nt-to-point lines connecting two PBXs SO that the users of both systems may communicate without dialing an outside call. Some systems have separate circuit boards for these tie-lines. Most systems use a circuit board which handles two, four or eight tie-lines. Digital Telephone Circuit Boards Most telephone systems now use digital telephones. This means that the analog voice signal converts to a digital form right in the telephone and travels back to the PBX cabinet in a digital form (combinations of ones and zeroes). For every 8, 16 or 32 of these telephones in the system it is necessary to have a digital telephone circuit board. This may also be called a digital station board. Each port on the board corresponds to a specific digital desktop telephone in the system. Every telephone has an associ- ated numeric location in the PBX cabinet indicating the port, the circuit card and the shelf. Analog Telephone Circuit Boards Most telephone systems are installed with at least one analog circuit board with ports for either 8 or 16 analog telephones. Many single-line telephones are analog. The voice signal is sent from the telephone to the PBX circuit board in an analog form. The Telecom Handbook 12 27 THE PBX The analog ports provide analog extensions required for us- ing fax machines and computer modems through the PBX. There are differences of opinion as to whether or not faxes and modems are best run through the PBX or through separate outside lines. It is believed that some PBXs may slow down the data transmissions. Most PBX manufacturers will not guarantee throughput of data beyond a certain speed. If you expect to have computers and fax machines go through the PBX, it is important to find out what speeds can be expected. You may decide to bypass the PBX. Some systems also use analog ports for interfacing with a Voice Mail or Automated Attendant system although smoother integration is accomplished with a digital interface. DTMF Circuit Boards DTMF (standing for Dual-Tone Multi-Frequency, also known as Touch-tone) 