Multimedia Resource Function (MRF)Offers Services Such as
ConferencingMRFC SIP User Interface toward S-CSCFMRFP Controls the
Media Server (MS)SIPMSMSMRFC
IMS started as a technology for 3rd Generation mobile networks
(under the auspices of the 3rd Generation Partnership Project
(3GPP), but it is now spreading to next generation wireline
networks and is going to be a key to Fixed Mobile Convergence
(FMC). It builds upon Session Initiation Protocol (SIP) which has
emerged as the crucial technology for controlling communications in
IP-based next generation networks (NGN).IMS is about services and
applications, enabling service providers to offer rich multimedia
services across wireless, packet and traditional circuit switched
networks. It is standards based and uses open interfaces and
functional components that can be assembled flexibly into hardware
and software systems to support real-time interactive services and
applications.The basic set of standards for IMS implementation were
released in 2004 and the first implementations are beginning in the
European wireless markets. The standards organizations are heavily
involved in developing standards to fill inevitable gaps and to add
new capabilities. However IMS is still untested in real-life major
carrier networks and its wide scale implementation is some years
away. That being said, it is the most likely evolution path for
next generation networks, including those for Emergency
Services.This presentation will illustrate the current standards
relating to IMS, define IMS and the show the functional elements
that comprise it. It will explain how elements interact and
illustrate example call flows.
Not much work has begun on relating Emergency Services to IMS.
This presentation will describe what has been done in 3GPP and
highlight areas where additional attention is required.
IMS comes from the need to evolve the TDM networks into robust,
extensible networks that can take advantage of emerging
technologies. The migration from circuit switching to packet
switching has played a significant role in the maturation of data
networking. Taking advantage of these same concepts, the voice
network can evolve into a multimedia network that allows the
combination and coordination of voice, video and data (sometimes
called the Triple Play). The drivers have to be business driven. In
the wireline networks there is a need to increase bandwidths
offered to users and replace the outdated TDM switches.In wireless
networks one clear driver is the ability to introduce services
rapidly and uniformly. All services should be available to the user
whether within the home network or roaming to another network.IMS
provides the architectural umbrella for wireline and wireless
convergence.There are a number of organizations whose activities
directly apply in defining IMS. The initial concept came from 3GPP
in Europe to be the evolution of GSM networks. 3GPP depended
heavily upon the work of IETF in defining SIP and related
protocols. It became clear that IMS had broader appeal and
organizations such as TISPAN began defining extensions needed for
the wireline network.
ATIS has incorporated IMS as the key concept in their NGN
project. IMS is being defined in phases. The early phases focused
specifically on GSM evolution. Releases kept building features and
functionality and expanding the scope of applicability beyond GSM.
Release 7 is to incorporate Emergency Services. It is important
that the U. S. Emergency Services Industry be involved in this
effort to assure its needs are recognized. While IMS has global
scope, the needs of Emergency Services in Europe are different that
those in the U.S. A common and coordinated approach to Emergency
Services will benefit the global community. IMS is defined as a
network architecture that defines functional elements. Each
functional element does not have to relate one-to-one to a physical
element. A number of functional elements can be incorporated into a
physical element depending upon a vendors implementation. The
Service Architecture defines standard methods for services to be
introduced while the Core Network defines the interactions between
functional elements. IMS is multimedia. Therefore you need to think
beyond a typical voice call. The context could be voice, video or
graphics; or a combination correlated between two or more parties.
