In this chapter, the Data-link layer (layer 2) handover procedures defined in IEEE 802.16j standard will be described first. And then we will present the handover procedures in Mobile IPv6 and Proxy Mobile IPv6, and show how the two IP layer mobile mechanisms cooperate with IEEE 802.16j. overview of MRS layer 2 handover.
MS MRS Serving BS Target BS
Figure 2.1 MRS layer 2 handover procedure
First, the handover preparation phase takes place. Three procedures are included - network topology advertisement, scanning, and association procedure. During network
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topology advertisement procedure, a BS periodically broadcasts MOB_NBR-ADV (Mobile Neighbor Advertisement) which contains information about the network topology MAC management message. This message tells the MRS the information about the network identification, provides information about neighboring BSs, and facilitates MRS synchronization with neighboring BSs. After obtaining information from this message, the MRS can make an immediate decision for a future handover. In the scanning procedure, MOB_SCN-REQ (Mobile Scanning Interval Allocation Request) and MOB_SCN-RSP (Mobile Scanning Interval Allocation Response) messages are exchanged. Through these two messages the MRS and the BSs it wants to scan can coordinate for the scanning. The MRS scans the neighboring BSs to seek and monitor one that is suitable for a handover candidate. The last one – the association procedure is an optional ranging procedure occurring during the scanning interval with respect to one of the neighbor BSs.
Secondly in handover decision and initiation phase, the MRS selects one or more candidate BSs. The MRS may make a decision to initiate the handover and sends a MOB_MSHO-REQ (Mobile MS Handover Request) message to current serving BS.
Before replying MOB_BSHO-RSP message with recommended target BSs to the MRS, the serving BS may send the MAC addresses and CIDs of the MSs under MRS to neighbor BSs over the backbone. The handover can also initiated by the serving BS which would send MOB_BSHO-REQ (Mobile BS Handover Request) message to the MRS.
The last phase is handover execution. The MRS chooses a target BS and then sends MOB_HO-IND (Mobile Handover Indication) message to indicate a handover to the serving BS. After this message, the serving BS officially terminates the connection to MRS so that no packet would be delivered by serving BS. And then the MRS switches to
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target link and performs downlink synchronization, raging, and network re-entry to the target BS. Next, the target BS sends RNG-RSP (ranging response) message to MRS to assign new CIDs for MSs, then MRS creates mapping between old and new CIDs for each MS. Once the handover execution is done, the target BS, which has become the serving BS, can start forwarding packets to the MRS and provide services.
2.2 Handover procedures with MRS in Mobile IPv6
In this section we will describe how MIPv6 works with IEEE 802.16j MRS mode.
While MRS enters a different subnet, the MSs connected to it must re-configure a CoA address, new IP address, and re-establish its IP connection based on the new IP address.
However, MSs cannot realize that they are under another network in the first place until they receive the router advertisement (RtrAdv) message periodically sent by the NAR.
The MS can also request the NAR to send router advertisement by sending router solicitation (RtrSol) message. Next, the MS uses information exchanged in these messages to create a new CoA and confirms this CoA by sending neighbor solicitation message. After the CoA confirmation is done by the NAR (the confirmation is called duplicate address detection, DAD), the MS sends a binding update message to its HA, which updates the binding cache upon receiving this message [9].
To sum up the procedure in MIPv6, after finishing the layer 2 handover of MRS, the MS performs the IP layer handover procedures including movement detection, new care of address (CoA) configuration and DAD. And binding update is sent by the MS to officially start connection to the NAR. We can see that the two layer handover mechanism operate sequentially, therefore this handover procedures lead to a long SDT and packet loss. Figure 2.2 shows the detail handover procedure using MIPv6.
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MS MRS Serving BS PAR Target BS NAR
MOB_NBR-ADV
Figure 2.2 Mobile IPv6 handover procedure in MRS mode
2.3 Handover procedures with MRS in Proxy Mobile IPv6
PMIPv6 is a network-based IP layer handover approach where the serving network controls IP mobility management on behalf of the MS. Hence, when PMIPv6 is used over IEEE 802.16j network, the IP layer mobility management is done by the AR so the MS never knows about the subnet changing and the occurrence of IP layer handover. The new AR (NAR) will start to perform the IP layer handover procedures on behalf of the MS via PMIPv6 after it receives the RtrSol (Router Solicitation) periodically sent by MS, which may arrive at any time after the MRS's attachment. Note that we use the term ―HA‖ and
―NAR‖ instead of ―LMA‖ and ―MAG‖ for the reason of consistency..
Figure 2.3 shows the PMIPv6 procedure [11]. After NAR detects the new MS is under its subnet by RtrSol message, it sends a PBU (Proxy Binding Update) to the MS’s home
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HA. The HA obtain the MS’s identifier form PBU and then allocates an appropriate home network prefix for the MS. Afterwards, the prefix is encapsulate in PBA (Proxy Binding Acknowledgement) message and sent back to NAR. At the same time the bi-directional tunnel is created between HA and NAR. Now the NAR sends RtrAdv (Router advertisement) message to the MS as a response of prior RtrSol message, and start forwarding data packets tunneled from HA. In PMIPv6 scheme, the packets forwarding from the PAR is not allowed since MOB_HO-IND message is exchanged, so the packet loss may occur during the periods of the layer 2 network re-entry (L2 RN) of MRS and the following PMIPv6 handover procedures. Since the IP layer handover of MSs are done one by one in this scheme, the SDT is affected by the amount of MSs under the MRS. In addition, because RtrSol is not sent upon layer 2 handover is done but is sent periodically, the interval of RtrSol also affect SDT. We can see that the NAR does not start handover procedure until it receives RtrSol, so the longest possible idle time is the whole interval time of RtrSol.
MS MRS Serving BS PAR Target BS NAR
MOB_NBR-ADV
Figure 2.3 Proxy Mobile IPv6 handover procedure in MRS mode
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