Hi All,
Here we would like to share the SIP call flow. Here we have also included PSTNs, so that the reader can co-relate the message of SIP and ISUP.
Here we would like to share the SIP call flow. Here we have also included PSTNs, so that the reader can co-relate the message of SIP and ISUP.
Scenario:
A Number wants to call B Number which is catered by PSTN B.
- PSTN
A to MSS X protocol used in ISUP.
- MSS
X to MSS Y protocol used in SIP.
- MSS
Y to PSTN B protocol is ISUP.
1) After receiving IAM from PSTN A with called party
number as B number MSS X after number analysis detects the B number has
to be routed to MSS Y which is connected by SIP. MSS X send a INVITE message.
Major components of INVITE are "Called Party", "Calling Party", "Bearer information", "Codec".
2) MSS Y receives INVITE and responds with 100 (Trying) message.
This response indicates that the request has been received by the next-hop server and that some unspecified action is being taken on behalf of this call (for example, a database is being consulted). At the same time after number analysis its send IAM to PSTN B.
3) MSS Y send 183 message to MSS X. This message is called as Session progress indicated that session is in progress. In response to 183 MSS A sends PRACK. PRACK (PRovisional ACKnowledgement) is like any another request within a dialog.
** PRACK is response for 1XX mesages
ACK is response for 2XX messages.
4) After analysis of B number PSTN B sends ACM with "Called Party status indicator = no indication" to MSS Y. MSS Y sends 200 message to MSS X in turn MSS X forwards ACM message to PSTN A.
5) When B number starts ringing PSTN B send CPG message with "Called Party status indicator = Subscriber free". Indicates that subscriber is free and ringing.
MSS Y send 180 ringing message to MSS X.
Which is communicated to PSTN A in CPG message and A number can hear a ringtone.
6) B number answers the call in response PSTN B send ANM message to MSS Y. MSS Y send 200(With ANM) message to MSS X. MSS X responds with ACK , also forwards ANM message to PSTN A. At this point of time speech path is through.
7) After conversation A number releases the call. REL is send from PSTN A to MSS X in response MSS X sends BYE message to MSS Y. MSS Y send the REL message to PSTN B.
8) PSTN B release the resources and respond with RLC message to MSS Y. MSS Y relays the 200(with RLC) message to MSS X. RLC is then forwarded to PSTN A. This complete the release of all resources used for call.
Comments and Appreciations are most welcome.
ChEEeeeEEeeeErs!!!
Telecom ChampTeam
telecomchamp@gmail.com
Major components of INVITE are "Called Party", "Calling Party", "Bearer information", "Codec".
2) MSS Y receives INVITE and responds with 100 (Trying) message.
This response indicates that the request has been received by the next-hop server and that some unspecified action is being taken on behalf of this call (for example, a database is being consulted). At the same time after number analysis its send IAM to PSTN B.
3) MSS Y send 183 message to MSS X. This message is called as Session progress indicated that session is in progress. In response to 183 MSS A sends PRACK. PRACK (PRovisional ACKnowledgement) is like any another request within a dialog.
** PRACK is response for 1XX mesages
ACK is response for 2XX messages.
4) After analysis of B number PSTN B sends ACM with "Called Party status indicator = no indication" to MSS Y. MSS Y sends 200 message to MSS X in turn MSS X forwards ACM message to PSTN A.
5) When B number starts ringing PSTN B send CPG message with "Called Party status indicator = Subscriber free". Indicates that subscriber is free and ringing.
MSS Y send 180 ringing message to MSS X.
Which is communicated to PSTN A in CPG message and A number can hear a ringtone.
6) B number answers the call in response PSTN B send ANM message to MSS Y. MSS Y send 200(With ANM) message to MSS X. MSS X responds with ACK , also forwards ANM message to PSTN A. At this point of time speech path is through.
7) After conversation A number releases the call. REL is send from PSTN A to MSS X in response MSS X sends BYE message to MSS Y. MSS Y send the REL message to PSTN B.
8) PSTN B release the resources and respond with RLC message to MSS Y. MSS Y relays the 200(with RLC) message to MSS X. RLC is then forwarded to PSTN A. This complete the release of all resources used for call.
