, or other radiated
energy source that has no obvious physical presence. Information
sent through a communications channel has a source from which
the information originates, and a destination to which the information
is delivered. Although information originates from a single source,
there may be more than one destination, depending upon how many
receive stations are linked to the channel and how much energy
the transmitted signal possesses.
In a digital communications channel, the information is represented
by individual data bits, which may be encapsulated into multibit
message units. A byte, which consists of eight bits, is an example
of a message unit that may be conveyed through a digital communications
channel. A collection of bytes may itself be grouped into a frame
or other higher-level message unit. Such multiple levels of encapsulation
facilitate the handling of messages in a complex data communications
network.
Any communications channel has a direction associated with it:
The message source is the transmitter, and the destination
is the receiver. A channel whose direction of transmission is
unchanging is referred to as a simplex channel. For example, a
radio station is a simplex channel because it always transmits
the signal to its listeners and never allows them to transmit
back.
A half-duplex channel is a single physical channel in which the
direction may be reversed. Messages may flow in two directions,
but never at the same time, in a half-duplex system. In a telephone
call, one party speaks while the other listens. After a pause,
the other party speaks and the first party listens. Speaking simultaneously
results in garbled sound that cannot be understood.
A full-duplex channel allows simultaneous message exchange in
both directions. It really consists of two simplex channels, a
forward channel and a reverse channel, linking the same points.
The transmission rate of the reverse channel may be slower if
it is used only for flow control of the forward channel.
Serial Communications
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Most digital messages are vastly longer than just a few bits.
Because it is neither practical nor economic to transfer all bits
of a long message simultaneously, the message is broken into smaller
parts and transmitted sequentially. Bit-serial transmission conveys
a message one bit at a time through a channel. Each bit represents
a part of the message. The individual bits are then reassembled
at the destination to compose the message. In general, one channel
will pass only one bit at a time. Thus, bit-serial transmission
is necessary in data communications if only a single channel is
available. Bit-serial transmission is normally just called serial
transmission and is the chosen communications method in many computer
peripherals.
Byte-serial transmission conveys eight bits at a time through
eight parallel channels. Although the raw transfer rate is eight
times faster than in bit-serial transmission, eight channels are
needed, and the cost may be as much as eight times higher to transmit
the message. When distances are short, it may nonetheless be both
feasible and economic to use parallel channels in return for high
data rates. The popular Centronics printer interface is a case
where byte-serial transmission is used. As another example, it
is common practice to use a 16-bit-wide data bus to transfer data
between a microprocessor and memory chips; this provides the equivalent
of 16 parallel channels. On the other hand, when communicating
with a timesharing system over a modem, only a single channel
is available, and bit-serial transmission is required. This figure
illustrates these ideas:
The baud rate refers to the signalling rate at which data is
sent through a channel and is measured in electrical transitions
per second. In the EIA232 serial interface standard, one signal
transition, at most, occurs per bit, and the baud rate and bit
rate are identical. In this case, a rate of 9600 baud corresponds
to a transfer of 9,600 data bits per second with a bit period
of 104 microseconds (1/9600 sec.). If two electrical transitions
were required for each bit, as is the case in non-return-to-zero
coding, then at a rate of 9600 baud, only 4800 bits per second
could be conveyed. The channel efficiency is the number of bits
of useful information passed through the channel per second. It
does not include framing, formatting, and error detecting bits
that may be added to the information bits before a message is
transmitted, and will always be less than one.
The data rate of a channel is often specified by its bit rate
(often thought erroneously to be the same as baud rate). However,
an equivalent measure channel capacity is bandwidth. In general,
the maximum data rate a channel can support is directly proportional
to the channel's bandwidth and inversely proportional to the channel's
noise level.
