Seamless Roaming Between VoWLAN and Cellular Networks HungJu Tze Department of Electrical and Computer Engineering University of Toronto Abstract This paper demonstrates the handoff mechanism specifically designed for seamless roaming between VoWLAN and cellular networks. This includes a comprehensive review of the current handoff technologies used in VoWLAN and cellular network as well as evaluations of different factors that affect VoWLAN QoS. The handoff is implemented in software on the mobile handset without modifying the existing network architectures. In result, this mechanism can offer users best features of both VoWLAN and cellular networks. Introduction WiFi (802.11) [1] is a flexible communication system that is able to deliver high bandwidth wireless data service at low cost. WiFi has proven to be a spectacular success and is one of the fastest growing segments in the telecom industry. WiFi is becoming the most pervasive wireless data service just as cellular networks made wireless voice as a part of most people s lives. The WiFi does not stop growing only as a commutation technology to offer wireless data service. With the Voice over IP (VoIP) technology, the WiFi goes beyond offering data service and offer voice service. Seamless voice and data communication have gradually become a reality on WiFi networks. The Voice over IP (VoIP) [2] technology is to treat voice as data packets and carry them over the Internet. Due to the price competition in voice telecom market between service providers, these service providers have to find a way to lower costs on delivering voice services. This is one of the biggest initiatives for service providers to launch VoIP technology on their networks. In addition to cost saving, VoIP introduces the opportunity for services integration. As IP is a flexible service model which splits transport and application services, new services can be created easily and integrated with the current voice service, such as multi-video conferencing, and unified communication service. In result, service providers will be able to generate new revenues by offering various kinds of new services on top of the traditional voice service. With the popularity of VoIP and WiFi, VoIP over WiFi (VoWLAN) is becoming an emerging technology [3] to deliver wireless voice services. VoWLAN combines the network cost savings of WiFi with the service model of VoIP. It will be a compelling idea to offer the best features of both WiFi and cellular networks to user by combing flexible and low cost of VoWLAN and large coverage of the cellular networks. Hence, a handoff technique has to be developed in order to integrate features of both networks. With this technique, users can choose to route voice calls through the internet when they re within a WiFi network for cost efficiencies and services. Users can hook up to a cellular network when they step out of the WiFi coverage area. Furthermore, users can maintain the voice call session while moving between different cells of cellular networks, between different spots of WiFi and between different WiFi spots and cellular cells.
Background This section will illustrate different techniques that maintain voice call sessions when moving from one network to another. This includes handoffs in cellular network and VoWLAN. Handoff in Cellular Networks When a user is moving between different cells in the cellular network, it requires a technique called handoff. This technique is to pass a voice session from one cell to another. The handoff is performed so quickly that users usually never notice. The handoff enables mobility between different cells for users without dropping the call in the cellular network. There are two major cellular technologies which employs different handoff techniques. The first one is called GSM (Global System for Mobile). The design principle of GSM is that a given slot on a given frequency channel can not be used by neighboring cells. When a phone which is in a call moves from one cell to another, at a certain point it has to switch between cells. This is called the hard handoff. In this approach, the GSM handset takes measurements of radio channel s signal strength and reports to Base Station Controller (BSC) and Mobile Switching Center (MSC) to pick a cell for handoff. As a result, the handoff will be all or nothing. The other cellular system is called CDMA (Code Division Multiple Access) which uses a completely different way to implement handoff. It uses a technique called soft hand-off. There are two base stations involved when handoff is happening. One is in the cell site where the phone is located and the other in the call site to which the call session is being passed. In CDMA, the handset will find the second cell and get itself connected. Unlike GSM, both base stations will hold off the call until the handoff is complete. The first site does not cut off the conversation until it receives the information that the second site is maintaining the call. Thus, the handoff reliability is higher and thus increases the range of the cell [4]. Handoff in VoWLAN Networks The WiFi network access point is similar to the base station in cellular networks. There are different approaches to achieve handoff in VoWLAN network in different layers of the OSI