Radio-over-Fiber (RoF) based network architectures would allow centralization of signal processing and signal conditioning functions and simple, cost-effective remote units at the cell site. RoF technology results in remote units with few components, however, certain aspects of the technology may inadvertently lead to high power consuming components. In this paper, we experimentally investigate the effect of electrical-optical-electrical (E/O/E) loss and signal bandwidth on the energy consumption of analog and digitized RoF (ARoF and DRoF) links. We also analyze the energy efficiency of multiple services on a single RoF distribution network. Our results show that E/O/E loss significantly degrades the energy efficiency of the ARoF links. DRoF is robust to the E/O/E loss on the optical link but is affected by the loss due to reconstruction of the radio frequency signal at higher Nyquist zones. We also show that increasing the bandwidth improves the energy efficiency. Finally, we demonstrate that the extra energy savings from having multiple services on a single RoF link depends on the wireless environment.

Passive optical network (PON) is regarded as a promising solution for the broadband bandwidth bottleneck problem. However, due to its passive nature, legacy PON is limited by the static power distribution, which makes it power inefficient. To address this problem, we propose QPAR [4], a Quasi-Passive and Reconfigurable node, which enables dynamic power and wavelength assignment so as to save optical power budget in PON. In this paper, we study the power gains that can be achieved in PON employing QPAR, as well as different factors that may facilitate or prevent real QPAR deployments. We conduct extensive simulations to demonstrate the merits of QPAR. Results show that QPAR can achieve high optical power saving by intelligently redistributing the unnecessary power assigned to “close” optical network units (ONUs) in the network. The saved power can either be used to connect more ONUs, or extend the network reach without increasing the optical power budget.

UltraFlow - also known as Optical Flow Switching (OFS) has been recently presented as an effective technology for large Internet data transfer. In this paper, we propose a novel scheduling mechanism for the Stanford UltraFlow access network, a novel optical access network architecture that offers dual-mode service to the end-users: legacy IP and OFS. The proposed scheme is a hybrid mechanism that combines the batch scheduling technique with existing methods so as to increase the average Flow throughput, while maintaining relatively low traffic latency. Extensive simulations highlight the advantages of the proposed solution and demonstrate its merits.

The paper describes different research concepts of which the combination leads to an improvement of the power consumption per subscriber in a wireline access network by 10× and the energy efficiency per transferred bit by 100×.

Next-generation access networks are expected to provide mobility, large data bandwidth, high quality of service (QoS), and ubiquitous coverage. Hybrid optical-wireless access networks will support these complementary features through high-capacity fiber subnetworks and flexible wireless subnetworks. As wireless access technologies progress, increased demand for access bandwidth and QoS will require a rapid evolution of fiber subnetworks and a tighter integration of the two technologies. This paper presents both radio-over-fiber transport technologies and novel architectures and technologies for hybrid optical-wireless networking. Basic optical systems for radio signals' transport use intensity modulation and direct detection. Advanced transport systems may utilize amplified links, coherent links, subcarrier transport, and digitized transport. They can be used to support powerful access solutions. On the networking side, diverse alternatives for hybrid access architectures are presented. Two wavelength division multiplexing (WDM)-based architectures, MARIN and GROWNet, are analyzed in detail. Both architectures dynamically support next-generation wireless access network traffic employing reconfigurable WDM technologies. MARIN is more technologically conservative and so is more suitable for near-future implementation. GROWNet is more technologically aggressive and is substantially more flexible and more powerful. Using hybrid network simulations, we demonstrate these architectures' capabilities to dynamically balance wireless traffic and to coordinate wireless transmission. We also review other research efforts in this field including ACCORDANCE, FiWi, FUTON, and WOBAN. Remaining challenges and research topics are also discussed to motivate further investigations into optical-wireless access networks.

In the past few years, Ethernet Passive Optical Networks (EPONs) have rapidly gained large popularity in broadband access networks. However the current standard has no management protocols aiming at reducing power consumption. In this paper, we propose an Energy Management Mechanism (EMM) within the IEEE 802.3ah control scheme. The main idea is to switch Optical Network Units (ONUs) to sleep mode and determine a suitable wakeup time schedule at the Optical Line Terminal (OLT). In the proposed EMM there is a trade-off between maximizing the power saving and guaranteeing the network performance. We compare two types of downstream scheduling schemes, Upstream Centric Scheduling (UCS) and Downstream Centric Scheduling (DCS), which are different in the way they assign 'active' and 'sleep' states to ONUs. Simulation results in terms of energy consumption and queuing delay are shown for the EMM based EPON system.

