ASSIGNMENT NO – 1

Ques-1: Discuss the various Digital Conversion Techniques or Methods which convert DIGITAL DATA to DIGITAL SIGNAL.

DIGITAL-TO-DIGITAL CONVERSION :

The signal that represents data can be digital or analog. We can represent digital data by using digital signals. The conversion of digital data to digital signal is known as Line

Coding.

LINE CODING:

Line coding consists of representing the digital signal to be transported by an amplitude- and time- discrete signal that is optimally tuned for the specific properties of the physical channel (and of the receiving equipment). The waveform pattern of voltage or current used to represent the 1s and 0s of a digital signal on a transmission link is called line encoding.

Digital Data: 01101110

Digital signal:

Types of Line Coding

Unipolar Scheme :

Unipolar encoding is a line code. A positive voltage represents a binary 1, and zero volts indicates a binary 0. It is the simplest line code, directly encoding the bitstream, and is analogous to on-off keying in modulation.

Non- Return -to- Zero:

A unipolar scheme was designed as a non-return-to-zero scheme, in which the positive voltage defines bit 1 and the zero voltage defines bit 0. It is called NRZ because the signal does not return to zero at the middle of the bit

In clock language, "one" transitions or remains high on the trailing clock edge of the previous bit and "zero" transitions or remains low on the trailing clock edge of the previous bit, or just the opposite. This allows for long series without change, which makes synchronization difficult. One solution is to not send bytes without transitions.Disadvantages of on-off keying are the wastage of power due to the transmitted DC level and also the power spectrum of the transmitted signal does not approach to zero at zero frequency.

Polar Scheme:

In Polar Scheme, the voltages are on the both sides of the time axis. For example, the voltage level for 0 can be positive and the voltage level for 1 can be negative.

Non- Return-to-Zero :In polar NRZ encoding , we use two levels of voltage amplitude. The two versions of polar NRZ are:

NRZ-L NRZ-I

In first variation NRZ-l(NRZ-Level),the level of the voltage determines the value of bit. The second variation NRZ-I(NRZ-Invert). The change or lack of change in level of voltage determines the value of bit.“Zero" is represented by no change in physical level."One" is represented by a change in physical level.In clock language, the level transitions on the trailing clock edge of the previous bit to represent a "zero."

Return-to-zero (RZ)

It describes a line code used in telecommunications signals in which the signal drops (returns) to zero between each pulse. This takes place even if a number of consecutive 0's or 1's occur in the signal. The signal is self-clocking. This means that a separate clock does not need to be sent alongside the signal, but suffers from using twice the bandwidth to achieve the same data-rate as compared to non-return-to-zero format.

The "zero" between each bit is a neutral or rest condition, such as a zero amplitude in pulse amplitude modulation (PAM), zero phase shift in phase-shift keying (PSK), or mid-frequency in frequency-shift keying (FSK). That "zero" condition is typically halfway between the significant condition representing a 1 bit and the other significant condition representing a 0 bit.

Although return-to-zero (RZ) contains a provision for synchronization, it still has a DC component resulting in “baseline wander” during long strings of 0 or 1 bits, just like the line code non-return-to-zero.

Biphase

The biphase mark code (also called FM1 code) is a type of encoding for binary data streams. When a binary data stream is sent without modification via a channel, there can be long series of logical ones or zeros without any transitions which makes clock recovery and synchronization difficult. Streams encoded in NRZ are affected by the same problem. Using biphase mark code makes synchronization easier by ensuring that there is at least one transition on the channel between every data bit; in this way it behaves much like the Manchester code scheme.

