Handover Protocols

In this section we state MRS handover which is relevant to our scheme. We simply describe mobile IP and depict fast mobile IPv6 protocol in predictive mode and reactive mode.

2.2.1 MRS Handover

IEEE 802.16j defines a handover process that MS or MRS needs to change the BS for higher signal quality or better QoS when it moves. The handover procedures can be decomposed into three phases: handover preparation, handover decision and initiation, and handover execution. The handover procedures of MRS are illustrated in Figure 2.2.

MRS Serving BS Target BS

Figure 2.2 Handover procedures of MRS

The handover preparation phase includes network topology advertisement, scanning, and association procedure. During network topology advertisement procedure, a BS needs to broadcast information regarding the network topology through MOB_NBR-ADV message. The purpose of the message is to provide a MRS with the current network identification and information about neighboring BSs, and to facilitate MRS synchronization with neighboring BSs. According to this information, the MRS can make an immediate decision for a future handover. If necessary, a MRS may perform a scanning procedure to find and monitor the suitable neighboring BSs as a target BS. Association procedure is an optional and initial ranging procedure occurred during the scanning interval with respect to one of the neighbor BSs. The handover decision and initiation begin when a MRS needs the handover from serving BS to target BS by sending a MOB_MSHO-REQ message. After receiving MOB_MSHO-REQ message, the serving BS replies MOB_BSHO-RSP message with recommended target BSs to the MRS and sends the MAC addresses and CIDs of the MSs under MRS to these target BSs through the backbone network. Afterward, handover execution occurs. The MRS selects the target BS and sends MOB_HO-IND

message to indicate a handover to the serving BS. After MRS sent MOB_HO-IND message, no packet transfer between the MRS and the serving BS is allowed. Then, MRS performs downlink synchronization, ranging, and network re-entry to the target BS. The target BS assigns new CIDs for MSs and sends it to MRS and then MRS creates mapping between old and new CID for each MS. After handover execution phase, the target BS becomes the serving BS and starts to provide service to the MRS.

2.2.2 Mobile IP

In the current network, both communication ends use IP address. If one side of communication alters the network domain, it will change the IP address, and the link would be interrupted, therefore it must reconnect to the network. We need a technology to help us transform IP address. The technique of mobile IP is developed to solve this problem in IPv4 architecture. With Mobile IP [6], as shown in Figure 2.3, it is unnecessary that a MS needs to change the address and causes interruption with correspondent node due to moving to a different network domain.

Figure 2.3 Mobile IP

A MS uses a home address (HA), which means a fixed IP, to represent itself.

When the MS moves to other network domain, it will obtain a care of address (CoA) from the foreign agent. Then, it may have a registration to link between HA and CoA.

The packets delivered to HA in local network is sent to home agent and then transferred to MS through the relation of HA and CoA. Through the CoA register technique, MS still can use HA to receive data with correspondent node even though it is not in home network.

2.2.3 Fast Mobile IPv6

Fast Mobile IPv6 (FMIPv6) [7] is defined to reduce the handover latency for the real-time traffic by movement detection and address configuration procedures. When MS moves to other network domain and changes to a new subnet, FMIPv6 enables a MS to quickly detect its entering to a new subnet and perform CoA configuration early by providing the subnet network prefix information of associated access router (AR). Figure 2.4 shows the FMIPv6 handover procedures.

NAR

Figure 2.4 FMIPv6 in predictive mode

After discovering a new neighbor BS, MS may perform scanning in order to determine the BSs that are available. Then, it selects one of the candidate BSs and obtains a new subnet prefix of the target BS by exchanging the RtSolPr message and PrRtAdv messages with previous AR (PAR). Upon receiving PrRtAdv message, the MS configures its CoA based on the subnet prefix obtained from the message.

When MS decides an impending handover, it notifies the router that there is a binding between previous CoA at the current subnet and new CoA at the target subnet by sending a FBU message to the PAR. Afterward, the PAR sends HI message to the new AR (NAR) for CoA confirmation procedure. After NAR receives HI message, it executes the CoA confirmation, duplicate address detection (DAD) procedure, and replies HAck message to the PAR. At the same time, the tunnel between the previous CoA of MS and its new CoA at the NAR is established.

The NAR receives the tunneled packets and stores them in a buffer until it receives FNA message from the MS. Then, it delivers the buffered packets to the MS.

The FNA message is sent after the MS conducts handover to the target BS and performs the network re-entry procedure. On receiving HAck message, PAR sends FBAck message to the MS. If the MS receives this message before its handover and sends MOB_HO-IND message as a final indication of handover, the predictive mode of FMIPv6 is enabled. The predictive FMIPv6 makes the MS to move to the new subnet and receive packets from the NAR quickly.

However, if the MS does not receive FBAck message before it is forced to move to the new subnet, reactive mode will occur. In reactive mode, the MS has to wait for packet rerouting to be executed then it can receive packets from the NAR. Figure 2.5 shows the FMIPv6 in reactive mode.

Figure 2.5 FMIPv6 in reactive mode