The European E1 and North American T1telephony standards govern high-rate voice and data transmissionseither directly to the end customer (access) or between centraloffices (transport). Because a failure in either case isunacceptable, and the repair technicians may not show up promptly,these systems include a local battery backup, redundant powersupplies, and 1+1 or 1+N redundancy for the signal-processingboards. This redundancy can take the form of one standby for everyworking board or, less expensively, one standby for every Nidentical working boards.
Figure 1 : E1/T1 systems feature separate Tx and Rx cableswith the ability to switch each to a redundant, standby electronicsboard.
In a typical E1/T1 system, separate coaxial or twisted-pairtelecommunications cables connect to the receiver and transmitterboards. In case of a failure, the system maintains operation byquickly switching to a standby protection board.
Figure 2 : A protection stage, transformer stage, andswitching stage are common to each E1/T1 line.
Upon looking at the transmitter or receiver path in greaterdetail, the signal line first enters the primary-protection stage,which includes voltage-limiting devices such as transient-voltagesuppressors (TVS) or gas-discharge tubes. The second stage is atransformer, which provides isolation, impedance matching, and anysignal-level adjustment necessary to meet the mask shape specifiedfor E1/T1 transmission.
Figure 3 : The transformer in an E1/T1 line helps to formthe output pulses per E1/T1 specifications.
Beyond the transformer are line drivers and receivers followedby the digital electronics necessary for data communications. Thesecomponents operate at 5 V and below and cannot tolerate anyovervoltage. Schottky diodes are placed between these componentsand the second stage with connections arranged to clamp anyexcessive voltage to the positive or negative supply rail.
With respect to reliability, the weak link in these signal pathsis the transmission boards. The first stage is generally locatedclose to the Tx and Rx connectors to eliminate excess voltageimmediately and prevent its coupling to other parts of the system.The second transformer stage is highly reliable. Therefore, theswitches must be placed between the electronics board and thetransformer. They can be electromechanical relays or solid-stateanalog switches.
Relays have been in use for years, providing contacts thatconnect to the main board in one position and to the protectionboard in the other. Disadvantages include the amount of space onthe board, because some boards have up to 24 protection lines, andpower dissipation. Power required by a single relay is not great,but when multiplied by the N lines in a large telecom system, itbecomes a substantial amount to generate and dissipate.
The second option, analog switches, is relatively new. Devicessuch as the MAX314 and MAX4606 offer the low on-resistance andlow-parasitic capacitance necessary to manage E1/T1 data rateswithout a significant insertion loss. Placed after the transformer,where the signal is already clamped to the bus voltage, they neednot withstand hundreds of volts. They are controlled by a simplelogic-level signal and draw almost zero current. The MTBF of ananalog switch, because it is a semiconductor with no mechanicalparts, is comparable with or better than any electromechanicalrelay.
To work, an analog switch requires a polarization supply voltagelarger than the absolute maximum rating of the signal it handles.For E1/T1 signals, ±5-V supplies are sufficient. Supplycurrent for the MAX314 is in the 1-µA range, so its negativesupply rail is easily generated with a simple charge-pump convertersuch as the MAX871.
The MAX871 comes in a SOT23 and requires an external ceramiccapacitor to derive -5 V from +5 V. These dual rails are essentialfor switch operation, so a redundant supply is necessary in case offailure. For the -5-V rail, two MAX871s decoupled by an outputdiode are sufficient.
The prospect for reducing electromechanical-relay dimensions isnot great, but the future of analog switches is promising. Maximhas already reduced the channel resistance in its MAX4661 and thefootprint in the MAX4624a SPDT device in a SOT23 package.
Maxim analog switches already withstand higher voltages thanthose specified for E1/T1 systems. The MAX314's working voltage ofup to ±20 V also suits it for protection boards in xDSLtransmission systems, which are similar to E1/T1 systems.