CONTROL UNIT

5.7 ADVANTAGES OFFERED BY THE LAMBDAPON

5.1 COM PARISON WITH MOBILE TELEPHONE SYSTEMS

The restriction in the number of available wavelengths for use in a WDM network can be compared to the restriction in available frequencies in mobile telephone systems. The frequencies used by mobile telephone networks need to be strictly controlled so as to avoid interference between them and to maintain orderly utilisation of them.

In m obile netw orks, restrictions on frequency availability place a lim it on the bandwidth of the radio channels available to the transmitters. This in turn limits the transmission rate of the radio transmitters and, hence, the number of users who can sim ultaneously use them. In 1979, researchers at AT&T Bell Laboratories [70] realised that if relatively low-power transmitters were used so that signals propagated only over short distances, the same radio frequencies could be used again with minimum interference in nearby areas as soon as the interference signals were sufficiently attenuated, yielding frequency reuse. This reuse allows a much higher number of mobile subscribers.

To minimise interference among users, there must be physical separation between cells that use the same frequencies. This is achieved by forming groups of cells into clusters which between them can use all the available radio channels. A typical cluster size is seven such that the geographical region to be served is divided into small hexagonal cells (as shown in Figure 5.1) [81].

"Although a seven-cell repeat pattern has been described, other cluster sizes are possible (such as four or twelve). Additionally, in reality the cluster sizes are not hexagonal but have irregular shapes determined by factors such as the propagation of radio waves over the terrain, obstacles (e.g. tall buildings), and the constraints on siting of base stations imposed by the geographical location to be covered.

Users are dynamically allocated a frequency, / , on which to transmit dependent on the cell which they are in. As a mobile user approaches the cell boundary, the signal strength falls. This is detected by the cellular system, which then switches the radio

link to a base station in an adjacent cell; this process is called hand-off or hand-over.

It happens automatically under system control; the user notices nothing, or possible a slight break in the conversation lasting about a third of a second.

There are some subtle features relating to the allocation of channels to the cells and to the mobile customers. Firstly, the more channels a cell has, the more subscribers it can support; hence, a cell in an urban area may be allocated more channels than a nearby cell serving a rural area. Secondly, the relationship between the number of channels and the number of subscribers supported is not linear; doubling the number of channels will more than double the number potential number of subscribers within cellular radio systems. The theory behind this is beyond the scope of this thesis, however, some representative figures can be quoted (more in depth coverage can be found in [8 2 ], and [83]. At 2% blocking probability (assuming the cellular system is

designed so that 98% of call attempts are successful), increasing the number of channels from 20 to 40 increases the telephone traffic capacity from 13 to 31 erlangs. It is also important to note that although there is not a fixed maximum number of subscribers, the quality of services decreases significantly as the number of subscribers in a cell increases beyond a certain limit (dependent on the number of frequency channels available).

5.2 WAVELENGTH REUSE IN WDM NETWORKS

Limiting the power of the optical signal transmitted down an optical fibre is not a suitable method of ensuring that geographical regions using the same wavelengths do not interfere (as is done in mobile networks). This is because the strength of the transmitted signal, in an optical fibre, is low and attenuates very slowly with distance (0.2 to 0.5 dB/km). As the signal strength deteriorates the noise inherent in the signal increases relative to the signal strength. This noise will interfere with any other signal sent at the same frequency. At the receiver the resulting noise build-up can render the signal indistinguishable from the noise.

However, in an optical fibre it is possible to "block out" signals using filters strategically placed throughout the network.

5.3 WAVELENGTH REUSE WITHIN A BROADBAND WDM-BASED PON

We propose to study the possibility of designing a circuit-switched WDM based optical network incorporating the advantages of cost sharing offered by broadcast passive optical networks. However, the WDM based PON should be capable of carrying both telephony and broadband symmetric traffic services requiring the transmission of very high bit-rates in both the upstream and downstream directions; unlike the traditional PONs mentioned in Chapter 3.

To overcome the limitation in the number of wavelengths available for dynamic allocation a novel method of augmenting the capacity of the PON by achieving a high degree of wavelength reuse is proposed. The wavelength reuse concept is both network topology and distance independent. The new wavelength reuse broadband

PON has been called the LambdaPON (after the Greek symbol Lambda for

wavelength). The network has been designed in such a manner as to allow customers to construct an optical connection between any two or more terminals. The optical path supports several levels of bandwidth making it possible for the network to supply bandwidth on demand, enabling flexible and rapid provisioning of bandwidth variable services. The network provides a very useful theoretical testbed platform over which other technologies can be overlaid and ideas tested.

Wavelength reuse is achieved by the use of dynamically controllable wavelength filters which offer the selective filtering of a single wavelength or group of wavelengths over time. The filters acts as very simplified active wavelength switches. Their function is merely to block the transmission of a wavelength or any subset of wavelengths, thereby localising the wavelength(s) to a desired area of the network and allowing the wavelength(s) reuse elsewhere. The speed at which the filter needs to operate is at the same rate as the time taken to set up a call (of the order ms).

5.4 THE LAMBDAPON UPGRADE AND WAVELENGTH REUSE