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6.4.2
Chapter 6
Peer-to-Peer Traffic
Many traffic studies show that peer-to-peer applications are becoming increasingly popular. PON architectures need therefore to support peer-to-peer applications, which require large bandwidth, and make the traffic pattern more symmetric between upstream and downstream paths with respect to other streaming or Internet client-server applications. A possible approach to deal with the increased bandwidth requirements of peerto-peer traffic, and with its symmetry, is to introduce (C)WDM in the upstream path, by equipping ONUs with slow tuneable transmitters, and the OLT with an array of fixed receivers, as sketched in Fig. 2.14 (and described in section 2.2.6). 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. Moreover, if WDM is introduced in both downstream and upstream paths it means that scheduling algorithms at the OLT should also be adapted. Besides, it is important that these algorithms take also into account multicast and peer-to-peer traffic. For peer-to-peer traffic, it is reasonable that intra-PON traffic, i.e., traffic coming from one ONU and directed to another ONU of the same PON, should be forwarded by the OLT back to ONUs as soon as possible. This goal can be reached providing a fast switching at the MAC layer. For example for EPONs, some slots (each slot can store several Ethernet packets) of the TDM stream can be reserved for peer-to-peer traffic; an efficient scheduling algorithm can be applied directly on these TDM slots, avoiding to receive individual Ethernet packets at the OLT. In general, the optimal solution would be to jointly schedule upstream and downstream transmissions, i.e., to couple the two normally disjoint upstream/downstream scheduling problems.