Disadvantages of 56K

• Introduction

• Only one A/D conversion allowed

• 40 to 50K more likely

• 56K for downloads only

• 53K limit in the U.S.

• Requires digital lines at the ISP

• Won't work with some PBXes

Won't work with some PBXes

Many offices use PBX (private branch exchange) telephone systems, a sort of electronic receptionist and switchboard. If your office telephone has its own extension, or if you must dial a number (usually 9 in the U.S.) to get an outside line, you are probably on a PBX. PBXes are also common in hotels.

It's likely that most PBX systems will prevent 56K modems from working by introducing extra analog-digital conversions. Workarounds include getting a separate phone line for the modem, or plugging the modem into the same phone jack as the fax machine, which is probably plugged into a direct outside line, because fax machines also have problems with PBX systems.