A communications protocol is an agreed-upon convention that defines
the order and meaning of bits in a serial transmission. It may
also specify a procedure for exchanging messages. A protocol will
define how many data bits compose a message unit, the framing
and formatting bits, any error-detecting bits that may be added,
and other information that governs control of the communications
hardware. Channel efficiency is determined by the protocol design
rather than by digital hardware considerations. Note that there
is a tradeoff between channel efficiency and reliability - protocols
that provide greater immunity to noise by adding error-detecting
and -correcting codes must necessarily become less efficient.
Asynchronous vs. Synchronous Transmission
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Serialized data is not generally sent at a uniform rate through
a channel. Instead, there is usually a burst of regularly spaced
binary data bits followed by a pause, after which the data flow
resumes. Packets of binary data are sent in this manner, possibly
with variable-length pauses between packets, until the message
has been fully transmitted. In order for the receiving end to
know the proper moment to read individual binary bits from the
channel, it must know exactly when a packet begins and how much
time elapses between bits. When this timing information is known,
the receiver is said to be synchronized with the transmitter,
and accurate data transfer becomes possible. Failure to remain
synchronized throughout a transmission will cause data to be corrupted
or lost.
Two basic techniques are employed to ensure correct synchronization.
In synchronous systems, separate channels are used to transmit
data and timing information. The timing channel transmits clock
pulses to the receiver. Upon receipt of a clock pulse, the receiver
reads the data channel and latches the bit value found on the
channel at that moment. The data channel is not read again until
the next clock pulse arrives. Because the transmitter originates
both the data and the timing pulses, the receiver will read the
data channel only when told to do so by the transmitter (via the
clock pulse), and synchronization is guaranteed.
Techniques exist to merge the timing signal with the data so that
only a single channel is required. This is especially useful when
synchronous transmissions are to be sent through a modem. Two
methods in which a data signal is self-timed are nonreturn-to-zero
and biphase Manchester coding. These both refer to methods for
encoding a data stream into an electrical waveform for transmission.
In asynchronous systems, a separate timing channel is not used.
The transmitter and receiver must be preset in advance to an agreed-upon
baud rate. A very accurate local oscillator within the receiver
will then generate an internal clock signal that is equal to the
transmitter's within a fraction of a percent. For the most common
serial protocol, data is sent in small packets of 10 or 11 bits,
eight of which constitute message information. When the channel
is idle, the signal voltage corresponds to a continuous logic
'1'. A data packet always begins with a logic '0' (the start bit)
to signal the receiver that a transmission is starting. The start
bit triggers an internal timer in the receiver that generates
the needed clock pulses. Following the start bit, eight bits of
message data are sent bit by bit at the agreed upon baud rate.
The packet is concluded with a parity bit and stop bit. One complete
packet is illustrated below:
The packet length is short in asynchronous systems to minimize
the risk that the local oscillators in the receiver and transmitter
will drift apart. When high-quality crystal oscillators are used,
synchronization can be guaranteed over an 11-bit period. Every
time a new packet is sent, the start bit resets the synchronization,
so the pause between packets can be arbitrarily long. Note that
the EIA232 standard defines electrical, timing, and mechanical
characteristics of a serial interface. However, it does not include
the asynchronous serial protocol shown in the previous figure,
or the ASCII alphabet described next.
The ASCII Character Set
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Characters sent through a serial interface generally follow the
ASCII (American Standard Code for Information Interchange) character
standard:
This standard relates binary codes to printable characters and
control codes. Fully 25 percent of the ASCII character set represents
nonprintable control codes, such as carriage return (CR) and line
feed (LF). Most modern character-oriented peripheral equipment
abides by the ASCII standard, and thus may be used interchangeably
with different computers.
Parity and Checksums
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Noise and momentary electrical disturbances may cause data to
be changed as it passes through a communications channel. If the
receiver fails to detect this, the received message will be incorrect,
resulting in possibly serious consequences. As a first line of
defense against data errors, they must be detected. If an error
can be flagged, it might be possible to request that the faulty
packet be resent, or to at least prevent the flawed data from
being taken as correct. If sufficient redundant information is
sent, one- or two-bit errors may be corrected by hardware within
the receiver before the corrupted data ever reaches its destination.
