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the internet encyclopedia volume phần 8

Điều này xảy ra bởi vì các thiết bị điện tử của một mạng điện thoại thông thường gây ra các đầu xa của hầu hết các kết nối cuộc gọi vang vọng lại một phiên bản làm suy yếu của tín hiệu truyền đến cuối nguồn gốc. Mặc dù tiếng vang này là luôn luôn hiện diện, | 652 Voice over Internet Protocol IP This occurs because the electronics of a typical telephone network cause the far end of almost every call connection to echo back a weakened version of the transmitted signal to the originating end. Although this echo is always present it is not noticeable until the round trip delay becomes long enough. All of these deficiencies of a transmission channel have the effect of reducing the channel capacity or throughput of a transmission link. Channel capacity refers to the theoretical upper limit on how much information in terms of bits per second can be transmitted through a channel Wozencraft Jacobs 1965 . Circuit-Switched Connections of a Call in a Conventional Telephony Network VOIP networks must interoperate with conventional circuit-switched networks in particular the ubiquitous public switched telephone network or PSTN. A VOIP network that cannot interoperate with and therefore exchange calls with the PSTN would be of little value to most telephone users. A network s value increases as it can be used to reach more users. Like other networks the PSTN uses a hub and spoke architecture of transmission links and switches Bell Laboratories 1977 1983 . Each user s telephone is typically connected by an individual circuit usually referred to as local loop to a central office switching hub. In a small town there may be only one central office whereas in a large major metropolitan area there could be several dozen central offices. In each central office hub there is usually at least one switch. The switches used in traditional telephony are called circuit switches. A circuit switch will route a call over a dedicated path from one transmission link to another for at least the duration of the call. See Figure 4. As will be described more fully below this contrasts with the packet switching used in the Internet which does not maintain a dedicated path connecting network links but instead passes information along shared paths on a .