1. August 12th, 2000 Adam Roach adam.roach@ericsson.com
SIP in 3GPP
August 12th, 2000
Adam Roach
Some slides are not self-explanatory. The comments found in this are should help to demonstrate the point being made.
For maximum benefit, I suggest you print these slides (with comments), and then step through the slide show (in full screen mode) so that the animations can be of assistance.

2. Disclaimer
Some terms used in this presentation are trademarks or registered trademarks of their respective owners. The use of these terms is not an endorsement of the related product or corporation; the terms are included only to give the audience concrete example with which they are (hopefully) already familiar.

3. 3GPP Network Model IP Multimedia Domain (multimedia by SIP,
EIR
MGCF
R-SGW *)
MRF
Multimedia
IP Networks
PSTN/
Legacy/External
Applications &
Services *)
Alternative
Legacy
Acces
mobile
Network
Network s
SCP
IP Multimedia Domain
(multimedia by SIP,
IETF RFC2543)
CSCF Mh Ms Mw
CAP Mm
HSS *) Cx
CSCF
CSCF Gr Mr Mg
PS Domain Gi Gi Gf Gc Gi
T-SGW *)
SGSN
GGSN Mc Iu Gi Gn Iu TE MT
UTRAN
CS Domain
(multimedia by ITU H.324M)
MGW
MGW R Uu Iu Nb Mc Mc
This picture describes the 3GPP reference model. Don’t concentrate too much on the details. What I’m illustrating here is that the 3G network is essentially broken up into three different domains: Packet Switched (which basically just provides a bitpipe into the network), Circuit Switched (which performs multimedia through H.324M -- I won’t be discussing this domain), and the IP Multimedia Domain.
The IP Multimedia Domain is what we’re interested in, since this is where 3G has decided to use SIP. Of particular interest is the CSCF (Call/Session Control Function), which can be considered the primary SIP node in the network. Nc
MSC server
GMSC server
T-SGW *)
CAP
CAP D C
Applications
& Services *)
HSS *)
R-SGW *) Mh

4. Different Kinds of CSCFs
Proxy CSCF: Provides emergency service breakout, triggers for locally-provided services, and number normalizing (per local dialing plan)
P-SCSF
Visited B
S-CSCF
Home A 1 2 7 10 11 12 13 15 17
P-CSCF
Visited A 18 A B
GGSN
SGSN
Radio Access Network
Home B 8
I-CSCF
HSS 9 14 6 3 4 5 16
Currently, 3GPP has defined three different functional behaviors which the CSCF will exhibit.
The Proxy CSCF (P-CSCF) provides a first point of contact for the handset. All signaling to and from the handset goes through the P-CSCF. In terms of SIP, it behaves as an outbound proxy.
The main purpose for this node is to provide emergency service breakout and to do some basic message manipulation to enable the visited domain operator to provide locally sensitive services (e.g. traffic reports, directory services, etc). It also does simple number internationalization (which allows the support of local dialin plans).
It will probably also play a role in quality of service reservations.

5. Different Kinds of CSCFs
P-SCSF
Visited B
S-CSCF
Home A 1 2 7 10 11 12 13 15 17
P-CSCF
Visited A 18 A B
GGSN
SGSN
Radio Access Network
Home B 8
I-CSCF
HSS 9 14 6 3 4 5 16
Interrogating CSCF: Queries the HSS to find the correct S-CSCF. First point of contact for incoming call signalling.
The Interrogating CSCF (I-CSCF) is mostly a load distribution node. Since DNS allows us simple statistical distribution among identical nodes, distributing load among the I-CSCFs is quite simple. But if all we relied on was statistical distribution, we wouldn’t be able to allocate subscriptions on appropriate serving nodes according to their capabilities, nor would be be assured of the ability to keep call state information between transactions.
So, the I-CSCF, in conjunction with the HSS, allocates subscription information onto appropriate Serving CSCFs. The HSS keeps track of this information so that all transactions and all calls for the same user go through the same service node.
The HSS stores user profile information; it’s somewhat similar to the HLR found in today’s cellular networks.

6. Different Kinds of CSCFs
P-SCSF
Visited B
S-CSCF
Home A 1 2 7 10 11 12 13 15 17
P-CSCF
Visited A 18 A B
GGSN
SGSN
Radio Access Network
Home B 8
I-CSCF
HSS 9 14 6 3 4 5 16
Serving CSCF: Provides subscriber services.
The Serving CSCF (S-CSCF), quite simply, provides users services.
Of course, SIP allows the terminal to provide many services itself. The S-CSCF will be useful in providing, for example: call forwarding when the terminal is not available, call barring, centralized speed dial lists, VPN services, etc.

