phones off (please)
Hardware 8
Basic Communications
Dr John Cowell
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Overview
 Communication issues
 speed (baud, bps, cps)
 direction (simplex, half-duplex and full-duplex)
 data bits and parity
 modes (asynchronous and synchronous)

packets and frames
 bandwidth and capacity
 Communication protocols
 serial and parallel communication
 other communication methods
 Data compression techniques
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Speed
 Maximum speed of data transmission is limited by the
hardware.
 the type of cable / line / media
 the capabilities of the device controllers
 Speed is measured in several (related) ways
 bits per second (bps)

The preferred way. The number of bits of information transmitted every
second. A 2400 bps second modem may have a baud rate of 1200 sending
symbols of 2 bits.
 baud rate
 the rate at which ‘symbols’ can be sent. Each symbol may be 1 or more bits.
 characters per second (cps)
 the number of characters transmitted every second
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Direction
 Communication takes place in certain directions
 Simplex transmission
 allows data to flow in a single direction only, e.g.


sending data to a basic printer
sending display information to screens in airports / stations
 Half-duplex transmission
 data can flow in both directions, but not at same time

e.g. CB radio or ‘walkie-talkies’
 Full-duplex transmission
 data can flow in both directions at the same time, e.g.


a (voice) telephone line
most network communications
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Data-bits
 Original ASCII only uses 7 bits to code characters
 most computers store a character in one byte (8 bits)

8th bit is always zero, so it does not have to be transmitted!
 only 7 bits need to be transmitted
 Original ASCII encoded only limited characters and so it
has been extended, e.g.
 8th bit is used to code an extra 128 characters
 quotes (‘ = 145), bullets (• = 149), pound-sign (£ = 163), etc.
 in this case 8 bits need to be transmitted
 International character sets (UNICODE) now use 16 bits to
encode each single character
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Parity
 Parity is a technique where an extra bit is included in each
transmitted character to detect errors
 e.g. for 7 bit ASCII an eighth parity bit is added
• In even-parity, the eighth bit is set to ‘0’ or ‘1’ so
that there are an even number of ‘1’s in the byte
ASCII ‘A’
1 0 0 0 0 0 1
0 1 0 0 0 0 0 1
ASCII ‘C’
1 0 0 0 0 1 1
1 1 0 0 0 0 1 1
• In odd-parity, the eighth bit is set to ‘0’ or ‘1’ so
that there are an odd number of ‘1’s in the byte
ASCII ‘A’
1 0 0 0 0 0 1
1 1 0 0 0 0 0 1
ASCII ‘C’
1 0 0 0 0 1 1
0 1 0 0 0 0 1 1
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Modes
 Transmitting data only works if the receiving device is ready
for it
 There are two aspects to being ready
 available
 in step
 There are two communication modes used to ensure that
the transmitting and receiving devices keep in step once
transmission has begun
 asynchronous and synchronous modes
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Asynchronous Mode
 A special start signal is transmitted at the start of each
group of message bits
 a group is usually just a single character
 Another special stop signal is transmitted at the end of each
group
 When the receiving device gets the start signal, it sets up the
timing mechanism to accept the group of message bits
 Usually, start / stop signals are additional (fixed value) bits,
e.g. start is a ‘0’ bit, stop is a ‘1’ bit’
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Synchronous Mode
 A larger group of message bits
 usually many characters
are transmitted together in a continuous stream
 There is a single start and stop signal at the beginning and
end of each message group
 there are no start / stop signals for each character
 the transmitting and receiving devices must synchronise their
clocks at the start of transmission

these must be accurate enough to keep time with each other
 Usually, error-checking bits are transmitted at the end of
each message to ensure accuracy
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Comparison
asynchronous
start
character
stop start
character
stop start
character
stop
synchronous
start
message (many characters)
error
stop
checking
 Synchronous mode is
+ faster (less start / stop signals)
 more complex
 more expensive
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Frames - 1
 Frames or packets are a further extension used in
synchronous transmission
 a transmitted message is divided into a series of message
groups called frames (carrying packets)
 frames may arrive at any time, in any order
 the receiving device has to rebuild the message by


testing the received frames
requesting the re-transmission of damaged/missing frames
control
information
control
start
information
start
packet 1
packet 2
error
stop
checking
error
stop
checking
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Frames - 2
 Control information includes
 source address
 destination address

important for routing
 actual number of data bytes
 sequence number
 important for when frames arrive out of order
 frame type
 start of message
 continuation of message
 end of message
 Not all bits transmitted are useful data
 data transfer rate is less than byte transfer rate
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Bandwidth
 For analogue media, bandwidth is the difference between
the highest and lowest frequencies at which a medium can
transmit
 measured in Hertz (cycles per second)
 for example, telephone bandwidth is from
300 Hz to 3300 Hz = 3000 Hz