A parity bit is added to a data packet for the purpose of error
detection. In the even-parity convention, the value of the parity
bit is chosen so that the total number of '1' digits in the combined
data plus parity packet is an even number. Upon receipt of the
packet, the parity needed for the data is recomputed by local
hardware and compared to the parity bit received with the data.
If any bit has changed state, the parity will not match, and an
error will have been detected. In fact, if an odd number of bits
(not just one) have been altered, the parity will not match. If
an even number of bits have been reversed, the parity will match
even though an error has occurred. However, a statistical analysis
of data communication errors has shown that a single-bit error
is much more probable than a multibit error in the presence of
random noise. Thus, parity is a reliable method of error detection.
Another approach to error detection involves the computation
of a checksum. In this case, the packets that constitute a message
are added arithmetically. A checksum number is appended to the
packet sequence so that the sum of data plus checksum is zero.
When received, the packet sequence may be added, along with the
checksum, by a local microprocessor. If the sum is nonzero, an
error has occurred. As long as the sum is zero, it is highly unlikely
(but not impossible) that any data has been corrupted during transmission.
Errors may not only be detected, but also corrected if additional
code is added to a packet sequence. If the error probability is
high or if it is not possible to request retransmission, this
may be worth doing. However, including error-correcting code in
a transmission lowers channel efficiency, and results in a noticeable
drop in channel throughput.
Data Compression
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If a typical message were statistically analyzed, it would be
found that certain characters are used much more frequently than
others. By analyzing a message before it is transmitted, short
binary codes may be assigned to frequently used characters and
longer codes to rarely used characters. In doing so, it is possible
to reduce the total number of characters sent without altering
the information in the message. Appropriate decoding at the receiver
will restore the message to its original form. This procedure,
known as data compression, may result in a 50 percent or greater
savings in the amount of data transmitted. Even though time is
necessary to analyze the message before it is transmitted, the
savings may be great enough so that the total time for compression,
transmission, and decompression will still be lower than it would
be when sending an uncompressed message.
Some kinds of data will compress much more than others. Data that
represents images, for example, will usually compress significantly,
perhaps by as much as 80 percent over its original size. Data
representing a computer program, on the other hand, may be reduced
only by 15 or 20 percent.
A compression method called Huffman coding is frequently used
in data communications, and particularly in fax transmission.
Clearly, most of the image data for a typical business letter
represents white paper, and only about 5 percent of the surface
represents black ink. It is possible to send a single code that,
for example, represents a consecutive string of 1000 white pixels
rather than a separate code for each white pixel. Consequently,
data compression will significantly reduce the total message length
for a faxed business letter. Were the letter made up of randomly
distributed black ink covering 50 percent of the white paper surface,
data compression would hold no advantages.
Data Encryption
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Privacy is a great concern in data communications. Faxed business
letters can be intercepted at will through tapped phone lines
or intercepted microwave transmissions without the knowledge of
the sender or receiver. To increase the security of this and other
data communications, including digitized telephone conversations,
the binary codes representing data may be scrambled in such a
way that unauthorized interception will produce an indecipherable
sequence of characters. Authorized receive stations will be equipped
with a decoder that enables the message to be restored. The process
of scrambling, transmitting, and descrambling is known as encryption.
Custom integrated circuits have been designed to perform this
task and are available at low cost. In some cases, they will be
incorporated into the main circuitry of a data communications
device and function without operator knowledge. In other cases,
an external circuit is used so that the device, and its encrypting/decrypting
technique, may be transported easily.
Data Storage Technology
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Normally, we think of communications science as dealing with the
contemporaneous exchange of information between distant parties.
However, many of the same techniques employed in data communications
are also applied to data storage to ensure that the retrieval
of information from a storage medium is accurate. We find, for
example, that similar kinds of error-correcting codes used to
protect digital telephone transmissions from noise are also used
to guarantee correct readback of digital data from compact audio
disks, CD-ROMs, and tape backup systems.