Fiber based access networks are recognized as promising technologies for solving broadband bandwidth bottlenecks. TDM-PONs, which are passive and non-reconfigurable, are the most widely deployed type of fiber access networks today. However, due to their passive nature, TDM-PONs encounter several issues such as inflexible coverage, lack of intelligence for control, and inability to counter security attacks. Based on current issues in optical access networks, we propose a novel non-volatile, reconfiguration node. The proposed remote node can reconfigure the network to adapt to varying degrees of deployment conditions and/or network attacks. Moreover, the proposed remote node incorporates a novel quasi-passive device that reconfigures using remotely supplied energy from CO. This quasi-passive device technology does not consume any energy once it reconfigures into a new latching states. Therefore, the proposed remote node has very low energy consumption and does not require local power supply to preserve the passivity of the passive distribution network. In particular, the proposed quasi-passive device technology is implemented using a novel tri-state Micro- Electro-Mechanical System (MEMS) actuator structure and the device demonstrates very small energy consumption requirement to enable energy be remotely supplied from CO. Also, the MEMS structure is shown to demonstrate acceptable insertion losses to makes it suitable for flexible energy distribution. Simulation study shows the proposed reconfigurable device would outperform traditional passive splitter in terms of maximum number of supportable users under realistic deployment conditions.

This paper presents a novel hierarchical and grid based reconfigurable optical-wireless network (GROW-Net) architecture. GROW-Net supports scalable and flexible integration of a large scale wireless mesh network in a municipal environment. Under the GROW-Net architecture, a joint evolution strategy is proposed to allow graceful upgrades in both optical backbone and wireless mesh network. To alleviate the known throughput bottleneck in mesh networks, a capacity enhancing technique based on flexible cell-splitting is proposed. The technique allocates wavelength resources to unutilized dark fibers using the unique structure of GROW-Net's reconfigurable and colorless access gateways. To analyze the effectiveness of the approach, throughput performance of the wireless mesh network is analyzed using a high fidelity simulator. The wireless mesh network in the simulation employs a distributed and cooperative medium access control protocol within a time division multiple access and time division duplex framework. Through the high fidelity simulation, results show the degree of throughput enhancement by using cell splitting method. The evolution strategy further enables the backbone to scale its bandwidth and adapt to future high throughput and very high throughput wireless technologies. The performance of the proposed backbone is demonstrated experimentally over the optical testbed.

In this paper, we propose cell breathing as a load balancing mechanism into WiMAX network with an integrated EPON backbone. Cell breathing is a well known cellular telephony concept that handles congestion by changing the coverage area of a fully loaded cell tower. In WiMAX, cell breathing can be achieved by changing the transmitting powers of the base stations. In the integrated EPON-WiMAX system, access gateways, i.e. EPON integrated base stations, report their residual downlink queue sizes to the central office using modified multi-point control protocol. Centralized gateway selection and power allocation can be made jointly in the central office for all connected access gateways. An iterative algorithm is proposed that equalizes the expected transmission time based on the reported queue sizes and adjusted transmission power level. The performance of the proposed cell breathing in integrated EPON-WiMAX system is simulated. The results show that cell breathing significantly outperforms fixed power scheme.

Various wireless and optical access technologies have been developed to address different issues in access networks. By combining the complementary characteristics of wireless and optical networks, a hybrid optical-wireless access network will enable broadband, ubiquitous, and cost-effective last-mile service to the users. In this paper, a reconfigurable optical backhaul leveraging both the standard Time Division Multiplexing Passive Optical Network (TDM-PON) technology and wavelength division multiplexed (WDM) ring is proposed to achieve a higher bandwidth efficiency than simply using point-to-point backhaul links. Furthermore, an integrated routing algorithm which can adapt to the change of overall demand among different service districts by taking advantage of the proposed optical backhaul is also proposed. An experimental testbed is implemented to evaluate the reconfigurable scheme and its feasibility. Also, simulation results show that the proposed integrated routing with load balancing can improve the performance in hybrid optical and wireless networks.

