The CSCFs within each IMS network act as proxies within the IMS domain, and therefore
follow the procedures outlined in RFC 3261 for both stateless and stateful proxies.
There are some instances when the I-CSCF, for example, may act as a stateful proxy.
TABLE 5.7 Mapping Between the SIP SDP and SS7 USI/HLC (SIP Origination)
SIP SDP SS7 User Service Information
High-Layer
Characteristics
Media Line
(m=)
Bandwidth
Line (b=)
Access Line
(a=)
Info
Transfer
rate
Info Transport
Capability
User Info
Layer 1
Protocol
Indicator
High-Layer
Characteristics
Identification
audio
RTP/AVP 0
Nothing or
64 Kbps
64 Kbps 3.1 KHz audio G.711 ??-law
audio
RTP/AVP
64 Kbps rtpmap PCMU/8000 64 Kbps 3.1 KHz audio G.711 ??-law
audio
RTP/AVP 9
AS:64 Kbps rtpmap: 9
G.722/8000
64 Kbps Unrestricted
digital info
with tones and
announcements
audio
RTP/AVP
AS: 64 Kbps Rtpmap
CLEARMODE/8000
64 Kbps Unrestricted
digital info.
image
udptl t38
Up to
64 Kbps
64 Kbps 3.1 KHz audio Facsimile
Group 2/3
image
tcptl t38
Up to
64Kbps
64 Kbps 3.1 KHz audio Facsimile
Group 2/3
audio
RTP/AVP
384 Kbps rtpmap
CLEARMODE/8000
384 Kbps Unrestricted
digital info
audio
RTP/AVP
1472 Kbps rtpmap
CLEARMODE/8000
1472 Kbps Unrestricted
digital info
Audio
RTP/AVP
1536 Kbps Rtpmap
clearmode/8000
1536
KBPS
Unrestricted
digital info
SS7 User Service Information SIP SDP
Info Transfer
Rate
Info Transport
Capability
User Info
Layer 1
Protocol
Indicator
High-Layer
Characteristics
Identification
Media Line
(m=)
Bandwidth
Line
(b=)
Access Line
(a=)
Speech Speech G.
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