Telecommunications - OSI Protocols - The physical layer

The physical layer provides the mechanism for a DTE to transmit data to a DCE; the data is seen as a stream of bits with no grouping into bytes.

MECHANICAL CHARACTERISTICS These define matters such as the pin layout and the size and shape of the latching block.
ELECTRICAL CHARACTERISTICS These specify whether the connection is balanced or unbalanced, the voltage levels used (and the tolerances), the input impedance's etc.
FUNCTIONAL CHARACTERISTICS These characteristics are the ones which say what circuit performs what function (possibly at what time). In general terms, there are four broad categories of functions - data, control, timing and earth.
PROCEDURAL CHARACTRISTICS These are the procedures required actually to use the interchange circuits. There is still some debate into which layer of the model some of these characteristics fit.

This layer defines the physical connection to the modem and may use V24, X21, X21bis, V35, etc.

(V24 and X21bis are similar, use the same 25 pin connector, and have equal status in ITU-T -CCITT).

The data is sent as a continuous serial synchronous or asynchronous data-stream.

Services provided to the data link layer

  1. Physical-connections;
  2. Physical-service-data-units;
  3. Physical-connection-endpoints;
  4. data-circuit identification;
  5. sequencing;
  6. Fault condition notification;
  7. quality of service parameters.

Functions within the physical layer

  1. Physical-connection activation and deactivation;
  2. Physical-service-data-unit transmission;
  3. Physical layer management.


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