CURRENT RESEARCH FOR MOBILITY MANAGEMENT IN 4G

There is considerable research being done on mobility management. Mobility solutions can be found by either developing improvements within the current architecture, or by revising the architecture to reflect the changing environment. The solutions propose different addressing and packet forwarding schemes. Almost all of them are IP based solutions, which allow interoperability and easy integration with the existing architectures. Within each of the solutions the relevance to mobility and their strengths and limitations, are discussed in brief:

1. The Internet Indirection Infrastructure is a scalable, self-organizing scheme which easily integrates with legacy systems. It proposes an architecture that offers a communication abstraction based on rendezvous points in an overlay network. When a host wants to send a packet, it forwards the packet to one of the servers it knows. A packet keeps traversing the network till the target server is reached; this leads to delay in route discovery and packet forwarding.
2. FARA (Forwarding directive, Association, and Rendezvous Architecture) is an ongoing project whose main purpose is to provide mobility by separating location from identity. One advantage is that neither an entirely new namespace nor a globally unique one is required for the entities. It allows several different forwarding mechanisms to co-exist in the network, resulting in variability in characteristics like mobility, identity, and anonymity. However, FARA model fails to take into consideration many packet forwarding issues like performance of network nodes, or the balance of anonymity vs. identity for communicating endnodes. It does not accommodate for security either.
3. Host Identity Payload (HIP) provides another way of breaking the binding between identities and topological locations of network nodes. HIP introduces new cryptographic identities that can be dynamically mapped to IP addresses. However, HIP Host Identity (HI). being a public key, is not practical in all actions; it is somewhat long, it needs to be hashed before being used in IPv6 applications. While providing support for mobility and multi-homing with a major architectural change in the addressing concept, the solution requires only small changes in current host implementations.
4. IST MIND develops the concepts and protocols generated in BRAIN by enabling hosts to cooperate with self-organizing wireless ad-hoc networks. It provides independent, interoperable solutions for local/micro-mobility from global mobility.
5. DRIVE specifies a multi-access architecture allowing for seamless intersystem- handover. The concept of a host-controlled flow control was developed to enable parallel usage of different access systems. The architecture is based on Hierarchical Mobile IP, extended by an AAA (Authentication, Authorization and Accounting) component. Over DRiVE extends the scenario with moving networks (e.g. vehicles, trains, etc.) in a multiradio/multi-access environment, defines a Mobile IP-based solution, and focuses on multicast support. The project has strong influence on the ongoing work within the IETF NEMO (Network Mobility) group.
6. The Architectural Principles of Ambient Networks require the integration of a multitude of different communication environments, rather than suffering from heterogeneity. The approach is to use network composition as the principle instead of terminals; networks as such can form the basic building block of the communication architecture. Network composition is a more powerful concept than the simple internetworking as enabled by the Internet Protocol. The current Internet assumes homogeneity in the environment in which to provide control. Ambient Networks have the potential to solve this issue of fragmented control.
7. Developing Standards for Seamless & Secure Mobility: Several industry consortia and standard development organizations such as the IEEE 802 LAN/MAN Standards Committee and the Internet Engineering Task Force (IETF) are expending considerable efforts to develop a common framework and extend existing mobility protocols in order to facilitate and optimize handover performance. Various activities are currently under way, including extensions to Mobile IP at the IETF, and the formation of the Media Independent Handover (MIH) working group in IEEE 802, in addition to several task groups within IEEE 802.11 in order to deal with roaming (IEEE 802. 11r) and interfacing to external networks (IEEE 802.11u) .
8. Interference Alignment Techniques for Wireless Interference Channels: The project is going at Samsung Advanced Institute of Technology, Korea from Nov. 2008 onwards.
9. Transmission Techniques for Multiple MIMO Relay Channels: This project is being developed at LG Electronics, Korea from Aug. 2008.
10. Physical Layer Design for High-speed wireless Systems (9^th): The project is going at Interstate Technology & Regulatory Council.

The best solution among the current and ongoing projects will be the one that successfully addresses all the related challenges as well as allows scalability for future possibilities. A few open issues, however, need to be addressed in most of the existing projects; i.e. synchronization of the entire network and sound QoS.

Enhancements to IPv6 Mobility Management Protocols Required by 4G Networks

Although the features mentioned are suited for 4G networks, recently, there has been almost universal recognition that IPv6 needs to be enhanced to meet the need for future 4G cellular environments. In particular, the absence of a location management hierarchy (IPv6 uses only simple location updates for location management) leads to concerns about the signalling scalability and handoff latency. This is especially significant when we consider that 4G aims at providing mobility support to potentially billions of mobile devices, within the stringent performance bounds associated with real time multimedia traffic.

