Local area networks using a spatial wavelength cross connect

Future photonic local area networks may be based upon single-hop architectures using wavelength division multiplexing. Due to optical device limitations however, the number of available channels may not be as large as desired. As a result, spatial wavelength reuse may be required to achieve networks with acceptable performance. One way of achieving this is to use a hierarchical design where a set of local wavelengths is reused in each local subnetwork. The remaining wavelengths are then used to provide connectivity between subnetworks. Although this design can result in very high network capacities, it relies on a high degree of traffic locality, which may not be present in typical local area networks. To address this issue, recently a LAN design was proposed that interconnects local subnetworks across a spatial wavelength cross connect `MANDALA'. The operation of this system however, relies on a single global control channel which may become a performance bottleneck even when the number of channels is very modest. In this paper, we propose some novel designs and protocols for achieving dynamic operation in such a cross connect LAN. The first option uses an associated control scheme for signaling within each local cluster and results in very simple user station designs. The second option employs a spatially reused common control channel for each cluster and requires additional station hardware. Three media access protocols are examined, namely, interleaved TDMA, and two reservation schemes. In the proposed designs, wavelength converter banks are introduced to eliminate the need for dynamic receiver tunability at the user stations. Both capacity and mean delay performance results are presented using analytic and simulation models. It is shown that the proposed systems yield various degrees of dynamic behavior and do not suffer from the performance degradations associated with that of `MANDALA'.     

Orthogonal signaling-based queue status investigation method in IEEE 802.11

IEEE 802.11 specifies four different medium access control (MAC) protocols to coordinate multiple access in a wireless local area network (WLAN). Since several tens of stations can operate in a WLAN, the performance of MAC protocols is important for overall network efficiency. It has been observed that the IEEE 802.11 MAC protocols can be improved by knowing which station has a non-empty queue, i.e., queue status. The point coordination function (PCF) can use this information to avoid polling a station that has no pending data. The HCF controlled channel access can adapt polling parameters based on queue status information, especially when scheduling a bursty and variable bit-rate traffic. Previously suggested methods are rather limited in terms of accuracy and efficiency.In this paper, we propose a novel method to investigate the queue status of multiple stations by exploiting orthogonal signaling. With synchronous transmission of orthogonal codes and symbol level signal processing, the method allows all of the associated stations to report their queue status at the same time. Challenges that can arise in the implementation of the proposed method are identified, and their solutions are suggested. The feasibility of detecting orthogonal signals is thoroughly tested on a realistic channel model. To demonstrate the performance improvement of a MAC protocol, we applied the proposed method to PCF. Both analysis and simulation show that the modified PCF significantly outperforms not only the original PCF but also other previously suggested PCF enhancements.     

IEEE 802.16e休眠模式算法的研究和改进

IEEE 802.16e为移动站点提供了大范围的无线接入,为了减少移动站点的功率消耗,它提出了一种节能机制休眠模式.移动站点在休眠间隔后进入监听间隔,检查其间是否有数据分组到达.如果有,它将进入清醒状态接收数据,否则,继续进入休眠间隔.草案中所提出的休眠模式指数增长算法在低业务量情况下,响应时间过长.文中提出了线性增长算法解决这一问题,理论分析了该算法的两个性能指标平均响应时间和平均能量消耗.并结合仿真实验,对比了这两种算法的性能,证实了后者具有较好的性能.进一步分析了休眠模式各个参数对上述两个性能指标的影响,对参数值的选取有重要帮助.     