7. SIP from terminal to network SIP between network call nodes
3GPP SIP Decisions
SIP from terminal to network
SIP between network call nodes
So, what decisions have actually been taken regarding SIP in the 3G standardization body?
Truthfully, not a whole lot. It has been decided to use SIP from the terminal to the network, and SIP between the network service nodes. In other words, all IP Multimedia call signaling will be performed via SIP.
Why is this all they’ve decided?
So far, 3G has been concentrating on network architecture and generalized information flows, as opposed to the gritty protocol details. The work in standardizing exactly how SIP is to be used is just now getting underway.

8. General Principles S-CSCFs may have different capabilities.
Firewalling must be possible.
Terminal and node behaviours should be consistent, regardless of whether the user is roaming.
Follow IETF standards as much as possible.
Service nodes are access independent.
Serving CSCFs may reside in either home or roamed-to network.
S-CSCFs may have different capabilities. In other words, an operator may have some somewhat expensive S-CSCFs that do business services (such as VPNs), and some relatively cheaper nodes that do residential services. The network needs to provide the capability to select an appropriate node based on the subscriber’s profile.
Firewalling between network domains must be possible. One major network operator concern is that allowing the structure of their network to be visible from the outside will open them up to certain types of security problems.
The behavior of the service nodes, and especially the terminals, should be the same regardless of whether the user is roaming. This is important, since it eliminates conditional behaviors that may only show up in odd circumstances, thus making the network easier to build, troubleshoot, and maintain.
There is a desire in the 3G group to, as much as possible, use IETF protocols for the IP Multimedia domain. This allows the ongoing work in the IETF to be leveraged in the 3G network, and should provide a smoother interoperation of 3G equipment with non-3G equipment.
The 3G network shouldn’t rely on and particular access method for its services (I’ll get into this more later).
Finally, a decision has been taken to allow the S-CSCF to be placed in the visited network (by the home operator), if it can support the user’s services. This is also covered later.

9. (Ideal) Access Independent Network
Worldcom
Service Node
Yahoo!
Service Node
PSTN
PSTN GW owned by traditional telco. Calling users subscribe; authorization/billing based on challenge/response model
Pay-per-view movie service using RTSP, hosted by backbone provider
Service Node
Peer IP backbone networks IP IP IP IP
GGSN
SGSN
CMTS
Broadband Access Providers (Wireless, Cable, DSL, etc.)
What do we mean by an access independent network? In an ideal world, this basically means that you have a group of interconnected IP networks with peering arrangements providing an IP network backbone, similar to today’s Internet. In fact, many people believe that this could be accomplished with today’s Internet.
Consumers gain access to this network through a variety of means: 3G wireless, DSL, cable modems, GPRS, wireless LANs, etc.
Bridges to the existing telephone network can be provided through media gateways. From a technical perspective, there’s no reason that these gateways can’t be provided by a third party -- as long as they are able to bill the customer for the service. This model is similar to the pay-per-subscription news web sites currently available.
New services, such as streaming media, may be offered by a variety of providers, possibly even the network backbone providers themselves.
Conversational services will be provided by both traditional and non traditional service providers, many of whom may also supply access methods.
Subscribers may even elect to run their own services from their own premises, when appropriate.
My own machine with a multicast chat server for all my friends, connected through my broadband access
Service Node

10. Short Term Network Picture
Access
PSTN IP
PSTN
Access IP
Firewall
Firewall
Call Control
IP Backbone
Call Control
Access
PSTN
In the short term, however, I expect the 3G network will look very similar to the business models provided by today’s wireless networks: services will be bundled with network and radio access. These same service providers will want to operate their own PSTN gateways, for the extra profits that can be gained from captive users. Different IP domains will have firewalls separating them.
Note, however, that this picture isn’t too far off from the ideal. You still have a variety of network access methods, and there’s no technical reason that third parties can’t provide multimedia service nodes.
Luckily, this provides a clear migration path towards an ideal network. Once consumers and the particularly savvy network and service operators begin to realize the really cool possibilities available, I’m sure we’ll see this migration begin. Once the shift has begun, an increasing number of service providers will have to shift to this model in order to remain competitive. IP IP
Firewall
Firewall
PSTN
Call Control
Call Control
Access

