Graduate school of computer science and Engineering Monmouth University West Long Branch, NJ, USA Email: {s0661493@monmouth.edu Tarik.guelzim@gmail.com} Abstract In the recent decade, our understanding of what a phone is has changed drastically. Moving from a simple gadget to make and receive calls, to a more sophisticated digital handset that acts as a cell phone, a personal digital assistant, PDA, a video camera, a messaging kit, a web browser and more [1]. The emergence of the IEEE 802.16e standard, VoIP and mobile-ip algorithms spawned a new generation of services that phone operators world wide will offer ranging from multimedia downloads to broadband internet access up to 20 Mbps, referred to as 4G or Fourth generation technology. Keywords VoIP, Mobile-IP, IEEE 802.11, IEEE 802.16e, WiFi, Fast Handoff, MAN, WLAN, H.323, 4G. 1. Introduction 4G technology promises the integration of the infamous IP protocol to manage the cell phone [2], this opens an even broader range of services consumers can benefit from with high availability and at a lower cost than the currently existing technologies. Sustaining voice services over IP networks shows a lot of potential in the Page 1 of 13
telecommunication industry [3]. Nevertheless, routing the data over the IP protocol requires a well known end to end IP address to the communicating hosts. This issue, however, has been intensively studied and solved by using mobile-ip model [6]. Such application of mobile VoIP has been also implemented using a combination of IEEE 802.11 and SIP [4] and simulation studies have solved many of the problems such as maintaining uninterrupted connectivity when a mobile phone moves from one station perimeter to another using the fast IP handoff technique described in [5]. This paper discusses the how this technology can be integrated with mobile phones and makes it an appealing choice for both phone operators and consumers. 2. Mobile IP (MIP) In order to support voice over IP in mobile networks, a lot of issues arise to keep the disruption time minimal when a mobile node has a communication in progress. Such issues can be inherited from the mobility concept itself where each mobile node (MN) changes its IP address every time it enters to a new base station s perimeter and changes networks along the way. A mechanism had to be defined to allow dynamic IP modification while preserving the mobility of the MN. IN [2], the author devised a mobility management scheme that makes use of the fast handoff technique described in [5], which uses an adaptive probabilistic algorithm to process handoff operation. Mobile IP was introduced to solve the mobility problem by maintaining two IP addresses. The first address, often referred to by the permanent home address is assigned to the mobile phone in the home network. The other IP address, called the care-of address (CoA) is assigned at the foreign network FA and represents the location of the mobile Page 2 of 13
node [6]. The binding between the permanent and the CoA addresses is maintained transparently between the HA and the FA. The following procedure summarizes the necessary steps for Mobile-IP: (a) The home and foreign agents located at the home network or mobile network respectively notify their presence by sending advertising packets. (b) The mobile node (MN) then decides whether to use the mobile IP mechanism depending on whether it is located inside or outside the home network. (c) The MN requests a CoA address when it moves to a foreign network and registers it with the home network to permit data packets to be routed to the right location. Figure 1: Mobile-IP (MIP) architecture (courtesy of Cisco networks) Page 3 of 13
2.1 drawbacks of Mobile IP The mobile IP technique suffers from some problems such as triangular routing phenomenon described in [7], and high handoff latency thus introducing huge performance overhead in terms of signaling load. The solution proposed by [7] to address this issue is to delegate the mobility management to regional domains/networks, instead of sending handoff information to the HA every time the mobile node moves, foreign networks can manage small movements locally by caching the MN s CoA. 3. voice over IP (VoIP) VoIP is the transmission of voice packet over an IP network. This technology emerged due to the popularity and reliability of the IP protocol as well as the increase broadband connection. In order to be transmitted, voice data is encoded using PCM (pulse code modulation) and sent over the trunk or the data line, these packets are then sent across the network with routing information to reach a VoIP aware hardware or software. To restore the audio data, the reverse process is applied on the received data and the original message is restored. Figure 2: VoIP with leased line over PSTN (courtesy of http://www.voipthailand.com ) Page 4 of 13
VoIP is an attractive model for service providers as well as consumers for the fact that it can utilize the pre-existing infrastructure of the internet. VoIP takes many form, but the predominant ones are PC to PC or PC to phone, which in turn means that the packets sent are routed from the IP network to the PSTN network and vice versa in the case of the 3G networks. 4. VoIP and H.323 Talking about VoIP cannot be done without mentioning the H.323 protocol, which is a complete suite that manages the by necessity for a standardized protocol soon to be named H.323 was created. This protocol defines the set of rules for voice data encoding schemes, call management, routing and so on. The H.323 defines a zone, which is a collection of terminals, is controlled by a gatekeeper. This latter manages the voice data that is put on the line to the internet which in turn is routed to the PSTN network via a gateway [8]. Figure 3: H.323 protocol stack (courtesy of Google images.com) Page 5 of 13
