Galen H. Sasaki

Cost-effective traffic grooming in WDM rings

We provide network designs for optical add-drop wavelength-division-multiplexed (OADM) rings that minimize overall network cost, rather than just the number of wavelengths needed. The network cost includes the cost of the transceivers required at the nodes as well as the number of wavelengths. The transceiver cost includes the cost of terminating equipment as well as higher-layer electronic processing equipment, which in practice can dominate over the cost of the number of wavelengths in the network. The networks support dynamic (i.e., time-varying) traffic streams that are at lower rates (e.g., OC-3, 155 Mb/s) than the lightpath capacities (e.g., OC-48, 2.5 Gb/s). A simple OADM ring is the point-to-point ring, where traffic is transported on WDM links optically, but switched through nodes electronically. Although the network is efficient in using link bandwidth, it has high electronic and opto-electronic processing costs. Two OADM ring networks are given that have similar performance but are less expensive. Two other OADM ring networks are considered that are nonblocking, where one has a wide-sense nonblocking property and the other has a rearrangeably nonblocking property. All the networks are compared using the cost criteria of number of wavelengths and number of transceivers.

Wavelength assignment in a WDM ring to minimize cost of embedded SONET rings

Discusses wavelength assignment for lightpaths. We study WDM networks in the form of rings and higher level networks as SONET/SDH self-healing rings. This view changes the goal of wavelength assignment (WLA) vs. previous work on the subject in a number of aspects. First, a pair of SONET add/drop multiplexers (ADMs) terminates each lightpath. These ADMs also terminate adjacent lightpaths to form rings, implying that the WLA has to support this type of sharing. Second, following Gerstal et al. (1998), we argue that the first-order optimization goal should be to minimize the overall network cost which is dominated by the number of required ADMs and not the number of wavelengths. These two minimization problems are intrinsically different, and there exist cases where the two minima cannot be simultaneously achieved. We derive a simple lower bound to the number of ADMs and show that this lower bound is not always achievable. Adding wavelength converters to the system does not improve the cost but splitting a lightpath and handling each part separately may reduce the total number of ADMs. We develop two heuristics to minimize the number of ADMs: cut-first, and assign-first. Both heuristics attempt to use the smallest number of ADMs possible. Cut-first always uses the minimum number of wavelengths, but may use more ADMs than necessary. However, the number of extra ADMs is proven to be bounded by the number of supported wavelengths and typically much less. We show instances where cut-first performs better than assign-first and vice versa. Finally, we present a set of transformations that take any WLA and improve its cost.

Wireless sensor placement for reliable and efficient data collection

Sensors can be paired with radio units and deployed to form a wireless ad-hoc sensor network. Actual deployments must consider the coverage that can be achieved with a given number of sensors: this coverage varies with the range of the radios and the maximum allowable distance between any point in the area and the nearest sensor. Deployments must also preserve connectivity in spite of possible failure or energy depletion in a subset of the units. This paper presents and analyzes a variety of regular deployment topologies, including circular and star deployments as well as deployments in square, triangular, and hexagonal grids.

Combined WDM and SONET network design

This paper considers grooming of low speed traffic into high speed lightpaths in a WDM based optical ring with a primary goal of reducing the cost of the entire system, which is dominated by the cost of the SONET transmission equipment connected to the optical ring. The paper attempts to enumerate the architectural options provided by SONET to arrive at a cost-effective solution, including unidirectional path-switched rings (UPSR) and bidirectional line-switched rings (BLSR), use of back-to-back connections between SONET ADMs to reduce the overall cost, and use of different ring speeds (OC-48 and OC-12). To demonstrate each of the architectures, a uniform traffic is considered and its grooming and resulting SONET architecture demonstrated. The paper deviates from earlier approaches which break the problem into two steps: traffic grooming and assignment of lightpaths to rings, in that it looks at the problem as a whole and tries to solve it in a single step. The paper also considers the characteristics of SONET UPSR and BLSR rings and how these affect the grooming. The paper derives lower and upper bounds to these problems for uniform traffic and shows how these improve on known results.

