IMS is a core network architecture defined by 3GPP to provide multimedia services in an IP-based, access independent manner. IMS defines a set of CSCF nodes that handle SIP signaling to enable multimedia sessions between user equipment regardless of the access network. IMS provides a common IP-based core that different access technologies can interface with, allowing for convergence of services across networks. Key objectives of IMS include enabling seamless roaming, quality of service guarantees, and more rapid development and deployment of new multimedia services for users.

1. Introduction to IMS-IP
Multimedia Subsystem
The IMS-Connectivity- One network Experience
Introduction
In the past few years, a number of things lead operators towards convergence. Mobile handsets and mobile application developers
have advanced very fast in the last few years and support lot of new technologies which sometimes network doesn't. Also, all kinds
of access devices are allowing people to access an IP centric voice infrastructure. Since services can be
accessed from a range of devices, users started demanding uniform access to services regardless of the type of
access. Also, wireline operators needed to consider ways to access wireless services to increase revenues.
Wireless operators, in turn, were already seeing handset sales peak and needed new ways to expand their
markets as well; the most obvious area being converged business services. Some new types of access have also
come up in the recent times. For example WLAN was not a compelling technology few years back but today it is
something that operators can't ignore. All this led to need of a common IP centric network core that is access
independent. The IP Multimedia Subsystem (IMS) defined by 3GPP provides such an enabling architecture that is
access independent, central in the move towards convergence. However since it was first introduced for wireless
network, Release 5 (the first 3GPP release containing IMS subsystem) was heavily biased towards 3 rd generation
wireless networks. Now each access type is being 'enabled' to work with an IMS core, be it DSL,
WLAN or GPRS.
IMS builds upon the soft-switch based network introduced for first time in release 4 of 3GPP and was introduced
for the first time in release 5. This white paper introduces IMS subsystem and includes details of the functional
elements, interface points, protocols and detailed call flows.

2. IMS envisages a network core that can work with various kinds of IMS is a subsystem in the 3G network and is the core signaling
access technologies. It is an overlay subsystem on top of existing network in an all IP network. It surfaced for the first time in
packet core. Release 5 of 3GPP and is therefore very much wireless centric.
The softswitch shown in Figure 1 is divided in to three call state From technology point of view SIP was considered flexible
control functions (CSCFs). These are analogous to a Mobile protocol but that was also a disadvantage since it doesn't have
Switching Centre (MSC) in the Release 4/ Release 99 any standard network topology So IMS was an attempt to build a
standard network topology using SIP. In other words, IMS is a
architecture and are primarily SIP servers with enhanced call
grand middleware between services and access networks and
handling functionality. These handle the call request from user
can be said to be the J2EE of telecom world.
equipment for purpose of establishing multimedia session
between UEs. One of the main objectives of IMS was to make switching
access to work with the same, be it DSL, WLAN, GPRS or any
distributed and hence various CSCF came into picture replacing
emerging technology, such as WiMAX.
the MSC server (release 4). Wireless networks have always
IMS is a subsystem in the 3G network and is the core signaling
network in an all IP network. It surfaced for the firstintime infollowed the
principle of centralized data repository home
Release 5 of 3GPP and is therefore very much wireless centric.
IMS envisages a network core that can work with various kinds of network and calls being handled by home network and hence
access technologies. It is an overlay subsystem on top of existing From technology point of view SIP was considered flexible
packet core. IMS also introduces a node known as HSS which is mainly an
protocol but that was also a disadvantage since it doesn't have
The softswitch shown in Figure 1 is divided in to three call state any standard network topology So IMS was an attempt to build aenhanced
control functions (CSCFs). These are analogous to a Mobile HLR. To ensure service availability at all times, calls
Switching Centre (MSC) in the Release 4/ Release 99 standard network topology using SIP. In other words, IMS is a
architecture and are primarily SIP servers with enhanced call are always main objectives ofnetwork. to make switching
One of the serviced in home IMS was
handling functionality. These handle the call request from user grand middlewarethe core network, access independence was
faster) pace than between services and access networks and
equipment for purpose of establishing multimedia session canthe said to be the J2EE evolve at came into picture replacing
distributed and hence various CSCF aworld.As be handsets typically of
between UEs. telecom different (and some times
one of the cornerstones of the IMS architecture and due to thisthe MSC server
(release 4). Wireless networks have always
followed it has been adopted by otherdata repository in homeattribute the
principle of centralized access technologies as
network and calls being handled by home network and hence
well also introduces a nodeIMS for their future networks. known as HSS which
is mainly an
enhanced HLR. To ensure service availability at all times, calls
Release 7 of IMS will address non-IMS end points as well and will
faster) pace than the core network, access independence wasline access
are always serviced in home network.
