By adopting Session Initiation Protocol (SIP) in WiMAX networks, when the mobile node (MN) moves to a foreign network, the MN sends a re-INVITE message to the corresponding node (CN) to re-establish the connection. This re-connection time is the most costly factor for a handoff. To effectively reduce the re-connection latency, a hierarchical SIP (HSIP) mobility management incorporated with MAC layer operations is proposed. As proposed in the HSIP architecture, several Base Stations (BSs) are collectively managed by an HSIP server to form an administration domain. When an MN roams within a domain, which is the most common mobility case, a re-INVITE message is not necessary, hence a significant traffic reduction can result. To demonstrate the applicability of the proposed HSIP mobility mechanism, an evaluation using the NS2 simulator was performed. Handoff delay and signaling overhead are investigated in both single-handoff and multiple-handoff occurrences. When the ratio of intra-domain to inter-domain handoffs is increased from 1 to 14, the proposed HSIP mobility mechanism can improve up to 13% in average handoff delay and 35% in average signaling overhead as compared with traditional SIP mobility management.
The maturity of IEEE 802.11 technology and its low setup cost has enabled it to be successfully applied to WiFi (Wireless Fidelity) networks. Many cities have now set up wireless network access points to offer users the convenience of always-on Internet [1]. The technology’s limited coverage and scalability, however, have limited wireless Internet access to specific areas. The specification of the IEEE 802.16 [2] WiMAX (Worldwide Interoperability for Microwave Access) standard provides for coverage of up to 40 miles and a transfer rate of up to 70 Mbps. The superior coverage and scalability means that it will gradually become the network technology of choice for future IP-based mobile communications networks [3]. Two fundamental standards are supported by IEEE 802.16. One is IEEE 802.16-2004, used for fixed wireless services, and the other is IEEE 802.16e [4], used for mobile communications. Users can either directly connect to the Base Station (BS) or indirectly connect to the BS through a Subscriber Station (SS).
In the All-IP network, support for mobility management is urgently needed. Currently, the Mobile IP [5] and SIP [6] protocols are the two most commonly used mobility mechanisms. Mobile IP may suffer from the triangular routing problem. In Mobile IP, when an MN moves to a foreign network, it is associated with a Care of Address (CoA) by the foreign network agent. The MN must register its CoA with the Home Agent (HA) which will receive all packets sent to the MN and then use tunneling to pass the packets to the MN through its current CoA. Using mobile IP, the involvement of the corresponding node (CN), the MN, and the HA may suffer from the triangular routing problem. If an MN’s movement causes frequent changes in its CoA, the corresponding handoffs could result in a large number of dropped packets. A dedicated software proxy module is required to pass packets sent to the MN’s fixed address to its new location. For services that promise consistently high QoS, this proxy forwarding implies a guaranteed delivery requirement, making it unsuitable for real-time applications.
The SIP protocol [6] is used for setting up and controlling voice transmission over the network. The SIP Mobility mechanism is used to meet the mobility requirement for SIP voice telephony. Using the SIP mechanism for mobility support, packet delay, or packet loss may interrupt mobile voice conversation [6]. Reducing handoff delay during mobile voice conversation is therefore essential to maintaining conversation quality. In the original SIP, if the MN is moving frequently, then the number of re-INVITE requests that the MN must send to the CN increases. This may increase system load or even cause handoff to fail.
In this paper, a hierarchical SIP (HSIP) architecture is proposed. In HSIP, a WiMAX network is partitioned into several domains, each contains an HSIP server and multiple BSs. And, each MN retains two addresses. One is the Local Address (LAddr) and the other is the External Address (EAddr). The LAddr of the MN is registered to the SIP Local Registrar and the EAddr of the MN is registered to the SIP Home Registrar. Through the address mapping mechanism, packets can be forwarded from the MN’s EAddr to its LAddr. Only if MN moves to a different domain, does it need to change its EAddr and send a re-INVITE to re-establish its connection with the CN. As the re-connection time is the most costly factor for a handoff, applying the proposed design, the number of re-INVITE requests could be reduced. Especially when the MN is moving within a domain, which is the most common mobility case, no re-INVITE message is necessary. Consequently, the handoff delay can be shortened.
The rest of this paper is organized as follows. Section 2 introduces related work in mobility management. Our proposed system architecture and mobility management are presented in Section 3. Simulation results and performance analysis are included in Section 4. Finally, conclusions are given.
This paper proposes a hierarchical SIP (HSIP) architecture for mobility management to improve the handoff efficiency. In HSIP, a WiMAX network is partitioned into several domains, each contains an HSIP server and multiple BSs. Within a domain, MN does not need to send a re-INVITE request to CN. Hence, signaling overhead and the handoff delay can be effectively reduced due to that most handoffs occur within a domain. The applicability of the proposed HSIP architecture is demonstrated by using the NS2 simulator. The performance of multiple handoffs before MN moves to a different domain is also examined. When the ratio of intra- domain to inter-domain handoffs is increased from 1 to 14, the proposed HSIP mobility mechanism offers an improvement up to 13% in average handoff delay and 35% in average signaling overhead as compared with the traditional SIP mobility scheme. As a result, the proposed HSIP architecture is feasible and favorable in dealing with the WiMAX handoffs.