Data Transfer in Digital Circuits
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Data is typically grouped into packets that are either 8, 16,
or 32 bits long, and passed between temporary holding units called
registers. Data within a register is available in parallel because
each bit exits the register on a separate conductor. To transfer
data from one register to another, the output conductors of one
register are switched onto a channel of parallel wires referred
to as a bus. The input conductors of another register, which is
also connected to the bus, capture the information:
Following a data transaction, the content of the source register
is reproduced in the destination register. It is important to
note that after any digital data transfer, the source and destination
registers are equal; the source register is not erased when the
data is sent.
The transmit and receive switches shown above are electronic and
operate in response to commands from a central control unit. It
is possible that two or more destination registers will be switched
on to receive data from a single source. However, only one source
may transmit data onto the bus at any time. If multiple sources
were to attempt transmission simultaneously, an electrical conflict
would occur when bits of opposite value are driven onto a single
bus conductor. Such a condition is referred to as a bus contention.
Not only will a bus contention result in the loss of information,
but it also may damage the electronic circuitry. As long as all
registers in a system are linked to one central control unit,
bus contentions should never occur if the circuit has been designed
properly. Note that the data buses within a typical microprocessor
are funda-mentally half-duplex channels.
Transmission over Short Distances
(< 2 feet)
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When the source and destination registers are part of an integrated
circuit (within a microprocessor chip, for example), they are
extremely close (thousandths of an inch). Consequently, the bus
signals are at very low power levels, may traverse a distance
in very little time, and are not very susceptible to external
noise and distortion. This is the ideal environment for digital
communications. However, it is not yet possible to integrate all
the necessary circuitry for a computer (i.e., CPU, memory, disk
control, video and display drivers, etc.) on a single chip. When
data is sent off-chip to another integrated circuit, the bus signals
must be amplified and conductors extended out of the chip through
external pins. Amplifiers may be added to the source register:
Bus signals that exit microprocessor chips and other VLSI circuitry
are electrically capable of traversing about one foot of conductor
on a printed circuit board, or less if many devices are connected
to it. Special buffer circuits may be added to boost the bus signals
sufficiently for transmission over several additional feet of
conductor length, or for distribution to many other chips (such
as memory chips).
Noise and Electrical Distortion
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Because of the very high switching rate and relatively low signal
strength found on data, address, and other buses within a computer,
direct extension of the buses beyond the confines of the main
circuit board or plug-in boards would pose serious problems. First,
long runs of electrical conductors, either on printed circuit
boards or through cables, act like receiving antennas for electrical
noise radiated by motors, switches, and electronic circuits:
Such noise becomes progressively worse as the length increases,
and may eventually impose an unacceptable error rate on the bus
signals. Just a single bit error in transferring an instruction
code from memory to a microprocessor chip may cause an invalid
instruction to be introduced into the instruction stream, in turn
causing the computer to totally cease operation.
A second problem involves the distortion of electrical signals
as they pass through metallic conductors. Signals that start at
the source as clean, rectangular pulses may be received as rounded
pulses with ringing at the rising and falling edges:
These effects are properties of transmission through metallic
conductors, and become more pronounced as the conductor length
increases. To compensate for distortion, signal power must be
increased or the transmission rate decreased.
Special amplifier circuits are designed for transmitting direct
(unmodulated) digital signals through cables. For the relatively
short distances between components on a printed circuit board
or along a computer backplane, the amplifiers are in simple IC
chips that operate from standard +5v power. The normal output
voltage from the amplifier for logic '1' is slightly higher than
the minimum needed to pass the logic '1' threshold. Correspondingly
for logic '0', it is slightly lower. The difference between the
actual output voltage and the threshold value is referred to as
the noise margin, and represents the amount of noise voltage that
can be added to the signal without creating an error:
Transmission over Medium Distances
(< 20 feet)
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Computer peripherals such as a printer or scanner generally include
mechanisms that cannot be situated within the computer itself.
