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A QoS system for a Wimax network
This article proposes a practical design for a system which manages the QoS feature of a Wimax network. The system will offer managed QoS services for a heteregenous access network with minimal human interaction, while taking into account the need to minimize complexity and line speed processing requirements. The latter is a key requirement; a Wimax base-station is expected to be a very processing intensive piece of machinery, with limited processing available for enhanced services such as QoS. The design also makes it possible to use standard modules such as schedulers which are already available from vendors such as Xilinx or Fujitsu.
The design follows the rough template proposed in QoS. We shall show how the templates may be actualized and how they contribute to end-to-end QoS management.
Objectives
The objectives of the QoS design are as given below. The first set are generic objectives which are true for any QoS system. The last few objectives are specific to the nature of the Wimax network.
- The QoS design will take into account the cost of offering service from the network and resource point of view.
- The QoS design will allow the user and the network to control and select the cost of service incurred for a particular session.
- The QoS design will help the user and the end-application to maximize the value to the user. We define value as Īnter%
where U(x) is the utility derived by the application from using an amount of resource x, C(x) is the amount of cost of the resource that is passed on to the user by the system and T is the overall duration of the service. The reader is referred to Scott? for a good overview of utility.
- The QoS design will utilize the enhanced link processing ability of the Wimax network i.e. AAS?, Hybrid ARQ, adaptive coding and modulation, etc.
- For the wimax system, the cost of supporting service is incurred at various points. There is the physical cost of the radio-resource, processing cost and various other features. The cost of service shall include the combined cost of all these if they are explicitly managed by the system.
- While wimax supports ACM, the number of modulations and coding schemes defined, and the SIR distance between the modes must be that high that dynamic reconfiguration between modes is pretty much ruled out, other than in the case of handover. As we know, the coding and modulation used for a given user, directly impacts the efficiency of usage by that user of the radio-link; channel capacity modeling is an essential part of QoS design. However, given the relative rigidity of the channel model and the mobility aspect of the system, the channel model must handle sudden, drastic changes in capacity, as opposed to continual fine-tuning
The QoS model
The diagram below shows a breakup of the QoS implementation. We shall now proceed to define the principal components.
Īnter% 
[[ Main.TCP |
The application includes both the source and the sink for the user data; also, by application we mean the application software, the endpoint transmission layer stack (TCP or UDP), the feedback stack if it is separate from the transmission layer stack i.e. RTCP. The application management logic, which tries to maximize the user benefit may be built into one of these i.e. the window management logic in TCP, or it may be a separate component i.e. the rate selector in an AMR voice codec. The application is a key entity because it is the only entity which knows the end-to-end path properties and also the utility function U().
The scheduler ]] schedules access to resources for individual service units. In this example, a service unit is an IP packet. There may be multiple schedulers for different classes of resources. For example, there may be a scheduler to control access to processing units for special processing, such as STC transcoding or hybrid ARQ. We assume that there is at least one scheduler to control access to radio-resources.
- The shaper or marker class assigns resources to individual service units. The mode of assignment may be individual and may vary from system to system. In some cases, it may be explicit and in some cases implicit. We shall see some practical examples soon.
Cost accounting
We now come to the crucial feature of our design, which is the accounting of resources. There are two ways of modeling the QoS system described here. One is that of a distributed game, with multiple applications fighting for limited resources. This is the approach used by Scott Shenker; we shall review it in our context and see how it fits. The other, more complex option, is to treat it as a planned economy model, with the QoS system doing the optimal allocation to maximize the aggregate welfare.
Categories: Wimax
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