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June 02, 2008
SIP Tips - How SIP Works Tutorial
By Tom Cross, Technology Columnist
Since too few people really know how SIP  works, here is the text explanation of each step of the way. After the IP phone is plugged into the LAN network and powers up via the electrical outlet or via POE (Power over Ethernet) cable, the phone needs to get an IP Address from the DHCP (Dynamic Host Configuration Protocol) server.
SIP Signaling
User dials number 303-594-1694. SIP-URI (Uniform Resource Identifier) is retrieved from DNS (Domain Name System): sip:tom@xyz.com;Transport=UDP .
The SIP INVITE along with the SDP (Session Description Protocol) is formatted into an Internet Message Format and encapsulated into Ethernet and sent via the LAN Switch to the router and encapsulated into IP or IP-MPLS and UDP ( News - Alert) and the SIP INVITE is sent via TCP, UDP and other protocols to the destination proxy. The caller receives a 100 (Trying) response indicates that the INVITE has been received and that the proxy is working “on behalf of” to route the INVITE to the destination. The caller receives a 180 (Ringing) and begins ear ringing using an audio ringback tone or by displaying a message on the telephone screen.
When the person called picks up the handset, the SIP phone sends a 200 (OK) response to indicate that the call has been answered. The 200 (OK) message contains a SDP (Session Description Protocol) media description of the type of session that the other party is willing to establish. An ACK message is sent.
SIP Media Session
User begins talking. Voice is transcoded into a CODEC, e.g. G.711, G.729 based on SDP. Voice is packetized. RTP is added and encapsulated in Ethernet and sent via the LAN Switch to the router and encapsulated into IP or IP-MPLS and UDP. If on-net IP network, the digital data is converted to an optical data stream and sent via optical fiber to an internet or internal router. If off-net, the digital data is channelized using a MG (Media Gateway ( News - Alert)) to a TDM (Time Division Multiplexed) channel and sent via an optical data stream to a Class 5 CO (Central Office) switch for connections to the PSTN (Public Switched Telephone Network). User hangs up and a BYE message is sent. A 200 ACK is received confirming disconnect.
Need help in training, technical writing, creating ‘award-winning’ customer case studies, customer presentations, effective web tutorials, SEM, PR and other services? Contact Tom Cross ( News - Alert) at 303-594-1694, cross@gocross.com or http://www.techtionary.com.
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Tom Cross is a technology columnist and a regular blogger for TMCnet. To read more of his articles, please visit his blog.
Multi-Protocol Label Switching (MPLS) | X | | In a Provider Provisioned Virtual Private Network, there may be more than one label. To begin with, IP packets may have a number of labels or "tags" attached to them. MPLS is just one type of label....more |
Central Office (CO) | X | | Central office or wire center where local telephone wires are interconnected to one another and to other central offices....more |
Dynamic Host Configuration Protocol (DHCP) | X | | DHCP provides a translation or conversion from 48-bit ethernet Source and Destination addresses (see above) to 32-bit IP-Internet Protocol Addresses. Why, ethernet was designed for use on LAN-Local A...more |
User Datagram Protocol (UDP) | X | | An alternative to TCP, UDP is a streaming (sent continuously) protocol that neither requires nor expects flow control or acknowledgments (NAK). Faster than TCP but less robust, it is commonly used fo...more |
Session Initiation Protocol (SIP) | X | | SIP is the real-time communication protocol for VoIP. SIP is a signaling protocol for Internet conferencing, telephony, presence, events notification (emergency calling) and instant messaging.
SIP...more |
(source: http://internetcommunications.tmcnet.com/topics/sip/articles/29748-sip-tips-how-sip-works-tutorial.htm)
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