Downlink packet scheduling and resource allocation in EPON WiMAX hybrid access networks
© Teng et al.; licensee Springer. 2012
Received: 14 June 2012
Accepted: 20 October 2012
Published: 2 November 2012
Multimedia applications over the Internet, such as IPTV and video-on-demand, have become fast growing applications in recent years. Such applications have stringent quality of services (QoS) constraints in terms of bandwidth, delay, and packet loss. As a consequence, broadband access networks play an important role for multimedia applications. There are two emerging technologies offering both high bandwidth and QoS support, namely Ethernet Passive Optical Network (EPON) and Worldwide Interoperability for Microwave Access (WiMAX). By integrating these two technologies, EPON-WiMAX integrated network can: (1) provide broadband access, (2) support mobile users, and (3) decrease network planning cost and operating cost. Thus, EPON-WiMAX integrated network is an ideal choice for multimedia applications with ubiquitous access. Although EPON-WiMAX integrated network has received growing attentions, however, most of previous works focus on the scheduling and bandwidth allocation in the upstream direction. Therefore, in this paper, we investigate the downlink scheduling and bandwidth allocation problem in EPON-WiMAX integrated networks. The objective of the study is to maximize the system throughput and guarantee the (QoS) so that the requirements of multimedia applications can be fulfilled. We proposed a two-stage downlink packet scheduling and resource allocation mechanism collaborating with application layer forward error correction (AL-FEC). We demonstrated the performance of our approach via simulations. Our simulation results indicated that the proposed mechanism increased the system throughput significantly, especially when AL-FEC is adopted.
Recently, a variety of popular multimedia applications have posted a high bandwidth requirement, such as high-definition television and video-on-demand services, which indicated that the broadband access technology has become more and more important. In wired networks, Ethernet Passive Optical Network (EPON) adopts optical fiber as the transmission medium. EPON is a point-to-multipoint fiber access network which supports up to 10 Gbps bandwidth. Since it can provide high bandwidth and is compatible with legacy Ethernet, it is considered as one of the solutions for the next generation wired broadband access technologies. In wireless networks, Worldwide Interoperability for Microwave Access (WiMAX)[2, 3] is a new generation of broadband wireless access technology which supports long-distance, high-bandwidth, quality of service QoS-guaranteed wireless communications. Therefore, it has been identified as one of the last mile solutions. Although EPON and WiMAX technologies are promising, it should be noted that deploying EPON or WiMAX still has some limitations. For example, deploying Fiber To The Home is expensive for Internet Service Providers. On the other hand, the data-transfer rate of mobile WiMAX subscriber stations (SSs) in current real-world implementation may only be maxing out around 70 Mbps over a 20 MHz channel which is much less than that of wired networks.
Therefore, Shen et al. proposed the integration of EPON and WiMAX networks to make up for each other's deficiencies. Advantages of the integrated EPON-WiMAX network include providing broadband Internet access, supporting mobile users, and reducing the network design and maintenance costs. In general, the architecture of the EPON-WiMAX network is a tree topology, consisting one EPON Optical Line Terminal (OLT), one 1:N passive optical splitter, and multiple EPON Optical Network Units (ONUs) which consequently connects to a WiMAX Base Station (BS). Based on how EPON ONU and WiMAX BS are constituted, the architecture can be classified into four categories, namely independent, hybrid, unified, and microwave-over-fiber. Among these four architectures, the hybrid architecture is the most promising with advantages of more flexible development, less deployment costs, and less technology restrictions. In the hybrid architecture, EPON ONU and WiMAX BS are integrated into one device logically or physically, referred to as ONU-BS. An ONU-BS consists of three components: EPON ONU, WiMAX BS and a central control unit. The central control unit is responsible for conversion between EPON and WiMAX networks, such as frame format conversion and QoS mapping. Traffic between OLT and ONU is transmitted on two separated wavelengths, typically 1310 nm (for upstream) and 1550 nm (for downstream). A MultiPoint Control Protocol (MPCP) is used as a control and signaling mechanism between OLT and ONU which is specified in the IEEE 802.3ah standard.
