Another UUNET Explanation

Frame-Relay, ATM, IP, X.25, DECnet, LAT, are all methods
  of encapsulation of information into discrete units and
  their subsequent delivery. Some are part of the PSTN,
  and some are not.

  Each protocol has its set of restrictions/features. None are
  "bad" per se (not even X.25, Joel :wink: Make sure not to confuse
  the protocol with the medium it uses. For instance, frame-relay
  DS3s will outperform ATM DS1s for data and voice routing :)*

  Many people who "hate" frame-relay really despise the predominant
  practice of overselling non Committed-Information-Rate circuits
  into an overloaded central hub location.

  Frame-Relay "routing" is L2, and IP routing is L3, so I won't
  really go into comparing them any more than, say, discussing
  the technical advantages of TCP vs. IP. They are two different things
  with similar form yet different functions.


* There are some purists who would claim that the additional overhead
  of switching frame-relay packets would increase latency over that inherent
  in an ATM switched network. They would say that my example above (which is
  just an example, so reading too much into it is an exercise in being a pedant)
  is flawed because the frame-DS3 is more latent than the ATM-DS1. Nonetheless
  I claim the inherent latency of both networks to be less than that noticeable
  by human ears. Nya. Nee. Nya. Nee. PooPoo.