- Research Article
- Open Access
A Selective Downlink Scheduling Algorithm to Enhance Quality of VOD Services for WAVE Networks
© The Author(s). 2009
Received: 1 April 2008
Accepted: 14 November 2008
Published: 30 December 2008
Providing quality-of-service- (QoS-) guaranteed video on demand (VOD) services over wireless access in vehicular environments (WAVEs) is a challenge as WAVE adopts enhanced distributive channel access (EDCA), a contention-based channel access mechanism, for air interface access control. This paper proposes a selective downlink scheduling (SDS) algorithm to enhance the quality of VOD for WAVE networks. According to the importance of video decoding, video packets are categorized into high and low priorities. The categorized packets are put into different queues in roadside units (RSUs) to contend for transmission opportunities. Aiming to improve video playback quality and reduce video playback delay, the proposed SDS algorithm schedules video packets based on their importance, playback deadline, and their real-time parameters of receiving onboard units (OBUs), such as velocity and remaining dwelling time. The effectiveness of SDS algorithm is verified by simulations.
Wireless access in vehicular environments (WAVEs)  is a next generation intelligent transportation technology that aims to improve transportation environments. The physical layer in WAVE is defined in IEEE 802.11  and amended by IEEE 802.11p . The spectrum of WAVE is allocated at the range of 5.86 5.92 GHz, and it is divided into seven channels each of 10 MHz bandwidth. Apart from improving transportation environment safety and intelligent management, the WAVE systems also provide a convenient means to provision data exchange services, such as video on demand (VOD), when people are traveling in vehicles. However, some specific features of WAVE networks, such as fast movement of vehicles and topology dynamic changes, render the provisioning of VOD services as a challenge. In this paper, we endeavor to tackle this problem by proposing a selective downlink scheduling (SDS) algorithm for VOD services in WAVE networks.
Compressed video streams are able to tolerate small amount of packet loss without degrading the quality of video playback very much, since some packets are less important for video decoding . This feature can be used for schedule algorithms to deliver important packets and discard unimportant packets when traffic is heavy, or a wireless channel is unreliable. In this paper, the proposed SDS algorithm is designed to improve video playback quality and reduce video playback delay, especially when traffic load is heavy.
The existing works related to the issues concerned in this paper can be classified into two categories. The first category focuses on traffic scheduling of contention-based wireless networks, such as [5–9]. The second category concerns the issues on video streaming in wireless networks [10–13]. The WAVE standard does not specify particular scheduling algorithms for its media access control (MAC). The conventional first-come-first-service (FCFS) algorithm adopted by  is still recommended to be used in WAVE in coordination with IEEE 802.11e enhanced distributed channel access (EDCA) technology [1, 14]. The earliest deadline first (EDF) algorithm is widely used in traffic scheduling on wireless channels for multimedia [7, 9]. EDF can improve the QoS of multimedia services and overall system utilization when the deadline of each packet in the traffic is known. In , Chang et al. proposed a maximum freedom last (MFL) scheduling algorithm specifically for dedicated short-range communication (DSRC) networks. MFL is an enhancement of EDF which achieves a low-handoff rate by taking DSRC network parameters, such as cell size and vehicle dwelling time, into account.
Video streaming over IEEE 802.11 networks attracted much research interest. The authors of [11–13] proposed mechanisms to transport H.264 and MPEG-4 over IEEE 802.11e/a networks. However, their mechanisms are based on hybrid coordination function controlled channel access (HCCA) . HCCA is a polling-based channel access mechanism which is different from contention-based EDCA used in WAVE. Furthermore, these mechanisms are only suitable for scalable video streams. Ksentini et al.  proposed a cross-layer architecture for H.264 over IEEE 802.11e. They utilized a specific feature of H.264 data partitioning (defined in the extended profile of H.264 standard). In their work, the bitstreaming of a frame is classified into three data partitions with different importance levels. The data partitions are delivered by different access categories (ACs) in IEEE 802.11e MAC. However, they only dealt with the different importance of data partitions within a frame, and they did not distinguish the importance between different video frame-types.
Inspired by EDF, MFL, and aforementioned video streaming schemes over wireless mechanisms, the SDS algorithm proposed in this paper aims to improve video playback quality and reduce video playback delay in WAVE networks. The SDS algorithm schedules video packets based on their importance, playback deadline, and their real-time parameters of receiving OBUs, such as velocity and remaining dwelling time. The SDS algorithm selectively drops unimportant video packets when it is not possible to transmit all packets due to limited dwelling time, heavy load, or undesired channel conditions. This selective packet dropping feature is of benefit to graceful quality degradation. In the SDS algorithm, video packets destined to different OBUs are coordinately scheduled by their playback deadline. This feature can be used to reduce the video playback delay of all OBUs. The proposed mechanism in this paper is not designed for only a specific video codec scheme (such as H.264), but it is also applicable to other popular video compression formats, such as H.263, MPEG-2.