IMS begin with 3GPP and ETSI, both which have their roots in
Europe. To introduce IMS in the U.S. there are some adoptions that
will be needed to accommodate U.S. nuances. It is likely that these
will be handled by TIA and ATIS. IMS relies heavily existing
standards developed by the IETF. Organizations representing IMS are
also heavily influencing the IETF activity to evolve or develop new
standards needed to complete the IMS picture. FMC Fixed Mobile
Convergence is the new buzz word of the industry. IMSs intended
applicability is across wireless, wireline, cable, enterprise and
other networks. One of the major attributes, which may have more
play in wireless, is the idea of service transparency no matter
what network you are in (e.g. the home or visited network). There
is a recognition that IMS must evolve into the networks and that
there needs to be interworking between existing functionality and
the newly envisioned capabilities. Media Gateways are the interface
between IMS and the legacy PSTN world. This allows calling between
the richly featured multimedia IMS environment to the existing
voice networks. The interworking between legacy endpoints and the
IMS network allow those endpoints to access the features and
functions that IMS can provide. Of course, this functionality may
be limited by the capabilities of the endpoint. For example,
network based features may be available to the legacy endpoint, but
not multimedia. IMS defines common support for Classes of Service,
Quality of Service, security and other attributes. However,
standards are still evolving in many of these areas. Since IMS is
built upon IP, it supports the flexibility and scalability to
support mobility, portability, service creation, etc. All of these
together may provide operational and financial improvements beyond
existing networks. IMS defines how to develop subscriber databases
which include User Profiles that enumerate identity, services,
security levels, etc. IMS defines common session control that
applies to any media. So the way a voice call is set up is
identical to how a video call is set up. Based upon the class of
service, different resources may be allocated in the network. IMS
specifies common OAM&P environment that allows the evolution of
operation support systems. IMS, in its self, does not define
services. However, it defines how services are accessed. These
services may be inherent in the IMS network or can provide gateways
to existing service platforms.IMS is designed to be applicable to
the evolution of all types of networks. The major wireless carriers
have committed to IMS as their next generations network. All of the
U.S. major wireline carriers have embraced IMS in their evolution
path. Cable network companies have not embraced IMS as of yet since
Cable Labs has just defined IP-based network topology that was
pre-IMS. It is thought that as cable networks evolve, they too will
embrace IMS.
One major advantage of IMSs commonality is that carriers from
each discipline can purchase equipment based upon the same
standards, thereby potentially decreasing the cost to provide
duplicate networks for different media services. IMS is being
defined as the convergence vehicle for all types of access
connectivity. IMS allows typical Centrex customers to migrate to
IP-based Centrex services and provides direct connectivity from the
myriad of IP PBXs that are being deployed. IMS provides a natural
evolution as LECs deploy their broadband access to the home as well
as providing cable providers a standardized approach for network
evolution. This theme extends to the introduction of WiFi or WiMax
technologies that are inherently IP based.
It is recognized that not all end points will be SIP enabled.
Therefore, legacy systems can take advantage of the IMS services by
entering the network through signaling and media gateways.
As shown in the wireless cloud, a network can evolve such that
it can take advantage of existing access techniques while evolving
to IP connectivity that can natively interconnect to the IMS core.
IMS is the only emerging concept that provides architectural
consistency for an evolving telecommunications network. It
encompasses the full range of capabilities required to evolve and
eventually replace the legacy TDM network. Not only does it deal
with call delivery, but registration, billing, operations and
administration.IMS is built upon SIP. In using SIP, IMS is able to
take advantage of the work of the IETF. In fact, much of the effort
in IETF today is as a result of the IMS influence. IMS defines
routing elements to include ingress from users, routing within
networks and routing between networks. It also acknowledges the
need to interwork between IMS networks and the PSTN.Databases are a
key component of the IMS structure ranging from those that home
subscriber information to those that provide services.Inherent in
the design of IMS is the concept of interoperability. One of the
initial concepts of IMS was to define the structure that would
allow new services to be introduced easily and seamlessly. That is,
no matter where the user is they should have access to all of the
services to which they have subscribed. And once services are used,
IMS provides the architecture to allow recording and charging.
This slide shows the functional elements of IMS. Those listed in
red are covered in this presentation. The others are important to
complete the picture, but are best left for discussion related to
the specific topic, e.g. security.This slide shows a single network
topology using IMS. A call originating from a SIP User Agent in the
IMS network may go to another SIP UA or egress to the PSTN.The HSS
contains all of the subscriber information. In the wireless network
it is the evolution of the HLR. In the wireline network it is the
equivalent of customer records provisioned on switches.Application
Servers are where the application reside. There may, for example,
be originating services or terminating services. The filtering
criteria is loaded into the S-CSCF when the subscriber registers
with the network.DNS is used to identify elements use in the
session set up.The CSCFs manage the session control: registration,
set up, tear down, feature activation.The P-CSCF is first point of
interaction with the User Agent. It also manages Quality of Service
and other conditions specific to a UA.The I-CSCF is used in network
to network signaling. The I-CSCF hides the network topology from an
external network.The S-CSCF is the primary signal processing engine
in IMS. It manages registration, checks for triggers for services
and performs routing (although routing instructions may come from
Application Servers).Media Resources may be conference services,
IVRs or other network services.If a call must egress to the PSTN
the BGCF selects the appropriate Media Gateway that can be
used.Media Gateways control the conversion from IP to PSTN TDM
signaling. Media Gateway Control Functions control the signaling
between IMS and the PSTN (e.g. IP to SS7).This slide illustrates
the IMS network to network connectivity. A call in the Visited
network goes to the home network and may be terminated to a SIP UA
within the network or egress to the PSTN.Within the Visited network
the P-CSCF and S-CSCF process the origination of the call and
select the destination network.Within the Home network the I-CSCF
receives the call signaling from the Visited network, chooses the
appropriate S-CSCF to process the call and the call is
completed.IMS registration is the procedure where the IMS user
requests authorization to use the IMS services in the IMS network.
