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A New Markov Model for Evaluating the ALR Dual-Threshold Policy

The paper proposes the analysis of the Dual- Threshold Policy, used by Adaptive Link Rate (ALR). In par- ticular, the paper firstly presents a new Markov model of the Dual-Threshold Policy. The new features of the model concern its ability to take into account the rate switching times and the buffer size of the Ethernet Network Interface Controller (NIC). Secondly, we have evaluated the performance of the Dual- Threshold Policy by means of the proposed model and of a new power consumption model of an Ethernet NIC implementing ALR. As performance parameter, we have considered the power savings, the packet loss probability, the mean packet delay, and the oscillation of link data rate. The results show that high rate switching times can make ALR unprofitable and that the current NICs buffer size is unsuitable for ALR. I. INTRODUCTION Energy consumption has become a key issue during the past few years. The Internet is rapidly becoming a major consumer of power with significant economic and environmental im- pacts. Since most of power consumption is due to the access networks, the key considerations to reduce power consumption of networking devices are that Ethernet links are mostly underutilized (1) and that idle and fully utilized Ethernet links almost consume the same amount of energy. In particular, the energy consumption of an Ethernet Network Interface Controller (NIC) is proportional to the link data rate, i.e. 10M, 100M, 1G. Adaptive Link Rate (ALR) is a method to reduce energy consumption adapting Ethernet link data rate to link utilization. ALR is currently a key component of the Energy Efficient Ethernet (EEE) task force (IEEE 802.3az) in IEEE 802.3 (2). The key issues of the ALR are to define a mechanism for quickly switching the link data rate (ALR Frame Handshake Mechanism) and to create a policy to control the link date rate switching so as to maximize power savings without sig- nificantly worsening the network performance (ALR Control Policy). The first issue has been addressed by IEEE 802.3az, propos- ing a new mechanism called Rapid PHY Selection (RPS). RPS is based on a MAC frame handshake to initiate a link data rate switch. Following the handshake, the link would resynchronize at the new link rate. The sum of the MAC handshake time and the resynchronization time gives the rate switching time. During the rate switching period the link is inactive and no packets can be transmitted on it. Since during this period new packets can arrive at the Ethernet NIC, this can lead to buffer overflows and consequently to packet losses. Regarding the ALR Control Policies, they are outside the scope of IEEE 802.3az and remain a fertile area for research and evolution. In the last few years, several Control Polices have been already proposed ((3), (4), and (5)). In (3), the authors proposed the Dual-Threshold Policy, which is based on output buffer occupancy, or queue length, threshold crossing. In this policy, there are two thresholds used to induce hysteresis into the system and to prevent a trivial oscillation between rates. The policy works as follows. If the output buffer queue length in a NIC exceeds the higher threshold, then the link rate must be transitioned to high. If the output queue decreases below the lower threshold, then the link rate can be reduced to low. The main shortcoming of the Dual-Threshold Policy is that the NIC may oscillate between data rates given smooth traffic input or bursty traffic, with long duration bursts causing increased response time and variability.