Multiple heterogeneous networks

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Limitations of 4G

Although the concept of 4G communications shows much promise, there are still limitations that must be addressed. One major limitation is operating area. Although networks are becoming more ubiquitous, there are still many areas not served. Rural areas and many buildings in metropolitan areas are not being served well by existing wireless networks. This limitation of today's networks will carry over into future generations of wireless systems. The hype that is being created by 3G networks is giving the general public unrealistic expectations of always on, always available, anywhere, anytime communications. The public must realize that although high-speed data communications will be delivered, it will not be equivalent to the wired Internet - at least not at first. If measures are not taken now to correct perception issues, 4G services are deployed, there may be a great deal of disappointment associated with the deployment of the technology, and perceptions could become negative. If this were to happen, neither 3G nor 4G may realize its full potential. Another limitation is cost. The equipment required to implement a next generation network is still very expensive. Carriers and providers have to plan carefully to make sure that expenses are kept realistic.

Some issue expected with the implementation of 4G with multiple heterogeneous networks are issues such as;

  • access,
  • handoff,
  • location coordination,
  • resource coordination to add new users,
  • support for multicasting,
  • support for quality of service,
  • wireless security and authentication,
  • network failure and backup, and
  • pricing and billing.

Network architectures will play a key role in implementing the features required to address these issues.


One of the most challenging problems facing deployment of 4G technology is how to access several different mobile and wireless networks. Figure 1 shows three possible architectures: using a multimode device, an overlay network, or a common access protocol.

Multimode devices

One configuration uses a single physical terminal with multiple interfaces to access services on different wireless networks. Early examples of this architecture include the existing Advanced Mobile Phone System/Code Division Multiple Access dual-function cell phone, Iridium's dual function satellite-cell phone, and the emerging Global System for Mobile telecommunications/Digital Enhanced Cordless Terminal dual-mode cordless phone. The multimode device architecture may improve call completion and expand effective coverage area. It should also provide reliable wireless coverage in case of network, link, or switch failure. The user, device, or network can initiate handoff between networks. The device itself incorporates most of the additional complexity without requiring wireless network modification or employing interworking devices. Each network can deploy a database that keeps track of user location, device capabilities, network conditions, and user preferences. The handling of quality-of-service (QoS) issues remains an open research question.

Overlay network

In this architecture, a user accesses an overlay network consisting of several universal access points. These UAPs in turn select a wireless network based on availability, QoS specifications, and userdefined choices. A UAP performs protocol and frequency translation, content adaptation, and QoS negotiation-renegotiation on behalf of users. The overlay Issues in network, rather than the user or device, performs handoffs as the user moves from one UAP to another. A UAP stores user, network, and device information, capabilities, and preferences. Because UAPs can keep track of the various resources a caller uses, this architecture supports single billing and subscription.

Common access protocol

This protocol becomes viable if wireless networks can support one or two standard access protocols. One possible solution, which will require interworking between different networks, uses wireless asynchronous transfer mode. To implement wireless ATM, every wireless network must allow transmission of ATM cells with additional headers or wireless ATM cells requiring changes in the wireless networks. One or more types of satellite-based networks might use one protocol while one or more terrestrial wireless networks use another protocol.


Supporting QoS in 4G networks will be a major challenge due to varying bit rates, channel characteristics, bandwidth allocation, fault-tolerance levels, and handoff support among heterogeneous wireless networks. QoS support can occur at the packet, transaction, circuit, user, and network levels.

  • Packet-level QoS applies to jitter, throughput, and error rate. Network resources such as buffer space and access protocol are likely influences.
  • Transaction-level QoS describes both the time it takes to complete a transaction and the packet loss rate. Certain transactions may be timesensitive, while others cannot tolerate any packet loss.
  • Circuit-level QoS includes call blocking for new as well as existing calls. It depends primarily on a network's ability to establish and maintain the end-to-end circuit. Call routing and location management are two important circuit-level attributes.
  • User-level QoS depends on user mobility and application type. The new location may not support the minimum QoS needed, even with adaptive applications. In a complete wireless solution, the end-to-end communication between two users will likely involve multiple wireless networks. Because QoS will vary across different networks, the QoS for such users will likely be the minimum level these networks support.

End-to-End QoS

Developers need to do much more work to address end-to-end QoS. They may need to modify many existing QoS schemes, including admission control,dynamic resource reservation, and QoS renegotiation to support 4G users' diverse QoS requirements. The overhead of implementing these QoS schemes at different levels requires careful evaluation. A wireless network could make its current QoS information available to all other wireless networks in either a distributed or centralized fashion so they can effectively use the available network resources. Additionally, deploying a global QoS scheme may support the diverse requirements of users with different mobility patterns. The effect of implementing a single QoS scheme across the networks instead of relying on each network's QoS scheme requires study.

Handoff delay

Handoff delay poses another important QoS-related issue in 4G wireless networks. Although likely to be smaller in intranetwork handoffs, the delay can be problematic in internetwork handoffs because of authentication procedures that require message exchange, multiple-database accesses, and negotiation-renegotiation due to a significant difference between needed and available QoS. During the handoff process, the user may experience a significant drop in QoS that will affect the performance of both upper-layer protocols and applications. Deploying a priority-based algorithm and using location-aware adaptive applications can reduce both handoff delay and QoS variability. When there is a potential for considerable variation between senders' and receivers' device capabilities, deploying a receiver-specific filter in part of the network close to the source can effectively reduce the amount of traffic and processing, perhaps satisfying other users' QoS needs. Although 4G wireless technology offers higher bit rates and the ability to roam across multiple heterogeneous wireless networks, several issues require further research and development. It is not clear if existing 1G and 2G providers would upgrade to 3G or wait for it to evolve into 4G, completely bypassing 3G. The answer probably lies in the perceived demand for 3G and the ongoing improvement in 2G networks to meet user demands until 4G arrives.