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International Telecommunication Union
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Bringing broadband to the home:
more steam for Internet access
Geneva, 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 networks based 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) utilize 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 destination by
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. Moreover, 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 independent 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", said
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 operator 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 access network and work is now starting in the ITU-T on standards for
TV multiplexing and modulation schemes for the broadcast overlay.
For further information please contact:
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