medium into IMS.
incorporate Release 1 of evolve at a different will bring in fixedAs the handsets
one of the cornerstones of the IMS architecture
typically TISPAN NGN which (and some times and due to this
IMS uses has been adopted by other and DIAMETER. Whileattribute it 3 main
protocols: SIP, H.248 access technologies as
Figure 1: IMS access independence
well for their future networks.
Release 7 of IMS will address non-IMS end points as well and will
SIP & DIAMETER came from IETF, H.248 was contributed
by ITU.
IMS is able to provide users with same service experience incorporate Release 1 of TISPAN NGN which will bring in fixed
line IMS gets adopted by various other organizations for theirAs access
regardless of the access network used. The IP Multimedia medium intomain protocols: SIP, H.248 and DIAMETER. While
IMS uses 3 IMS.
future networks, IMS is getting changed so that it is not too
subsystem (IMS) defines an enabling architecture that is access SIP & DIAMETER came from IETF, H.248 was contributed
wirelessby ITU. specific as it is now. For example signaling payload
independent. This is central in moving towards true
As IMS gets adopted by variousneeded in wireless since aircompression on air
convergence. Having defined this core network architecture that interface is other organizations for their
future networks, IMS is getting changed so that it is not too
IMS is able to provide users with same service experience interface traffic has to be optimized to the maximum. Also, IMS is
is access independent, efforts are on to 'enable' different types of wireless specific as it is now. For example signaling payload
regardless of the access network used. The IP Multimedia all IPV6 but wire air networks have mostly wireless now aircompression on line
access to work with the same, be it DSL, WLAN, that is accesssubsystem (IMS) interface is needed inIPV4 right sinceand
defines an enabling architecture GPRS or any interface traffic has tohandle presence of maximum. and IPV6 inhence IMS
independent. This is as WiMAX.emerging technology, suchcentral in moving needs to be optimized to the both IPV4 Also, IMS is
towards true all IPV6 but wire line networks have mostly IPV4 right now and
convergence. Having defined this core network architecture that network. Wire line to handle presence of both firewall issues.hence IMS needs
is access independent, efforts are on to 'enable' different types of IP networks also have NAT &IPV4 and IPV6 in
network. Wire line IP networks also have NAT & firewall issues.
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3. So these problems are getting addressed in TISPAN with IMS as From network topology point of view, IMS creates signaling
the base architecture. To solve all the above mentioned problems infrastructure in the form of various SIP proxies which together
in wire line networks, TISPAN has added two main components: perform call control like MSC in the old architecture.
RACS and NASS.
CSCFs perform a number of functions. They perform multimedia
session control function which is similar but far more enhanced
Network transformation than the call control functionality done in MSC. They also
perform address translation / DNS look up / ENUM support etc.
Now let's look at the network transformation. All the networks are to translate the called user identity into a directly reachable
divided mainly into 3 layers: transportation/access, session address on the IP network. This function is similar to the digit
control & services. So the first convergence has to happen at translation function in the MSC. CSCF also transfers control to
transportation level. This results in access independence. Next an APP server, potentially via. a service broker, similar to the IN
step is that session control has to converge. Different networks concept used in MSC and SCP. For most of these functionalities
handle call in different way so in order to have convergence SIP protocol is used. Role of the VLR and HLR is replaced by
at session control layer, there has to be a common signaling centralized subscriber profile storage space i.e. HSS. HSS is
core architecture which is used by all access mediums. IMS used by CSCF whenever subscriber profile information is
achieves this. needed by the CSCF.
Figure 3: IMS Architecture, various Nodes
Figure 2: Network transformation Why IMS is needed
It is possible to have an IMS installation and use the roaming
As IMS is fundamentally data centric and follows the “home principle built into access network e.g. GPRS roaming to access
control model”, mobile networks are going to be the first adapters the same IMS network from anywhere in the world. This is
to IMS and can easily build a bridge to IMS network from non-IMS depicted in the following figure;
networks of today e.g. pure circuit switched network like GSM.
