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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.
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.
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