The IMS network authenticates and authorizes the user to access the
IMS network. The UA/UE initiates the registration process when the
terminal is connected or otherwise introduced into the network. The
SIP registration is passed to the S-CSCF. If the user happens to be
roaming in another network then the P-CSCF in the Visited network
would pass the registration to the S-CSCF in the Home network
through a I-CSCF. Users are always registered in the Home network.
The S-CSCF forwards the request to the HSS via the Multimedia Auth
Request (MAR) message to 1) download authentication data via the
Multimedia Auth Answer (MAA) message and 2) inform the HSS that
this S-CSCF is in control and any other queries to the HSS should
be returned to this S-CSCF. The S-CSCF creates a SIP 401
Unauthorized response that includes a challenge that the IMS
terminal should answer. The IMS terminal sends a new Register that
contains the response to the challenge. The S-CSCF validates the
user and sends a Session Auth Request (SAR) message to the HSS
informing it that the user is now registered and requesting the
user profile, including services, that come in a Session Auth
Answer message (SAA).Now that a user is registered with the network
there is a need for notification of state changes. For example, a
user registration may be valid for a fixed period of time and then
the network requires the user to register. Or the user or network
element may go out of service and need to inform the other of some
state change. This is done by having the UA/UE subscribe to the
registration state. Not only does the UA/UE subscribe, but the
P-CSCF serving the UA/UE subscribes so it can be informed. When the
IMS terminal has completed registration the P-CSCF sends a
Subscribe request for the registration event. The request is
directed at the S-CSCF (which is in the Home network).The S-CSCF
receives the request and installs that subscription, i.e. the
S-CSCF takes the role of a notifier. The S-CSCF sends a Notify
request to the P-CSCF. This request includes Public User Identities
and the registration state. When the IMS terminal has completed
registration it sends a Subscribe request for the registration
event. The request is directed at the S-CSCF (which is in the Home
network).The S-CSCF receives the request and installs that
subscription, i.e. the S-CSCF takes the role of a notifier. The
S-CSCF sends a Notify request to the user. This request includes
Public User Identities and the registration state.
In case the S-CSCF has to shutdown or there is some other
stimulus the S-CSCF will inform the user (and the P-CSCF) of the
event.This example illustrates how IMS multimedia calls may be
coordinated between parties. The first is a voice call originated
by User A. The second is a video call originated by User B and the
third is a data call originated by User A.The voice call originates
from user A and enters the IMS network X at the P-CSCFThe P-CSCF
passes the call to the S-CSCFThe S-CSCF interrogates the
Application Server for originating servicesThe S-CSCF forwards the
call to the I-CSCF of network Y.The I-CSCF interrogates the HSS to
determine the S-CSCF and passes the call to it.The S-CSCF
interrogates the Application Server for terminating services.The
S-CSCF passes the call to the P-CSCF assigned for the user and the
voice call is completed.Now a video call is set up from User B to
User A and the signaling path is reversed.
