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Publisher : Elsevier - Science Direct (الزویر - ساینس دایرکت)
Journal : Computer Communications, Volume 27, Issue 8, May 2004, Pages 697–707
Providing seamless mobility support is one of the most challenging problems towards the system integration of fourth generation (4G) wireless networks. Because of the transparency to the lower layer characteristics, application-layer mobility management protocol like the Session Initiation Protocol (SIP) has been considered as the right candidate for handling mobility in the heterogeneous 4G wireless networks. SIP is capable of providing support for not only terminal mobility but also for session mobility, personal mobility and service mobility. However, the performance of SIP, operating at the highest layer of the protocol stack, is only as good as the performance of the underlying transport layers in such a heterogeneous environment. In this paper we analyze the handoff performance of SIP in a IP-based 4G network with Universal Mobile Telecommunication System (UMTS) and Wireless LAN (WLAN) access networks. Analytical results show that the handoff to a UMTS access network introduces a minimum delay of 1.4048 s for 128 kbps channel, while for handoff to a WLAN access network the minimum delay is 0.2 ms. In the former case the minimum delay is unacceptable for streaming multimedia traffic and requires the deployment of soft-handoff techniques in order to reduce the handoff delay to a desirable maximum limit of 100 ms.
Fuelled by the advancement of wireless technologies and the emergence of multimedia data services, cellular wireless networks have evolved to their third generation (3G) in just two decades. However, comprehensive 3G wireless networks are yet to be available due to the costly deployment and upgrade of already deployed system equipment. It may also be possible that 3G technology will never be fully deployed. Other predictions foresee a ‘generation jump’ directly to 4G wireless networks  and . The major task towards 4G architecture is system integration  and , where a unified wireless access system is to be established through the integration of the services offered by current access technologies such as General Packet Radio Service (GPRS), CDMA2000 or Wireless LAN (WLAN) as well as future wireless access technologies such as Universal Mobile Telecommunication System (UMTS). The trend towards packet switched technologies and increasingly general use and acceptance of the Internet Protocol (IP) indicate that different wireless access networks are to be connected to an IP-based core network, namely the Internet. Conceptually, a 4G wireless network architecture can be viewed as many overlapping wireless Internet access domains as shown in Fig. 1. In this heterogeneous environment, a mobile host (MH) is equipped with multiple (often called multi-mode) wireless interfaces to connect to any or all wireless access networks anytime anywhere. Therefore, providing seamless mobility support is one of the most challenging problems for the system integration in 4G wireless networks. Several mobility protocols have been proposed for wireless Internet , , , , ,  and . Although these protocols have the common goal of location transparency, they differ a lot from each other due to choices made during design and implementation phases. These protocols can be broadly classified based on the layer of their operation, such as those operating in the network layer , transport layer  and application layer . The dependency of these mobility protocols on the access networks reduces progressively as we move up on the protocol stack . Among them, Mobile IP  and Session Initiation Protocol (SIP)  have been standardized by Internet Engineering Task Force (IETF)  as the mobility solutions for the network layer and application layer, respectively. Although Mobile IP seems to be the architecturally right protocol for providing IP Mobility in the wireless Internet, it requires significant changes in the underlying networking infrastructure. Application layer protocols, however, are transparent to the lower layer characteristics. They maintain the true end-to-end semantics of a connection and are expected to be the right candidate for handling mobility in a heterogeneous environment. Indeed, SIP has been accepted by the third Generation Partnership Project (3GPP) as a signaling protocol for setting up real-time multimedia sessions. SIP is capable of supporting not only terminal mobility but also session mobility, personal mobility and service mobility. Therefore, SIP seems to be an attractive candidate as an application layer mobility management protocol for heterogeneous 4G wireless networks. However, SIP uses TCP or UDP to carry its signaling messages and hence is limited by the performance of TCP or UDP over wireless links. In addition, SIP entails application layer processing of the messages, which may introduce considerable delay. These are the prime factors behind the handoff delay while using SIP as the mobility management protocol. European Telecommunications Standards Institute (ETSI)  has defined in a quantitative way four different classes of performance—best, high, medium, and best effort—for voice traffic and streaming media over IP networks . The first two classes specify the type of IP telephony services that have the potential to provide a user experience better than the Public Switched Telephone Network (PSTN). Medium class has the potential to provide a user experience similar to common wireless mobile telephony services. Best effort class includes the type of services that will provide a usable communications service but may not provide performance guarantees. The specification for the end-to-end media packet delay for the best and high classes of services is less than 100 ms, while for medium and best effort classes the delay is less than 150 and 400 ms, respectively. In fact, a handoff delay of more than 200–250 ms makes voice conversations annoying. Clearly, the handoff delay, being a component of the total end-to-end delay, should also abide by these delay limits. Thus, it is evident that for quality of service (QoS) sensitive streaming multimedia traffic belonging to either best or medium class, the handoff delay should be less than 100 ms. In this paper we investigate the performance of SIP as a mobility management protocol in a heterogeneous access networking environment predicted for 4G wireless networks. In particular, we perform a case study of SIP-based handoff delay analysis using SIP to handle terminal mobility in a IP-based network. Two different types of access technologies, viz. UMTS and IEEE 802.11b based WLAN, have been considered for the IP-based network. Analytical results show that for WLAN networks the handoff delay is suitable for streaming media but for UMTS network the minimum handoff delay does not meet the specifications. More precisely, handoff to a UMTS network from either another UMTS network or a WLAN, introduces a minimum delay of 1.4048 s for 128 kbps channel, while a handoff to a WLAN access network from another WLAN or a UMTS network, the minimum delay is 0.2 ms. Clearly, in the former case the minimum delay is unacceptable for streaming multimedia traffic and requires the deployment of soft-handoff techniques to reduce the handoff delay and keep it within a desirable maximum limit of 100 ms. The rest of the paper is organized as follows. In Section 2, we describe the system architecture for the case study. In Section 3, we model and analyze the handoff performance of SIP for the sample architecture and present numeric results to evaluate the performance of SIP-based terminal mobility management. Section 4 concludes the paper.
نتیجه گیری انگلیسی
Several mobility protocols have been proposed for Wireless Internet targeting different layers of the network protocol stack to achieve different goals . Although each of them has the same goal of providing location transparency, the dependency of the mobility protocols on the underlying layers reduces as they operate higher in the protocol stack. In the next generation networks, a variety of wireless network technologies are supposed to co-exist, and hence no single network specific mobility protocol is expected to work for all of them. The design of a uniform mobility protocol that will work across all the different networks, requires tremendous efforts. The only solution seems to implement the mobility management functionality in the application layer, where there is least amount of dependency on the lower layers. SIP, accepted widely as a signaling protocol but capable of providing mobility support at the application layer, satisfies this criterion. We have performed a case study for a heterogeneous network with IP backbone having a combination of UMTS and WLAN access networks, to evaluate the performance of SIP-based mobility management. Results show that the minimum handoff delay introduced by the GPRS radio access of UMTS network is 1.4048 s for 128 kbps channel bandwidth, while the corresponding delay is around 0.2 ms for a 11 Mbps WLAN access network. Thus, the handoff delay while moving to a GPRS radio network, unlike the WLAN access network, is unacceptable for streaming multimedia. The major bottleneck found in our case study is the GPRS wireless access. Thus in order to comply with the maximum limit of the handoff delay, soft-handoff and advance resource reservation techniques need to be deployed.