For the integration of EPON and WiMAX, packet scheduling and bandwidth allocation in upstream direction have received much more attention. Yang et al. presented a converged network architecture based on the concept of virtual ONU-BS (VOB) and proposed a QoS-aware dynamic bandwidth allocation (DBA) scheme. The proposed QoS-aware DBA scheme is operated in a hierarchical manner and therefore can support bandwidth fairness at the VOB level as well as class of traffic fairness at the SS level. Jung et al. investigated three possible integrated architectures and proposed a centralized scheduling (CS) mechanism to enhance end-to-end delay and provide better QoS provisioning for the lower priority traffic. Relatively less attention has been focused on downlink packet scheduling and bandwidth allocation on the integrated EPON WiMAX network. Emphasizing on inter-cell cooperative transmission, Gong et al. proposed three schemes to optimize ONU-BS user association and resources allocation (BUA-RA) in terms of minimizing the number of rejected connection requests. However, QoS guarantee for different classes of traffic was not considered, which is essential in mobile WiMAX.
In this study, a two-stage design of downlink packet scheduling and resources allocation was proposed for the EPON-WiMAX hybrid access networks. In the first-stage, packet scheduling from OLT to ONU-BSs was considered to balance the load among ONU-BSs to ensure each ONU-BS had abundant resources to transmit packets to its SSs. In the second-stage, packet scheduling from ONU-BS to SSs was considered. To provide end-to-end bounded-delay and fair allocation of bandwidth, WF2Q was adopted to schedule packets to be transmitted on each downlink sub-frame. Since channel condition can vary considerably across users in a wireless environment, packet loss is one of the main obstacles to fulfill QoS of traffic classes. To enhance end-to-end reliability and resources utilization of each ONU-BS, a combination of application layer forward error correction (AL-FEC) and modulation and coding scheme (MCS) was proposed. As a result, the overall system throughput was improved and packet transmission delay was reduced while QoS of each traffic class could be guaranteed. The performance of our approach is demonstrated via simulations. The simulation results indicated that the proposed mechanism increased the overall system throughput significantly while guaranteeing the QoS of traffic classes, especially when AL-FEC is adopted.
The rest of the article is organized as follows. We first present a literature survey on EPON-WiMAX integrated networks in Section 2. Section 3 gives an overview of the EPON WiMAX architecture, WiMAX handover schemes, and problem formulation. In Section 4, we proposed a two-stage scheduling algorithm and the use of AL-FEC to improve the system throughput while guaranteeing the QoS of each traffic class. Simulation results are presented in Section 5. Finally, conclusion and future work are given in Section 6.
2. Related works
For offering wider bandwidth and mobility with low costs to users, much research effort has been conducted on the integration of EPON and WiMAX. Shen et al. proposed four architectures for integration of EPON and WiMAX, namely independent, hybrid, unified, and microwave-over-fiber, in which related research issues are elaborated. Ghazisaidi and Maier proposed a techno-economic model to evaluate the cost-performance trade-offs of EPON and WiMAX networks. An optimal ONU-BS placement model was presented in, which jointly take BS-User association and resource breakdown assignment into consideration. Kim et al. proposed a distributed antenna based EPON-WiMAX integration architecture with a cost-efficient cell planning mechanism which was used to optimally control the size of overlapped cell coverage areas.
For integration of EPON and WiMAX, packet scheduling and bandwidth allocation in upstream direction have received much more attention. Yang et al. presented a converged network architecture based on the concept of VOB which is a logical form of the hybrid architecture presented in. The authors also proposed a QoS-aware DBA scheme which can support bandwidth fairness at the VOB level and class of traffic fairness at the SS level. Jung et al. investigated three possible integrated architectures: independent ONU-BS, combined ONU-BS, and hybrid ONU-BS. To address the problem of the independent scheduling, the authors proposed a CS mechanism to enhance end-to-end delay and provide better QoS support. Ranaweera et al. investigated different DBA algorithms, intra ONU-BS scheduling algorithms, and QoS mapping mechanisms on the QoS performance of the EPON-WiMAX converged network. A hybrid priority weighted fair scheduling was also proposed to avoid bandwidth starvation in the lower priority traffic classes. Dias Piquet and Saldanha Fonseca assessed the performance of a standard-compliant WiMAX uplink scheduler and showed that the scheduler can provide QoS support to the SSs.
