Data Communication

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Data Communication

• E. Amir Ezzat
• Amir.ezzat_at_rashpetco.com
• 2 012 73324840

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Contents

• Data Transmission Circuits
• Parallel and Serial Data Transmission
• Asynchronous Serial Transmission
• Synchronous Serial Transmission
• Data Communication Terminology
• Channel, baud rate, bits per second, bandwidth
• Protocols
• Asynchronous and synchronous protocols
• Data Multiplexing
• Time division multiplexing and frequency division
  multiplexing
• Modems
• Summary

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Data Transmission

• Data transmission is the transfer of data from
  point-to-point often represented as an
  electromagnetic signal over a physical
  point-to-point or point-to-multipoint
  communication channel
• A communication channel refers to the medium used
  to convey information from a sender (or
  transmitter) to a receiver, and it can use fully
  or partially the medium.
• Examples of channels copper wires, optical
  fibbers or wireless communication channels.

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Data Communication Channels

• The following is a discussion on the THREE main
  types of transmission circuits (channels),
  simplex, half duplex and full duplex.
• Simplex
• Data in a simplex channel is always one way.
  Simplex channels are not often used because it is
  not possible to send back error or control
  signals to the transmit end. An example of a
  simplex channel in a computer system is the
  interface between the keyboard and the computer,
  in that key codes need only be sent one way from
  the keyboard to the computer system.
• Half Duplex
• A half duplex channel can send and receive, but
  not at the same time. Its like a one-lane bridge
  where two way traffic must give way in order to
  cross. Only one end transmits at a time, the
  other end receives.
• Full Duplex
• Data can travel in both directions
  simultaneously. There is no need to switch from
  transmit to receive mode like in half duplex. Its
  like a two lane bridge on a two-lane highway.

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Parallel and Serial Data

• Data may be transmitted between two points in two
  different ways. Lets consider sending 8 bits of
  digital data (1 byte)
• Parallel transmission
• Each bit uses a separate wire
• To transfer data on a parallel link, a separate
  line is used as a clock signal. This serves to
  inform the receiver when data is available. In
  addition, another line may be used by the
  receiver to inform the sender that the data has
  been used, and its ready for the next data.

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Parallel and Serial Data

• Serial
• Each bit is sent over a single wire, one after
  the other
• Usually no signal lines are used to convey clock
  (timing information)
• There are two types of serial transmission,
  essentially having to do with how the clock is
  embedded into the serial data
• Asynchronous serial transmission
• Synchronous serial transmission
• If no clock information was sent, the receiver
  would misinterpret the arriving data (due to bits
  being lost, going too slow).
• Parallel transmission is obviously faster, in
  that all bits are sent at the same time, whereas
  serial transmission is slower, because only one
  bit can be sent at a time. Parallel transmission
  is very costly for anything except short links.

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Asynchronous Serial Transmission(RS232 Example)

• Because no signal lines are used to convey clock
  (timing) information, this method groups data
  together into a sequence of bits (five to eight),
  then prefixes them with a start bit and a stop
  bit. This is the method most widely used for PC
  or simple terminal serial communications.
• In asynchronous serial communication, the
  electrical interface is held in the mark position
  between characters. The start of transmission of
  a character is signaled by a drop in signal level
  to the space level. At this point, the receiver
  starts its clock. After one bit time (the start
  bit) come 8 bits of true data followed by one or
  more stop bits at the mark level.
• The receiver tries to sample the signal in the
  middle of each bit time. The byte will be read
  correctly if the line is still in the intended
  state when the last stop bit is read.
• Thus the transmitter and receiver only have to
  have approximately the same clock rate. A little
  arithmetic will show that for a 10 bit sequence,
  the last bit will be interpreted correctly even
  if the sender and receiver clocks differ by as
  much as 5.
• It is relatively simple, and therefore
  inexpensive. However, it has a high overhead, in
  that each byte carries at least two extra bits a
  20 loss of line bandwidth.

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Synchronous Serial Transmission (PS2 Example)

• The PS/2 mouse and keyboard implement a
  bidirectional synchronous serial protocol.
• The bus is "idle" when both lines are high
  (open-collector).  This is the only state where
  the keyboard/mouse is allowed begin transmitting
  data.  The host has ultimate control over the bus
  and may inhibit communication at any time by
  pulling the Clock line low.
• The device (slave) always generates the clock
  signal.  If the host wants to send data, it must
  first inhibit communication from the device by
  pulling Clock low.  The host then pulls Data low
  and releases Clock.  This is the
  "Request-to-Send" state and signals the device to
  start generating clock pulses.
• Summary Bus StatesData high, Clock high
   Idle state.Data high, Clock low
   Communication Inhibited.Data low, Clock
  high  Host Request-to-Send

