Physical Aspects of the Media

When we look at the Data Communications model, we see that all communications must travel through some sort of medium. The Physical layer of the OSI model is concerned with the physical attributes of the medium such as cable type, frequency, voltages, etc.... This section describes the physical aspects of the medium and how it affects the transfer of data.

Layer 1 Physical layer is concerned with the medium

Data communications over a medium can be divided into two basic types:

  1. serial data communication
  2. parallel data communication

Serial Data Communications

The physical connection determines how many bits (1's or 0's) can be transmitted at a single instance of time. If only 1 bit of information can be transmitted over the data transmission medium at a time then it is considered a Serial Communication.

Parallel Data Communications

If more than 1 bit of information is transmitted over the data transmission medium at the same time then it is considered a Parallel Communication.

Communications Advantages Disadvantages
Parallel Fast Transfer Rates Short distances only
Serial Long Distances Slow transfer rates

The transfer rate comparison is relative to serial versus parallel. Serial data transfer is used for high transfer rates over long distances. Data can be transferred at much faster rates over long distances using serial methods than using parallel data transfer. At shorter distances, typically under 15 feet, parallel data transfers are used.

A personal computer uses 64 bit high speed parallel address and data busses to transfer data within the confines of the computer. If 64 bit high speed data transfers were attempted for long distances, cable characteristics would cause the signals to lead or lag each other and all the bits would not arrive at the exact same time. The bits would arrive staggered and the data would be corrupted.

Serial data communication only transfers one bit at a time and does not have the timing problems (race conditions) of parallel data communcations.

Transmission Media Catagories

There are 2 basic categories of Transmission Media:

  • Guided Transmission Media - uses a "cabling" system that guides or bounds the data signals along a specific path. The data signals are bound by the "cabling" system. Guided Media is also known as Bound Media. Cabling is meant in a generic sense in the previous sentences and is not meant to be interpreted as copper wire cabling only.

  • Unguided Transmission Media - consists of a means for the data signals to travel but nothing to guide them along a specific path. The data signals are not bound to a cabling media and as such are often called Unbound Media.

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Copyright July 2013 Eugene Blanchard