11. Selection of Serving Node Location (Home Execution)
I-CSCF
Home
P-CSCF
Visited
GGSN
SGSN
Radio Access Network
S-CSCF
HSS
Profile 3 4 6 7
8. OK
5. REGISTER
2. REGISTER
9. OK
This slide demonstrates the basic registration flows found when a serving CSCF is located in the home network.
Of particular interest are steps 3 and 4: during these, the home service provider determines where to send the registration. In this case, it is a S-CSCF in the home domain.
1. REGISTER
10. OK

12. Selection of Serving Node Location (Non-Roaming)
Home
HSS
Location
Profile 3 4 6 7
8. OK
I-CSCF
S-CSCF
5. REGISTER
2. REGISTER
9. OK
Notice that the registration flow for the non-roaming case is identical.
P-CSCF
1. REGISTER
10. OK
GGSN
SGSN
Radio Access Network

13. Selection of Serving Node Location (Visited Execution)
I-CSCF
Home
P-CSCF
Visited
GGSN
SGSN
Radio Access Network
S-CSCF
HSS
Profile
Selection of Serving Node Location (Visited Execution) 3 4
5. REGISTER
12. OK 9 10
2. REGISTER
13. OK
11. OK
8. REGISTER 6 7
In the case that a serving CSCF is selected in the visited network, the behaviors of each node are identical as in the previous case: the only difference is that, in steps 3 and 4, the home network determines that the next hop for registration lies in the visited network.
Once the registration arrives there, the I-CSCF does what all I-CSCFs do: it works with the location function in the HSS to determine where to send the registration next (in this case, the serving CSCF).
14. OK
1. REGISTER

14. Visited-Domain Provided Service (e.g. 411 Directory Assitance)
S-CSCF
I-CSCF
HSS
P-CSCF
Visited
MGCF/
T-SGW
MGW
GGSN
SGSN
Radio Access Network
PSTN 4 3
5. INVITE
6. INVITE
2. INVITE
9. INVITE
10. IAM
Updates URI
11. ANM
This demonstrates the mechanism by which the P-CSCF can be used to trigger (but not provide) location sensitive services. This call flow shows how one might implement, for example, a location-sensitive directory assistance service. 7 8
1. INVITE tel:1411

15. Non-Conversational Services
Web
Server
IP Backbone
Media
Server
GGSN
SGSN
Radio Access Network
Since wireless handsets will have an IP connection, Non-conversational services will be accessed in the same way as they are from normal PCs and wired devices.

16. Future Issues to be Decided
Handling of Instant Messages (SMS). SIP “MESSAGE” seems a natural fit, but other ideas are being proposed
Uses of MRF (Media Resource Function) node. Possible uses include:
Conference Bridge (both dial-in and spontaneous)
Transcoding
Streaming Media/Announcments

17. Wireless Services Implications
Called
Home
Caller
Home
Value added
services
Value added
services
API
API
SIP
SIP
(S-CSCF)
(S-CSCF)
Access
signaling
signaling
Access
WireLine
WireLine
Another feature that the decision to use SIP as a call control method affords that wasn’t present in previous wireless networks is a complete decoupling of the services provided with the network that the user is currently in. No longer will the limitations of the visited network have an impact on the services available to the user.
This frees the network operators to deploy new services much faster, since they will no longer need to wait for support from their network partners.
It also allows service providers to deploy proprietary service nodes in their network, allowing innovative services which would not otherwise be possible, giving the truly creative service providers an edge in attracting and retaining customers.
There are two reasons this technique is possible: by not relying on the visited network to provide service triggering, the service provider is allowed to hide trade secrets which may be relevant to these new services. Secondly, the service provider doesn’t need to be constrained to the type of information which can be expressed via the standardized service API.
WLan
WLan
TA?
UTRAN
GPRS
TA?
GPRS
UTRAN
TA?
FW?
FW?
Bearer Services
oriented
Bearer Services
oriented
Bearer Services
oriented
IP backbone
Video/Audio/Signaling
Video/Audio/Signaling

18. 3G is dedicated to using SIP for call control.
Conclusions
3G is dedicated to using SIP for call control.
This development will allow a seamless integration of “smart” wireless terminals into the exploding information/communication IP networks.
If deployed correctly, the transition to 3G networks will be a revolution in wireless communications unlike any previously seen.
3G has taken the decision to use SIP for the IP Multimedia call control.
This decision allows the integration of wireless terminals into the Internet, bringing them up to the same information access level as personal computers, but with the additional benefit of being portable and wireless.
This basically ties the success of wireless terminals to the ever-growing success of the Internet. If done correctly (i.e. without crippling barriers), this promises to be the most radical wireless revolution yet seen.