Relatively less attention has been focused on downlink packet scheduling and bandwidth allocation on the integration of EPON and WiMAX. In, a cross-layer design for video multicasting was proposed, where a modified MDC on scalable video streams at the application layer, superposition coding at physical layer as well as inter-cell cooperative transmission are jointly considered. Emphasizing on inter-cell cooperative transmission, Gong et al. proposed three schemes to optimize BUA-RA in terms of minimizing the number of rejected connection requests and time slot usage. However, this study cannot provide QoS guarantee of different traffic classes. Therefore, in this study, we proposed two-stage design of downlink packet scheduling and resources allocation to improve the system throughput and provide QoS guarantee for each traffic class.
3. System model
3.1. Network environment
3.2. Problem formulation
The meaning of the symbols used in Equations ( 1 )–( 10 )
The total number of ONU-BS in the system
The total number of SS in the system
The maximum number of connections per SS
The maximum resource (time slots) of ONU-BS m
The resource (time slots) allocated to SS n by ONU-BS m
Whether ONU-BS m is the serving BS of SS n
The actual bandwidth (bps) allocated to SS n
R n req
The minimum required bandwidth of SS n
MRR j req
The minimum required bandwidth of the j th connection
MLTR j req
The bandwidth that is needed for completely transmitting all the packets before deadline by the j th connection
The SINR value of SS n
The MCS adopted by SS n
The code rate of the error correction code used by SS n
The packet loss rate of SS n
The required packet loss rate by WiMAX
To solve this problem, we proposed a two-phase solution in the following section. In the first phase, we solved the formula (10) by the proposed MCS and FEC selecting algorithm MFSA algorithm that determines which combination of the MCS and AL-FEC was the best solution in terms of minimum resource (time slots) requirements. In the second phase, two-stage scheduling was proposed to maximize the network throughput. The first-stage scheduling algorithm was used to determine the factors which allowed the system to reach load balancing while the aggregated network throughput is maximized by the second-stage scheduling.
4. Proposed algorithms
Based on the various MCSs that were selected, the transmission rate and packet error rate also varied. Specifically, when a high data rate MCS (such as 64QAM) is selected by BS for transmitting packets to SS, fewer time slots are consumed when transmitting, but higher packet loss rate could be encountered. On the other hand, if a low data rate modulation (such as BPSK) is selected by BS for transmitting information to SS, then more time slots are consumed, but the packet loss rate would decline. Gopala and Gamal therefore introduced the concept of opportunistic transmission: if the AL-FEC was used to help reduce the packet loss rate, then the BS could employ a better modulation to transmit the packet. As a consequence, for a system with M MCSs and F AL-FECs, the BS was offered M*F possible combinations of MCS and AL-FEC to choose from when transmitting packets to an SS. The objective of opportunistic transmission was to select the best combination which could minimize the consuming of time slots while satisfying the packet loss rate guarantee. In this section, we proposed a simple algorithm to achieve this goal.
4.2. First-stage scheduling
4.3 Second-stage scheduling
In Equations (17) and (18), i denotes the type of service, j the index of the connections of that service type, k the index of packets in the queue of that connection, MRR i req is the minimum required bandwidth of service type i, and MLTR i req is the required bandwidth to meet the delay requirement of real time traffic (e.g., rtPS). Equation (17) calculated the minimum required bandwidth for each service type i. For example, the sum total of the MMR of rtPS connections was the MRR required for the service type of rtPS. Equation (18) calculated the required bandwidth for service types (such as rtPS) with delay requirement, where packet _ size k is the length of packet k in the queue of connection j, in bits; D remain (τ k ) was the remaining latency time with tolerable delay of packet k in the queue of connection j, in seconds.
