hat are not required in a backplane bus. For example, in order to use ethernet in an architec- ture similar to that of Figure 6.47, every subrack controller must operate versus the bus like an ethernet switch. Moreover, in order to operate correctly, an ethernet bus needs an appropriate, although potentially quite simple, physical layer that has to be constructed via the embedded cards electronics. This is for sure a powerful solution, but sometimes too complex for the requirements. Specific bus protocols exist for backplane applications that are able to mange backplane busses fulfilling all their requirements. The two most diffused among these protocols are the protocols of the PCI and of the VME family. Protocols in each family come from the family generator (e.g., original PCI) via tailoring for a specific requirement, as number of managed cards. It is out of the scope of this chapter to detail the working principles of the protocols belonging to these two families; thus, we limit our analysis to a comparison of PCI and VME pros and cons whose key results are reported in Table 6.20, and to a brief comment on the set of protocols belonging to the two families. TABLE 6.20 Comparison between Two Backplane Protocols: PCI and VME Property Address space 32 bit Up to 64 bit Data path Up to 64 bit Up to 64 bit Bus clock speed 33, 66, or MHz Asynchronous no standard clock Data rate Up to 1.06 Gbyte/s Up to 512 Mbyte/s Hot-swap capabilities Operating voltages 3.3 or 5V 3.3 or V Reduces cost Can be high: licensing costs as well as lower volumes Telecommunication Networks Highest number of cards on a segment: VME64. VME technology is the clear leader for the number of cards found in a segment (or continuous trace). VME is able to fit up to 21 cards on a standard 19' backplane. It is important to note that, although PCI can have only four cards (eight for Compact PCI and two for PCI-X) on a segment, the technology can support up to 256 segments. However, this does have drawbacks because segments are arranged in a hierarchi