In recent years, the reduced cost and increased capacity of memory have resulted in a growing number of buffers in switches and routers. Consequently, today's networks suffer from bufferbloat, a term that refers to excess frame buffering resulting in high latency, high jitter, and low throughput. Although ring aggregation is an efficient topology for forwarding traffic from multiple, widely deployed user nodes to a core network, a fairness scheme is needed to achieve throughput fairness and avoid bufferbloat, because frames are forwarded along ring nodes. N Rate N+1 Color Marking (NRN+1CM) was proposed to achieve per-flow fairness in ring aggregation networks. The key idea of NRN+1CM is to assign a color that indicates the dropping priority of a frame according to the flow-input rate. When congestion occurs, frames are selectively discarded based on their color and the frame-dropping threshold. Through the notification process for the frame-dropping threshold, frames are discarded at upstream nodes in advance, avoiding the accumulation of a queuing delay. The performance of NRN+1CM was analyzed theoretically and evaluated with computer simulations. However, its ability to avoid bufferbloat has not yet been proven mathematically. This paper uses an M(n)/M/1/K queue model to demonstrate how bufferbloat is avoided with NRN+1CM's frame-dropping threshold-notification process. The M(n)/M/1/K queue is an M/M/1/K queuing system with balking. The state probabilities and average queue size of each ring node were calculated with the model, proving that the average queue size is suppressed in several frames, but not in the most congested queue. Computer simulation results confirm the validity of the queue model. Consequently, it was logically deducted from the proposed M(n)/M/1/K model that bufferbloat is successfully avoided with NRN+1CM independent of the network conditions including the number of nodes, buffer sizes, and the number and types of flows.

Efficiently locating nodes and allocating demand has been a significant problem for telecommunication network carriers. Most of location models focused on where to locate nodes and how to assign increasing demand with optical access networks. However, the population in industrialized countries will decline over the coming decades. Recent advance in the optical amplifier technology has enabled node integration; an excess telecommunication node is closed and integrated to another node. Node integration in low-demand areas will improve the efficiency of access networks in this approaching age of depopulation. A dynamic node integration problem (DNIP) has been developed to organize the optimal plan for node integration. The problem of the DNIP was that it cannot consider the requirements of network carriers. In actual situations, network carriers often want to specify the way each node is managed, regardless of the mathematical optimality of the solution. This paper proposes a requirement modeling language (RML) for the DNIP, with which the requirements of network carriers can be described. The described statements are used to solve the DNIP, and consequently the calculated optimal solution always satisfies the requirements. The validity of the proposed method was evaluated with computer simulations in a case study.

Our group has developed terahertz(THz)-waves emitting devices utilizing single crystals of high temperature superconductor Bi2Sr2CaCu2O8+δ (Bi2212). The working principle of the device is based on the AC Josephson effect which is originated in the intrinsic Josephson junctions (IJJs) constructed in Bi2212 single crystals. In principle, based on the superconducting gap of the compound and the AC Josephson effect, the emission frequency range from 0.1 to 15 THz can be generated by simply adjusting bias voltages to the IJJs. In order to improve the device performances, we have performed continuous improvement to the device structures. In this paper, we present our recent approaches to high performance Bi2212 THz-waves emitters. Firstly, approaches to the reduction of self Joule heating of the devices is described. In virtue of improved device structures using Bi2212 crystal chips, the device characteristics, such as the radiation frequency and the output power, become better than previous structures. Secondly, developments of THz-waves emitting devices using IJJs-mesas coupled with external structures are explained. The results clearly indicate that the external structures are very useful not only to obtain desired radiation frequencies higher than 1 THz but also to control radiation frequency characteristics. Finally, approaches to further understanding of the spontaneous synchronization of IJJs is presented. The device characteristics obtained through the approaches would play important roles in future developments of THz-waves emitting devices by use of Bi2212 single crystals.

