Protocols and technologies used in transport networks: PDH, SDH, SONET, DWDM.
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The plesiochronous digital hierarchy (PDH) is a technology used in telecommunications networks to transport large quantities of data over digital transport equipment such as fibre optic and microwave radio systems.
PDH allows transmission of data streams that are nominally running at the same rate, but allowing some variation on the speed around a nominal rate. By analogy, any two watches are nominally running at the same rate, clocking up 60 seconds every minute.
The data rate is controlled by a clock in the equipment generating the data. In order to transport multiple data streams from one place to another over a common transmission medium, they are multiplexed in groups of four.
Because each of the four data streams is not necessarily running at the same rate, some compensation has to be introduced.
Synchronous Digital Hierarchy (SDH) is a group of fiber optic transmission rates that transport digital signals with different capacities. SDH technology enables low-bit rate data streams to combine with high-rate data streams.
SDH is placed at the bottom of the protocol stack in the physical layer, along with the very fiber upon which it transmits its signal.
Any IP traffic that is destined to be transmitted across a fiber-based SDH network will be framed by a Layer 2 protocol before being ready to take its orders from the SDH equipment.
SDH is the International Telecommunication Union's Telecommunication Standardization Sector version of the Synchronous Optical Network (SONET) standard.
Both technologies provide faster and cheaper network interconnections than traditional Plesiochronous Digital Hierarchy (PDH) equipment.
SDH systems were developed in the late 1980s and early 1990s to replace PDH technology. The primary goal was to eliminate potential synchronization issues for bulk data and telephone exchanges. The data transfer rate also increased in a simple and more flexible optical fiber-based network infrastructure
What is the difference between PDH technology and SDH transmission? PDH technology came with complex multiplexing -- bit stuffing or interleaving -- where additional bits of data are stuffed into the data stream to achieve synchronicity. High transmission bandwidth costs also constrained PDH.
By contrast, SDH technology ushered in an era of multiplexing and synchronization without bit stuffing. Instead, the telecommunications industry based SDH systems on byte interleaving that ensures exact timings, while providing a similar level of flexibility.
SDH uses the following Synchronous Transport Modules (STMs) and rates: STM-1 (155 megabits per second) STM-4 (622 Mbps) STM-16 (2.5 gigabits per second) STM-64 (10 Gbps)
SONET stands for Synchronous Optical Network. SONET is a communication protocol, developed by Bellcore – that is used to transmit a large amount of data over relatively large distances using optical fibre. With SONET, multiple digital data streams are transferred at the same time over the optical fibre.
SONET includes four functional layers: Path Layer: It is responsible for the movement of signals from its optical source to its optical destination. STS Mux/Demux provides path layer functions.
Line Layer: It is responsible for the movement of signal across a physical line. STS Mux/Demux and Add/Drop Mux provides Line layer functions.
Section Layer: It is responsible for the movement of signal across a physical section. Each device of network provides section layer functions.
Photonic Layer: It corresponds to the physical layer of the OSI model. It includes physical specifications for the optical fibre channel (presence of light = 1 and absence of light = 0).
Advantages of SONET: 1. Transmits data to large distances 2. Low electromagnetic interference 3. High data rates 4. Large Bandwidth
Dense wavelength division multiplexing (DWDM) is a fiber-optic transmission technique. It involves the process of multiplexing many different wavelength signals onto a single fiber. Each fiber has a set of parallel optical channels and each one uses slightly different light wavelengths.
What is the benefit of Dense wavelength division multiplexing DWDM )? Dense Wavelength Division Multiplexing (DWDM) has long been the technology of choice for transporting large amounts of data between sites.
The main advantage is that it increases bandwidth by allowing different data streams to be sent simultaneously over a single optical fiber network.
Today's DWDM systems typically support 96 channels spaced at 0.8 nm apart within the 1550 nm C-Band spectrum. Because of this, DWDM systems can transmit a huge quantity of data through a single fiber link as they allow for many more wavelengths to be packed onto the same fiber.