This paper presents a novel integrated metro and access network based on centralized resource sharing. In its access segment, MARIN adopts a flexible and scalable architecture that employs centralized tunable transmitters and passive cyclic wavelength router in the central office. A tunable transmitter and fixed receiver scheme is proposed in the hybrid metro edge router - access central office to simultaneously achieve access resource sharing and dynamic bandwidth allocation in the integrated network. A wavelength reservation scheme and QoS aware scheduling scheme are proposed and discussed. Then, the proposed network is simulated and compared against legacy buffer insertion ring, such as resilient packet ring. The results demonstrate enhanced throughput performance through statistical multiplexing gain. An experiment testbed is constructed to demonstrate the physical feasibility of the proposed architecture.

Optical Burst Transport network is a novel WDM ring architecture for Metropolitan Area Networks (MAN). It leverages the advantages of optical burst switching and WDM rings, while using a token-based medium access scheme. In this paper, we investigate protection together with spatial reuse property. Specifically we show how protection in OBT can be different from conventional protection and propose three algorithms for implementing a fast and low implementation cost, unidirectional 1:1 protection scheme. We analyzed the performance of these schemes via simulation and found an optimum algorithm for minimizing data loss and maximizing data delivery during the fault.

The last mile continues to be a major bottleneck in the Internet. Its low bandwidth and flexibility prevents the deployment of new services and the development of new applications. In this paper we present a summary of current efforts in access networks research, focusing in particular on fiber optic solutions. We present the Stanford University aCCESS (SUCCESS) initiative within the Photonics & Networking Research Laboratory (PNRL). As part of this initiative, two novel network architectures have been developed, SUCCESS-HPON and SUCCESS-DWA, which propose a smooth migration path from current TDM-PONs to future higher bandwidth, cost-efficient, scalable WDM-PONs. In addition, we present SUCCESS-LCO, a spectral-shaping line coding technique that enables a cost-effective shorter-term capacity upgrade of existing TDM-PONs. We discuss as well what we believe are the main open research areas in optical access networks.

Driven by the aggressive progress of optical access network development and deployment, we envision that the bandwidth upgrade in the metro-area network (MAN) will take place in the near future. Instead of upgrading the system performance independently in the metro and access networks, we propose a highly integrated metro-access network - the metro-access ring integrated network (MARIN). With MARIN architecture, the optical access network is integrated into MAN, such that the capacity upgrade in both networks can be achieved simultaneously without interfering the operation of existing MAN system. By interconnecting multiple access ring networks, all-optical path can be formed to route the MAN traffic between any metro hubs connected to MARIN. Fast tunable lasers are used in the central office of each access ring network to form a hybrid TDM/WDM access network architecture, and facilitate resource sharing, desirable network scalability, and routing efficiency of the metro traffic, which is the focus of this paper. wavelength routed optical burst switching (WR-OBS) is employed for route management of metro traffic. In MARIN, each access ring network emulates a distributed optical burst switch, controlled by the local central office. With WR-OBS for bandwidth and route allocation, the simulation results show that using multiple tunable lasers for simultaneous burst transmission and high-speed reconfigurable device at the joint node, the network performance of MARIN can be significantly improved.

The end-to-end QoS of streaming video on a joint metro and access network is investigated. SUCCESS DWA PON is a highly scalable broadband access network, which can bridge current TDM PON and future WDM PON smoothly and cost-effectively. The IEEE 802.17 Resilient Packet Ring is an emerging MAN standard that designs for efficient packet-based traffic transmission over rings. A realistic video service model is constructed on this joint network, where one of the RPR nodes is dedicated to feed VBR video streams to the end-users in the presence of self- similar Internet traffic. Two improvements are proposed to improve the observed shortcomings in the video streaming performance. To improve the QoS of the streaming video, an additional transit queue is added in the RPR mode to reduce end-to-end delay. In addition, the RPR fairness algorithm is modified to improve the class-C traffic throughput. Simulation results show combined video QoS and total throughput enhancement with the proposed modifications.

