rocesses where n is the number of voice channels and p is the amount of offered traffic. Blocking prob- ability is also known as the grade of service (GoS). GoS has traditionally been a parameter of great importance in network design and has also been used in evaluating the competitive advantage of different long-distance telephone operators. GoS levels have also been regulated by governments in many countries as a minimum measure of performance for monopoly oper- ators. In particular, in countries where the incumbent telephone company was government- owned, quality of service provision could be a political issue, and therefore the regulator used GoS to ensure high quality service provision. The Erlang-B formula is complex to calculate manually. It can easily be calculated however using a simple computer program and is found on numerous websites. The Westbay Engineers site [5] has a simple interface for calculation of GoS based on offered traffic amount and avail- able voice circuits. Alternatively it can calculate the number of necessary lines for a desired level of GoS. Example 3.1 One thousand users each generate on average one call every 2000 S, or on average, one call is generated every 2 S. Each call goes on for an average of 100 S. What is the amount of offered traffic? What is the GoS for 50 lines and 64 lines? Answer M=100 S A=2 S p = 100/2=50 E For 50 circuits: GoS = 11% For 64 circuits: GoS = 1% Voice Communications Case Study 3.1: Transatlantic Cable 1 (TAT-1) The first transatlantic submarine telegraph cable was laid in 1858, only 14 years after Morse's first official message was sent. The line was very slow (it took 16.5 h to send a 96-word message from Queen Victoria to President Buchanan [6]). With the invention of telephony, transatlantic telephony was considered, but clearly the required capacity was not available. The main problem was signal dissipation through the coaxial cable, which meant that a transmitted voice signal would be received well below noise level. With no fixed line telephony, 