model. The first approach will be mobile IP. It is an approach to support mobility in the layer 3 by achieving access point to access point handoff. Mobile IP is based on the concept that a mobile node has a home address associated with a home network. Each time the mobile node connects to a foreign network, it obtains a temporary address which is known as a Care of Address (CoA). The CoA is valid while the mobile node is attached to the foreign network domain. It is deleted or purged from the foreign network once the mobile node leaves the domain. Obviously, this approach requires collaboration of various network elements in the wireless network to support mobility. As an alternative to Mobile IP, SIP supports handoff for VoWLAN applications by providing handoff capabilities at the application layer. SIP (Session Initialization Protocol) is one of the most popular signaling protocols used for implementing VoIP. It is designed to be used in applications that require session setup and parameter negotiations before actual communication. When initializing the VoIP call between clients, the client device will use SIP signaling to negotiate for the communication parameters such as codec, and response time. When the signaling is finished, the RTP media stream will be established to transfer voice as data stream. The SIP approach to mobility is to resend the same INVITE SIP message [5] originally used for the caller to initiate a call with modified parameters. The re-invitation informs the other call participant about the new IP address, which then is used by the RTP session for any subsequent audio transfer. Though the communication actually is broken and reestablished in this process, the end user can get a seamless experience if the call session reestablishments can be done automatically and quickly. The SIP approach can reuse the existing network architectures to achieve mobility. However, is not
as efficient as the Mobile IP approach as SIP costs twice as much as time for client to associate with an access point and the time to use of the Address Resolution Protocol (ARP). The SIP approach will complement the mobile IP approach when user is moving between two different WiFi networks. Roaming in VoWLAN & cellular networks Knowing that cellular networks always have greater coverage than WiFi, it is assumed that a cellular network is always available when users steps out of the WiFi coverage area. As more and more handsets are both cellular and WiFi enabled, it is then made possible to have voice roaming between two different networks. In order to achieve seamlessly roaming of voice services between these two networks, it is necessary to develop a handoff technique similar to handoffs in VoWLAN or cellular network to maintain call session when moving between these two different networks. In all handoff processes, the handheld device has to be intelligently aware of the correct situation to perform the handoff when user moves out of the network coverage. This includes monitoring the signal strength from both networks and determines when to perform the handoff based on the data. Knowing the threshold in order to perform handoff correspondingly is the first requirement for achieving seamless roaming. The second requirement for seamless voice roaming is to perform the handoff. Handoff between VoWLAN and Cellular Networks will be challenging compared to a single network which is designed for handoff. WiFi and Cellular Networks are never designed to handover a call to each other at the network level. VoWLAN Capacity analysis Combine VoIP and WLAN is an obvious course of action considering the popular deployment of the technology. However, the VoIP performance issue arises when moving from a wired network to a wireless network. In contrast to wired network, wireless network will introduce new variable as VoIP is originally designed only for wired network. The bandwidth required by VoIP will be different depending on the encoding method of the voice stream. While G.711 is the mainstream for toll-quality voice services, a number of codex are developed for more efficient compression. G.723 is the codec in the ITU standard H.323 and offers high compression but its voice quality is similar to cellular phones. G.729 is a codec which is designed for balance between voice quality and bandwidth requirement. G.729 makes VoIP possible even with a dial-up or ISDN internet connection. Codec Bandwidth IP bandwidth (2Way) G.711 64Kbps 160Kbps G.723.1 6.4Kbps 34Kbps G.729 8Kbps 48Kbps Table 1: codec comparison Most of VoIP applications still choose G.711 as their primary codec simply because of compatibility issues. Even though G.729 is a more efficient codec for voice coding, it requires additional standard such as T.38 to support fax and other telephony features. As the bandwidth of last mile increases every year, it is likely to be seen that G.711 is still the primary codec used for VoIP applications. In an IP network, voice codec are placed into packets with a duration of 5, 10, or 20 msec of sample voice. These samples are then encapsulated into a VoIP packet. To determine the real bandwidth required by the Voice on IP network, the overhead of IPv4, UDP and RTP have to be included. The two way VoIP bandwidth requirement can then be calculated.
