Mobile networks have always focused on voice as the primary application, and that was certainly the case for analog networks (first generation [1G]), Time Division Multiple Access (TDMA) networks (2G), and even Code Division Multiple Access (CDMA) networks (3G). Now with the introduction of 4G networks, multimedia applications will assume primary importance. One of the most interesting and compelling applications in this area is mobile TV. Given the enormous success of mobile services and the popularity of TV, this combination is a natural one. Variations on this theme include broadcast, multicast, and unicast of both real-time and stored content. Enabling these types of services on a broad scale clearly introduces some significant challenges, including latency, throughput, and network capacity.
Mobile TV is just one example of the Internet Everywhere experience. At the heart of this experience are all-IP end-to-end mobile networks based on open systems. These networks will emerge from several different sources. WiMAX has emerged out of the IEEE 802.16 committee, Universal Mobile Telecommunications Service (UMTS) SAE/LTE out of the Third-Generation Partnership Project (3GPP), and 3GPP2 is working on a Radio Access Network (RAN) evolution for CDMA. It is expected that all three of these technologies, along with Wi-Fi, will serve as the basis for 4G networks. Wi-Fi is certainly central to this effort because it is the dominant indoor radio technology, and a substantial part of mobile calls originate from within the home or business (72 percent in the United States).
Some of the important characteristics of next-generation mobile networks follow:
• They must be engineered from the beginning to be all-IP end-to-end-representing a major change from the circuit-switched architectures that have dominated in the past and an essential step toward enabling multimedia applications.
• We will see a move away from closed RAN architectures and toward open systems with interoperability. The Internet has always been based on the concept of open systems, and this concept is being introduced in the mobile world. Base stations will be built by RAN vendors and mobile gateways will be built by IP vendors. Open interfaces will allow network integrators to bring these pieces together to build a robust mobile network. The move to open systems offers the following advantages:
– It allows the radio domain and IP domains to evolve separately.
– It gets the mobile industry on the price-performance curve of the router industry.
– It provides much greater flexibility in vendor selection.
– New radio technologies can be more easily introduced into the network.
• The next generation of mobile technology will be based on Orthogonal Frequency Division Multiple Access (OFDMA) and Multiple Input Multiple Output (MIMO) antenna technology. There is now near unanimous agreement that OFDMA is the preferred approach to handling packet traffic, and MIMO increases throughput in good signal-noise conditions.
• Much higher speed services (up to 50 Mbps per sector on the downlink and 25 Mbps per sector on the uplink [assumes a 10-MHz carrier]) will enable support for multimedia applications that require high throughput and very low latency. Uplink performance will always be challenging because client devices are power-constrained.
• Costs will decrease as networks go all-IP end-to-end, and lower costs will be an essential component of a 4G deployment. It is not just the extensive use of IP technology and simplified networks that will decrease costs, but the move to open systems. The one constant in networking is that open systems decrease costs significantly. A variety of IEEE technologies have experienced such a decrease.
• A much more reasonable intellectual property rights (IPR) licensing environment than exists with 3G technologies will emerge. Such licensing is always challenging in the wireless world, where patents are seen as a revenue source. The goal is to focus on lower costs, better transparency, and more predictability in royalty payments.
• Security will be an essential part of a 4G network architecture. The Internet Everywhere Experience will allow the mobile subscriber to access a whole host of Internet related services, but with that flexibility comes the risk associated with Internet connectivity. Next generation solutions must have a carefully thought out security approach that protects both the network and the subscriber.
Future Expectations
Fourth-generation network technology is not so much a new modulation technology as it is a way of architecting networks. These networks will use a variety of mobile packet radio technologies along with Wi-Fi to offer a ubiquitous broadband experience for the mobile subscriber.
WiMAX has been at the forefront of the move to all-IP end-to-end networks based on open systems, and this technology is already being deployed in fixed wireless applications. The 3GPP and 3GPP2 roadmaps also show a clear direction toward all-IP end-to-end networks and open systems. In all three cases OFDMA and MIMO are seen as critical ingredients (Figure 1). These three mobile technologies are all moving in the same direction, but they will probably always be slightly different for political, commercial and IPR reasons.
Figure 1. Strong Industry Direction Toward OFDMA
The mobile gateway will emerge as these networks begin to develop. The mobile gateway, the first-hop router in an all-IP end-to-end mobile network, connects base stations with the core network. Because a mobile gateway also communicates with base stations, it must support IP protocols specific to that radio technology. Sitting behind these gateways are the IP core network and the services domain. Services in an all-IP end-to-end network are both IP Multimedia Subsystem (IMS) and non-IMS based. A critical feature of next-generation networks is that they are much simpler than 3G networks. Next-generation networks typically have only two types of devices: base stations and mobile gateways-and that is all-with the latter incorporating many of the functions that are found in numerous different boxes in networks today.
