Abstract:
Researchers and vendors are expressing a growing interest in 4G wireless networks that support global roaming across multiple wireless and mobile networks—for example, from a cellular network to a satellite-based network to a high-bandwidth wireless LAN. The 4G mobile system is an all IP-based network and provides the user access to different radio access technologies. In this environment, roaming is seamless and users are always connected to the best network.
This paper aims to provide an insight into the issues related to Mobility Management in 4G networks, and focuses on the enhancements required to make IPv6 the underlying protocol in 4th generation networks.
Introduction:
Traditional phone networks (2G cellular networks) such as GSM, used mainly for voice transmission, are essentially circuit-switched. 2.5G networks, such as GPRS, are an
extension of 2G networks, in that they use circuit switching for voice and packet switching for data transmission. Circuit switched technology requires that the user be billed by airtime rather than the amount of data transmitted since that bandwidth is reserved for the user. Packet switched technology utilizes bandwidth much more efficiently, allowing each user’s packets to compete for available bandwidth, and billing users for the amount of data transmitted. Thus a move towards using packet-switched, and therefore IP networks, is natural.
3G networks were proposed to eliminate many problems faced by 2G and 2.5G networks, like low speeds and incompatible technologies (TDMA/CDMA) in different countries. Expectations for 3G included increased bandwidth. A 4G or 4th generation network is the name given to an IP-based mobile system that provides access through a collection of radio interfaces. 4G network promises seamless roaming/handover and best connected service, combining multiple radio access interfaces (such as HIPERLAN, WLAN, Bluetooth, GPRS) into a single network that subscribers may use. With this feature, users will have access to different services, increased coverage, the convenience of a single device, one bill with reduced total access cost, and more reliable wireless access even with the failure or loss of one or more networks.
At the moment, 4G is simply an initiative by R&D labs to move beyond the limitations, and deal with the problems of 3G. At the most general level, 4G architecture will include three basic areas of connectivity:Personal Area Networking (such as Bluetooth), local high-speed access points on the network including wireless LAN technologies (such as IEEE 802.11 and HIPERLAN), and cellular connectivity. 4G supports global roaming where each device will be able to interact with Internet based information that will be modified on the fly for the network being used by the device at that moment. In short, the roots of 4G networks lie in the idea of pervasive computing.
4G Characteristics:
The defining features of 4G networks are listed below:
· High Speed – 4G systems should offer a peak speed of more than 100Mbits per
second in stationary mode with an average of 20Mbits per second when travelling.
· High Network capacity - Should be at least 10 times that of 3G systems, enabling high-definition video to stream to phones.
· Fast/Seamless handover across multiple networks - 4G wireless networks should support global roaming across multiple wireless and mobile networks.
· Next-generation multimedia support - The underlying network for 4G must be able to support fast speed and large volume data transmission at a lower cost than today.
4G Networks and IPv6:
The goal of 4G is to replace the current proliferation of core mobile networks with a single worldwide core network standard, based on IP for control, video, packet data, and voice. The objective is to offer seamless multimedia services to users accessing an all IP-based infrastructure through heterogeneous access technologies. IP is assumed to act as an adhesive for providing global connectivity and mobility among networks. An all IP-based 4G wireless network has inherent advantages over its predecessors. It is compatible with, and independent of the underlying radio access technology. It replaces the old Signaling System 7 (SS7) telecomm protocol, which is considered massively redundant as it consumes a larger part of network bandwidth even when there is no signalling traffic. IP networks, on the otherhand, are connectionless and use the slots only when they have data to send. Hence there is optimum usage of the available bandwidth.
Mobility Management Issues in 4G Networks:
Mobility is a critical aspect of 4G. There are three main issues regarding mobility
management in 4G networks:
1) The 1st issue deals with optimal choice of access technology. A user may be offered connectivity from more than one technology and one has to consider how the terminal and an overlay network choose the radio access technology suitable for services the user is accessing.