When encoding, the symbol rate must be twice the bitrate of the original signal. Every bit of the original data is represented as two logical states which, together, form a bit. Every logical 1 in the input is represented as two different bits (10 or 01) in the output. The input logical 0 is represented as two equal bits (00 or 11) in the output. Every logical level at the start of a cell is inversion of the level at the end of the previous cell. In BMC output the logical 1 and 0 are represented with the same voltage amplitude but opposite polarities, as shown in the following image:

BMC coding provides a better synchronization since there is a change in the polarity at least every two bits. It is not necessary to know the polarity of the sent signal since the information is not kept in the actual values of the voltage but in their change: in other words it does not matter whether a logical 1 or 0 is received, but only whether the polarity is the same or is different from the previous value; this makes synchronization even easier. Finally, BMC coded signals have zero average DC voltage, thus reducing the necessary transmitting power and minimizing the amount of electromagnetic noise produced by the transmission line. All these positive aspects are achieved at the expense of doubling clock frequency.

BMC is essentially a form of frequency-shift keying, where the channel frequency of a data 1 bit is double the channel frequency of a logical 0 bit.

BMC is used as the encoding method in AES3 and S/PDIF. Many magnetic stripe cards also use BMC encoding, often called F2F (frequency/double frequency) or Aiken Biphase. That standard is described in ISO/IEC 7811. SMPTE time code also uses BMC.

BMC is also the original "frequency modulation" used on single-density floppy disks, before being replaced by "double-density" modified frequency modulation.

Manchester codeIn telecommunication, Manchester code (also known as Phase Encoding, or PE) is a line code in which the encoding of each data bit has at least one transition and occupies the same time. It therefore has no DC component, and is self-clocking, which means that it may be inductively or capacitively coupled, and that a clock signal can be recovered from the encoded data.

Manchester code is widely used (e.g. in Ethernet; see also RFID). There are more complex codes, such as 8B/10B encoding, that use less bandwidth to achieve the same data rate but may be less tolerant of frequency errors and jitter in the transmitter and receiver reference clocks.

Features

Manchester code ensures frequent line voltage transitions, directly proportional to the clock rate. This helps clock recovery.

The DC component of the encoded signal is not dependent on the data and therefore carries no information, allowing the signal to be conveyed conveniently by media (e.g. Ethernet) which usually do not convey a DC component.

Description

An example of Manchester encoding showing both conventions

Extracting the original data from the received encoded bit (from Manchester as per 802.3):

original data XOR clock = Manchester value 0 0 0 0 1 1 1 0 1 1 1 0

Summary:

Each bit is transmitted in a fixed time (the "period"). A 0 is expressed by a low-to-high transition, a 1 by high-to-low transition (according to G.E.

Thomas' convention -- in the IEEE 802.3 convention, the reverse is true).

The transitions which signify 0 or 1 occur at the midpoint of a period.

Transitions at the start of a period are overhead and don't signify data.

Manchester code always has a transition at the middle of each bit period and may (depending on the information to be transmitted) have a transition at the start of the period also. The direction of the mid-bit transition indicates the data. Transitions at the period boundaries do not carry information. They exist only to place the signal in the correct state to allow the mid-bit transition. The existence of guaranteed transitions allows the signal to be self-clocking, and also allows the receiver to align correctly; the receiver can identify if it is misaligned by half a bit period, as there will no longer always be a transition during each bit period. The price of

these benefits is a doubling of the bandwidth requirement compared to simpler NRZ coding schemes (or see also NRZI).

In the Thomas convention, the result is that the first half of a bit period matches the information bit and the second half is its complement.

Differential Manchester encodingDifferential Manchester encoding (also known as CDP; Conditioned Diphase encoding) is a method of encoding data in which data and clock signals are combined to form a single self-synchronizing data stream. It is a differential encoding, using the presence or absence of transitions to indicate logical value. This gives it several advantages over standard Manchester encoding:

Detecting transitions is often less error-prone than comparing against a threshold in a noisy environment.

Because only the presence of a transition is important, polarity is not. Differential coding schemes will work exactly the same if the signal is inverted (wires swapped). (Other line codes with this property include NRZI, bipolar encoding, biphase mark code, coded mark inversion, and MLT-3 encoding).

A '1' bit is indicated by making the first half of the signal equal to the last half of the previous bit's signal i.e. no transition at the start of the bit-time. A '0' bit is indicated by making the first half of the signal opposite to the last half of the previous bit's signal i.e. a zero bit is indicated by a transition at the beginning of the bit-time. In the middle of the bit-time there is always a transition, whether from high to low, or low to high. A reversed scheme is possible, and no advantage is given by using either scheme.