If the network is made up of different RF technologies, then client devices must also support multiple RF modes. This scenario is already occurring as cellular technologies are introduced into laptops and Wi-Fi technology begins to emerge in mobile phones. In addition to handsets, these networks will require residential gateways to support the delivery of mobile broadband to the home. Indoor coverage has always been a challenge in the mobile world and will become even more problematic as very high-speed 4G services emerge. In addition to residential gateways, 4G networks will also make extensive use of picocell and microcell technologies to deliver very high data rates in high-usage areas.
RF Challenges in a 4G World
Fourth-generation networks will offer very high data rates on both the uplink and downlinks. High-speed mobile services encounter some significant RF challenges, including difficulty in reaching in-building subscribers with enough power to support high-speed services and the limitation in transmit power from a battery-powered client device. To address these difficulties, 4G will use macrocells, microcells, Wi-Fi access points, and femtocells extensively.
Macrocells will still be the backbone of these networks because they can cover a very large area, but their performance is poorer than that of microcells. Only users in very close proximity to a macrocell will actually be able to get the true 4G data rates.
Microcell technology will be used in densely packed urban areas. These base stations will be much smaller than traditional macrocells and will enable very high data rates over a much more limited area. Small cells bring subscribers closer to the base station, thus significantly increasing performance. These cells will most likely sit on lamp posts, telephone poles, cable strands, etc. and use wireless mesh, DSL, or cable technology for the backhaul (Figure 2).
Figure 2. Wi-Fi Microcell Base Station
A few options have emerged for in-home coverage, including Wi-Fi access points and femtocells, which use licensed radio technology and will appear over the next few years. Both devices use the subscriber's cable or DSL service for backhaul. Cellular coverage can be enabled over Wi-Fi access points using dual-mode phones and technologies such as UMA or IMS.
The Linksys® division of Cisco is the largest provider of Wi-Fi access points in the world and will help enable next-generation networks.
Femtocell technology is an exciting new area in which subscribers can have a very small base station in their home that also uses DSL or cable for backhaul. These devices use licensed radio and operate at power levels of only a few milliwatts. The most significant difference between this approach and Wi-Fi is that a standard cell phone can be used. Both of these approaches allow for significantly better indoor mobile phone experience than what the macro cellular network can typically provide.
In addition to providing users a much better service and offloading expensive macro cellular networks, these approaches will allow a variety of new broadband services. A traditional problem with cellular networks is the loss that occurs when RF signals have to penetrate the walls of a home. Although this loss will limit the high-speed services that can be offered, these in-home devices remove that limitation. Mobile services will be limited mainly by the throughput of the broadband backhaul (DSL or cable) into the home. These devices will also reduce the backhaul expense in the macro cellular network-a major part of the cost of today's voice-oriented mobile network that promises to become a significant problem in the multimedia networks of tomorrow.
The 4G network will put a huge premium on smaller base stations (femtocells, picocells, and microcells) that cover more limited areas, but at much greater throughput.
The 4G networks will be knit together from a variety of different-sized base stations using a variety of different all-IP radio technologies. But regardless of the size of the base station or the modulation technology, backhaul will be a major expense.
Backhaul in a 4G World
Backhaul is already a major expense for mobile operators in a 2G and 3G world, where base stations typically need only a few T1s worth of bandwidth It will become an even bigger problem in a 4G world, where each base station will need several tens of megabits of bandwidth on the backhaul. Residential gateways and femtocells can definitely help by offloading traffic from the cellular network, but a clear strategy toward optimizing existing backhaul is a requirement for the future.
IP can play a major role here, and the move to IP in the RAN does not need to wait for 4G. Bringing IP into today's network offers enormous benefits, including:
• Optimizing transport by allowing 2G and 3G (with HSPA) to share transport
• Optimizing transport by not using bandwidth during silent periods in speech-Because voice is inherently half-duplex, this approach offers a significant bandwidth savings.
• Enabling remote maintenance of cell sites-Remote maintenance becomes even more important in 4G networks because the number of cell sites will need to increase to get the desired throughput.
• Offering the ability to use other types of backhaul, including Fast Ethernet, Gigabit Ethernet, and wireless-These options become much more important as the network moves to 4G because they offer much more bandwidth for the price.
Cisco offers a RAN optimization solution that can be deployed today to solve 2G and 3G problems and easily evolve to address 4G deployments. In today's network, it supports both 2G and 3G radios and backhauls mostly voice traffic across T1 lines. In a 4G world, it will support both 3G and 4G radios and backhaul voice and multimedia traffic over a variety of transport options, including wired solutions using Fast Ethernet and Gigabit Ethernet and wireless solutions using point-to-point and mesh architectures. The mesh approach is a very interesting option when microcells are deployed in an urban area with a very dense footprint.
An aggressive move here not only sets the stage for a 4G future, but also can reduce backhaul costs in today's network by as much as 50 percent.
Cisco offers a portfolio of products in this space that can help operators save money today and prepare for the future.
Mobile Gateway
The backhauling of 4G traffic brings us to the first-hop router, which basically provides a mobile gateway function. A mobile gateway supports the different radio technologies that will be deployed as part of a 4G network. One of the most talked-about radio technologies in recent years has been WiMAX, which uses an Access Services Network Gateway (ASN-GW) as the first-hop router.
