A Tutorial on IEEE 802.11ax High Efficiency WLANs - noureldin mohamed

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IEEE COMMUNICATIONS SURVEYS & TUTORIALS, VOL. 22, NO. 2, SECOND QUARTER 2020 Software-defined Allocation and Virtualization for Broadband Multi - Beam Satellite Communication Networks based on High Throughput Satellites Noureldin Mohamed , Student Member, IEEE , Charles E. Leiserson, Fellow Member, IEEE I. INTRODUCTION A Abstract— The increase in the number of satellite lthough the Internet has changed people's daily lives, communication system users requires more spectrum and bandwidth resources. In the future, multi-beam broadband almost two-thirds of people do not use wired or wireless satellite systems should have greater flexibility and be able to internet. The satellite network, which has global coverage and dynamically adjust to changes in business volume. A method for is not constrained by geographical conditions, has attracted effectively sharing spectrum can effectively use spectrum widespread attention from the research and industry community. resources. Cognitive radio spectrum sharing technology is an They prefer to use low-altitude satellites to reduce propagation effective method for improving spectrum efficiency and realizing delay and achieve real-time communication. Additionally, effective utilization of spectrum resources. on the other hand, the through the use of addressing, routing and other technologies, next-generation of high-throughput satellite (HTS) used for you can provide transportation services with Quality of Service broadband user access is closely related to the use of the Ka band (QoS) regulations in next-generation satellite networks. and more frequencies. This is related to accessing so-called "terabit connections" to support increased bit rate requirements. However, the existing satellite networks do not upgrade Therefore, there are many challenges, such as updating hardware/software flexibly and depend on closed and planned configuration, introducing new communication technologies and infrastructures. It brought great challenges for the rapid networks, providing truly excellent services, interoperability of introduction of new communication technologies and networks, satellite network equipment, integration of Satellite, and hampered the provision of truly differentiated services for a terrestrial networks. Software-Defined Network (SDN) has wide variety of growing satellite network applications, and flexibility, Programmability, and logic centralization increase the provided satellite communication equipment provided by use of network resources and simplify the network Management Microsoft The interoperability between them brings significant reduces operating costs and promotes development and obstacles. Different operators (or based on different innovation. This paper studies the spectrum broadband satellite network system dedicated to cognitive radio sharing software. The communication technologies) impede the smooth integration of technology enables effective management of satellite resources, heterogeneous satellites and terrestrial networks. On the other improves the use of satellite resources, and improves the hand, terrestrial network architectures adopt new paradigms, performance of multi-beam satellite communication systems such as Software Defined Networking (SDN) and Network along with high-throughput satellite. The principle of multiple Function Virtualization (NFV). The SDN focuses on the knowledge packages sharing software-defined packages discusses ambitious vision of the centralized network logic, which makes the system of satellite broadband networks. Analyze spectrum the network nodes programmable, thereby providing a certain efficiency and productivity. level of abstraction that is accessed through the control interface Manuscript received October 22, 2019; revised December 2, 2019; accepted (API). The SDN template provides the opportunity to manage January 18, 2020. Date of publication February 06, 2020; date of current network services by summarizing the main functions. This can version May 28, 2020. The work of N. Mohamed was supported by The Engineering and Physical Sciences Research Council (EPSRC) Project under be done by separating the network control plane from the data Grant 2016CBN9149. (Corresponding author: Noureldin Mohamed) . level, and by network simulation. The goal of NFV is to N.Mohamed with the School of computer science and technology, Beijing implement some network functionality in software packages so Institute of Technology, Beijing 100081, China (e-mail:1820181066@bit.edu.cn, noureldinmohamedabdelaal@gmail.com ). that services can be served using these software packages C.E.Leiserson is with the school of Engineering and, Massachusetts Institute simultaneously. In this case, network functions such as of Technology, Cambridge, Massachusetts, United States (e-mail: switching, routing, and security services are no longer applied cei@mit.edu). through dedicated hardware, but through software running on commercial-purpose x-86 devices. There are some major Index Terms— HTS system, Ka band, QN-band, Software players in the SDN / NFV field (i.e. CISCO, VMware, Alcatel- Defined Networking, Network Functions Virtualization, Lucent, HP, IBM, etc.). Has or will be able to integrate (all or Extremely High Frequencies, Smart Gateways, Extremely part of it) And