The Physical Layer

1. As I explained last time, the physical layer of the OSI model is concerned with the physical details of the mechamisms used to transport digital data between a pair of machines.

2. Transmission methods can first be divided into two categories:

   guided

   - which includes systems in which the signals send from one machine to another are guided along some path through a wire or optical fiber, and

   unguided

   - which includes systems using radio waves, microwaves, infrared transmission or any other form of electromagnetic radiaton propagating through open space.

   We will devote most of our attention to guided transmission systems.

3. To guide the propagation of an electrical signal one typically uses a pair of conductors and signals by varying the voltage difference between the two conductors (Bell's original phone system apparently used just one conductor and depended on the ground for a "ground", but...).

   The most common packaging of such conductors used in current networks are:

   Twisted Pair

   which should look a lot like telephone wire to you (if you have ever messed around with the phone wiring in your house).

   

   Basically, as its name suggests, "twisted pair" is just two independently insulated conductors twisted toghether. Typically, it is packaged with several pairs (2,3, or 4) encased together within a plastic casing.

   Coaxial cable

   which should look a lot like cable TV wire to you.

   

   Here the two conductors are arranged so that one (the inner conductor) is completely surrounded by the other (with a layer of insulation in between the two and another layer of insulation surrounding the whole package). While the inner conductor is typically a solid wire, the outer conductor in a coaxial cable is frequently a wire mesh (providing more flexibility).

   If you recall at least as much of the physics of electricty and magnetism as I do (which is not much), you should know that electric current flowing through a conductor produces a magnetic field and that exposing a conductor to a varying magnetic field produces current in the conductor.

   As a result, exposing wires to magnetic fields can produce current that would interfere with date signals transmitted through those wires. Worse yet, wires carrying signals are generating magnetic fields. So, network wires can interfere with each other. The particular physical construction of each of the types of cabes used in data networks (i.e. the twisting or nesting of one conductor within another) is designed to minimize the degree to such interference (although I won't pretend to remember enough physics to explain precisely why).

4. An alternative to electrical signaling is to use light waves guided by optical fiber.

   FIber is structured somewhat like coaxial cable. A transparent "inner core" is surrounded by another transparent layer called the cladding. The cladding is in turn surrounded by a protective plastic cover.

   

   The difference in the indices refraction of the core and cladding is designed so that a significant portion of a beam of light projected into one end of the fiber will be reflected at the boundary between the core and cladding. As a result, light can travel from one end of the fiber to the other.