Also, there are already more than 40 Million 3G network
subscribers on UMTS Release 4 based 3G network. So moving
these customers onto IMS based network would not be
a big task.
Reference Architecture
From service description point of view, IMS is primarily an overlay
on top of existing and future packet networks using SIP,
MEGACO & DIAMETER as the main protocols. It reuses SIP
protocol as defined by IETF and has added some extensions to
fulfill mobility and other requirements. As far as interoperability
with existing CS domain networks is concerned, IMS provides
reference points from where IMS network can connect to circuit
switched network using gateways.
Figure 4: Using transport plane roaming
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4. This approach suffers from the disadvantage that user plane Short description of Nodes
traffic (media) will not be routed efficiently since GPRS
infrastructure will not be application signaling aware and also Following is short description of various nodes in IMS:
since user plane traffic has to come to the home network in order 1. P-CSCF (Proxy-CSCF): It is the first point of contact for
to get routed elsewhere. mobile equipment in the visited network. It accepts the
registration request from user equipment and forwards the
So ideally IMS should be deployed as shown below, where user same to the home network. It enforces quality of service in the
gets connected to the IMS domain of the local service provider visited network under the control of the home network's S-
and then that IMS domain knows the home domain to contact CSCF and may also provide local control for emergency
which will eventually provide the service. services in the visited network. It's role is similar to the
outbound proxy (OBP) in SIP networks. It is also responsible
for generating billing information for S-CSCF for subscriber's
usage of visited network resources.
2. I-CSCF (Interrogating-CSCF): It is the first point of contact in
a home network whenever a call is to be terminated on an IMS
user. It is optional element in an IMS network and P-CSCF
may directly also contact the S-CSCF. It may be used to
support load balancing between various S-CSCFs as
configured in HSS. It may also provide topology hiding and
firewall functionality.
3. S-CSCF (Serving-CSCF): It performs multimedia session
control for calls originated by or terminated to users in the
home network that is served by S-CSCF. In a network there
can be more than one S-CSCF. S-CSCFs can also be added
to flexibly increase capacity of the home network. It may also
transfer the control of call to the P-CSCF in case user has
asked for a service that can be better served locally in the
visited network e.g. finding list of restaurants in the vicinity of
user. S-CSCF is mainly responsible for billing the subscriber.
Figure 5: Using IMS Roaming
Following requirements are achieved by IMS: One of the key points here is that although P-CSCF is the first
point of contact for a UE, it is the S-CSCF that handles user
1. Ensuring QoS: On the public internet quality of service is request and hence enables user to use his subscribed feature
unpredictable. Together with the underlying transport always irrespective of the capabilities of the visited network.
network, IMS attempts to provide predictable quality of This is also called Virtual Home Environment (VHE).
service by regulating and controlling usage of bearer
resources. UEs use SIP signaling to negotiate the media
attributes that are needed for call and then request bearer 4. HSS (Home Subscriber Sub System): It is a replacement of
network to provide the same. IMS ensures that only HLR and AUC in the traditional wireless network. It stores
authorized media streams are carried as per the negotiated subscriber profile and also some authentication information
media attributes and hence has view of currently available that is used by S-CSCF for authenticating the subscriber at
network resources all the time. the time of registration.
5. App Server: App server hosts services that are provided to
subscriber when they desire. Application Servers are invoked
2. Secure communication: Security is necessary in any when S-CSCF notices that criteria related to a call are met
network and IMS also ensures this. It provides data integrity and hence app server needs to take control of the call.
protection by using IPSec. Users are authenticated to prevent
unauthorized users from entering the network. Similarly when
a call attempt is made, PDF generates authentication token 6. Breakout Gateway Control Function (BGCF): When
that is used by UE when requesting resources from the S-CSCF determines that the call needs to breakout of the IMS
network thus restricting resource theft. domain and go into other networks like PSTN, it passes call to
BGCF. BGCF then determines whether call needs to
breakout in the same domain or it needs to pass the call to
another BGCF in some other IMS domain. If former is the
3. Roaming: IMS supports roaming to ensure that user plane
case it passes control to the MGCF which then uses media
traffic is routed more efficiently instead of relying on user place
and signaling gateways (MGW and SGW respectively) to
roaming.
pass call to other network.
