Synchronous And Asynchronous Transmissions

Synchronous and Asynchronous Transmissions

Introduction • Digital signal = waveform from a finite set • Signal has time definition – Time information to be known by the receiver is essential • Time info seperately sent – short distance • Signal itself carries time info – long distance

• Line coding provides the solution • The best design carries – Inherent Synchronisation information – Lesser bandwidth – Robustness to system inaccuracies

Pulse Transmission • Signal f(t) = rect(t/T); the spectrum will be F(ω ) = = T

sin(ωT / 2) ωT / 2

• The energy will be preserved in the bandwidth 1/2T. – The data rate

Rmax = 2.BW

R = signaling rate = 1/T BW = available bandwidth

• When channel bandwidth limited to 1/2T inter-symbol interference (ISI) occurs • Square shape signal is undesirable and infinite duration signal is required to confine bandwidth • Band limiting is done with pulse shaping filter and delay is associated to make the filter realisable

Inter symbol Interference • Causes • • • •

Timing inaccuracies Insufficient bandwidth Amplitude distortion Phase distortion

• Timing inaccuracies cause at Tx and Rx • At Tx, do not confirm the ringing frequencies – not significant due to sharp filter cutoffs • At Rx, timing derived from noisy, distorted signal leads to inaccuracy – Sensitivity will be less when bandwidth is larger than Nyquist rate

• Insufficient Bandwidth • When BW is reduced, riniging frequency is reduced and ISI increased • In some systems, ISI is purposely introduced – partial response signaling

Inter symbol Interference… • Amplitude distortion and phase distortion – both modifies the waveform • Amplitude distortion • The frequency response of the channel cannot always be predicted • Different frequency component will be amplified with different gain • Compensated with amplitude equalisers

• Phase distortion • Due to different delays for different frequency components • Componsated using phase equalisers

Transmission of pulses • Two modes of transmission • Synchronous • Asynchronous

• Asynchronous • Group of bits or characters are transmitted with predefined interval between them • But, the time of transmission is unrelated to each other • Nominal clock is enough to recover the signal – the sample clock is reestablished for reception of each group

• Synchronous • Digital signals are sent continuously at a constant rate • Synchronised clock is required for an infinite period of time

Asynchronous transmission • • • • • •

Between transmissions, line inactive (idle) The beginning is identified by start bit – one or more(accurate sampling clock) The line returns to idle state when stop bit is received – one or more Nominal clock is enough Optimal for low speed data Sample time is middle to start bit • Due to noise and distortion, drift occurs – Asynchronous become poor for high noise environment

• Offset in the clock frequency larger drift over time makes asynchronous become poor at high data rates • Maximum length in each symbol group and

•

Advantages • Easy determination of sample time, • Automatic character framing (for high rate – one after another; low rate – idle time), • highly applicable for varying data rates

Asynchronous transmission… • Isochronous • During the transmission of asynchronous transmission on synchronous line • Adjusting the information rate by inserting null codes

• Drawback • Poor performance – higher error in noisy environment • Data rates upto 1200bps (voiced band modem) are optimal • Long distance communication via asynchronous is obsolete – cost and performance of synchronous being advantageous

Synchronous transmission • Used in interoffice digital transmission links – T1 lines • Line codes designed to ensure the synchronisation of local clock with incoming signaling rate • To provide continuous indication of signal boundaries artificial transitions are introduced – transmission overhead and loss in capacity • Techniques • • • • •

Source code restriction Dedicated timing bits Bit insertion Data scrambling Forced bit errors

• Line code inserting the transitions themselves – sixth techniques

Synchronous transmission… • Source code restriction • Insert suuficient number of signal transitions and restrict the length • Drawback » Loss of information not suitable for random data » In T1 system loss is one part of 256. » Eliminated by the introduction of binary eight-zero substitution technique

• Dedicated timing bits • Periodically insert transition bearing bits independent to source data • Channel capacity is reduced • Dataphone digital Service (DDS) over T1 line offers capacity 56kbps – out 8 bits 7 bits carries user data ; one bit for timing • In fiber systems timing bits are 1 in 5 bits to 1 in 20 bits

Synchronous transmission… • Bit insertion • Instead at regular interval, inserted only when required • Loss in capacity is minimum • Analogous to zero-bit insertion (flag at end) in High Level Data Link Control (HDLC) protocol – insert a zero when five ones comes continuously • Drawback – Delay due to insertion requires buffer to smooth the arrival rate – In time division multiplex system, alteration in character stucture makes boundary maintenance critical

Synchronous transmission… • Data scrambling • Similar to encryption • Data is scrambled to randomise the data pattern • Useful to get strong timing components for data with low transition densities • Not used in T-carrier systems, but used in 274Mbps T4M coaxial transmission and optical fiber systems • Drawback – Do not prevent long strings of 0’s

Synchronous transmission… • Forced bit errors • Errors are forcibly introduced to make transition – intentional bit errors are less frequent than random bit errors – performance not much affected • Drawback – The bit error would not be controlled when it is introduced by transmission link, – The significance of bit will be known only when the transition is introduced byt the source

• In ARQ, the error introduced by the transmission link, creates repeated retransmission