• Data is transmited 1 byte at a time
• 1 start bit.  This is always 0.
• 8 data bits, least significant bit first.
• 1 parity bit (odd parity - The number of 1's in
  the data bits plus the parity bit always add up
  to an odd number. This is used for error
  detection.).
• 1 stop bit.  This is always 1.
• 1 acknowledge bit (host-to-device communication
  only)

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Synchronous Serial Transmission

• In fast speed synchronous communications, data is
  not sent in individual bytes, but as frames of
  large data blocks. Frame sizes vary from a few
  bytes through 1500 bytes for Ethernet.
• The clock is embedded in the data stream
  encoding, or provided on separate clock lines
  such that the sender and receiver are always in
  synchronization during a frame transmission. Most
  modern WAN framing is built on the High-Level
  Data Link Control (HDLC) frame structure. An HDLC
  frame has the following general structure
• The flag is a sequence 01111110 which delimits
  the start of the frame. A technique known as bit
  stuffing is used to insert additional zeros into
  the data so that a flag sequence never appears
  anywhere but at the start and end of a frame.
  These extra bits are "unstuffed" again by the
  receiver.

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Synchronous Serial Transmission

• The address field is usually one byte, but may be
  more. It is used to indicate the sender or
  intended receiver of the frame. It is possible to
  have multiple stations connected to a single
  wire, and to design the system so that each
  receiver only "sees" frames with its own address.
  By this means multiple stations can communicate
  with each other using just one line (for instance
  on a Local Area Network).
• The control field is one or more bytes. It
  contains information on the type of frame (for
  instance, whether this is a frame containing user
  data or a supervisory frame which performs some
  sort of link control function). It also often
  contains a rotating sequence number that allows
  the receiver to check that no frame has been
  lost.
• The "payload" of the frame is the data field. The
  data in this field is completely transparent. In
  fact, it does not even have to be organized in 8
  bit bytes, it is a purely arbitrary collection of
  bits.
• Following the data field are two bytes comprising
  the Cyclic Redundancy Check(CRC). The value of
  these bytes is the result of an arithmetic
  calculation based on every bit of data between
  the flags. When the frame is received, the
  calculation is repeated and compared with the
  received CRC bytes. If the answers match then we
  are sure to a very high degree of certainty that
  the frame has been received exactly as
  transmitted. If there is a CRC error the received
  frame is usually discarded.
• Finally, the frame is terminated by another flag
  character.
• Synchronous communication is usually much more
  efficient in use of bandwidth than Asynch. The
  data field is usually large in comparison to the
  flag, control, address, and CRC fields, so there
  is very little overhead.

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Serial Communication
Name Sync/Async Type Duplex Max devices Maxspeed(Kbps) Maxdistance(feet) Pincount (not including ground)
RS-232 async peer full 2 115.2 30 2 (or 4 with HW handshake)
RS-422 async multi-drop half 10 10000 4,000 1 (unidirectional only, additional pins for each bidirectional comm.)
RS-485 async multi-point half 32 10000 4,000 2
I2C sync multi-master half Limitation based on bus capacitance and bit rate 3400 lt10 2
SPI sync multi-master full Limitation based on bus capacitance and bit rate gt1000 lt10 31(Additional pins needed for every slave if slave count is more than one)
Microwire sync master/slave full Limitation based on bus capacitance and bit rate gt625 lt10 31(Additional pins needed for every slave if slave count is more than one)
1-Wire async master/slave half Limitation based on bus capacitance and bit rate 16 1,000 1
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Data Communication Terminology

• Channel
• A channel is a portion of the communications
  medium allocated to the sender and receiver for
  conveying information between them. The
  communications medium is often subdivided into a
  number of separate paths, each of which is used
  by a sender and receiver for communication
  purposes.
• Baud Rate
• Baud rate is the same as symbol rate and is a
  measure of the number of line changes which occur
  every second. Each symbol can represent or convey
  one (binary encoded signal) or several bits of
  data. For a binary signal of 20Hz, this is
  equivalent to 20 baud (there are 20 changes per
  second).
• Bits Per Second
• This is an expression of the number of data bits
  per second. Where a binary signal is being used,
  this is the same as the baud rate. When the
  signal is changed to another form, it will not be
  equal to the baud rate, as each line change can
  represent more than one bit (either two or four
  bits).
• Bandwidth
• Bandwidth is the frequency range of a channel,
  measured as the difference between the highest
  and lowest frequencies that the channel supports.
  The maximum transmission speed is dependant upon
  the available bandwidth. The larger the
  bandwidth, the higher the transmission speed.

