Bits Bytes and Packets

1
Bits Bytes and Packets

• Unit 5
• Communication Technology

2
Data representation

• All digital data is represented as bits (ones and
  zeros)
• The bits are encoded by varying the electrical
  voltage.

Time
Voltage
3
Bytes

• The stream of data bits needs to be structured in
  some way.
• A bit stream is normally divided in groups of 7
  bits, sometimes referred to as bytes.
• Error checking is important because, during
  transmission factors such as electrical
  interference may corrupt single or multiple bits
  so what is sent as a one may be interpreted at
  the receiving end as a zero and vice versa.

1
0
1
1
1
0
0
byte
bits
4
Terminology

• Simplex A single direction of data transmission.
• Half Duplex(HDX) Data may be transferred in both
• directions but not simultaneously.
• Full Duplex (FDX) Data may be transferred in both
• directions simultaneously Data may be
  transferred in both
• directions simultaneously.

5
Terminology

• Data Terminal Equipment (DTE) - a terminal
• Data Communication Equipment (DCE) a modem -
  Modulator Demodulator

6
Terminology

• Asynchronous transmission Data transmitted at a
  rate
• independent on a clock signal
• Synchronous transmission Data transmitted at a
  rate dependent
• on a clock signal
• Start Bit A data bit used in Asynchronous
  transmission to
• determine the start of a data stream
• Stop Bit A data bit used in Asynchronous
  transmission to
• determine the end of a data stream
• Parity Bit A data bit appended to a data stream
  for use for
• error checking.

7
Parity

• In each group of 7 bits (byte) a parity bit is
  added
• This is used for error checking.
• If using even parity, a one or zero is added to
  the 7 data bits to make the total number of ones
  even.
• Odd parity is occasionally used, this involves
  adding a one or a zero to make the total number
  of ones odd.
• The sending end adds the parity bit, the
  receiving end checks it.
• If the receiving end finds a odd number of bits
  in a 8 bit byte (when using even parity) is knows
  there has been an error.

8
Parity example
There are 4 ones in the data, so with even parity
a zero is added to keep the number of ones even
0
1
1
0
1
1
0
0
7 data bits
There are 3 ones in the data, so with even parity
a 1 is added to make the number of ones even
1
1
1
0
0
1
0
0
9
Parity
Using even parity, what should the parity bit be
on these bytes?
?
0
0
0
0
1
0
0
?
1
1
0
0
1
1
0
?
0
1
0
0
0
1
0
Parity is a simple method of error detection but
it only works with certain types of error. What
happens if two bits within the byte are wrongly
set?
10
Asynchronous transmission

• In the simplest form of data transmission, data
  is split up into bytes, and a parity bit is added
  
• Start and stop bits are also added to help the
  receiving end identify successive bytes. Start
  bits are zeros, stop bits are ones
• Therefore to send 7 bits of data asynchronously
  10 bits are required

0
?
1
7 data bits
Stop bit
Start bit
Parity bit
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Asynchronous transmission

• The addition of the start and stop bits reduces
  the efficiency of transmission, adding a 20
  overhead.
• Asynchronous transmission is therefore only
  suitable if the volume of data being transferred
  is low.
• Lack of timing information restricts asynchronous
  transmission to low speed.
• Timing is important because the receiving end
  must know the exact amount of time the sending
  end uses to send each bit.

12
Synchronous transmission

• Synchronous transmission is used to overcome the
  limitations of asynchronous transmission.
• Rather than sending individual bytes, a block of
  bytes is sent.
• Timing information precedes the block of data so
  the receiving end can lock on to the timing of
  the sender.
• With synchronous transmission data and control
  (timing) bits are put together in a structure
  called a frame or packet.

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Synchronous data packet

• The exact format of the packet depends upon the
  protocol in use.
• Typically it might looks something like this

Up to 50 data bytes
This byte marks the end of the packet
These are synchronisation bytes
Packet
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Synchronous transmission

• Typically more efficient than asynchronous
  because the ratio of data to control information
  is higher.
• Higher speeds can be achieved because the
  synchronisation information sent allows the
  receiving end to lock on to the precise timing
  using a known bit pattern.
• Why limit the amount of data sent in each packet?
  Wouldnt it be more efficient to have really
  large packet sizes?