IT4101 Data Communications

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PREPARING FOR THE BIT
IT4101 Data CommunicationsNetworks
dcnet_at_ict.cmb.ac.lk
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Data communicationsandNetworks
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• C H Log2( 1 S/N) bits per second.
• In this equation
•   (a)   C is the bandwidth of the signal.
• (b)   H is the bandwidth of the Channel.
• (c)   C is the capacity of the Channel.
• (d)   H is the Bandwidth of the signal.
• (e)   S/N is applied in decibels.

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Bit Rate and the Bandwidth of a Channel

• A signal can be decomposed into an infinite
  number of Fourier components.

• Transmission facilities diminish the power of the
  Fourier components as they are transmitted.

• Above a certain frequency (fc cutoff frequency)
  no Fourier components will pass through.

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Maximum Data Rate of a Noisy Channel

• The unwanted signals present on a channel are
  collectively called as noise.

• The amount of noise present, is measured by the
  ratio known as Signal-to-Noise Ratio. In
  practice, it is given as
• 10 log10 S/N in decibels (written as dB).

Where S Signal Power N Noise Power
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Maximum Data Rate of a Noisy Channel Ctd.

• The equation which provides the relationship
  between the maximum data rate and the bandwidth
  of the channel is

Maximum Data Rate H log2 (1 S/N) bits/sec
Where H Bandwidth of the channel S/N
Signal-to-Noise Ratio of the channel.
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2. In Asynchronous transmission  (a) Complete
Synchronization is achieved using the Start and
Stop bits. (b) Complete Synchronization is
achieved using the Start, Stop bits and
communication Control characters. (c) Complete
Synchronization is achieved using the Start, Stop
and Parity bits. (d) Using parity bits can
increase the efficiency. (e) The efficiency is
lower than that is in Synchronous Transmission.
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Synchronization

• Since the information are encoded into digital
  signals at the transmitting side, for the
  receiver to interpret the incoming information
  correctly, the receiver must be able to identify
  the exact boundaries of each bit, each byte, and
  each block of information.

• Enabling this capability in a transmission link
  is called synchronization.

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Synchronization Ctd.

• Synchronization in a transmission link is
  achieved in three levels

• Bit or Clock Synchronization

• Byte or Character Synchronization

• Block or Frame Synchronization.

• There are two approaches followed in common
  practice.

• Asynchronous Transmission
• Synchronous Transmission

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Asynchronous Transmission

• The bit rate is known in advance by both
  transmitting and receiving ends.

• The level of the link is maintained at high
  when no bits are transmitted.

• The number of data bits, use and the type of the
  parity bit, and the number of stop bits are also
  agreed upon in advance, and each transmitted
  element (byte, character,etc.) is encapsulated
  with a Start bit and one or more Stop bits.

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Asynchronous Transmission Ctd.
Time
Start of Transmission
1 1 0 0 0 1 0 1
Stop Bit(s)
8 data bits
Start Bit
Even Parity Bit
Direction of Transmission
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Asynchronous Transmission Ctd.
Stop Bit(s)
Start Bit
Stop Bit(s)
Start Bit
1 1 0 0 0 1 0 1
1 0 0 0 0 1 0 1
1st Byte
2nd Byte
Direction of Transmission
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Asynchronous Transmission Ctd.

• The bit rates known by both ends enable the Bit
  or Clock synchronization.

• The use of start and Stop bits enables the Byte
  or Character synchronization.

• The communication control characters such as STX,
  ETX are used to achieve the Block or Frame
  synchronization.

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Asynchronous Transmission Ctd.

• When one Start bit, one Parity bit and one Stop
  bit is used for each transmitted element, for
  example a byte, the actual throughput can be
  examined by the efficiency.

• In the above case the efficiency is
• (8/11) x 100 72.72

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• C H Log2( 1 S/N) bits per second.
• In this equation
•   (a)   C is the bandwidth of the signal.
• (b)   H is the bandwidth of the Channel.
• (c)   C is the capacity of the Channel.
• (d)   H is the Bandwidth of the signal.
• (e)   S/N is applied in decibels.

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2. In Asynchronous transmission  (a) Complete
Synchronization is achieved using the Start and
Stop bits. (b) Complete Synchronization is
achieved using the Start, Stop bits and
communication Control characters. (c) Complete
Synchronization is achieved using the Start, Stop
and Parity bits. (d) Using parity bits can
increase the efficiency. (e) The efficiency is
lower than that is in Synchronous Transmission.
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Contact
External Degree Unit (EDU) of the Institute of
Computer Technology No. 221/2A, Dharmapala
Mawatha, Colombo 7. Phone [phone] Fax
074-720512 Email bit_at_ict.cmb.ac.lk htt
p//www.bit.lk