The telex network is a switched network of teleprinters similar to a telephone network, for the purposes of sending text-based messages. The term refers to the network, not the teleprinters; point-to-point teleprinter systems had been in use long before telex exchanges were formed starting in the 1930s. Teleprinters evolved from telegraph systems, and like the telegraph they used the presence or absence of a pre-defined level of current to represent the mark or space symbols. This is as opposed to the analog telephone system, which used differing voltages to encode frequency information. For this reason, telex exchanges were entirely separate from the telephone system, with their own signalling standards, exchanges and system of "telex numbers" (the counterpart of a telephone number). When telephone and telex exchange equipment was co-located, which was not uncommon, the different signalling systems would sometimes cause interference.