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Chapter 6. Ethernet > Autonegotiation

6.6. Autonegotiation

We have looked at four versions—10BaseT, 100BaseT4, 100BaseTX, and 100BaseT2—of twisted pair–based Ethernet transmission systems all using the same RJ-45 connector. Although each system has its own unique physical layer and different physical layers do not communicate with each other, it will be very beneficial for a pair of Ethernet transceivers to at least recognize each other and to find out the optimal common physical layer with which they can communicate when they are connected. Because the 10BaseT transmission system requires the simplest transceiver circuits, an autonegotiation procedure has been defined in the Ethernet standards based on its link integrity test pulse sequence, and no packet or upper layer protocol overhead is required. The autonegotiation function allows Ethernet transceivers at both ends of a link segment to advertise abilities, to acknowledge receipt and understanding of common modes of operation that both devices share, and to reject the use of operational modes that are not shared. Where more than one common mode exists between the two devices, a Priority Resolution function is provided to allow the devices to resolve to a single mode of operation. The autonegotiation function also provides a Parallel Detection function to allow an Ethernet transceiver to be recognized properly in case it may not have the autonegotiation capability.

The Priority Resolution function includes Table 6.7, which represents the relative priorities of different twisted pair–based Ethernet transmission systems, where the system in the higher row poses the higher priority. The rationale for this hierarchy is straightforward. 10BaseT is the lowest common denominator and therefore has the lowest priority. Full-duplex solutions are always higher in priority than their half-duplex counterparts. 100BaseT2 is ahead of 100BaseTX and 100BaseT4 because 100BaseT2 runs across a broader spectrum of copper cabling and requires only two pairs. 100BaseT4 is ahead of 100BaseTX because 100BASET4 runs across a broader spectrum of copper cabling. It should be noticed that the 1000BaseT transmission system, although not examined here because it will probably not be used in homes in the near future, has a higher priority than 100-Mbps technologies.


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