Submarine multiplexed network demonstrates 10-gigabit speeds
DAVE WILSON
Historically, undersea communications systems have been point-to-point links. However, with improved fiber-optic technology developed in the mid-1980s, fibers within a cable can be branched, allowing three or more landing points instead of two. This flexibility has been demonstrated in a trial conducted by Alcatel Submarine Networks, Greenwich, UK.
Alcatel has shown that when traffic paths within a fiber are branched, traffic can be re-routed as user demand increases or decreases. Additionally, the technique offers extra reliability--should one path fail, traffic can be rerouted along another.
This technique is called wavelength add/drop multiplexing. Combined with wavelength-division multiplexing, it has allowed the company to develop 10-gigabit-per-second transmission speeds on a commercial undersea telecommunications system.
From the user`s perspective, these wavelength-division multiplexing techniques will allow the capacity of the system on one traffic path to be quadrupled, from the equivalent of 30,000 simultaneous telephone calls to 120,000. On an intercontinental system of a maximum of four fiber pairs, this would mean that nearly 500,000 telephone calls could be made simultaneously over the same cable.
The Alcatel trials were carried out over several days on the longest of the three segments of Rioja, the first optically-amplified submarine network in Europe, with landing points in Spain, the United Kingdom, Belgium and the Netherlands.
In the experiments, the 930-kilometer link between Santander, Spain, and Land`s End, UK, was used to evaluate four-channel wavelength transmissions with 2.5-Gbit/sec capacity in each channel--a total of 10-Gbit/sec wavelength-division multiplexing.
Speaking at the Optical Amplifiers and Amplification `95 Conference in Switzerland in June, Nigel Taylor of Alcatel said that the segment comprised two fiber pairs and 10 repeaters separated by an average span of 90 km. The four optical fibers in the cable were concatenated to give a total transmission distance of 3700 km, over which error-free performance was obtained on each of the four channels. Concatenation of fiber-optic cable means that cable segments are linked together in serial fashion or put into a series or a chain to increase the effective length of the fiber-optic cable for the test.
Multi-wavelength transmission with narrow channel spacing and optical network facilities required the deployment of new optical components. More important, developers needed to design add-and-drop filters with a high rejection ratio, allowing branching units to drop one channel from the multiplex and to add another signal of the same wavelength without affecting the remaining channels.
Then they had to deploy a high-wavelength-stability transmitter to prevent one channel from disturbing the transmission of the others. Lastly, a high-selectivity receive filter was required to avoid crosstalk and extinction ratio degeneration from adjacent channels.
"Transmitters and filters are the sort of components that are already used today for long-haul transmission and are an extension of what`s already available," says Alcatel`s Taylor. Nevertheless, since the closest two wavelengths in the Alcatel trial were just 1 nanometer apart, channel filtering still had to be precise.
The development of the branching units was more complex. According to Alcatel, three types of branching units can be implemented in a wavelength-division multiplexing system.
A broadcast branching unit is one in which all wavelengths present in a trunk are broadcast in every spur. An alternative is a fixed-filtering branching unit, in which a set of wavelengths can be added or dropped by using fixed filters, allowing wavelength reusability. Third, developers can employ a tunable filtering branching unit, in which any wavelength can be added or dropped according to need.
Although Alcatel admits that designs based on tunable filters offer the highest levels of network flexibility, in this trial it demonstrated the wavelength add/drop multiplexing capability by wavelength branching using a fixed-filtering branching unit. The wavelength add/drop multiplexing technique used in the trial was implemented with wavelength-selectable filter components. The technique provided the required functionality for the passive wavelength branching unit, which will enable wavelength routing on future submarine networks.
According to Taylor, the fixed filtering branching unit prevents the line from being loaded with unwanted data and degrading other channels. Taylor says that Alcatel rejected the tunable system approach for this system because, at present, tunable units are more complex to implement and involve more system management issues. Furthermore, the market has not evolved to the degree of sophistication required to take full advantage of the methodology.
Using the wavelength-division multiplexing technique, one of the four channels was dropped halfway along the transmission path after 1850 km, and new data was inserted at the same wavelength for propagation over another 1850 km. Such an approach allows individual wavelengths to be routed within a network, giving additional flexibility to the network configuration.
For point-to-point transmission experiments, the four fibers in Segment 1 were simply concatenated. For channel add/drop experiments, the branching unit was inserted halfway along Segment 1 concatenation, and Channel 2 was dropped and routed over 474 km of the partially laid segment (United Kingdom to Belgium). Another channel of the same wavelength was added in Land`s End, UK, and transmitted over the second half of Segment 1 fiber concatenation.
A technique of power pre-emphasis in the transmit terminal station was used to overcome the natural bandwidth limitation of the erbium-doped fiber amplifiers employed in the repeaters. System output values of about 20 decibels were recorded for all channels by optimizing the channel pre-emphasis and repeater output power levels. q
Dave Wilson writes from London.