A Multilayered Hybrid Architecture to Support Vertical Handover between IEEE802.11 and UMTS R. Good
[email protected]
N. Ventura
[email protected]
Department of Electrical Engineering University of Cape Town Rondebosch, South Africa will play an important role in the development of the 4G networks.
ABSTRACT Telecommunications advances have created the need for a high speed, ubiquitous network capable of catering for diverse application domains. This Next Generation or 4G network can be achieved through the interworking of several existing architectures to form a seamless global network. An important issue involved in interworking is vertical handover.
3G wireless refers to a group of standards for mobile cellular systems. The International Mobile Telecommunications 2000 (IMT2000) is the global definition of 3G wireless systems, as specified by the International Telecommunications Union (ITU) [9]. Several standards have been developed from this definition, the most predominant being the Universal Mobile Telecommunications Standard (UMTS), which is based on the widely deployed Global System for Mobile Communications (GSM).
This paper reviews mobility protocols, Mobile IP and the Session Initiation Protocol (SIP), and compares their ability to implement vertical handovers. A multilayered, hybrid architecture is proposed and described, which incorporates both SIP and Mobile IP. A Mobile IP framework is introduced and evaluated, based on its ability to implement vertical handover.
WLANs are capable of delivering high data rates, but have a limited range and allow limited mobility. Contrastingly UMTS networks deliver data rates up to only 2Mbps, but cover large geographical areas and allow high mobility. Clearly the integration of these technologies would be an important step towards the ubiquitous 4G mobile data network. This heterogeneous network would provide high data rates to all subscribers with range of a “hotspot”, while also allowing them to fall back on the widely available UMTS network, providing “always on” connectivity.
Categories and Subject Descriptors C.2.1 [Computer-Communication Networks]: Network Architecture and Design – Wireless Communication
General Terms Design, Experimentation, Performance, Management
Keywords
The issue of vertical handover must be addressed to achieve this integration. A handover occurs when an end user changes its point of attachment to a network. The nature of this point of attachment depends on the network architecture. For instance an end user in an 802.11 network attaches to an access point, while an end user in a GSM network attaches to a base station. Horizontal handover occurs when an end user moves between networks of the same architecture and establishes a connection with a similar point of attachment, while vertical handover occurs when an end user changes its point of attachment from one network to another architecturally different network.
Vertical Handover, Interworking, 4G
1. INTRODUCTION In recent years the nature of the Telecommunications industry has been radically altered. Technological advancement has seen the miniaturization of computing equipment allowing for powerful portable devices. Furthermore, the development and proliferation of wireless networking has allowed these devices to achieve true mobility without wired constraints. In 1997 the IEEE released the 802.11 standard, which gave rise to a number of other standards all forming the 802.11 family [11]. These standards were designed to facilitate the flexible creation of Wireless Local Area Networks (WLAN) and allow for the introduction of several new services. One of these services is the introduction of public wireless access networks, more commonly known as “hotspots”. The great success and massive recent deployment of WLAN technology indicates that these networks
The handover procedure between WLAN and UMTS networks must be both fast (low latency), and smooth (low packet drop) to support the wide range of necessary applications. To achieve these seamless handovers, several mobility protocols have been proposed. These protocols can be broadly classified based on the layer of their operation. The Internet Engineering Task Force (IETF) has standardized two of these protocols: namely Mobile IP and SIP [3]. Mobile IP is implemented at the network layer, while SIP is implemented at the application layer.
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The remainder of the document is organized as follows. Section II introduces Mobile IP as a mobility management protocol and explains its operation. This section analyses the protocols ability to achieve vertical handover between WLAN and UMTS
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networks. Section III introduces SIP as an application layer solution to provide mobility support in heterogeneous networks. Section IV proposes a hybrid architecture incorporating both SIP and Mobile IP as the best candidate for mobility management in heterogeneous networks. The implementation and architecture of such a network is discussed. Section V gives an overview of the developed evaluation framework, and analyses the test results. Section VI presents conclusions drawn from the design of the hybrid system, and the development of the evaluation framework.
2. MOBILITY MANAGEMENT USING MOBILE IP Mobile IP was introduced to provide seamless, transparent data services to applications running on mobile devices. It is an extension to standard IP that makes mobility transparent to higher level protocols, allowing upper layer sessions to continue despite network layer movement [4].
Fig. 1. Mobile IP system overview
2.1 System Overview
2.2 Disadvantages of Mobile IP
Mobile IP introduces several new concepts. A mobile node (MN) is any host that can connect to a network and move into foreign subnets. Mobile IP achieves transparent mobility by allowing a MN to have two addresses: a home address (HoA), and a care of address (CoA) [6]. The HoA is static and is used to identify TCP connections, while the CoA changes at each new point of attachment and is the MNs location significant address.
