Home > Articles

This chapter is from the book

SIP Call Flow

Basic SIP session setup involves a SIP UA client sending a request to the SIP URL of the called endpoint (UAS), inviting it to a session. If the UAC knows the IP address of the UAS, it can send the request. Otherwise, the UAC sends the request to a proxy or redirect server to locate the user. That server might forward the request to other servers until the user is located. After the SIP address is resolved to an IP address, the request is sent to the UAS. If the user takes the call, capabilities are negotiated and the call commences. If the user does not take the call, it can be forwarded to voice mail or another number. The following sections outline various scenarios in more detail.

Call Flow Between Two SIP Gateways

Cisco routers, including CME routers, can act as SIP gateways for calls that originate from non-SIP phones. The gateways function as SIP UAs and set up a SIP session between them for each call. Figure 4-1 shows two routers handling analog phones, using SIP between them. In this example, SIP GW-A originates the calls and acts as a UAC, and SIP GW-B acts as a UAS. The signaling from the PBX to the gateway is just normal analog call signaling. Only the two gateways exchange SIP messages.

Figure 4-1

Figure 4-1 Call Flow Between Two SIP Gateways

In Figure 4-1, the analog phone on the left initiates a call to the analog phone on the right.

After the first phone initiates the call, the call flow proceeds as follows:

  1. The PBX sends a call setup signal to GW-A, which then sends a SIP INVITE message to GW-B. This INVITE contains SDP information for capabilities negotiation. GW-A also sends a Call Proceeding message to the PBX.
  2. GW-B exchanges call setup message with its PBX and sends SIP responses 100 (Trying) and 180 (Ringing) to GW-A.
  3. GW-A translates these messages into analog signaling messages for its PBX.
  4. When the user on the right picks up the call, his PBX sends a Connect message to GW-B, which then forwards a SIP 200 (OK) response to GW-A. This OK response contains SDP information with the capabilities that both devices support.
  5. GW-A delivers a Connect message to its PBX. When the PBX acknowledges that with a Connect ACK, it sends a SIP ACK message to GW-B.
  6. GW-B sends a Connect acknowledgement to its PBX, and the call is active. At this point, normal voice steams exist between the two phones and the gateways, and RTP voice streams exist between the two gateways.
  7. The user on the left hangs up the phone. His PBX sends a Call Disconnect message to GW-A. GW-A then sends a SIP BYE message to GW-B and a Release message to the PBX. The PBX responds with a Release Complete message.
  8. GW-B sends a Call Disconnect message to its PBX, which responds with a Release message.
  9. GW-B forwards a SIP 200 (OK) response to GW-A and a Release Complete message to its PBX. The call is now completely terminated.

Call Flow Using a Proxy Server

SIP UAs register with a proxy server or a registrar. Proxy servers then act as an intermediary for SIP calls. Cisco routers that are acting as SIP gateways can use the services of a SIP proxy server, either contacting the server or receiving requests from it. They can additionally register E.164 numbers with a proxy server or a registrar.

Proxy servers can either leave the signaling path when the call is connected or can enable "Record-route" to stay in the signaling path. If Record-route is disabled, the proxy server does not know of any changes to the call or when the call is disconnected. Figure 4-2 shows call flow when Record-route is disabled.

Figure 4-2

Figure 4-2 SIP Call Flow Using a Proxy Server

In Figure 4-2, a SIP endpoint places a call using a proxy server. The figure shows several types of endpoints:

  • A PC and a PDA running a SIP application
  • A SIP phone
  • A cell phone that uses SIP

In Figure 4-2, one of these endpoints places a call to an analog phone behind SIP gateway GW-B. The call flow proceeds as follows:

  1. The UAC sends an INVITE to its proxy server. In this INVITE, the Request-URI field contains the address of the called phone number as part of the SIP address. SDP information is included with this INVITE.
  2. The proxy server creates a new INVITE, copying the information from the old INVITE, but replacing the Request-URI with the address of GW-B—the UAS.
  3. When GW-B receives the INVITE, it initiates a call setup with the PBX. It sends a SIP response 100 (Trying) to the proxy server which, in this example, sends a 100 response to the SIP UAC. The proxy server is not required to send this response.
  4. The PBX sets up an analog call with the end user and sends call progress messages to GW-B. When GW-B receives the Alerting message, it sends a SIP 180 (Ringing) message to the proxy server. The proxy server sends the same message to the UAC.
  5. When the end user picks up the phone, the PBX sends a Connect message to GW-B. GW-B then sends a SIP 200 (OK) response to the proxy server, which sends it to the UAC. SDP information for the remote end is included in this OK response. The proxy server is not configured to be stateful—that is, Record Route is disabled. Therefore, the proxy server leaves the signaling path, and all further SIP signaling is directly between the UAC and GW-B.
  6. The SIP UAC acknowledges the OK response, and a two-way RTP stream is established between the UAC and GW-B, the UAS. A two-way voice stream is established between GW-B and the PBX.
  7. When the UAC hangs up, it exchanges SIP BYE and OK signals with GW-B. GW-B terminates the call with the PBX.