There are three main areas where IPv6 needs to be enhanced before being used as the core networking protocol in 4G networks:

1. Paging Support: The base IPv6 specification does not provide any form of paging support. Hence to maintain connectivity with the backbone infrastructure, the mobile node needs to generate location updates every time it changes its point of attachment, even if it is currently in dormant or standby mode. Excessive signaling caused by frequent motion leads to a significant wastage of the mobile node's battery power, especially in environments with smaller cell areas (such as 802.11 based cellular topologies). It is thus impractical to rely completely on location updates and is essential to define some sort of flexible paging support in the intra-domain mobility management scheme.
2. Scalability: IPv6 allows nodes to move within the Internet topology while maintaining reachability and on-going connections between mobile and correspondent nodes. To do this a mobile node (MN) sends Binding Updates (BUs) to its Home Agent (HA) and all Correspondent Nodes (CNs) it communicates with, every time it moves. Authenticating binding updates requires approximately 1.5 round trip times between the mobile node and each correspondent node (for the entire return routability procedure in a best case scenario, i.e. no packet losses). In addition one round trip time is needed to update the HA; this can be done simultaneously while updating CNs. These round trip delays will disrupt active connections every time a handoff to a new radio access technology is performed. Elimination of this additional delay element from the time-critical handover period will significantly improve the performance of IPv6. Moreover, in the case of wireless links, such a solution reduces the number of messages sent over the air interface to all CNs and the HA. A local anchor point will allow Mobile IPv6 to benefit from reduced mobility signaling with external networks. For these reasons a new Mobile IPv6 node, called the Mobility Anchor Point (MAP) is being suggested, that can be located at any level in a hierarchical network of routers. Unlike Foreign Agents in IPv4, a MAP is not required on each subnet. The MAP will limit the amount of Mobile IPv6 signaling outside the local domain. The introduction of the MAP provides a solution to the aforementioned problems in the following way:
   1. The MN sends binding updates to the local MAP rather than the HA (which is typically further away) and CNs.
   2. Only one binding update message needs to be transmitted by the MN before traffic from the HA and all CNs is re-routed to its new location. This is independent of the number of CNs that the MN is communicating with. Thus by decreasing signaling traffic by having an intermediate level in the hierarchy helps accommodate a larger number of mobile nodes in the system.
3. Technologies: A mobile node switches from one network to another network in one of two cases: (a) when the signal from the network it is currently in starts to become weak or (b) when the mobile host detects another network which is better suited to its application compared to its current network.

The decision of the mobile device on the suitability of the network can be based on signal strength, network bandwidth or certain policies which the user might have stored in his profile based on which subsequent switching between networks of different access technologies may occur. For example, when a user is streaming a video, he/she may use WLAN and when he/she is listening to highly compressed audio, she might switch to GPRS.

Further, another issue that needs to be resolved is that of informing the source (HA/CN) when the MN has moved. In such a situation, the MN does a location update to its HA, which then takes charge of sending IP datagrams to the MN's new location using standard Mobile-IP mechanisms.

In line with the 4G vision of bringing together wide-area networks and local-area packet-based technologies, mobile terminals are being designed with multiple physical or software-defined interfaces. This is expected to allow users to seamlessly switch between different access technologies, often, with overlapping areas of coverage and dramatically different cell sizes. Mobility management protocols should then be capable of handling vertical handoffs.

Mobility Management in 4G

The 4G mobility management includes mobility related features, absent in previous generation networks, such as: Moving Networks, Seamless Roaming and Vertical Handover.

Mobility Management Operations

The operation of mobility management is divided into two related parts, location management and handoff management.

Location Management

Location management involves two operations; location registration and call delivery as shown in figure 10. Location registration involves the mobile terminal periodically updating the network about its new location (access point). This allows the network to keep a track of the mobile terminal. In the second operation the network is queried for the user location profile and the current position of the mobile host is retrieved. Current techniques for location management involve database architecture design and the transmission of signaling messages between various components of a signaling network. Since location management deals with database and signaling issues, many of the issues are not protocol dependent and can be applied to various networks such as PLMN (Public Land Mobile Network) based networks, PSTN (Public Switched Telephone Network). ISDN (Integrated Services Digital Network). IP, Frame Relay, X.25, or ATM (Asynchronous Transfer Mode) Networks depending on the requirements.

