## 1 Introduction
In network congestion, it is significant to reduce packet jitter in a wireless network considering the demand of the real-time applications. Most existing coding-aware routing protocols such as ROCX [6], DCAR [8], PNCAR [9], TCAR [11], NCQR [12], and LCMR [13] assume that all flows participating in the network coding have the same transmission rate, which is untrue for most applications. In fact, multi-rate environments often appear. In multi-rate environments, network coding is an entirely new problem. Existing multi-rate based coding-aware routing protocols such as RCR [7], CPTT [10] also use ONC scheme [5] in selected coding node. RCR [7] is derived from the observation that multi-path routing with traffic splitting can increase coding gain when it combines with the coding-aware routing mechanism. CPTT [10] includes the effect of multiple rates on the design of a new routing metric for path selection. However, neither of them considers the queue state of the selected coding node in packet coding algorithm. On the other hand, considering the queue state of the coding node, if the node delays packets to wait for the arrival of a future coding opportunity, this may result in severe delay, jitter and packet loss because packets need to wait to be network-coded with each other and consume the finite buffer. This may reduce the network performance, especially for real-time traffic. In fact, the reduced jitter achieved through network coding may be offset by jitter induced by waiting for coding opportunities in coding queue. An intuitive and straightforward solution is to allow opportunistic transmission of some packets without network coding to improve long-term jitter benefits. Should the coding node wait for a coding opportunity or just transmit the packet without coding? This is the fundamental question we seek to answer. Figure 2: **(a)** Routing without coding consideration; **(b)** Routing with coding consideration at node 2. For the solution of above question, there is a method for queue-length control [14]: the packets are forwarded without coding if the queue length exceeds a threshold value. This way is much simpler, but the jitter of a packet may be large if the threshold is not right choice. In related work [15, 16, 17, 18], the queue state of coding node is mainly considered regardless of the coding aware routing. [PERSON] _et al._[15] formulated a Markov model of the queue state and proposed a queue-length control policy to strike the optimal trade-off between power efficiency and QoS. [PERSON] _et al._[16] show that if the queue state can be modeled as a Markov process, the queue lengths is enough to be observed as an optimal control action and this can be further detailed in [17, 18]. However, in these researches, there is an identical fact that the assumed topology is a basic coding structure or a line network. In addition, in works [16, 17, 18], there are different queues for different flows, and there is no restriction on the length of these queues. In some situations, the optimal thresholds may not achieve. In this paper, on the basis of routing discovery in coding-aware routing, from the point of view of data transmission, we propose a low-jitter wireless transmission algorithm based on buffer management named BLJCAR, which considers the queue state in packet coding algorithm. BLJCAR consists of two stages: network jitter training and data transmission. In the data transmission phase, BLJCAR decides packets in coding node according to the queue-length based threshold policy instead of regular ONC policy as used in existing coding-aware routing algorithms. The network jitter training phase is introduced before the data transmission phase to search for the optimal threshold of the queue-length control policy. BLJCAR is a unified framework for the single rate case and multiple rate case. Note that the ONC policy can be seen as a special instance of the queue-length based threshold policy where the threshold is 0.