Digital Transmission

Definition of Terms

• Line coding is the process of converting digital data to a digital signal.
• Line coding methods must eliminate the dc component and provide a means of synchronization between the sender and the receiver. 
• NRZ, RZ, Manchester, and differential Manchester encoding are the most popular polar encoding methods. 
• AMI is a popular bipolar encoding method. 
• In NRZ-L the level of the voltage determines the value of the bit. In NRZ-I the inversion or the lack of inversion determines the value of the bit.
• NRZ-L and NRZ-I both have an average signal rate of N/2 Bd.
• NRZ-L and NRZ-I both have a DC component problem.
• In Manchester and differential Manchester encoding, the transition at the middle of the bit is used for synchronization.
• The minimum bandwidth of Manchester and differential Manchester is 2 times that of NRZ.
• In bipolar encoding, we use three levels: positive, zero, and negative.
• Block coding provides redundancy to ensure synchronization and inherent errordetection. Block coding is normally referred to as mB/nB coding; it replaces each m-bit group with an n-bit group. 
• In mBnL schemes, a pattern of m data elements is encoded as a pattern of n signal elements in which 2m ≤ Ln.
• Block coding can improve the performance of line coding through redundancy and error correction. 
• Block coding involves grouping the bits, substitution, and line coding.
• Block coding is normally referred to as mB/nB coding; it replaces each m-bit group with an n-bit group.
• B8ZS substitutes eight consecutive zeros with 000VB0VB.
• HDB3 substitutes four consecutive zeros with 000V or B00V depending on the number of nonzero pulses after the last substitution.
• Scrambling provides synchronization without increasing the number of bits. Two common scrambling techniques are B8ZS and HDB3.
• The number of different values allowed in a signal is the signal level. The number of symbols that represent data is the data level.
• Bit rate is a function of the pulse rate and data level.
• The most common technique to change an analog signal to digital data (digitization) is called pulse code modulation (PCM).
• The first step in PCM is sampling. The analog signal is sampled every Ts s, where Ts is the sample interval or period. The inverse of the sampling interval is called the sampling rate or sampling frequency and denoted by fs, where fs =lITs. There are three sampling methods: ideal, natural, and flat-top.
• PCM involves sampling, quantizing, and line coding. 
• According to the Nyquist theorem, to reproduce the original analog signal, one necessary condition is that the sampling rate be at least twice the highest frequency in the original signal.
• Other sampling techniques have been developed to reduce the complexity of PCM. The simplest is delta modulation. PCM finds the value of the signal amplitude for each sample; DM finds the change from the previous sample.
• Digital transmission can be either parallel or serial in mode. 
• While there is only one way to send parallel data, there are three subclasses of serial transmission: asynchronous, synchronous, and isochronous.
• In parallel transmission, a group of bits is sent simultaneously, with each bit on a separate line. 
• In serial transmission, there is only one line and the bits are sent sequentially.

Three techniques involve digital-to-digital conversion

• Line coding
• Block coding
• Scrambling

Five categories of Line coding

• Unipolar - the signal levels are on one side of the time axis, either above or below. Traditionally, a unipolar scheme was designed as a non-return-to-zero (NRZ) scheme in which the positive voltage defines bit 1 and the zero voltage defines bit O.
• Polar - the voltages are on the both sides of the time axis. In polar NRZ encoding, we use two levels of voltage amplitude. We can have two versions of polar NRZ: NRZ-L and NRZ-I.
• Bipolar - there are three voltage levels: positive, negative, and zero. The voltage level for one data element is at zero, while the voltage level for the other element alternates between positive and negative.
• Multilevel - The desire to increase the data speed or decrease the required bandwidth has resulted in the creation of many schemes. The goal is to increase the number of bits per baud by encoding a pattern of m data elements into a pattern of n signal elements.
• Multitransition- MLT-3, a scheme that maps one bit to one signal element. The signal rate is the same as that for NRZ-I, but with greater complexity (three levels and complex transition rules). It turns out that the shape of the signal in this scheme helps to reduce the required bandwidth.

Line coding schemes

Summary of line coding schemes

Common Block coding methods

• 4B/5B - A block coding technique in which 4 bits are encoded into a 5-bit code. The four binary/five binary (4B/5B) coding scheme was designed to be used in combination
  with NRZ-I.
• 8B/10B - A block coding technique in which 8 bits are encoded into a lO-bit code.
• 8B/6T - A three-level line encoding scheme that encodes a block of 8 bits into a signal
  of 6 ternary pulses.

Two common scrambling techniques

• B8ZS - bipolar with 8-zero substitution (B8ZS), a scrambling technique in which a stream of 8
  zeros are replaced by a predefined pattern to improve bit synchronization.
• HDB3 - High-density bipolar 3-zero (HDB3) is commonly used outside of North America. Four consecutive zero-level voltages are replaced with a sequence of OOOV or BOOV. The reason for two different substitutions is to maintain the even number of nonzero pulses after each substitution.

The three sampling methods

• Ideal - pulses from the analog signal are sampled. This is an ideal sampling method and cannot be easily implemented.
• Natural - a high-speed switch is turned on for only the small period of time when the sampling occurs.
• Flat-top - The most common sampling method, called sample and hold, however, creates flat-top samples by using a circuit.

Three subclasses of serial transmission

• Asynchronous -  send 1 start bit (0) at the beginning and 1 or more stop bits (1 s) at the end of each byte.
• Synchronous - send bits one after another without start or stop bits or gaps. It is the responsibility of the receiver to group the bits.
• Isochronous - provides synchronized for the entire stream of bits must. In other words, it guarantees that the data arrive at a fixed rate.

Data transmission and modes

Note: You can proceed to take the multiple choice exam regarding this topic. Digital Transmission - Set 1 MCQs

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credit: Behrouz A. Forouzan©2013 www.FroydWess.com

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