Physical Layer
Physical layer deals with
communication media. This layer receive frame from data link layer and convert
them in bits. It load these bits on actual communication media.
Depending on media type these bit
values are converted in single. Some use audio tones, while others utilize
state transitions—changes in voltage from high to low and low to high. Specific
protocols are needed for each type of media to explain the proper bit patterns
to be used, how data is encoded into media signals.
Fiber Cabling
Two types of fiber are used for
connections: multimode and single-mode.
Multimode fiber
o Multimode fiber has thickness of
either 850 or 1300 nm (nanometres).
o Light signals are provided by an
LED.
o Multimode fiber can achieve
speeds in hundreds of MBPS.
o Multiple signals can be
transmitted per fiber.
Single-mode fiber
o Single mode fiber has a thickness
of 1300 or 1550 nm.
o Light signals are provided by
lasers.
o Single mode fiber can achieve
speed up 100Gbps.
o Only one signal cab be
transmitted per fiber.
Ø Loss factor
Loss factor is used to describe any signal loss in
the fiber before the light source gets to the end of the fiber.
Ø Connector loss
Connector loss is a loss that occurs when a
connector joins two pieces of fibres.
Ø Attenuation
Attenuation is a signal loose due to distance.
Ø Microbending
Microbending describe distortion in light source
due to wrinkle in the fiber, typically where the cable is slightly bent.
Ø Macrobending
Macrobending describe leakage of the light source
from the fiber, usually from a bend in the fiber cable.
Ø Transmission of signals across
the fiber are defined by two standards SONET and SDH. Both of these standards
define the physical layer framing used to transmit light sources, which also
includes overhead for the transmission.
