chẳng hạn như các gói tin gửi đi và bị mất, và sự chậm trễ và jitter phát sinh (Về mặt lý thuyết, nếu chất lượng của cuộc gọi là không thể chấp nhận được, CA là thông báo, CA hướng dẫn cả hai bên để chấm dứt các cuộc gọi) Nếu điện thoại hoặc bị treo lên, cổng nó được kết nối (R1 hoặc R2) thông báo cho CA của sự kiện này. | Digitizing and Packetizing Voice 19 While the call is in progress the end points R1 and R2 in this example collect and analyze the call statistics such as packets sent and lost and delay and jitter incurred Theoretically if the quality of the call is unacceptable the CA is notified and the CA instructs both parties to terminate the call. If either phone hangs up the gateway it is connected to R1 or R2 notifies the CA of this event. The CA instructs both parties that call termination procedures must be performed and call resources must be released. In the centralized call control model the end points are not responsible for call control functions therefore they are simpler devices to build configure and maintain. On the other hand the CA is a critical component within the centralized model and to avoid a single point of failure it requires deployment of fault-tolerance technologies. It is easier to manage a centralized model than to manage the distributed model because only the CAs need to be configured and maintained. Implementing new services features and policies is also easier in the centralized model. Digitizing and Packetizing Voice Upon completion of this section you will be able to identify the steps involved in converting an analog voice signal to a digital voice signal explain the Nyquist theorem the reason for taking 8000 voice samples per second and explain the method for quantization of voice samples. Furthermore you will be familiar with standard voice compression algorithms their bandwidth requirements and the quality of the results they yield. Knowing the purpose of DSP in voice gateways is the last objective of this section. Basic Voice Encoding Converting Analog to Digital Converting analog voice signal to digital format and transmitting it over digital facilities such as T1 E1 had been created and put into use before Bell a North American telco invented VoIP technology in 1950s. If you use digital PBX phones in your office you must realize that .
