The two basic types of business systems are in common use.  The Key Telephone System (KTS) serves small businesses where a few people need access to any one of several communication paths to a switching system.  By using push buttons with indicator lights on the telephone instrument, the user can select an idle line to make a call.  The user may also identify an incoming call on any of the lines and connect to that line to answer the call by operating the appropriate button.  In FIG.  1, the ABC warehouse (8020) uses a key telephone system (8001).  It is connected to a switching machine (not shown) in an end central office (8010) through a digital T1 line (8021).  An Access Bank. TM. ( the trademark for the present invention) channel bank (8002) converts the T1 line to 12 or 24 loop-start dialog telephone lines (8022) that connect the Key Telephone System (8001).  Since the warehouse telephone stations are operated by human beings, the communications path represents a human-to-machine interface and is called a KTS Line. 
The second category of business system is known as a PBX or Private Branch Exchange.  This is a switching machine, similar to those connecting subscribers or trunks in central offices.  PBX's located on the subscriber premises are considered "branches" or subsidiaries of the central office switching system.  They are "private" because they are dedicated to the business subscriber for the use of in-house personnel, instead of being shared with many business and residential users, like the switches in telco central offices.  As indicated in FIG.  1, the ABC sales office (8023) and factory (8024) use PBX's (8011, 8008) for their business telephone systems.  The communication path between the PBX's (8011, 8008) and their CO's (8004, 8010) are machine-to-machine interfaces, thus they are PBX trunks (8025, 8026) and require trunk circuits. 
PBX trunks may be connected as many individual analog lines (8026) from a central office (8010).  Alternatively, PBX trunks may be provided on a digital T1 line (8027) from the Central office (8004).  An Access Bank. TM.  channel bank (8005) is used to convert the T1 line to 12 or 24 ground-start telephone lines (8025) within, or near, the ABC Sales Office (8023).  PBX stations are connected to the PBX switching machine, just like residential subscribers connect to the end CO switch, and are known as station lines.  PBX station users can dial one another and be connected by the PBX switch.  If a PBX user wishes to call a telephone located outside the company, the PBX switch selects an idle trunk over which the call is dialed, or in some cases a PBX attendant places the call using a special control box or turret (not shown), just like telephone operators used to do from their switch boards.  Similarly, incoming calls over the CO trunks are received by the PBX switching system and extend to the desired station automatically or by an attendant. 
Note that, unlike key telephone system users, the PBX station cannot select a particular trunk to answer or to initiate an outgoing call.  The PBX switching system does that.  However, as can be seen in the ABC Factory (8024) in addition to individual station lines, the PBX also serves a Key Telephone System (KTS) (8028).  This provides the users within a department the convenience and features of the KTS (8028) in answering and making calls through the PBX switch.  This arrangement is referred to as a key system installed "behind" a PBX.  The communication path between PBX and KTS is a human-to-machine interface and is called a key system line. 
Three types of special arrangements shown in FIG.  1 are popular in business communications.  PBX users might want trunks to a central office which are not from the CO serving them.  Such a transmission facility is known as a Foreign Exchange (FX) Trunk (8029).  Companies might find it convenient to directly interconnect their PBX's at different locations without switching over shared public trunks.  These arrangement are called tie trunks, and provide full period dedicated private circuits for interconnection.  The ABC Sales Office (8023) and ABC Factory (8024) are connected by a digital T1 line (8036) to transmit the trunk circuits.  Access Banks. TM.  at the sales office (8034) and Factory (8035) serve to convert the T1 line (8036) to 12 or 24 analog tie trunk circuit paths (8033). 
There might be a need for PBX stations to be located at distant places, far removed from the premises where the business system serves the majority of the user personnel.  Such arrangements can be engineered and treated just like any other station serving the PBX.  This is known as an Off-Premises Extension (OPX).  A T1 line (8007) is used to carry off-premise extension channels from the Digital PBX (8008) to the Warehouse (8030).  An Access Bank. TM. ( 8009) in the Warehouse converts the T1 line (8007) to 24 individual telephone connections (8031). 
For providing residential telephone service, a T1 line (8012) is used to carry 24 telephone channels to a common point of distribution (8014).  Only 4 wires (2 pairs) are used to connect this point to the Central office (8013) with a digital T1 line (8012), rather than 48 wires (24 pairs) required when using analog transmission.  An Access Bank. TM. ( 8015) is used to convert the T1 line to 24 residential telephone lines (8037) at the point of distribution (8014).