Article about IMS by David Geer

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Computer

I N D U S T R Y T R E N D S

Published by the IEEE Computer Society

or several years, telecommuni-

cations providers have touted

the potential of converged net-

works that offer a wide range

of voice, data, and multimedia

services, all over a single IP infra-

structure.

These networks—which differ from

today’s disparate voice, data, signaling,

and control networks—would be

based on open standards rather than

proprietary approaches and let users

work with applications or one another

across wireless or wireline platforms.

However, these networks have been

just a vision until recently. Now,

though, a growing number of telecom-

munications carriers and equipment

vendors—including Alcatel, British

Telecommunication, Ericsson, Lucent

Technologies, Motorola, and Nokia—

are beginning to release devices and ser-

vices based on a convergence approach

called IP Multimedia Subsystem.

IMS is the culmination of technol-

ogy standards by the Internet Engi-

neering Task Force (IETF) and two

Third Generation Partnership Project

groups (the 3GPP and 3GPP2).

IMS supports many types of com-

munications, including instant messag-

ing (IM), push-to-talk cellular walkie-

talkie service, videoconferencing, and

video on demand. It also provides

authentication and, for wireless ser-

vices, roaming capabilities.

Converged networks offer carriers—

which are always looking for income

sources—the advantages of using a sin-

gle network infrastructure to quickly

implement new, revenue-producing ser-

vices that customers can work with

regardless of the platform they use, said

Alain Mouttham, chief technical officer

of SIPquest, an IMS application vendor.

In the process, customers would gain

access to a variety of seamlessly acces-

sible services.

However, proponents must over-

come several key technical hurdles and

convince potential users of IMS’s

advantages.

DRIVING FORCES

“The acceptance of new non-voice

mobile offerings shows that many

users want richer, multimedia commu-

nications services available wherever

they are,” said Greg Patterson, senior

member of Bell South’s architecture

and technology technical staff.

Major wireless and wireline network

service providers are supporting IMS to

generate revenue and control expenses.

Large networking-equipment vendors

and smaller companies that provide

applications and ancillary equipment

are also adopting the technology,

Patterson noted.

In the past, providers offered con-

verged services via time-consuming,

expensive integration that required

them to build support for the tech-

nologies behind each service.

Proponents say IMS eliminates the

need for such difficult integration

because it provides a common service

environment on which to make ser-

vices available.

ALL ABOUT IMS

The 3GPP—a group of standards

bodies, mostly from Europe, that sets

third-generation telecommunications

specifications—initially developed

IMS for W-CDMA (wideband code-

division multiple-access), the 3G ver-

sion of GSM (Global System for

Mobile Communications) cellular

technology.

The 3GPP2—a collaborative project

of North American and Asian stan-

dards organizations—subsequently

adapted IMS for use with CDMA2000,

the 3G version of CDMA.

In essence, IMS, using open inter-

faces, would serve as its own underly-

ing network platform for routing com-

munications based on various net-

working protocols.

Figure 1 shows a basic IMS archi-

tecture.

IMS protocols

IMS works with several important

protocols.

SIP. IMS uses the IETF’s Session

Initiation Protocol for session control

and signaling. Thus, any device with

SIP-compatible software and an SIP

address, which is

a kind of uniform

resource identifier

, can participate in

IMS sessions.

SIP establishes IP connections and

specifies how to initiate, modify, or ter-

minate an interactive user session

involving multimedia.

COPS. IMS uses the IETF’s Common

Open Policy Service protocol to ensure

quality of service, which is important

for telephony and other traffic that

doesn’t tolerate latency. COPS enables

the communication of QoS and other

Building Converged

Networks with

IMS Technology

David Geer

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work Solutions Group. This enables

users to obtain the services they request

from applications to which they have

access.

Security

IMS isn’t designed to prevent hacker

attacks, explained Marchetti. Outside

technologies, such as firewalls, must

provide that capability, he explained.

IMS uses authentication for wire-

line-access security.

IMS uses the IETF’s HTTP Digest

Authentication protocol for mobile

network-access security, explained

Greg Carter, IMS solutions manager

for mobile-telecommunications vendor

Ericsson.

Using HTTP Basic Authentication,

IMS transmissions between client and

server would be unencrypted and

could be intercepted. HTTP Digest

Authentication lets a client prove to a

server that it knows the password

without having to send the password

in the clear. The client performs a com-

traffic policy information between a

policy server and clients.

Diameter. IMS’s home subscriber

server (HSS) is the system’s master

database of information about sub-

scribers, including their names and

locations, services they have permis-

sion to access, and data to be used

with the authentication and autho-

rization processes.

IMS uses the IETF’s Diameter proto-

col to let devices access the HSS and

then provide the necessary authentica-

tion, authorization, and, for billable

communications, accounting services,

said Emerio Marchetti, service provider

Sprint Nextel’s senior director of net-

work engineering operations.

IMS network elements

There are several major IMS net-

work elements in addition to the HSS.

Call session control functions. The

three CSCFs process SIP signaling

packets and provide control and rout-

ing functions for sessions. The serving

CSCF handles session control services

and application invocation. The inter-

rogating CSCF is the entry point for

communications in the home network.

The proxy CSCF is the entry point for

an outside network and is used when

roaming takes place.

SIP application server. This server

hosts and executes IMS-based services.

Access gateway. This element pro-

vides an interface between the wireless

or wireline network used for data

transmission and the IP-based net-

work used for processing.

PSTN gateway. The public switched

telephone network gateway interfaces

with the traditional circuit-switched

telephone network by providing sig-

nal and coding translation between IP

and phone systems.

Breakout gateway control function.

