Transmission Modes

􀂄For various reasons, data streams are often considered to be composed of various elements:
􀁺Bits –0 or 1
􀁺Characters –eight bit sequences
􀁺Blocks or frames –potentially variable numbers of bits
􀂄It is necessary to determine the start and end of these elements. The technique for doing this is synchronization.
􀂄There are two transmission modes:
􀁺asynchronousand synchronous.
􀂄We will examine bit synchronization for both synchronous and asynchronous transmissions

Asynchronous Transmission
􀂄Used primarily when the data to be transmitted is generated at random intervals. E.g.: a user typing at a keyboard communicating with a computer.
􀂄Generally used in applications where the data to be transferred consists of characters, each character being encoded using 7 or 8 binary bits, common coding schemesbeing ASCII and EBCDIC.
􀂄As data is transferred randomly there may be long intervals during which no data signal is present on the line. The receiver must be able to resynchroniseat the start of each new character received.

Disadvantage of Asynchronous Transmission
􀂄The use of the start and stop bits for each byte transferred means the method is inefficient in its use of transmission capacity.
􀂄The bit synchronisation method becomes less reliable as the bit rate increases.
􀂄So we look at some of the synchronous transmission schemes

Synchronous Transmission
Used for large blocks of data at higher bit rates. A frame of data is transmitted as a contiguous bit stream with no delay between each 8-bit element.
􀂄The receiver clock operates in synchronism with the received signal.
􀂄There are two methods of achieving this:
􀁺Embeddingthe clock information into the transmitted signal and having the receiver extract it.
􀃎Requires either return to zero or transition orientated scheme.
􀁺The receiver keeps a localclock, which is kept synchronizedwith the received signal using a Digital Phase Lock Loop (DPLL)
􀃎May be used for NRZ schemes
􀃎Requires sufficient transitions to keep synchronization
•Uses bit stuffing (more later)

Media Types
There are a number of transmission media types used for Data Communication. The choice of medium depends on:
Distance to be covered
Desired Bite Rate (in bits per second, bps)
Cost Considerations
Media are often categorised as:
Guided
Wireless
Satellite

Multilevel Modulation Techniques

􀂄So far we have had Bit rate = Baud Rate, where each signal element corresponded to just one bit of data, but more bits per signal element can be encoded.
􀂄Common Techniques include:
􀁺Quadraturephase shift keying (QPSK)or 4 PSK which allows 4 phase changes
􀁺QuadratureAmplitude Modulation (QAM)which changes phase and amplitude
􀂄The more bits per signal element the higher the throughput is, but the more complex the scheme is.

Phase Modulation Schemes

􀂄Phase shift keying (PSK):Uses two fixed signals with a 180 phase difference to each other to indicate a 0 or a 1 respectively.
􀁺Complex demodulation circuitry needed to recover the reference phase.
􀂄Differential PSK (DPSK):Uses a phase shift of 90 relative to the current signal to indicate a binary 0, and a 270 phase shift for binary 1.
􀁺This has simpler demodulation equipment.

Modulation Schemes

􀂄A single frequency signal, known as the Carrier, is selected to lie within acceptable range of frequencies.
􀂄Amplitude, Frequency or Phase is then varied, or keyed, in accordance with the data signal to be transmitted.
􀁺Amplitude shift keying(ASK or AM) is rarely used because of attenuation problems.
􀁺Frequency shift keying(FSK or FM), is used with lower bit rate modems. Relatively simple demodulation circuitry.
􀂄The Signalling Rateor Baud Rateis the number of times per second the amplitude, frequency or phase of the transmitted signal changes.

Encoding schemes: Transition Orientated

Manchester encoding: binary 1: transition from A/2 to –A/2 in middle of bit time interval, binary 0: transition from –A/2 to +A/2 in middle of bit time interval.

Differential Manchester encoding: There is a transition at the centre of each bit, but there is only a transition at the startof a 0 bit.

NRZ inverted (NZRI):starting at a fixed signal level, binary 1: denoted by a transition and binary 0: by no transition.
 The maximum frequency of a NZRI signal is half that of Bipolar and Manchester encoded signals so it only requires half the transmission bandwidth
Use NRZI in Wide Area Networks (WANs) while the other methods are generally used in LANs.

Encoding schemes: Polar Schemes

􀂄Polar encoding schemes rely on the voltage level to make a determination of whether a binary 1 or a 0 was sent.
􀂄Common Schemes:
􀁺UnipolarNRZ:binary 1: +A volts, binary 0: 0 volts.
􀁺Polar NRZ:binary 1: +A/2 volts, binary 0: –A/2 volts.
􀃎Half the power requirement of unipolarNRZ
􀁺Bipolar:Consecutive binary 1s: +/-A/2 volts, binary 0: 0 volts
􀃎Produces a frequency spectrum with less low frequency components.
􀂄Drawbacks to polar encoding schemes are:
􀁺Long strings of either 1s or 0s can cause loss of timing information
􀁺Systematic errors in polarity can cause all 1s to be read as 0s and vice versa

Converting Bits to Signals

There are two fundamentally different ways to produce digital signals
DC or lowpasswhere bits are represented using square waves.
Common encoding schemesinclude:

• Non-return-to-zero (NRZ)

• Bipolar

• Manchester

Bandpassor modulatedwhere bits are represented using fixed frequency sinusoids. This is used when the channel does not pass low frequency signals.
Common modulation techniquesinclude:

• Amplitude shift keying (ASK)

• Frequency shift keying (FSK)

• Phase shift keying (PSK)