Strategies for Improving the Quality of Polling Service in Wireless Metropolitan Area Network

. Abstract. Four kinds of service types are defined in IEEE 802.16. In order to provide the Quality of Service (Qos) for different services, the system must use a reasonable resource allocation method and scheduling algorithm to efficiently and fairly allocate bandwidth resources. Although in the IEEE 802.16 MAC, for the uplink real-time polling service (rtPS) and non-real-time polling service (nrtPS) business type of data transmission are used to polling, but do not provide business-based services. In this paper, the distinction between priority service of rtPS and nrtPS is carried out, and the simulation experiment is used to analyze the performance characteristics of the protocol under the distinction of high and low priority. Wherein the theoretical values of the average delay and the information packets are compared with the experimental values. The average query cycle and throughput are also evaluated. Which proves the validity of the improved service strategy and improves the service characteristics of the system.


Introduction
In the fields of industrial control, computer time division multiplexing, communication system protocol and computer network protocol, the control mode of polling system has been widely used because of its fairness and usability.The analysis and research of the polling system are also developing continuously.With the deepening of the research, the application of the polling system has been further expanded.The strategy of the polling service can be divided into three categories: exhaustive service, gated service and limited-k service.In practice, prioritybased services have a wide range of needs, so the distinction priority service is very necessary.In the broadband network, it is necessary to provide end-to-end service quality assurance for different types of service flows to satisfy the requirements of the traffic flow for broadband, delay jitter and packet loss rate.IEEE802.16 [1,2] , also known as the IEEE Wireless MAN air interface standard, it regulates the underlying standards (including the physical layer and the media access control layer) of the wireless access system in the 2-66 GHz frequency range, and analyzes the system coexistence background, gives the system design, configuration and frequency use scheme.
With the rapid growth of wireless data services and multimedia applications, providing QoS support in the system business has become a basic requirement that must be met.The MAC protocol in IEEE 802.16 defines four types of services, specifying different QoS parameters and basic bandwidth allocation mechanisms for them.
However, the standard does not specify the principles of resource allocation and scheduling between services and similar services.Therefore, how to make effective use of system resources and ensure that different QoS requirements of various services become important and challenging research work.In this paper, we focus on the scheduling problem of real-time polling service (rtPS)   (3) Safety Sub layer (SS), responsible for authentication and encryption.
IEEE 802.16 MAC layer implementation of the principle of QoS [3] is to map the MAC packet transmission to the business flow, and map to the connection that has ߦ (n+1)}.The state of the system can be described by the Markov chain, which is aperiodic and experienced., , ,  ,   , , , Where F(z) is the probability generation function of the transmission time distribution for the information packets arriving in any time slot according to the exhaustive service rule.And it is assumed that the memory capacity of each site is large enough and don't cause loss of information packets.

Mean queue length
The derivatives of Eqs ( 1) and ( 2) give the mean queue length as: The mean queue length of the low priority service is: (3) The mean queue length of the high priority service is: (4)

The average delay
The average delay of the information packet is the time from the arrival of the information packet into the site memory until it was sent.
The average delay for each site's low priority business information packet is: (5) The average delay for each site's high priority business information packet is: (6)

The average query cycle and throughput
The average query cycle for N sites is: The throughput of the system is the number of , , , , lim , , packets transmitted by the system per unit of time.
The throughput of the system is: (8)

Method of improving QoS guarantee
Based on the traffic priority of the polling mechanism: in the original IEEE 802.16 MAC, the IP head of the Taxonomy of Service (ToS) field that has 3bit priority sub field mapping into the above four different Qos level, see Table 2.The same level of business uses the same type of scheduling, no longer distinguish priority, here we will further subdivide the rtPS business and nrtPS business to distinguish priority services."011" of rtPS and "001" of nrtPS in the ToS field are the high priority service identifier in the same service, and "100" of rtPS and "010" of nrtPS are used as low priority service identifier in the same business.In the IEEE 802.16 MAC polling mode, the BS periodically queries the different business queues of the SS site.First, the higher priority [8,9] business queue is authorized to send the information in the exhaustive service mode, that is, it sends the full information packets of the queue and the packets of information to be reached during the transmission, until the class of business queues on the query SS site is empty.When the SS high priority service queue is empty, the BS starts to query the service queue with the lower priority and sends the information according to the policy of the gated service.That is, it sends the information packets that arrive before the transmission start.The information packets arriving during transmission will wait for the next chance to send.
In this way, the service of the polling mode in the IEEE 802.16 MAC is distinguished by the different service policies of the exhaustive service and the gated service, which makes it have better flexibility and QoS guarantee.ToS sub field settings, providing services to the SS site in business priority.Table 3 shows that the average information packets increases with the increase of the arrival rate, the information packet arrival rate is from 0.0005 to 0.003, and the information packets stored in the high priority service and the low priority service is increased by more than 20 times, but the distinction between high-priority and low-priority services remains clear.Table 4 shows the same rule on the average delay of the information packet.High-priority and low-priority services are clearly differentiated as the arrival rate and system load increase.Table 5 verifies the consistency between theoretical and experimental analysis of query cycles and throughput.

