Thomas E. Stern

Congestion control for packet voice by selective packet discarding

In order to reduce the time delays as well as multiplexer memory requirements in packet voice systems, a family of congestion control schemes is proposed. They are all based on the selective discarding of packets whose loss will produce the least degradation in quality of the reconstructed voice signal. A mathematical model of the system is analyzed and queue length distributions are derived. These are used to compute performance measures, including mean waiting time and fractional packet loss. Performance curves for some typical systems are presented, and it is shown that the control procedures can achieve significant improvement over uncontrolled systems, reducing the mean waiting time and total packet loss (at transmitting and receiving ends). Congestion control with a resume level is also analyzed, showing that without increasing the fractional packet loss, the mean and variance of the queue can be reduced by selecting an appropriate resume level. The performance improvements are confirmed by the results of some informal subjective testing. >

Protection cycles in mesh WDM networks

A fault recovery system that is fast and reliable is essential to todays networks, as it can be used to minimize the impact of the fault on the operation of the network and the services it provides. This paper proposes a methodology for performing automatic protection switching (APS) in optical networks with arbitrary mesh topologies in order to protect the network from fiber link failures. All fiber links interconnecting the optical switches are assumed to be bidirectional. In the scenario considered, the layout of the protection fibers and the setup of the protection switches is implemented in nonreal time, during the setup of the network. When a fiber link fails, the connections that use that link are automatically restored and their signals are routed to their original destination using the protection fibers and protection switches. The protection process proposed is fast, distributed, and autonomous. It restores the network in real time, without relying on a central manager or a centralized database. It is also independent of the topology and the connection state of the network at the time of the failure.

Throughput analysis, optimal buffer allocation, and traffic imbalance study of a generic nonblocking packet switch

A general model is presented to study the performance of a family of space-domain packet switches, implementing both input and output queuing and varying degrees of speedup. Based on this model, the impact of the speedup factor on the switch performance is analyzed. In particular, the maximum switch throughput, and the average system delay for any given degree of speedup are obtained. The results demonstrate that the switch can achieve 99% throughput with a modest speedup factor of four. Packet blocking probability for systems with finite buffers can also be derived from this model, and the impact of buffer allocation on blocking probability is investigated. Given a fixed buffer budget, this analysis obtains an optimal placement of buffers among input and output ports to minimize the blocking probability. The model is also extended to cover a nonhomogeneous system, where traffic intensity at each input varies and destination distribution is not uniform. Using this model, the effect of traffic imbalance on the maximum switch throughput is studied. It is seen that input imbalance has a more adverse effect on throughput than output imbalance. >

An architecture for a wavelength-interchanging cross-connect utilizing parametric wavelength converters

This paper proposes an architecture for a wavelength-interchanging cross-connect (WIXC) that can be used as a switching node of strictly transparent and scalable networks with all-optical routing and all-optical wavelength conversion capabilities. This architecture utilizes all-optical parametric wavelength converters based on difference-frequency-generation (DFG) or four-wave mixing (FWM), although this work focuses only on the implementation using difference-frequency-generation wavelength converters. The proposed WIXC architecture exploits the unique wavelength mapping properties of parametric wavelength converters: mirror image mapping and simultaneous multichannel wavelength conversion. The derivation of this architecture involves application of a space/wavelength transformation to the classical Benes switch fabric. The connection setup for the resulting architecture follows the well established looping algorithm, and the architecture is scalable in both the ports and the wavelengths. The scaling occurs in an orderly fashion, which allows modular upgrades of WIXCs for cost-effective evolution of the networks. The unique properties of the parametric wavelength converter including transparent and multichannel conversion capabilities result in a WIXC architecture that requires fewer wavelength converters while maintaining scalability and transparency.

Algorithms for routing in a linear lightwave network

Routing algorithms are proposed for setting up calls on a circuit-switched basis in linear lightwave networks (LLN), i.e., networks composed only of linear components, including controllable power combiners and dividers, and possibly linear (non-regenerative) optical amplifiers. The overall problem is decomposed into three subproblems: (1) physical path allocation, (2) checking for violations of the special optical constraints on the allocated physical path, and (3) channel assignment. Only point to point connections are considered. The physical path allocation technique uses the K-shortest path algorithm and tries to minimize the number of sources potentially interfering with each other, as a result of the incoming call. A channel assignment heuristic that tends to spread out calls evenly among the available channels works better than one that tries to maximize channel reuse.<<ETX>>

