Jitter-Aware Packet Scheduler for Concurrent Multipath Transmission in Heterogeneous Wireless Networks Min-Cheng Chan, Chien-Chao Tseng and Li-Hsing Yen Institute of Computer Science and Engineering, National Chiao Tung University, Taiwan, R.O.C. mcjan@cs.nctu.edu.tw, cctseng@cs.nctu.edu.tw, lhyen@cs.nctu.edu.tw Abstract—Mobile devices equipped with multiple network inter- faces have the potential to increase transmission throughput by exploiting concurrent multipath transmission (CMT). However, packet scheduling for CMT is challenging since the diversity of latencies among transmission paths can easily yield out-of- order packet receptions and cause receiver buffer blocking if the receiver buffer is not large enough. Previous studies proposed several solutions attempting to eliminate out-of-order receptions and receiver buffer blocking. Delay-Aware Packet scheduling (DAPS) is the only one among them that considers delay diversity at the sender side when scheduling packets. However, DAPS assumes quasi-static path delay and thus does not perform well if path delay changes dynamically. In this paper, we analyze how jitter affects the performance of DAPS. Furthermore, we propose and implement a jitter-aware packet scheduler named JAPS. Performance evaluations show that JAPS outperforms existing algorithms in terms of throughput under different settings of data volume, receiver buffer size, network jitter and bandwidth ratio. Index Terms—Concurrent multipath transmission, scheduling algorithm, heterogeneous wireless networks. I. I NTRODUCTION As wireless communication technologies continue to ad- vance, more and more people rely on their mobile devices to access online services. Modern mobile devices such as smart phones are equipped with multiple network interfaces that allow them to access the Internet through heterogeneous wireless networks. This ability also motivates the study of using multiple heterogeneous networks concurrently to provide larger trans- mission throughput. For example, Multipath TCP (MPTCP) [1] is an extension of TCP that supports multipath transmission and multihoming. CMT-SCTP [2], a multihoming-capable Stream Control Transport Protocol (SCTP) [3], is another approach that supports concurrent multipath transmission. The main challenge of concurrent multipath transmission is that, if we transmit data concurrently over multiple heteroge- neous links using simple scheduling algorithm such as Round- robin, the asymmetry of delay and bandwidth may result in out- of-order receptions. Due to limited memory and receiver buffer in mobile devices, out-of-order receptions may further causes receiver buffer blocking [4]. Kuhn et al. [5] analyzed the receive buffer blocking problem and concluded that with simple Round- robin scheduling algorithm the maximum blocking time would Chien-Chao Tseng is the corresponding author of this paper. increases as the diversity of link delay and bandwidth increase. As a consequence of out-of-order receptions, the transmission throughput may severely degrade, even worse than that of using a single path. Some previous approaches to this problem manipulate re- ceiver buffer, such as increasing buffer size [6] or splitting the buffer [7]. Other prior researches used specific retransmission policy [8]–[10] to reduce the chance of receiver buffer blocking. However, the above-mentioned solutions did not take the root cause (i.e. the diversity of links) into consideration. DAPS [5] uses a more proactive approach at the sender side, which estimates the round trip time (RTT) of each available link and predicts the arrival sequence when scheduling packets over available paths. However, DAPS is based on an accurate delay estimation, which is challenging when being used in networks with high jitter such as heterogeneous wireless networks since unexpected delay bursts will lead to under- estimation of network delay. Jitter-aware Packet Scheduler (JAPS), a simple but effective mechanism is proposed in this paper to remedy the effect of out-of-order receptions due to delay bursts. When delay bursts are detected at the sender side, JAPS immediately retransmits delayed packets through the fast path in order to let the duplicated packet arrives earlier than the original one. However, since retransmission shares the same bandwidth resource with normal data traffic, scheduling retransmission becomes a critical problem. We also implemented a prototype of JAPS and evaluated the performance in terms of throughputs under different data volumes, receiver buffer sizes, network jitters and bandwidth ratios. The evaluation result shows that JAPS outperforms DAPS and Round-robin when used in a network environment with high jitter. The rest of the paper is organized as follows. Section II introduces prior works that try to solve problems caused by link asymmetry. Section III analyzes the effect of jitter under the constraint of only two paths and introduces the proposed algorithm: JAPS. Section IV elaborates more about implementation details of JAPS. Section V further extends the result in section III to three or more paths. Section VI evaluates the performance enhancement provided by JAPS. Finally, We