Prioritizing different types of traffic

connection reservation and two (the above mentioned [Cho98] and [Nag95]) were based on prediction. It was concluded that per-connection bandwidth reservation is too expensive unless it is used in a situation where the mobility patterns are known (e.g. in a highway⁷). Out of the prediction based schemes the local scheme [Cho98]

outperformed the information exchange based scheme [Nag95] and was superior es- pecially in terms of complexity and computational demands.

In terms of the existing WiMAX Forum network architecture both approaches, local and information exchange based, are possible but the local approach seems more feasible for Mobile WiMAX Base Stations due to its simplicity and local nature.

The WiMAX Forum network architecture does not provide any kind of a framework for information exchange between BSs which means that a new protocol would have to be introduced for an information exchange based dynamic reservation scheme resulting in lower scalability.

A potential area of research that could contribute to enhancing handover resource reservation is mobility pattern estimation. One example was presented in [Liu98]

where a hierarchical user mobility model to estimate the mobility pattern of a user was designed.

The advantage of using such differentiation is that the higher priority schemes can receive even better QoS. It is far more irritating to be dropped out of a conversation than to wait a little longer for your FTP download and therefore the possibility to ensure that handovers for non-real-time connections are dropped before dropping handovers for real-time connections is very beneficial. The slight disadvantage that this approach introduces is additional complexity.

3.2.2.2 Previous research

There has been some research on traffic prioritization. In [Jay00] a framework for QoS provisioning for multimedia services was proposed by using different treatment for real-time and non-real time traffic on the link layer. [Zen00] proposed a han- dover scheme where priority reservation for voice handover was used. In the scheme resources are reserved for both voice and data handovers but the voice handovers have priority over data handovers.

In [Xha04] a framework for dynamic priority queuing was presented. Although in the paper, priority was based on the received signal strength and the remaining time in the overlapping region between two cells, it could potentially be used for prioritizing also different types of traffic.

In [Che05] the idea of prioritizing handovers based on their traffic type presented in [Zen00] was applied to a dynamic environment with a dynamic multiple-threshold bandwidth reservation (DMTBR) scheme. The scheme uses a dynamic guard band for handovers while maintaining relative priorities for different traffic classes. It is capable of granting differential priorities not only to different traffic classes but also to new and handover traffic for each class by dynamically adjusting three bandwidth reservation thresholds. The scheme assumes two traffic classes non-real-time (nrt) and real-time (rt). The thresholds used in the scheme are presented in Figure 3.3.

Figure 3.3: Multiple-threshold bandwidth reservation [Che05].

As resource reservation increases, the resources reserved after the guard band for new real time connections has been passed, can be used by new rt connections⁸ and by nrt and rt handovers. In the same way the resources reserved after the guard band for non-real-time handovers can be only used by nrt and rt handovers. Finally, the resources reserved after the guard band for real-time handovers can only be used by rt real-time handovers. All new nrt connections will be blocked after the new real-time connection guard band has been passed which will happen in the example in Figure 3.3.

The proposed scheme works locally by first estimating initial values for the thresh- olds based on instantaneous mobility and traffic load situation. The thresholds are further adapted according to instantaneous QoS measures such as dropped han- dovers and blocked new calls in a similar way as in [Lee03]. Throttling (blocking new calls randomly) is also used when network becomes congested.

8The reserved resources could be used e.g. for the MCS changes for the rt connections.

Load Balancing with Handovers in Mobile WiMAX

Now that we have a good understanding of the background behind load balancing with handovers and behind rescue handover and traffic prioritization and have also covered the key system aspects of Mobile WiMAX in relations to these, we can start to consider how load balancing and rescue handover and traffic prioritization could actually be applied to Mobile WiMAX. In this chapter, we will first design a basic load balancing scheme by applying an existing scheme from prior research to Mobile WiMAX and see how it could be enhanced in the Mobile WiMAX system.

Second we will examine how the basic algorithm could be further complemented with handover and traffic prioritization.

4.1 A Load balancing algorithm for Mobile WiMAX

We will begin by applying the algorithm presented in [Vel04] to Mobile WiMAX.

The scheme was originally introduced for WLAN networks so it will need some modifications and adjustments from our part in order to be compatible with Mobile WiMAX.