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Introduction
Push to talk is the common name for half-
duplex voice services activated by pressing
a button. PoC is the name of the open spec-
ifications 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 seg-
ment, 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)—evi-
dence that Ericsson is taking concrete steps
toward realizing IP services on IMS in mo-
bile 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 applica-
tions. 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 de-
scription 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 negotia-
tion 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 end-
to-end negotiation, new media coders/de-
coders (codec) may be introduced in termi-
nals as they become available.
Signaling compression
When designing SIP, the Internet Engi-
neering 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 re-
source 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 commu-
nication service, push to talk also enjoys the range and wide area cover-
age 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 con-
sisting 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 ser-
vice 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