communicate with the de facto open framework high frequencies, Propagation impairment mitigation techniques, Site diversity, Smart gateway, Cognitive Radio; SDN / NFV Standard to date: OpenStack. The SDN / NFV Full frequency reuse, Radio resource management. model permits taking advantage of the high flexibility, measurability and speedy preparation options inherent during a IEEE COMMUNICATIONS SURVEYS & TUTORIALS, VOL. 22, NO. 2, SECOND QUARTER 2020 programmable setting. The processing power is given by B. NETWORK VIRTUALIZATION multiple virtual machines (VMs) on the physical server (or Virtual network simulations can create and isolate multiple "node"). If there's a necessity to boost the process capabilities independent virtual networks on shared network infrastructure of the device (i.e. router / virtual adapter and GW), the virtual and coexistence. A virtual network is a logical network that setting permits (very) to instantly add VMs to specific physical contains certain virtual elements (network devices (or nodes) nodes or portion additional resources (such as computer and links). Virtual nodes are summaries of network devices that hardware cores/chains and memory Random access and storage) are usually hosted on one physical node. It performs network to the VM of that node. The construct of SDN additionally functions, such as forwarding and forwarding, by occupying applies to the rear finish layer (storage): a superior SAN some managed node resources. Resources for virtual network (storage space network) topology may be simulated by the equipment are diverse, such as CPU, variable memory, network package, i.e. associate degree SDN controller that manages one interface, storage, switching, etc. A virtual link is an abstraction or a gaggle of DAS (Direct connected Storage) teams. Disks in from a network link created on one or more physical links or goods devices will produce a computer storage space network physical paths. It consumes transmission resources (i.e. with low value and simplified management functions. This, at bandwidth for physical links) and exchanges resources for the side of the implementation of the SDN within the CDN physical nodes that are traversed. (Content Delivery Network), opens new horizons for the implementation of affordable integrated Earth services. The C. NETWORK FUNCTION VIRTUALIZATION introduction of SDN and NFV models within the direction of The telecom industry has always preferred to use dedicated the present development of HTS will bring huge advantages to equipment to provide network functionality. However, this service suppliers and users, therefore making associate degree model inevitably leads to long market delays and higher costs. innovative “demand-driven network” which will support The NFV concept challenges this model. In fact, NFV correct new services and optimize the optimum use of resources. advocates the virtualization of network functionality in Moreover, the network virtualization provided by these new software modules running on standard IT infrastructure (such models will effectively notice the chance of integration between as turnkey commercial servers), which can be grouped and/or satellites and terrestrial resources and will promote the linked to create services. Since virtual network functions can be expansion of satellite applications. this text can discuss the implemented on one or more virtual machines, this method implementation of the SDN model in future HTS systems and takes advantage of the server virtualization experience learned determine the foremost fascinating use cases and views. half from the cloud computing industry. The main benefits of NFV two introduces the integrated practicality of the terrestrial SDN are reduced CAPEX and OPEX and improved network agility. satellite networks and also the expected advantages for the whole SatCom community (stakeholders, consumers, etc.). In D. SATELLITE NETWORK ARCHITECTURE Section Three, the satellite network reference set up was analyzed, and in Section foursome, specific applications for SDN / NFV networks were known. half 5 concludes. This work is a typical broadband satellite network (BSN), which provides multi-beam coverage with front and backlinks. II. BACKGROUND The BSN ground segment combines multiple hubs, interconnected with some contact points (presence points) or A. INTRODUCTION TO gateways to external networks (usually the Internet) through a SOFTWARE-DEFINED NETWORKS dedicated backbone network (Figure 2). Recently, the SDN has become a new way to network Usually, the hub supports two-way traffic on one or more programming and management, as the logic of the central packets. It combines the forward link transmission unit (FL-TU) control level is separated from the level of data forwarding. The and the reverse link reception unit (RL-RU), as well as the SDN architecture defines a new entity (called a controller) that gateway (GW) to the terrestrial network and network control integrates the control intelligence of one or more network center (NCC) and network management center (NMC). FL-TU elements (adapters), as shown in Figure 1. Various open performs basic domain functions through adaptive coding and interfaces are established for communication between the modulation (ACM), such as DVB-S2 coding and modulation. control level and the data level (southern interface), and Gates are usually full-featured IP routers with powerful