ated power increases thereby increasing the number of ports. A number of ports on the order of 1000 coincide with an overall switch capacity of 5 THz, on the order of mag- nitude of the capacity of a practical OXC switching 10 Gbit/s channels where the matrix has to be continuously fed with power to maintain the state. In this case, a Clos fabric dissipates about 1.4kW, that is a great power, but still manageable from a standard optical platform. For example, such a switch fabric could fit in a standard ATCA sub-rack, since it can dissipate up to 200W per slot and has a maximum of 12 slots. Considering that one slot will be occupied by the sub-rack controller and one from the card managing the con- nection of the local agent with the system central controller, 10 slots remain for a total dis- sipated power of 2kW that is sufficient to host the Clos switch fabric. A crossbar dissipates 8kW, a power that would require nonstandard cooling methods and would fit with difficulties on a standard telecom platform, as demonstrated by the previous example. Increasing the number of ports provides many more advantages for the Clos fabric. For example, if we consider an OXC switching 40 Gbit/s channels, a switch fabric with a capac- ity of 20 THz is needed, that is, a fabric with 4000 ports. A Clos fabric dissipates 10.5kW SO that it is still possible to host it into five ATCA sub- racks occupying one rack and a half without any nonstandard power management. A crossbar matrix dissipates 124kW, which is simply too much for this kind of platforms. 7.2.2.6 Clos Switch Fabric Modularity The price to pay for a small number of crosspoints with respect to a crossbar fabric is a more difficult scalability of a Clos fabric. As a matter of fact, while similar crossbar elements can be combined easily to obtain a larger crossbar, this is not true for a Clos fabric. Since it is impossible to increase the number of ports of a Clos switch fabric by maintain- ing the Clos architecture, other scaling strategies have to be found. A trivial way