Communication Systems/Line Codes

Line Codes

In addition to pulse shaping, there are a number of useful line codes that we can use to help reduce our errors or to have other positive effects on our signal.

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. The common types of line encoding are unipolar, polar, bipolar and Manchester encoding.

Line codes are used commonly in computer communication networks over short distances.

There are numerous ways digital information can be coded onto a transmission medium. Some of the more common include:

Each of the various line formats has a particular advantage and disadvantage. It is not possible to select one, which will meet all needs. The format may be selected to meet one or more of the following criteria:

• Minimize transmission hardware

• Facilitate synchronization

• Ease error detection and correction

• Minimize spectral content

• Eliminate a dc component

The Manchester code is quite popular. It is known as a self-clocking code because there is always a transition during the bit interval. Consequently, long strings of zeros or ones do not cause clocking problems.

Non-Return to Zero Codes (NRZ)

Non-Return to Zero (NRZ) codes are a bit awkwardly named, especially considering that the unipolar NRZ code does return to a zero value. In essence, an NRZ code is just a simple square wave, assigning one value to a binary 1, and another amplitude to a binary 0.

NRZ codes are more bandwidth efficient than bipolar ones. However, their spectral components go all the way down to 0 Hz. This prevents them from being used on transmission lines which are transformer coupled, or for some other reason cannot carry DC.

Unipolar NRZ

Unipolar NRZ is simply a square wave with +V being a binary 1, and 0V being a binary 0. NRZ is convenient because computer circuits use unipolar NRZ internally, and it requires little effort to expand this system outside the computer. Unipolar NRZ has a DC term, but a relatively narrow bandwidth.

Bipolar NRZ

Bipolar NRZ operates using a bipolar voltage supply rail. Marks typically are represented using negative voltages (e.g., -9V), while spaces with positive voltages (e.g., +9V). For example, RS-232C/EIA-232 signaling relies on bipolar NRZ.

-5V +5V -5V +5V -5V -5V +5V -5V -5V
 1  0   1   0   1   1   0   1   1

Bipolar NRZ has similar bandwidth and DC balance issues as unipolar NRZ.

AMI

AMI (alternate mark inversion) is another example of a bipolar line code. Each successive mark is inverted and the average or DC level of the line is therefore zero.

AMI is usually implemented as RZ pulses, but NRZ and NRZ-I variants exist as well.

One of the weaknesses of this approach is that long strings of zeros cause the receivers to lose lock. It is therefore necessary to impose other rules on the signal to prevent this. For example, combining NRZ-M with AMI yields MLT-3, the line coding system used with 100-base-T Ethernet.

CDI

The CDI(Conditioned Diphase Interface) bipolar line code is actually a slightly different form of the original FM line coding used in single-density disk drives and audio cassette tapes. Marks are encoded as alternate polarity full period pulses. Spaces are encoded by half a period at the negative voltage and half period at the positive voltage. This coding scheme has the advantage that it requires less logic to implement than HDB3.

Manchester

Manchester codes were an invention that allows for timing information to be sent along with the data. In an NRZ code, if there is a long stream of ones or zeros, the receiver could conceivably suffer so much compound jitter that it would either lose or gain an entire bit time, and then be out of sync with the transmitter. This is because a long stream of 1s or 0s would not "change state" at all, but instead would simply stay at a single value. Manchester codes say that every single bittime will have a transition in the middle of the bit time, so that a receiver could find that transition, and "lock on" to the signal again, if it started to stray away from center. Because there are more transitions, however, manchester codes also require the highest bandwidth of all the line codes.

Differential Codes

Some communication channels (such as phase-modulated sine waves and differential twisted pairs) have the characteristic that transitions between the 2 symbols used can be easily distinguished, but when first starting to receive it is difficult to tell which of the 2 states it is in. For example, full-speed USB uses a twisted pair and transmits +3.3 V on one line and 0 V on the other line for "1", but 0 V on the one line and +3.3 V on the other line for "0". Because some cables have an extra half-twist in them, it is impossible for a device that was just plugged in to tell whether the symbol it is currently receiving is a "1" or a "0".

Differential codes still work, not even noticing when the 2 wires get switched.

Differential codes, in general, look exactly the same on a oscilloscope or spectrum analyzer as the non-differential code they are based on, and so use exactly the same bandwidth and have exactly the same bitrate.

Differential codes that work when the 2 wires get switched include:

• Differential Manchester encoding—based on Manchester encoding
• Non-Return-to-Zero Inverted (NRZI) -- based on non-return-to-zero (NRZ)

(A few non-differential codes also work even when the 2 wires get switched—such as bipolar encoding, and MLT-3 encoding).

Differential Manchester

Differential Manchester encoding, also called biphase mark code (BMC) or FM1, is a line code in which data and clock signals are combined to form a single 2-level self-synchronizing data stream. It is a differential encoding, using the presence or absence of transitions to indicate logical value. It has the following advantages over some other line codes: • A transition is guaranteed at least once every bit, allowing the receiving device to perform clock recovery. • Detecting transitions is often less error-prone than comparing against a threshold in a noisy environment. • Unlike with Manchester encoding, only the presence of a transition is important, not the polarity. 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, coded mark inversion, and MLT-3 encoding). • If the high and low signal levels have the same voltage with opposite polarity, 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.

Comparison

Code	Bandwidth	Timing	DC value
Unipolar NRZ	Low bandwidth	No timing information	High DC component
Bipolar NRZ	Lower bandwidth	No timing information	No DC component
Differential NRZ	Lower bandwidth	No timing information	Little or no DC component
Manchester	High bandwidth	Good clock recovery	No DC component
Differential Manchester	Moderate bandwidth	Good clock recovery	No DC Component

Further reading

• Wikipedia:differential coding