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Call setup in H323 relies in Q931 protocol.
Q931 is a subscriber protocol developed for ISDN.
As it’s a subscriber protocol, A-side category is defined in the switch and is not transported in the call setup messages, so, there is no field within the Q931 stack to write down A-side category.
In the case of a Class 4 switch replacement, A-side category is not transported, so, a transit call that enters with a defined category (payphone for example) will be transported and will leave the network with a default category as it was not transported.
Only with the aid of a proprietary protocol, the incoming gateway can inform to the outgoing gateway which category must signal, but this also influencites the routing desitions if the routing is done taking into consideration A-side category.Hmmm…so help me understand what you are looking for. The H.225 spec does have a non-standard field to transport this type of information. Is your issue that its not standardized yet? There are many folks working on defining “signaling” transparency across various protocols.
In a Class 4 switch with all the trunks with CCS#7, the #7 signaling message IAM has a mandatory field called A-side category.
If we are replacing a Class 4 TDM switch with a VoIP solution with H323 signaling, the interworking CCS#7 – H323 looses the A-side category as this field is not defined in Q931 stack.
The outgoing call in the other side of the class 4 replacement must write in the outgoing IAM A-side category field, but as it was not transported, the outgoing gateway has to write down a default value, so, transparency is not possible, at least with pure H323 signaling.
Suppose that our Class 4 VoIP replacement receives a Q763 IAM message with A-side category “payphone”, this value is not mapped, so, the outgoing gateway has to build an outgoing IAM, but, as A-side category has not been informed, the outgoing Q763 IAM leaves the VoIP Class 4 with A-side category “Ordinary Subscriber”. If this call should be routed or billed according to A-side category, the solution is not usable.I understand the technical issues, but I want to understand why non-standard fields cannot be used. It does get the job done, just not with other vendors.
So at the end of the day, if I told you I could pass elements such as NOA,BCI,GNO etc… but only to and from elements from a single vendor would this allay your fears.
Please dont take this the wrong way. I am just trying to understand the point you are trying to make.Eduardo,
You are right. The category of calling party is cannot be propogated over H.323. SIP is th e solution.
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