Ballot theorems applied to the transient analysis of nD/D/1 queues

The problem of transporting constant-bit-rate (CBR) traffic through a packet network is analyzed. In the system considered, CBR traffic is packetized and packets from several similar sources are multiplexed on a transmission link. The bit streams are recreated at the receiving end by demultiplexing the packets and then playing out the packets of each CBR stream. Traffic fluctuations may cause gaps to appear in the playout process. Their frequency can be reduced by adding a smoothing delay to each stream. The queueing system analyzed has periodic arrivals and deterministic service times. A method of analysis, based on the ballot theorems of Takacs (1967), is presented to provide steady-state delay distributions as well as a transient analysis of the system to predict the statistics of the time for a gap to develop in the CBR stream as a function of the smoothing delay     

Shuffle Net: an application of generalized perfect shuffles tomultihop lightwave networks

A multihop wavelength-division multiplexing (WDM) approach, referred to as Shuffle Net, for achieving concurrency in distributed lightwave networks is proposed. A Shuffle Net can be configua19 red with each user having as few as one fixed-wavelength transmitter and one fixed-wavelength receiver, avoiding both wavelength-agility and pretransmission-coordination problems. The network can achieve at least 40% of the maximum efficiency possible with wavelength-agile transmitters and receivers. To transmit a packet from one user to another may require routing the packet through intermediate users, each repeating the packet on a new wavelength until the packet is finally transmitted on a wavelength that the destination user receives. For such a multihop lightwave network, the transmit and receive wavelengths must be assigned to users to provide both a path between all users and the efficient utilization of all wavelength channels     

A dynamic rate control mechanism for source coded traffic in a fastpacket network

To achieve better statistical gain for voice and video traffic and to relieve congestion in fast packet networks, a dynamic rate control mechanism is proposed. An analytical model is developed to evaluate the performance of this control mechanism for voice traffic. The feedback delay for the source node to obtain the network congestion information is represented in the model. The study indicates that significant improvement in statistical gain can be realized for smaller capacity links (e.g., links that can accommodate less than 24 voice calls) with a reasonable feedback time (about 100 ms). The tradeoff for increasing the statistical gain is temporary degradation of voice quality to a lower rate. It is shown that whether the feedback delay is exponentially distributed or constant does not significantly affect performance in terms of fractional packet loss and average received coding rate. It is also shown that using the number of calls in talkspurt or the packet queue length as measures of congestion provides comparable performance     

Queueing in high-performance packet switching

The authors study the performance of four different approaches for providing the queuing necessary to smooth fluctuations in packet arrivals to a high-performance packet switch. They are (1) input queuing, where a separate buffer is provided at each input to the switch; (2) input smoothing, where a frame of b packets is stored at each of the input line to the switch and simultaneously launched into a switch fabric of size Nb×Nb; (3) output queuing, where packets are queued in a separate first-in first-out (FIFO) buffer located at each output of the switch; and (4) completely shared buffering, where all queuing is done at the outputs and all buffers are completely shared among all the output lines. Input queues saturate at an offered load that depends on the service policy and the number of inputs N, but is approximately 0.586 with FIFO buffers when N is large. Output queuing and completely shared buffering both achieve the optimal throughput-delay performance for any packet switch. However, compared to output queuing, completely shared buffering requires less buffer memory at the expense of an increase in switch fabric size     

Implication of dropping packets from the front of a queue

When congestion occurs in a packet queuing system, packets can be dropped from the rear or the front of the queue. It is demonstrated that the probability of a packet being dropped is the same in systems with rear and front packet dropping. It is shown that the probability of a packet being delayed longer than a given value in a system with front dropping is less than or equal to that in a system with rear dropping. It is further illustrated that front dropping not only improves the delay performance on an internodal link, but also provides the overall loss performance for time constrained traffic such as packet voice     

A new local area network architecture using a centralized bus

LOCAL AREA NETWORKS are currently enjoying tremendous popularity as a means for providing wideband interconnection and communications among data terminals, host computers and other types of digital equipment located throughout a single building or a campus of buildings. Such networks are typically based on bus, ring, or star architectures, each of which manifests its own set of advantages and disadvantages. In this paper, an architectural approach is described that draws upon and integrates the advantages found separately in these three different architectures, while avoiding the major disadvantages found in any one. This new architecture employs a centrally located short bus that provides an extremely efficient packet-switching service to the devices attached to the network. Bandwidth on the short bus is dynamically allocated in response to instantaneous demands by means of a highly efficient but flexible prioritybased bus contention scheme. The approach permits multiple priority classes with fair allocation of bandwidth within each class, along with a capability for integrated circuit and packet switching. The architecture can also make use of existing twisted-pair building wiring, and at the same time take advantage of emerging optical-fiber technology. In addition, the architecture provides a means to expand the network beyond a local area, resulting in a wide-area network capability.     

