思科路由器上的DTMF中继

双音多频(DTMF)信令用于传统的模拟普通电话业务(PoTS)电话，将数字发送到交换中心，以实现呼叫路由或其他特殊功能。DTMF数字还用于特殊功能，如语音邮件登录、自动座席(AA)和交互式语音应答(IVR)系统。DTMF数字是能够在PSTN上进行tone拨号的频率组合。脉冲拨号在历史上曾用于转盘电话。根据所使用的信令机制，IP语音系统有许多利用DTMF数字的方法。思科路由器上所有传统的TDM (time division multiplexing)接口默认支持通过DTMF数字。思科VoIP (Voice over IP)拨号点默认不支持DTMF数字。根据使用的信令协议，可以在一个VoIP拨号对等体上配置多个dtmf中继能力。VoIP拨号端将通过带内(IB)或带外(OOB)的DTMF数字。在频带中，DTMF- relay将在语音媒体流的RTP报头中使用一个特殊的有效载荷类型标识符(PTI)来区分DTMF垫和实际语音通信中的数字。 An out of band mechanism will pass DTMF digits in the signaling protocol (SCCP, SIP, MGCP, or H.323) instead of using the RTP media stream. There are four DTMF relay mechanisms that are supported on VoIP dial-peers: • H245-alphanumeric • H245-signal • SIP-Notify • Rtp-nte H.245 mechanisms are only available on H.323 dial-peers. H.245 is part of the H.323 protocol suite. is the default protocol used on VoIP dial-peers. SIP Notify is only used in the SIP signaling protocol. RTP NTE stands for Real-Time Protocol (RTP) Named Telephone Events (NTE). NTE are mechanisms used in the IETF (Internet Engineering Task Force) RFC (request for comments) 2833. RFC2833 is the de facto standard used in the industry standard for multi-vendor interoperability in SIP environments. The next blog entry will focus on the method in which DTMF digits are passed from Cisco phones.