The Cisco ASN-GW solution will support the WiMAX Forum's Network Working Group (NWG) specification that will emerge early in 2007. This specification defines the interface between an ASN-GW and a base station. The ASN-GW supports all bearer management functions, and the base station is responsible for radio control. Cisco has been very active in the NWG and was one of the founding members of that organization. WiMAX will represent the first instance of full vendor interoperability in the RAN. It is expected that other standards bodies will follow the lead of the WiMAX Forum and push for open interfaces with interoperability in their next-generation all-IP network implementations.
The mobile gateway in a UMTS SAE/LTE deployment will use a function known as a User Plane Entity (UPE) + Mobility Management Entity (MME), which communicates with a base station over an S1 interface. In either case the functions provided are somewhat similar and include mobility management, authentication, header compression, security, paging, billing, content filtering, edge proxy support, etc. The mobile gateway and the base station are the only two pieces of RAN equipment in a next-generation network (see Figure 3). This rather dramatic simplification helps decrease network costs and makes interoperability much easier. When open systems are added to this architecture, the cost savings will be impressive. A significant amount of capital expenditures (CapEx) go into the RAN, and moving to open systems will result in significantly reduced network costs. Lower costs are essential if 4G is to deliver broadband mobile services cost-effectively.
Figure 3. 4G Mobile Network Architecture
Another interesting feature of all-IP end-to-end networks is that all traffic must pass through the packet gateways because all traffic is, of course, packet-based, putting a premium on scalability, performance, and high availability. All three of these characteristics are hallmarks of the Cisco product line of service provider class routers.
A good general rule is that as the network moves from 3G to 4G, it will require from two to three orders of magnitude more packet gateway capacity.
This scenario puts a premium on selecting mobile gateways that can scale. The most scalable solution is usually based on a service module architecture where additional blades can be plugged into a chassis to meet demand. Cisco has a strong portfolio of gateways for the mobile market, including packet data serving nodes (PDSNs) for CDMA, gateway GPRS support nodes (GGSNs) for UMTS/Global System for Mobile Communications (GSM), and ASN-GWs for WiMAX.
Applications That Will Drive the Adoption of 4G
The bandwidths that are being envisioned for 4G are in the tens of Mbps on both the uplink and downlink-a significant amount of bandwidth. What types of applications consume that much bandwidth? Applications such as mobile TV do, and this technology is actually seen as part of a broader initiative on the part of service providers to deliver video content to users that can be consumed at the time of their choosing, the place of their choosing, and through the device of their choosing. The options include the TV, laptop computer, or mobile device.
Today's networks are designed around voice, which consumes only about 12 kbps on the airlink. Expectation is that high-quality video to a handset (along with audio) will consume on the order of 20 times that much bandwidth, an amount that will severely limit the number of video subscribers on a 3G base station sector to no more than a handful-clearly not the answer for a mass-market service. Successful mobile video will require at least an order of magnitude more bps per square kilometer than what can be provided by traditional networks, so improved modulation technologies such as OFDMA, new antenna technologies such as MIMO, and extensive use of microcells, picocells, and femtocells will be required. Femtocells allow users to get closer to the base station and thus get a stronger signal, which is essential to supporting high data rates, and a dense footprint of small cells will improve the bps per square kilometer. The video revolution will completely change the way mobile networks are built.
Conclusion
Fourth-generation networks based on packet radio technology represent a major change for the mobile industry. It is not just a move away from voice as the primary application-but a complete change in the way networks are built. This technology represents the end of circuit switching, the end of closed RAN architectures, and probably the end of the vertically integrated supplier model. It will introduce a multivendor architecture into the RAN. The network integrator can now put networks together using best-of-class suppliers.
Fourth-generation networks will also mean the end of complex network architectures that emerged as a result of data being an add-on to a circuit-switched voice network. New networks will be all-IP end-to-end and will consist of only two RAN elements: the base station and the mobile gateway. A variety of different-sized base stations will emerge to address the challenges of delivering high-speed services, and a variety of different-sized mobile gateways will likewise emerge to address different deployment scenarios. All traffic will run through these gateways-a scenario that will put a premium on scalability, availability, and manageability.
Fourth-generation networks will embrace a variety of mobile radio technologies along with Wi-Fi. The different mobile technologies are all moving in the same direction, but will probably never truly merge because of politics, commercial, and IPR reasons. Although the latter will continue to be a challenge for the industry as 4G emerges, the entire industry is focused on bringing this variable under control.
The applications that will accelerate the next build-out are video in nature, and foremost among them is mobile TV, an application that is expected to be very popular-and represents a real problem for 3G networks. It will not take many users streaming video down to a handset before today's networks will not be able to meet demands; next-generation networks will have far more capacity.
Fourth-generation networks will probably develop sooner than expected-but operators can start preparing now. IP can be introduced into the network in ways that not only set the stage for tomorrow, but also offer operators significant savings today. Cisco will be there to help mobile operators transition to an all-new architecture focused on all the old applications and a host of new ones.
