Purushotham Kamath

The need for media access control in optical CDMA networks

Optical CDMA local area networks allow shared access to a broadcast medium. Every node on the network is assigned an optical orthogonal codeword (OOC) to transmit or receive on. OOCs are designed to be pseudo-orthogonal, i.e., the correlation (and therefore the interference) between pairs of codewords is constrained. This paper demonstrates that the use of optical CDMA does not preclude the need for a media access control (MAC) layer protocol to resolve contention for the shared media. OOCs have low spectral efficiency. As more codewords are transmitted simultaneously, the interference between codewords increases and the network throughput falls. This paper analyzes a network architecture where there is virtually no MAC layer, except for choice of the codeset, and shows that its throughput degrades and collapses under moderate to heavy load. We propose an alternate architecture called interference avoidance where nodes on the network use media access mechanisms to avoid causing interference on the line, thereby improving network throughput. Interference avoidance is analyzed and it is shown that it can provide up to 30% improvement in throughput with low delays and no throughput collapse. We validate our analysis through simulation with realistic network traffic traces.

Generation of high bandwidth network traffic traces

High bandwidth network traffic traces are needed to understand the behavior of high speed networks (such as the Internet backbone). However, the implementation of a mechanism to collect such traces is difficult in practice. In the absence of real traces, tools to generate high bandwidth traces would aid the study of high speed network behavior. We describe three methods of generating high bandwidth network traces: scaling low bandwidth network traffic traces; merging multiple low bandwidth traces; generating traces through simulation by scaling a structural model of real world traces. We evaluate the generated traces and discuss the advantages and disadvantages of each method. We also discuss some of the issues involved in generating traces by the structural model method.

All-optical wavelength and time 2-D code converter for dynamically-reconfigurable O-CDMA networks using a PPLN waveguide

We demonstrate all-optical wavelength and time code conversion for O-CDMA networks at 2.5-Gbit/s with 10-Gchip/s. Difference-frequency generation provides wavelength-shifting and fiber-Bragg gratings introduce cyclic time-shifts to the incoming code, generating a new time/wavelength code with less than 0.7-dB power penalty.

Algorithms for interference sensing in optical CDMA networks

Optical CDMA local area networks allow shared access to a broadcast medium. Every node is assigned an optical orthogonal codeword (OOC) to transmit or receive on. Optical CDMA systems have low throughput under moderate to heavy offered load due to interference between codewords. Interference sensing is a media access architecture where nodes on the network sense the amount of interference on the line before transmission. Nodes defer transmissions if there is interference on the line. This paper discusses and analyzes three algorithms for interference sensing. Through simulation it is shown that these algorithms reduce or eliminate throughput degradation at high loads. The study shows that simple algorithms such as selfish or threshold based algorithms are sufficient to eliminate throughput degradation.

An Optical Booster for Internet Routers

Although optical technologies have been effectively employed to increase the capacity of communication links, it has proven difficult to apply these technologies towards increasing the capacity of Internet routers, which implement the central forwarding and routing functions of the Internet. Motivated by the need for future routers that will forward packets among several high-speed links, this work considers the design of an Internet router that can forward packets from a terabit-per-second link without internal congestion or packet loss. The router consists of an optical booster integrated with a conventional (mostly electronic) Internet router. The optical booster processes Internet Protocol packets analogously to the hosting router, but it can avoid the time-consuming lookup function and keep packets in an entirely optical format. An optically boosted router is an inexpensive, straightforward upgrade that can be deployed readily in a backbone IP network, and provides optical processing throughput even when not deployed ubiquitously.

Analysis of an optical burst switching router with tunable multiwavelength recirculating buffers

Optical burst switching (OBS) presents challenges to the design of optical routers. This paper considers how to dimension a router of N input data ports with an additional M fiber delay lines (FDLs) in an OBS network. The router incorporates tunable FDLs that can vary their size to fit the burst being buffered. Tunable FDLs can be approximated using a set of static FDLs of unequal sizes. For this, the size of static FDL set is monotonically increased, in step size increments, from minimum burst size until the throughput increase is equal to the corresponding tunable FDL configuration. Simulation results for a 32-input port router with 256 tunable delays achieve up to 20% higher throughput than static delays at high input port load. Multiple recirculations are a critical requirement; when packets can circulate only once through the buffer, no measurable improvement is achieved after the number of FDLs becomes equal to the number of input data ports. When recirculation is permitted, throughput increases by up to 40%, depending on a combination of the number of FDLs added and the recirculation limit, which must increase in tandem (e.g., for 32 buffers with eight recirculations or 256 buffers with 16 recirculations). For a given number of FDLs, there is an optimal recirculation limit beyond which there is no measurable throughput benefit. By varying the recirculation limit or number of FDLs, tunable buffering can match the gain achieved by wavelength conversion, possibly at lower hardware cost.

Dynamically reconfigurable all-optical correlators to support ultra-fast internet routing

To implement an effective optical bypass for an electronic router, the key design decision is to combine a software algorithm with a small set of dynamically configurable fiber-Bragg-grating based optical correlators. A conceptual diagram showing how the optical bypass is implemented in an IP router is shown.

Experimental Demonstration of an Interference-Avoidance-Based Protocol for O-CDMA Networks

We demonstrate the transmission scheduling algorithm in an O-CDMA network to avoid congestion collapse in an O-CDMA network. Our result shows that transmission scheduling increases the performance of the system by orders of magnitude.

Experimental demonstration of interference avoidance protocol (transmission scheduling) in O-CDMA networks

We experimentally demonstrate a transmission scheduling algorithm to avoid congestion collapse in O-CDMA networks. Our result shows that transmission scheduling increases the performance of the system by orders of magnitude.