6 Mapping SIP Responses to SS7 Cause Codes (continued)
126 Chapter 5
The parameters and their respective values are discussed further in the text that
follows. The intent is not to provide an exhaustive accounting here, but enough information
as to understand the role these parameters play within each domain and how
they interact between the legacy network and the IMS.
Called Party Number This is a mandatory parameter from the SS7 network identifying
the digits dialed by the calling party and that is used to derive the REQUEST-URI
used in the SIP INVITE. The calling party telephone number is converted into a TEL
URI for the SIP messaging.
Calling Party Number The calling party number is mapped to the P-ASSERTEDIDENTITY
header in SIP. From this header the MGCF can determine the calling party
from the SIP domain. The calling party number parameter in the SS7 message is part
of the IAM.
There are two parameters in the Calling Party Number field of SS7 that must also
be set if being sent back into the SS7 network. The outgoing SS7 message is populated
with the Numbering Plan Indicator set to ISDN (telephony) Numbering Plan. The
Nature of Address Indicator is also examined to determine if the number is a national
or an international number. The MGCF must be able to read the country code (CC) portion
of the SIP TEL URI found in the P-ASSERTED-IDENTITY to determine this.
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