Unit 5 Lecture #1

What do you understand by An Error in transmission?  When transmission of digital signal takes place between two systems such as computers. The signals get contaminated due to the addition of noise to it.  the noise can introduce an error in the binary bits travelling from one system to the another.  That means a 0 may change to 1 or a 1 may change to 0  These error can become a serious threat to the accuracy of the digital system.

What are the causes of error transmission Transmission errors are caused by following reasons  thermal noise  impulse noise (e.g. relays)  signal distortion during transmission (attenuation)  crosstalk  voice amplitude signal compression (companding)  quantization noise (PCM)  jitter (variations in signal timings)  receiver and transmitter out of synch

What are the types of transmission error? Errors are introduced in the data bits during their transmission across a data network. These can be categorized as: 1.Content errors 2.Flow integrity errors

Content errors Content errors are the errors in the content of a message, e.g. a binary 1 may be received as a binary 0. A block of data may have single bit error or multiple errors. A single bit error is an error in a data unit in which only one bit has been altered Multiple errors in a transmitted data block are referred to as burst error. Length of burst error is defined from the first corrupted bit to the last corrupted bit.

Single-bit error Figure

Multiple-bit error Figure

Single-bit and burst error

Example 1: a 1/100 s burst of impulse noise on a transmission with a data rate of 1200 bps might change all or some of the 12 bits of information Question 1 : calculate the no of bits damaged/changed when a 1 ms burst of impulse noise happens in a transmission with a data rate of 1 Mbps.

There can be several causes of content errors. o Signal impairment – Distortion of electrical signal and noise are the main causes of content errors. An impulse can last long enough to corrupt several bits and therefore leads to burst errors. o Loss of synchronization – The receiver samples the received signal to extract the digital information by using a clock that is synchronized to the clock of the transmitter.

Scramblers – Scramblers is used to ensure synchronization of transmitter and receiver clocks.Its randomizes the serial data before its transmission. String of zeros and ones are thus avoided in the in the transmission signal Scrambles are done using a shift register and exclusive OR gates.

Scrambles have unfortunate property of multiplying errors. If an errors is received, it gets multiplied as it shifts through the shift register. The errors multiplied depend on the number of taps taken from the shift register.

Transmission channel switching Transmission channel of telecommunication network are usually protected, i.e. if the main path fails due to any reason (cut in fiber, fading of radio signal), the signals are switched over to stand by path (alternate fiber route, alternate radio frequency). The switchover may take about 10 to 50 ms, and it causes momentary loss of digital signal. These momentary disturbances are unnoticeable for voice channels but lead to burst errors in data signals.

Flow integrity errors Data is sent as data packets across a data network. It is not necessary that data packets sent by a source to the destination take the same path. In figure packet 1 goes via nodes A, B and D. Packet 2 goes directly from node A to node D. Packet 3 starts its journey from nodes A to D via nodes B and C. Flow of these data packets from source to the destination may be affected in several ways.

The packets may be delivered out-of-sequence. Packet 2 takes shorter path and therefore it may arrive at node D ahead of packet 1. A data packet may not arrive at destination at all. Packet 3 reaches up to node C, but is lost thereafter.

Flow integrity error B A C D