trunks and DID trunks on the same board. This creates efficien- cies in the use of the ports and therefore can lower the cost in terms of the total number of circuit boards to be purchased. It can also save space (sometimes called real estate) within the PBX cabinet. Note: The trunks that use the above types of circuit boards are sometimes collectively referred to as copper trunks or ana- log trunks to distinguish them from the outside lines that are delivered on the T-1s, which is common in PBXs today. Copper trunks are still used for back up. The cost of T-1s has dropped dramatically in recent years, SO T-1 has become the more preva- lent method of delivering the outside lines to the PBX. T-1 Circuit Boards A T-1 is a high-capacity circuit using two pairs of wires, en- abling the transmission of up to 24 voice conversations at one time. In order for it to do this, a piece of hardware called a mul- tiplexer is required at each end. The T-1 circuit board is a multiplexer that fits right into the PBX shelf. In some older PBXs the T-1 circuit card must be placed on a shelf specifically designed to handle it. If you are going to use a T-1, you may need to buy a separate shelf for it. Each PBX has limits, SO find out how many separate T-1s your PBX can hold. You may hear the term channel bank which is a multiplexer external to the PBX. The channel bank can be used if your PBX does not have T-1 circuit board capability. PRI/ISDN Circuit Boards Most PBXs can accept ISDN (Integrated Services Digital Net- work) lines. The circuit board may be for a PRI (Primary Rate 26 di The Telecom Handbook CHAPTER 2 Interface) ISDN line which has 23 B channels plus one D chan- nel. A few PBXs can also accept BRI (Basic Rate Interface) ISDN lines with 2 B channels plus one D channel although the BRI lines are seldom found in a PBX. The ISDN lines can be used for voice conversations and data transmissions. T-1/PRI Circuit Boards On newer PBXs, the same circuit board is used for both T-1 and PRI. Tie-Line Circuit Boards Tie-lines are poi