Message Encoding – Draft

As introduced, to evaluate and understand networking we need to assess the communication devices, the messages, the medium and the protocols.  To provide an example of message encoding I provide Wikipedia’s desciptionManchester Encoding below:

Manchester code

In telecommunication, Manchester code (also known as Phase Encoding, or PE) is a form of data communications line code in which each bit of data is signified by at least one voltage level transition. Manchester encoding is therefore considered to be self-clocking, which means that accurate synchronization of a data stream is possible. Each bit is transmitted over a predefined time period.

However, there are today many more sophisticated codes (8B/10B encoding) which accomplish the same aims with less bandwidth overhead, and less synchronization ambiguity in pathological cases.

Manchester coding has been adopted into many efficient and widely used telecommunications standards, such as Ethernet.

Manchester coding provides a simple way to encode arbitrary binary sequences without ever having long periods without level transitions, thus preventing the loss of clock synchronization, or bit errors from low-frequency drift on poorly-equalized analog links (see ones-density).

If transmitted as an AC signal it ensures that the DC component of the encoded signal is zero, again preventing baseline drift of the repeated signal, making it easy to regenerate and preventing waste of energy.

Description

 

 

An example of Manchester encoding showing both conventions

Summary:

  • Time is divided into periods, and one bit is transmitted per 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 signifying 0 or 1 occur at the midpoint of a period
  • Transitions at the beginning of a period are overhead and don’t signify data

Manchester codes always have a transition at the middle of each bit period, and depending on the state of the signal, may have a transition at the beginning of the period as well. The direction of the mid-bit transition is what carries the data, with a low-to-high transition indicating one binary value, and a high-to-low transition indicating the other. Transitions that don’t occur mid-bit don’t carry useful information, and exist only to place the signal in a state where the necessary mid-bit transition can take place. Though this allows the signal to be self-clocking, it doubles the bandwidth requirements compared to NRZ coding schemes (or see also NRZI).

In the Thomas convention, this results in the first half of a bit period matching the true bit value and the second half being the complement of the true bit value. This is in contrast with non-return-to-zero coding schemes.

 

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