Ericsson CDMA2000 User Manual

Introduction

Push to talk is the common name for half­duplex voice services activated by pressing
a button. PoC is the name of the open spec­ifications for this service. In the consumer
segment, push to talk allows users to stay in
touch with friends and coordinate leisure
activities, such as visits to the cinema or
simultaneous communication with a group
of family members. In the enterprise seg­ment, it can be used to share information in

a group—for instance, a field technician can
use it to ask colleagues for help or advice.

Ericsson’s Instant Talk solution complies
with the PoC specifications. Its users can
make person-to-person calls or create an
ad hoc group call. At present, it is available
for implementation in GPRS/EDGE and
CDMA2000 networks. WCDMA solutions
will follow. PoC and Instant Talk are based
on the IP Multimedia System (IMS)—evi­dence that Ericsson is taking concrete steps
toward realizing IP services on IMS in mo­bile networks.

1,2

SIP-based service

Ericsson Instant Talk is an important step
toward making IP-based services an integral
part of the mobile service offering and of
making the Internet protocol a dominant
transport technology for mobile applica­tions. The solution is controlled by SIP,
whose philosophy of client-to-client control
over multimedia session establishment
while drawing on support from servers in
the network distinguishes it from other
common application layer protocols (FTP,
HTTP).

3

Because SIP can establish, modify and
terminate multimedia sessions between two
or more clients, it facilitates one-to-many
communication, which is a key feature of
Instant Talk. Clients use the session de­scription protocol (SDP), contained in the
body of SIP messages, to describe what kinds
of media they can use and how the media
should be transported. End-to-end negotia­tion of media types and transport permits
tandem-free operation—that is, the coded
bit stream that contains the talk burst is sent
directly to the encoder in the receiving
terminal without being decoded in the base
station subsystem (BSS). Thanks to SIP end­to-end negotiation, new media coders/de­coders (codec) may be introduced in termi­nals as they become available.

Signaling compression

When designing SIP, the Internet Engi­neering Task Force (IETF) had the large
bandwidth and low latencies of the public
Internet in mind, which is to say the size of
SIP messages was not a priority. But in the
context of mobile environments the size of
SIP messages can be a drawback. In mobile
systems, radio spectrum is an expensive re­source that must be used wisely. Operators
must always try to maximize data transport

16

Ericsson Review No. 1, 2004

Ericsson Instant Talk

Niclas Medman, Krister Svanbro and Per Synnergren

Push to talk is a quick and informal way of communicating person-to-person
and with groups. With a simple push of a button, users can activate voice
communication with friends and family—much the same way as using
walkie-talkies or private mobile radios. But because it is a mobile commu­nication service, push to talk also enjoys the range and wide area cover­age of traditional mobile services.

Ericsson Instant Talk is a voice-over-IP (VoIP) service set up using the
session initiation protocol (SIP). The technical realization is based on a
total business approach that gives operators a complete solution con­sisting of the Instant Talk Application Server, the IP Multimedia (IPMM)
system, terminal clients, and professional services.

Ericsson’s practice of basing products on open standards—to ensure
interoperability and rapid uptake of service—also applies to Instant Talk.
The solution is fully compliant with the push to talk over cellular (PoC)
specification. It has also been built with service evolution in mind, giving
operators the ability to add new features and services to enrich the ser­vice and the IPMM.

3GPP Third-generation Partnership Project
AMR Adaptive multirate
ASCII American Standard Code for Infor-

mation Interchange
AUC Authentication center
BSS Base station subsystem
CDMA Code-division multiple access
CS Circuit-switched
CSCF Call/session control function
DNS Domain name service
EDGE Enhanced data rates for GSM evolu-

tion
EVRC Enhanced variable rate codec
FTP File transfer protocol
GLMS Group list management server
GPRS General packet radio service
HLR Home location register
HSS Home subscriber server
HTTP Hypertext transfer protocol

