The Physical Layer is responsible for the transparent transmission of information over a communications channel, and the layer contains all the necessary functions to achieve this end. An important point that should be emphasised is that the Physical Layer functions are entirely independent of the actual physical medium in use. Moreover the specific medium being used, be this copper wire, co-axial cable or fibre optic cable, is completely hidden from the Data Link Layer.

Basically, the design issues here can be grouped as follows:

• Mechanical - This deals with the type of connector, physical dimensions, allocation of the function of each of the pins in the physical connector and so on
• Electrical - This is concerned with the electrical characteristics e.g. how many volts should be used to represent a 1 and how many for a 0, impedances and so on
• Functional - These specifications deal with the meaning of voltage levels on certain pins of the connector
• Procedural - These specifications define the rules applying to various functions, the sequence in which certain events may occur

At this layer the RS-232-C can be cited as an example of an appropriate protocol.

Services provide to the Data Link Layer

• Physical connections - This provision of a transmission of a raw bit stream between Data Link entities. A physical connection effectively enacts a "data circuit" between physical end points.
• Physical service data units - These comprise one bit is serial transmission and n bits in parallel transmission. A physical connection can either be full or half duplex.
• Physical connection end-points - The Physical Layer provides identifiers of physical connection end-points which may be used by the Data Link Layer.
• Sequencing - The Physical Layer delivers bits in the same order in which they were submitted for transmission.
• Data circuit identification - This layer provides identifiers which uniquely specify the data circuits between two systems.
• Fault condition notification - The Data Link Layer is informed of fault conditions detected within this layer.
• Quality of service parameters - The Data Link Layer is provided with error rates, service availability, transmission rates and delays.

The following functions are performed by the Physical Layer:

• Set up and release of physical connection between entities in the Data Link Layer.
• The transmission of bit streams - these can either be synchronous or asynchronous.
• The protocols at this level deal with some aspects of the management activities in the layer.

Thus in conclusion the Physical Layer design can be properly considered to be within the domain of the electrical engineer. Moreover, it can be said to be the lowest layer of the Reference Model, which defines the physical and electrical characteristics of the interface to the network and provides a transparent transmission path for the information across the chosen physical medium.