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Protocols and Synchronization

• Protocols
• A protocol is a set of rules which governs how
  data is sent from one point to another. In data
  communications, there are widely accepted
  protocols for sending data. Both the sender and
  receiver must use the same protocol when
  communicating.
• BY CONVENTION, THE LEAST SIGNIFICANT BIT IS
  TRANSMITTED FIRST (Network order)
• ASYNCHRONOUS PROTOCOLS
• Asynchronous systems send data bytes between the
  sender and receiver. Each data byte is preceded
  with a start bit, and suffixed with a stop bit.
  These extra bits serve to synchronize the
  receiver with the sender.
• Transmission of these extra bits (2 per byte)
  reduce data throughput. Synchronization is
  achieved for each character only. When the sender
  has no data to transmit, the line is idle and the
  sender and receiver are NOT in synchronization.
  Asynchronous protocols are suited for low speed
  data communications.
• SYNCHRONOUS PROTOCOLS
• Synchronous protocols involve sending timing
  information along with the data bytes, so that
  the receiver can remain in synchronization with
  the sender. When the sender has no data to
  transmit, the sender transmits idle flags (a
  sequence of alternating 0's and 1's) to maintain
  sender/receiver synchronization. Data bytes are
  packaged into chunks called packets, with address
  fields being added at the front (header) and
  checksums at the rear of the packet.

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Data Multiplexing

• A multiplexer is a device which shares a
  communication link between a number of devices
  (users).
• Rather than provide a separate circuit for each
  device, the multiplexer combines each low speed
  circuit onto a single high speed link. The cost
  of the single high speed link is less than the
  required number of low speed links.
• It does this by time or frequency division.

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Time Division Multiplexing

• In time division, the communications link is
  subdivided in terms of time.
• Each sub-circuit is given the channel for a
  limited amount of time, before it is switched
  over to the next user, and so on
• In the picture bellow it can be seen that each
  sub-channel occupies the entire bandwidth of the
  channel, but only for a portion of the time

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Frequency Division Multiplexing

• In frequency division multiplexing, each
  sub-channel is separated by frequency (each
  sub-channel is allocated part of the bandwidth of
  the main channel)
• The speed or bandwidth of the main link is the
  sum of the individual sub-channel speeds or
  bandwidth.

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Modems

• Modems are devices which allow digital data
  signals to be transmitted across an analogue
  link.
• Modem stands for modulator/demodulator. A modem
  changes the digital signal to an analogue
  frequency, and sends this tone across the
  analogue link. At the other end, another modem
  receives the signal and converts it back to
  digital.

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Modulation Techniques

• Modulation techniques are methods used to encode
  digital information in an analogue world.
• There are three basic modulation techniques
• AM (amplitude modulation)
• FM (frequency modulation)
• PM (phase modulation)
• All 3 modulation techniques employ a carrier
  signal. A carrier signal is a single frequency
  that is used to carry the intelligence (data).
• For digital, the intelligence is either a 1 or 0.
  
• When we modulate the carrier , we are changing
  its characteristics to correspond to either a 1
  or 0.

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Amplitude Modulation

• Modifies the amplitude of the carrier to
  represent 1s or 0s
• a 1 is represented by the presence of the carrier
  for a predefined period of 3 cycles of carrier.
• Absence or no carrier indicates a 0
• Pros
• Simple to design and implement
• Cons
• Noise spikes on transmission medium interfere
  with the carrier signal.
• Loss of connection is read as 0s.

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Frequency Modulation

• Modifies the frequency of the carrier to
  represent the 1s or 0s.
• a 0 is represented by the original carrier
  frequency
• a 1 by a much higher frequency ( the cycles are
  spaced closer together)
• Pros
• Immunity to noise on transmission medium.
• Always a signal present. Loss of signal easily
  detected
• Cons
• Requires 2 frequencies
• Detection circuit needs to recognize both
  frequencies when signal is lost.

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Phase Modulation

• Phase Modulation modifies the phase of the
  carrier to represent a 1 or 0.
• The carrier phase is switched at every occurrence
  of a 1 bit but remains unaffected for a 0 bit.
• The phase of the signal is measured relative to
  the phase of the preceding bit. The bits are
  timed to coincide with a specific number of
  carrier cycles (3 in this example 1 bit)
• Pros
• Only 1 frequency used
• Easy to detect loss of carrier
• Cons
• Complex circuitry required to generate and detect
  phase changes

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Summary

• In simplex circuits, data only travels one way.
  In half-duplex circuits, data travels in both
  directions but not at the same time. In
  full-duplex circuits, data can travel in both
  directions at the same time.
• Parallel circuits use a separate wire for each
  bit of data, and also use wires to convey timing
  information. Serial circuits use the same wire
  for all data bits, and timing information is sent
  along with the data. Parallel transmission is
  faster. Examples of parallel circuits in
  computers are the address, data and control bus.
• In asynchronous communication, each data element
  like a character is prefixed with a start and
  stop bit. In synchronous communication the data
  is accompanied (either explicitly or implicitly)
  by a clock signal.
• A modem is a device which allows computer data to
  be sent over the telephone (dial-up) networks