Conclusions
This paper based on the four original types of service scheduling makes some improvements in wireless
is the arrival rate of the low priority service for each site.The generation function, mean value and variance of each station for low priority service arrival process are A(z), ߣ ଶ (i=1,2,…,N)=λ=‫ܣ‬ ᇱ (1) and ߪ ఒ ଶ ‫ܣ=‬ ᇱᇱ (1)+ ߣ-ߣ ଶ .The low priority queues of the site i (i = 1, 2, ..., N) obtain the transmission right at time ‫ݐ‬ and send the information packets arriving before time ‫ݐ‬ by the gated service rule, the transmission service time of an information packet in the queue follows the identical independent probability distribution with the generation function, mean value and variance are B(z),ߚ=‫ܤ‬ ᇱ (1) and ߪ ఉ ଶ ‫ܤ=‬ ᇱ (1)+ ߚ-ߚ ଶ .When the site i completes the low-priority service information packet transmission before the station time ‫ݐ‬ arrives, after a switch time ߛ [its generation function, the mean value and variance are R(z), ߛ=ܴ ᇱ (1) and ߪ ఉ ଶ ‫ܤ=‬ ᇱᇱ (1)+ ߚ -ߚ ଶ ], AP(BS or Master) through the multi-point scheduling method to check whether the high priority queue is empty, and query i + 1 site.If the high priority service queue is not empty, the high priority service of each station is unified according to the exhaustive service rule at time ‫ݐ‬ * to send information packets, and the generation function, the mean value and variance are ‫ܤ‬ (z) ߚ ‫ܤ=‬ ᇱ (1) and ߪ ఉ ଶ ‫ܤ=‬ ᇱᇱ (1)+ ߚ -ߚ ଶ .After the high priority business of each station is empty, the i + 1 site starts the low priority service transmission at time ‫ݐ‬ ାଵ .The transmission of the high priority service and the low priority service of each site are alternately accepted service by the AP (BS or Master) and are accessed according to the rules of the exhaustive service and the gated service respectively.At the time of ‫ݐ‬ , ‫ݐ‬ * and ‫ݐ‬ ାଵ ‫ݐ(‬ ‫ݐ<‬ * ‫ݐ<‬ ାଵ )can be expressed by the variables {ߦ
metropolitan area network IEEE 802.16 MAC protocol, and further provides polling scheduling strategies based on service priority.Then, the theoretical value of the average number of information packets and the average delay of the information packet is compared with the experimental value, in the case of high priority business and low priority business.And the theoretical value and the experimental value of the query cycle and throughput are basically consistent.So, they are proved that the improved scheduling strategy improves the flexibility of protocol scheduling and better guarantees the QoS of different service flows.Compared with the traditional wired network, although the existing wireless network service quality and performance there are still some gaps.However, with the popularity of wireless networks and the development of a variety of business applications, the research and improvement of MAC layer QoS mechanism in wireless metropolitan area network has clear practical significance and application value.In the future work, we will continue to study the priority of the polling service strategies, the use of new hybrid services, such as gatedexhaustive service, gated-gated service, exhaustiveexhaustive service to guarantee the service quality of data service, and improve the utilization rate of system resources.

4 Traffic service strategy based on service priority for wireless metropolitan area networks 4.1 Protocol model analysis
U

Table 2 .
Correspondence table for ToS field and IEEE

Table 3 .
Average information packets stored

Table 4 .
Information packets average delay

Table 5 .
Average query cycle and throughput