28-09-2012, 02:51 PM
Digital Hierarchies
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There are two hierarchical structures that exist for digital networks:
1. Plesiochronous Digital Hierarchies
2. Synchronous Digital Hierarchies
Plesiochronous Hierarchies
In a Plesiochronous hierarchy, the higher level multiplex functions include "bit stuffing" techniques. This allows the input bit streams from I/O channels to use "free-running" clocks. As such, the user's clock rate is propagated (plus a little "Jitter") through the higher level multiplexer. Slip rates requirements between End-User multiplex equipment must still be met, for adequate performance of voice and (particularly) data.
Synchronous Digital Hierarchy
In the later 1980s, synchronous network hierarchies were defined. In Synchronous networks, all multiplex functions operate using clocks derived from a common source.
SDH is a transport hierarchy based on multiples of 155.52 Mbit/s. The basic unit of SDH is STM-1.
Different SDH rates are given below:
STM-1 = 155.520 Mbit/s2
STM-4 = 622.080 Mbit/s
STM-16 = 2,488.320 Mbit/s (~2.5 Gbit/s)
STM-64 = 9,953.280 Mbit/s (~10 Gbit/s)
Each rate is an exact multiple of the lower rate, therefore the hierarchy is synchronous.
Optical Fibre
An optical fiber (or optical fibre) is a flexible, transparent fiber made of glass (silica) or plastic, slightly thicker than a human hair. It functions as a waveguide, or “light pipe”, to transmit light between the two ends of the fiber. Optical fibers are widely used in fiber-optic communications, which permits transmission over longer distances and at higher bandwidths (data rates) than other forms of communication.
Types:
Single-Mode Fibres
Multi-Mode Fibres
Principle of Operation:
An optical fiber is a cylindrical dielectric waveguide (non-conducting waveguide) that transmits light along its axis, by the process of total internal reflection. The fiber consists of a core surrounded by a cladding layer, both of which are made of dielectric materials. To confine the optical signal in the core, the refractive index of the core must be greater than that of the cladding. The boundary between the core and cladding may either be abrupt, in step-index fiber, or gradual, in graded-index fiber.