Laying down the packet network requires planning so you can maintain quality of service.

It's pretty clear that the next generation of digital wireless networks will be IP-based. IP offers interoperability between diverse networks and the delivery of a new generation of IP-delivered subscriber services. The method for migrating in a seamless fashion from a circuit-based digital wireless network to one that is packet-based is the question at hand. One thing is known. Implementing these technologies will not be wholesale migrations. Rather they will begin with beta trials to establish a road map with respect to software and hardware reconfigurations.

A Phased Implementation
There are many issues that must be resolved in migrating from a 1G circuit-based network to a 3G packet-based network. Phase I migration begins by establishing an architecture to support a packet radio system. Initially, this is accomplished by migrating the base-station transceivers and the software that packetizes calls coming from the radio air network (RAN). By enabling packet-data services, it's possible to bypass the MSC, which allows data traffic to be routed directly from the RAN to an IP network. To do this, a "data overlay" is required to support the transport of packetized data and to bypass the MSC. One approach requires the establishment of a GPRS serving node (GSN) and a gateway GPRS serving node (GGSN) capability. These serve to support the receipt and transport of packet data from the RAN to an IP network. Such a strategy allows you to leverage IP-enabling data services as the foundation for a core IP network.

During Phase II, you might select additional data elements to support the core network. This would be a logical expansion of the data-services network because the core network will be delivering IP-based services to the subscriber along with any voice services.

Phase III adds network elements to the core network for delivering IP-based services, including voice over IP (VoIP). You would build out the core network with Internet working gateways, packet voice control, signaling and new feature servers. In this phase, the circuit-based MSC would continue to decline in favor of the gains delivered by packetized voice, IP-based authentication, security and mobile management. The addition of a public-switched telephone network (PSTN) and media gateways enhance the MSC functions for interface to the PSTN. These gateways also serve as a VoIP gateway supporting the interface between the IP network and the PSTN. The gateway function converts the voice call encapsulated in an air-interface frame and an IP end point such as IP-enabled device (for example, an IP phone).

During Phase III, you will see the introduction of feature servers to support service creation, service delivery, subscriber authentication, etc., which will position you to support end-to-end IP-based services.

Good to Go
Service providers face numerous issues as they approach the migration process for a circuit-switched network. First, you aren't likely to discard equipment before its programmed life is complete. Second, migration to a new service level is predicated on a deliberate planning cycle, which incorporates the demands of subscribers and the ability of network enhancements to support subscriber demands. Finally, before any wholesale introduction of network enhancements, there must be an adequate trial period to determine necessary modifications to ensure the success of the final network upgrades of either hardware or software.

Bell Atlantic Mobile (BAM) is a good example of a planned move from a circuit-based network to a packet-based network. Today, BAM provides analog and digital mobile services, which are based on digital mobile-switching facilities. BAM's 3G migration plan will involve a graduated process. Initially, BAM will upgrade its base-station infrastructure to support the 1X radio-transmission-technology (1XRTT) standard. This standard is based upon the cdma2000 standard that allows its networks to support more subscribers and improve data performance based upon standards being approved by the International Telecommunications Union (ITU). Although this move will improve data transmission greatly, the improvement won't be perceptible to many network users until more services are accessed from the Internet. A key planning issue has been the ongoing improvement of network reliability through enhancement of the call-control and mobility-management process for data services. This has meant upgrading the core system software to include packet-scheduling algorithms, which manage and control the wireless system.

BAM Migration for Expanded Traffic Load
Gerry Flynn, BAM executive director for technology development, explained that BAM's migration has been engineered based upon its current subscriber base and 30% penetration rate. To meet the anticipated growth rate, BAM will continue to expand its service footprint to accommodate additional subscribers. This expansion will affect the overall voice- and data-transmission rates, and the 1XRTT migration will allow BAM to carry this expanded traffic load.

Next year, BAM will continue to upgrade its software in its core systems based upon ITU 2000/cdma2000 standards, which will bring BAM's networks up to 144kb/s packet-transmission levels. This will allow BAM to support improved throughput requirements in response to an expected subscriber demand for more Internet-delivered services.

