Digital Transmission

2

A Taxonomy of Transmission Modes

• Defn: A transmission mode is the manner in which data is sent over the underlying medium

• Transmission modes can be divided into two fundamental categories:

• Serial — one bit is sent at a time– Serial transmission is further categorized according to

timing of transmissions

• Parallel — multiple bits are sent at the same time

Data transmission and modes

Parallel Transmission

• Parallel transmission allows transfers of multiple data bits at the same time over separate media

• In general, parallel transmission is used with a wired medium that uses multiple, independent wires

• Furthermore, the signals on all wires are synchronized so that a bit travels across each of the wires at precisely the same time

Parallel transmission

Serial Transmission

• Serial transmission– sends one bit at a time

• It may seem that anyone would choose parallel transmission for high speeds– However, most communication systems use serial mode

• There are two main reasons(1)serial networks can be extended over long distances at less cost

(2)using only one physical wire means that there is never a timing problem caused by one wire being slightly longer than another

• Sender and receiver must contain a hardware that converts data from the parallel form used in the device to the serial form used on the wire

Serial transmission

Timing of Serial Transmission

• Serial transmission mechanisms can be divided into three broad categories (depending on how transmissions are spaced in time):

• Asynchronous transmission can occur at any time– with an arbitrary delay between the transmission of two data

items

• Synchronous transmission occurs continuously – with no gap between the transmission of two data items

• Isochronous transmission occurs at regular intervals – with a fixed gap between the transmission of two data items

In asynchronous transmission, we send 1 start bit (0) at the beginning and 1 or more stop bits (1s) at the end of each

byte. There may be a gap between each byte.

Note

Asynchronous here means “asynchronous at the byte level,”

but the bits are still synchronized; their durations are the same.

Note

Asynchronous transmission

In synchronous transmission, we send bits one after another without start or

stop bits or gaps. It is the responsibility of the receiver to group the bits. The bits

are usually sent as bytes and many bytes are grouped in a frame. A frame is identified with a start and an end byte.

Note

Synchronous transmission

Isochronous

• In isochronous transmission we cannot have uneven gaps between frames.

• Transmission of bits is fixed with equal gaps.

Simplex, Half-Duplex, and Full-Duplex Transmission

• A communications channel is classified as one of three types:

(depending on the direction of transfer)

– Simplex

– Full-Duplex

– Half-Duplex

• Simplex: a simplex mechanism can only transfer data in a single direction

– It is analogous to broadcast radio or television

– Figure 9.8a illustrates simplex communication

• Full-Duplex: allows transmission in two directions simultaneously

– It is analogous to a voice telephone conversation

• in which a participant can speak even if they are able to hear background music at the other end

Simplex, Half-Duplex, and Full-Duplex Transmission

Simplex, Half-Duplex, and Full-Duplex Transmission

• Half-Duplex: A half-duplex mechanism involves a shared transmission medium

– The shared medium can be used for communication in each direction

– But the communication cannot proceed simultaneously

– It is analogous to using walkie-talkies where only one side can transmit at a time

• An additional mechanism is needed at each end of a half-duplex communication that coordinates transmission

– to insure that only one side transmits at a given time

BitRate

• Bit rates measure the number of data bits (that is 0′s and 1′s) transmitted in one second in a communication channel. – A figure of 2400 bits per second means 2400 zeros or

ones can be transmitted in one second, hence the abbreviation “bps.”

– Individual characters (for example letters or numbers) that are also referred to as bytes are composed of several bits.

Baud Rate or Signaling Rate

• A baud rate is the number of times a signal in a communications channel changes state or varies.– Named after Emile Baudot – For example, a 2400 baud rate means that the

channel can change states up to 2400 times per second.

– The term “change state” means that it can change from 0 to 1 or from 1 to 0 up to X (in this case, 2400) times per second.

– It also refers to the actual state of the connection, such as voltage, frequency, or phase

Symbol Time

• The symbol duration time is the amount of time it takes to complete one symbol change.

• Because baud stands for the number of signals per second, the symbol duration time tries to calculate the time between each signal in a second

Baud Vs Bit Rate

• The difference between Bit and Baud rate is complicated and intertwining. Both are dependent and inter-related.