Dual-hop LANs using a wavelength routing cross connect

The use of wavelength-division-multiplexing (WDM) may dramatically increase the traffic capacity in future local and metropolitan area networks. However, because of device limitations, the number of available wavelengths may initially be less than desired. As a result, spatial reuse may be required in order to obtain designs which will support a reasonable number of stations. There are several ways in which this may be accomplished (Matsumoto, 1993; Janoska and Todd, 1994; Kannan et al., 1994).In (Matsumoto, 1993), a LAN design was considered where media access is performed across a spatial wavelength cross connect. Although the traffic capacity may be much higher than in conventional LANs, many operational aspects in this type of system are much more complex. In this paper, we investigate the use of a dual-hop architecture as a way of simplifying the operation of such a system. The network consists of two stages. In the first, the wavelength agility of the user stations is used to route packets from a given local optical network (LON) to a destination LON. When packets arrive at a destination LON, they are buffered and transmitted onto the required destination wavelength. There are a number of advantages to this design including the elimination of protocols which would require both dynamic station transmitter and receiver tunability. This design also takes advantage of increasingly available commercial ATM buffer/switch components. Several hybrid electro-optic options are considered. In addition to the electronic buffering versions, we also show that the LON buffering stage could be implemented using the almost all-optical approaches discussed in (Haas, 1993; Tsukada et al., 1994; Hunter et al., 1994; Thompson and Giordano, 1987; Gabriagues et al., 1994). Simplified control strategies are proposed and wavelength sharing is used to decrease the number of required buffers. The performance of the system is compared with others using various traffic models.     

An energy-efficient permutation routing protocol for single-hop radio networks

A radio network (RN) is a distributed system where each station or node is a small hand-held commodity device called a station. Typically, each station has access to a few channels for transmitting and receiving messages. By RN(p, k), we denote a radio network with p stations, where each station has access to k channels. In a single-hop RN, every station is within the transmission range of every other station. Each station consumes power while transmitting or receiving a message, even when it receives a message that is not destined for it. It is extremely important that the stations consume power only when it is necessary since it is not possible to recharge batteries when the stations are on a mission. We are interested in designing an energy-efficient protocol for permutation routing, which is one of the most fundamental problems in any distributed system. An instance of the permutation routing problem involves p stations of an RN, each storing n/p items. Each item has a unique destination address which is the identity of the destination station to which the item should be sent. The goal is to route all the items to their destinations while consuming as little energy as possible. We show that the permutation routing problem of n packets on an RN(p, k) can be solved in 2n/k+(p/k)/sup 2/+p+2k/sup 2/ slots and each station needs to be awake for at most 6n/p+2p/k+8k slots. When k/spl Lt/p/spl Lt/n, our protocol is more efficient, both in terms of total number of slots and the number of slots each station is awake compared to a previously published protocol by Nakano et al. (2001).     

Analysis of power saving with continuous connectivity

Always-on mobile users need high bandwidth channels with negligible access delay and limited power consumption. Such a continuous connectivity mode requires the management of high-speed channels, which can turn into substantial operational costs (i.e., power consumption rate) even in presence of low traffic, unless a power saving mechanism is enforced. In this paper, we analyze the impact of 3GPP-defined power saving mechanisms on the performance of users with continuous connectivity. We develop a model for packet transmission and operational costs. We model each downlink mobile user’s traffic by means of an M/G/1 queue, and the base station’s downlink traffic as an M/G/1 PS queue with multiple classes and inhomogeneous vacations. The model is validated through packet-level simulations. Our results show that consistent power saving can be achieved in the wireless access network, as high as 75% for mobiles and 55% for base stations.     

On the use of learning automata in the control of broadcast networks: a methodology

Due to its fixed assignment nature, the well-known time division multiple access (TDMA) protocol suffers from poor performance when the offered traffic is bursty. In this paper, an adaptive TDMA protocol, which is capable of operating efficiently under bursty traffic conditions, is introduced. According to the proposed protocol, the station which is granted permission to transmit at each time slot is selected by means of learning automata (LA). The choice probability of the selected station is updated by taking into account the network feedback information. The system which consists of the LA and the network is analyzed and it is proven that the choice probability of each station asymptotically tends to be proportional to the probability that this station is not idle. Although there is no centralized control of the stations and the traffic characteristics are unknown and time-variable, each station tends to take a fraction of the bandwidth proportional to its needs. Furthermore, extensive simulation results are presented, which indicate that the proposed protocol achieves a significantly higher performance than other well-known TDMA protocols when operating under bursty traffic conditions.     

