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Introduction
The mobile WiMAX [1] supports optimized handover. Optimized handover helps to ensure that the real-time applications (such as VoIP) are efficiently supported without service degradation by achieving latencies less than 50 ms.
HO Procedure
Handover procedure as per NWG Stage 3 [4] is divided into two phases:-
a. HO Preparation Phase - In this phase MS is connected to serving BS and starts the handover related signaling to reserve the resources in the target BS.
b. HO Action Phase - This phase begins when the MS leaves the serving BS and performs network re-entry in the target BS.
As part of HO Action Phase, MS performs either full network re-entry or optimized network re-entry in target BS. This is done on the basis of the information available at target BS. Target BS can receive the MS service and operational information over the backbone network from the previous serving BS.
HO Preparation Phase
As part of HO preparation phase, MS service and operational information is transferred from current serving BS to target BS using backbone network messages. This can be done in the following ways:
1. HO_Request message from serving BS to target BS
2. Context retrival procedure initiated at target BS
HO Action Phase
As part of HO action phase, target BS on the basis of the information received from serving BS, will send HO Process Optimization TLV as part of RNG-RSP message. This TLV helps in identifying the message exchange that can be omitted during network re-entry.
HO Process Optimization TLV
This TLV identifies re-entry process management messages that may be omitted during the current HO attempt due to the availability of MS service and operational context information. The target BS may elect to use MS service and operational information obtained over the backbone network to build and send unsolicited SBC-RSP and/or REG-RSP management messages to update MS operational information. The MS shall not enter Normal Operation with Target BS until completing receiving all network re-entry, MAC management message responses as indicated in HO Process Optimization.
Message that can be omitted during network re-entry
By using HO Process optimization TLV following message exchange can be omitted during network re-entry.
a. SBC-REQ/RSP management messages
b. PKM Authentication phase except TEK phase
c. PKM TEK creation phase
d. REG-REQ/RSP management
e. Network Address Acquisition management messages
f. Time of Day Acquisition management messages
g. TFTP management messages
Security consideration in optimized handover
Security procedure is optimized using the following consideration:-
a) Validating the AK is usually combined with the procedure of ranging which include 802.16e RNG-REQ and RNG-RSP with CMAC tuple.
b) Sharing TEK within a same mobility domain is possible when Handover procedure between two base stations can transfer TEK context information. If the TEK is shared among BSs, the set of BSs are considered as same security entities within a same trusted domain
References
[1] IEEE 802.16
[2] IEEE 802.16e
[3] WiMAX forum network architecture (Stage 2)
[4] WiMAX forum network architecture (Stage 3)
For suggestions and comments, please contact sapan.goel@hsc.com
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