Our first thought might be just to extend the computer's internal
buses with a cable of sufficient length to reach the peripheral.
Doing so, however, would expose all bus transactions to external
noise and distortion even though only a very small percentage
of these transactions concern the distant peripheral to which
the bus is connected.
If a peripheral can be located within 20 feet of the computer,
however, relatively simple electronics may be added to make data
transfer through a cable efficient and reliable. To accomplish
this, a bus interface circuit is installed in the computer:
It consists of a holding register for peripheral data, timing
and formatting circuitry for external data transmission, and signal
amplifiers to boost the signal sufficiently for transmission through
a cable. When communication with the peripheral is necessary,
data is first deposited in the holding register by the microprocessor.
This data will then be reformatted, sent with error-detecting
codes, and transmitted at a relatively slow rate by digital hardware
in the bus interface circuit. In addition, the signal power is
greatly boosted before transmission through the cable. These steps
ensure that the data will not be corrupted by noise or distortion
during its passage through the cable. In addition, because only
data destined for the peripheral is sent, the party-line transactions
taking place on the computer's buses are not unnecessarily exposed
to noise.
Data sent in this manner may be transmitted in byte-serial format
if the cable has eight parallel channels (at least 10 conductors
for half-duplex operation), or in bit-serial format if only a
single channel is available.
Transmission over Long Distances
(< 4000 feet)
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When relatively long distances are involved in reaching a peripheral
device, driver circuits must be inserted after the bus interface
unit to compensate for the electrical effects of long cables:
This is the only change needed if a single peripheral is used.
However, if many peripherals are connected, or if other computer
stations are to be linked, a local area network (LAN) is required,
and it becomes necessary to drastically change both the electrical
drivers and the protocol to send messages through the cable. Because
multiconductor cable is expensive, bit-serial transmission is
almost always used when the distance exceeds 20 feet.
In either a simple extension cable or a LAN, a balanced electrical
system is used for transmitting digital data through the channel.
This type of system involves at least two wires per channel, neither
of which is a ground. Note that a common ground return cannot
be shared by multiple channels in the same cable as would be possible
in an unbalanced system.
The basic idea behind a balanced circuit is that a digital signal
is sent on two wires simultaneously, one wire expressing a positive
voltage image of the signal and the other a negative voltage image.
When both wires reach the destination, the signals are subtracted
by a summing amplifier, producing a signal swing of twice the
value found on either incoming line. If the cable is exposed to
radiated electrical noise, a small voltage of the same polarity
is added to both wires in the cable. When the signals are subtracted
by the summing amplifier, the noise cancels and the signal emerges
from the cable without noise:
A great deal of technology has been developed for LAN systems
to minimize the amount of cable required and maximize the throughput.
The costs of a LAN have been concentrated in the electrical interface
card that would be installed in PCs or peripherals to drive the
cable, and in the communications software, not in the cable itself
(whose cost has been minimized). Thus, the cost and complexity
of a LAN are not particularly affected by the distance between
stations.
Transmission over Very Long Distances
(greater than 4000 feet)
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Data communications through the telephone network can reach any
point in the world. The volume of overseas fax transmissions is
increasing constantly, and computer networks that link thousands
of businesses, governments, and universities are pervasive. Transmissions
over such distances are not generally accomplished with a direct-wire
digital link, but rather with digitally-modulated analog carrier
signals. This technique makes it possible to use existing analog
telephone voice channels for digital data, although at considerably
reduced data rates compared to a direct digital link.
Transmission of data from your personal computer to a timesharing
service over phone lines requires that data signals be converted
to audible tones by a modem. An audio sine wave carrier is used,
and, depending on the baud rate and protocol, will encode data
by varying the frequency, phase, or amplitude of the carrier.
The receiver's modem accepts the modulated sine wave and extracts
the digital data from it. Several modulation techniques typically
used in encoding digital data for analog transmission are shown
below:
Similar techniques may be used in digital storage devices such
as hard disk drives to encode data for storage using an analog
medium.
