ent bus protocol: In this case, applicable only to slotted busses, when the CSMA-CD protocol senses no carrier on the bus, an available packet is transmitted in the available time slot only with probability p, while with probability (1 - p) the access point waits a time slot to perform transmission. Telecommunication Networks When the inner bus structure is realized via optical transmission, WDM multiplexing can be exploited to increase the inner bus capacity: This can be done in several ways, both with respect to the used WDM comb and with respect to the adopted protocol. The CSMA-CD protocol is almost the same that is used in single-channel solutions, but for the fact that an additional mechanism is needed to select on what wavelength the access point has to transmit. Basically two possible solutions exist: either the access point selects a random wavelength and then accesses it as in a single-chan- nel system, or the protocol first selects the channel while sensing all the available wavelength and then it uses the standard access method on the selected channel. Multichannel switch fabrics based on a WDM inner bus would be very attrac- tive due to the possibility of pushing the bus capacity to very high values. Unfortunately, the practical implementation is thwarted by the cost and complex- ity of tunable WDM lasers (see Chapter 5) that greatly impact on the practical value of this solution. Naturally, this situation could be completely changed by the availability of low-cost, high-speed tunable lasers, like those that are under development for tunable XFP transceivers. In order to give an idea of the performance of switch fabric adopting different kinds of internal bus protocols, we will use the performance evaluation model reported in [29] assuming the following hypotheses: The switch fabric uses a slotted inner bus. The packet arrival process at each input port is a Poisson process. All the arrivals processes at different input ports are independent and have the same parameters. A packet arriving to a switch