An example of Differential Manchester encoding

A related method is Manchester encoding in which the meaningful transitions are the mid-bit ones, and these encode data by their direction (positive-negative is one value, negative-positive is the other).

Differential Manchester is specified in the IEEE 802.5 standard for token ring LANs, and is used for many other applications, including magnetic and optical storage.

Note: In differential Manchester encoding, if a "1" is represented by one transition, then a "0" is represented by two transitions and vice versa.

Bipolar encodingIn telecommunication, bipolar encoding is a type of line code (a method of encoding digital information to make it resistant to certain forms of signal loss during transmission). A duobinary signal is such an encoding.

A binary 0 is encoded as zero volts as in unipolar encoding. A binary 1 is encoded alternately as a positive voltage and a negative voltage. This prevents a significant build-up of DC, as the positive and negative pulses average to zero volts. Little or no DC-component is considered an advantage because the cable may then be used for longer distances and to carry power for intermediate equipment such as line repeaters. The DC-component can be easily and cheaply removed before the signal reaches the decoding circuitry.

Bipolar encoding is preferable to non-return-to-zero where signal transitions are required to maintain synchronization between the transmitter and receiver. Other systems must synchronize using some form of out-of-band communication, or add frame synchronization sequences that don't carry data to the signal. These alternative approaches require either an additional transmission medium for the clock signal or a loss of performance due to overhead, respectively. A bipolar encoding is an often good

compromise: runs of ones will not cause a lack of transitions, however long sequences of zeroes are still an issue. Long sequences of zero bits result in no transitions and a loss of synchronization. Where frequent transitions are a requirement, a self-clocking encoding such as return-to-zero or some other more complicated line code may be more appropriate, though they introduce significant overhead.

Alternate Mark Inversion

When used on a T-carrier, the code is known as Alternate Mark Inversion because, in this context, a binary '1' is referred to as a "mark", while a binary '0' is called a "space". The coding was used extensively in first-generation PCM networks, and is still commonly seen on older multiplexing equipment today, but successful transmission relies on no long runs of zeroes being present. No more than 15 consecutive zeros should ever be sent to ensure synchronization. The modification of bit 7 causes a change to voice that is undetectable by the human ear, but it is an unacceptable corruption of a data stream. Data channels are required to use some other form of pulse-stuffing, such as always setting bit 8 to '1', in order to maintain a sufficient density of ones. If the characteristics of the input data do not follow the pattern that every eighth bit is '1', the coder using alternate mark inversion adds a '1' after seven consecutive zeros to maintain synchronisation. On the decoder side, this extra '1' added by the coder is removed, resulting that the correct data arrives for the receiver. Due to this, the data sent between the coder and the decoder is longer than the original data by less than 1% on average. Of course, this lowers the effective data throughput to 56 kbit/s per channel.

2B1QTwo-binary, one-quaternary (2B1Q) is a physical layer encoding used for Integrated Services Digital Network (ISDN) Basic Rate Interface (BRI) implementations. 2B1Q uses four signal levels, which are −450 mV, −150 mV, 150 mV and 450 mV, each (1Q) equivalent to two bits (2B). A competing encoding technique, also used for ISDN basic rate interfaces, is 4B3T.

To minimize error propagation, bit pairs (dibits) are assigned to voltage levels according to a Gray code, as follows:

Dibit Signal level10 +450 mV11 +150 mV01 −150 mV00 −450 mV

If the voltage is misread as an adjacent level, this will only cause a 1-bit error in the decoded data. 2B1Q is also used for some variants of HDSL.