The BGCF offers routing functional-

ity based on telephone numbers and is

used only in communications between

an IMS network and a phone in a tra-

ditional circuit-switched system.

Policy decision function. The PDF

assigns bandwidth, resources, and

packet priorities for sessions based on

both QoS requirements and demand

on the system.

Foreign and home agents. For

mobile users, the foreign agent is a

router that serves a device being used

away from its home network. For

transmissions to the device, the home

agent acts as a router on the home net-

work that tunnels packets to a tempo-

rary care-of address. The foreign agent

picks up the data from this address and

delivers it to the device.

Interfaces

IMS uses open interfaces to let dif-

ferent sets of functions interact, even

across platforms. SIP is the technol-

ogy’s primary open interface, accord-

ing to Sprint Nextel’s Marchetti.

The standardized IMS service con-

trol interface enables connectivity

between the session manager and SIP

application servers, explained Mike

Morris, director of converged core

solutions in Lucent Technologies’ Net-

November 2005

Media

gateway

Service

control

Media

server

Home

subscriber

server

Session

control

Circuit-switched

and mobile telephone

networks

world

Source: Infitel

Signaling: SIP

Payload: RTP/UDP/IP

Figure 1. IMS supports many types of communications over various kinds of wireline and

wireless networks. IMS integrates all services on an IP-based infrastructure—using IP,

the User Datagram Protocol (UDP), or the Real-Time Transport Protocol (RTP)—and

provides session control and signaling via the Session Initiation Protocol (SIP).

Page 3

Computer

dependency on a single vendor and lets

companies look for the best-of-breed

and/or best-priced products.

New services and applications

IMS is important to telecommuni-

cations carriers because it lets them sell

new revenue-generating services that

would otherwise be too expensive or

complex to provide, according to

Sprint Nextel’s Marchetti.

Service providers could introduce

new applications more quickly than

before, simply by writing them to the

single set of IMS standards, rather than

to the standards for the multiple net-

works on which they might be used.

This capability would let providers

meet changing customer demands and

yield profits more quickly.

Carriers could also more easily cus-

tomize services and applications for

sale to customers.

Based on potential customer

demand, the most popular new IMS

services will probably be push-to-talk,

already available on General Packet

Radio Services networks; presence,

which provides information on other

users’ availability on a network; video

sharing during voice sessions; the shar-

ing of other types of content such as

photos; and IM, explained Marchetti.

CHALLENGES

Because IMS is an enabling platform,

rather than an application, potential

users may not clearly understand its

potential benefits and savings, and this

could slow adoption, explained

Ericsson’s Carter.

Also, while wireline service pro-

viders are beginning to move to IP,

which is necessary for IMS adoption,

mobile carriers are still focused on

their cellular networks and will be

putation based on the password and a

random value supplied by the server.

The result is transmitted to the server,

which performs the same computation

and, if it obtains the identical answer,

authenticates the client.

IMS also provides access security by

using authentication via information

in GSM-based mobile devices’ sub-

scriber identification modules.

IMS also uses authentication for

wireline-communications security.

ADVANTAGES

IMS offers numerous advantages.

For example, IMS makes manage-

ment easier because an administrator

has just one IP network to handle, not

separate networks for each service, said

Dick Martens, director of product mar-

keting and strategy for Infitel, a tele-

communications application vendor.

Versatility

IMS supports IP-based services such

as video, Internet telephony, and con-

ferencing over wireline, cellular, Wi-Fi,

WiMax, Ethernet, optical, DSL, cable,

and other types of networks.

Among other advantages, this lets

users choose the type of network that

gives them the best price, bandwidth,

or availability at a particular time and

location.

Easy service integration

IMS would let users easily integrate

multiple services on a single IP-based

network.

Without IMS, companies would

have to reengineer many functions each

time they add a new application. For

example, companies would need sepa-

rate voice-mail infrastructures for wire-

line and wireless telephone systems.

Not having to build new network

infrastructures for each service saves

equipment. IMS would thus let users

more easily adopt a wide range of

applications to create a customized

suite of communications capabilities.

In addition, companies using IMS

could mix products from various ven-

dors in their systems. This reduces

slower to support the new technology,

said senior analyst Keith Nissen with

In-Stat, a market research firm.

Eventually, though, mobile service

providers will increasingly adopt IP

and IMS as a way to offer different

types of multimedia services over a sin-

gle 3G network, explained Henry

Goldberg, In-Stat senior analyst for

networking.

Carter said that while IMS provides

open network interfaces, the technol-

ogy will still have to work with inter-

faces to specific cellular phones.

“There needs to be a standard IMS

client interface on the phone,” he

explained. “Otherwise, for each IMS

application, there will be a different

implementation in the handset and a

different user interface, defeating the

purpose of IMS.”

Also, noted 3GPP project manager

Alain Sultan, “IMS is a brand new tech-

nology. It still needs lots of testing.”

oldberg said, “Wider adoption of

3G cellular technology will pro-

vide additional bandwidth to

support IMS networks, which will

enable new applications such as video

sharing.”

Wireline and wireless IMS systems

will also gain user-location capabilities,

which will particularly help emergency

services, observed Sultan.

According to Bell South’s Patterson,

as companies increasingly adopt IMS,

they may add proprietary extensions,

which would reduce or eliminate some

of the technology’s benefits.

Added Goldberg, “Providers must

overcome a list of challenges to fully

migrate to a converged-networks

architecture for their wireline and

wireless businesses.” I

David Geer is a freelance technology

journalist based in Ashtabula, Ohio. Con-

tact him at david@geercom.com.

I n d u s t r y T r e n d s

Editor: Lee Garber, Computer,

l.garber@computer.org

IMS would let users

easily integrate multiple

services on a single

IP-based network.