VoIP and Voice over WLANs
Miscellaneous VoIP protocols

SIP works in conjunction with Resource Reservation Protocol (RSVP), Real-time Transport Protocol (RTP), Real-time Control Protocol (RTCP), Real-time Streaming Protocol (RTSP), Session Announcement Protocol (SAP), and Session Description Protocol (SDP).

H.323 also works in conjunction with RTP and RTCP. Modern voice gateways usually have two parts. The first part is the signaling gateway and the second is the media gateway. The signaling gateway communicates with the media gateway using MGCP. MGCP can interoperate with both SIP and H.323. Figure shows the signaling and transport protocols used for delivering VoIP in a SIP or H.323 network.

Signaling System 7 (SS7)
Another protocol that should be mentioned is Signaling System 7 (SS7). SS7 is the Common Channel Signaling (CCS) system that is used with circuit-switched networks, such as Integrated Services Digital Network (ISDN) and the PSTN. Bellcore developed SS7. It separates signaling information from user data. A specified channel, called the D channel, is used exclusively to carry signaling information for all other channels in the system. This type of signaling is called out-of-band because it does not use the user bandwidth.

For VoIP to be able to route calls to the PSTN, it must be able to interface with SS7, which the PSTN uses.

ENUM
The IETF Telephone Number Resolution working group, known as ENUM, is devising a scheme to map E.164 telephone numbers to IP addresses using the Internet DNS. The objective is to allow any application, including a SIP application, to discover resources associated with a unique phone number. A SIP phone or proxy server would use number domain translation and DNS resolution to discover a DNS resource. These DNS resources would provide a SIP address at which the dialed number could be reached.