6. Architectures of PON-Based Open Access Networks
6.3 Multi-OLT PON-Based OAN ( M-OAN)
A PON also has sufficient bandwidth to achieve the required bandwidth demand of an OAN.
However, the current PON architecture with a single OLT is not efficient enough to integrate all the service providers in an OAN [16]. In the chapter 3 of this thesis, a multi-OLT PON structure has been proposed for combining the FTTH and WSNs in a single optical network [18,19,20]. However, the multi-OLT PON structure is not suitable for an OAN as in the multi-OLT PON an ONU is considered as an access terminal only for a single service provider, i.e., FTTH network or WSNs, but in an OAN every access terminal is shared by all the service providers in the network.
In this section, a new multi-OLT PON structure for the OAN is proposed to make all the access terminals or ONUs of the PON-based OAN are available to every service provider. In this M-OAN, every ONU will be shared by all the service providers and each OLT will be performed as a CO of a particular service provider.
6.3.1 Network Architecture of an M-OAN
Fig.6.3 Architecture of a multi-OLT PON-based OAN.
A system model for an M-OAN is shown in Fig. 6.3. For convenience, only four OLTs for four service providers have been considered in this figure. Here, the OLT1 is for the FTTH network, the OLT2 is for the WSNs, the OLT3 is for the HDTV/VoD, and the OLT4 is for the FNs. On the other hand, 16 ONUs and each of the ONU is shared by all the four service providers are shown in the figure to represent the proposed M-OAN system model. However, the number of OLTs can be increased depending on the number of service providers in the OAN. Here, all the OLTs contain a
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common polling table to provide synchronization in both the upstream and downstream transmission channels of the TDM PON. As well as the definition of the OAN it is clear that in the OAN all the access terminals are open to any service provider. That is why, all the four service providers, FTTH, WSNs, HDTV/VoD, and FNs, in an OAN can be connected to each of the ONUs.
In the upstream direction of the M-OAN, data packets from all the ONUs will be received by all the OLTs but every OLT accepts the data packets from a designated service provider. In contrast, in the downstream direction of the M-OAN, all the ONUs accept the broadcasted data packets from every OLT.
6.3.2 Upstream Frame Format of an M-OAN for the Modified LS Scheme
To improve the bandwidth sharing efficiency in a PON system several bandwidth sharing algorithms have been proposed. However, this thesis analyzes the performance of an M-OAN using the modified version of the existing LS scheme. The main objective of this analysis using the LS scheme is just to show that the performances of the proposed network architecture of the OAN are better than those of the S-OAN. The proposed ALDBAM scheme with some modification can be a more suitable algorithm for the M-OAN because the ALDBAM scheme utilizes the multi-OLT PON effect that can be matched with the M-OAN. But, at this stage, this thesis only emphasizes on the modified version of the LS scheme. Figure 6.4 shows the bandwidth sharing principle among the ONUs in the upstream direction for the proposed M-OAN using the modified version of the existing LS DBA algorithm [17]. Here, the maximum transmission window max in the LS scheme is divided by four service providers and the four maximum transmission windows, max_1, max_2, max_3, and
max_4, of the four service providers reflect their maximum packet lengths. The number of service providers connected to an ONU is not fixed, as shown the Fig. 6.4 the four service providers are connected to the ONU1 while three service providers are connected to the ONU N.
Fig. 6.4 Upstream frame format of an M-OAN for the modified LS scheme.
In a DBA scheme, the length of a time cycle is flexible and maintains an upper bound, i.e.,
max cycle cycle T
T . The maximum transmission window for each ONU is max Tcyclemax N, here, N is the
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number of ONUs. In the M-OAN, the max of an ONU has been divided into all the service providers connected to that ONU. Length of the maximum transmission window for each service provider depends on their packet lengths and the number of service providers connected to that ONU. For the upstream transmission in the proposed scheme, four service providers are divided into two groups depending on the generated packet lengths of each service provider. This scheme assumes that the FTTH terminals and the FN are in one group while the WSN and the HDTV/VoD are in another group. The max for each service provider is calculated as follows:
max_1
max_4and
max_2
max_3 (6.1)3 max_
2 max_
4 max_
1 max_
max
(6.2) here, max_1 is the maximum transmission window for the FTTH terminals, max_2 is the maximum transmission window for the WSNs, max_3 is the maximum transmission window for the HDTV/VoD, and max_4 is the maximum transmission window for the FNs.6.3.3 Timing Diagram of an M-OAN
Fig. 6.5 Timing diagram of an M-OAN.
Figure 6.5 shows the timing diagram to schedule the data transmissions among the ONUs and OLTs of an M-OAN. A modified version of the interleaved polling algorithm IPACT is used to avoid collisions and data overlapping. Here, all the OLTs sequentially transmit the Gate messages to every ONU, and an OLT receives the corresponding Report message of a service provider from every ONU.
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In the figure, four OLTs transmits the four Gate messages G11, G21, G31, and G41 to the ONU1 from the OLT1, OLT2, OLT3, and OLT4, while every ONU transmits the four Report messages and data packets R1 & D1, R2 & D2, R3 & D3, and R4 & D4 to the OLT1, OLT2, OLT3, and OLT4, respectively.
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