Ericsson CDMA2000 specifications Introduction, SIP-based service, Signaling compression

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 familyÑmuch 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.

EricssonÕs practice of basing products on open standardsÑto ensure interoperability and rapid uptake of serviceÑalso 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.

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

BOX A, TERMS AND ABBREVIATIONS

3GPP	Third-generation Partnership Project	IETF	Internet Engineering Task Force
AMR	Adaptive multirate	IMS	IP multimedia subsystem
ASCII	American Standard Code for Infor-	IP	Internet protocol
	mation Interchange	IPMM	IP Multimedia
AUC	Authentication center	IS/IT	Information system/information
BSS	Base station subsystem		technology
CDMA	Code-division multiple access	MRF	Media resource function
CS	Circuit-switched	OMA	Open Mobile Alliance
CSCF	Call/session control function	PoC	Push to talk over cellular
DNS	Domain name service	QoS	Quality of service
EDGE	Enhanced data rates for GSM evolu-	RTCP	RTP control protocol
	tion	RTP	Real-time transport protocol
EVRC	Enhanced variable rate codec	SDP	Session description protocol
FTP	File transfer protocol	SIP	Session initiation protocol
GLMS	Group list management server	UDP	User datagram protocol
GPRS	General packet radio service	UDVM	Universal decompression virtual
HLR	Home location register		machine
HSS	Home subscriber server	URI	Uniform resource indicator
HTTP	Hypertext transfer protocol	WCDMA	Wideband CDMA

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

EricssonÕs 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)Ñevi- 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 operationÑthat 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