SIP-I vs SIP-T
The Global voice network is migrating towards IP based communication system. However the existing PSTN network will remain for another couple of years. So the interconnect function between Voice over IP and PSTN plays major role in the voice market these days.
Like translation between two languages (English<->Dutch), Mapping of two systems comes up as basic interconnect function.
SIP-I and SIP-T are to two similar approaches for interworking between ISUP networks and SIP networks in other words generally PSTN and VoIP networks. In specific, they facilitate conveying ISUP parameters through a SIP network so that calls that originate and terminate on the ISUP network can transit via a SIP network with no loss of information.
SIP-I and SIP-T both define the mapping of messages, parameters, and error codes between SIP and ISUP Networks. Both of them are fully interoperable with compliant SIP network components on the SIP network.
The way SIP-I and SIP-T allow transparent transit of ISUP parameters through a SIP network is by attaching a literal copy of the original ISUP message to the SIP message at the ingress PSTN gateway; this ISUP message appears as another body on the SIP message.
Differences between SIP-I and SIP-T are:
SIP-I was developed by the ITU in 2004 (defined in ITU-T Q.1912.5) where as SIP-T was developed by the IETF (Internet Engineering Task Force) who developed SIP.
SIP-I defines a mapping from SIP to BICC in additional to ISUP, while SIP-T addresses only the ISUP
SIP-T is inherently designed for interoperation with native SIP terminals, while SIP-I is restricted for use between PSTN gateways only
SIP-I is more accurate and explicitly defines the parameters between ISUP and SIP and on top it defines the supplementary services for telecommunication interconnection in detail, which is not support by SIP-T.
SIP-I is widely accepted by manufacturers and carriers especially soft switch and Session Border controller (SBC) vendors.
RFC3261 (Session Initiation Protocol), ITU-T Q.1912.5(Definition of SIP-I) and RFC3372 (SIP-T)