# Supporting Heterogenous Traffic on Top of Point-to-Multipoint Light-Trees +
## 2 Optical Layer Supporting P2P and P2 MP Traffic  
P2P and P2 MP communications are illustrated in Figure 1a,b. Figure 1a presents a particular scenario of P2P connectivity, where all the sources are in the same location and the destinations are in multiple remote locations, i.e., a hub-and-spoke arrangement, which is typical in, e.g., access networks. Figure 1b shows the same scenario for P2 MP connectivity, where one single source communicates with destinations in multiple remote locations. Some differences are worth highlighting: (i) from the pure connectivity viewpoint, in a P2P connection, the signal sent by a source is received only by its destination counterpart, while in a P2 MP connection, the sourced signal is received by all the destinations; (ii) one single source is used and part of the communication resources can be shared in P2 MP connectivity, whereas dedicated sources and connectivity resources are needed in P2P connectivity. Therefore, in the case of \(N\) destinations, \(2\times N\)_terminals_ with dedicated connectivity resources are involved in the case of P2P connectivity, whereas only \(N+1\)_terminals_ are involved in the case of P2 MP connectivity. P2 MP can be easily implemented on the optical layer by creating light-trees connecting one hub node with multiple leaves [7]. However, most services involve P2P traffic and thus the theoretical benefits of P2 MP in terms of resource savings can be difficult to be collected in practice. For this very reason, we explore the application of P2 MP communications at the optical layer while supporting P2P traffic, and possibly P2 MP traffic, at the packet layer (Figure 1c). Here, the number and dimensioning of the optical transceivers are key  
Figure 1: P2P (**a**) and P2 MP (**b**) connectivity. Supporting P2P and P2 MP traffic on top of optical P2 MP (**c**). performance indicators of the cost in terms of CAPEX and OPEX for the network operator compared to the traditional P2P connectivity with dedicated lightpaths and transceivers. In addition, the efficiency of the solution and the simplicity of the architecture should also be considered in the analysis. Two optical technologies have been selected as they can be used for optical P2 MP and support P2P traffic at the packet layer: DSCM and OCS. In the case of DSCM (Figure 2a), the spectrum is separated into a number of Nyquist digital subcarriers (SC), e.g., 16, 4 GBd SCs, and multiplexed together at the hub node to create a 64 GBd signal. The hub node sends the SCs to all the leaves, where lower-capacity transceivers listen in to their assigned frequencies to receive their dedicated SCs. In the example, we represent four destinations all operating a sub-set of four SCs. To simplify the hardware implementation of DSCM, the SCs assigned to each destination must be contiguous in the spectrum, which means that P2 MP traffic can be supported only for destinations with neighboring SCs. In the case of OCS (Figure 2b), a single optical signal is generated at the hub node and sent to the leaves. The optical constellation is sliced, and every slice is used to support P2P or P2 MP traffic. In the example in Figure 2b, five optical constellation slices are defined (OCS 1 to 5), where four of them support P2P traffic between the source node and one of the destinations, whereas OCS5 supports P2 MP traffic. It is worth noting that the effective bitrate of an OCS can be controlled by selecting the number of CPs that are assigned to it. In the example in Figure 2b, the optical signal is modulated using 64-QAM, so 64 CPs are available. From them, OCS1 is assigned 32 points, whereas OCS 3 and 4 are assigned just 4. Only the CPs assigned to one OCS can be used for communication to the corresponding destination. Figure 2: Application of DSCM (**a**) and OCS (**b**) for optical P2 MP (updated from [16]). It seems clear that implementing optical P2 MP using DSCM or OCS can bring benefits in terms of cost, efficiency, and simplicity of architecture.