While Mobile IP does offer a solution to the problem of transparent mobility, it suffers from several drawbacks. From the functional description of Mobile IP it should be clear that when a MN is visiting a foreign network, all of its packets are routed via the HA through a tunnel, while packets being sent from the MN are sent directly to their destination. This results in a triangular situation where packets to the MN experience larger latency than packets being sent from the MN. This difference in latencies causes unacceptable conditions for most real time applications. Route optimization is the proposed solution to the triangular routing problem [7]. The basic principle behind this mechanism is the addition of a binding cache at the corresponding node, allowing packets to be sent directly to the MN. Route optimization requires significant additions to the corresponding node, which means that wide deployment can not be expected in the near future.
Two new network elements must be introduced for the correct functioning of Mobile IP: a Home Agent (HA) and a Foreign Agent (FA). The HA is a specialised router on the home network, which forwards traffic to its current point of attachment when it is away from the home network. A FA is a specialised router in the foreign network that cooperates with the HA to deliver traffic to the visiting MN. Collectively these elements are known as mobility agents. When a MN moves into a foreign network it must register its CoA before sending or receiving any packets. Mobile IP defines two modes for obtaining a CoA. In the first mode, foreign agent CoA the MN uses the address of the FA it is registered with as its CoA. In the second mode, co-located CoA the MN acquires its own local IP address, which it uses as its CoA. In this mode an external mechanism such as DHCP is needed to assign valid local IP addresses.
Mobile IP uses IP in IP encapsulation to tunnel packets to a roaming MN. This increases packet overhead, which can drastically increase packet latency. A MN in a foreign network can often be mistaken as a malicious user because its source IP address is not recognized. This is due to the fact that the corresponding node associates with the MNs HoA, and does not recognize packets sent from the MNs CoA. These packets will not pass through firewall and ingress filtering mechanisms.
As the MN moves through subnets, the HA creates bindings, which map the MNs HoA to its current CoA. As part of the registration process when a MN acquires a new CoA, it notifies the HA that it is reachable through a new address. Each of these bindings has an associated lifetime, ensuring dormant bindings are not maintained.
2.3 Mobile IPv6 As IP has developed along two different paths, so has Mobile IP. Up until now in this paper what has been referred to as Mobile IP is in fact Mobile IPv4, and a second standard Mobile IPv6 has also been developed. Mobile IPv6 incorporates a larger address space and additional built in enhancements. While it seems clear that IPv6 and Mobile IPv6 are the technologies of the future, this is still a very distant future as IPv6 is having difficulties spreading due to the world wide IPv4 infrastructure in place [6]. This as well as the fact that operators are interested in reusing their investment in the IPv4 network, means that IPv4 and more importantly Mobile IPv4 is still an important area of research.
The HA then intercepts all packets destined for the MN, using proxy and gratuitous ARP and sends them along a tunnel to the MN. The HA constructs a new packet that contains the MNs CoA as its destination address. This new header then encapsulates the original packet, causing the MNs HoA to have no effect on the routing until the packet reaches the CoA. This process is known as IP in IP encapsulation [8]. Figure 1 shows how a MN is able to maintain upper layer sessions while visiting a foreign network.
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Standard SIP supports terminal mobility, but must implement session, service and personal mobility to provide IP mobility support. How SIP achieves this mobility is best illustrated using an example. A corresponding node is trying to establish a connection with a MN and sends a SIP INVITE message. If the MN is attached to the home network an ACK message is sent and a SIP session is established. If the MN is away from the home network, this is detected and the redirect server sends the current point of attachment of the MN to the corresponding node. The corresponding node then sends an INVITE message to this new location to establish a SIP session. This is shown in figure 3.
This paper focuses on Mobile IPv4, which from here on will be referred to as Mobile IP for simplicity.
2.4 Vertical Handover When interworking WLAN and UMTS networks the link layer technologies are drastically different and certain changes need to be made to the Mobile IP implementation. Firstly a mechanism is needed to monitor the WLAN and UMTS interfaces, and based on certain criteria switch between the two. Secondly traffic streams need to be carefully monitored when performing handovers due to the change in bit rate. Additional packet buffers are needed to accommodate the differing bandwidths. A further mechanism is needed to monitor network loads to ensure QoS provisioning when moving between networks. The IP Multimedia Subsystem has been introduced into the UMTS architecture as of Release 5. IMS is an overlay architecture that provides multimedia services over IP networks, and will introduce several QoS provisioning mechanisms in later releases; this is beyond the scope of this document. Figure 2 shows the necessary system architecture.
