General Information on Interconnecting SIP Providers with ISDN Systems - Gateways
When interconnecting an ISDN System with a SIP Service Provider several aspects of the communication flow need to be considered. During a call different phases of signalling occur and the call has multiple attributes.
The signalling consists of messages that are exchanged between the participants. The attributes are Information Elements in ISDN and Headers in SIP.
ISDN Messages
The main messages in ISDN are:
| Message | Mandatory | Meaning | Attributes |
|---|---|---|---|
| SETUP | mandatory | Create a new Call | Destination and Source Address, Audio Channel, Codec, ... |
| SETUP_ACKNOWLEDGE | optional | Call has been acknowledged, more Digits in Destination missing | |
| PROCEEDING | optional | All Information to establish the call has been provided | Audio availability, Audio channel |
| ALERTING | optional | call has been delivered, remote end is ringing | Audio availability, Audio channel |
| CONNECT | mandatory | remote end picked up, call is established | Connected Party Address, Audio Channel |
| CONNECT_ACKNOWLEDGE | optional | Connect has been received | |
| DISCONNECT | mandatory | Call has been disconnect by one party | Reason |
| RELEASE | mandatory | ressources have been released for this call | Reason |
| RELEASE_COMPLETE | mandatory | call is completely released now, no further message for this call is accepter | Reason |
There are more messages that are mainly used for supplementary services.
SIP Messages
the main messages in SIP are:
| Message | Mandatory | Meaning | Attributes |
|---|---|---|---|
| REGISTER | optional | registering IP at a SIP Server | Destination and Source Address, Authentification |
| INVITE | mandatory | Session/Call initiation | Destination and Source Address, Authentification, SDP with Media information |
| ACK | optional | Messages was received | SDP with Media information |
| 180 Alerting | optional | Session is in Progress | SDP with Media information |
183 Progress | optional | Session is in Progress | SDP with Media information |
| 200 OK | mandatory | Session is established | SDP with Media information |
| BYE | mandatory | Session is terminated | Reason |
| 4XX / 5XX Status | mandatory | Session couldn't be established | Reason |
| CANCEL | mandatory | Session has been cancelled by initiator | Reason |
There are more messages that are used for special cases.
beroNet Gateway Mapping of Messages and Attributes
The beroNet Gateway translates between SIP and ISDN Messages and tries to map these in an optimal way. There are a few configuration options that can define the behavior of this mapping. Especially the callprogress table can be used to change the mapping of Progress messages. The causecode table can be used to map certain SIP Error Responses to ISDN disconnect reasons and vice versa.
The basic mapping looks like:
| SIP Message | ISDN Message | behaviour changing settings | main attribute settings |
|---|---|---|---|
| INVITE | SETUP | - | callerid settings, codec settings |
| 180 Alerting | ALERTING | Callprogress Table | SDP |
| 183 Progress | PROCEEDING | Callprogress Table | SDP |
| 200 OK | CONNECT | - | |
| BYE | DISCONNECT | - | |
| 4XX / 5XX | DISCONNECT | wait_for_cancel | cause codes |
| CANCEL | DISCONNECT | - | cause codes |
Troubleshooting Tools
The Fulltrace can be used to trouble shoot SIP to ISDN conversions. It contains the files:
- tcpdump.pcap
- ISDNdebug-1.pcap - ISDNdebug-8.pcap
These files are wireshark readable and show the real messages that where transmitted over the IP or ISDN Interfaces.
If you need scheduled remote assistance, you can request our on-demand support services: https://www.beronet.com/support