To optimize the network capacity of WiFi when deploying VoIP, the throughput [6] of WiFi has to be taken into consideration. The throughput of WiFi will vary depending on the modulation density and MAC protocol. The small coverage of a particular WiFi network can offer higher data rate modulation while larger coverage will use lower rate modulation. Modulation HCF(Mbps) DCF/EDCF(Mbps) 54Mbps 40.5 19.9 22Mbps 16.5 8.1 11Mbps 8.25 4 5.5 Mbps 4.125 2.04 1Mbps 0.75 0.37 Table 2: modulation and MAC protocols There are two different kinds of MAC protocol. The Ethernet protocol, DCF and EDCF, limit capacity at approximately 37% of the peak data rate. DCF/EDCF protocols do not effectively manage network latencies as capacity limit is approached. In contrast, scheduled TDMA protocols such as HCF can carry approximately 75% of network capacity. HCF control latencies and provide QoS by providing fair weighted queuing so that all users will receive service under full-network-load conditions. Quality of service mechanism 802.11e [7] has also to be in place to ensure the bandwidth is secured for voice service. Theoretically, 802.11b s bandwidth is capable to support more than 10 G.711 calls simultaneously through one single access point. Experiments are performed using G.711 on single 802.11b network configured with DCF Mac protocol. The packet loss is used to measure voice call quality on WiFi network. The result is shown in Figure 1 Packet Loss (%) 40 35 30 25 20 15 10 5 0 0 2 4 6 8 Number of VoIP call Figure 1 VoIP call and Packet Loss Surprisingly, the WiFi will only be capable to support five calls simultaneously to maintain acceptable voice quality call. Another series of experiment is performed using higher compression voice codec G.729 to see if more calls can be handled by the WiFI. The result doesn t change as access point congestion depends on number of packets the access point can process than on the actual bandwidth. Voice packets are relatively small and sent frequently which explains the low throughput for the voice packets. Due to this characteristic of WiFi, it is common to put more multiple voice packets in the single packets to increase capacity. However, this will increase the delay as packet size is getting bigger. The other approach is to design a voice codec which is able to adapt to the packet loss. The
VoWLAN network has to be planned to take all these factors into consideration. The VoIP call can be handoff to cellular networks to avoid quality degradation of voice call once the packet loss is presented due to high number of VoIP call in the cell. Interference on WiFi Signal Similar to cellular networks, WiFi signal strength is one of the most important attribute to determine the situation for handoff. The signal strength of WiFi decreases as distance increases. Unlike wired lines, wireless links usually have high bit error rate and unpredictable link behavior. The fluctuation of wireless signal strength can be caused by several factors; weather change, user movement, orientation and obstacle. As weather is an uncontrollable variable, it is assumed that the impact of weather change to the signal strength will not be in the scope of experiments. It is assume that only one WiFi network is presented in the 2.5 GHz band though theoretically three 802.1 networks can exists in the same time without interference. Other network such as bluetooth seems to have little interference on the performance on VoWLAN [8]. Hence, interference caused by other networks in the same band won t be taken into consideration. User movement is defined as the user s movement away from the WiFi access point. User orientation is defined as the body of the user blocking the direct path between the end device and access point. The effect of obstacle is referred as the obstacle blocking between the end device and the access point. All experiments were performed in an open-space environment to eliminate all possible interference, particularly multipath and fading effect. The wireless end-device is a notebook with software to measure the signal strength of WiFi. The access point is a 802.11g enabled device. All experiments were conducted multiple times in order to achieve higher accuracy. The benefit of wireless is to enable to user to carry an end device around. It is a common scheme that user orientation will occur when the user moves. The user orientation will cause the user s body to block the line of sight between the end device and the access point. -70 Signal Strength(dbm) -75-80 -85-90 -95 No user blocking User blocking Figure 2: User orientation Figure 2 illustrates the effect of user orientation. Obviously, user orientation will definitely cause a decrease in the signal strength up to 10dbm. This factor has to be taken into account to determine the threshold for handoff. In contrast, the user movement does not seem to affect the signal strength. The signal strength stays the same when the user walks (0.3m/sec) and runs (0.6m/sec). In figure 3, it is clear the user movement won t affect the signal strength when the distance between the end device and the access point stays constant.
