tailieunhanh - Fundamentals of Telecommunications P2

These are operational terms, and they will be used throughout this text. Simplex is oneway 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. | INTRODUCTORY TOPICS IN TELECOMMUNICATIONS 9 of trunks required and making switches smaller. Of course network users will do a lot of complaining about poor service. Let s just suppose the PSTN does just that cuts back on the number of circuits. Now during the BH period a user may dial a number and receive either a voice announcement or a rapid-cadence tone telling the 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 . 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 probability 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 objectives are usually stated as B or 1 . This means that during the busy hour 1 in 100 calls can be expected to meet blockage. Simplex Half-Duplex- and Full Duplex These are operational terms and they will be used throughout this text. Simplex is oneway 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 .