o the client layer, the adaptation function would be to translate it in the client layer protocol. In order to match the bandwidth request coming from the client layer with a physical layer channel, the adaptation function can operate in two ways, depending on the type of channels that can be transmitted through the ODN. A first possibility assumes that the capacity of the ODN channels are prescribed, SO that there exists a list of possible channel bit rates, and all the ODN channels have to travel at a bit rate comprised in the list. It is possible to increase flexibility to include a large number of channel bit rates, but it is sure that sooner or later, a client requesting a connection at a non included bit rate will come out. The adaptation function in this case has the role to optimize the frame efficiency by wisely using the available bit rates. Alternatively, it is not impossible to conceive the continuous creation of channels with a bit rate exactly equal to the requested capacity plus the redundancy introduced by a frame header. In this case, however, the adaptation function is simplified, but all the management of the tributary channels over all the ODN is quite complicated. For example, dynamic TDM multiplexers and demultiplexers have to be considered and grooming operations are much more complex since they face a potentially unlimited set of tributary bit rates. On the ground of the network architecture that we have introduced, a possible sub- layering of the ODN physical layer could be represented as in Figure 10.5, even if there is not yet sufficient agreement on this structure to start even a preliminary study opening the way to its standardization. The upper layer of the ODN is the channel layer, where data coming from the client lay- ers are organized in channels that are the elementary entity of the ODN. As we have seen, there is a plurality of possible channel speeds and several available survivability strategies that can be applied at the channel layer. Connection Client layer (e.g., MPLS) Con