The rest of the paper is outlined as follows. Section 2 presents some preliminaries, and Section 3 discusses the system model. In which the system architecture and system operations are discussed. The proposed scheduling algorithm is presented in Section 4. In Section 5, the scheduling algorithm is evaluated by simulations, and performance of the algorithms is analyzed. Finally, Section 6 concludes the paper.
2.1. Video Frame and Packet
In video compression technologies, such as MEPG and H.264 , encoded pictures (or frames) are arranged in groups of pictures (GOPs). An encoded video stream consists of successive GOPs. A GOP can contain the following frame types: I-frame, P-frame, and B-frame. The order of intraframes and interframes is specified in a GOP. An I-frame is a reference picture which is intracoded corresponding to a fixed image and it is independent of other pictures. A P-frame is predictive-coded frame which contains motion-compensated difference information from the preceding I- or P-frame. A B-frame is bidirectionally predictive-coded frame which contains different information from the preceding and following I- or P-frame within a GOP. I-frame and P-frame are often referred to as anchor frames. A GOP always begins with an I-frame. Afterwards, several P-frames follow. The B-frames are inserted between two consecutive anchor frames.
Since I-frames contain all the information for image reconstruction without the need of referring to other frames, the more I-frames a video stream has, the more resilient it is for transmission over wireless links. However, having more I-frames increases the video stream size. In order to save bandwidth, it is common that video streams are encoded with only one I-frame per GOP. In WAVE, the errors of video stream decoding can occur in OBUs due to packet loss or transmission error. A successfully received I-frame can correct any errors caused by any preceding frames. Therefore, I-frames are much more important than P- and B-frames. In the cases of heavy traffic load or severe packet loss, it is wise to deliver I-frames to OBUs first to enhance the quality of video playback.
It should be noted that the sequence of video frames for transmission is different from the sequence of video playback. Taking the encoding pattern "IBBP " as an example, the actual transmission sequence is "IPBB ", that is, the P-frame is transmitted before the B-frames. This is due to the fact that decoding B-frames requires both I- and P-frames received in an ONU's buffer. In this paper, the playback deadline of a frame or packet means the latest time that a frame (or packet) has to be available in an ONU's decoding buffer.
2.2. Wave Channel Operations
Detailed description of multiple channel operations in WAVE networks can be found at . In order to better understand the operations of the proposed SDS algorithm in Section 3, we summarize the typical channel operations as follows. First, upon power on, an OBU monitors the CCH until a WAVE service advertisement (WSA) sent by an RSU is received. A WSA carries the information of available SCHs and their access parameters, such as channel numbers. Based on the WSA information, the OBU then synchronizes with the RSU, and the OBU can exchange data with the RSU in SCHs.
The discussion above is based on single-channel WAVE devices. With a single channel, an OBU can work on either CCH or SCH at a time. If two or more channels are facilitated in a WAVE device, the operations in CCH interval and SCH interval can be conducted simultaneously. In this paper, we consider single-channel WAVE devices only as this is common for OBUs.
3. System Model of VOD in WAVE Networks
3.1. System Architecture
Inside the CCH, there is a CCH classifier which categorizes packets into four queues identified by access category index (ACI). These four queues have different traffic specifications, such as access priorities, which are defined in IEEE 802.11e EDCA. Each access category has its own channel access function and uses specified EDCA parameters. These parameters are (1) arbitration interframe space (AIFS) which defines the minimum time interval between the wireless medium becoming idle and the start of another transmission; (2) contention windows (CW) which defines a random number generation window for random collision backoff mechanism; (3) transmit opportunity (TXOP) limit which defines the maximum time duration for which a WAVE device can transmit after obtaining a TXOP. The sum of TXOPs of these four queues is a CCH interval, that is, , where is the TXOP of the queue in the CCH channel. The SCH scheduler schedules these four access category queues based on their access parameters. There is no specific scheduling algorithm defined in the WAVE standard, and the scheduling operations may differ in different implementations.
In the proposed scheduling architecture, SCH's packet queuing procedure is similar to that of CCH's. The SCH classifier is in charge of putting packets into these four access category queues. It is the same as CCH that . SCH differs from CCH in the following manners: (1) the queue (Figure 4(c)) is dedicated for video streaming packets, and it is further separated into subqueues for each OBU to store high- and low-priority video packets; (2) the SCH scheduler implements the proposed SDS algorithm (as discussed in Section 4).
