New Specifications For Optical Fiber
ITU recommends new standards
19 November 2001
Two major building blocks for all-optical networks have been agreed by the International Telecommunication Union with the adoption of two draft new global standards for increasing the efficiency and survivability of optical fibre access networksbased on Passive Optical Network (PON) techniques.
The draft new standards are designated ITU-T Recommendations G.983.4 and G.983.5.A passive optical network (PON) is a system that brings optical fiber cabling and signals all or most of the way to the end user in residential and new small/medium business networks.
Depending on where the PON terminates, the system can be described as fibre-to-the-curb (FTTC), fiber-to-the-building (FTTB), Fibre To The Cabinet (FTTCab), Fibre To The Office (FTTO) or fiber-to-the-home (FTTH). Passive Optical Networks (PONs) use light of different colours (wavelengths)over strands of glass (optical fibres) to transmit large amounts of information between customers and network/service providers.
The passive simply describes the fact that optical transmission has no power requirements or active electronic devices once the signal is going through the network.
With PONs, signals are carried by lasers and sent to their appropriate destinationby devices that act like highway interchanges, without the use of any electrical power, eliminating expensive powered equipment between the provider and the customer.
PONs offer customers video applications, high-speed Internet access, multimedia and other high-bandwidth capabilities.
Although the technique of PONs has been known for around 20 years as an alternative to traditional wire pair and coaxial cable, it is only now, with the need for fast internet access, that they are looking attractive for mass deployment in, for example, new building developments. Line rates are up to 622 Mbit/s in both the upstream direction (customer to network/service provider), and the downstream(network/service provider to customer) direction - over three to four orders of magnitude (or 1,000 to 10,000 times) faster than a state-of-the-art modem which provides for network access at up to 56 kbit/s.
In addition to speed, another advantage of optical technology is that it is flexible and is expected to require less maintenance than older cable technologies.
The costs of fibre and much of the equipment located with the service provider is shared among several customers, making it more cost attractive. Because PON is indpendent from bit rates, signal format (digital or analogue), and protocols (SONET/SDH, Internet Protocol, Ethernet or ATM), only the equipment needed for delivering specific services needs to be added at the ends of the network when the time comes to add new services to existing customers or to add new customers. As services can be mixed or upgraded cost-effectively as required, PONs offer the type of scability - an important consideration for operators who want to expand capacity in line with market demand. Such a degree of flexibility is unmatched in most of today's network architectures.The draft new standard G.983.4 specifies a Dynamic Bandwidth Assignment (DBA) mechanism which improves the efficiency of the PON by dynamically adjusting the bandwidth among the Optical Network Units (ONUs) that are near end users or in homes, for example, in response to bursty traffic requirements.. The practical benefits of DBA are twofold. Firstly, network operators can add more customers to the PON due to the more efficient utilization. Secondly, customers can enjoy enhanced services, such as those requiring bandwidth peaks beyond the traditional fixed allocation.The second draft new standard G.983.5 specifies a number of protection options for PONs which will enable enhanced survivability for e.g. Fibre To The Cabinet (FTTCab) and the delivery of highly reliable services in the case of e.g. Fibre To The Office (FTTO).These draft new standards complement G.983.3 which was approved earlier this year. The G.983.3 standard adds an additional wavelength band to the downstream direction of a Broadband - Passive Optical Network (B-PON). Until now, only two wavelengths have been specified, one for each direction of transmission. The new wavelength band could, for example, allow separate wavelengths for interactive and broadcast services over an optical distribution network.
“Together with the basic PON standard (ITU-T G.983.1) and the related PON management and control interface standard (ITU-T G.983.2) issued in 1998 and 2000 respectively, the ITU-T now furnishes a consistent set of five PON standards, which allows services such as multiple-line telephony and high speed internet access at, for example, 100 Mbit/s rates to be carried on one pair of wavelengths and video services on one or more additional wavelengths” says Peter Wery, Chairman of ITU-T Study Group 15.
This additional wavelength band is transparent and, with optical amplification and further power division in the feeder network, could allow a national, regional or local broadcast TV overlay.
The television signals can be broadcast in a number of formats although the new generation of digital set-top boxes would give best performance and could offer up to 400 TV channels on a single wavelength.
This functionality allows a network oprator to offer broadcast services on the same infrastructure, generating a new revenue stream to maximize network investment.
Vendors already have products which support high-speed data and internet over the fibre accessnetwork and work is now starting in the ITU-T on standards for TV multiplexing and modulation schemes for the broadcast overlay.
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