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Alternatively, it may use wavelength-tuneable transmitters and receivers, which can address any wavelength in a certain range in principle. The network management and control system commands to which downstream and to which upstream wavelength channel each ONU transceiver is switched.

The network management command signals are transported via an out-of-band wavelength channel in the 1. The APON signal channel selected by the ONU is converted into a bidirectional electrical broadband data signal by the transceiver; this is done by a cable modem controller tuned to the appropriate frequency band for multiplexing with the electrical CATV signal. The upstream data signal is usually put below the lowest frequency CATV signal so below 40—50 MHz and the downstream signal in empty frequency bands positioned between the CATV broadcast channels.

The signals are carried by the coaxial network in which only the electrical amplifiers need to be adapted to handle the broadband data signals to the customer homes, where the CATV signal is separated from the bidirectional data signals. The latter signals are processed by a cable modem, which interacts with the cable modem controller at the ONU site. As soon as the traffic to be sent upstream by an ONU grows and does not fit anymore within its wavelength channel, the network management system can command the ONU to be allocated to another wavelength channel, in which still sufficient free capacity is available.


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If WDM is used and ONUs are provided with tuneable transceivers, nodes can be re-tuned re-configured to less loaded wavelengths to equalize traffic among all available wavelengths, thereby increasing the network capacity. Although tuneability is required at the ONUs to dynamically assign wavelengths to transceivers, this tuneability does not need to be fast since it must track slow traffic changes i.

Nowadays, slow tuneable filters and lasers are becoming cheap devices so that these solutions can be considered affordable in a PON scenario in the near future. Reconfigurable PONs build different logical topologies over the fibre network which can properly match traffic conditions and follow slow traffic changes.

A decision problem concerning how to build these logical PONs arises.

Next-Generation FTTH Passive Optical Networks

Thus, efficient reconfiguration mechanisms that solve this problem need to be studied. A preliminary analysis of a possible upgrade for current PON solutions with capabilities of reconfiguration was carried out.

A simple and cost effective approach could be to introduce CWDM only in the downstream and leave the upstream untouched upstream based on TDM only as shown in Fig. If so, the OLT must be equipped with an array of lasers to transmit on the different wavelengths, and ONUs should have slow tuneable hence cheap filters to receive on a selectable single wavelength.

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As a consequence, one logical PON is built for each laser of the Fig. Good solutions should equalize the offered traffic among all wavelengths and avoid overload-conditions. Since the OLT schedules all downstream traffic i. Although this problem can be solved optimally, it has been demonstrated to be NP-hard. Since tuneability is slow, the ONUs go offline during the process of retuning for what we call a blackout period, during which they cannot receive packets.

Therefore, when looking for traffic-matched logical topologies, it is also important to consider the reconfiguration cost i. Reconfiguration algorithms should not only look at traffic equalization but also at possible traffic disruption. Both the optimal solution and heuristic approaches to a similar problem have been proposed and studied in [Neri05].

In order to improve bandwidth allocation also for upstream traffic for example, for peer-to-peer traffic C WDM can be introduced in the upstream path by equipping ONUs with slow tuneable transmitters and the OLT with an array of fixed receivers, as sketched in Fig. Again a decision problem arises for deciding to which wavelength ONU transmitters should be tuned. However, while for downstream traffic the OLT knows the traffic matrix, for upstream traffic this information is distributed among nodes.

On the contrary, blackout times are not as critical as in the downstream case since transmitters could be retuned in a distributed fashion without any packet loss i. These users move across the geographical area served by the network e. The corresponding RAP may switch on more microwave carriers to provide this additional capacity over the air and also has to claim more capacity from the ONU.

This local extra capacity can be provided by re-allocation of the wavelength channels over the ONUs which is done by a flexible wavelength router positioned in the field. Similar to the architecture of the wavelength-reconfigurable fibre-coax network in Fig. The four downstream wavelengths are located in the 1,—1, nm range, with GHz spacing, and the four upstream ones in the 1,—1, nm range with the same spacing. The flexible wavelength router directs the downstream wavelength channels each to one or more of its output ports and thus via a split network to a subset of ONUs.

OA flex. The flexible router can select one of these upstream wavelengths and direct it to the ONUs that can modulate the signal with the upstream data and return it by means of a reflective modulator via the router to the local exchange. The flexible wavelength router can be implemented with a wavelength de-multiplexer separating the wavelength channels, followed by power splitters, optical switches and power couplers in order to guide the channels to the selected output port s.

Depending on the granularity of the wavelength allocation process, the flexible router may be positioned at various splitting levels in the network. Using a similar strategy to assign wavelength channels to the ONUs as depicted in Fig. It was also assumed that the system deployed seven wavelength channels, and that the calls arrived according to a Poisson process where the call duration and length were uniformly distributed. When wavelength re-allocation would not be possible i. On the other hand, in the static WDM case when the 49 hot spots were evenly spread over the seven wavelength channels, the blocking probability is much lower i.

Unfortunately, a network operator cannot know beforehand where the hot spots will be positioned, so in this static WDM situation the system blocking probability will be anywhere between the best case and the worst case, and no guarantee for a certain blocking performance can be given. However, when dynamic re-allocation of the wavelength channels is possible, the system can adapt to the actual hot spot distribution. The blocking performance may be even better and stable when positioning the flexible router at the third splitting point; however, this implies that the costs of the router are shared by less ONUs which results in a higher cost per user.

