Excerpts from cmu.comp.sys.mac: 5-Apr-93 Re: SE Serial Port Speed???? by
Samuel John Kass@andrew. 
actually a 14.4 kbps modem using standard compression (v3.2 v4.2.bis?)
cna reach 57,600 bps, however I have not seen any server modems that
have hardware compression.  I have been told the annex modems here break
up at ~36k, but I have never seen faster than 14.4kbps.