Cost effective traffic grooming in WDM rings

We provide network designs for optical wavelength division multiplexed (OWDM) rings that minimize overall network cost, rather than just the number of wavelengths needed. The network cost includes the cost of the transceivers required at the nodes as well as the number of wavelengths. The transceiver cost includes the cost of terminating equipment as well as higher-layer electronic processing equipment, and in practice, can dominate over the cost of the number of wavelengths in the network. The networks support dynamic (time varying) traffic streams that are at lower rates (e.g., OC-3, 155 Mb/s) than the lightpath capacities (e.g., OC-48, 2.5 Gb/s). A simple OWDM ring is the point-to-point ring, where traffic is transported on WDM links optically, but switched through nodes electronically. Although the network is efficient in using link bandwidth, it has high electronic and opto-electronic processing costs. Two OWDM ring networks are given that have similar performance but are less expensive. Two other OWDM ring networks are considered that are nonblocking, where one has a wide sense nonblocking property and the other has a rearrangeably nonblocking property. All the networks are compared using the cost criteria of number of wavelengths and number of transceivers.

Scheduling transmissions in WDM broadcast-and-select networks

Considers a broadcast-and-select, wavelength division multiplexed (WDM), optical communication network that is packet switched and time slotted. The amount of time it takes transmitters and receivers to tune from one wavelength to another is assumed to be T slots. The authors consider all-to-all transmission schedules, which are defined to be ones that schedule a packet transmission between each input-output pair. They present upper and lower bounds for the minimum length of such schedules. In particular, if each of N inputs has a tunable transmitter and each of N outputs has a tunable receiver then the minimum length is between (N+o(N))(/spl radic/T+1) and ((N+o(N))/spl radic/T. This provides some insight into the relationship between packet delay and T. The authors also consider schedules that do not allow packet transmissions while a transmitter or receiver is tuning from one wavelength to another. >

Fault tolerant multiwavelength optical rings with limited wavelength conversion

This paper presents methods for recovering from channel failures, link failures, and node failures in wavelength-division multiplexed (WDM) point-to-point links and ring networks with limited wavelength conversion/switching capabilities at the nodes. Different recovery schemes are presented to handle each type of failure. Each scheme is evaluated based on the network hardware configuration required to support it and the performance and management overheads associated with fault recovery. Although similar recovery techniques have been used in conventional networks such as SONET, the constraints due to limited wavelength conversion require new and more complex solutions.

The interface between IP and WDM and its effect on the cost of survivability

A summary of research on survivable IP networks overlaid over WDM networks is presented. The WDM networks are part of optical transport service providers, who lease lightpath services to institutions with IP networks. The lightpath services realize IP links for IP networks, and they have different protection grades such as unprotected and protected. The research included considering new network survivability requirements and incorporating them into network design problems. The cost of survivable IP over WDM networks is compared over three scenarios. Each succeeding scenario has the WDM network provide more flexible services, and the IP and WDM networks become more integrated. We consider the problem of setting up lightpaths for an IP network so that the network will remain connected after a fiber link fault. Algorithms to find the lightpaths and minimize cost are given. The network costs under the three scenarios are compared by simulations.

Scheduling of periodic connections with flexibility

Applications that require periodic optical connectivity may be serviced by scheduled lightpaths. Many applications are flexible about when exactly the scheduled lightpaths should occur. Lightpath service providers can exploit this flexibility to schedule connections for better utilization of resources, and presumably lower prices for clients. To find the relationship between network resources and time flexibility, a simple WDM link and a leaky bucket traffic model are considered. The network resources, and in particular the required number of wavelengths W, are shown to be a function of the traffic parameters and the time flexibility. For random traffic, simulations are used to measure the sensitivity of W with time flexibility.

A minimal cost WDM network for incremental traffic

A wavelength division multiplexed network is considered for arbitrary topologies. The network allows optical signals to pass through nodes, which often results in less electronic and opto-electronic equipment than networks that do not, i.e., networks that only switch traffic electronically. A disadvantage of having optical pass through is that there is less capability to switch tributary traffic streams and so there may be more blocking. However, it is shown that the network operates as well as a network with only electronic switching under a particular incremental traffic model. Examples are given that shows that the network can have lower cost when the cost is dominated by the line terminating equipment. A simple heuristic design algorithm is also given to configure the network to minimize its cost.