IMS Overview - EAAC-recommendations

rabidwestvirginiaNetworking and Communications

Oct 26, 2013 (3 years and 9 months ago)

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IP Multimedia Subsystem (IMS)


The IP Multimedia Subsystem (IMS) is a framework for the delivery of Internet Protocol (IP) multimedia
services.


Development of IMS began in 1999 in the international standards development organization called 3
rd

Generation Partnership Project (3GPP). The initial standardization efforts were focused on the evolution
of the GSM wireless network from the legacy circuit switched telecommunications model to a packet
based IP based telecommunications model. Later rele
ases of the IMS standards were expanded to
incorporate other access technologies as well as to facilitate the support of fixed
-
mobile convergence
(FMC). The IMS now supports a full set of access technologies including the following:


-

GSM family of access
technologies (GSM, EDGE, GPRS)

-

CDMA2000 family of access technologies

-

Long Term Evolution (LTE) wireless access technology

-

Wireless LAN

networks

(e.g., WiFi networks)

-

Packet cable

networks

-

Fixed wired networks

-

Femto cells


Also, the IMS in conjunction with

the LTE wireless access technology has been selected as the
architectural framework for the next generation Public Safety network in the United States in order to
provide an end
-
to
-
end nationwide interoperable
interconnected
network.


In order to facilita
te interoperability and interconnection with the Internet and other global networks,
the IMS uses ITU and IETF protocols wherever possible. For example:

-

The IMS uses the Session Initiation Protocol (SIP) for call session transaction processing.

-

The Diam
eter protocol is used for functions such as user profiles, policy controls, and charging
information.

-

The H.248 protocol is used for multimedia streams.


The IMS provides a horizontal control layer that isolates the various access networks from the
applic
ations being developed at the service layer. This layer of isolation provides for simplification of
the application development since the application developer is not required to know the control details
of each individual type of access network. This la
yer of isolation also allows the application to be
interoperable across the entire set of access technologies.


The types of functions performed within the IMS include the following:

-

Subscriber profiles

-

Authentication services

-

Authorization services

-

Network policy enforcement

-

Quality of service

-

Priority services

-

Emergency call handling

-

Multimedia emergency call handling (currently in development)

-

Multimedia content control

-

Charging control including support of pre
-
paid accounts

-

Voice call continuity

between packet switch networks and legacy circuit switched networks

-

Packet based text messaging (to replace the existing circuit switched SMS service)

-

IP session security

-

Interconnectivity with legacy PSTN networks


The IMS emergency services specificatio
ns define a complete end to end system for emergency call
support between end users device and a PSAP over managed IP networks (i.e., not the Internet). The
following describes the capabilities provided in the 3GPP specifications.


The IMS specifications
define not only use of IETF protocols (with enhancements) in an IMS
environment, but the full end to end system aspects including emergency session establishment
(multimedia sessions using the multimedia emergency services specification is currently under
development), bearer establishment and management for the type of bearer used for the emergency
session, quality of service/quality of experience (which includes priority handling of bearers where
appropriate and managing end
-
to
-
end latency), routing and
mobile device location determination,
media gateway for interconnection, interoperability and control of multimedia communications, policy
control, signaling compression for minimizing the SIP signaling load over the air interface, and interface
to the ESI
net (ATIS enhancements to the NENA i3 spec are currently in development to account for IMS
-
based originating networks).


The 3GPP “Evolved Packet System” or EPS is effectively a connection
-
oriented transmission network
and, as such, it requires the establi
shment of a “virtual” connection between two endpoints before any
traffic can be sent between them. This virtual connection is called an “EPS Bearer”, which emphasizes
the fact that the virtual connection provides a “bearer service”, i.e. a transport servi
ce with specific QoS
attributes. “EPS bearer” has a QoS Class Index (QCI) that describes the type of service that makes use of
the virtual connection (e.g. conversational voice, streaming video, signaling, best effort, etc); optionally,
the bearer may hav
e a “flow specification” that describes the guaranteed and maximum bitrate (GBR,
MBR) of the aggregate traffic flow that goes through the virtual connection. There is also a filter
specification that describes the traffic flows (in terms of IP addresses, p
rotocols, port numbers, etc) for
which the transport service is provided between the two endpoints.


The IMS is used to set up 3GPP Evolved Packet System (EPS) bearers. The characteristics of these bearers
are specified by a QoS class identifier (QCI), fo
r example QCI1 which is a guaranteed bit
-
rate bearer for
VoLTE media, QCI5 which is a high
-
priority non
-
guaranteed bit
-
rate bearer for IMS signaling messages,
QCI6 for Video (Buffered Streaming) and TCP
-
based (e.g., www, e
-
mail, chat, ftp, p2p file sharing
,
progressive video, etc.).


QCI classes 1
-
4 are Guaranteed Bit Rate classes, and QCI classes 5
-
9 are Non
-
guaranteed bit rate classes.
Each QCI class also has a Packet Delay Budget and Packet Error Loss characteristic.

The LTE Radio Access Network is res
ponsible for admission control for these bearers, and will manage
admission to allow for sufficient capacity across the air interface and the transport network resources.
Once admitted, the RAN will use the QoS parameters to invoke the appropriate scheduli
ng policies to
meet the required end
-
to
-
end delay and packet loss rate targets. The RAN also has a number of tools to
minimize packet sizes (robust header compression (ROHC) and radio link control (RLC) unacknowledged
mode with short packet data convergenc
e protocol (PDCP)/RLC sequence numbers) to maximize the
number of mobile phones that can transmit/receive VoIP packets in a given cell. In addition, the RAN has
to ensure that end
-
to
-
end delay budgets and packet
-
loss rates are met for media packet (for exa
mple, a
200ms mouth to ear delay on VoIP packets). There are link
-
adaptation algorithms to statistically ensure
that the VoIP packets are successfully transmitted within the loss targets. The packet loss rate and
packet delay budget must also take into acc
ount cell handover. Another very important aspect of VoIP
support on the radio access network is the need to maximize user talk time; to help with this, LTE allows
UEs to turn off their transceiver periodically between VoIP frames.


The QoS parameter ARP c
ontains information about the priority level. The priority level defines the
relative importance of a resource request. This allows deciding whether a bearer establishment or
modification request can be accepted or needs to be rejected in case of resource
limitations (typically
used for admission control of GBR traffic). The range of the ARP priority level is 1 to 15 with 1 as the
highest level of priority.


Emergency bearer services (i.e. a session for the IMS emergency call) are provided by the serving
network to support IMS emergency when the network is configured to support emergency services. The
Policy Control Rule Authorization and QoS Rule generation f
unction selects QoS parameters that allow
prioritization of IMS Emergency calls. If an IMS Emergency call is prioritized the QoS parameters shall
contain an ARP value that is reserved for intra
-
operator use of IMS Emergency calls. Within the IMS
architect
ure, the Policy Control Resource Function, or PCRF, determines if a particular session is an IMS
emergency session (e.g. emergency call or PSAP callback) and makes authorization and policy decisions
that restricts the traffic to emergency destinations, IMS

signalling and the traffic to retrieve user location
information (in the user plane) for emergency services. If the mobile also has Multimedia Priority
Service, the PCRF generates the corresponding Policy/QoS rule(s) with the ARP/QCI parameters as
appropr
iate for the prioritized service.


In a non
-
IMS system that only uses “IETF protocols”, none of these critical functions are performed,
thereby not having that capability to guarantee priority.