无线异构EH蜂窝网络中移动台关联的性能分析

在无线EH蜂窝网络中,建立了一个异构网络模型,这个模型由宏基站、小基站、中继站以及移动台组成。所有站点仅由周围无线环境中收集的能量供电。基于这个模型,研究了一种基于加权信噪比决策的新型移动关联方法。在方法中,移动台不仅可以通过直连链路关联到宏基站和小基站,也可以通过中继链路关联到宏基站,关联取决于信道状态信息和由权重函数和信噪比门限的乘积所定义的加权信噪比门限。此外,推导了系统中断概率和平均信道容量的表达式,仿真结果表明,WSNR-MA方法在多类型站点间的业务负载均衡中具有灵活性,比现有的方法能够获得更大的系统信道容量和更好的中断性能,具有积极的作用。     

Load Balancing in IEEE 802.11 Networks

Because wireless stations independently select which access points to camp on, the total wireless station traffic on all available IEEE 802.11 network APs might be unevenly distributed. This load-balancing problem can lead to overloading and network congestion. This survey examines the problem, along with state-of-the-art network- and wireless-station-based solutions. It also presents experimental results using off-the-shelf IEEE 802.11 devices. As the results show, effectively balancing AP traffic loads can increase overall system throughputs.     

A CSMA/CD compatible MAC for real-time transmissions based on varying collision intervals

This paper suggests a CSMA/CD compatible MAC protocol for real-time transmissions in a shared Home or Small Office Local Area Network. The new MAC is based on the distributed assignment of special TAG numbers to the stations transmitting real-time traffic. These TAG numbers determine a Round Robin order of transmissions among the real-time stations. They also help in resolving collisions among real time stations by setting the length of the Jam signal, transmitted in case of a collision, to be a function of the TAG number. In a collision the station with the highest TAG number, and so with the longest Jam, is persisting with its Jam transmission for the longest until all the other stations defer. Thus, the collision terminates and the longest persisting station can transmit its packet successfully. The new protocol enables stations implementing the IEEE 802.3 MAC standard to transmit on the same network with stations implementing the new protocol. After introducing the protocol, we compute an upper bound on the access delay that the protocol guarantees and prove the correctness of the distributed TAG assignment procedure. Finally, we simulate the protocol in a network consisting stations implementing the new protocol together with stations implementing the standard IEEE 802.3 MAC. We show that the access delays of the stations transmitting real-time traffic are indeed bounded as predicted.     

Analysis of power saving and its impact on web traffic in cellular networks with continuous connectivity

In this work, we analyze the power saving and its impact on web traffic performance when customers adopt the continuous connectivity paradigm. To this end, we provide a model for packet transmission and cost. We model each mobile user’s traffic with a realistic web traffic profile, and study the aggregate behavior of the users attached to a base station by means of a processor-shared queueing system. In particular, we evaluate user access delay, download time and expected economy of energy in the cell. Our study shows that dramatic energy saving can be achieved by mobile devices and base stations, e.g., as much as 70%–90% of the energy cost in cells with realistic traffic load and the considered parameter settings.     

