HWN* Handover Management
In the HWN* context, each communication mode exhibits different capacity characteristics therefore selecting the most appropriate mode for a particular service request is critical to guarantee QoS to the end user. A balanced traffic load distribution between the available modes is also important to avoid one mode becoming excessively loaded, which leads to an unstable state. The HWN* requires an algorithm that includes the service transfer function between the cellular communication modes and MANET communication modes. Another novelty of the HWN* handover mobility management is that user segregation is provided to guarantee QoS to particular user classes by the prioritisation of user requests when the HWN* system approaches a congested state. As the RN provides stable infrastructure support for a MANET, it will be beneficial to switch more service from the cellular service to the MANET service. The HWN* concentrates on the mode selection considering differentiated user negotiation, multiple handover triggers and congestion control. Here the handover mobility management procedure refers to a process of transferring a MT from its serving BS to another BS or from one medium access interface to another interface. The RN using cellular frequencies relays the handover traffic but doe s not have traffic admission functionality. While for the MANET mode and MANET with RN mode, the procedures refer to a new path finding process similar to rerouting, which is described as part of the cascaded ADCR routing algorithm. To mutually mitigate the traffic burden between communication modes, the HWN* dynamically allocates the radio resources without a fixed plan.
Three service classes are devised to describe subscriber behaviour: High Profile Users (HPUs), Normal Profile Users (NPUs) and Low Profile Users (LPUs). Principally, HPUs get the guaranteed QoS service and the class is assured with any amount of bandwidth, firm end-to-end delay bounds and limited queue loss for data flow. Next are NPUs with less QoS guaranteed medium access opportunities compared to HPUs, but the users will get a close service quality as the one received by HPUs. LPUs are a best-effort class with absence of QoS specification using currently available medium resources. HPUs have the highest access priority in any of the communication modes of the HWN*, and traffic admission of NPUs and LPUs has to consider the impact of ongoing HPUs sessions. NPUs are also configured to have a higher probability than LPUs in terms of resource acquisition and this probability is decided by an Association Level (AL) set, which will be described later. Inter-system and intra-system mobility management are addressed separately to reduce system complexity.
MANET RN and MANET modes can potentially become the primary communications methods if distributed management, link reliability and security problems of the MANET mode can be solved. Specifically for the MANET mode, the issue is that data relaying via a third party MT is not safe as discussed in MCN review. Data relaying via provider owned fixed RN provides a permanent solution. The RN integration largely moderates the problem by providing reliable relays between communication parities. Upon communication request, HPUs are configured to search for cellular based service, NPUs sfor a MANET RN service while LPUs for MANET service by default. The signalling, path discovery and route establishment for MANET and MANET RN modes are completed before the BSON and BSON RN modes, if all four modes are available. Inter-system and intra-system traffic handovers are only triggered when essential to avoid unnecessary network management overhead.
The control entity for inter system mobility cooperation is called the HWN* Mobility Controller, which is responsible for managing the modification of a route in an attempt to maintain or enhance the QoS level. The unit is located in each BS and a BS periodically monitors receives, specifies, filters and analyses frequencies in use by nearby MTs, RNs and BSs, geographically locates MTs so that the terrain blocks interfering signals, and may use directional antennas to reduce unwanted signals. The BS also gathers the information on the MANET with RN mode channel availability from its associated RN. A MT, either requiring differentiated service or not, makes a distributed decision on inter-network and intra-network handovers based on information gathered from its associated BS mobility controller as well as information gathered after probing one hop point to point direct communication. To obtain an effective handover process, while reducing the unnecessary handover rate, it is proposed to setup a dedicated Status Check Point embedded in the HWN* Mobility Controller where the necessary measurements are taken and then fed back to the handover algorithms of the nodes involved. The check result indicates the likelihood of a handover, which depends on interference level and physical layer information such as Bit Error Rates (BER), velocity, buffer size, etc. Since the HPUs applications have higher priorities over NPUs and LPUs, subscribers from this profile are more likely to get an accept ticket, which is issued by the Negotiation Unit in continuing with traffic handover. If an accept ticket is not issued, the MT will not use the status check data to request continuing with the handover process unless the status check point data necessitates handover. Then the mobility controller will decide to accept, queue or reject the MT handover request. The negotiation process between HPUs, NPUs and LPUs is based on Association Level (AL), which makes decisions and feeds back to the HWN* Mobility Controller. The AL is a set of parameters monitoring channel availabilities, an AL that scores higher than the threshold means that the channels are already occupied by ongoing sessions. The AL set is further sub-classified as AL in the BSON, BSON RN, MANET and MANET.
The handover algorithms in the HWN* should allow subscribers to seamlessly move without dropping their communication session and considers differentiated QoS issues, for example, it must guarantee QoS for HPUs that agree to pay more than NPUs and LPUs. Two handover types are therefore used as described previously, Intra Network Handover occurs when a MT enters into another entity that belongs to the same network with a cellular TDMA MAC interface. An Inter Network Handover happens when a MT leaves the serving network and communicates with another entity that belongs to a different network. For all service classes, the intra network handover is selected before considering inter network handover as less traffic management overhead may be produced. In intra network handover, after obtaining pass tickets from the Status Check Pointand the Negotiation Unit, the Network Selector entity that embedded in the mobility controller informs the MT if the RN should participate in handovers or not, then the MT makes a local decision. The inter network handover is seen as a switching process between ad hoc and cellular services which has different properties and procedures compared to the intra network handover. The Status Check Point is activated first to avoid extra expense and the Negotiation Unit keeps monitoring the channel availability status and updating the AL in a short interval to grant or reject handovers. The Network Selector always tries to divert the traffic back to intra network handover before it is informed that the intra network handover is not possible. If the MT is currently communicating in ad hoc modes, the selector will search for available B Ss in neighbouring cells. If the MT is using cellular modes, it will look for either direct point-to-point communication or search for a fixed RNs involved multi-hop communications. The Network Selector also uses several short network search expire time s for both intra-network search and inter-network search to make sure a MT is not long isolated during the network selection process
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