Digital phone = Voice over Internet ProtocolVoice over Internet Protocol (VoIP) is a general term for a family of transmission technologies for delivery of voice communications over IP networks such as the Internet or other packet-switched Other terms frequently encountered and synonymous with VoIP are IP telephony, Internet telephony, voice over broadband (VoBB), broadband telephony, and broadband Internet telephony refers to communications services—voice, facsimile, and/or voice-messaging applications—that are transported via the Internet, rather than the public switched telephone network (PSTN) The basic steps involved in originating an Internet telephone call are conversion of the analog voice signal to digital format and compression/translation of the signal into Internet protocol (IP) packets for transmission over the Internet; the process is reversed at the receiving VoIP systems employ session control protocols to control the set-up and tear-down of calls as well as audio codecs which encode speech allowing transmission over an IP network as digital audio via an audio Codec use is varied between different implementations of VoIP (and often a range of codecs are used); some implementations rely on narrowband and compressed speech, while others support high fidelity stereo Voice over IP has been implemented in various ways using both proprietary and open protocols and Examples of available VoIP implementations include:VoIP ImplementationsSIP/RTP IMS H323 Skype Further examples and comparisons are available from the following Wikipedia article: Comparison of VoIP softwareConsumer marketA major development starting in 2004 has been the introduction of mass-market VoIP services over broadband Internet access services, in which subscribers make and receive calls as they would over the PSTN Full phone service VoIP phone companies provide inbound and outbound calling with Direct Inbound D Many offer unlimited calling to the US, and some to Canada or selected countries in Europe or Asia as well, for a flat monthly fee as well as free calling between subscribers using the same These services have a wide variety of features which can be more or less similar to traditional POTS数字电话 = IP电话(简称VoIP,源自英语Voice over Internet Protocol;又名宽频电话或网络电话)是一种透过互联网或其他使用IP技术的网路,来实现新型的电话通讯。过去IP电话主要应用在大型公司的内联网内,技术人员可以复用同一个网路提供数据及语音服务,除了简化管理,更可提高生产力。随着互联网日渐普及,以及跨境通讯数量大幅飈升,IP电话亦被应用在长途电话业务上。由於世界各主要大城市的通信公司竞争日剧,以及各国电信相关法令松绑,IP电话也开始应用於固网通信,其低通话成本、低建设成本、易扩充性及日渐优良化的通话质量等主要特点,被目前国际电信企业看成是传统电信业务的有力竞争者。IP电话的技术IP电话通过把语音信号经过数字化处理、压缩编码打包、透过网路传输、然后解压、把数字信号还原成声音,让通话对方听到。话音从源端到达目的端的基本过程是:声电转换:通过压电陶瓷等类似装置将声波变换为电信号 量化采样:将模拟电信号按照某种采样方法(比如脉冲编码调制,即PCM)转换成数字信号 封包:将一定时长的数字化之后的语音信号组合为一帧,随后,按照国际电联(ITU-T)的标准,这些话音帧被封装到一个RTP(即实时传输协议,Realtime Transport Protocol)报文中,并被进一步封装到UDP报文和IP报文中。 传输:IP报文在IP网络由源端传递到目的端 去抖动:去除因封包在网路中传输速度不均匀所造成的抖动音 拆包 电声转换 一个完整的、可以大规模商用运营的IP电话系统包括如下一些技术(暂不完全):寻址 话音编解码 回声消除和回声抑制 传输 IP报文时延控制功能 去抖动 IP报文的去抖动(de-jitter)功能 话音编码方案目前世界多个标准组织和工业实体提出了很多话音编码方案。 其中包括国际电信联盟的G711(速率64kbps),G1(速率3kbps或者3kbps),G729A(速率8kbps)编码方案。 微软、Intel等业界巨头也有自己的编码方案。VoIP 通讯协定目前常用的协定如 H323、SIP、MEGACO 和 MGCP。H323是常见的VoIP标准,由ITU-T於1996年提出,原本是用於区域网路(LAN)上的视讯会议,后来被应用於VoIP网路电话上。H323定义了一个综合性的规范,使网路上的终端设备遵循这些规范,得以顺利进行沟通,包括语音压缩格式(G711、G729、G1)、影像压缩格式(H261、H263)、呼叫信令(H225)、控制信令(H245)、注册与认证等(RAS:Registeration Admission Status)。H323 架构由4个元件所组成,包括终端设备(Terminal)、闸道器(Gateway)、闸道管理员(Gatekeeper)、多点控制单元(MCU:Multipoint Control Unit),可进行单点对单点或单点对多点的通讯。对於VoIP的应用而言,H323的子协定多且复杂性高,在许多技术上的问题受限,不容易针对新的应用作扩展。因此,IETF(Internet Engineering Task Force)分别在1999年8月提出MGCP(Media Gateway Control Protocol)协定与1999年3月的SIP(Session Initiation Protocol)新架构,试图简化H323的复杂性,且在语音传递功能提供较高的延展性。