 (1,544,000 bits. A bit is a zero or a one.) - Definition compliments of Newton's Telecom Dictionary it The Telecom Handbook CHAPTER 2 data bus. Each PBX manufacturer uses a proprietary method for transmission both from the telephones and inside the PBX. There- fore it is not possible to use the telephone from one manufacturer with the PBX of another. While it has been anticipated that the transition to IP would eliminate this, it appears that a proprietary telephone that works with the manufacturers specific operating system will be still necessary to access all system capabilities. This enables the PBX manufacturers to hold onto the profitable business of selling desktop telephones to work with their PBXs. Connecting the PBX to the Outside World Through a T-1 In most newer PBXs, there is a T-1 circuit board enabling the T-1 to be delivered right into the PBX where the 24 separate channels of the T-1 are separated enabling the identification and transmission of up to 24 separate conversations. If the PBX is older, or if there is not enough space left in the PBX for a T-1 circuit board, an external device called a channel bank, as men- tioned earlier, can serve the same purpose, separating the channels of the T-1. In this case, the PBX is equipped with trunk circuit boards requiring a separate port in the PBX be used for each of the 24 channels of the T-1 since to the PBX, this looks no differ- ent that 24 separate analog outside lines (also known as trunks). Earlier PBXs and How they Worked Up through the 1960's, many PBXs still in place had a cordboard as the attendant console. If you know where to look (old hotels and apartment buildings, for example,) you still see a few today. The cordboard served the basic functions of allowing the extension user to signal the switchboard operator, enabling the operator to set up a circuit and connecting the extension user to an outside line. From this cordboard comes the term tip and ring. This refers to two separate wires on the plug which, when plugged in, meet with two 