基于3G网络的交通气象信息自动监测系统

针对交通气象信息监测系统中存在的问题,研究并设计实现了一种全自动交通气象信息监测系统。该系统由一组分布于公路沿线的交通气象监测站与交通管理中心服务器组成。站点与管理中心服务器之间通过3G网络实现数据通信。监测站点不仅可以自动监测和向管理中心服务器上传气象数据,而且具有对各种恶劣天气现象进行智能判决、即时上报的功能。交通管理中心接收所有监测站点的气象数据,进行分析判决、图形化显示,实现恶劣天气现象的报警,同时将数据存储于数据库中以备统计查询。监测站点采用低功耗微处理器,并以太阳能电池与蓄电池供电,从而站点的布设不受地理条件限制。实验表明:该系统能准确实时获取气象信息,达到设计要求。     

A ring traffic monitor

This paper describes the design and construction of a ‘traffic monitor’ for the Cambridge ring; this is a device that connects to a ring and enables selected data to be captured for performance monitoring or protocol debugging.The hardware consists of a M68000 VMEBUS system with specialized hardware to select and capture minipackets from the ring.The software currently running on this system allows a user to select a pair of station addresses and capture and display all ‘traffic’ between these two stations.     

Loss probabilities of hand-in traffic under various protocols: II. Model comparisons

Mobile telephone traffic business demands that a given base station be able to handle both newly originating traffic and hand-off traffic from neighboring base stations, with priority to be given to the latter, existing traffic. This paper examines three access control protocols that give such traffic priority by specifying a reserve capacity R amongst the N_tot channels, where R need not be an integer. Assuming a Poisson arrival process and independent exponential service times, it is shown via birth–death processes, sample path constructions and stochastic monotonicity results for Markov processes that pairwise comparison of the protocols considered is possible, and, in particular, that one of them is ‘best’ in terms of the smallest loss probability of one type of traffic for a given rate of loss for the other type. Results proved via sample path constructions hold under weaker conditions of stationary ergodic arrival processes and exponential service times.     

Stability and throughput optimization of multichannel slotted non-persistent CSMA protocol

This paper describes two protocols suitable for a multichannel, multiaccess slotted non-persistent CSMA environment for infinite population, analyses their stability and examines their throughput optimization. The common sensing multichannel slotted non-persistent CSMA (CSCSMA) protocol is an extension of the slotted single channel non-persistent CSMA protocol with an appropriate policy for the selection of the channel in which a station (re)transmits. This policy restricts the control information among the stations, and consequently reduces the cost of the station interface. Pakes's Lemma criteria are applied to define sufficient conditions for ergodicity of the Markov chain, which describes the evolution of the busy station population, and to specify regions in which the multichannel system is stable. Control parameters are the retransmission probabilities. Optimization rules are derived which show that the optimal retransmission probabilities may be expressed as a function of the number of busy stations. The separate sensing multichannel slotted non-persistent CSMA protocol (SSCSMA), using a different policy, distinguishes the channels into two groups: the retransmission channel group devoted in collision resolution; and the transmission channel group for the access of free stations. Stability regions are defined using the results for CSCSMA, and rules for optimal allocation of channels among the two groups are derived for improvement of system performance.     

A New Token-Based Channel Access Protocol for Wavelength Division Multiplexed Multiprocessor Interconnects

This paper presents a new token-based channel access protocol for wavelength division multiplexed optically interconnected multiprocessors. Our empirical study of access protocols based on slotted time division multiplexed data and control channels reveals that such protocols typically suffer from excessive slot synchronization latency due to static slot preallocation. The proposed token-based time division multiple access protocol minimizes latency by allowing dynamic allocation of slots to use channels efficiently. Simulation results indicate that the proposed scheme can significantly increase the performance for protocols based on preallocation and also those based on preallocation-controlled reservation of multiple channels.     