Nevertheless, the H.323 was sought to be very complex which led in turn to the emergence of the SIP, which is a lightweight implementation of the discussed protocol. Both SIP and H.323 can be used as part of the mobile VoIP protocol stack to control, maintain and deliver voice packets to the corresponding node CN across heterogeneous networks. 5. AT&T s igsm network for mobile nodes. Mobile-IP is a very elaborate technique to preserve uninterrupted data flow regardless of the targets mobility, And also VoIP is a technology that uses the infamous IP protocol to route PCM packets over the internet s network, thus, presenting a cheaper alternative to existing telephone networks. In 2000, AT&T laboratories have conducted research on how to merge VoIP services over GSM networks [3]. The architecture of their system consisted of using an optimized H.323 protocol over GSM networks. To start a communication, one must be inside the IP network and then it sends a request to the H.323 gatekeeper. This latter performs call control protocol including registration with the system as well as release while the mobile node moves from one IP network to another. The proposed system, igsm, also defines the translation mechanisms between the IP network and the PSTN or mobile phone networks as well as data marshalling between them [3]. This system however never went into a large scale implementation because some security concerns. 6. The dynamic mobility management model (DSR) Page 6 of 13
To support voice delivery to mobile nodes, various protocols are combined together to form a system i.e. Mobile IP, H.323, handoff techniques. However, these protocols impose a lot of messaging overhead and arise many security issues since the MN credentials are kept in all the foreign networks it moved from or into. A dynamic shadow registration (DSR) was proposed to solve those disadvantages and limitations [6]. Its main purpose is to provide a seamless VoIP handoff scheme. In order for the handoff to be transparent to the mobile node, the time it requires to be accomplished must be minimized. The proposed DSR is a probabilistic protocol that predicts the mobile node s next foreign network. This method requires an authentication, authorization and accounting (AAA) server in the home network, which is contacted by the foreign network before the handoff takes place. The home AAA server calculates the next hops for the MN based on probabilistic computation and sends the credentials of the MN to the AAAF of the FA s that are likely to be moved to the home AAA. This latter sends a security handshake to the network with the highest probability (p (i) 1) and starts the registration process. By the time the MN arrives at the new foreign network, the PCM packets can reach the new location of the node without any interruption. 7. Using the DSR scheme to MIP DSR is a clever model that solved many of the problems inherited by current technologies such as long hand off times and the interception of the MN s security credentials. Mobile IP defines two building blocks. The Home agent that controls and tunnels the PCM data to the MN s location and a foreign agent that leases the new care of address CoA to the MN. However, a new problem arises when the MN moves at high Page 7 of 13
speeds, 130 km/hr for example, while in the train or driving a car. The speeds results in possible abruption in PCM routing due to insufficient time to process a handoff. The authors in [6] devised a formula for calculating the total time required to accomplish a handoff: T = RTT<MN, DHCP> + RTT<MN,AAAF> + TT<MN,AAAH> The simulation of this process in [6] is summarized in the following table: Term Time required (ms) Description RTT<MN, DHCP> 10 Time to send and receive data over a wireless link RTT<MN,AAAF> 12 Time to send and receive data over a foreign network RTT<MN, AAAH> 25 Time to send and receive data from the home network T = 47ms is the time required for a handoff using the DSR technique with Mobile-IP. This rate brings the error rate to a mere 0.1018% over 10k of data sent [6], which is still high according to the industries standards but it is compensated by the lower overhead and the security property of the system. 8. IEEE 802.16 standard and the migration to 4G broadband networks. Even though 3G networks are not fully deployed yet, standards have been devised for the next generation network 4G that will feed broadband internet access to IP based and mobile nodes. Generally discussed under the umbrella of WiMAX (world wide interoperability for microwave access), it allows the existing network infrastructures to work and operate interchangeably at broadband rates. WiMAX is aimed at Wide Area Networks (WAN) and promises coverage in the range of 4-6 miles. This technology is a Page 8 of 13