In this paper we study the tunable resources scheduling problem for WDM-PON under the Stanford University aCCESS - hybrid WDM/TDM PON (SUCCESS-HPON) architecture, a next generation optical access network. In SUCCESS-HPON a few tunable transmitters and receivers at the OLT are shared by all the users for both downstream and upstream transmission using a centralized light sources approach. Each ONU is assigned a single wavelength for both downstream and upstream data transmission; for the latter, a continuous wave is provided by the OLT for the ONUs to amplitude-modulate upstream data onto it and send it back to the OLT. Through the use of novel scheduling algorithms, it is possible to provide service to all users in the network with just a few tunable transmitters and receivers, making the network very cost-efficient. The performance of these algorithms has been extensively studied through simulations. In this paper we attempt to prove 100% throughput guarantee on a particular scheduling algorithm. We show that the maximum weight matching (MWM) algorithm for admissible traffic with the strong law of large numbers property is stable and can guarantee 100% throughput in a simplified model of SUCCESS-HPON. We derive this result by converting the scheduling problem under consideration to a generalization of the well-known crossbar input-queued switch scheduling problem, and then use a fluid model of a discrete time switch together with the extended Birkhoff-von Neumann (BvN) decomposition theorem. The MWM algorithm can be easily implemented on the SUCCESS-HPON architecture since the number of wavelengths, which determines the complexity of implementation of MWM algorithm, is usually small. This proof of MWM in SUCCESS-HPON is meaningful in that it suggests a practical scheduling algorithm with 100% throughput guarantee as well as determines a theoretical bound. As a byproduct of this research, we prove the extended BvN decomposition theorem, which may be useful in proving 100% throughput guarantee of MWM algorithm in resource scheduling (allocation) problems where certain number of users share the same kind of resources, for example, as in the case that K users are sharing Ntype-A resources and Mtype-B resources.

We propose a novel metro-access integrated network that integrates hybrid TDM/WDM optical access networks into MAN, such that the capacity upgrade in both networks can be achieved simultaneously. By interconnecting multiple access ring networks, MARIN provides alternative paths for MAN traffic to be routed on it with high aggregated throughput. Thus it enables the deployed infrastructure, installed hardware, and available bandwidth of the optical access networks to be shared and leveraged by MAN. Cyclic wavelength selective switch, fast tunable laser, and optical parametric wavelength conversion are used in MARIN system for network reconfigurability, resource sharing in the central office, and wavelength set conversion, respectively. The key components are demonstrated with experimental testbed and their performance is evaluated.

In this paper, we introduce a novel passive and wavelength selective remote node specifically designed to enable security countermeasure in PONs. The central office could physically isolate problem makers from the network without disrupting regular links by issuing a wavelength specific control signal to this device. With its wavelength selective nature, the countermeasure-enabled passive device is also a natural candidate to a WDM-PON remote node. To demonstrate its feasibility, a discrete version of the device is implemented. In addition to enable countermeasure capability, a new kind of PON restoration technique that employs a hybrid optical-wireless network is proposed. Finally, cost and performance considerations are discussed in an 8-channel planer lightwave circuit example.

Novel line codes combined with WDM techniques allow simple and cost-effective overlay of higher bitrate services in existing fiber optical access networks. The solution leaves the existing services and field-deployed fibers untouched. The design of the spectral shaping line codes is presented, along with the analyses for different code rates and processing window sizes. The computation can be simplified by employing vector quantization for the trigonometric functions. DC balance is achieved by forwarding the DC residual to the following window. The spectral shape can be manipulated in various ways to optimize the system performance. Experiments have been conducted to demonstrate and compare several different optimized line codes. For a specific tolerable interference level, the optimal line code can easily be determined which maximizes the data throughput. The service overlay by the line coding technique provides a very flexible and economic way to upgrade existing passive optical networks.

In this paper we study the problem of scheduling variable-length frames in WDM-PON under Stanford University access (SUCCESS), a next-generation hybrid WDM/TDM optical access network architecture. The SUCCESS WDM-PON architecture has unique features that have direct impact on the design of scheduling algorithms: first, tunable transmitters and receivers at OLT are shared by ONU to reduce transceiver counts; second, the tunable transmitters not only generate downstream data traffic but also provide ONU with optical continuous wave (CW) bursts for upstream transmissions. To provide efficient bidirectional transmissions between OLT and ONU, we propose a batch scheduling algorithm based on the sequential scheduling algorithm previously studied. The key idea is to provide room for optimization and priority queueing by scheduling over more than one frame. In the batch scheduling, frames arriving at OLT during a batch period are stored in virtual output queues (VOQ) and scheduled at the end of the batch period. Through simulation with various configurations, we demonstrate that the proposed batch scheduling algorithm, compared to the original sequential scheduling algorithm, provides higher throughput, especially when the system load is high, and better fairness between upstream and downstream transmissions.

