Present-day access networks carry a large variety of services to the customer along a wide range of last-mile transport media, such as twisted-pair copper cables, coaxial cables, and wireless drops. Besides the increase in number of users connected, the surging demand for capacity in present-day access networks is fueled by a rapid growth of the usage of interactive services. The most noteworthy is web browsing, with an increasing hunger for streaming audio and video applications. Fibre is pushed ever deeper in the network, closer to the user, in order to meet this demand. Fibre to the Home is in most cases been considered to be still too costly, both from component and installation point of view; it offers, however, the most future-proof solution to the capacity demands which may emerge. As an upgrade to existing access networks, fibre is being used to feed signals to the last part of the customer access networks; examples of these hybrid access networks are fibre-coax and fibre-wireless networks. Wavelength Division Multiplexing (WDM) can advantageously be deployed in the next upgrading step, to increase capacity In the fibre feeder part by introducing multiple wavelength channels without having to change the installed costly fibre plant. But apart from just boosting the capacity, WDM may have even more impact by using wavelength routing techniques. In shared-fibre feeder networks, like a tree-and-branch one with large splitting factor, transport capacity may be shifted from less busy network parts to those spots with a much higher need. Thus capacity can be delivered on demand, and the operator can more efficiently direct his network resources and hence maximize his revenues. Another way to exploit the wavelength domain is by using different wavelength channels for carrying different services. By leasing wavelength channels, several service providers can be hosted by a common fibre infrastructure.
In this talk, after reviewing the basic options for wavelength routing, two flexibly wavelength-reconfigurable hybrid access networks will be described in detail: a fibre-coax network as explored in the ACTS AC028 TOBASCO project, and a fibre-wireless network as in AC349 PRISMA.
The TOBASCO project has trialed flexible WDM technology In a fibre-coax CATV network in Ghent (Belgium), providing residential households with broadband interactive services via the coaxial cable user access network. The main application envisaged is tele-working; e. g., by connecting students and university staff members from their homes with the university computer servers, or by giving small businesses like travel agencies a fast internet connection. The fibre feeder network is laid out as a widely-split broadcasting network, and is carrying multiple ATM-based 622 Mbit/s wavelength channels. At the ONUS, wavelength selection is done; this selection is controlled from the headend site via a network management and control overlay network. When the traffic at an ONU exceeds its allowable share of the present wavelength channel, another wavelength channel may be assigned to this ONU in which sufficient capacity is still ava~lable. Thus the distribution of network resources among the ONUS is optimally adjusted to the local temporal traffic loads.
The PRISMA project takes the technology one step further into the fibre-wireless domain, where broadband mobile services (UMTS, nomad~c computing) will be trialed. To support the temporary increase of the density of users and/or of the service capacity requirements in a wireless cell (i. e, the occurrence of a “hot spot”), extra microwaves may be added to the Base Station Transceiver (BTS) in that cell. The feeder capacity to the BTS is then adjusted by redirecting the appropr~ate wavelength channel capacity to its ONU. The wavelength channel selection Is done by means of an adjustable wavelength router installed in the field, which by avoiding wavelength-selective devices at the ONU leads to a more cost-effective system implementation. First studies have shown that this flexible WDM approach significantly reduces the call blocking probability; in particular the blocking probability for “hot spot” cells can be reduced to that of regular cells. Thus, given that in practice the peak traffic demands of the cells do not coincide in time, dynamically reconfiguring the feeder capacity on a wavelength basis provides a new way to achieve statistical multiplexing gain in fibre-based hybrid access networks.
In the presentation, the basic system concepts investigated in both projects will be described, and some network layouts shown. The implementation of a number of key system modules will be discussed, as well as the results from some laboratory experiments and field trials.