The services to be offered by 3G Networks were listed by the ITU-T in their IMT-2000 Concept: Based on high-quality, anytime anywhere communication, support would be offered for data transport, Internet and Intranet access and, in particular, multimedia services. The first thing to observe is that “anytime anywhere communication” has not quite become reality. While a mobile network is available almost anywhere—UMTS, cdma2000, WLAN hotspots, WiMAX, etc.—the user is not necessarily able to access it: technologies are heterogeneous and incompatible, inter-technology roaming is not fully possible as yet.
On the other hand, devices are increasingly multi-homed: they support several radio interfaces, e.g. HSPA,WLANand Bluetooth, in order to always be able to connect. It is noteworthy however, that these radio interfaces are operated independently. The user has to actively select which interface shall be used and it is not possible to handover a session for most them.
More radio interface technologies and also more networks are being deployed and will be deployed in the future. At first sight, this seems to point towards an exacerbation of today’s situation: in order to be always connected users need more hardware, more multi-homing, a multitude of subscriptions and, in particular, technical expertise. Looking more closely, however, it becomes apparent that the situation might indeed improve. The general tendency is towards a collaboration of heterogeneous technologies and networks, something known as convergence of heterogeneous Access Networks.
The ultimate vision is a high-level architecture for 4G Networks as seen by the ITU [ITU M.1645] and as shown in the diagram besides;
The Access Networks of the different technologies are connected to a common IP-based Core Network. These Access Networks include mobile Access Networks of many denominations as well as fixed Access Networks such as DSL and cable, and pure digital broadcast networks, i.e. digital TV.
Beyond this architectural view, what does it mean that the heterogeneous Access Networks converge? In the 4G context, different degrees of convergence can be discussed, in particular:
Common Service Network
In its simplest form, convergence means access to a common network supporting services, which can be called the Service Network. In other words, no matter what the access technology, the user can access the same services. 3GPP designed their service delivery support infrastructure, the IMS, to be access independent. A variety of standardization organizations are working intensely on specifying how their “Access Networks” can collaboratewith theIMS.For example, theWiMAXForum[WFArch] specifies howto access theIMS from WiMAX, and 3GPP2 have adapted the IMS specifications to be compatible with cdma2000. The fixed world also supports access to the IMS in the context of Fixed-Mobile Convergence, for example ETSI, the ITU, the Japanese ARIB as well as the consortium of cable operators, CableLabs. The IMS is therefore an example of a common Service Network.
Single Identity and Subscription
The subscriber has a single subscription and the MS has a single identity—a URI or a telephone number—in all Access Networks. The subscriber can be reached in all Access Networks and can roam between them if his MS supports the corresponding technology. He receives a single bill. From an operator.s perspective, this scenario raises an interesting question: with which operator does the user have the subscription? Subscriptions usually generate more value to the operator than providing roaming to somebody else’s users. Technically, this question translates into who owns the HSS/AAA Server? Where—in the Core Network or in (which?) Access Network—is this server located?
3GPP in the Evolved Packet Core (EPC) [3GPP 22.278], 3GPP2 as Converged Access Network (CAN) [3GPP2 X.S0054] are already working on detailed solutions for Core Networks which feature an HSS/AAA Server to which heterogeneous ANs can be attached. The WiMAX Forum complements this approach neatly by specifying a Mobile WiMAX Access Network which can be attached to any Core Network and which it expects to feature a AAA Server or equivalent [WF Arch]. The concept of a single subscription for heterogeneous ANs is also on the books of the standardization organizations for fixed networks, e.g. ETSI with Next Generation Network (ETSINGN) and CableLabs with their PacketCable 2.0 technology. They typically envisage an AAA Server respectively HSS to be located both in the Core Network and the Service Network.
Automatic selection of Access Network
Here, the user no longer needs to select the most appropriate Access Network manually. Instead, a multi-homed MS chooses what is most appropriate automatically, in collaboration with the network. For example, it would pickDECTor Bluetooth while at home,UMTSwhen moving andWLANwhen in reach of a hotspot. Obviously, the algorithm for selecting what isthe “most appropriate” Access Network is of utmost interest: who decides according to which criteria? Which configuration options will be available to the user? Criteria for selecting the Access Network could be cost, QoS necessary to the desired application, current network load and how long the access is expected to be available—there is no use in choosingWLAN when the user is only driving through a hotspot.
Automatic selection of Access Networks in a network-assisted, i.e. operator-controlled way will be part of the next release of the 3GPP System. While the other standardization organizations can be expected to include this functionality as well, the respective specifications are not yet at an advanced stage.
Handover and seamless handover between Access Networks
In the most sophisticated form of convergence, handover—or even seamless handover, is possible between heterogeneous Access Networks. This includes the basic mobility functionality, and, furthermore, support for transferring security credentials—i.e. a single authentication for all Access Networks—and QoS reservation to the new access. Seamless handover is currently being addressed by 3GPP EPC and 3GPP2 CAN. seamless mobility between circuit-switched and packet-based Access Networks is even being specified. Full details are, however, still missing. Other standardization organizations are also planning to work on the issue.
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