4. Faster service development and easier access to service:
IMS achieves the latter by way of access independence so
that user can access same service independent of the access 7. Media Gateway (MGW): Role of Media Gateway (MGW) is to
medium used. Also, since IMS cleanly separates, transport, convert media from other networks into RTP as it enters the
signaling / call control and service layer, it helps easier and IMS domain and vice versa. It is controlled by MGCF.
faster development of services.
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5. 8. Media Gateway Control Function (MGCF): MGCF gets call S-CSCF (by using the domain look up mechanisms for UE's
from BGCF and passes it on to the PSTN. It receives / sends home domain) via I-CSCF of the home domain.
signaling from signaling gateway and performs interworking
between the SIP signaling and signaling used in As the REGISTER messages traverses the network, various
other networks. nodes in the message path e.g. p-cscf and i-cscf and other
proxies, add themselves to the “Path” header in SIP REGISTER
message. Hence when this message reaches S-CSCF, home
9. Signaling Gateway (SGW): Signaling gateway receives SS7 network is aware of the path that needs to be taken to reach the
or other forms of signaling from the network that it interfaces UE. In other words, home network is aware of where the user has
with and sends it in similar format over IP to MGCF e.g. M3UA roamed.
over SCTP. SGW doesn't need to convert the signaling
to SIP since this kind of interworking is performed by MGCF.
Similarly in the 200 OK for REGISTER message, S-CSCF sends
How it works
a path that call origination requests must take from UE whenever
Let's take an example call flow involving P, I & S CSCF. it makes a call. This information is present in the “Service-Route”
header and is stored at the UE.
Following figure shows two IMS domains having various CSCF.
UE-B is a domain B subscriber currently roamed into network A
and similarly UE-A is a subscriber from domain A, which has Assuming that users A and B that belong to IMS domains A & B
roamed into domain B. respectively, have roamed into IMS domains B & A respectively
and have registered with their home networks, following is the
path that would be taken by signaling and media when user A
calls user B:
Signaling
Media
IP Connectivity
Step 1-c
Step 1-b
Step 2-a
Step 3-a
Step 2-b
Figure 6: IMS Networks with roaming subscriber
Step 1-a
Following figure shows registration of roaming users A with their
home networks:
Figure 8: IMS Call Flow High Level
Following summarizes the steps taken in this call flow:
Step 1:
UE-A prepares an INVITE message and sends it to the P-
CSCF-B since it is the default outbound proxy for UE-A.
P-CSCF sends the same to S-CSCF, in accordance with
the “service-route” header present in the “200 OK”
message received for REGISTER from S-CSCF-A.
Step 2:
S-CSCF sends INVITE to domain B S-CSCF.
Step 3:
S-CSCF-B remembers the 'Path” header received in
REGISTER message from UE-B and uses the same to route
the INVITE to UE-B. Message is routed through I-CSCF-A
and P-CSCF-A.
Figure 7: REGISTER Message Flow Subsequent messages are exchanges in similar way and call
is established. However media flows between UE directly
As shown in the call flow, UE sends the REGISTER message to through their P-CSCFs.
its local P-CSCF which then forwards the same to home domain
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6. Now let us look at each IMS node in detail:
IMS Architecture Details P-CSCF
This section explains the IMS nodes and reference points P-CSCF is the first point of contact for a UE in the IMS network. It
in detail. is similar to the out bound proxy. It acts as proxy when it passes
messages sent to and received from the UE and sometimes also
acts as UA when it has to take some action on its own e.g.
IP Connectivity releasing the call in case of media or signaling inactivity or when
bearer network can't provide enough resources as are needed
In order to be connected to an IMS domain, user has to be for call. Following tasks are performed by P-CSCF:
connected to the IP world first. Release 5 of IMS assumes GPRS
for the IP connectivity but in release 6 when WLAN was
introduced, IMS adopted the concept of IP Connectivity access
network (IP-CAN) as any access network that can connect the To forward REGISTER message received from UE to the I-
user equipment (UE) to the P-CSCF. Future releases will test CSCF of domain to which UE belongs.
more variants of the IP-CAN.
To forward the requests / responses to and from the UE for
terminating and originating call. Also similar routing of
So here is how multi access IMS network will finally look like: messages needs to be done for independent transactions
that are not related to a call.