There are several network technologies available today, which can be viewed as complementary. For ex., WLAN is best suited for high data rate indoor coverage. Whereas GPRS or UMTS, are best suited for nation wide coverage and can be regarded as wide area networks. Thus a user needs to make the optimal choice of radio access technology among all those available. A handover algorithm would assure that the best overall wireless link is chosen. The network selection strategy should take into consideration the type of application being run by the user at the time of handover. This ensures stability as well as optimal bandwidth for interactive and background services.
2) The 2nd issue deals with mobility between access technologies. This includes fast, seamless vertical handovers, quality of service (QoS), security and accounting. IPv6 provides mobility in a manner that resembles only simple portability. To enhance Mobility in IPv6, ‘micro-mobility’ protocols (such as Hawaii
3) The 3rd issue concerns the adaptation of multimedia transmission across 4G networks. Indeed multimedia will be a main service feature of 4G networks, and changing radio access networks may in particular result in drastic changes in the network condition. In cellular networks such as UMTS, users compete for scarce and expensive bandwidth. Variable bit rate services provide a way to ensure service provisioning at lower costs. In addition the radio environment requires that the services are adaptive and robust against varying radio conditions as it has dynamics.
High variations in the network Quality of Service (QoS) leads to significant variations of the multimedia quality. Avoiding this requires choosing an adaptive encoding framework for multimedia transmission. The network should signal QoS variations to allow the application to be aware in real time of the network conditions.
Mobility Management in IPV6:
Features of mobility management in Ipv6:
Ø 128-bit address space provides a sufficiently large number of addresses
Ø High quality support for real-time audio and video transmission, short/bursty connections of web applications, peer-to-peer applications, etcFaster packet delivery, decreased cost of processing – no header checksum at each relay, fragmentation only at endpoints.
Ø Smooth handoff when the mobile host travels from one subnet to another, causing a change in its Care-of Address.
Enhancements to IPv6 Mobility Management protocols required by 4G networks:
As it has been Universally recognized, IPv6 needs to be enhanced to meet the need for future 4G cellular environments. There are three main areas where IPv6 needs to be enhanced before being used as the core networking protocol in 4G networks:
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. Hence it is essential to define some sort of flexible paging support in the intra-domain mobility management scheme.
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 sends Binding Updates (BUs) to its Home Agent (HA) and all Correspondent Nodes (CNs) it communicates with, every time it moves.
Heterogeneous access technologies:
A mobile node switches from one network based on an access network technology
like GPRS to another network based on a different access technology like WLAN in one of two cases: a. When the signal from the network currently 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 switching between networks of different access technologies may occur. For example, when a user is streaming a video, she may use WLAN and when she is listening to highly compressed audio, she might switch to GPRS.
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. However, these mechanisms are inadequate.
In line with the 4G vision of bringing together wide-area (such as GPRS) and
local-area packet-based (such as 802.11) 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.
Scalability Support – HMIPv6:
IPv6 hosts have a home agent and co-located care of address. As they move from domain to domain or subnet to subnet, the send binding updates (or BUs) to inform their respective home agent and their corresponding hosts of the change in binding between their permanent IPv6 address and their co-located care of address. The HA may then tunnel packets from corresponding hosts to the mobile host at the new CCOA. When the binding updates reach the corresponding hosts, the corresponding hosts may send packets directly to the mobile hosts.
Although this approach provides the convenience of a single IPv6 address that is
independent of the point of attachment of the mobile host, it is not scalable. As the number of mobile hosts in a given domain increase, the number of binding updates increases. This in turn causes more signalling within the domains and across the internet. This overhead may lead to longer network delays. To work around this problem of IPv6. A shortfall of vanilla IPv6 is that it deals with intra-site mobility and inter-site mobility in the same way.