Call Flow Using Multiple Servers

SIP UAs and SIP proxy servers can contact a redirect server to determine where to send an INVITE. They typically do this when the called number is outside the local domain. The redirect server returns the most detailed information it has—either endpoint location(s) or the location of the next-hop server. Then it relies on the proxy server or UAC to route its INVITE appropriately.

Figure 4-3 shows the call flow in a more complex network with registrar, redirect, and proxy servers. (Recall that these are functional units and can all reside in the same device.) The figure shows the messages that are necessary to route the initial INVITE method to the UAS. After GW-B, the UAS, receives the INVITE, call flow is similar to the previous examples.

Figure 4-3

Figure 4-3 SIP Call Flow with Multiple Servers

In Figure 4-3, one of the SIP endpoints in Network A calls an analog phone behind gateway GW-B in Network B. The following steps take place:

  1. The gateway, GW-B, registers the E.164 phone numbers of its analog phones with the registrar server.
  2. The registrar server replies with a 200 (OK) response.
  3. The UAC sends an INVITE method to its proxy server, Proxy-A.
  4. The proxy server recognizes that the destination number is outside its domain. It sends the INVITE to the redirect server.
  5. The redirect server replies with a 300-series message listing the SIP address of the next-hop proxy server, Proxy-B.
  6. Proxy-A sends an INVITE message to Proxy-B.
  7. Proxy-B requests the location of the called number from its registrar server.
  8. The registrar server responds with the SIP address of GW-B.
  9. Proxy-B sends an INVITE to GW-B.

Following these steps, GW-B sets up the call with the PBX. It sends responses to Proxy-B, which forwards them through Proxy-A to the calling endpoint. If Record-route is enabled, all further signaling goes through the proxies. If not, call signaling proceeds as shown in Figure 4-2.

Call Flow Using Cisco CallManager 5.x

CallManager 5.x supports SIP phones and is an integral part of a SIP network. It can play different roles, such as registrar server and B2BUA.

Figure 4-4 illustrates a call flow scenario with CallManager acting as a B2BUA.

Figure 4-4

Figure 4-4 Call Flow with CallManager 5.x

In Figure 4-4, a SIP phone is registered to a CallManager. The SIP phone places a call to an analog phone off a PBX behind the router/gateway GW-B. A SIP trunk exists between CallManager and the gateway. CallManager acts as a B2BUA—it terminates each leg of the call during the signaling phases, yet it allows the RTP stream to go directly between the two endpoints. This is accomplished by the way SDP information is sent.

  1. A SIP phone that is registered to CallManager calls the analog phone. It sends an INVITE containing standard SDP information to CallManager. CallManager responds with a 100 Trying message. In this step, CallManager is acting as a UAS.
  2. CallManager sends an INVITE over its SIP trunk to the remote SIP gateway, GW-B. This INVITE has a different Call-ID number than the one from the phone. In addition, this INVITE does not contain SDP fields. CallManager acts as a UAC in this step.
  3. GW-B answers with a 100 Trying message and initiates a call to the PBX. (That signaling is not shown.) GW-B sends its SDP parameters in a 183 Session Progress message to CallManager. Included in this are the session parameters that the gateway supports.
  4. CallManager sends a 183 Session Progress message to the SIP phone. This message contains an SDP portion with the capabilities that both endpoints support. For instance, suppose that the original SDP message of the phone indicated that it supported G.711 and G.729 codecs, but the gateway SDP message said that it supported only G.729. In that case, the 183 message from CallManager to the phone would list only G.729. It would also list the IP address of GW-B as the originator address in SDP field 'c.'
  5. When the analog phone picks up, GW-B sends a 200 OK message containing its SDP information. CallManager acknowledges it with an ACK that contains the SDP information that both endpoints support. The IP address of the SIP phone is also included as the originator address in the SDP field 'c.'
  6. CallManager sends a 200 OK message with SDP information to the phone. The phone acknowledges that message. Now that each endpoint has the IP address of the other, the two can establish an RTP stream between them for the duration of the call.
  7. In Figure 4-4, the analog phone hangs up, so GW-B sends a SIP BYE method to CallManager.
  8. CallManager replies with a 200 OK response and then sends a BYE to the SIP phone. The phone responds with a 200 OK message.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020