8B/6T

8B/6T means send 8 data bits as six ternary (one of three voltage levels) signals. 3/4 (6/8) wave transitions transitions per bit i.e. the carrier just needs to be running at 3/4 of the speed of the data rate. The incoming data stream is split into 8-bit patterns. Each 8-bit data pattern with two voltage levels 0 volts and V volts is examined. This 8-bit pattern is then converted into a 6-bit pattern but using three voltage levels -V, 0 and V volts, so each 8-bit pattern has a unique 6T code. For example the bit pattern 0000 0000 (0x00) uses the code +-00+- and 0000 1110 (0x)E) uses the code -+0-0+. There are 36 = 729 possible patterns (symbols). The rules for the symbols are that there must be at least two voltage transitions (to maintain clock synchronisation) and the average DC voltage must be zero (this is called 'DC balance' that is the overall DC voltage is summed up to 0v, the +V and -V transitions are evenly balanced either side of 0V) which stops any polarisation on the cable.

The maximum frequency that the 6T codes could generate on one carrier is 37.5MHz. FCC rules do not allow anything above 30MHz on cables and Category 3 cable does not allow anything above 16MHz (which is what 100BaseT4 was designed for). The 100BaseT4 standard uses 8B/6T encoding on three pairs in a round robin fashion such that the maximum carrier frequency on any single pair is 37.5/3 = 12.5MHz.

PAM-5

This employs multi-level amplitude signalling. To encode 8 bits, 28 = 256 codes or symbols, are required since there are 256 possible pattern combinations. A five level signal (e.g. -2v, -1v, 0v, 1v and 2v) called Pulse Amplitude Modulation 5 is used (This works in a similar manner to

MLT-3). Bearing in mind that there are 4 separate pairs being used for transmission and reception of data, this gives us a possibility of 54 = 625 codes to choose from when using all four pairs. Actually only four levels are used for data, the fifth level (0v) is used for the 4-dimensional 8-state Trellis Forward Error Correction used to recover the transmitted signal from the high noise.

If you plot time (nanoseconds) against voltage you will see an 'eye pattern' effect showing the different signal levels. Comparing a plot for MLT-3 against PAM-5 will demonstrate how that the separate levels for PAM-5 are less discreet. This is why extra convolution coding is used called Trellis coding, which uses Viterbi decoding for error detection and correction.

2 bits are represented per symbol and the symbol rate is 125Mbps in each direction on a pair because the clock rate is set at 125MHz. This gives 250Mbps data per pair and therefore 1000Mbps for the whole cable.

This type of encoding is used by Gigabit Ethernet. The data signals have distinct and measurable amplitude and phases relative to a 'marker signal'. Using this two way matrix allows more data bits per cycle, in the case of Gigabit Ethernet 1000Mbps is squeezed into 125MHz signals. The electronics are more complex and the technology is more susceptible to noise.

MLT-3 encodingMLT-3 encoding (Multi-Level Transmit) is a line code (a signaling method used in a telecommunication system for transmission purposes) that uses three voltage levels. An MLT-3 interface emits less electromagnetic interference and requires less bandwidth than most other binary or ternary interfaces that operate at the same bit rate (see PCM for discussion on bandwidth / quantization tradeoffs), such as Manchester code or Alternate Mark Inversion.

MLT-3 cycles through the voltage levels -1, +1, and 0. It moves to the next state to transmit a 1 bit, and stays in the same state to transmit a 0 bit. Similar to simple NRZ encoding, MLT-3 has a coding efficiency of 1 bit/baud, however it requires four transitions (baud) to complete a full cycle (from low-to-middle, middle-to-high, high-to-middle, middle-to-low). Thus, the maximum fundamental frequency is reduced to one fourth of the baud rate. This makes signal transmission more amenable to copper wires.

MLT-3 was first introduced by Crescendo Communications as a coding scheme for FDDI copper interconnect (TP-PMD, aka CDDI). Later, the same technology was used in the 100BASE-TX physical medium dependent sublayer, given the considerable similarities between FDDI and 100BASE-[TF]X physical media attachment layer (section 25.3 of IEEE802.3-2002 specifies that ANSI X3.263:1995 TP-PMD should be consulted, with minor exceptions).

Signaling specified by 100BASE-T4 Ethernet, while it has three levels, is not compatible with MLT-3. It uses selective base-2 to base-3 conversion with direct mapping of base-3 digits to line levels (8B6T code).