Islands in the Bit Stream

IMT-2000 promises data rates of up to 2Mb/s. That means tomorrow's network architectures will look a lot different.

IMT-2000 is an ITU initiative that establishes goals and defines various operating characteristics for 3G systems. The "2000" suffix highlights one feature: the ability to transfer data at rates of up to 2,000kb/s for local coverage areas and up to 384kb/s over wider areas.

The ITU plans to deploy IMT-2000 around 2002. Various organizations around the world have proposed several 3G architectures and technologies. To qualify for the IMT-2000 initiative, the proposed architectures and technologies must meet the ITU operating requirements. The three dominant, competing proposals are UMTS, UWC-136 and cdma2000 UMTS' and cdma2000's air interfaces are based on CDMA modulation techniques, while UWC-136 is based on TDMA.

The ITU has realized that IMT-2000 probably won't be a single system; instead, it's likely to be a combination of systems. The push to ensure that all three technologies are interoperable with one another resulted in the 3G Partnership Project 2 (3GPP2), which works with various standard organizations to ensure compatibility among IMT-2000 systems.

NTT DoCoMo in Japan is one 3G pioneer. Driven by a need to increase capacity, it began experimenting with W-CDMA and is conducting multiple trials designed to prove the W-CDMA concept and, at the same time, establish NTT DoCoMo as the early adopter and leading provider of 3G wireless services. The earlier trials were based on NTT DoCoMo's specification of W-CDMA, and its announced plans to migrate to the 3GPP-defined UMTS specification. Because the architectures of UMTS and NTT DoCoMo's version of W-CDMA are quite similar, NTT DoCoMo and the vendors that participated in the trials have a head start on implementing and providing UMTS services.

Inside the UMTS Network
The 3GPP's UMTS specifications define the air interface, radio access network and the supporting network interface. UMTS supports voice, circuit-emulation and IP packet-data transport between mobiles and the network. Because UMTS is based on CDMA, it supports soft, softer and hard handoffs, which enable uninterrupted data delivery during handoffs.

The UMTS radio access network consists of three node types: node B, radio-network controller (RNC) and core network (CN). Node B's functions are similar to the BTS, providing an interface between the air and network interfaces. The Iub interface connects node Bs to the RNC, which communicates with the CN via the Iu interface. Additionally, for handoffs, RNCs may communicate with other RNCs via the Iur interface. UMTS interfaces are based on ATM.

UMTS interfaces operate in three planes: transport, control-signaling and user. The transport plane is implemented using ATM transport, which uses ATM adaptation layers AAL2 and AAL5. The AAL2 is used to transport bearer data and non-access stratum signaling traffic. The AAL5 is used primarily for control signaling between UMTS nodes. User-plane protocols exist above the AAL2 protocol layer and support transportation of voice and data traffic to and from mobile terminals over AAL2. Macro-diversity functions, which enable soft and softer handoffs, also are implemented within these protocol layers.

The control-signaling plane consists of protocols such as node B application part and radio access network application part. The control-signaling protocols provide management and call-control functions.

The user plane, or bearer data, exists in two domains: circuit-switched and packet-switched. The circuit-switched domain consists of voice and unrestricted digital information (UDI) services. The voice traffic is encoded using adaptive multirate compression, which provides efficient use of bandwidth based on various factors. The UDI service emulates data circuits with rates of up to 2Mb/s. The packet-switched domain provides IP packet-delivery services.

Co-Existence is Key
3G's acceptance depends heavily on the migration strategy from 2G and 2.5G to 3G. A "forklift" upgrade of the infrastructure will stop deployment of 3G services. Reusability and compatibility with 2G and 2.5G services are key to protecting wireless providers' investments in 2G and 2.5G technologies. This co-existence also is important to ensure roaming between 3G and 2G because 3G's roll-out won't be immediate nor ubiquitous. Not all providers will offer 3G, so the ability to roam between the two will ensure wide acceptance when 3G is available only in isolated "islands."

Migration and co-existence strategies have been considered for cdmaOne, cdma2000, GSM, TDMA, UMTS and UWC-136. One example is how UMTS is designed to provide interworking with GPRS and GSM.

The existing MSC and serving GPRS support nodes are shared by GSM, GPRS and UMTS. GSM and GPRS providers that have UMTS licenses can deploy 3G services incrementally. For example, they can begin by offering 3G services in test markets without having to design a completely new network infrastructure. Moreover, their ability to operate using existing GPRS and GSM services ensures both continuity of 3G services networkwide and faster 3G acceptance among subscribers.

Because UMTS and other 3G services are considered new services operating in new bands, they require new licenses. Not every provider will have the financial resources or incentives to bid for licenses, so many providers will continue to use 2G and 2.5G even as others migrate to 3G. Nevertheless, 3G's higher data rates will put 2G providers at a disadvantage. Those that don't get 3G licenses will require technologies to enhance their services in order to offer 3G-like services.

For GPRS and GSM, the 3G-like technology is EDGE, an air interface used in TDMA. EDGE includes new modulation techniques and new coding schemes, which result in data rates of up to 384kb/s. Enhanced GPRS (E-GPRS) provides GSM voice and higher data-rate GPRS services in existing GSM bands.

The fundamentals of EDGE technology also are being adopted by the UWC-136 program. UWC-136 is based on GPRS-136 HS EDGE, which uses IS-136 for voice and EDGE for data at rates of up to 2Mb/s for local service areas and 384kb/s for wide areas. To increase service density, UWC-136 defines a new mode called EDGE Compact, which considers both frequency and time domain in reuse during cell planning. By providing an upgrade path from existing TDMA infrastructures, UWC-136 provides a cost-effective means for TDMA-based IS-136 operators to migrate to 3G.

As the wireless industry deploys 3G, wireline networks are evolving to a converged architecture. A tight integration between wireless and wireline networks is expected and will enable providers to offer services seamlessly between the two. We might soon see cross-pollination of technologies such as voice over IP.

Because 3G provides high bandwidth and a wide variety of bearer types, it likely will spawn exciting mobile services such as high-speed, full-feature Internet access and full-motion multimedia services. Users' devices will evolve to accommodate 3G services, and with the convergence of the wireline communications network, 3G wireless services are limitless.

Bantukul (dan.bantukul@tekelec.com) is Tekelec senior product manager.