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Chapter 4(Only 4.2 – 4.3)

Digital Transmission

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Advantages of Digital Transmission Low cost technology Longer distances using repeaters Security & Privacy

Encryption Noise can be easily removed Cheaper and easier digital Multiplexing

(Combining data from different sources into one link)

Allows processing of both analog data (voice, video) and digital data by the computer

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Figure 4.1 Digital Transmission of Digital Data

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Digital Transmission of Digital Dat

1= 3V

0= 0V

1= +3V

0= -3V

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4.3 Transmission Mode4.3 Transmission Mode

Parallel Transmission

Serial Transmission

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Figure 4.31 Data transmission and modes

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Figure 4.32 Parallel transmission

Advantage: speed

Disadvantage: high cost (needs a wire for each bit)

limited to short distances

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Figure 4.3 Serial transmission

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Serial Transmission Modes Timing problems require a mechanism to

synchronize (coordinate) the Transmitter and receiver

The receiver must know the rate at which bits are being received so that it can read the value of the line at appropriate intervals to determine the value of each received bit

Two ways to handle the synchronization problem Asynchronous serial transmission Synchronous serial transmission

Modem operates in these two modes.

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Figure 4.4 Lack of synchronization

Receiver clock is faster than the sender clock

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Asynchronous Mode Data transmitted one character (byte) at a time without

any clock line that synchronizes both the sender and the receiver clocks.

Each character (byte) is treated independently Sender and receiver clocks do not have to be synchronized

(same clock speed) all the times but only for the duration of the character (agreed-upon rate)

For each character (byte) at least two bits are added : Start bit = 0 at the beginning (to alert the receiver that a new

character is arriving) One or more Stop bits =1 Each byte may be followed by a gap which can be an idle line or

additional stop bits

Used widely between PC and Keyboard, PC and Modems, PC and Printers, or Fax machines

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Figure 4.34 Asynchronous transmission

Gap is either an idle channel or additional stop bits

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In asynchronous transmission, we send 1 start bit (0) at the beginning and 1 or more stop bits (1s) at the end of each

byte. There may be a gap between each byte.

Note

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Asynchronous here means “asynchronous at the byte level,”

but the bits are still synchronized; their durations are the same.

Note

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Advantages Simple Cheap

Disadvantages Not effective usage of channel bandwidth

Overhead >= 2/(8-bit ASCII code + 2 ) = 20% Not suitable for long blocks of data

because the receiver’s clock might drift out of synchronization with the transmitter’s clock.

Used with slow transmissions (one character at a time )

Asynchronous Mode

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Binary bits are combined into blocks of arbitrary length called frame No gaps between blocks and no start or stop bits Synchronization - two methods:

Separate clock line between the sender and receiver (one send the clock and the other uses it as its clock)

Works for only short distances – Noise will affect it for long distances

Embed clock information in the data signal (encoding data into different form). This is called self-synchronizing digital signal

Receiver has the responsibility of recognizing the frame boundaries How the receiver can detect the beginning/end of a block (frame)?

Use bit pattern to mark start (preamble) and end (postamble) of data

This is called Flag: 01111110 Data + control information is called frame Used in high-speed communications such as data transmission

between computers

Synchronous Mode

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Figure 4.35 Synchronous transmission

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In synchronous transmission, we send bits one after another without start or

stop bits or gaps. It is the responsibility of the receiver to group the bits.

Note

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Example on self-synchronizing signal - Manchester encoding

In Manchester encoding, the transition at the middle of the bit provides clocking and data

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Frame example

Advantage In total much less overhead per frame more efficient

than asynchronous Used for high speed transmissions, e.g., between

computers, between modems, and routers

Synchronous Mode – cont.