HORNET is a tunable transmitter fixed receiver metro ring network, designed for efficient packet transport. By extending the MAC protocol to include a reservation mechanism, the packet based infrastructure is enhanced to support fixed bit rate circuits. We propose several reservation mechanisms and select one of the mechanisms to simulate and demonstrate experimentally. Our reservation scheme can setup fine granularity circuits, on the order of one to a few Mb/s, and setup and teardown times on the order of the ring propagation delay. The scheme is fully distributed and coupled with HORNET's wavelength sharing architecture allows simultaneous, point-to-point circuits between any and all network nodes.

Metropolitan area networks are currently undergoing an evolution aimed at more efficiently transport of data-oriented traffic. However, the incoming generation of metro networks is based on conventional technology, which prevents them scaling cost-effectively to ultrahigh capacities. We have developed a new architecture and set of protocols for the next generation of metro networks. The architecture, named HORNET (hybrid optoelectronic ring network), is a packet-over-wavelength-division multiplexing ring network that utilizes fast-tunable packet transmitters and wavelength routing to enable it to scale cost-effectively to ultrahigh capacities. A control-channel-based media access control (MAC) protocol enables the network nodes to share the bandwidth of the network while preventing collisions. The MAC protocol is designed to transport variable-sized packets and to provide fairness control to all network end users. The efficiency and the fairness of the MAC protocol is demonstrated with custom-designed simulations. The implementation of the MAC protocol and the survivability of the network have been demonstrated in a laboratory experimental testbed. The article summarizes the accomplishments of the HORNET project, including the design, analysis, and demonstration of a metro architecture and a set of protocols. The HORNET architecture is an excellent candidate for next-generation high-capacity metro networks.

Optical carrier sense multiple access with collision avoidance (CSMA/CA) is a media access control (MAC) protocol proposed for future metropolitan area wavelength division multiplexing (WDM) ring networks with a fixed receiver and a tunable transmitter at access nodes. In this paper, we focus on the unslotted version of the optical CSMA/CA MAC which is a fully-distributed and asynchronous protocol. We present the results of design and performance evaluation of fairness control schemes based on longest queue first (LQF) scheduling and two random routing algorithms - full random routing (FRR) and partial random routing (PRR). Through extensive network-level simulation of a WDM ring network with 10 nodes and 10 wavelengths on a 100 km ring at 10 Gbit/s line rate, we demonstrate a combination of the LQF scheduling and the PRR with a retransmission counter provides good fairness (fairness index of 0.9995) with high bandwidth efficiency and small delay spread, under highly unbalanced traffic conditions.

As data traffic increases exponentially, IP over WDM transport will replace conventional SONET transport in metropolitan area networks. Such networks will require new media access control (MAC) protocols to efficiently share network bandwidth among multiple network nodes. This paper describes and evaluates novel and practical carrier sense multiple access with collision avoidance (CSMA/CA) MAC protocols for IP over WDM ring networks that handle variable size IP packets without complex variable optical delays or centralized algorithms. Simulation results show that the proposed IP-MAC protocols are efficient and comparable to our selected benchmark and will be implemented in the IP-HORNET testbed.

Current metropolitan area networks (MANs) based on the SONET transport are not developing at the rate required to support the phenomenal increase in data traffic. To address the needs of future MANs, the Optical Communications Research Laboratory at Stanford University and Sprint Advanced Technology Laboratories are building HORNET (Hybrid Optoelectronic Ring NETwork). HORNET has a multiple access architecture, in which nodes access any WDM channel using a novel media access control protocol and fast tunable laser transmitters. HORNET transports data packets directly over the WDM ring, eliminating the SONET transport. This paper presents the HORNET architecture, the node design consisting of novel packet-over-WDM components, and the experimental testbed with results.

The impact of finite intermediate frequency (IF) on the performance of heterodyne ASK lightwave systems is examined and quantified in the presence of laser phase noise and shot noise. For negligible linewidths, it is shown that certain finite choices of IF (R/sub b/,3R/sub b//2,2R/sub b/,5R/sub b//2, etc.) lead to the same ideal bit-error-rate (BER) performance as infinite choices of IF. Results indicate that for negligible linewidths the worst case sensitivity penalty is 0.9 dB for proper heterodyne detection and occurs when f/sub IF/=1.25 R/sub b/. For nonnegligible linewidths (e.g., when /spl Delta//spl nu/T/spl ges/0.04) the sensitivity penalty is always less than 0.9 dB for finite choices of IF. The analysis presented does lead to a closed-form signal-to-noise ratio (SNR) expression at the decision gate of the receiver which can readily be used for BER and sensitivity penalty computations. The SNR expression provided includes all the key system parameters of interest such as system bit rate (R/sub b/), the peak IF SNR (/spl xi/), laser linewidth (/spl Delta//spl nu/), and the IF filter expansion factor (/spl alpha/). The findings of this work suggest that the number of channels in a multichannel heterodyne ASK lightwave system can be increased substantially by properly choosing a small value for the IF at the expense of a small penalty <1 dB. On the negative side, IF frequency stabilization becomes a more critical requirement in multichannel systems employing small values of IF.

