There are two main difficulties. First, WANs and LANs have different objectives, with WANs being oriented toward voice traffic and LANs being oriented towards data traffic. Second, the global telephone system has given WANs a single (approximately) common standard, while LANs have been developed without a strong central authority, resulting in many of standards and protocols, almost none of which are similar to the ATM standards.
The ATM physical layer deals with these difficulties by splitting into the Transmission Convergence (TC) and Physical-Medium Dependent (PMD) sub-layers. The TC sub-layer is responsible for dealing with framing differences, while the PMD sub-layer deals with the actual physical-medium differences.
The easiest way to answer this question is to think in terms of twenty questions. It would take an infinite number of questions to narrow down an infinite set of messages to a single message; that is, in general, it would be impossible to remove uncertainty.
This page last modified on 14 November 2004.