[SIPForum-discussion] SIP to PSTN

[Abhisek Acharya]

Question--:In SIP to PSTN Call Flow In-between GW are there, Someone tell
me after sending ACM message "from SSP to GW" then GW what  it will send
180/183, if it is sending 183-Res to SIP then Two-way RTP will flowing in
between SIP to GW  and one-way media will flowing SSP to GW"

ANSWER--

1.The above one is a very very tricky situation.When ACM message comes from
ISUP side ideally it means the phone is ringing on the ISUP side and the
calling party should hear the ring-back tone.

2.Again the ring back can be  local ring back or remote back.if 18x
response contains sdp body then this should be remote ring back and if it
does not contain sdp body then this will be local ring back.

3.Also at times ACM message comes bit early than expected just to satisfy
T9 timer which is otherwise called as early ACM.In this case the called
party phone may not be ringing.It may be in roaming (in case of called
subscriber is out of its HLR/MSC).In that case 183 response goes to SIP
side and a subsequesnt CPG from ISUP side will comes from ISUP side to
denote ringing.


Ideally in case of 183 response two way RTP should not flow from the SIP
end and GW is this is only for ringback.One way audio should flow.Are you
sure two way audio is flowing from SIP entity to GW?I believe while
ringback is there calling party is only hearing the ringback so  RTP should
flow only one way.

First Question:-

"In between two user agent a C5 switch is present and one is sip user
another is isup user , So who is responsible to convert SIP message to ISUP
message in C5 switch. Actually what C5 are doing there & What are the
functionality/Element contain C5 switch in particular situation".

ANSWER

C stands for CLASS which means Custom Local Area Signaling System and this
is an ancient term since the old days of telecom when Graham bell started
the legacy and Catagorisation started by AT&T folks where C5 stands for end
switches and C4 switches are interconnecting switches where they connect
multiple C5 switches.

But in modern day telephony where various protocols are involved and
interworking is a MUST then there is entity which is called as TANDEM
switch/NGN switch which takes care of the SIGNALING conversion from one
protocol to other protocol.Like ISUP to SIP.SIGNALING conversion is doen by
the NGN switch and the media is also converted from CIRCUIT SWITCHED MEDIA
to PACKETISED MEDIA and this is done by with the help of  MEDIAGATEWAYS.

Ideally C5 elements maintain the end point phone informations like its
number,its balance,etc.Ideally modern day MSCs are clasic examples of
CLASS5 switches and old day telephone exchanges are also the
exmaples.Please let me know if something is not clear here.

SSP---------NGN---------SIP


Abhisek Acharya
GS-LAB,PUNE


On Tue, Jan 15, 2013 at 5:07 PM, Suryakanta Behera <skbehera321 at gmail.com>wrote:

> Hi,
>
> I have a question. Can Anyone help
>
> "In between two user agent a C5 switch is present and one is sip user
> another is isup user , So who is responsible to convert SIP message to ISUP
> message in C5 switch. Actually what C5 are doing there & What are the
> functionality/Element contain C5 switch in particular situation".
>
> Another Question
> "In SIP to PSTN Call Flow In-between GW are there, Someone tell me after
> sending ACM message "from SSP to GW" then GW what  it will send 180/183, if
> it is sending 183-Res to SIP then Two-way RTP will flowing in between SIP
> to GW  and one-way media will flowing SSP to GW"
>
> Is it Correct ! If Correct, how two-way RTP will flowing in between SIP to
> GW  and one-way media will flowing SSP to GW.
>
>
>
> Please, describe briefly about this....
>
> Thanks & Regards
> Surya
>
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