IMS Network Components
IMS, IP Multimedia Subsystem is an architectural
framework to deliver IP multimedia services to end users.
Designed by the wireless standards body 3GPP, IMS was first destined
[R5] to carry GSM networks beyond GPRS to deliver "Internet
Services" over.
Further revisions by standards bodies 3GPP, 3GPP2 and TISPAN provided
support of networks other than GPRS, such as Wireless LAN, CDMA2000
and Fixed line.
IMS is:
- Hardware independent
- Implemented inside a network
- Uses IP Protocol
- Uses SIP – Internet protocol IETF for session initiation.
IMS will provide a standard method of developing and deploying
applications across both wireless and wireline terminals.
In this way, IMS aids Fxed Mobile Convergence [FMC]. A horizontal
control layer isolates the access network from the service layer,
such that each service does not require its own control functions.
The control layer acts as a common horizontal layer.
There are alternative and overlapping technologies for enabling
the access and provision of services across wired and wireless networks:
- Generic Access Network
- Softswitches
- Naked SIP
However, IMS is widely supported as the preferred access mechanism
going forward.
IP Multimedia Core Network Subsystem
The IP Multimedia Core Network Subsystem is a collection of different
functions, linked by standardized interfaces. When grouped, these
form a single IMS administrative network.
- A function is not a node [hardware box]
- An implementer is free to combine 2 functions in 1 node, or
to split a single function into 2 or more nodes.
- Each node can be present multiple times in a single network,
for load balancing or organizational issues.
Network Access
Users connect to an IMS network using various methods, using standard
Internet Protocol [IP].
Direct IMS terminals [mobile phones, PDAs, computers] register
directly into an IMS network, even when they're roaming in another
network or country. All terminals must use IPv6 and run SIP User
Agents.
IMS Supports
- Fixed access - DSL, cable modems, Ethernet,
- Mobile access - W-CDMA, CDMA2000, GSM, GPRS
- Wireless access - WLAN, WiMAX
- Legacy systems - POTS, H.323 and non IMS-compatible VoIP systems
are supported through gateways.
Core IMS Network Components
The core IMS network consists of
Home Subscriber Server
The HSS [Home Subscriber Server or User Profile Server Function
UPSF] is the master user database that supports the IMS network
entities that handle the calls/sessions. The HSS is similar to the
GSM HLR and AUC in that it:
- Contains the user profiles
- Performs authentication and authorization of the user
- Provides information about the physical location of user.
An SLF [Subscriber Location Function] is needed to map user addresses
when multiple HSSs are used. Both the HSS and the SLF communicate
through the DIAMETER protocol.
User Identities
Identities used in normal 3GPP networks include:
- International Mobile Subscriber Identity [IMSI] - unique user
identity that is stored in the SIM.
- Temporary Mobile Subscriber Identity [TMSI] - generated per
geographical location to preserve user privacy
- International Mobile Equipment Identity [IMEI] - a unique,
phone specific, device identity stored inthe
phone.
- Mobile Subscriber ISDN Number [MSISDN] - the telephone number
of a user.
IMSI/TMSI are both used for user identification.
With IMS, the following additional identities are implemented:
- IP Multimedia Private Identity [IMPI] - unique to the phone
- IP Multimedia Public Identity [IMPU]
These are not phone numbers, but rather URIs [Unique Resource
Identifiers], that can be either digits [phone numbers] or alphanumeric
identifiers [sip email address].
You can have multiple IMPU per IMPI. The IMPU can also be shared
with another phone, allowing multiple devices to map to the same
identity.
The HSS user database contains, but is not limited to, the IMPU,
IMPI, IMSI, and MSISDN.
Call/Session Control
SIP servers or proxies serve several roles, collectively called
CSCF [Call Session Control Function]. These are used to process
SIP signalling packets in the IMS. The primary SIP functions include:
- P-CSCF [Proxy-CSCF]
- S-CSCF [Serving-CSCF]
- I-CSCF [Interrogating-CSCF]
P-CSCF [Proxy-CSCF]
A SIP proxy that is the first point of contact for the IMS terminal.
It can be located either in the visited network [in full IMS networks]
or in the home network [when the visited network isn't IMS compliant
yet]. Some networks might use a Session Border Controller for this
function.
The terminal discovers its P-CSCF with either DHCP, or assigned
in the PDP Context [in GPRS]. This assignment occurs during registration,
and does not change for the duration of the session. The P-CSCF
performs multiple functions:
- sits on the path of all signalling messages
- can inspect every message
- authenticates the user and establishes an IPsec security association
with the IMS terminal.
- prevents spoofing attacks and replay attacks and protects the
privacy of the user.
- Other nodes trust the P-CSCF, and do not have to authenticate
the user again.
- compress and decompress SIP messages using SigComp, which reduces
the round-trip over slow radio links
- may include a PDF [Policy Decision Function], which authorizes
media plane resources e.g. quality of service [QoS] over the media
plane.
- used for policy control, bandwidth management, etc The
PDF can also be a separate function.
- generates charging records
S-CSCF [Serving-CSCF]
A SIP server, located in the home network, that acts as both the
central node of the signalling plane and also performs session control.
The S-CSCF uses DIAMETER Cx and Dx interfaces to the HSS to download
and upload user profiles. It has no local storage of the user. All
necessary information is loaded from the HSS. The S-CSCF:
- handles SIP registrations - binds the user location [ IP address
of the terminal] and the SIP address
- sits on the path of all signaling messages, and can inspect
every message
- decides to which application server[s] the SIP message will
be forwarded, in order to provide their services
- provides routing services, typically using ENUM lookups
- enforces the policy of the network operator
There can be multiple S-CSCFs in the network for load distribution
and high availability reasons.