STARNET is a broadband backbone optical wavelength-division multiplexing (WDM) local area network (LAN). Based on a physical passive star topology, STARNET offers all users two logical subnetworks: a high-speed reconfigurable packet-switched data subnetwork and a moderate-speed fix-tuned packet-switched control subnetwork. Thus, STARNET supports traffic with a wide range of speed and continuity characteristics. We report the analysis and implementation of an entire STARNET two-node network, from the optical to the computer layer, at the Optical Communications Research Laboratory (OCRL) of Stanford University. To implement the two logical subnetworks, we designed and implemented two different techniques: combined modulation and multichannel subcarrier multiplexing (MSCM). OCRL has already demonstrated several combined modulation techniques such as phase shift-keyed and amplitude shift-keyed (PSK/ASK), and differential phase shift-keyed and amplitude shift-keyed (DPSK/ASK), yielding combined ASK/DPSK modulation receiver sensitivities better than -32 dBm. OCRL has designed and implemented a high-speed high-performance packet-switched STARNET computer interface which enables high-throughput transfer to/from host computer, low latency switching, traffic prioritization, and capability of multicasting and broadcasting. With this interface board, OCRL has achieved average transmit and receive throughputs of 685 Mb/s and 571 Mb/s, respectively, out of the 800 Mb/s theoretical maximum of the host computer bus. The incurred packet latency due to the interface for a specified multihop network configuration has been simulated to be 24 /spl mu/s. Using simulation and experimental results, it is shown that STARNET is highly suitable for high-speed multimedia network applications.

The implementation of optical packet-switched networks requires that the problems of resource contention, signalling and local and global synchronization be resolved. A possible optical solution to resource contention is based on the use of switching matrices suitably connected with optical delay lines. Signalling could be dealt with using subcarrier multiplexing of packet headers. Synchronization could take advantage of clock tone multiplexing techniques, digital processing for ultra-fast clock recovery, and new distributed techniques for global packet-slot alignment. To explore the practical feasibility and effectiveness of these key techniques, a consortium was formed among the University of Massachusetts, Stanford University, and GTE Laboratories. The consortium, funded by ARPA, has three main goals: investigating networking issues involved in optical contention resolution (University of Massachusetts), constructing an experimental contention-resolution optical (CRO) device (GTE Laboratories), and building a packet-switched optical network prototype employing a CRO and novel signaling/synchronization techniques (Stanford University). This paper describes the details of the project and provides an overview of the main results obtained so far.

The goals and basic node configuration of STARNET, an optical broadband local area network based on a physical passive star topology, are presented. Over a single physical network, STARNET offers all users both a moderate-speed packet network and a high-speed WDM circuit interconnect. Several topological and protocol solutions based on these two data transport facilities are available to the users. An ongoing experimental effort aimed at the realization of a STARNET prototype is described. The prototype nodes are FDDI and SONET OC-48 data-format-compatible. The first four network layers will be fully implemented and the nodes will serve high performance workstations.>

A number of optical signal processing functions that might be potentially important for future lightwave communication networks are described. An optical network with a distribution capacity of 100 HDTV channels is considered along with how such a network can be implemented using the following functional subsystems: frequency converters; transmitter banks; modified (wavelength division multiplexing) WDM demultiplexers; and tunable optical receivers. Discussed are the key network-level issues: the power budget, the channel separation, and the overall rationale for selection of multiplexing techniques. A hardware implementation of the functional subsystems using three basic building blocks-tunable amplifiers/filters, phase locked loops, and comb generators-is discussed.>

This paper compares the noise immunities of feedback communication systems for two particular types of systems noise: Gaussian white noise, which is typical of radio and telephone communication, and Poisson-distributed noise, which is typical of laser communication. The comparison shows that as the error probability requirement is made more stringent, the use of feedback will save commensurately more energy in both cases.