• But the simplest explanation is that a Bit Rate is how many data bits are transmitted per second.

• A baud Rate is the number of times per second a signal in a communications channel changes.

Pulse Shaping

• In a digital communication system, digital information can be sent on a carrier through changes in its fundamental characteristics such as: phase, frequency, and amplitude.

• Its purpose is to make the transmitted signal better suited to the communication channel by limiting the effective bandwidth of the transmission.

• By filtering the transmitted pulses this way, the intersymbol interference caused by the channel can be kept in control.

• In RF communication, filter plays an important part because it is effective at eliminating spectral leakage, reducing channel width, and eliminating interference from adjacent symbols (Inter Symbol Interference, ISI).

Pulse shaping

Pulse shaping

• the sync pulse, shown, meets both of these requirements because it efficiently utilizes the frequency domain to utilize a smaller portion of the frequency domain,

• The sinc pulse is periodic in nature and is has a maximum amplitude in the middle of the symbol time.

• In addition, it appears as a square wave in the frequency domain and thus can effectively limit a communications channel to a specific frequency range.

Need of pulse shaping

• In communications systems, two important requirements of a wireless communications channel demand the use of a pulse shaping filter. These requirements are: – 1) generating bandlimited channels– 2) reducing inter symbol interference (ISI) from multi-

path signal reflections

• Both requirements can be accomplished by a pulse shaping filter which is applied to each symbol.

Reducing Channel Bandwidth

• In a multi-channel communications systems, limiting all the power of a modulated carrier to just the carrier bandwidth is extremely important for several reasons.

• First, the transmission power is reduced when the signal has a more concentrated frequency range.

• In addition, limiting a channel to a specific frequency band eliminates adjacent channel interference.

Reducing ISI

• In bandlimited channels, intersymbol interference (ISI) can be caused by multi-path fading as signals are transmitted over long distances and through various mediums.

• More specifically, this characteristic of the physical environment causes some symbols to be spread beyond their given time interval.

• As a result, they can interfere with the following or preceding transmitted symbols.

• One solution to this problem is the application of the pulse shaping filter that we have previously described.

• By applying this filter to each symbol that is generated, we are able to reduce channel bandwidth while reducing ISI.

Line Coding

• Given some binary information, the binary bits are not transmit through the channel as 1’s and 0’s but is used to generate a voltage signal that represents the information we would like to transmit.

• There are different forms of signals (called Line Codes) that can be used to represent the information.

• The terms Return to Zero (RZ) and Non–Return to Zero (NRZ) will be used in describing these signal.

Types of Line coding

• On–Off (NRZ)

• Polar (NRZ)

• On–Off (RZ)

• Polar (RZ)

• Bipolar (RZ)

• Manchester (also called bi-phase)

On–Off (NRZ)

• In this form of line codes, a bit of 1 is represented by some positive voltage (+5 volts for example) and a bit of 0 by 0 volts (justifying calling this signal On–Off).

• The pulses corresponding to binary 1 remain at the positive voltage for the whole duration of the bit period (it does not return to zero at any time during the bit period justifying calling this line code NRZ).

Polar (NRZ)

• A bit of 1 is represented by some positive voltage (+5 volts for example) and a bit of 0 is represented by negative of that voltage (so it would be –5 volts).

• The pulses corresponding to binary 1 and binary 0 remain at the positive and negative voltages, respectively, for the whole duration of the bit period (they do not return to zero).

• The advantage of this line code over the On–Off (NRZ) is that it has zero–DC value when the number of binary 1’s is equal to the number of binary 0’s.

On–Off (RZ)

• A bit of 1 is represented by some positive voltage (+5 volts for example) for half of the bit period and zero in the other half of the bit period and a bit of 0 is represented by zero for the whole bit period.

• This is why this line code is a return–to–zero line code (because any pulse corresponding to binary 1 always returns back to zero).

• The advantage of this line code over the previous line codes is that a long sequence of ones always has transitions at the center of each bit and therefore bit synchronization becomes easy for long sequences of ones.