The HSS assigns the S-CSCF to a user, when queried by the I-CSCF.
I-CSCF [Interrogating-CSCF]
Another SIP function located at the edge of an administrative domain.
Its IP address is published in the DNS of the domain [using NAPTR
and SRV type of DNS records], so that remote servers can find it,
and use it as a forwarding point [e.g. registering] for SIP packets
to this domain.
The I-CSCF queries the HSS using the DIAMETER Cx interface to retrieve
the user location [Dx interface is used from I-CSCF to SLF to locate
the needed HSS only], and then routes the SIP request to its assigned
S-CSCF.
The IBCF is used as gateway to external networks, and provides
NAT and Firewall functions [pinholing].
Application Servers [AS]
Application servers host and execute services, and interface with
the S-CSCF using SIP.
For example, in the Voice Call Continuity Function [VCC Server],
the AS can operate in either:
- SIP proxy mode
- SIP US [user agent] mode
- SIP B2BUA [back-to-back user agent] mode.
An AS can be located in the home network or in an external third-party
network. If located in the home network, it can query the HSS with
the DIAMETER Sh interface [for a SIP-AS] or the MAP interface [for
IM-SSF].
SIP AS: Native IMS application server
IM-SSF: an IP Multimedia Service Switching Function interfaces
with CAMEL Application Servers using CAP
Media Servers
An MRF [Media Resource Function] provides a source of media in
the home network. It can be used for:
- Playing of announcements [audio and video]
- Multimedia conferencing [e.g. mixing of audio and video streams]
- Text-to-speech conversion [TTS] and speech recognition
- Realtime transcoding of multimedia data [i.e. conversion between
different codecs]
- Digit detection
- Custom ring-back tones
- Lawful intercept
- Auto attendants [and announcements]
Each MRF is further divided into :
An MRFC [Media Resource Function Controller] is
a signalling plane node that acts as a SIP User Agent to the S-CSCF,
and which controls the MRFP with a H.248 interface
An MRFP [Media Resource Function Processor] is
a media plane node that implements all media-related functions.
Breakout Gateway
A BGCF [Breakout Gateway Control Function] is a SIP server that
includes routing functionality based on telephone numbers. It is
only used when calling from the IMS to a phone in a circuit switched
network, such as the PSTN or the PLMN.
PSTN Gateways
A PSTN/CS gateway interfaces with PSTN circuit switched [CS] networks.
- For signalling, CS networks use ISUP [or BICC] over MTP, while
IMS uses SIP over IP.
- For media, CS networks use PCM, while IMS uses RTP.
An SGW [Signalling Gateway] interfaces with the signalling plane
of the CS. It transforms lower layer protocols as SCTP [which is
an IP protocol] into MTP [which is an SS7 protocol], to pass ISUP
from the MGCF to the CS network.
An MGCF [Media Gateway Controller Function] does call control
protocol conversion between SIP and ISUP and interfaces with the
SGW over SCTP. It also controls the resources in an MGW with an
H.248 interface.
An MGW [Media Gateway] interfaces with the media plane of the CS
network, by converting between RTP and PCM. It can also transcode
when the codecs don't match [e.g. IMS might use AMR, PSTN might
use G.711].
Media Resources
Media Resources are those components that operate on the media
plane and are under the control of IMS Core functions. Specifically:
Border GateWay [BGW]
A BGW [Border GateWay] operates under the control of the IBCF
and interfaces with the media plane of another NGN - eg another
SP [Service Provider] who has similarly migrated to IMS - and provides
a media proxy of the RTP packets.
It can also transcode when the codecs and/or packetisation period
don't match [e.g. SPa uses G.729 30ms and SPb uses G.711 20ms].
Charging
Charging is either done offline or online:
- Offline charging - applied to users who pay
for their services periodically [e.g., at the end of the month].
- Online charging - credit-based chargingused
for prepaid services, or real-time credit control of postpaid
services.
Both may be applied to the same session.
Offline Charging
All the SIP network entities [P-CSCF, I-CSCF, S-CSCF, BGCF, MRFC,
MGCF, AS] involved in the session use the DIAMETER Rf interface
to send accounting information to a CCF [Charging Collector Function]
located in the same domain.
The CCF will collect all this information, build a CDR [Call Detail
Record], and send this to the billing system [BS] of the domain.
Each session carries an ICID [IMS Charging Identifier] as a unique
identifier. IOI [Inter Operator Identifier] parameters define the
originating and terminating networks.
Each domain has its own charging network. Billing systems in different
domains will also exchange information, so that roaming charges
can be applied.
Online charging
The S-CSCF talks to an SCF [Session Charging Function].
The SCF can signal the S-CSCF to terminate the session when the
user runs out of credits during a session.
The AS and MRFC use the DIAMETER Ro interface towards an ECF [Event
Charging Function].
When IEC [Immediate Event Charging] is used, a number of credit
units is immediately deducted from the user's account by the ECF.
The MRFC or AS is then authorized to provide the service. The service
is not authorized when not enough credit units are available.
When ECUR [Event Charging with Unit Reservation] is used, the
ECF first reserves a number of credit units in the user's account
and then authorizes the MRFC or the AS.
After the service is over, the number of spent credit units is
reported and deducted from the account; the reserved credit units
are then cleared.
Reference:Wikipedia
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