SIP Requests and Descriptions
In typical VoLTE point of view here is a list of all SIP messages and their meaning. In the rightmost column you can find the RFC number.
| SIP Request | Description | Definition |
| INVITE | Indicates that a client is being invited to participate in a call session | RFC 3261 |
| ACK | Confirms that the client has received a final response to an INVITE request | RFC 3261 |
| BYE | Terminates a call; can be sent by either the caller or the called party | RFC 3261 |
| CANCEL | Cancels any pending request | RFC 3261 |
| OPTIONS | Queries the capabilities of servers | RFC 3261 |
| PRACK | Provisional acknowledgement | RFC 3262 |
| SUBSCRIBE | Subscribes to event notification | RFC 3265 |
| NOTIFY | Notifies the subscriber of a new Event | RFC 3265 |
| PUBLISH | Publishes an event to the Server | RFC 3903 |
| INFO | Sends mid-session information that does not modify the session state | RFC 6086 |
| REFER | Asks recipient to issue a SIP request (call transfer) | RFC 3515 |
| MESSAGE | Transports instant messages using SIP | RFC 3428 |
| UPDATE | Modifies the state of a session without changing the state of the dialog. | RFC 3311 |
SIP Codes and Descriptions
SIP is based around request/response transactions, in a similar manner to the Hypertext Transfer Protocol (HTTP). Each transaction consists of a SIP request (which will be one of several request methods), and at least one response.
These codes are grouped according to their first digit as “provisional”, “success”, “redirection”, “client error”, “server error” or “global failure” codes, corresponding to a first digit of 1–6; these are expressed as, for example, “1xx” for provisional responses with a code of 100–199.
| SIP Code | Description |
| 1xx | Provisional Responses |
| 100 | Trying |
| 180 | Ringing |
| 181 | Call is Being Forwarded |
| 181 | Queued |
| 183 | Session Progress |
| 199 | Early Dialog Terminated |
| 2xx | Successful Response |
| 200 | OK |
| 202 | Accepted |
| 204 | No Notification |
| 3xx | Redirection Response |
| 300 | Multiple Choices |
| 301 | Moved Permanently |
| 302 | Moved Temporarily |
| 305 | Use Proxy |
| 380 | Alternative Service |
| 4xx | Client Failure Response |
| 400 | Bad Request |
| 401 | Unauthorized |
| 402 | Payment Required |
| 403 | Forbidden |
| 404 | Not Found |
| 405 | Method Not Allowed |
| 406 | Not Acceptable |
| 407 | Proxy Authentication Required |
| 408 | Request Timeout |
| 410 | Gone |
| 412 | Conditional Request Failed |
| 413 | Request Entity Too Large |
| 414 | Request-URI Too Long |
| 415 | Unsupported Media Type |
| 416 | Unsupported URI Scheme |
| 417 | Unknown Resource-Priority |
| 420 | Bad Extension |
| 421 | Extension Required |
| 422 | Session Interval Too Small |
| 423 | Interval Too Brief |
| 428 | Use Identity Header |
| 429 | Provide Referer Identity |
| 430 | Flow Failed |
| 433 | Anonymity Disallowed |
| 436 | Bad Identity-Info |
| 437 | Unsupported Certificate |
| 438 | Invalid Identity Header |
| 439 | First Hop Lacks Outbound Support |
| 440 | Max-Breadth Exceeded |
| 469 | Bad Info Package |
| 470 | Consent Needed |
| 480 | Temporarily Unavailable |
| 481 | Call/Transaction Does Not Exist |
| 482 | Loop Detected |
| 483 | Too Many Hops |
| 484 | Address Incomplete |
| 485 | Ambiguous |
| 486 | Busy Here |
| 487 | Request Terminated |
| 488 | Not Acceptable Here |
| 489 | Bad Event |
| 491 | Request Pending |
| 493 | Undecipherable |
| 494 | Security Agreement Required |
| 5xx | Server Failure Response |
| 500 | Server Internal Error |
| 501 | Not Implemented |
| 502 | Bad Gateway |
| 503 | Service Unavailable |
| 504 | Server Time-out |
| 505 | Version Not Supported |
| 513 | Message Too Large |
| 580 | Precondition Failure |
| 6xx | Global Failure Response |
| 600 | Busy Everywhere |
| 603 | Decline |
| 604 | Does Not Exist Anywhere |
| 606 | Not Acceptable |
ChEEeeeEEeeeErs!!!
Telecom ChampTeam
telecomchamp@gmail.com
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