Local Mobility Management in HMIPv6:
Networks are divided into domains and subnets, with each administrative domain having a Mobility Anchor Point (MAP) at the highest level. Intra domain mobility of a mobile host is handled separately from inter-domain mobility. When the MH changes points of attachment within the same domain, the MAP of that domain is informed of the change in Care of address of the MH through binding updates. Binding updates are also sent to correspondent hosts within the same domain. The MAP functions as a foreign agent by intercepting IP datagrams destined for the MH and forwarding them to the appropriate CoA inside the domain. This way, intra domain handoff can be performed transparent to the MH’s Home Agent or external correspondent hosts, that is the MH does not need to send its HA or Chs binding updates. This reduces signalling traffic due to reduced binding updates. It also reduces handoff latency as far-off home agents and correspondent hosts need not be updated every time a mobile host changes point of attachment. This may be crucial to ensuring minimal handoff latency to ensure QoS for real-time data. When a mobile host moves between domains, IPv6 mobility management is used. hierarchy. The concept is illustrated in the diagrams below.
This hierarchical setup can be extended to multiple levels where subnets have MAPs and each MH has a virtual care of address associated with it at each level of the hierarchy.
Drawback of HMIPv6:
Due to the hierarchical nature of the protocol, each anchor point is only aware of the next anchor point down in the hierarchy. Each node stores a mapping of source : destination addresses which are a node’s previous and next nodes in the hierarchical structure. These addresses are called VCoAs ( virtual care of address). Only the lowest anchor point in the hierarchy stores a mapping of VCoA to PCoA which is the physical care-of-address of the MH in the foreign environment.
Support for Vertical Handoffs:
The current IPv6 specification does not support vertical handoffs. Since IP is the common protocol, everything below it is abstracted from the application. From the application, it is always connected as handoffs occur. To provide this support in IPv6 a daemon can be run at the network layer which takes care of switching between different radio access technologies. The mobile device might be having separate interface cards for each of the networks or may use a single multimode card which works in different modes at different times.
The protocol stacks of each of the different radio access technology are stored in the
mobile device. The daemon in the network layer will then choose which radio access network (RAN) to use on the basis of network speed, quality of service, cost of usage and other similar criteria. The selection policies are customizable and changes between different RANs are automatic and transparent to the user and depend on coverage and network load conditions.
Figure: Multiple protocol stacks being maintained at the Mobile Host
After selecting the RAN, the daemon then initializes the appropriate protocol stack
(GPRS or WLAN etc) before starting to use that interface. This way the IP datagrams being passed down get encapsulated in the correct format of the radio access technology in use. This model allows the device to utilize any interface as long as the hardware is present (or introduced through a flash port or PCMCIA port) by just installing the necessary stack protocols.
Other ways such as dynamically downloading the protocol stack from the network were discarded due to the complexity and the latency added in the process which made it unsuitable for real time applications. Also, memory not being such a big constraint, it was decided that maintaining multiple protocol stacks at the mobile node itself is a good way of adding vertical handoff functionality to the network layer without introducing much latency.Another enhancement suggested was to make the application transparent to any kind of delays due to horizontal or vertical handoffs. This has been suggested in the next subsection.
Conclusion:
IP based mobile telecommunications networks are the next big leap in the mobile telecoms industry. 4G or fourth generation mobile networks will allow users to roam over a variety of radio access networks such as WLAN, W-CDMA, CDMA2000 by integrating mobility management mechanisms and vertical handoff schemes at the network layer. Current or recently proposed network layer protocols such as IPv6 are not scalable when the number of mobile hosts becomes large. IPv6 does not cope well or differentiate between local (intra-site) mobility and global (inter-site) mobility. As most user mobility is expected to be local, most binding updates will be generated by locally mobile hosts. To reduce the signaling and processing overhead induced by a large number a binding updates, intra-site mobility needs to be transparent to correspondent hosts and home agents.
Hierarchical Mobile IPv6 is a scheme which exploits domain hierarchy. Administrative entities called Mobility Anchor Points (MAP) are added to border routers of each domain. When a host moves within a domain, it sends binding updates to the MAP to change its care-of-address to permanent-IP address mapping. This change is transparent to the Home Agent of the MH and to correspondent hosts. This reduces signaling traffic. Inter-domain mobility is handled by standard IPv6 mobility management schemes. Vertical handoffs between different radio access networks should take place transparent to the application layer. This can be achieved by setting up data streams on both radio access networks during handoff, synchronizing the two and then passing the data stream from the new radio access network to the application layer. This was illustrated in the section on soft handoffs.
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