Private Telephone Networks : Telephone Stations, Local Wiring

Telephone Stations
Telephone stations are telephone instruments that are connected to a telephone network for the purpose of telephony. When these telephone stations are used as part of the private network, they are often identified by the type of system they are connected with. For example, telephone stations that are connected to key systems are called key telephones.

Telephone stations can vary from simple POTS telephones (sometimes called 2500 telephones) to complex Internet telephones (IP Telephones). Telephone stations usually receive their power from the telephone line (loop current) but may receive it from an external source (such as the PBX switching system).

Figure 1 shows a typical telephone station that is used in a PBX system. This diagram shows the difference between standard analog telephone stations and more advanced PBX stations. This diagram shows that analog telephones receive their power directly from the telephone line and digital PBX telephones require a control section that gets its power from the PBX system. Analog telephones also use in-band signaling to sense commands (e.g., ring signals) and to send commands (e.g., send dialed digits). Digital telephones use out-of-band signaling on separate communication lines to transfer their control information (e.g., calling number identification).


Figure 1: Analog and Digital Telephone Stations

Local Wiring
The local wiring for private telephone systems includes the lines between the telephone stations and switching assemblies or interconnection equipment. Local wiring systems may include interconnection points or wiring rooms. Incoming trunks enter the building in a main distribution frame (MDF). Trunks between multiple floors or other buildings are called intermediate distribution frames (IDFs). These IDF areas are often referred to as “wiring closets” because in the past telecommunications wiring was seldom considered when designing a building and the space available for terminating cables was usually in utility closets.

Private telephone system wiring is usually connected through unshielded twisted pair (UTP) wire. There are exceptions where shielded cable, coaxial cable, or fiber is used. Shielded cable is used in areas the may cause or be sensitive to electromagnetic interference (EMI) such as hospital radiology areas or near high voltage equipment in manufacturing plants. Coaxial cable and fiber is used for high-speed data inter-connection trunks or for LAN backbone systems that are part of the private telephone system.

Figure 2 shows the typical wiring systems that are used for private telephone systems. Key systems required many pairs (12 or 25 pairs typical) of lines for each telephone. These telephones were wired to a punch block (splice point) near the telephones. The punch block was connected to the key service unit (KSU). Analog PBX systems usually required 4 wire connections from extension telephones to the PBX switching unit. Two wires were used for audio and two (or more) wires for status or special feature lines. Digital PBX systems connect to each extension using 4 wires. Two wires are used for audio (analog or digital) and two are used for external power. This diagram shows that some digital PBX systems may use multiplexed digital lines to allow multiple phones to share a single line (e.g., a line between building). When this occurs, it is called a remote peripheral equipment (RPE). The RPE separates (demultiplexes) the digital line from the PBX to multiple digital stations.


Figure 2: PBX Local Wiring