The term signalling refers to electrical energy, generated by various methods to carry the binary information. These methods are called modulation or encoding. Modulation is used in analogue signalling and encoding is used in digital signalling. Digital signals are represented by definite states meaning a binary "0" or a "1". A digital value changes instantaneously from one value to the next. An analogue signal gradually changes from one value to the next. Take for instance the speedometer of a motor car. A analogue meter change from one reading to the next by gradually moving the needle from one reading to the next on a linear basis. It is never on an exact state. A digital readout will only commence to next state when the next defined value of the output is true (predefined by the electronic circuitry constantly measuring an analogue input).
By using pulses of light or different electrical voltages, digital signalling can be accomplished.
There are two types of encoding that can be used with digital signalling:
Current state
The presence or absence of a signal defines the characteristics of the current state method. Example: if there is a binary "1" make the output voltage equal to +V volt. If there is a binary "0" make the output voltage equal to 0 volt.
Figure 14 Current state signalling
Current state is used by the following signal encoding schemes:
Current transition
The transition between voltages is measured at the receiver that is decoding the binary information. In this case the transition from one voltage to the next would indicate a binary "1", but could also be the other way around. A transition in voltage could also indicate a binary "0".
Figure 15 Current transition
The following signal encoding schemes use the state transition method:
Only AMI and HDB3 will be discussed.
Bipolar-Alternate mark inversion
A binary "0" is encoded as 0 volt, known as a space. A binary "1" known as a mark is encoded as a alternating positive-, negative-voltage. With bipolar encoding long strings of zeros would result in the receiver loosing synchronisation due to the fact that no line transitions are taking place.
Figure 16 AMI Signal conversion
HDB3
HDB3 (High Density Bipolar Three) is commonly used with E1 (2Mb/s) trunks, as signalling scheme. It conforms to the ITU-G.703, G.704 recommendations. The following figure demonstrate how the HDB3 coding scheme works.
Figure 17 HDB3 Signal conversion
The main advantage of HDB3 is that there will always be line changes present even if no data is transmitted. Meaning the receiver will always have a reference to which it can derive a clocking signal. After every four consecutive zero's a violation pulse (V) will be generated by the transmitter.
Advantages and disadvantages of digital signalling
Using digital signalling can be far more advantageous than analogue signalling. Digital signals is less susceptible to noise and interference than its analogue counterpart. The equipment is simpler and cheaper to manufacture. Digital signals are much more attenuated than analogue signals compared over the same distance travelled. This is because of the high frequency at which digital signals are transmitted.