but available bandwidth on copper wire  2 MHz
ADSL technology exploits this gap
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Capacity
 For digital media, bandwidth is the rate at which data can
be transmitted
 usually measured in bits-per-second
 it is sometimes referred to as the capacity of the link
 limited by parameters of transmission medium
 With all data transmission, not all data sent is useful e.g.
 start/stop characters, control info., error-checking, etc. all
slow down the effective data rate
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Serial Communication
 In serial communication, each bit is transmitted one at a
time over a single wire
01000100
01000001
01100011
01000001
D
A
T
A
• Only a single wire (plus an earth) is needed for
each direction of transmission, i.e.
– simplex requires just one wire plus earth
– full-duplex requires two wires plus earth
• this makes it very cheap
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Serial Uses
 Because serial communication is so cheap, it is widely used
for slow speed peripherals
 modems
 slow printers
 mice
 other input devices

barcode readers, magnetic card readers, etc.
 Unfortunately, there are a wide variety of cable standards
and specifications e.g.
 RS232 , RS432
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Parallel Communications
 In parallel communication, all eight bits in one byte
(character) are send over eight different wires all at once
0
1
0
0
0
1
0
0
0
1
0
0
0
0
0
1
0
1
1
0
0
0
1
1
0
1
0
0
0
0
0
1
D
A
T
A
– faster than serial, but more expensive
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Parallel Uses
 Printers
 original PC parallel ports were simplex ports capable of
transmitting information only

suitable for printers
 modern printers are often half- or full-duplex so that they can
send status information such as ‘out-of-paper’
 Add-on peripheral devices
 external hard disk drives
 external CD-ROM drives
 tape backup devices
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Other Protocols
 Parallel and serial connectors are point to point
 need one port (connector) per device
 USB (Universal Serial Bus) combines serial
communications with bus technology
 multiple devices (up to 128) from one port
 is now the ‘standard’
 Infra red technology is similar in concept
 multiple ‘serial’ devices from one port
 no physical connection (wires) required
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Data Compression Techniques
 The effective data rate of a communications link can usually
be increased through the use of data compression (and
decompression) techniques
 as the communications link is the slowest link in the chain, a
processor at either end of the link can afford to spend time
converting data into a compressed form
 Data is examined prior to transmission and then recoded so
that unnecessary redundant and duplicate bits are
eliminated
 text can often be compressed by 75% or more
 graphics (bitmaps) can often be compressed over 90%
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Text Compression
 Some compression methods are used for all types of information
(but especially text)
 ‘lossless’ compression / decompression
 all information is preserved (perfectly) on decompression
 Huffman coding
 determines how often each character occurs
 The more common the character, the shorter the code that replaces
it.
 Lempel-Ziv-Welch (LZW)
 searches for repeated strings in a document

e.g. ‘communications’, ‘compression’
 replaces these with special short codes
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Graphics Compression
 Other techniques can be used for graphics files
 including ‘lossy’ compression / decompression

decompressed image is an approximation of original
 Run length encoding (RLE)
 many graphics files have long sections of identical value bytes, e.g.
0’s for an area of black image
 repeated sequences are replaced by a code for

‘the next 2000 bytes are all zero’
 JPEG (Joint Photographics Experts Group)
 a ‘lossy’ technique designed specially for photographic images,
taking into account capabilities of the human eye in distinguishing
adjacent colours and pixels
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MPEG
 A set of standards for video and audio compression
developed by the Moving Picture Experts Group. Started in
1988.
 Achieves video compression between 25:1 and 50:1
 MPEG-1 - coding video at about 1.5MBits/s. The audio
layer 3 is MP3.
 MPEG-2 – coding for transmission rates above 4Mbits/s.
Used for DVD and digital TV and HDTV.
 MPEG-3 – never released – incorporated into MPEG-2.
 MPEG-4 – used for Blu-ray disk encoding.
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MPEG Compression
 Uses 5 different compression techniques:
1. A frequency based transform – Discrete Cosine Transform
(DCT).
2. Quantization, lossy compression, removes detail which may
not be too noticeable to the viewer.
3. Huffman coding (lossless compression) using code table based
on encoded data.
4. Motion compensated predictive coding – comparing the
differences between successive images.
5. Bi-directional prediction – images are predicted from images
before and after the image.
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MPEG encoding
 There are many references on the web giving great detail.
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Summary
 Communication issues
 speed (baud, bps, cps)
 direction (simplex, half-duplex and full-duplex)
 data bits and parity
 modes (asynchronous and synchronous)

packets and frames
 bandwidth and capacity
 Communication protocols
 serial communication
 parallel communication
 other communication methods
 Data compression techniques
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