he user that all trunks are busy (ATB) and to try again later. From a technical standpoint, the user has encountered blockage. This would be due to one of two reasons, or may be due to both causes. These are: insufficient switch capacity and not enough trunks to assign during the BH. There is a more in-depth discussion of the busy hour in Section 4.2.1. Networks are sized/dimensioned for a traffic load expected during the busy hour. The sizing is based on probability, usually expressed as a decimal or percentage. That prob- ability percentage or decimal is called the grade of service. The IEEE (Ref. 2) defines grade of service as "the proportion of total calls, usually during the busy hour, that cannot be completed immediately or served within a prescribed time." Grade of service and blocking probability are synonymous. Blocking probability objec- tives are usually stated as B = 0.01% or 1%. This means that during the busy hour, 1 in 100 calls can be expected to meet blockage. 1.3 INTRODUCTORY TOPICS IN TELECOMMUNICATIONS 1.3.5 Simplex, Half-Duplex, and Full Duplex These are operational terms, and they will be used throughout this text. Simplex is one- way operation; there is no reply channel provided. Radio and television broadcasting are simplex. Certain types of data circuits might be based on simplex operation. Half-duplex is a two-way service. It is defined as transmission over a circuit capable of transmitting in either direction, but only in one direction at a time. Full duplex or just duplex defines simultaneous two-way independent transmission on a circuit in both directions. All PSTN-type circuits discussed in this text are considered using full-duplex operation unless otherwise specified. 1.3.6 One-Way and Two-Way Circuits Trunks can be configured for either one-way or two-way 7 operation. A third option is a hybrid where one-way circuits predominate and a number of two-way circuits are provided for overflow situations. Figure 1.8a shows two-way trunk operation. In this case, any trunk can be selected 