IETF Internet Engineering Task Force
IMS IP multimedia subsystem
IP Internet protocol
IPMM IP Multimedia
IS/IT Information system/information

technology
MRF Media resource function
OMA Open Mobile Alliance
PoC Push to talk over cellular
QoS Quality of service
RTCP RTP control protocol
RTP Real-time transport protocol
SDP Session description protocol
SIP Session initiation protocol
UDP User datagram protocol
UDVM Universal decompression virtual

machine
URI Uniform resource indicator
WCDMA Wideband CDMA

BOX A, TERMS AND ABBREVIATIONS

Ericsson Review No. 1, 2004

17

efficiency over the air interface. This prob­lem has been addressed through signaling
compression (SigComp), another IETF stan­dard initiated by Ericsson. SigComp is a ver­satile compression framework that can use
any compression algorithm to compress
ASCII-based protocols, such as SIP, to a frac­tion of their original size, reducing both bit
rate requirements and transport latency.

SigComp sessions are initialized during
the registration phase of the Instant Talk
service. The SigComp compressor generates
a decompression byte code that contains a
preferred compression algorithm and trans­mits it to the decompressor. The decom­pressor, called the universal decompression
virtual machine (UDVM), decodes the byte
code and adopts the decompression algo­rithm, making it ready to receive com­pressed SIP messages. Extended operations
(an important feature) enable SigComp to
learn from SIP messages.4Therefore,
SigComp becomes increasingly efficient as
compression and SIP signaling proceed.

SigComp is a critical component of
Ericsson Instant Talk because it can signif­icantly reduce the number of bits sent over
the wireless link (compression ratios of 8:1
are not unusual).5It greatly reduces the
transmission delay of SIP messages, and con­sequently, the time it takes to establish an
Instant Talk session.

Technical realization

Ericsson has taken a total business approach
in its technical realization of Intant Talk,
providing the infrastructure, terminal
clients and professional services, and ensur­ing availability of terminals. Guided by this
approach, Ericsson will continue to opti­mize performance by implementing en­hancements in the radio access network, mo­bile core network, application servers and
clients. Ericsson’s ability and willingness to
influence the nodes in the end-to-end path
of telecommunications systems set it apart
from other vendors of push to talk solutions.
The commercial launch of Ericsson Instant
Talk will include vertical service assurance.

One more important factor is that
Ericsson bases its products on open stan­dards. Therefore, handset interoperability
will not be an issue. Operators and end-users
will be able to choose terminals and termi­nal vendors freely.

Instant Talk consists of three main parts:
the IPMM, the Instant Talk application
server, and the handset client.

IP multimedia system

Figure 1 shows a schematic drawing of the
nodes involved in the Instant Talk service.
At the heart of the solution is the Ericsson
IPMM system (Figure 2), which complies
with the principles of the IMS standard
(3GPP) that was drafted to bring SIP-based
communications to the wireless market.

2

The IMS can be deemed a “new” domain that
has been added to the mobile core network
to support a wide range of SIP-based appli­cations, such as Instant Talk, instant mes­saging, and presence services.

The IPMM architecture includes the
call/session control function (CSCF), the
media resource function (MRF), and the
home subscriber server (HSS). The CSCF is
the teminal’s first point of contact in the
IPMM domain. All SIP signaling is routed
through the CSCF, which also performs
SigComp. The role of the CSCF is to handle
subscriber registration, and to support the
establishment, modification and release of
Instant Talk sessions. The CSCF ensures
interoperability with telephone systems and
network addressing mechanisms by query­ing domain name service (DNS) servers to
map SIP uniform resource identifiers (URI)
or E.164 numbers to network addresses.

In an IPMM-based service, media is di­rected to, replicated in, and distributed from

Access network

(GPRS,

CDMA2000

WCDMA)

Instant Talk client

Instant Talk
application

server

Core

network

SIP

Signaling

RTP

Media

Call/session
control
function
(CSCF)

Home
subscriber
server
(HSS)

Media
resource 
function
(MRF)

Figure 1

Ericsson Instant Talk high-level architecture.