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Tín hiệu được sử dụng cho tín hiệu từ tổng đài đến router, truyền tín hiệu giữa các bộ định tuyến, và truyền tín hiệu từ các bộ định tuyến để PBX. Nếu không có các thủ tục này, các cuộc gọi sẽ không được tốt. | Cisco Voice over IP Chapter 9 1009 Signaling is used for signaling from the PBX to the router signaling between routers and signaling from the router to the PBX. Without these procedures calls would not be possible. Signaling Between Routers and PBXs When signaling from PBX to router lifting the handset produces an off-hook connection appears as a trunk line to the PBX which signals the router to seize the PBX then forwards the dialed digits to the router in the same manner the digits would be forwarded to a telephone company switch or another PBX. The signaling interface may be any of the common signaling methods used to seize a trunk line such as FXS FXO E M or T-1 E-1 signaling. As you can see in Figure the PBX seizes a trunk line to the router and forwards the dialed digits. Within the router the dial plan maps the dialed digits to an IP address and initiates a call establishment request to the remote peer router that is indicated by an IP address Figure .This control channel is used to set up the Real-time Transport Protocol RTP audio streams and the RSVP protocol may be used to request a guaranteed QoS. When the remote router receives the call request it signals a line seizure to the PBX. After the PBX acknowledges this seizure the router forwards the dialed digits to the PBX and signals a call acknowledgment to the originating router. Figure shows this line seizure. Figure PBX-to-Router Signaling Figure Router-to-Router Signaling 1010 Chapter 9 Cisco Voice over IP Figure Router-to-PBX Signaling VoIP Protocols With VoIP signaling end stations perform signaling and session successfully emulate voice services across an IP network enhancements to the signaling stacks are required. For example an agent is added to the router for standards-based support of the audio and signaling protocol is used for call establishment and teardown between .