0 Distance(m) Signal Strength(dbm) -20-40 -60-80 0 20 40 60 80 100 120 140-100 Stop Walking Running Figure 3 User movement In the urban environment, obstacle such as people and cars can be moving in middle of the end device and the access point. This is similar to user orientation but the obstacle can usually be bigger. Five series of experiments were performed by placing car to place in different distances to the end device. The distance between the end device and the access point is 100 meters. In Figure 4, the obstacle does affect the signal strength approximately 10dbm when the obstacle is close to either the access point or the end device. The obstacle could cause serious signal strength decrease when the obstacle is moving dynamically closer to either mobile client or base station. This is another important factor which has to be taken into account to determine the threshold. Signal Strength (dbm) -65-70 -75-80 -85-90 -95 No Blocking Obstacle Blocking 1M 10M 50M 90M 99M Figure 4 Obstacle In the real world, wireless NIC will not be able to remain connected to wireless network when the signal strength is below -96dbm. When determining threshold for handoff, the handset designers have to account interference caused by obstacles and by user orientation in the worst case scenarios. The threshold can be derived by adding the possible interference to the minimum signal strength that the wireless NIC can remain connected. When the signal strength is below the threshold, the VoWLAN call has to be handover to cellular network to avoid possible VoIP call drops. The call on cellular can be handoff to VoWLAN if the signal strength is above the threshold. The precise threshold can be further justified depending on the WiFi network planning.
Design of Handoff Technique in roaming Voice roaming requires a way to transfer a call from one network to another in a reliable manner. As VoWLAN and Cellular are two completely different types of networks, the handoff have to be implemented at the higher level compared to handoffs in cellular/vowlan networks. In practice, the WiFi/Cellular dual mode mobile device will be assigned two addresses (telephone numbers or SIP addresses) for both VoIP and cellular phone. The unified numbering service in voice IP can integrate two addresses from VoIP and cellular to be one single reachable number. The unified numbering service can be configured to allow all incoming calls to reach the appropriate number in VoWLAN and cellular networks. Furthermore, two telephone numbers from VoWLAN and cellular networks also grants the mobile device to simply use call transfer feature to perform the hand-off when roaming between networks. There are two types of call transfer in VoWLAN: blind call transfer and consult call transfer. The blind call transfer tries to transfer a call directly from the calling number to the desired number without monitoring the process. In the case of moving from VoWLAN to cellular, the blind call transfer will transfer the number from VoWLAN to cellular. The blind transfer will put the call on VoWLAN on mute during the call transfer to cellular phone. Therefore, there will be no voice going through until the voice is routed to cellular phone. On the other hand, consulting call transfer will keep the call session in VoWLAN and avoid interruption of the conversation while transferring the call to cellular. Obviously, it is necessary to perform consulting call transfer when roaming between VoWLAN to cellular networks. Therefore, consult call transfer feature on both networks will provide reliable hand-off to enable roaming. The software in the mobile device will have to hide the complicated call transfer mechanism to provide seamlessly roaming experience. The handoff should take place when the WiFi signal is below the threshold or high packet loss due to mass usage in WiFi cell. The VoWLAN call should not get dropped until the call session on cellular network is created. The call on cellular should be routed to VoWLAN by using the same mechanism once the WiFi signal strength is above the threshold and link quality is acceptable. Conclusion It goes without saying that both VoIP and WiFi are emerging technologies that are gaining a lot of attentions. This is because of the low cost and flexibility offered by these technologies. The marriage between WiFi and VoIP will be able to combine all these features to provide a complementary solution to the current cellular voice service. The handoff between VoWLAN and cellular will let users have voice service from best features of both networks; coverage in cellular, cost efficiency and flexibility in WiFi. The handoff mechanism demonstrated in this paper make seamless roaming between VoWLAN and cellular networks possible without modifying the existing network architectures. Dual mode mobile handsets have to be designed in a way to hide all these complexity to provide seamless roaming experience. As the result, VoWLAN and cellular can integrated to provide users flexible, affordable, and reliable voice services.
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