Developments in Optical Access Networks

Written on 3:18 PM by ooe

Optical fibre access network technology has become more complex over the last few years with bit rates steadily increasing. PON systems are available with bit rates in the range 155 Mbit/s to, currently, 622 Mbit/s. Newer systems, both PON and point-to-point, will allow this trend to continue with 2.5 Gbit/s systems becoming practicable in the near future. With this increase in technology has come an increase in the opportunities for fibre system to cost-reduce more of the network, including the metro space – a fact that increases the appeal of the technology. Some of the newer PON systems are being designed using Ethernet as the underlying technology with a promise of reducing the price of the technology still further. How this will occur in practice, however, is still unclear. What is certain is that optical access technologies are expanding and are sure to find an opening in real networks in the foreseeable future.

Developing equipment for the access network has always been a risk for the equipment manufacturers. The size of the network means that equipment volumes can be huge but the equipment is extremely cost sensitive. Furthermore, the sheer cost of equipment rollout means that network operators do not attempt such a major upgrade of their networks lightly. Indeed, to date there have been no ubiquitous upgrades of the UK access network, rather there have been many small, ad hoc changes. Things are now beginning to change with the introduction of ADSL, although this still utilises the most costly and fault-prone parts of the copper access network.

Whilst some new services are being rolled out to businesses that can afford them, these roll-outs are focused on those customers wanting to use the bandwidths immediately with little effort on overall network upgrades. In the UK, services such as the short haul data service can provide upto 2 Gbit/s to customers separated by no more than 40 km, whilst conventional data networks can supply in the order of an STM1's worth (155 Mbit/s) to high bandwidth customers, again mainly business related.

In terms of residential customers, progress in upgrading the network has been slow, mainly because of the perceived lack of value of bandwidth upgrades. The residential market has been getting ever more cost sensitive, with the restrictions in permitted price increases and a glut of bandwidth in the core now hurting many CLEC companies quite significantly. Against this backdrop comes ADSL, enabling users to get 500 kbit/s downstream and 256 kbit/s upstream at an extremely competitive price.

Back in 1989 people predicted the introduction of Passive Optical Network (PON) technology into the network [1]; however, this roll-out did not occur for several reasons, the main one being that there was no perceived market for the types of services that optical fibre technology could bring. This is now beginning to change.

Current bandwidth upgrades will not satisfy customers for ever. There is a question as to when residential customers will require more bandwidth than that offered by ADSL but it seems to be required sooner rather than later. During the last 10 years, there has been a step change in the computing power available to the average residential consumer. Processing power has increased from 8 Mbit/s systems to the current 2 Gbit/s systems (with 3 Gbit/s around the corner) and there is little sign of the pace of technological advancement slowing. This is already leading to customers being able to manipulate video images in real time. What they cannot do is transport this data, in real time, across the telecommunications network.

Following a similar line, point-to-point Ethernet technologies are also receiving attention with a variety of systems becoming available from different manufacturers.

If developments continue at the same pace, then there will be a need for affordable interconnections at speeds currently unattainable by ADSL. Whilst VDSL could cope in the medium term, questions are already being asked about its suitability, given the high operational costs of the required street located electronics and questions relating to radio interference from overhead copper cables.

Against this backdrop the future development of fibre systems for the access network seems to be important. Fibre access network products are already becoming available but most of these are expensive, difficult to implement and are not full service compatible.

Whilst fibre technology has yet to mature enough to make cheap access systems, the major problem is the lack of volume in the sector to date. It is against this backdrop that there is a need to develop affordable, flexible systems. A variety of companies and people, however, are rising to the challenge. It has become noticeable that there are more start-up companies designing and building equipment for this market sector than ever before and that existing major players are now moving into this field with mainstream products.

Whilst standardisation of systems has been shown to lead to a steep reduction in their price, along with increased availability, standardisation too early can lead to poor products. Hence pre-standards discussions in an open environment can be a useful first step. This can be brought about at specialist fora at which the associated problems and challenges can be discussed and indeed such fora have been created and are making good progress in setting initial requirements and solving technical problems.

The best known standards forum is FSAN. Until recently the FSAN group has been the mainstay of optical fibre access system development. Seven operators, including BT, first met in the spring of 1995 in order to begin the FSAN initiative. FSAN stands for Full Service access network and had an initial focus on passive optical network system (PONs). BT had previously been experimenting with ATM-based PON systems [2] but these systems remained largely in-house.

The network operators invited equipment providers to join the group in order to progress activities in a manner suited to the production and specification of such equipment. Using this mechanism the number of contributors to the FSAN forum could be controlled, preventing the organisation from becoming unwieldy. It is still the situation today that equipment vendors only join FSAN by invitation, although they are free to solicit such invitations.

The IEEE are also now active in this area and have their own forum, the Ethernet in the First Mile (EFM) group, trying to create access network standards around existing Ethernet standards.

These groups will now be discussed in some detail.

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