By 2002, BAM expects that it will be able to take advantage of its switch vendor's ongoing core system improvements, which will support improved spectral efficiencies. It's expected that cdma2000 technology will increase voice capacity by up to two times present system carrying capacities. Further, data rates will be increased by as much as 10 times carrying capacity. BAM plans for future data rates of from 1Mb/s to 2Mb/s to support future WAP-based applications when base-station upgrades will take on the appearance of wireless routers handling larger bandwidths for Internet communications.

A More Aggressive Approach
Last September, Sprint PCS announced the availability of circuit-switched data that included the Sprint PCS Wireless Web. This service allowed Sprint customers to select Internet content in real time, using phones equipped with mini browsers. In planning for this offering, Sprint leveraged its CDMA digital network, which provided an easy upgrade path for circuit-switched data. To accomplish this move, Sprint PCS upgraded all of the software in its base stations and MSCs. It also integrated into its network a system called an inter-working function (IWF). The IWF platform serves as a data gateway between wireless CDMA and wireline networks. In a normal data call, the call would be routed through a modem to the PSTN; however, the IWF gateway provides a mobile caller with direct digital access to the Internet bypassing the PSTN. By migrating its network to data with a gateway (IWF) to handle call-processing functions for a data call, Sprint was able to establish a Wireless Web data service accessible through a Sprint PCS handset.

Oliver Valente, Sprint vice president of technology and advanced systems development, explained that the design concept allows Sprint PCS to move to the next level of service, 3G, through a combination of hardware and software upgrades to its base stations as well as software upgrades to the MSCs. This gradual migration will allow Sprint PCS to offer a fully packetized data network with speeds of up to 144kb/s. Further, these planned upgrades will double voice capacity, effectively increasing the battery life of the subscriber's handset.

Although the roadmap to data has been unique for Sprint, Valente explained that Sprint's approach allowed it to plan ahead for packet data, avoiding the necessity for a major investment in network facilities to support migration to 3G data.

The Changing Configuration of the Legacy Network

Figure 1 shows a legacy wireless facility configured with a legacy wireless/wireline switching facility illustrates a planned migration from a hybrid legacy network to one that has been fully de-coupled and is fully supported by separate functional processors in a server-centric IP network. The migration might take place over several years and would not be a "fork lift" move.

This migration process from circuit-switched to packet-switched service begins with the upgrading of the base station and the base-station software. The first level in Phase I of this graduated process shows the enhanced base-station transceivers that support packetizing the voice traffic emanating from the radio-access network (RAN). In this configuration, the base-station transceivers are connected to a centralized controller via terrestrial T1s where a partial routing of data packets might take place using an internet-working function. This might take the form of a special dedicated processor designed to bypass data traffic at the MSC for routing to an IP network interface.

During a second-level migration period, the centralized base-station-controller function would be gradually replaced by a router function that links to the internal IP network and supports the IN services in the switching center. During this transition period, data traffic is routed to an IP network gateway while voice traffic is routed via a legacy wire-line switch.

IP Network Servers Take Over
In Figure 2, you begin to see the ongoing de-coupling of the legacy wireless switch with much of the mobile switch functions taken over by servers on the internal IP network. These de-coupled functions are then made resident on standardized computing platforms distributed across an IP backbone network within the switching center. To some degree, legacy switching centers already are functioning in this manner because special features such as voice mail, short message service (SMS) and fax service already run on dedicated computer platforms. These dedicated computer platforms are linked to the MSC via an internal IP network arrangement.

However, there is a more aggressive migration of all IN functions to dedicated servers. This would include SMS, HLR/VLR authentication support, enhanced services such as prepaid and voice messaging.

Dedicated computers also serve to interface to the SS7 public switched network and the SS7 STP. Another special IP server supports VoIP as well as serves as an interface to the Internet for delivery of Internet-based subscriber services.

During Phase II, the legacy switching facilities would begin to be phased out as the IP network begins to assume more of the functions formerly managed by the proprietary switching engines.

These legacy-related functions would be absorbed into the IP network, becoming part of the IP-network dynamics that flow across and control the network.

To date, the Internet has created many opportunities for the Internet-service providers and is now influencing the development of a new class of IP-based service offerings for the wireless-service provider. The migration of the legacy switching center to that of an IP server-centric network will only serve to accelerate this migration to enhanced Internet-based subscriber services. The movement toward IP server-centric networks is upon us, and in a short time, we can expect to see a new array of compatible wireless devices required to access new and exciting IP base applications.

Llana (llana@bellatlantic.net) is a contributing writer based in King of Prussia, PA.