Digital Hierarchies 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. Name Rate DS0 64 KBPS E MBPS E MBPS E MBPS E MBPS

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 Mbit/s. The basic unit of SDH is STM-1. Different SDH rates are given below: STM-1 = Mbit/s2 STM-4 = Mbit/s STM-16 = 2, Mbit/s (~2.5 Gbit/s) STM-64 = 9, Mbit/s (~10 Gbit/s) Each rate is an exact multiple of the lower rate, therefore the hierarchy is synchronous.

Synchronous Digital Hierarchy STM-1 Frame: TOH: Transport Overhead (RSOH + AU4P + MSOH) MSOH: Multiplex Section Overhead RSOH: Regeneration Section Overhead AU4P: AU-4 Pointers VC4: Virtual Container-4 payload (POH + VC-4 Data) POH: Path Overhead

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: 1.Single-Mode Fibres 2.Multi-Mode Fibres

Optical Fibres Advantages: 1.Immunity to Electromagnetic Interference(EMI) 2.High Bandwidth 3.Less Losses 4.Low cost in comparison to conductor cables.

Optical 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.

Optical Fibres Index of refraction: The index of refraction is a way of measuring the speed of light in a material. The index of refraction of a vacuum is 1, by definition. The typical value for the cladding of an optical fiber is 1.52(n2). The core value is typically 1.62(n1). The larger the index of refraction, the slower light travels in that medium.

Optical Fibres Total internal reflection: When light traveling in an optically dense medium hits a boundary at a steep angle (larger than the critical angle for the boundary), the light will be completely reflected. This is called total internal reflection. This effect is used in optical fibers to confine light in the core. Light travels through the fiber core, bouncing back and forth off the boundary between the core and cladding.

Optical Fibres Multimode Fibres: Fiber with large core diameter (greater than 10 micrometers) may be analyzed by geometrical optics. Such fiber is called multi-mode fiber, from the electromagnetic analysis. In a step-index multi-mode fiber, rays of light are guided along the fiber core by total internal reflection. A high numerical aperture allows light to propagate down the fiber in rays both close to the axis and at various angles, allowing efficient coupling of light into the fiber.

Optical Fibres Single Mode Fibres: As an optical waveguide, the fiber supports one or more confined transverse modes by which light can propagate along the fiber. Fiber supporting only one mode is called single- mode or mono-mode fiber. The most common type of single-mode fiber has a core diameter of 8–10 micrometers and is designed for use in the near infrared. The mode structure depends on the wavelength of the light used, so that this fiber actually supports a small number of additional modes at visible wavelengths.

Optical Fibres Step Index Fibres & Graded Index Fibres:

Optical Fibres Attenuation: Attenuation in fiber optics, also known as transmission loss, is the reduction in intensity of the light beam (or signal) with respect to distance traveled through a transmission medium. Attenuation coefficients in fiber optics usually use units of dB/km through the medium due to the relatively high quality of transparency of modern optical transmission media. Empirical research has shown that attenuation in optical fiber is caused primarily by both scattering and absorption. Minimum attenuation can be achieved at 1350 & 1550 nm.

Optical Fibres Standards used by RailTel: ITU G.652 Covers single-mode NDSF (non-dispersion-shifted fiber). This fiber is in most of the cable that was installed in the 1980s. Optimized in the 1,310-nm range. Low water peak fiber has been specifically processed to reduce the water peak at 1400 nm to allow use in that range. -Atten </= 0.5 / 0.4 at 1310 / 1550nm Macrobend </= 0.5 dB at 1550nm PMD </= 0.5 ps/sqrt(km) ITU G.653 Covers single-mode dispersion-shifted optical fiber. Dispersion is minimized in the 1,550-nm wavelength range. At this range attenuation is also minimized, so longer distance cables are possible.

Optical Fibres 24 Fibre cable used by RailTel:

Optical Fibres DWDM Dense wavelength division multiplexing (DWDM) is a technology that puts data from different sources together on an optical fiber, with each signal carried at the same time on its own separate light wavelength. Using DWDM, up to 80 (and theoretically more) separate wavelengths or channels of data can be multiplexed into a light stream transmitted on a single optical fiber. Each channel carries a time division multiplexed (TDM) signal. In a system with each channel carrying 2.5 Gbps (billion bits per second), up to 200 billion bits can be delivered a second by the optical fiber. DWDM is also sometimes called wave division multiplexing (WDM).