Michael G. Hluchyj

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. >

ShuffleNet: an application of generalized perfect shuffles to multihop lightwave networks

The authors propose a multihop wavelength-division-multiplexing (WDM) approach, referred to as ShuffleNet, for achieving concurrency in distributed lightwave networks. ShuffleNet can be configured with each user having as few as one fixed-wavelength transmitter and one fixed-wavelength receiver, avoiding both wavelength agility and pretransmission coordination problems. Still, 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, however, 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 class of assignment schemes is proposed which is based on a generalization of the perfect shuffle and achieves high efficiency for uniform traffic loads. Physically, the network may take on a variety of topologies, including a bus, tree, or star.<<ETX>>

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. >

Queueing in space-division packet switching

The authors study the performance of four different approaches for providing the queuing necessary to smooth fluctuations in packet arrivals to a space-division packet switch. They are (1) input queueing, where a separate first-in, first-out (FIFO) buffer is provided at each input to switch; (2) input smoothing, where a frame of b packets is stored at each of the N input lines to the switch and simultaneously launched into a switch fabric of size Nb*Nb; (3) output queuing, where packets are queued in a separate 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.<<ETX>>

Queueing analysis of continuous bit-stream transport in packet networks

The problem of transporting continuous bit-stream oriented (CBO) traffic through an all-packet network is examined. In the system considered, CBO traffic is packetized by collecting bits generated by a source during a fixed interval of time (packetization time) and packets from K such 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 bit-stream in sequence. The queuing system analyzed is a single-server queue with periodic arrivals and deterministic service times; the steady-state distributions of the queue length and delay are derived. The method of analysis is based on the ballot theorems and has a computational complexity of O(K), as compared to other proposed methods with complexities of O(K/sup 3/) or greater. It is shown that the delay, measured in units of the packetization time, decreases as the link bandwidth increases. Therefore, for the large capacity links, the contribution of the queuing delay to the total network delay is small compared to the packetization time.<<ETX>>

A dynamic rate control mechanism for integrated networks

To achieve better statistical gain for voice and video traffic and to relieve congestion in integrated networks, a dynamic rate control mechanism is proposed. This mechanism, using a variable rate coder, adjusts the source coding rate based on network feedback information. An analytical model is developed to evaluate the performance of the mechanism for voice traffic. The feedback delay for the source node to obtain the network congestion information is presented in the model. Significant improvement in statistical gain can be realized for smaller capacity links (e.g., links that can accommodate less than 24 calls) with a reasonable feedback time (about 100 ms); but this increase causes a temporary degradation in voice quality to a lower rate. The authors show 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. Using the number of calls in talkspurt or the packet queue length as measures of congestion provides comparable performance.<<ETX>>

Frame loss due to buffer overflows in fast packet networks

An analytical method is derived to estimate the protocol data unit (PDU) frame loss probability as a function of buffer size when identical sources are multiplexed on a fast packet link. The generation of PDUs from each source is governed by an on-off source model, and upper and lower bounds on the frame loss probability are derived. The analysis takes into account the correlated nature of fast packet losses within a PDU due to buffer overflows. The required buffer size for a given frame loss probability is directly proportional to the expected PDU frame length, and the loss probability decreases exponentially with increasing buffer size.<<ETX>>