[SIPForum-discussion] SIP-I and SIP-T

[vijay kant gupta]

Hi ,

I just Paste an article.

Regards
Vijay


SIP-I and SIP-T refer to two very similar approaches for interworking
ISUP networks with SIP networks. In particular, they provide the means
for conveying ISUP-specific parameters through a SIP network so that
calls that originate and terminate on the ISUP network can transit a
SIP network with no loss of information.

SIP-T was developed by the IETF — the same body that developed the SIP
protocol itself — around the same time the most recent version of SIP
was being developed (mid-2002). It is defined by RFC 3372, RFC 3398,
RFC 3578, and RFC 3204.

SIP-I was developed by the ITU in 2004, and made use of most of the
constructs defined in the IETF SIP-T effort. It is defined by ITU-T
Q.1912.5.

SIP-I and SIP-T both define the mapping of messages, parameters, and
error codes between SIP and ISUP. Both of them are fully interoperable
with compliant SIP network components on the SIP network.

The key differences between SIP-I and SIP-T are:

    SIP-I defines a mapping from SIP to BICC (in additional to ISUP),
while SIP-T addresses only the ISUP case, and
    SIP-T is inherently designed for interoperation with native SIP
terminals, while SIP-I is restricted for use between PSTN gateways
only.

SIP-I and SIP-T also define somewhat different mappings of information
between the protocols, mostly in terms of converting from SIP error
codes to ISUP cause codes.

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 (typically, a peer
to an SDP body).

The SIP network ignores the extra ISUP body, processing the SIP
message as it normally would. After the SIP service network performs
any necessary modifications to the SIP message, it arrives at the PSTN
egress gateway. This egress gateway uses the attached ISUP message as
the basis for the ISUP message it will send; however, it first makes
modifications necessary to match changes made to the SIP message
during its traversal of the SIP network.



As mentioned before, with SIP-T, the messages may also terminate on
the native SIP terminals in the network, which will ignore the extra
ISUP body. Additionally, messages may originate from these SIP phones
and terminate on the PSTN gateways, which will then generate a new
ISUP message for the PSTN.

Putting this together in a call flow, a typical successful call setup
from a PSTN terminal to another PSTN terminal through a SIP network
can look something like this: