The Lam bdaPON w avelength reuse concept can be applied to upgrade both the SuperPON and the traditional PON structure. Figure 5.5 shows the upgrade proposal applied to the SuperPON structure. The SuperPON electronic head end is replaced by an identical optical head-end chip. The optical head-end chip consists of a number of passive routing nodes interconnected via waveguides etched into the chip. A dynamic tunable filter is affixed to each of the waveguides. By interconnecting several of the PRNs together, the system can be designed to serve a large number of custom er (of the order 256 as previously explained). The maximum number of PRNs which can be interconnected is dependent on the offered traffic load and grade of service required, the num ber o f w avelengths available, the topology o f the netw ork and the components used.
The upgrade of the traditional PON uses the same components and concept, however in this case the PRNs are interconnected by optical fibres. The SuperPON upgrade techniques are best suited to serving urban areas with high population densities because additional waveguides can be used to interconnect the PRNs to form a highly meshed architecture, thereby allowing greater reuse of the wavelength channels. On the other hand, the PON upgrade is suitable for less densely populated areas in which small scattered com m unities exist. Long lengths of fibre can then be used to interconnect the PRNs. Electronic Head-end
/ \
Large Split (= 3000) Upstream and Downstretim G bit/s 256 way split K E Y : I I P assive R outing N od e (PR N O D ynam ic Tunable FiltersFigure 5.5 Schematic Diagram Showing W avelength Reuse W ithin A SuperPON 5.6 RESOLVING CONTENTION FOR THE NETW ORK CONTROL UNIT Since the NRM serves several thousand users; there is the possibility that more than one user may simultaneously transmit a control packet to the NRM. If two or more users were to simultaneously transm it packets to the NRM, assuming they were all equidistant from the NRM , the packets w ould overlap resulting in them being
irrecoverably corrupted. When this occurs contention is said to have occurred.
Users whose packets have contended must be inform ed. Hence, users must be informed of the status of any packets which they transmitted to the NRM. In order to
deal with contentions, a passive star coupler is placed prior to the NRM (as shown in Figure 5.6) and a slightly modified version of the ALOHA protocol (further details of this protocol is given in chapter 7) is used to inform the users of their success or failure in accessing the database.
The role of a passive star coupler is to combine any optical signals entering any one of its input ports and divide the signal equally among its output ports. By placing a star coupler just prior to the NRM, any packets received on A,i will be combined with packets received on other incoming fibres and retransmitted downstream to the users and to the NRM. Hence, by monitoring this wavelength channel and receiving their originally transmitted packet after the round-trip delay, users who have transmitted a
status or termination packet can determine whether or not the packet has contended with other packets. If the packet has contended, the users are required to retransm it the original packet after a random time interval (as stipulated by the ALOHA protocol). Fibres To & From PRN w / / Fibres To & From PRN 3 Fibres To & From PRN 2 Passive Star Coupler V Fibres To & From PRN 1
Figure 5.6 Resolving contention for the NRM
The database discards any contended packets it receives. If the packet has not contended the database looks up the inform ation requested and broadcasts the information to all the users on the status packet as previously discussed. The actual design and operations o f Network Routing M anager are beyond the scope o f this PhD, however several network systems have been proposed which require the use of
The degree of contention for the NRM is dependent on the length of the packets transmitted, the transmission rate used, the number of users sharing the database and the number of call requests during the peak busy hour period. A detailed calculation of the expected contention for the NRM and its performance is calculated in Chapter 7.
5.7 ADVANTAGES OFFERED BY THE LAMBDAPON
Now that the concept of the LambdaPON has been explained, it is worthwhile revisiting the advantages offered by the network.
1) Provides a possible upgrade path for both the PON and the SuperPON networks. 2) Offers a high bandwidth (capability of GHz) transparent optical path to
customers dynamically. Hence customers need only pay for what they use. 3) Wavelengths are not pre-assigned to the customers but allocated to customers
dynamically as required allowing network flexibility. If a particular area of the network becomes traffic congested, additional wavelengths can be assigned to relieve the congestion. This, unfortunately, dictates the need for tunable receivers at customer terminals and hence requires the price of these receivers to be low in order for the network to be cost effective.
4) The wavelength channels can be used to provide either point-to-point routing between customers requiring the use of two wavelengths, or used as a channel over which additional multiplexing techniques can be imposed. By imposing two TDM time slots onto each wavelength, a point-to-point link between two
customers only requires the use of 1 wavelength.
5) If two wavelengths are used to interconnect two customers, the routing of information between the customers can then be performed entirely in the optical domain with no electronic switching or buffering of the signals in the network
required. Once transmitted signals remain unmodified (i.e. there is no
conversion, regeneration or any other non-linear operation) until it is are received by the receiver at the other end.
6) The network operates in a optically circuit-switched mode and hence once the
wavelengths have been allocated, the delay is negligible since no buffering or optoelectronic conversion is required. Call set-up time is limited only by the network controller units processing speed, the tuning speed of the transceivers and the propagation delay (typically a few ms).
7) Customers can be allocated bandwidth on demand since all the devices used to route the information are bit-rate independent and passive in nature. This enables customers to chose a required bandwidth for a desired service for any duration.
8) Switching within the network is achieved by the use of controllable tunable wavelength filters which are required to operate only at the speed of the caU set up process.
9) To provide network resilience and reduce the propagation delay during call set up and contention for the network control unit, the network control information
can be stored using distributed network control units. However, this has its
drawbacks; namely increased control and management complexity and increased hardware requirements.
10) The network does not require wavelength conversion or synchronisation if two wavelengths are used. However, wavelength stabilisation is necessary.
11) The concept can be applied to any network topology. For highly meshed architectures, route diversity is provided resulting in network resilience. By modifying the core network passive routing nodes to perform both space and wavelength multiplexing, the development of an end-to-end transparent optical network is possible.
In effect the LambdaPON is a very flexible structure which allocates a "black pipe" to a customer for the duration of his/her call. The structure does not impose any constraints on bandwidth or type of information transmitted over the wavelength channel. Hence the LambdaPON provides the ideal theoretical testbed platform overwhich other ideas can be tested and technology overlaid.
5.8 CONCLUSIONS
A novel wavelength reuse concept has been proposed offering a high degree of wavelength reuse depending on the network topology and wavelength allocation algorithm. The network has been designed in such a way as to provide a very flexible structure which imposes no constraints on the bandwidth or service requirements of the customer. The next chapters will now delve into greater details of the LambdaPON e.g. component requirements and network performance parameters.
It is assumed throughout that there is a network manager and signalling system that sets up calls on request from user stations. The function of the Network Control Unit is to determine the physical path to be allocated to the call, set up the tunable filter parameters (i.e. the wavelengths to be blocked) along the route and finally to assign the wavelengths and keep track of them and shut down the call. The physical route and the wavelength assigned is assumed to remain unchanged throughout the calls duration.