 L.A. Tie lines usually connect separate offices of the same organization. A single tie line can handle only one conversation at a time. If there will be many calls going back and forth between the two PBXs, you need a sufficient num- ber of tie lines to handle the volume (you can put the tie lines on a high capacity circuit such as a T-1). If you have a network connecting more than two PBXs, you may call over several con- secutive tie lines, taking a route through one or more PBXs before reaching the final destination. Many PBXs are intelligent enough to recognize the location of the three or four-digit extension num- ber you have dialed. They send the call automatically to the correct PBX and telephone at that site. In this case, there is no need for you to dial an access code. The PBXs are still sending access codes to each other behind the scenes to route the call, but this is transparent to you. This capability to dial among PBXs on a net- work by simply dialing the extension number is sometimes called a uniform numbering plan. A tie line is sometimes called a tie trunk. It is a dedicated circuit between two points. Off-Premise Extensions An off-premise extension is another type of point-to-point line with a PBX on one end and a single line telephone or a key tele- phone system on the other. The idea is that if a call comes into the PBX, that call can then be extended to a distant location, across the street or across the country. To the caller, it appears that the person is at the location he called. The person at the off-premise extension can pick up the telephone and dial any extension on 214 The Telecom Handbook Figure 10.4 Off-Premise Extension EXT. 5555 on the PBX Incoming Call on DID Trunk Off-Premise Extension Circuit (OPX) Caller Dials Directs Call to the 212-883-5555 Off-Premise Extension Local Telephone Co. Telephone Rings Central Office OUTSIDE LINES the distant PBX, just as if he was on site. He can also, if set up to do so, draw dial tone and place an outside call through the PBX. If the off-pr