In this blog i explain complete set of messages send between ISP and Gateway for a call made from my mobile to my desk phone extension 399. I did a debug ISDN Q931 on my gateway inorder to capture the messages. Even though a deep level analysis are mostly done by TAC, its always good to understand what exactly happens in the backend .

Debug ISDN Q.931

Feb 17 12:43:38.835: ISDN Se0/1/0:15 Q931: RX <- SETUP pd = 8 callref = 0x25C9
Sending Complete
Bearer Capability i = 0x8090A3
Standard = CCITT
Transfer Capability = Speech
Transfer Mode = Circuit
Transfer Rate = 64 kbit/s
Channel ID i = 0xA1838F
Preferred, Channel 15
Calling Party Number i = 0x2183, ‘94301628’
Plan:ISDN, Type:National
Called Party Number i = 0xC1, ‘399’
Plan:ISDN, Type:Subscriber(local)

Feb 17 12:43:38.851: ISDN Se0/1/0:15 Q931: TX -> CALL_PROC pd = 8 callref = 0xA5C9
Channel ID i = 0xA98387
Exclusive, Channel 7

Feb 17 12:43:38.852: ISDN Se0/1/0:15 Q931: TX -> ALERTING pd = 8 callref = 0xA5C9
Progress Ind i = 0x8088 – In-band info or appropriate now available

Feb 17 12:43:44.403: ISDN Se0/1/0:15 Q931: TX -> CONNECT pd = 8 callref = 0xA5C9

Feb 17 12:43:44.487: ISDN Se0/1/0:15 Q931: RX <- CONNECT_ACK pd = 8 callref = 0x25C9

Feb 17 12:43:50.133: ISDN Se0/1/0:15 Q931: TX -> DISCONNECT pd = 8 callref = 0xA5C9
Cause i = 0x8090 – Normal call clearing

Feb 17 12:43:50.217: ISDN Se0/1/0:15 Q931: RX <- RELEASE pd = 8 callref = 0x25C9

Feb 17 12:43:50.224: ISDN Se0/1/0:15 Q931: TX -> RELEASE_COMP pd = 8 callref = 0xA5C9

I use translator X to read the messages in a readable format.

SETUP

When i tried to call 399 from mobile i received a setup message from ISP and this is the first message used to initiate a call.

CALL_PROC

From the above picture you can see that my router send CALL_PROC message to notify the ISP that call being processed and no more setup message is required for me.

ALERTING

My router send an ALERTING message to ISP and its used to notify the caller that i have initiated an indication on 399 about the incoming call.

CONNECT

When i answered the call from my desk phone my router send a CONNECT message out to the ISP. It inform the caller that the call has been answered

DISCONNECT

Notifies the ISP that the called party has ended the call.

RELEASE

It indicates that the call has been terminated

RELEASE_COMP

It indicates that the sender has terminated the call and that the Call Reference Value(CRV) is reusable