Before sending video packets to a channel router, the priority of each video packet is marked by the priority marking module. Some reserved bits in video bitstream header can be used to identify the importance of a packet. Taking H.264 over RTP  as an example, the with values 30 and 31 are not specified in the standard . As shown in Figure 4(b), we can mark high-priority video packets as type 30, and low priority video packets as type 31.
3.2. System Operations
Before delivering packets in an SCH interval, the RSU has to decide how to use the SCH interval for all OBUs. This is where scheduling algorithms come to play. In the proposed architecture, the SDS algorithm schedules downlink packets based on a service list to specify the sequence of the OBUs to be served and which packets in the queues are granted to deliver to OBUs. As shown in Figure 5, OBU 3 will be served first and then OBU 1. There are 21 packets ( ) in the high-priority queue and 31 packet ( ) in the low-priority queue granted to be delivered to OBU 3. For OBU 1, only high-priority packets are granted to be delivered in this SCH interval. The high-priority packets of OBU 1 are not granted, and they are probably expired and thus discarded. The SDS algorithm generates or updates the service list in the RSU just before each SCH interval. The service list is indexed by OBU IDs and it is ordered by a weight factor (as discussed in Section 4 below).
After the service list is generated or updated, as shown in Figure 5(b), the RSU serves the OBUs according to this list one by one by following the order. The granted video packets are delivered within the current SCH interval. The above operational procedure repeated in each WAVE sync interval.
4. Proposed SDS Algorithm
As aforementioned, an RSU delivers downlink packets to OBUs based on a service list. The first task for the SDS algorithm to do is to generate or update the service list, so that the RSU can use it to deliver packets in SCH intervals.
that is, either the service interval is entirely allocated to OBU or the RSU allocates the OBU's with its remaining dwelling time . Based on a GP's playback deadline, we can calculate a tolerable playback delay of this GP. For example, the tolerable playback delay of a high-priority GP is . The RSU also keeps the time that each GP already waited in the queue, that is, queuing delay. For example, the queuing delay of a high-priority GP is . Based on the total size of a GP and the transmission rate to OBU , the total time needed to transmit a GP to OBU can be calculated, for example, for GP it is or , for high-and low-priority GPs, respectively.
where is the time needed to send the first high-priority GPs to OBU , and is the tolerable playback deadline of the first high-priority GPs. The equation implies that the OBUs with small dwelling time, small tolerable playback delay, larger queuing delay, and larger amount of high-priority packets will be scheduled first.
Initialize the service list: remove an OBU from the list if it is no longer connected to the RSU;
In each sub-queue, discard the packets if their playback deadlines are reached.
Grant the packets in all high priority sub-queues and then all low priority sub-queues in a round-robin
The algorithm first initializes the service list by removing disconnected OBUs and adding newly connected OBUs (line 1). It then discards the packets with playback deadlines are reached (line 2). The algorithm calculates the scheduling weight factors (line 3) and sorts the OBUs by their factors (line 4). After that, for each OBU, all the transmission time required for transmitting high- and low-priority GPs with deadlines within current WAVE sync interval is calculated (line 5). This step is to find the packets which will be expired if they are not transmitted within current sync interval. Line 6 calculates the total time required to transmit all high- and low-priority GPs. Line 7 initializes to . If the unallocated time is enough for transmitting the high-priority GPs (line 8), all high-priority GPs will be granted (line 9). Line 10 updates the unallocated time . If there is still enough time for low-priority GPs (line 11), all packets in the low-priority GPs will be granted (line 12). If the unallocated time is not enough to transmit all low-priority GPs (line 15), will be used for allocating the GPs according to the order yielded in line 4, until the TXOP is fully allocated, that is, . It is the same if is not enough for all high-priority GPs (line 19), and only part of high-priority GPs can be granted according to the order yielded in line 4. If there is still some time left after all the high- and low-priority GPs with the playback deadlines within current are granted (line 22), the remaining time will be used to grant the packets in all high-priority subqueues and then all low-priority subqueues in a round-robin manner in the sorted OBU order. The reason to use round-robin here is for computational simplicity, since these packets' playback deadlines are not reached until next SCH interval and they can be transmitted in next sync interval if they are not scheduled in current .