Suppose that the ONU-BS has total_dl_slots time slots available in one frame, as shown in Equation (19), it will preferentially allocate all the total_dl_slots time slots to UGS connections first. If there exists a remaining bandwidth remaining_slot UGS , it will determine the weights of rtPS according to Equation (21), and then allocate the remaining_slot UGS to rtPS connections based on their weights. W rtPS is the maximum value of the minimum required bandwidth of rtPS and the delay tolerant required bandwidth, whereas the WnrtPS is the minimum required bandwidth of nrtPS, as shown in Equations (24) and (25). If rtPS does not use up the bandwidth allocated to it, with remaining_slot rtPS remaining, then nrtPS connections will have available to them the bandwidth allocated to them based on their weights plus the remaining bandwidth not used up by rtPS, remaining_slot rtPS , as shown in Equation (23). Finally, if nrtPS does not use up its bandwidth, then the ONU-BS will search out the SSs with better modulation and determine if there are packets in their rtPS and nrtPS connections that have not been transmitted. If there are such packets, then it will allocate the remaining bandwidth remaining_slot nrtPS to the rtPS and nrtPS connections of the SSs with better modulation. Otherwise, it will allocate all the remaining bandwidth to the BE connections of all the SSs.
Upon receiving the allocated bandwidth, all the service types other than rtPS will commence scheduling according to the SINR values of their connections based on a greedy algorithm. That is, each traffic class will transmit preferentially packets to SSs which consume smallest number of time slots in order to increase the overall throughput. As for rtPS, it will commence scheduling according to the SINR values of its connections, then it will search the queue for packets which are about to fail to satisfy the delay guarantee. If there are such packets, it will allocate the bandwidth preferentially to them. Otherwise, it will transmit preferentially the packets which consume smallest number of time slots, just like the other service types.
5. Numerical results
5.1. System parameters
System parameters used in simulations
EPON data rate
EPON frame duration
WiMAX system bandwidth
WiMAX frame duration
Number of OLT
Number of ONU-BS
Number of SS
Number of connection
Specification of traffic classes
Connections per SS
Min. reserve rate: traffic rate
Min. reserve rate: traffic rate
5.2. Numerical results of the proposed MFSA
The best MCS selected by MFSA without adopting AL-FEC
Bytes per slot
The best combination of MCS and AL-FEC selected by MFSA
AL-FEC code rate = (k/n)
Bytes Per Slot
5.3 Simulation results and discussions
Traffic parameters for each traffic class
20 ms (CBR)
20 ms (VBR)
Scenario 1: 232 kbps
Scenario 1: 580 bytes
20 ms (VBR)
Scenario 2: 352 kbps
Scenario 2: 880 bytes
20 ms (CBR)
We compared four methods under these two Scenarios. The first method, denoted by the original, represents the baseline method in which a two-stage scheduling and AL-FEC were not applied. The second method, referred to as 1s_sch, adopts the first-stage scheduling such that the OLT would, based on the loads of both ONU-BSs, dynamically transmit the packets to the OSS via the ONU-BS with a smaller load. The third method, referred to as 2s_sch, adopts both the first-stage and second-stage scheduling algorithms at the OLT and ONU-BS, respectively. Lastly, the fourth method, denoted by 2s_sch_FEC, adopted MFSA and two-stage scheduling algorithms so that best combination of MCS and AL-FEC was selected and load balancing among ONU-BSs as well as WF2Q were applied to achieve the best system throughput.
5.3.1. UGS traffic
5.3.2. rtPS traffic
5.3.3. nrtPS traffic
5.3.5. The Overall System throughput
EPON-WiMAX is a promising solution for broadband access network with mobility support. In this study, we focused on the downlink scheduling problem from OLT to ONU-BS, referred to as the first-stage scheduling, and from ONU-BS to SSs, referred to as the second-stage scheduling. In the first-stage scheduling, the OLT tried to balance the traffic load of ONU-BSs based on their queue length. In the second-stage scheduling, WF2Q was used to determine which packet in the queue was to be transmitted preferentially at a certain time. We have also proposed the MFSA algorithm to determine the optimal combination of modulation and AL-FEC would be used for receiving packets by SSs under various SINRs so that the resource (time slots) consumption could be minimized. Our simulation results have shown the effectiveness of the two-stage scheduling, MFSA, and the applied AL-FEC. Specifically, these schemes were able to improve the system throughput significantly while guaranteeing the QoS of different traffic class.
In this study, we assumed that one SS would only receive packets from one BS at any one moment. In future studies, we will be more focused on studying the cooperative transmission among multiple BSs by using Space-Time Coding, such that the system throughput could be more enhanced.
The authors would like to thank the National Science Council of the Republic of China, Taiwan for financially supporting this research under Contract No. NSC 100-2221-E-194-012-MY3 and NSC 100-2221-E-194-027-MY3.
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