To perform signaling compression and decompression for
messages destined to and received from the UE.
To handle emergency calls. In release 5 emergency calls are
P-CSCF not handled by P-CSCF and UE is redirected to the CS-CN.
P-CSCF
But in release 6 onwards it will be possible for P-CSCF to
handle the call since P-CSCF is the one who has best idea
about location of UE and hence can handle emergency call in
best possible way.
To send charging related information to the charging
collection function (CCF).
To provide integrity protection of SIP signaling by maintaining
IP SEC association with UE so that data is transferred with UE
in a secure fashion.
To perform media policing. This entails ensuring that UE is
using the same media types / formats as those which were
negotiated when call was established. It also inspects the
Figure 9: Multi access IMS SDP present in INVITE message being sent and can reject
this if UE is trying to use media types / formats that it is not
Detailed Description of Functional Elements allowed to.
Following is a diagram that shows all IMS components from the
point of view of transport, signaling core and service plane To maintain session timers. P-CSCF maintains session timers
separation. so that there are no hanging sessions and if UE loses bearer
network connectivity, call can be cleared. It also maintains
RTCP timers for the same purpose.
PDF
PDF applies policy logic to the session and media related
information received from P-CSCF. It performs following tasks:
Generates an authorization token that identifies the session
for which P-CSCF makes authorization request.
Provides an authorization decision according to the stored
session and media related information on receiving a bearer
authorization request from the GGSN / IP-CAN.
Updates the authorization decision at session modifications,
which changes session and media-related information.
Enables the usage of an authorized bearer (e.g., Packet Data
Protocol or PDP context).
Figure 10: Various planes in IMS Architecture
Informs the P-CSCF when the bearer (e.g., PDP context) is
In this diagram HSS is shown overlapping in both the signaling as
lost or modified. A modification indication is only given when
well as services plane since HSS is used by both.
the bearer is upgraded or downgraded from or to 0 kbit/s.
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7. Passes an IMS-charging identifier to the GGSN and to pass a HSS
GPRS-charging identifier to the P-CSCF. HSS is the data storage for an IMS domain. It stores data related
to user identities, registration information, access parameters
I-CSCF and service-triggering information. For authentication, it stores a
secret key for each mobile subscriber, which is used to generate
I-CSCF is the first point of contact for an operator's network. For dynamic security data for each mobile subscriber. Data are used
example when a call has to be terminated to a subscriber, I- for mutual authentication of the International Mobile Subscriber
CSCF of that network has to be contacted first. It performs Identity (IMSI) and the network.
following functions:
Contacts HSS to obtain the name of S-CSCF that is serving a There may be more than one HSS in the network in which case
user. SLF is used to locate the relevant HSS.
Assigns an S-CSCF based on received capabilities from the
HSS. An S-CSCF is assigned if there is no S-CSCF allocated.
SLF
Forwards SIP requests or responses to the S-CSCF.
It is used to resolve the HSS applicable for a particular subscriber
Sends accounting-related information to the CCF. when there are multiple HSS in the network.
Provides topology hiding functionality.
MRFC / MRFP
S-CSCF
These come into picture when specialized media processing is
S-CSCF is the CSCF that finally serves call termination to / required for a call. For example if announcements, conferencing
call origination from a UE. It also handles REGISTER request etc is needed then media resource function (MRF) is needed.
sent by UE when it is registering from visited network. There MRFC is contacted by S-CSCF or AS via Sip signaling and it in
may be multiple S-CSCF each performing similar / different turn communicates with via H.248 interface to invoke the
roles. media services.
Functions performed by S-CSCF are:
To authenticate users by means of the IMS Authentication and BGCF
Key Agreement (AKA) scheme. It comes in picture when call has to leave IMS network and enter
some other network e.g. PSTN. The Breakout Gateway Control
To download user information and service-related data from
Function (BGCF) is responsible for choosing where a breakout
the HSS during registration. to the CS domain occurs. The outcome of a selection process
can be either a breakout in the same network or another network.
To route mobile-terminating traffic to the P-CSCF and to route
If the breakout happens in the same network, then the BGCF
mobile originated traffic to the I-CSCF, the Breakout Gateway selects a Media Gateway Control Function (MGCF) to handle a
Control Function (BGCF) or the application server (AS). session further. If the breakout takes place in another network,
then the BGCF forwards a session to another BGCF in a
To perform session control. selected network.