OpenFlow is the standard the actual. On the console's northern functions and protocol groups (for example, support for interface, you can use the network-level data path view to different routing protocols, network address translation, access deploy the application. SDN opens new opportunities. Most control list (ACL), firewall services, SNMP, QoS, etc.). NCC importantly, it simplifies network management and allows provides control functions; usually access control and resource automatic on-demand networking and optimal use of network control/allocation of satellite terminals (ST) in the forward resources. direction and return links. The NMC performs all management functions, namely network component configuration (ST, hub), error, performance, billing management, and security. A performance improvement factor (PEP) designed to improve IEEE COMMUNICATIONS SURVEYS & TUTORIALS, VOL. 22, NO. 2, SECOND QUARTER 2020 TCP performance can also be found on satellite links in the hub a certain distance (usually tens of kilometers for EHF), ensuring (or carried over to PoP or near the end-user). Successfully an area between precipitation is minimal. Therefore, if heavy providing satellite communication services for end-users rain occurs in one GW region, there may not be rain (or very includes one or more real business participants, each of whom small) in another area; the site is used in the best spread plays one or more roles (they bear a series of responsibilities). conditions to create the link. Figure 2. Satellite communication architecture. There are two possible configurations of site diversity: balanced and unbalanced. Balanced Variation is a site Figure 1. Network vision with the software-defined network diversification scheme consisting of two or more ground (SDN) approach. stations with the same performance. In unbalanced diversity, the performance of the earth station varies over diversity. In particular, it made the performance of an earth station (main station) high enough to significantly reduce the performance Three main roles are involved: requirements of another earth station (substation). The Satellite operator (SO): owns the satellite and starts operating. performance gain obtained through site diversity can be very It leases satellite capacity at the transmitter and receiver level high, but it is worth noting that in the commercial HTS case, (physical layer) to one or more SNOs. the application of this technique requires a complete iteration of Satellite network operator (SNO): Use one or more satellite the GW, so the ground segment cost is not affordable. The transmitters and receivers and one or more satellite hubs to Smart GW relies on the synchronous GW aggregate connected operate the broadband satellite network. Divide the bandwidth to the terrestrial fiber network and is used for the location at the transmitter and receiver level to provide satellite guidance diversity diagram of the feed link so that the feed link data can and return links for Layer 2 operators. NCC controls this be routed in a way to compensate for the deep fading of one (or bandwidth sharing. Through NMC, SNO provides a more) gateway. GWs in the pond are found in various nutrient management interface for purchased resources. beams. It is the spatial reuse of the feed link bandwidth that is Satellite Virtual Network Operator (SVNO): Based on the fully reusable by GW. satellite links signed with one or more SNOs, it builds and Various configurations for Smart GW can be used: provides high-end end-to-end value-added services that can be • The first configuration is designed to ensure continuous obtained through satellite access. service without repetition. In this structure, each GW is transmitted through a carrier linked to different user packets. III. GATEWAYS DIVERSITY OPTIONS FOR HTS Therefore, when a GW fails, only a portion of the user's bandwidth will be lost, and the GW can service another GW Spatial diversity technology relies on the spatial redirection of intermittent user terminal (above the usual DVB-S2 TDMA, radio paths around fading sources; the reason for this is that using shared FDMA resources with other terminals in the same precipitation is intermittent and uneven in space and time. Two user pack). This SG architecture differs from the HTS currently major GW diversity systems can be used within the HTS system: running, for, in the current HTS, each user packet is connected single site diversity and smart portal. The first is a well thought to one GW unit. It is worth noting that this solution can out topic, based on the simultaneous use of two simultaneous maximize service continuity, but it will reduce the total system ground stations located within the same feed point beam and at capacity during a power outage. IEEE COMMUNICATIONS SURVEYS & TUTORIALS, VOL. 22, NO. 2, SECOND QUARTER 2020 overall performance of the satellite link by utilizing the limited size and range of dense rain cells. These brain cells can extend only a few kilometers in both the horizontal and vertical directions and tend to get smaller as the rain intensifies. With sufficient physical isolation between ground terminals, the probability of both sites exceeding a certain level of attenuation due to precipitation is much less than the probability of exceeding the attenuation level at one location. Site diversity is one of the most powerful PIMTs, but achieving performance improvements