5 digital datacomm
TRANSCRIPT
![Page 1: 5 digital datacomm](https://reader036.vdocument.in/reader036/viewer/2022062405/554f47d0b4c905524c8b4746/html5/thumbnails/1.jpg)
Digital Data Communications Techniques
Key point
![Page 2: 5 digital datacomm](https://reader036.vdocument.in/reader036/viewer/2022062405/554f47d0b4c905524c8b4746/html5/thumbnails/2.jpg)
2
Asynchronous and Synchronous Transmission• Timing problems require a mechanism to
synchronize the transmitter and receiver—Timing (rate, duration, spacing) must be same
for transmitter and receiver.
• Two solutions—Asynchronous—Synchronous
![Page 3: 5 digital datacomm](https://reader036.vdocument.in/reader036/viewer/2022062405/554f47d0b4c905524c8b4746/html5/thumbnails/3.jpg)
3
Asynchronous• Data transmitted one character at a time
—5 to 8 bits
• Timing only needs maintaining within each character
• Resynchronize with each character
![Page 4: 5 digital datacomm](https://reader036.vdocument.in/reader036/viewer/2022062405/554f47d0b4c905524c8b4746/html5/thumbnails/4.jpg)
4
Asynchronous (diagram)
![Page 5: 5 digital datacomm](https://reader036.vdocument.in/reader036/viewer/2022062405/554f47d0b4c905524c8b4746/html5/thumbnails/5.jpg)
5
Effect of timing error
• Assume a data rate of 10,000 bits per second (10kbps); therefore, each bit is of 0.1 millisecond (ms), or 100 us, duration. Assume that the receiver is fast by 6%, or 6 us per bit time. Thus the receiver samples the incoming character every 94 us (based on the transmitter’s clock). As can be seen, the last sample is erroneous.
![Page 6: 5 digital datacomm](https://reader036.vdocument.in/reader036/viewer/2022062405/554f47d0b4c905524c8b4746/html5/thumbnails/6.jpg)
6
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
![Page 7: 5 digital datacomm](https://reader036.vdocument.in/reader036/viewer/2022062405/554f47d0b4c905524c8b4746/html5/thumbnails/7.jpg)
7
Synchronous - Bit Level• Block of data transmitted without start or stop bits• To prevent timing drift between transmitter and
receiver, their clocks must be synchronized—Can use separate clock line between transmitter and
receiver• 1 side eg. tramsmitter pulses the line regularly one short
pulse a time• Good over short distances• Subject to impairments (long distances)
—Embed clock signal in data• Manchester encoding (digital signals)• Carrier frequency (analog)
– Synchronize receiver based on phase of carrier
![Page 8: 5 digital datacomm](https://reader036.vdocument.in/reader036/viewer/2022062405/554f47d0b4c905524c8b4746/html5/thumbnails/8.jpg)
8
Synchronous - Block Level• Need to indicate start and end of block• Use preamble and postamble• More efficient than asynchronous
![Page 9: 5 digital datacomm](https://reader036.vdocument.in/reader036/viewer/2022062405/554f47d0b4c905524c8b4746/html5/thumbnails/9.jpg)
9
Synchronous (diagram)
![Page 10: 5 digital datacomm](https://reader036.vdocument.in/reader036/viewer/2022062405/554f47d0b4c905524c8b4746/html5/thumbnails/10.jpg)
10
Types of Error• An error occurs when a bit is altered between
transmission and reception• Two general types of error:
—Single bit errors• One bit altered• Adjacent (nearby) bits not affected• White noise (eg)
—Burst errors• Length B• Contiguous (adjacent) 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
(eg)
![Page 11: 5 digital datacomm](https://reader036.vdocument.in/reader036/viewer/2022062405/554f47d0b4c905524c8b4746/html5/thumbnails/11.jpg)
11
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 undetectedExample : odd parity
Tx = 1110001 + odd (1) = 11110001 Rx = 11110001 no error
Rx = 11100001 error
![Page 12: 5 digital datacomm](https://reader036.vdocument.in/reader036/viewer/2022062405/554f47d0b4c905524c8b4746/html5/thumbnails/12.jpg)
12
Cyclic Redundancy Check (CRC)• For a block of k bits transmitter generates n bit sequence
• Transmit k+n bits which is exactly divisible by some number
• Receive divides frame by that number—If no remainder, assume no error
![Page 13: 5 digital datacomm](https://reader036.vdocument.in/reader036/viewer/2022062405/554f47d0b4c905524c8b4746/html5/thumbnails/13.jpg)
CRC
13
![Page 14: 5 digital datacomm](https://reader036.vdocument.in/reader036/viewer/2022062405/554f47d0b4c905524c8b4746/html5/thumbnails/14.jpg)
14
Binary Division / modulo 2 arithmetic
•At transmitter•String sent = data + CRC bits
•At receiver• if remainder = 0 no error•If remainder not 0 error
![Page 15: 5 digital datacomm](https://reader036.vdocument.in/reader036/viewer/2022062405/554f47d0b4c905524c8b4746/html5/thumbnails/15.jpg)
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 one frame transmission time
• Would result in retransmission of frame in error plus many subsequent frames
• Need to correct errors on basis of bits received
![Page 16: 5 digital datacomm](https://reader036.vdocument.in/reader036/viewer/2022062405/554f47d0b4c905524c8b4746/html5/thumbnails/16.jpg)
Error correction
16
k = message, FCS = frame check sequence,FEC=forward error correction n = total bit to be transmit (k+FCS or k + FEC))
On transmission end , each k bit block of data is mapped into an n-bit block (n>k) called codeword, using FEC encoder, the codeword is then transmitted. Due to the impairments, which may produce bit errors in the signal, the block of data is passed through an FEC decoder with one of four possible outcome
DIAGRAM
![Page 17: 5 digital datacomm](https://reader036.vdocument.in/reader036/viewer/2022062405/554f47d0b4c905524c8b4746/html5/thumbnails/17.jpg)
FOUR POSSIBLE OUTCOME
17
1. NO bit error2. Detect and correct those errors3. Detect but not correct errors4. Error occur but Not detected and not correct errors
![Page 18: 5 digital datacomm](https://reader036.vdocument.in/reader036/viewer/2022062405/554f47d0b4c905524c8b4746/html5/thumbnails/18.jpg)
Block Code Principles
18
HAMMING DISTANCE = d( V1, V2 ) which v1 disagree v2Rule = select lowest d(v1,v2) as original codeword
example v1 = 011011 v2= 110001 , so d (v1,v2)= 3
Example
For k =2 and n = 5
Data block codeword received codeword = 00100 00 00000 d(v1,v2) = 1
01 00111 d(v1,v2) = 210 11001 d(v1,v2)=311 11110 d(v1,v2)=4
![Page 19: 5 digital datacomm](https://reader036.vdocument.in/reader036/viewer/2022062405/554f47d0b4c905524c8b4746/html5/thumbnails/19.jpg)
Block code principles
19
![Page 20: 5 digital datacomm](https://reader036.vdocument.in/reader036/viewer/2022062405/554f47d0b4c905524c8b4746/html5/thumbnails/20.jpg)
20
ERROR CORRECTION PROCESS
![Page 21: 5 digital datacomm](https://reader036.vdocument.in/reader036/viewer/2022062405/554f47d0b4c905524c8b4746/html5/thumbnails/21.jpg)
21
Key PointSynchronization• The receiver must know the rate at which
bits are being received so that it can sample the line at appropriate intervals to determine the value of each received bit.