To interact with service platforms. Interaction means the
capability to decide when a request or response needs to be
routed to a specific AS for further processing.
MGCF
To translate an E.I64 number to a SIP universal resource
identifier (URI) using ENUM. MGCF provides the call control functionality for calls that are
terminated into / originated from non-IMS networks. It uses
To select an emergency centre when the operator supports gateways to convert media and signaling functionality from other
IMS emergency sessions. This is a Release 6 feature. network formats to IP. it gets the IP converted common channel
signaling information and performs interworking between this
To execute media policing. The S-CSCF is able to check the and SIP protocol since CSCFs in IMS use SIP for call control.
content of the SDP payload and check whether it contains
media types or codecs, which are not allowed for a user.
When the proposed SDP does not fit the operator's policy or
user's subscription, the S-CSCF rejects the request. Gateway (MGW + SGW)
Signaling and media gateway convert the signaling and media
To maintain session timers. It allows the S-CSCF to detect from non-IMS networks to IMS networks respectively. Signaling
gateway converts the transport of SS7 from MTP3 to IP and
and free resources used up by hanging sessions.
transports the ISUP protocol PDU on to IP network towards the
To send accounting-related information to the CCF for offline MGCF, as shown in the diagram:
charging purposes and to the Online Charging System (OCS)
for online charging purposes.
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8. Let us look at the each reference point in detail:
Gm reference point
It is the reference point between UE and the P-CSCF, which is
UE's first contact in the IMS network. UE uses this reference
point for the following actions / procedures:
During registration UE sends REGISTER and receives the
response to REGISTER message. It also establishes Path
and service route header to be used later when receiving /
making calls. UE also establishes security association with P-
CSCF and uses this interface to exchange authentication
data with P-CSCF during registration procedure.
During session initiation / termination same interface is used
to send / receive SIP signaling messages. P-CSCF may
inspect messages received on this interface to verify the
same with PDF to ensure disciplined usage of resources.
Figure 11: Signaling Gateway & MGCF Protocol Layers Transaction procedures allow UE to create independent
transactions on this interface. These can be used to
exchange capabilities (OPTIONS) or to send messages in an
internet chat (MESSAGE).
Application Server / Services
Application servers are used when S-CSCF is serving a call and Mw reference point
it finds that call criteria meets the service criteria. In this case call This is the SIP based signaling interface between various SIP
control is transferred to the application server. Application server servers viz. S-CSCF, I-CSCF & P-CSCF. Following actions /
hosts enhanced service like prepaid service and executes the procedures apply to this interface / reference point:
same on received call and handles it accordingly.
During UE registration process this interface is used to send
message from P-CSCF to I-CSCF and further to S-CSCF. In
Reference Points the reverse direction when responses flow, same interface is
used to send responses as well. The same is used by S-
Following diagram shows reference points and interfaces
CSCF to perform network-initiated de-registration and also
between all IMS nodes, it is taken from 3GPP specification 3GPP
TS 23.002: when user has to be re-authenticated by network. In case of
re-authentication, P-CSCF needs to delete the user related
state information that it might have accumulated in last
registration.
For mobile originated as well mobile terminated calls this
reference point is used between S-CSCFs. The same is also
used by P-CSCF to release a call if P-CSCF is notified by PDF
about degradation / loss of bearer from underlying transport.
It is also used for transaction procedures that allow UE to
create independent transactions on this interface. These can
be used to exchange capabilities (OPTIONS) or to send
messages in an internet chat (MESSAGE).
W
ISC reference point
ISC stands for IMS service and is based on SIP protocol. It is
between S-CSCF and AS and is used for following:
When S-CSCF receives a call that matches service criteria, it
forwards call to AS by sending INVITE. AS may modify the
SIP message and may send the call back to S-CSCF using
the same interface.
Figure 12: IMS reference points
AS may initiate a call towards S-CSCF as per service logic.
AS may also initiate a transaction e.g. for sending messages
using MESSAGE.
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9. Cx reference point which decides whether it should use a MGCF or BGCF in
Cx interface is between CSCFs and HSS. It is based on another IMS domain for breakout to occur. Protocol used is SIP.
