g e n e s i s t e l e m a n a g e m e n t . c o m

A PDH fiber link consists of two fibers, one for data in each direction, and a terminal station at each end to drive light into one fiber, and detect it from the other, whilst sending and receiving data to nearby equipment with electrical signals. If the link is more than about 50 km long, then regenerator stations must be installed, to detect the light from the fiber on the first leg of the link, recover the data bits and use this to modulate a laser to drive the second leg.

PDH systems are point-to-point, bi-directional digital data links at rates of up to 140 Megabits per second. With careful monitoring - especially of the laser-diodes, which have limited life spans - they are generally highly reliable and form much of the inter-exchange, inter-capitals, and submarine cable networks now in use.

A single PDH link might carry 140 Mb/s (Megabits per second), which is composed of four 34 Mb/s 'tributary' streams, each of which is composed of three 8 Mb/s streams, each of which might be composed of four 2 Mb/s streams. All these streams must be synchronized - the presentation of bits in all the streams must be exactly in step.

PDH systems suffer from a number of drawbacks, which lead to the development of SDH:

• The requirement for synchronization was highly inconvenient. There are fundamental problems with synchronizing telecommunications equipment all over a country or between countries.
• A PDH link is basically an end-to-end pipe, and if it was desired to access, for instance, a 2 Mb/s stream in the middle of a link then costly multiplexers must be installed at that site to break the 140 Mb/s stream into four 34 Mb/s streams and then to break one of those down into 8 and then into the desired 2 Mb/s stream. The hardware required to do this expensive, hard to reconfigure and bulky - and is known as a 'multiplexer-mountain'.
• The entire link can fail, at least in one direction, with the failure of any one component or following damage to the buried fiber.
• Methods of monitoring and managing PDH systems were proprietary rather than standardized.

Early SDH links used 1310 nm infrared lasers, but all now use 1550 nm. At this longer wavelength, the fiber is more transparent and so regenerators need only be situated at 100 km intervals rather than 50 km - reducing costs and improving reliability. Data rates of 155, 622 Mb/s, 2.5 Gb/s and 10 Gb/s are possible The most common data rate for SDH is 2.5Gb/s. A 10Gb/s system comprises four sets of 2.5Gb/s equipment time-division-multiplexed to turn a single laser's light on or off every 1/10,000,000,000 of a second. The resulting pulses of light measure only 20mm long in the fiber.

The SDH data-stream embodies data for monitoring the link's error rate, and for managing all SDH equipment remotely. However the higher-level performance monitoring and management protocols are not standardized. Consequently, one manufacturer's SDH equipment and remote monitoring software will typically not be compatible with another's in terms of management - although the equipment of differing manufacturers interoperates correctly in terms of carriage of data.

The three most important benefits of SDH systems are:

• The tributary data-streams which make up the full payload have some extra bits available which enables 'slippage' without loss of data - in other words, the tributary data-streams do not need to be exactly synchronized with the main SDH system, and can vary their rate freely and be delivered at that rate at their destination.
• Advances in electronic design, and SDH's complex but elegant architecture, make it much easier for small data-streams to be accessed within a larger one. This is achieved by SDH 'Add-Drop- Mulitplexers' which are much more compact, reliable, flexible and remotely manageable than the 'multiplexer-mountain' which would have been required with PDH.
• SDH supports the creation of fault-tolerant fiber 'rings'. For instance four Add-Drop Mulitiplexers (ADMs) in 4 cities may be linked together by 4 pairs of fiber, where each fiber in the pair carries light in the opposite direction to the other. Data can be sent from any ADM to any other in both the clockwise direction and the anti-clockwise direction. Should the fiber be severed at any one point, and then the ADMs automatically reconfigure themselves within milliseconds to restore communications using the available fibers. Similarly, if one ADM fails entirely, the other three reconfigure themselves to maintain communications using the two remaining active pairs of fiber, which link them.