ss in the user plane. In these situations, some other gain-simplicity, efficiency, robustness-was at stake. To understand how signalling protocols have evolved from the first Signalling in the Circuit-Switched Network Signalling plane FIGURE 1-5 Two different planes exist in a telephone call. User plane (voice) telephone system back in the nineteenth century up to the present is to understand which gains were achieved and what exactly triggered protocol design at a certain point of time. SIP is both one such protocol and also a departure from the evolutionary path. Local and central battery systems After years of using telegraphs, on March 10, 1876, Alexander Graham Bell patented electrical voice trans- mission using a continuous current, and the telephone was born. Signalling in the early telephone systems was very simple. When the user picked up the receiver, a circuit closed. The terminal then supplied a current to the circuit. The action of closing the circuit meant its seizure. human operator at the switchboard answered upon seizure of the circuit and routing was accomplished by telling the operator the identity of the callee. The operator switched the call manually. This is the Local Battery system. Local Battery systems were problematic. Terminals contained batteries, making maintenance more difficult because it was left in the hands of users. Since battery technology was not as advanced as in modern termi- nals, users found the job complicated and were not eager to take it on. The Central Battery system was devised to make things easier on the users. Central Battery systems supply current to the terminals from the exchange. System automation was the next step in making telephones eas- ier to use. From the installation of the very first exchange in 1878, circuit- switching had been performed manually. In 1889, Almon B. Strowger applied for a patent for an automatic telephone electromechanical switch, and human intervention was no longer needed within the exchanges. Users acquired a dial tone upon seizure o