Info about Telephones
Implementation of T1 technology
T1 cannot operate over analog Plain Old Telephone Service (POTS) telephone lines. Instead, it must be deployed using specially conditioned copper twisted- pair lines, with two pairs of wires (four wires) being used for a single T1 circuit. To support full-duplex communication, two of these four wires are used for transmission (TX interface) and the other two for receiving (RX interface). T1 lines typically terminate at the customer premises with an RJ-48 connector, which looks like an RJ-45 connector but is pinned differently. T1 lines are generally unshielded twisted-pair (UTP) cabling but other media can be used, including coaxial cabling or fiber-optic cabling.
T1 usually cannot run over existing local loop wiring because:
• Bridge taps installed by telcos to trunk telephone traffic in neighborhood wiring causes distortion of T1 signals, so these must be removed to allow the circuit to carry T1 signals.
• Loading coils, which are used to reduce signal distortion for analog phone lines, have the opposite effect of increasing distortion of digital signals, and these also must be removed.
To deploy T1 as a solution for multiplexing voice traffic, a T1 channel bank is generally installed at the customer premises. This channel bank can be connected to a Private Branch Exchange (PBX), which then connects to digital telephone and fax equipment. For WAN data links the scenario is usually somewhat different, using customer premises equipment (CPE) such as
• A T1 CSU/DSU for connecting bridges or routers to T1 circuits
• T1 bridges and routers with integrated T1 CSU/DSUs
• A T1 multiplexer (MUX), a multiplexer for aggregating several T1 circuits for even higher-speed communication
• T1 access routers, which support multiple remote access links over a single T1 line
To test T1 equipment such as channel banks and CSU/DSUs, use a cable simulator, which is a passive device that simulates a standard 22-gauge twisted-pair T1 line that is 1310 feet (400 meters) long (the alternative is to use 1310 feet of actual 22-gauge twisted-pair wiring). Connect two cable simulators between your CPE and your T1 test equipment using the TX and RX interfaces to analyze your device's performance. A "wet" T1 line carries a small DC current of about 140 mA (milliamperes) at several hundred volts for powering the CSU/DSU at the customer premises. "Dry" lines carry no current, so CSU/DSUs must be powered from the customer premises. Do not touch a T1 line-a wet line can give you a serious shock!