In recent years, Ethernet fabrics have been developed with a view to using resources efficiently and simplifying the operation of data center networks. With Ethernet fabrics, frames are forwarded along the shortest paths based on routing tables without blocking ports. Ethernet fabrics are expected to be employed in more general networks including carrier access networks. In particular, the use of shortest path bridging MAC (SPBM) is expected to allow smooth migration from existing networks. With SPBM, networks can be flexibly constructed on demand in any network topology. If an arbitrary topology is constructed, traffic paths can overlap on specific links and throughput unfairness occurs. However, it is difficult to achieve accurate weighted fairness with existing schemes. This paper proposes employing weighted N rate N+1 color marking (WNRN+1CM) in SPBM networks to achieve per-flow weighted fairness. WNRN+1CM was developed to realize weighted fairness in layer-2 ring networks and the applicability to other network topologies has not yet been discussed. The outline of WNRN+1CM in SPBM is as follows. The weight and the maximum rate are provided for each flow at edge bridges. When edge bridges receive frames from outside the SPBM domain, they assign colors to frames according to the input rate and the weight of each flow. The color indicates the dropping priority. If the input rate exceeds the maximum rate, frames are discarded to limit the throughput. Core bridges selectively discard frames based on their color and the dropping threshold when congestion occurs. The bandwidth is allocated based on the weights. The performance of WNRN+1CM is evaluated with a theoretical analysis and computer simulations. WNRN+1CM can achieve weighted fairness in aggregation networks and multipoint networks. The throughput ratio matches the weights and the flow throughputs are limited to their maximum rate regardless of changes in traffic.

Ring aggregation networks are widely employed for metro access networks. A layer-2 ring with Ethernet Ring Protection is a popular topology for carrier services. Since frames are forwarded along ring nodes, a fairness scheme is required to achieve throughput fairness. Although per-node fairness algorithms have been developed for the Resilient Packet Ring, the per-node fairness is insufficient if there is bias in a flow distribution. To achieve per-flow fairness, N Rate N+1 Color Marking (NRN+1CM) was proposed. However, NRN+1CM can achieve fairness in case there are sufficient numbers of available bits on a frame header. It cannot be employed if the frame header cannot be overwritten. Therefore, the application range of NRN+1CM is limited. This paper proposes a Signaling based Discard with Flags (SDF) scheme for per-flow fairness. The objective of SDF is to eliminate the drawback of NRN+1CM. The key idea is to attach a flag to frames according to the input rate and to discard them selectively based on the flags and a dropping threshold. The flag is removed before the frame is transmitted to another node. The dropping threshold is cyclically updated by signaling between ring nodes and a master node. The SDF performance was confirmed by employing a theoretical analysis and computer simulations. The performance of SDF was comparable to that of NRN+1CM. It was verified that SDF can achieve per-flow throughput fairness without using a frame header in ring aggregation networks.

With shortest path bridging MAC (SPBM), shortest path trees are computed based on link metrics from each node to all other participating nodes. When an edge bridge receives a frame, it selects a path along which to forward the frame to its destination node from multiple shortest paths. Blocking ports are eliminated to allow full use of the network links. This approach is expected to use network resources efficiently and to simplify the operating procedure. However, there is only one multipath distribution point in the SPBM network. This type of network can be defined as an end-to-end multipath network. Edge bridges need to split flows to achieve the load balancing of the entire network. This paper proposes a rate-based path selection scheme that can be employed for end-to-end multipath networks including SPBM. The proposed scheme assumes that a path with a low average rate will be congested because the TCP flow rates decrease on a congested path. When a new flow arrives at an edge bridge, it selects the path with the highest average rate since this should provide the new flow with the highest rate. The performance of the proposed scheme is confirmed by computer simulations. The appropriate timeout value is estimated from the expected round trip time (RTT). If an appropriate timeout value is used, the proposed scheme can realize good load balancing. The proposed scheme improves the efficiency of link utilization and throughput fairness. The performance is not affected by differences in the RTT or traffic congestion outside the SPBM network.