Fig. 3.Esatblishing a session with SIP mobility management If during the session the MN moves to another network, SIP must allow sessions to remain active. When a MN moves to a new network, it sends an INVITE message to the corresponding node using the same call identifier as the original call setup. In this way a new session is reestablished. This is shown in figure 4. Fig. 2.UMTS –WLAN interworking with Mobile IP mobility management
3. MOBILITY MANAGEMENT USING SIP SIP is an application layer signaling protocol standardized by the IETF used for creating, modifying and terminating sessions with one or more participants. SIP has been proposed as the correct candidate for handling mobility in a heterogeneous environment for several reasons. SIP is an application layer technology, which means its functionality is transparent to all lower layers. It is a very simple, robust and scaleable protocol. Furthermore, SIP is an end-to-end orientated signaling protocol which means all logic is stored in the end points.
Fig. 4 Session continuity using SIP mobility management Results have shown SIP to be far more efficient than Mobile IP, regarding packet loss and handover delay, as a mobility manager handling real time session [10]
3.1 System Overview SIP introduces several new concepts and network elements. User agents are end points that use SIP to manage communication sessions. Proxy servers are SIP entities that perform several functions including routing of sessions invitations and AAA. A registrar is a special SIP entity that receives registrations from users, and stores information about their current location in a location database. A redirect server is an entity that receives requests for a particular user agent and replies with the current location of that user agent.
3.2 Disadvantages of SIP As stated earlier SIP is an application layer signaling protocol that can be used to support mobility. However, it must be made clear that SIP does not support network mobility; each time a MN enters a foreign network it is assigned a new IP address and reestablishes all sessions. This change in IP address destroys all TCP connections established by the MN. This inability to support TCP connections is disastrous for non real time applications as a new TCP connection results in all previous data being discarded.
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The MN must implement a Mobile Policy Table (MPT), which is used to decide which mobility protocol to use when a handover occurs. In the MPT we can specify that traffic using a certain port must be handled by a specific mobility protocol. In this way we can divide the traffic into real time and non real time sessions, assigning SIP to real time sessions, and Mobile IP to non real time sessions.
3.3 Vertical Handover The IP Multimedia Subsystem architecture plays an important role in SIP based handover as it has adopted SIP as the signaling protocol for providing multimedia services. The IMS introduces a new element called the Call Session Control Function, which acts as a SIP server in the UMTS network. The system architecture used to implement vertical handover is shown in figure 5.
This proposed architecture will take advantage of the functionality of both SIP and Mobile IP and provide efficient macro-mobility support for a heterogeneous architecture, while Cellular IP or Hierarchical Mobile IP would be used to implement micro-mobility. The entire architecture is illustrated in figure 6.
5. EVALUATION FRAMEWORK A Mobile IP evaluation framework was developed as part of the proposed solution to confirm some of the aforementioned conclusions and to perform further tests. A framework as opposed to a simulation was chosen for several reasons. While the architecture could have been created and tested in a network modeling environment, an evaluation framework allows the developer to address issues on all levels of the design process. Furthermore when implementing a framework many problems that go undetected in a simulation, become readily apparent. The aim of the framework was to have a working system that used Mobile IP to implement vertical handover between heterogeneous networks as shown in figure 2.
Fig. 5 UMTS WLAN interworking with SIP mobility management
4. HYBRID MOBILE IP/SIP ARCHITECTURE
5.1 System Architecture
It should become apparent that Mobile IP and SIP are architecturally suited for different applications. A practical solution would be to incorporate both SIP and Mobile IP into a single architecture, where the two protocols could complement one another. This multilayered architecture would use SIP mobility for the real time domain, and Mobile IP for the non real time domain.
When translating the model into a practical framework several abstractions had to be made. The currently deployed UMTS network does not support Mobile IP making a physical implementation impossible. Consequently the UMTS network was represented by an 802.3 LAN. While this simplification removes some of the issues involved with interfacing to the UMTS network, it still allows vertical handover to be addressed because from the transport layer upwards these protocols are identical.
Integrating these two architectures involves several issues. Network Address Translation (NAT) is a technique in which the source or destination addresses of an IP packet are rewritten as they pass through a router. When integrating SIP and Mobile IP, NAT functionality must be implemented by border routers to modify SIP header fields so that the IMS in the UMTS network will be able to process these messages.
The Internet cloud is replaced by a low performance software router. In the WLAN foreign network the functionality of the access point and router is incorporated into the FA. The framework structure is shown in figure 7.
