Digital Data Communications
Techniques
Transmission Mode
• How many bits can be sent in one clock cycle?
• Serial mode
—Only one bit is sent in one clock cycle
•...
Parallel Transmission
Serial Transmission
• Used for long distance transmission. Why?
• Two types: Asynchronous and synchronous
• Asynchronous
•...
Asynchronous Serial
Asynchronous …
• Data transmitted on character at a time
—5 to 8 bits
• Timing only needs maintaining within each
characte...
Asynchronous (diagram)
Asynchronous - Behavior
• In a steady stream, interval between characters
is uniform (length of stop element)
• In idle st...
Synchronous Serial
• Bits are sent one after another without gap
—No start and stop bits
• Receiver must organize bits int...
Synchronous
Types of Error
• An error occurs when a bit is altered between
transmission and reception
• Single bit errors
—One bit alt...
Error Detection Process
Error Detection
• Additional bits added by transmitter for error
detection code
• Parity
—Value of parity bit is such that...
Cyclic Redundancy Check
• For a block of k bits transmitter generates n bit
sequence
• Transmit k+n bits which is exactly ...
Error Correction
• Correction of detected errors usually requires
data block to be retransmitted
• Not appropriate for wir...
Error Correction Process
Diagram
The End
Upcoming SlideShare
Loading in …5
×

Datalink controls

357 views

Published on

all about how data is error is detected and corrected through transmission

Published in: Art & Photos, Technology, Business
0 Comments
0 Likes
Statistics
Notes
  • Be the first to comment

  • Be the first to like this

No Downloads
Views
Total views
357
On SlideShare
0
From Embeds
0
Number of Embeds
3
Actions
Shares
0
Downloads
9
Comments
0
Likes
0
Embeds 0
No embeds

No notes for slide

Datalink controls

  1. 1. Digital Data Communications Techniques
  2. 2. Transmission Mode • How many bits can be sent in one clock cycle? • Serial mode —Only one bit is sent in one clock cycle • Parallel mode —n bits are sent in one clock cycle —Uses n wires to send n bits
  3. 3. Parallel Transmission
  4. 4. Serial Transmission • Used for long distance transmission. Why? • Two types: Asynchronous and synchronous • Asynchronous • One byte of data sent serially • Start bit = 0, Stop bit=1  In total, ten bits are sent • Next byte may not follow previous byte immediately – i.e. there may be idle time between bytes • It is asynchronous only at the byte level. Individual bits are still synchronized.
  5. 5. Asynchronous Serial
  6. 6. Asynchronous … • Data transmitted on character at a time —5 to 8 bits • Timing only needs maintaining within each character • Resynchronize with each character
  7. 7. Asynchronous (diagram)
  8. 8. Asynchronous - Behavior • In a steady stream, interval between characters is uniform (length of stop element) • In idle state, receiver looks for transition 1 to 0 • Then samples next seven intervals (char length) • Then looks for next 1 to 0 for next char • Simple • Cheap • Overhead of 2 or 3 bits per char (~20%) • Good for data with large gaps (keyboard)
  9. 9. Synchronous Serial • Bits are sent one after another without gap —No start and stop bits • Receiver must organize bits into bytes • Synchronous sends more data bits than asynchronous • Synchronous: used for high speed transmission • Asynchronous: for low speed transmission
  10. 10. Synchronous
  11. 11. Types of Error • An error occurs when a bit is altered between transmission and reception • Single bit errors —One bit altered —Adjacent bits not affected —White noise • Burst errors —Length B —Contiguous sequence of B bits in which first last and any number of intermediate bits in error —Impulse noise —Fading in wireless —Effect greater at higher data rates
  12. 12. Error Detection Process
  13. 13. Error Detection • Additional bits added by transmitter for error detection code • Parity —Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones —Even number of bit errors goes undetected
  14. 14. Cyclic Redundancy Check • For a block of k bits transmitter generates n bit sequence • Transmit k+n bits which is exactly divisible by some number • Receiver divides frame by that number —If no remainder, assume no error
  15. 15. Error Correction • Correction of detected errors usually requires data block to be retransmitted • Not appropriate for wireless applications —Bit error rate is high • Lots of retransmissions —Propagation delay can be long (satellite) compared with frame transmission time • Would result in retransmission of frame in error plus many subsequent frames • Need to correct errors on basis of bits received
  16. 16. Error Correction Process Diagram
  17. 17. The End

×