 

Figure 1: Location management operations

Some key research issues for location management include:

• Addressing, i.e. how to represent and assign address information to mobile nodes. The problem is becoming more severe since the 4G mobile communication systems will be based on the internetworking and interoperability of diverse and heterogeneous networks of different operators and/or technologies. A global addressing scheme is needed, e.g. IPv6 address, to locate the roaming nodes.
• Database Structure, i.e. how to organize the storage and distribution of the location information of mobile nodes. Database structure can be either centralized or distributed, or the hybrid of these two schemes. Tradeoff is needed between access speed, storage overhead, and traffic overhead due to the access to the related databases. Caching is also an important technique for the improvement of access performance.
• Location Update Time, i.e. when a mobile node should update its location information by renewing its entries in corresponding databases. Schemes for location update can be either static or dynamic. In a static scheme location update is triggered by some fixed conditions like time period or network topology change. A dynamic scheme is more personalized and adaptive, and based on some situations such as counter, distance, timer, personal profile, or even predicted factors.
• Paging Scheme, i.e. how to determine the exact location of a mobile node within a limited time. Obviously an adequate tradeoff is needed between time overhead and bandwidth overhead. There are also both static and dynamic schemes for location paging. In static cases paging is simply done to the whole certain area where the mobile node must be in. For a dynamic method, the main problem is to firstly organize the paging areas into groups and then recognize the best sequence of the separated areas for paging, based on information like distance, probability, moving velocity, etc.

Handoff Management

Handoff management equals controlling the change of a mobile node's attachment point to a network in order to maintain connection with the moving node during active data transmission.

Operations of handoff management include (Figure 2):

• Handoff Triggering, i.e. to initiate handoff process according to some conditions. Possible conditions may include e.g. signal strength deterioration, workload overload, bandwidth decrease or insufficiency, new better connection available, cost and quality tradeoff, flow stream characteristic, network topology change, etc. Triggering may even happen according to a user's explicit control or heuristic advice from local monitor software.
• Connection Re-establishing, i.e. the process to generate new connection between the mobile node and the new attachment point and/or link channel. The main task of the operation relates to the discovery and assignment of new connection resource. This behaviour may be based on either network-active or mobile-active procedure, depending on which is needed to find the new resource essential to the new establishment of connection.
• Packet Routing, i.e. to change the delivering route of the succeeding data to the new connection path after the new connection has been successfully established.
  
  

 

Figure 2: Handoff of management operations

Wireless networks vary widely in both service capabilities and technological aspects, so no single wireless network technology can fulfill the different requirements on latency, coverage, data rate, and cost. An efficient strategy is necessary for the management of such a wireless overlay architecture and mobility within the framework. In homogeneous environments, traditional horizontal handoff can be employed for intra-technology mobility. In heterogeneous environments, vertical handoff should be used for inter-technology mobility. Vertical handoff may be occur either upward (i.e. to a larger cell size and lower bandwidth) or downward (i.e. to a smaller cell size and higher bandwidth); and the mobile device does not necessarily move out of the coverage area of the original cell. Some packet-level QoS parameters become more important to real-time multimedia services, including packet latency, packet loss rate, throughput, signalling bandwidth overhead, and device power consumption.

Besides the basic functions that implement the goal of handoff management, there are many other requirements on performance and packet-level QoS that should be carefully taken into account when trying to design or select a handoff management scheme, including

• Fast Handoff, i.e. the handoff operations should be quick enough in order to ensure that the mobile node can receive data packets at its new location within a reasonable time interval and so reduce the packet delay as much as possible. This is extremely important to real-time services.
• Seamless Handoff, i.e. the handoff algorithm should minimize the packet loss rate into zero or near zero. Fast handoff and seamless handoff together are sometimes referred to as smooth handoff. While the former concerns mainly packet delay, the latter focuses more on packet loss.
• Routing Efficiency, i.e. the routing path between corresponding node and mobile node should be optimized in order to exclude possible redundant transfer or bypass path as triangle routing. Some distinct but complementary techniques exist for handoff management to achieve its performance and QoS requirements above, including:
• Buffering and Forwarding, i.e. the old attachment point can cache packets during the MN in handoff procedure, and then forward to the new attachment point after the operation of connection re-establishing of mobile node's handoff.
• Movement Detection and Prediction, i.e. mobile node's movement between different access points can be detected and predicted so that the next network that will soon be visited is able to prepare in advance and packets can even be delivered there before and/or during handoff simultaneously to the old attachment point.
• Handoff Control, i.e. to adopt different mechanisms for the handoff control. Typical examples include e.g. layer two or layer three triggered handoff, hard or soft handoff, mobile-controlled or network-controlled handoff, etc.
• Domain-Based Mobility Management, i.e. to divide the mobility into intra-domain mobility and inter-domain mobility according to whether the mobile host's movement happens within one domain or between different domains