Multicasting in a linear lightwave network

Dynamic routing algorithms are proposed for setting up multicast connections in a linear lightwave network (LLN). The problem of finding a physical path for the multicast connection so as to satisfy all the constraints in the LLN is shown to be NP-complete, and a heuristic approach is presented. An algorithm is presented that decomposes the LLN into edge disjoint trees with at least one spanning tree. A multicast call is allocated a physical path on one of the trees, using the smallest component tree (SCT) or the minimum interference tree (MIT) criterion. Finally, the call is allocated the least used channel from among channels that can be allocated to it. The best performance (low blocking probability) is obtained when the LLN is decomposed into many spanning trees, each of them having a small diameter. It is also found that the selection of trees for each call using the MIT criterion exhibits better performance than with the SCT criterion.<<ETX>>

Cascadability of passband-flattened arrayed waveguide-grating filters in WDM optical networks

The cascadability of passband-flattened arrayed waveguide-grating (AWG) filters is studied using experimental and theoretical transfer functions. The formalism is general and can be used to cascade any type of filter at any channel spacing. For example, modeling indicates that transmission through 100 such AWG (de)multiplexers at 200-GHz channel spacing, assuming 10-Gb/s data streams introduces distortion-induced penalties below the widely acceptable 0.3-dB limit, provided that certain filter design requirements are satisfied. All simulations focus on the filter cascadability and central frequency misalignment effects, and neglect nonlinearities and crosstalk.

The wavelength dilation concept in lightwave networks-implementation and system considerations

The dilation concept in switching networks is generalized and extended to the wavelength dimension, resulting in a new class of dynamic wavelength-routing cross-connects. These wavelength-dilated switches (WDSs) are reconfigurable and fault-tolerant, and they feature suppressed crosstalk and denser wavelength-spacings. They are scalable, with switch-complexity of alpha N log/sub 2/ N, where alpha is the dilation constant, typically 4.6, and N is the number of ports. It is shown, that by careful wavelength grouping, the crosstalk is reduced significantly. A switch architecture is suggested to fully exploit the advantages of this method. An example is given showing how such a fabric can be used to reduce the coherent crosstalk in wavelength-routing switches based on acoustooptic tunable filters (AOTF), making them practical for networks with subnanometer wavelength spacings. The WDS can be used in a broad range of applications from interconnection networks to wide-area networks (WANs) on a national scale. Large cross connects can support tens of thousands of channels using only few tens of wavelengths. >

Accurate modeling of optical multiplexer/demultiplexer concatenation in transparent multiwavelength optical networks

This paper presents an accurate theoretical model for the study of concatenation of optical multiplexers/demultiplexers (MUXs/DMUXs) in transparent multiwavelength optical networks. The model is based on a semianalytical technique for the evaluation of the error probability of the network topology. The error-probability evaluation takes into account arbitrary pulse shapes, arbitrary optical MUX/DMUX, and electronic low-pass filter transfer functions, and non-Gaussian photocurrent statistics at the output of the direct-detection receiver. To illustrate the model, the cascadability of arrayed waveguide grating (AWG) routers in a transparent network element chain is studied. The performance of the actual network is compared to the performance of a reference network with ideal optical MUXs/DMUXs. The optical power penalty at an error probability of 10/sup -9/ is calculated as a function of the number of cascaded AWG routers, the bandwidth of AWG routers, and the laser carrier frequency offset from the channels nominal frequency.

Routing in a linear lightwave network

Dynamic routing of point-to-point connections in a waveband selective linear lightwave network is addressed. Linear lightwave networks are all optical networks in which only linear operations are performed on signals in a waveband selective manner. Special constraints arise because of the linearity in the linear lightwave network. The overall problem of finding a path satisfying all the routing constraints for point-to-point connections is shown to be very complex. Owing to the complexity, the overall routing problem is decomposed into several subproblems. In particular, given a request for a point-to-point connection a waveband is first chosen for the call. Two heuristics, MAXBAND which allocates the most used band to a call and another MINBAND (least used band) are studied. Then, the problem of routing in a given waveband is further divided into smaller subproblems of finding a path in the waveband, checking for feasibility of the path in the chosen waveband and channel allocation (within the waveband). For finding paths in a waveband, K-SP, BLOW-UP and MIN-INT algorithms are proposed. A recursive algorithm checks for feasibility of the path on the waveband. Two channel allocation schemes (within a single waveband) MIN and MAX are presented. Simulations show that using MAXBAND (waveband), MIN-INT (path on waveband) and MIN (channel within waveband) policies resulted in the best performance (least blocking). >