• Long sequence of zeros is still difficult to be synchronized.

ON-Off (RZ)

Polar (RZ)

• In this line code, a bit of 1 is represented by some positive voltage (+5 volts for example) for half of the bit period and zero in the other half of the bit period and a bit of 0 is represented by the negative of that voltage for half of the period and zero for the other half.

• The advantage of this line code over the previous ones is that long sequences of ones or zeros have transitions at the center of each bit and therefore bit synchronization becomes easy for long sequences of ones or zeros.

• Also, this line code has zero DC when the number of ones and zeros is the same.

Polar (RZ)

Bi-Polar (RZ)

• A bit of 0 is represented by zero volts for the whole bit period. A bit of 1 is represented by some positive voltage (+5 volts for example) for half of the bit period and zero in the other half of the bit period.

• However, the next bit of one (wither it is the next bit or 1000 bits later is represented by the negative of the voltage for half of the bit period and zero for the second half.

• So, the bits of 1’s are represented by alternating positive and negative pulses.

• This insures that the DC value of the signal is always zero even if we have non–equal number of ones and zeros.

Bi-Polar (RZ)

Manchester (Bi-Phase)

• In this line code, a bit of 0 is represented by +5 volts for the first half of the bit period and -5 volts for the second half while a bit of 1 is represented by a -5 volts for the first half of the bit and +5 for the second half of the bit.

• Note that there are transmissions that may occur at the beginning or end of a bit when two consecutive bits have the same bit value (a 1 followed by a 1 or a 0 followed by a 0).

• These transitions however do not carry any information. So, we can say that a up transition in the middle of the bit represents a bit of 1 and a down transition in the middle of the bit represents a bit of 0.

Manchester (Bi-Phase)

Synchronization• Information about the communication channel, such as the channel

phase response, is necessary for the construction of the various receivers

• In many practical situations, this information is not known a priori and the relevant channel parameters have to be estimated from the received signal

• The three main channel parameters required by most receivers are the carrier frequency, the carrier phase, and the symbol timing of the received signal.

• The carrier frequency of the received signal may be different from that of the nominal value of the transmitter carrier frequency.– Due to:-

• Deviation in frequency• Doppler effect

• This transmission delay introduces a mismatch between the symbol timing at the transmitter and that at the receiver

Synchronization

• We need to know the symbol timing at the receiver (or equivalently, the transmission delay) in order to eliminate the performance degradation due to the timing mismatch.

• The process of estimating these parameter is called synchronization.

Synchronization

• The process of estimating the carrier phase is known as carrier phase synchronization, which can be accomplished by a phase-locked loop (PLL) circuit.

• The process of estimating the transmission delay is known as symbol timing synchronization, which can be accomplished by a delay-locked loop (DLL) circuit.

• The same PLL circuit used for carrier synchronization can also be employed to track the carrier frequency mismatch when it is significant.

Bit Error-Rate

• In digital transmission, the number of bit errors is the number of received bits of a data stream over a communication channel that have been altered due to noise, interference, distortion or bit synchronization errors.

• The bit error rate or bit error ratio (BER) is the number of bit errors divided by the total number of transferred bits during a studied time interval. BER is a unitless performance measure, often expressed as a percentage.

• The bit error probability pe is the expectation value of the BER.

Factors effecting BER

• In a communication system, the receiver side BER may be affected by transmission channel– noise– interference– Distortion– bit synchronization problems– attenuation– wireless multi-path fading, etc.

• The BER may be improved by – choosing a strong signal strength – choosing a slow and robust modulation scheme or– line coding scheme, – applying channel coding schemes such as redundant forward

error correction codes.

Calculation of bit-error probabilities with addition of Gaussian Noise

• In a noisy channel, the BER is often expressed as a function of the normalized carrier-to-noise ratio measure denoted Eb/N0, (energy per bit to noise power spectral density ratio), or Es/N0 (energy per modulation symbol to noise spectral density).

• In the case of QPSK modulation and AWGN channel, the BER as function of the Eb/N0 is given by: – BER = 1 / 2erfc(Eb / N0 / sqrt(2))