Finally, User A sets up a data call to User B using the same
signaling path.This slide comes directly from one of the 3GPP
specifications and defines protocol interfaces required to deliver
a call to the legacy Emergency Services Network or an IP-capable
PSAP. Note that PSAP selection is left to implementation. Also the
IP PSAPs are treated a peer network since the interface is from a
S-CSCF to the PSAP and a P-CSCF is not included to manage the PSAP
interface.Serious work on Emergency Services has not begun within
IMS standards. The initial assumption is that Emergency Services
would follow legacy methods. This slide illustrates the conceptual
model currently defined in 3GPP. The IP Connectivity Access Network
(which represents the wireless access, cable access, etc.) forwards
the call to the P-CSCF in the IMS Core Network and the call is
routed to the S-CSCF.The S-CSCF performs PSAP selection. However,
3GPP currently defines this as left to the implementation. There is
current work within IETF to define this.The emergency call may be
delivered to the legacy Emergency Services Network through Media
Gateways.The emergency call may be delivered to a PSAP capable of
directly handling SIP calls. Note that a significant amount of work
is required to define the interactions between the IMS network and
a IP-capable PSAP. (Note that the IMS Core network treats the PSAP
as a foreign network since the interface is from a S-CSCF to the
PSAP and a P-CSCF is not included.)This slide illustrates a
potential architecture where the Emergency Services Network is IMS
enabled.The emergency call comes into the IMS ESNet from another
IMS network with its location object.The S-CSCF interrogates the
PSAP Selection Application Server for routing instructions. The AS
must convert the location object to a PSAP URI or other designation
that maybe used by the S-CSCF to route the call.The call is
delivered to the IP-capable PSAP with location.It is possible that
the call could be delivered directly to the Legacy PSAP via CAMA
trunks in the Media Gateway. The issue with this is that now the
location is lost and the Legacy PSAP would have to have a key to
query for location information (e.g. ALI).Note that the concept of
managing the interactions between an IMS Emergency Services Network
and the Legacy Emergency Service Network requires much further
thought. For example, legacy station sets will need to terminate to
IP PSAPs and IP terminals may need to terminate to an IP PSAP.
In recognizing that a legacy ESNets may have to exist in
parallel with IMS ESNet, questions regarding how the two interwork
for various call flows arise. This slide, and the following 3
slides, illustrate topics for consideration in call delivery.
Four salient components are shown interacting with the IMS
ESNet.A VSP VoIP network (or potentially the Intenet) that is
capable of delivering the emergency call to the IMS ESNet with the
location of the caller (Presence Identification Format Location
Object (PIDF-LO).A Legacy PSTN that may consist of Class 5 end
offices and only pass the callers number (ANI) in the signaling.An
IP capable PSAP that is the next generation PSAP that is able to
receive a SIP call that includes the callers location (PIDF-LO).The
Legacy ESNet or a Legacy PSAP which interconnects using CAMA or
E-MF and is only able to receive the callers number (ANI).This
slide represents the long term vision of emergency services. That
is, a call originates with its location (PIDF-LO) and enters the
ESNet via a SIP INVITE. The location is used to select the PSAP and
the call is delivered to the IP capable PSAP with the callers
location. If a call originates from a VoIP network capable
providing the location of the caller (PIDF-LO) and must be
delivered to the Legacy ESNet or a Legacy PSAP then there are
issues that need to be resolved related to the Legacy PSAP
obtaining the location of the caller. The call originates in a VoIP
network where the callers location is included in the SIP INVITE.
When the call enters the ESNet the PIDF-LO may be used to select
the Legacy Emergency Services Network or the Legacy PSAP. The call
must then traverse a Media Gateway to get to the Legacy ESNet or
PSAP. In doing so the callers location is lost. Therefore, a query
mechanism is required to obtain the callers location.Calls from the
PSTN or legacy end offices can only pass the callers number.
Therefore if a call were to come from a Legacy EO to the ESNet it
would only have ANI. If the call is destined to a IP PSAP, the IMS
ESNet would require a mechanism to acquire the callers location,
use it to select the PSAP and forward the call to the IP PSAP with
location (PIDF-LO)
This flow only illustrates a wireline call. However a wireless
call can be extrapolated. For the Wireline Compatibly Mode, an ESRK
would be passed to the IMS ESNet and the flow would have the same
attributes as wireline. For Hybrid CAS calls, 20 digits would be
delivered to the IMS ESNet and the ESNet would have to use the ESRD
to obtain the callers cell site information.
Also, wireless Phase 2 exacerbates the problem since the PSAP
may need to obtain updated location information. If a call
originates from a Legacy EO and must be terminated to a Legacy
ESNet or PSAP then the issue of obtaining the location for routing
and acquiring the location at the PSAP must be addressed. The IMS
ESNet would require a mechanism to acquire the callers location,
use it to select the PSAP and forward the call to the Legacy PSAP.
When the call is received at the Legacy PSAP, a query mechanism is
required to obtain the callers location.
Call delivery is only one of the things that must be resolved in
order to implement a VoIP ESNet. Many of the Emergency Services
functions must be replicated. A call may need to be bridged from
one PSAP to another PSAP. It would be desirable to have the
original callers location information transferred as well.