Fig. 7 Evaluation Framework System Overview Fig. 6 Hybrid Mobile IP/SIP system architecture
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test was conducted for 60 seconds and while proceeding, a vertical handover was forced and the MN moved from the home network to the foreign network.
The MN, HA, FA and software router were all implemented on desktop machines, on a Linux platform; the MN and FA were equipped with wireless interfaces and IP mobility was emulated by forcing the MN to change from one interface to another. Dynamics Mobile IP [1] was used to implement the Mobile IP behaviour of the various elements. Dynamics was chosen because of its flexibility and ease of use.
The 60 second period was divided into 10 segments of 6 second duration. A histogram is produced that shows how long the sending host spends in the send() call waiting for ACK packets to arrive for each segment. The results are shown in figure 9; the first two segments representing call setup have been omitted to isolate the handover mechanism.
The framework used Foreign Agent CoA mode, because colocated CoA mode requires an external DHCP server. Dynamics offers both triangular and reverse routing; reverse routing was chosen for the platform because of its security implications and reliable handling of real time sessions.
From the graph it can be deduced that the vertical handover occurred during time segment 5 due to the increased time spent in the waiting call. When the MN moves to the foreign network, it must follow the procedure of registration, tunneling, etc.; consequently the sending host must wait longer for the ACK packets to arrive. The TCP session continues in time segment 6, 7 and 8 showing that TCP connections are maintained during vertical handover.
External functions were created that used the Dynamics APIs to force the behaviour of the MN. These functions can force the MN to associate with either the home or foreign network. Using the wireless extensions API it is trivial to monitor the wireless interface and force a handover based on certain statistics (signal to noise ratio, signal strength, etc.). In this way the MN uses the 802.3 wired interface, unless an 802.11 wireless network with sufficient signal strength is available; detection of such a network will force a vertical handover. Ideally network load would be taken into account to help assist the handover decision; this has not been implemented in the current framework. Additional buffers to cater for different network data rates have also been omitted from the design for simplicity.
5.2 Test Results To verify the correct functionality of the framework a simple ping test was used. Packets were sent from the corresponding node to the MN, and a handover was forced. These results are shown in figure 8. Note the difference in round trip time for the home and foreign networks. In this small, isolated evaluation framework the packet latency is tripled and in a larger, more complex network this latency can expect to increase further. Fig. 9 TCP stream test results fro vertical handover While TCP is not the standard protocol for real time transfer, this analysis does give an indication of the performance of Mobile IP when handling real time sessions. With a handover disruption of well over 1 second it can be deduced that Mobile IP is not suitable for multimedia sessions, but the maintaining of TCP connectivity means that non real time sessions are sufficiently supported.
6. CONCLUSIONS The stringent user requirements in today’s connected world have introduced the need for a new generation, high speed network capable of supporting vast numbers of applications and “always on” connectivity. While this 4G network has not been defined at this time it is likely that it will involve the interworking of most existing network architectures to form a single IP based infrastructure. This paper investigated the interworking of 802.11 and UMTS cellular networks, and has attempted to address the issue of vertical handover in heterogeneous networks.
Fig. 8 Ping verification test To test the performance of the Mobile IP framework the bench marking traffic generator Netperf [2] was used. Netperf was used to set up a TCP stream between the corresponding node and the MN. This kind of test gives an indication of how fast one system can send data and/or how fast the other system can receive it. The
Two network architectures capable of implementing vertical handover have been introduced: a SIP based system, and a Mobile IP based system. Mobile IP drawbacks make that system unsuitable for real time applications, while SIPs inability to
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[5] S.M. Faccin, P. Lalawaney, B. Patil “IP Multimedia Services: Analysis of Mobile IP and SIP Interactions in 3G Networks”, 2004.
support TCP connections makes that system unsuitable for non real time sessions. A hybrid architecture has been proposed incorporating both SIP and Mobile IP as mobility managers. This allows the protocols to complement one another, each managing a specific application domain.
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A Mobile IP network was developed in the form of an evaluation framework on a Linux platform. A fully functional system capable of forcing a MN to associate either with an 802.11 wireless network, or an 802.3 wired network was created. This framework illustrates many of the issues surrounding Mobile IP and vertical handover.
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With the massive popularity of portable devices supporting IP connectivity, the issue of mobility becomes a serious concern. A user must be able to roam through different networks while maintaining seamless connectivity. The integration of Mobile IP and SIP helps to achieve this goal providing transparent mobility services for both real time and non real time applications.
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7. REFERENCES
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