5. Simulation and Result Analysis
5.1. Simulation Setup
Since the focus of the proposed algorithm is to schedule the packets in the RSU queues, handover is not considered directly in this paper. In the simulations, we assumed that the video packets associated to an OBU are always available in its currently connected RSU. If the OBU is not associated with the RSU due to mobility, the proposed algorithm will discard all the packets related to the OBU in the RSU queues.
Parameters of video streams used in the simulations.
Statistics of video streams used in the simulations (high priority frame and packet).
No. of Frm
Maximum Frm size
Average Frm size
No. of Pkts
Maximum Pkt size
Average Pkt size
Silence of the lambs
Star wars IV
Statistics of video streams used in the simulations (low priority frame and packet).
No. of Frm
Maximum Frm size
Average Frm Size
No. of Pkts
Maximum Pkt size
Average Pkt Size
Silence of the lambs
Star wars IV
The ratio between high- and low-priority packets.
Ratio between high- and low-priority packets
Silence of lambs
Star wars IV
In the simulations, we assume that all OBUs request VOD services after they enter the simulated highway. The requested video streams are randomly selected from the aforementioned four benchmark video streams. Except for the video data ( ), other types of downlink traffics are generated by following rules: (1) voice traffic is generated randomly in the range of (32 kbps, 128 kbps) (2) self-similar BE traffic is generated randomly in the range of (0 kbps, 128 kbps) (3) background traffic is generated randomly in the range of (0 kbps, 256 kbps).
In the simulations, the proposed SDS algorithm is compared with the conventional FCFS , EDF , and MFL  algorithms in terms of video packet delivery ratio and video playback delay. We run the simulation 50 rounds and collect the results for analysis.
5.2. Delivery Ratio of Video Packets
Figure 7 shows the average delivery ratio of high-priority packets. When the total traffic load is under 80%, there is no high-priority packet loss. After the traffic load reaches a saturation point (100% of traffic load), the delivery ratio drops. The FCFS algorithm drops the fastest. This is because it does not distinguish the difference in packets. By distinguishing playback deadlines, the EDF and MFL perform better than FCFS in all video streams. Amongst EDF and MFL, MFL performs better due to the fact that it considered some specific features of vehicular networks, such as remaining dwelling time and tolerable maximum transmission delay. As EDF and MFL do not take the importance of different types of video packet into account, they treat high- and low-priority packets in the same way. The SDS algorithm outperforms the others due to the fact that the high-priority packets are preferentially treated. In all four video streams, the SDS algorithm always achieves the highest delivery ratio. Even though the overall traffic load is 160%, the SDS algorithm's average delivery ratio is still higher than 80% in all video streams. This reflects the specific feature of the SDS algorithm, that is, to deliver high-priority packets first.
Apart from the delivery ratio of high-priority packets, we are also interested to analyze the delivery ratio of low-priority packets. As shown in Figure 8, when the traffic load becomes higher than the saturation point, the delivery ratios of all algorithms drop quickly for all video streams. As can be observed, the delivery ratio of the SDS algorithm is lower than EDF and MFL in most of the cases. It is even lower than FCFS in video stream Tokyo Olympics after the traffic load is larger than 140%. This reflects the fact that the high-delivery ratio of high-priority packets is at the cost of low-delivery ratio of low-priority packets when the traffic load is very heavy.
5.3. Video Playback Delay
Apart from higher delivery ratio of high-priority packets, another main objective of the proposed SDS algorithm is to reduce video playback delay. In the situations of heavy traffic load and severe channel condition, video playback delay is unavoidable. In this simulation, video playback delay is defined as the accumulated time of the video discontinuity during the overall video stream playback. Video discontinuity is caused because the required high-priority packets (which constitute I-frames) are undelivered within their playback deadlines.
In this paper, we have proposed a selective downlink scheduling algorithm, SDS, for WAVE networks in order to improve the quality of VOD services. Video packets are marked into high and low priorities before they are fed into WAVE MAC. Marked video packets are put into different access category queues. Our proposed SDS algorithm schedules the video packets according to their playback deadline and received OBU's network parameters, such as the time remained to communicate with current RSU and so on. The SDS algorithm is evaluated through simulations, showing its effectiveness and efficiency for video streaming over WAVE networks. In this work, we only focus on increasing the delivery ratio of high-priority packets and reducing video playback delay. We believe that these two parameters are fundamental for the quality of video streaming over WAVE. Other video quality evaluations, such as peak signal-to-noise ratio (PSNR) and mean opinion score (MOS), will be studied in our future work.
The authors would like to gratefully acknowledge the UK Engineering and Physical Sciences Research Council (EPSRC) Project PANDA (EP/D061881/1) and Taiwan National Science Council Grant NSC 97-2219-E-006-004.
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