DIAMETER protocol and is used by I-CSCF and S-CSCF
whenever they require access to subscriber profile data. I-CSCF
needs this to get the S-CSCF assigned to the subscriber. S- Mj reference point
CSCF also needs this interface for a variety of reasons e.g. for
When BGCF receives a call from CSCF, and it selects MGCF for
getting authentication information when it needs to authenticate
the breakout, it uses this interface to communicate with MGCF.
a user that is registering through the P-CSCF.
Protocol used is SIP. Evidently, it is used in conjunction with Mi
reference point.
Dx reference point
When multiple HSSs are present in the system, CSCFs need to Mk reference point
contact SLF for knowing which HSS to use for handling a given In case BGCF needs to communicate with another BGCF
subscriber. This interface is based on DIAMETER protocol and is instead of MGCF for the breakout to occur, it uses Mk reference
always used in conjunction with Cx interface. It uses the point to communicate. Protocol used by this interface is SIP.
redirection logic of DIAMETER protocol.
Mn reference point
This is the interface between CSCF and MGW for creating
context and setting up bearer for IMS to CS call and vice versa.
Protocol used is H.248
Mp reference point
This is the interface between MRFC and MRFP and is used to
setup media channels for specialized media handling when
CSCF require so. Protocol used on this interface is H.248
Mr reference point
This is the interface between CSCF and MRFC and is used when
specialized media handling is required for the call. Protocol used
is SIP.
Go reference point
Figure 13: Cx and Dx Reference Points Operators want to ensure that the QoS parameters along with
source & destination addresses of the intended IMS media traffic
matches the negotiated values at the IMS level. This requires
Sh reference point
communication between the IMS (control plane) and the GPRS
This based on the DIAMETER protocol and is between CSCF network (user plane). The Go reference point is used for this
and HSS for fetching user data. It has commands like user-data- purpose. In addition, the charging correlation was added as an
request (UDR) to request data and gets the same in User-data- additional functionality.
answer (UDA). This interface can also be used by AS to
subscribe for updates in user data in the HSS.
The protocol used is the Common Open Policy Service (COPS)
protocol. Go procedures can be divided into two main
Mm reference point categories:
This reference point is used by CSCF to send the call to other IP
multimedia networks which are not IMS. Protocol used is SIP. Media authorizationPolicy Enforcement Point (PEP) (e.g.,
GGSN) uses the Go reference point to ask whether a
requested bearer activation request (by UE) can be accepted
Mg reference point from the PDF that acts as a policy decision point. The PEP
It is the interface between MGCF and CSCF and is used for also uses the Go reference point to notify the policy decision
session control procedure for calls that are destined to / received point about necessary bearer modification and bearer
from circuit switched networks. It uses SIP signaling. It is job of releases (e.g., PDP context).
MGCF to convert CS domain signaling into SIP signaling and
send the same to CSCF using this interface.
Charging correlation via the Go reference point in IMS is
achieved by passing an IMS Charging correlation via the Go
Mi reference point reference point in IMS is achieved by passing an IMS
charging identifier to the IMS. With this procedure it is
When call needs to breakout of IMS domain and go into other possible to later merge GPRS charging and IMS charging
network then CSCF uses this interface to send call to BGCF information in a billing system.
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10. Gq reference point Globallogic expertise in IMS Technology
When a stand-alone PDF is deployed the Gq reference point is Globallogic is developing various IMS nodes for leading telecom
used to transport policy set-up information between the equipment vendor and a telecom solution provider . It also has substantial
application function and the PDF. The term "application function" expertise in performing quality assurance and testing of software being written
is used because it is intended that a PDF could authorize other for IMS architecture.
traffic than IMS traffic. In the IMS case the P-CSCF plays the role
of an application function.
About the Author
Himanshoo Kumar Saxena has over a decade of Software product
development experience in convergence domain. He has been
involved in development of various convergence products
including some IMS nodes from concept to finish. He currently
plays role of Consultant Architect at Globallogic.
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Figure 14: Go and Gq Reference Points
Media authorization and correlation of Go and Gq interface is
also shown in the figure 14.
As shown in the above diagram, during media characteristics
negotiation procedures, PCSCF queries PDF to authorize the
media properties that UE is willing to use for call. Later when UE
tries to get the bearer for actually sending media (e.g. PDP
activation through a GGSN), bearer controller (GGSN), acting as
the PEP (policy enforcement point), queries the PDP to ensure
that this usage is properly authorized.
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