Technical Helpweb

Abstract

Along with traditional TDM services, such as ISDN PRI and BRI services, many telephony service providers now offer SIP trunk services, used for connecting a company's telephony infrastructure to the PSTN via the Internet. With Dialogic® Host Media Processing (HMP) Software, a company can develop variety of cost-effective, flexible solutions to utilize most telephony features offered by public SIP providers.

SIP is a relatively new and evolving protocol, and some interoperability issues may arise when connecting Dialogic HMP Software-based exchanges to a public SIP trunk. This note provides answers for common questions you may be asked or may encounter when dealing with an Internet Telephony Service Provider (ITSP).

Detail

The Dialogic HMP Software uses the Dialogic® Global Call API library that supports both First party call control (1PCC) and Third Party call control (3PCC) modes.

This article mainly describes Dialogic HMP Software behavior when configured in1PCC mode; 3PCC provides more flexibility when SDP manipulations are required.

1. Call-setup Questions:

Q: On an outgoing INVITE from Dialogic HMP Software, what is the behavior of Dialogic HMP Software if a 300 (Multiple Choices) or 302 (Moved Temporarily) response is received?

A: Dialogic HMP Software does not automatically re-transmit the call to a new address. Instead, it clears the current session with BYE and sends a GCEV_DISCONNECT event to the application; the application may then retrieve the response code using the gc_ResultValue() API. If the disconnect result value is one of IPEC_SIPReasonStatus3xxx_yyyyy, the application can then retrieve the new Contact URI form the data block associated with the GCEV_DISCONNECT and may create a new call using that URI. The original call must be dropped and released as usual. 

Q: On an outgoing INVITE from Dialogic HMP Software, will Dialogic HMP Software re-try the INVITE if a 422 (Session interval too small) response is received?

A: Dialogic HMP Software will not retry INVITE automatically, it will act as above. The application may repeat the call later on.
 

Q: On an incoming INVITE to Dialogic HMP Software, can Dialogic HMP Software respond with a 300 or 302 code?

A: Yes, a Dialogic HMP Software application can send any valid 3xx or 4xx responses using gc_DropCall when in Offered state. The application must provide a new redirected address (when applicable) in a Contact: header.

Q: Can the Dialogic HMP Software SIP API provide support for UA servers?
A: Yes, a Dialogic HMP Software-based application can act as both UAC and UAS, with some limitations in the latter case. See the Dialogic® Global Call IP Technology Guide at http://www.dialogic.com/manuals/docs/globalcall_for_ip_hmp_v10.pdf for information on REGISTER and UPDATE SIP message support.
 

Q: Can Dialogic HMP Software set / retrieve non-standard or arbitrary SIP headers?

A: In most cases, yes. Dialogic HMP Software provides an application interface for setting arbitrary values for any valid headers. Dialogic HMP Software can also set or read non-standard SIP headers via generic SIP Header interface, see Chapter 4.9 in the Dialogic® Global Call manual: http://www.dialogic.com/manuals/docs/globalcall_for_ip_hmp_v10.pdf.

2. Support of optional SIP features

 

Q: Does Dialogic HMP Software support RFC 3325 (SIP-Asserted identifier)?

A: Dialogic HMP Software currently does not directly support this RFC. The application can implement this feature when acting as UAC via a generic headers mechanism, as far as Digest Authentication is supported by the UA.

Q: Does Dialogic HMP Software support RFC 4028 (SIP session timers)?

A: This feature can be supported at the application level. This means that the application is responsible for maintaining session timers and updating the session as needed. For 1PCC, only re-Invite can be used for updating the session, on 3PCC an UPDATE message can also be sent.

Q: Does Dialogic HMP Software support RFC 3256 (SUBSCIRBE/NOTIFY)?

A: Yes, Dialogic HMP Software supports SUBSCRIBE and subscribed and non-subscribed NOTIFY messages in any direction in both 1PCC and 3PCC modes. An arbitrary MIME body is supported as well.

3. Audio codec Support

Q: Is there a list of supported codecs in the Dialogic HMP Software documentation?

A: Yes, please refer http://www.dialogic.com/manuals/docs/ip_media_api_hmp_v15.pdf, p.222-223 for a list of supported codecs.

4. DTMF handling

Q: What types of DTMF transmission are supported on Dialogic HMP Software?

A: Dialogic HMP Software supports in-band, RFC2833 and out-of-band DTMF. For the first two modes, a voice resource is necessary to detect or generate DTMF digits.

Q: Is payload type negotiation supported per the 2833 spec?

A: Yes.

Q: Is Dialogic HMP Software able to send DTMF after a 183 but prior to an OK?

A: Yes, as long as media is established. A Dialogic HMP Software-based application must set capabilities before sending the digits (upon OFFERED, for example). Commonly, this is known as “early media”, or “SIP fast start”, and Dialogic HMP Software can send any audio data as soon as remote capabilities are known.

5. SDP manipulation

 

Q: Can Dialogic HMP Software issue a RE-INVITE with a 0.0.0.0 SDP?

A: Yes, Dialogic HMP Software supports both 0.0.0.0 (using the older RFC2543) and “a=Inactive” (using the newer RFC 3261) methods to put a session on hold, in both directions.

Q: Can Dialogic HMP Software change IP address / port for RTP on the fly, within previously established dialog? 

A: Yes, but only in 3PCC mode. 

6. Faxing

 

Q: Does Dialogic HMP Software support T.38 real time faxing? V.17? T.37?

A: Yes for T.38.  V.17 is supported with G.711 codecs only.  There is currently no support for T.37.

Q: Can Dialogic HMP Software map PSTN cause values to SIP response codes?

A: Yes, flexibly. Dialogic HMP Software can provide a cause value for most PRI messages, and the Dialogic HMP Software application can send any valid SIP response code to an incoming INVITE.

8. Where can I find / try the Dialogic HMP Software?

You can download and try Dialogic HMP Software from http://www.dialogic.com/products/ip_enabled/download/hmp_download.htm. Please read "Licensing Information" at the location above to obtain and configure a 4-port "all features" trial license.

Product List

Dialogic® Host Media Processing (HMP) Software


Acronyms:

SIP – Session Initiation Protocol
TSP – Telephony Service Provider
ITSP Internet Telephony Service Provider
PSTN – Public Switched Telephone Network
SDP - Session Description Protocol