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CCS#7 – H323 : A-side Cat issue

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  • #20811
    Eduardo Moreira

    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.

    Eduardo Moreira

    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.



    You are right. The category of calling party is cannot be propogated over H.323. SIP is th e solution.

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