of wavelengths indicates an absolute throughput increasing almost linearly with the number of channels. 7.3.3 Delay in Bus-Based Switch Fabrics Analyzing bus structures in the LAN context, after the assessment of the throughput of each bus protocol, the distribution of the delay experienced by the packets while travers- ing the LAN is evaluated. This is also an important issue in switch fabrics, but in the last case the overall fabric passing through delay cannot be identified with the bus transmission delay. As a matter of fact, the overall packet delay while traversing a bus-based switch fabric is the sum of several addenda: The packet waiting time in the input queue The electronic elaboration time related to packet transmission, which is composed by the time needed to assess the transmission possibility (e.g., to sense the carrier presence on the channel) and the time needed to condition the packet for trans- mission (e.g., tagging it with the fabric internal header) The bus transmission propagation Telecommunication Networks The average packet waiting time due to collisions in the shared medium The packet waiting time in the output queue (if it exists) before transmission on the line Thus, a key role in determining the average packet delay and its standard deviation is played by the input and output queues policy and by the queues' length. For simplicity (and also because it is a very common case) let us assume that there is no output queue in the switch fabric and the packet directly exits toward the line card that, if needed, stores it and launches it out of the switching device. The switch fabric can then be modeled as a set of parallel queues whose servant is con- stituted by the fabric bus. In this model, the time elapsing for the packet preparation and physical transmission along the bus plays the role of the service time for each queue. This model is effective in representing the switch fabric behavior, but it is difficult to manage mathematically essentially because the service processes of each input