Transferring between IP capable and Legacy PSAPs must be addressed.
Specifications are required for routing to an alternate PSAP if the
primary is not available. If PSAP selection fails (corrupted data,
data not yet provisioned) a method must be incorporated to choose a
default PSAP that can manage calls. Wireless Phase 2 information
often is not available at the time the call is set up. In addition
the PSAP may want to request a location update. These mechanism
must be considered. NRIC VII and others have noted considerations
for TTY/TDD over IP. This topic needs careful consideration with
regard to PSAPs. The ability for 3rd party call centers and Relay
Centers to provide emergency calls on behalf callers needs to be
specified.The HSS is the central repository for user-related
information. In wireless networks it is the evolution of the HLR.
The HSS contains all the user-related subscription data required to
handle multimedia sessions. These data include, among other items,
location information (not the physical location), security
information (including authentication and authorization), user
profile information (including the services that the user is
subscribed to) and the S-CSCF that is allocated to the user.A
network may contain more that one HSS in the case the number of
subscribers is too high to be handled by a single HSS. All data
related to a particular user are stored within a single HSS. The
HSS is typically implemented using a redundant configuration.
Application Servers host and execute services. Depending upon the
application they may operate as a SIP Redirect Server, Proxy, User
Agent or Back to Back User Agent (B2BUA).
There may be three categories of AS: SIP AS, OSA-CS and
IM-SSF.
SIP AS This is a native Application server that hosts and
executes IMS services based upon SIP. New IMS applications will be
developed in the SIP AS.
OSA-SCS (Open Services Access Service Capability Server) This
Application Server provides an interface to the OSA framework
Applications Server. In essence it provides a gateway function.
IM-SSF (IMS Service Switching Function) This is a specialized
Applications Server that alls reuse of CAMEL (Customized
Applications for Mobile network Enhanced Logic) services. These
services are specific to GSM and European networks. A similar
function will provide access to U.S. Advanced Intelligent Network
(AIN) services.In general CSCF provide the SIP routing logic in the
IMS network.
P-CSCF The P-CSCF is the first point of contact between the IMS
terminal and the network . All signaling from/to the IMS terminal
go through the P-CSCF. Thje P-CSCF is allocated to the IMS terminal
during registration and provides functions such as security,
authentication, and the correctness of the SIP requests. The P-CSCF
may include a Policy Decision Function (PDF) that authorizes media
plane resources and manages Quality of Service over the media
plane.
I-CSCF The is a SIP Proxy located at the edge of an
administrative domain. The address of the I-CSCF is listed in the
DNS records of the domain. When a SIP server follows SIP procedures
to find the next SIP hop for a particular message the SIP server
obtains the address of an I-CSCF of the destination domain.
S-CSCF The S-CSCF is the central note of the IMS signaling
plain. It acts as a registrar in that when the IMS terminal
registers the S-CSCF obtains SSP information from the HSS. All
signaling passes through a S-CSCF. The S-CSCF inspects every SIP
message and determines whether the SIP signaling should visit one
or more Application Servers. Those ASs would potentially provide a
service to the user.IMS networks must be able to deliver calls to
and receive calls from the PSTN. In order to do this there is a
need to interwork signaling (e.g. SIP to ISUP) and bearer channels
(e.g. RTP to TDM).
BGCF The BGCF provides routing functionality based on telephone
numbers. The BGCF is only used in a circuit switched network, such
as the PSTN. Its basic functions are 1) select an appropriate
network where interworking with the circuit switched (CS) domain is
to occur or 2) select an appropriate PSTN/CS gateway (i.e.
MGCF).
MGCF The MGCF is the central node of the PSTN/CS gateway. It
implements a state machine that does protocol conversion and maps
SIP to ISUP. It also controls the resources of the Media
Gateway.
SGW The Signaling Gateway performs the lower layer protocol
conversion. In this presentation it is assumed part of the
MGCF.
MGW The Media Gateway interfaces to the media plane of the CS
network. One side the MGW is able to send and receive IMS media
over RTP and on the other side the MGW uses one or more PCM time
slots to connect to the CS network. The Media Resource Function
provides a source of media in the IMS network. This may be the
ability to play announcements, mix media streams (for
conferencing), transcode between different codecs, and do any sort
of media analysis.