viable alternative to provide VoIP services for MN s in the sense that it supports ATM, IPv4 as well as IPv6. The diagram below summarizes the WiMAX spectrum of targeted clients and the technologies it uses: Figure 4: IEEE 802.16 standard (courtesy of http://www.research.analysys.com ) What makes this standard very appealing for VoIP applications is its backward compatibility with existing technologies. The ability to operate in the 5MHz frequency spectrum enhances the maximum data rate to deliver the data. According to [1], mobile WiMAX can deliver data up to 30 Mbps for audio and video. IEEE 802.16 and IEEE 802.11x can work together to deliver voice over IP over a WiMAX network. It is done by integrating the H.323 and SIP protocols into the phones to manage internet telephony and route voice streams PCMs from a WiFi network when a mobile node is in a hot spot to a Page 9 of 13
WiMAX network back to another hot spot where the user is registered in using either SIP or H.323 or to the existing PSTN network. 8.1 Fast IP handoff for VoIP in WLAN networks WiMAX support for VoIP relies heavily on the widespread of WiFi networks in a metropolitan area network (MAN), however, preserving an ongoing session when a MN moves from one WiFi network to another is a real disadvantage because the handoff latency is very large due in large to the IP adjustment in access points. IP-IAPP as described in [5], also referred to by the Fast IP handoff, was proposed to resolve the latency issue and to guarantee a transparent mobility from one network to another. The mechanism of IP-IAPP works as follows: (a) Every MN gets an IP address from the home access point (HAP). (b) Once the MN moves closer to a foreign AP, it sends a trigger to start the IP-IAPP mobility management procedure to resolve the new IP. (c) The FAP now acts as the new agent for the MN and offers advanced routing services to it. This entire procedure is accomplished in less than 50ms according to the simulations made by [5]. Another method was described in [9] to support network layer mobility using transparent proxy caching mechanisms, which in turn requires minimum change in the existing infrastructure of the WLAN networks. The authors at [9] make use of standard DHCP and DNS servers to forward packets to the new location of the MN. The node binds to a Page 10 of 13
foreign network by sending a DHCP request and the proxy server in the new domain issues a DNS update of the new IP address assigned. This method requires no new protocols since it only introduces a proxy and a DNS server in each domain. In order for the PCMs to reach the MN, the proxy server intercepts the packets and forward them based on the DNS lookup entry. However, the performance of this system is tied heavily to the hardware used and the bandwidth of the network, which make it less susceptible to be a good solution for bandwidth hungry application such as voice or video streaming. Conclusion In this paper, we presented the different protocols or combination of protocols to service VoIP in a mobile IP environment. We described the igsm architecture devised by AT&T in [3] to integrate voice over IP over existing GSM networks. However, this method does not guarantee broadband access for voice applications that require a high bandwidth. Also, to offer VoIP in mobile networks, it is primordial to keep the ongoing sessions uninterrupted by implementing H.323 or SIP or their variations into the Mobile-IP model, where a MN can move freely from one network to another without any degradation in the voice stream caused by voice packets not been able to reach their new physical location and that by introducing a home agent, a foreign agent and a bind mechanism to resolve the home address to the Care-Of-Address. To ensure a high quality of service with minimal hand off time, we presented the DSR protocol [6] which uses a probabilistic algorithm to switch the control to a new foreign agent FA by calculating the possible candidate of foreign networks the mobile node might move into. IEEE 802.16 and its successor IEEE 802.16e were also introduced by the industry standard to offer broadband Page 11 of 13
access to mobile nodes in a metropolitan area network as well as how to integrate existing technologies such as WLANs to server as routing network segments. 4G networks promise to make data available everywhere at high broadband speed everywhere, and VoIP will sure be one of the beneficiaries of such advancements in networking and open the doors to the mass production of IP based personal mobile solutions. References [1] K.R. Santhi, G. Senthil Kumaran, Migration to 4G: Mobile based Solutions, IEEE computer society, 2006. [2] Chung-Kuo Chang, A Mobile-IP Based mobility System for wireless Metropolitan Area Networks, IEEE computer society, 2005. [3] Herman C.H. Rao, Yi-Bing Lin, Sheng-Lin Cho, igsm: VoIP service for Mobile Networks, IEEE communications Magazine, April 2000. [4] Sangki Yun, Hyogon Kim, Inhye Kang, Squeezing 100+ VoIP out of 802.11b WLANs, IEE communication Magazine 2006. [5] Ioanna Samprakou, Christo Bourasm Theodore Karoubalis, Fast IP handoff support for VoIP and multimedia applications in 802.11 WLANs, IEEE 2005. [6] Eunjung Lee, Jinsuk Back, Shou-hsuan Stephen Huang, A dynamic mobility management scheme for VoIP services, IEEE 2006. [7] Jun-Zhao sun, Jaakko Sauvola, Mobile IP applicability: When do we really need it?, IEEE 2004. [8] Andrew S. Tanenbaum, Computer Networks, Prentice Hall PTR 2003, [9] Wei Wu, Nilanjan Banejee, Kalyan Basu, Sajal K, Das, Network assisted IP mobility in wireless LANs, IEEE 2003 Page 12 of 13
Page 13 of 13