In recent times, wireless local area networks (WLAN) technology based on the 802.11 family has become a resource in solving communication needs in multiple applications. Some of the applications that have gained momentum are those related to the communications that not only transport LAN data or web type Internet access, but also mainly IP-based voice and video. This is possible because of the availability of proprietary and standardized (IEEE802.1le) techniques for traffic prioritization. This circumstance has enabled the possibility to have not only WLAN-based handhelds, but also dual-mode terminals including cellular and WLAN.
There are many scenarios in which wireless convergent voice and video over IP are being considered for both private and public applications. In private communications, companies already using WLAN for LAN data-only are considering the replacement of their obsolete TDM PBXs with new communications methods capable of handling VoIP, and therefore merging the data and voice networks.
In this scenario, the availability of dual-mode cellular and WLAN voice handsets enables the possibility to migrate the cordless communications to the WLAN domain, to take advantage of the lower rates in the fixed networks with respect to the cellular. There are different approaches to define reference architectures for a communication convergence solution based on dual-mode handsets. The objective of these kinds of solutions is to provide a seamless mobility user experience, whether the user is under cellular (GSM or CDMA) or WLAN coverage, ensuring service continuity for both, voice and data, when roving between GSM and WiFi areas. These solutions offer high value to the end-user in terms of convenient improvements by providing unique number and unique handset. Although still in an early phase for adoption and waiting for a wide availability of dual-mode handsets, there are already several initiatives being carried out by main service providers around the world, some of them with commercial deployments. Three different architecture alternatives have been conceived to support cellular/wireless convergence.
Enterprise-Based Architecture
Some PBX manufacturers are implementing “wireless extensions” over WiFi. These are proprietary solutions based on dual-mode handsets, running a specific voice client in charge of interwork with the communication server at the PBX structure. As a matter of fact, this is the same principle that is applied earlier to pure WLAN-based handsets. From a user experience standpoint, the handset not only allows mobility around the campus, but enables access to the PBX supplementary services as well.
Unlicensed Mobile Access
This is a mobile oriented solution promoting basically an alternative access to GSM/GPRS network. From a commercial point of view, it can be seen as a complement to low-quality coverage areas, a lower cost alternative access network, or simply as a way of accelerating fixed mobile substitution. Dual-mode handsets with specific Unlicensed Mobile Access (UMA) clients are linked to WiFi access points which, in turn, are connected throughout a broadband link (ADSL or cable-modem) to a specific UNC (UMA Network Controller), working very much like a BSC (Base Station Controller) in a cellular network. In fact, it is connected to the core network using the same interfaces, A for GSM circuit-switching network, and Gb for GPRS data network. The main advantages of this architecture are total service continuity (voice, SMS, and data sessions) and handover between cellular and wireless areas. However, it lacks flexibility for local switching and horizontal handover in campus deployments. UMA is being specified by 3GPP body on GAN (Generic Access Network) and supported by the FMCA (Fixed Mobile Convergence Alliance).
SIP/WiFi Architecture
The third alternative for fixed mobile convergence is a network-based architecture, aligned with IMS (IP Multimedia Subsystem) standards and specified by 3GPP in the VCC (Voice Call Continuity) Technical Report and supported by the FMCA. It is a pure SIP-based solution, with a multimedia SIP client installed on the dual-mode handset. Presence, numbering, and routing decisions are taken at a network application Server, connected to a VoIP network and, optionally, to a cellular network via a CAMEL signaling interface. At the access layer, a variable number of WiFi access points are deployed at customer premises (or at public hotspot) depending on the area to be covered, the number of users, and voice traffic. These access points are controlled by a WiFi Network Controller, implementing Fast Roaming capabilities for horizontal handover. This handover is achieved by means of break-before-make and acceleration of authentication using an authentication proxy. Moreover, the WiFi access network support mechanisms to separate Virtual WLANs and differentiate voice and data packets. Vertical handover, from WiFi to GSM, and the other way around, uses a “make-before-break” procedure, which is controlled and instructed by the Application Server, and synchronized with the SIP client running on the dual-mode handsets. This handover is based on signal quality measurements and uses hysteresis between two thresholds.