Using a packet switch for circuit-switched traffic: a queueingsystem with periodic input traffic

Consideration is given to the effects of time-multiplexed stream traffic on the performance of a store-and-forward packet switch. Substantially reducing the amount of buffering in the switch results in only a small probability that an existing circuit will be disrupted during the length of its connection. For example, with a circuit-switched frame of length 1000 and 100% loading, reducing the buffer size from 999 packets to 83 results in only a 10^-6 circuit-disruption probability     

Input Versus Output Queueing on a Space-Division Packet Switch

Two simple models of queueing on anN times Nspace-division packet switch are examined. The switch operates synchronously with fixed-length packets; during each time slot, packets may arrive on any inputs addressed to any outputs. Because packet arrivals to the switch are unscheduled, more than one packet may arrive for the same output during the same time slot, making queueing unavoidable. Mean queue lengths are always greater for queueing on inputs than for queueing on outputs, and the output queues saturate only as the utilization approaches unity. Input queues, on the other hand, saturate at a utilization that depends onN, but is approximately(2 -sqrt{2}) = 0.586whenNis large. If output trunk utilization is the primary consideration, it is possible to slightly increase utilization of the output trunks-upto(1 - e^{-1}) = 0.632asN rightarrow infty-by dropping interfering packets at the end of each time slot, rather than storing them in the input queues. This improvement is possible, however, only when the utilization of the input trunks exceeds a second critical threshold-approximatelyln (1 +sqrt{2}) = 0.881for largeN.     

The Knockout Switch: A Simple, Modular Architecture for High-Performance Packet Switching

A new, high-performance packet-switching architecture, called the Knockout Switch, is proposed. The Knockout Switch uses a fully interconnected switch fabric topology (i.e., each input has a direct path to every output) so that no switch blocking occurs where packets destined for one output interfere with (i.e., block or delay) packets going to different Outputs. It is only at each output of the switch that one encounters the unavoidable congestion caused by multiple packets simultaneously arriving on different inputs all destined for the same output. Taking advantage of the inevitability of lost packets in a packet-switching network, the Knockout Switch uses a novel concentrator design at each output to reduce the number of separate buffers needed to receive simultaneously arriving packets. Following the concentrator, a shared buffer architecture provides complete sharing of all buffer memory at each output and ensures that all packets are placed on the output line on a first-in first-out basis. The Knockout Switch architecture has low latency, and is self-routing and nonblocking. Moreover, its Simple interconnection topology allows for easy modular growth along with minimal disruption and easy repair for any fault. Possible applications include interconnects for multiprocessing systems, high-speed local and metropolitan area networks, and local or toll switches for integrated traffic loads.     

A Knockout Switch for Variable-Length Packets

The Knockout Switch is a new packet switch architecture recently proposed for high-speed local and metropolitan area networks, multiprocessor interconnects, and local or toll switches for integrated traffic loads. We describe an approach to extend the original Knockout Switch to work with variable-length packets. This new architecture employs an input broadcast bus arrangement to achieve complete interconnection of the inputs and outputs. Consequently, there is no congestion in the switch fabric other than the unavoidable conflict of multiple simultaneous packets destined for the same output. It is with this output contention that the Knockout principle is fully utilized to efficiently concentrate and store contending packets while maintaining the first-in first-out discipline of the packet sequence; and yet the fabric speed required is no more than the input/output line speeds, Under these design goals, no switch can yield better delay/ throughout performance. These are the most important attributes that have been preserved in the current proposal from the original Knockout Switch. For anN times Nswitch configuration, the variable-length packet Knockout Switch consists ofNinput broadcast buses, and anN:Lconcentrator (L ll N) and a shared buffer for each output. The design of each subsystem is discussed with emphasis on possible VLSI realization. Using today's technology, we should be able to implement the proposed switch with both input/output lines and internal hardware operating at 50 Mbits/s. The dimension of the switch (N times N) can grow modularly from say 32 × 32 to 1024 × 1024, rendering a total throughput in the range of tens of gigabits per second. Future upgrading of the line interfaces to much higher speed without modification to the internal switch hardware is also possible with a modest restriction on the minimum length of new packets.