Saturated throughput analysis of IEEE 802.11e EDCA

IEEE 802.11e standard has been published to introduce quality of service (QoS) support to the conventional IEEE 802.11 wireless local area network (WLAN). Enhanced distributed channel access (EDCA) is used as the fundamental access mechanism for the medium access control (MAC) layer in IEEE 802.11e. In this paper, a novel Markov chain based model with a simple architecture for EDCA performance analysis under the saturated traffic load is proposed. Compared with the existing analytical models of EDCA, the proposed model incorporates more features of EDCA into the analysis. Firstly, we analyze the effect of using different arbitration interframe spaces (AIFSs) on the performance of EDCA. That is, the time interval from the end of the busy channel can be classified into different contention zones based on the different AIFSs used by different sets of stations, and these different sets of stations may have different transmission probabilities in the same contention zone. Secondly, we analyze the possibility that a station’s backoff procedure may be suspended due to transmission from other stations. We consider that the contention zone specific transmission probability caused by the use of different AIFSs can affect the occurrence and the duration of the backoff suspension procedure. Based on the proposed model, saturated throughput of EDCA is analyzed. Simulation study is performed, which demonstrates that the proposed model has better accuracy than those in the literature.     

基于竞争终端个数区间的IEEE 802.11性能优化

IEEE 802.11的MAC协议采用基于CSMA/CA的DCF机制,研究发现,上述协议的性能随无线局域网中竞争终端个数的增加而迅速恶化.当竞争终端个数在一定范围内变化时,使用相同的优化协议参数,系统的性能都能接近最优.因此,设计了一个基于状态检测与竞争终端个数区间的自适应性能优化机制,DOOR(dynamicootimization on range).根据相关性能模型的分析,先将竞争终端的个数分为若干区间,并分别计算出各区间的优化协议参数.当系统检测到竞争终端个数发生变化时,根据其所处的区间,对相关的参数进行动态调整,从而有效地改善了协议的整体性能.同时还给出了相关理论模型和计算的详细说明,并尝试给出了划分区间的基本原则与方法.最后,实验仿真结果验证了新的方法能够根据竞争终端个数的变化对系统性能进行整体优化,在吞吐量和延迟等方面明显优于标准的IEEE 802.11协议.     

Does Accessibility Affect Retail Prices and Competition? An Empirical Application

This paper attempts to link the concepts of accessibility and the firm’s conduct in the regional retail market in Spain. We use a database that includes sale price, service station location, level of traffic and type of road. We show that accessibility has two main effects on final prices. The accessibility of own-brand gas stations increased their prices while the accessibility of rival gas stations causes price reductions. If we include the value of time, then no rational consumer should travel further than his nearest petrol station in search of lower prices. Finally, our paper shows that service stations can establish a dominant position if consumers do not have access to other retailers within a 17-min radius.     

Performance evaluation of bandwidth allocation methods in a geostationary satellite channel in the presence of internet traffic

Resource allocation represents an important issue for the next generation TCP/IP Quality of Service-based satellite networks. Many schemes, proposed in the recent literature, consider Internet traffic as the superposition of traffic sources without distinguishing between User Datagram Protocol (UDP) and Transmission Control Protocol (TCP) flows, even if UDP and TCP imply very different traffic characteristics. The basic idea of this work is that a resource allocation algorithm which is conscious of the difference may be more efficient because it can make use of the different behaviour of TCP and UDP in the presence of network congestion. Actually TCP reduces the source flow rate and, as a consequence, also the bandwidth occupancy when there is network congestion. The use of this feature within the bandwidth allocation scheme allows reducing the bandwidth waste due to over provisioning and using the residual bandwidth for other sources. The advantage is particularly clear over satellite channels where fading often affects the communication: having some residual bandwidth available for stations which have experienced fading can improve the satellite network performance.This paper presents a detailed performance evaluation of a bandwidth allocation scheme, called E-CAP-ABASC and studied for the satellite environment. The bandwidth is assigned to the earth stations that compose the network by a master station on the basis of a cost function whose main part is represented by a closed-form of the packet loss probabilities for the TCP and UDP traffic. The use of two different packet loss probability models for TCP and UDP allows exploiting the different features of the two traffic types, so improving the overall performance either in terms of packet loss or, on the other hand, in terms of the traffic admitted.The performance evaluation is carried out by varying the link degradation due to fading, the traffic load, and the flow balance between UDP and TCP. The results show a good performance of E-CAP-ABASC, compared with two other schemes. Advantages and drawbacks are discussed.