Locating the router at the second splitting point is a good compromise between adequate improvement of the system blocking performance and system costs per ONU. From this splitting point, there is a fibre for each ONU connected to the network. In principle the tree topology consists of cascaded splitting points and topologies with a single splitting point are in general termed as star topology. However, due to the special relation between OLT and ONU there is directivity and therefore this topology, if applied to access networks, is commonly termed as tree topology.

The main advantage of this topology is that the splitting is concentrated on a single point; thus it is simple to detect a network problem. Another advantage is that all ONUs have the same power budget which means that they all will receive roughly the same optical signal quality. This architecture allows that the OLT equipment, which is the enabling equipment for processing capacity and transport capacity, to be shared among all ONUs and makes only use of fairly mature low cost optical components.

In addition, the point-to-multipoint connectivity of a PON also reduces fibre congestion potential at the OLT site compared to a pure point-to-point approach. The number of ONUs that can be supported is limited either by the splitting loss in the star coupler or the required bandwidth per user. Note that all users need to fit to the capacity of the OLT splitter link and thus the potential of sharing capacites limits the number of active users. The star topology single splitting point topology is attractive due to the fact that the transfer from a narrowband last mile technology up to 2 Mbps per customer to a fibre based broad-band access network at least 1 Gbps shared capacity per ONU is easy and effective.

If the number of subscribers increases, the star network can be easily broken up into several subnets adding another splitter and OLT to the splitter link, yielding a versatile and flexible architecture for network expansion. Commonly star topologies involve a passive optical broadcast star splitter device, which simply splits all the optical power independent of wavelength and port.

The basic architectures however use TDM only, i. Stars present a weakness in terms of reliability. Failure of the central device can bring down the whole network, but a total failure of a passive broadcast element is unlikely. However, many partial failure scenarios exist, including amplifier failures, port connection failures at the access nodes, transmitter or receiver failures at access nodes e. Such failures generally lead to failure of one or more arms of the star. In case of access networks a failure of the OLT to splitter link is fatal as that efficiently affects all communication.

Another drawback is that the available bandwidth is shared by all customers on the trunk connection from OLT to the optical splitter. The average capacity share per customer is thus the number of wavelengths multiplied by the bit rate per wavelength divided through the total number of customers.

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For example eight wavelengths at 2. If this value shall be guaranteed, the customer traffic must be limited to this rate which in turn deteriorates the network utilisation as typically not all customers access that capacity constantly. Each final subscriber is connected to it by means of a tap coupler that extracts a small part of the power that is being transmitted from the OLT. The two advantages of this topology are that it is the one that uses minimal amount of optical fibre if ONUs are directly connected to the tap coupler and allows flexible deployments as a new ONUs can be connected to the network very easily by adding one more tap.

The main problems are: on the one hand, that the signal is degraded when passing through each tap coupler, and therefore the ONUs located far from the OLT are receiving weak and degraded signals; on the other hand, that the required total fibre length is high for covering a two-dimensional area. As there are two possible ways to reach the OLT, it is still possible to establish and maintain a data link in case of a fibre cut. However, it requires two fibres to be used at the OLT and more complicated equipment at each ONU with switching capabilities to be able to send and receive the signals being transmitted from the two directions of the fibre ring.

It also shows the same problem as the bus topology in terms of power budget. When the optical signal passes through each ONU, the signal is degraded and attenuated.

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This factor is the most restrictive one in terms of transmission capabilities and restricts the number of ONUs that can be connected to the ring. Capacity is also shared among all ONUs if resilience is used, thus the second fibre from the OLT does not increase the network capacity, i.


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In case one fibre is cut, the other can be used to transmit. However, when deploying the structure, these two fibres need to be installed on separated trunks to avoid both being cut at the same time. In latter case the maximum capacity per ONU trunk is halved. However, assuming a realistic splitting ration by far exceeding at the star splitter, this restriction in general does not reduce the maximum number of customers supported at a guaranteed bit rate. Combination of these basic topologies can offer granularity and a higher density as well as better applicability to cover two dimensional areas.

The most promising approach is to mix a primary ring and secondary trees to offer resilience in the distribution feeder and optimization of fibre length deployment in the access segment Fig. Another interesting approach is the double ring, which offers resilience on both, the primary and secondary segments of the access network Fig. However, this implies a more complex access protocol and more fibre deployment. The considered network architecture is shown in Fig. Therefore AWG-based single hop networks offer superior bandwidth utilisation compared to passive star coupler architectures.

Therefore this architecture provides peer connectivity as it is typically required for a Metropolitan Area Network MAN. Several proposals of such application exist, e. Normally, N simultaneous transmissions can be simultaneously supported by one AWG if the number of ports equals the number of wavelengths, yielding a connectivity of N.

Each wavelength channel can be divided into fixed-duration time slots and data packets fit into the so created TDM channels. Accordingly each node in a group may transmit and receive on any TDM channel available. To assure that no collisions occur among channels of the sender and receiver groups, a specific scheduling algorithm is required — see section 6. Current TDM based PON networks are emerging as viable and cost effective access solutions for service providers facing extremely growing bandwidth demands per connection.