requires significantly higher system costs. Within this framework, we will focus on the balanced diversity of the site because it is the most common configuration used to implement the SG concept. Figure 3. Single site diversity. • The second configuration is a system designed to not fully utilize one GW capacity in clear sky conditions; therefore, when one GW encounters deep atmospheric fading (or even complete outages), its flow can be allocated to other GWs in the network (conditions encountered Good channel spread). This implementation requires a complex airborne key matrix to achieve a reconfigurable (or almost complete) channel communication between GW and user point. Since the system capacity is very large compared to "clear sky" operating conditions, the second SG solution appears to be ineffective, but in actual commercial applications, the service provider needs to spend several years selling the entire system capacity, so Smart GWs can be implemented using parts that do not sell Figure 4. Geometry of LEO satellite links. to clients. In the long run, when the service provider sells all the system capacity, other solutions must be identified to maintain guaranteed service availability using pre-provided SGs V. USES CASES technology (for example, GW iteration can be entered using a USE CASE 1: INTER-HUB HANDOVER WITH SITE single site diversity configuration). All GWs in the cluster are DIVERSITY connected by a network control center that manages and monitors traffic. Description and current practice - In satellite communications, the use of higher frequency bands such as Ka or Q / V makes Compared to a single site diversity diagram, the use of system the adaptive coding and modulation (ACM) mechanisms resources in the smart GW is more efficient; in fact, all gates mandatory, which can offset the signal attenuation caused by can work simultaneously, and the system capacity is used in an meteorological events such as clouds or rain. In the case of very ideal way without the need for a full iteration of the GW. On high attenuation, the throughput attenuation due to ACM and the other hand, defining efficient flow control / switching the resulting network congestion may be inconsistent with the algorithms and GW network synchronization can be very QoS restrictions for some streams (VoIP, video conferencing). complicated. Besides, in the second structure, the switch matrix If weather degradation is caused by weather interference on on the board should be used. GPS positioning, a strong encryption mechanism is inevitable. If downgrading includes hub localization, you should consider IV. WIDE AREA DIVERSITY using another remote location. The concept of a single ST Site diversity is a generic term used to describe the use of two connection (successful or unsuccessful) is called multiple axes (or more) terrestrial terminals geographically separated in a called site diversity. space communication link to overcome the downlink path Site diversity deployment can follow two different methods as attenuation effects during heavy rainfall. Location diversity, described above. The N + P method relies on repeated P axes, also known as path diversity or space diversity, can improve the which can replace the failed location to achieve complete user IEEE COMMUNICATIONS SURVEYS & TUTORIALS, VOL. 22, NO. 2, SECOND QUARTER 2020 switching (HO). The N + 0 frequency multiplexing method is •Traffic control: Determination of active services, performance, used to serve the carriers from different axes to the terminal, and resources received (satellites and backbone). and the wrong location results in the loss of the corresponding • Satellite and ground-based network performance indicators. frequency band segment. If the continuity of network services •SNO / SVNO Strategy: Urgent or superuser requirements. must be guaranteed and performance compromised, both Once specific flows or ST handover are defined, the application situations will present challenges. Indeed, axis modulation will automatically: (case N + P) or carrier modulation (N + 0) must be sent to ST • Notify the relevant ST and FL / RL-TU to change its and implemented. At the same time, the routing table in the frequency if necessary. Earth Network must be updated. Moreover, the delivery • Update the forwarding rules in the GWs and backbone decision issue is complex, as it may involve hundreds of STs, network. Two options can be considered: considering multiple criteria, such as traffic control, network • Direct route guidance: The flow is directed directly from its knowledge, and channel quality changes. The current satellite new center to the closest PoP, network follows the N + P method and delivers the entire This is often achieved due to SDN related programmable package at once. functionalities that are added to the packet-processing pipe. as an example, OpenFlow can dynamically deploy forwarding rules matching packets based on: • Incoming network interface. • IP/MAC addresses. • Classes of services or protocols used. • Rate of identified flow or group. • Deep packet inspection (DPI) using legacy functions. lastly, SDN doubtless contributes to present and future satellite networks by easing the management of inter-hub handover enabled by site diversity, and by extending its capabilities. USE CASE 2: MIDDLEBOXES VIRTUALIZATION Central boxes are very common in Internet architecture, especially in certain networks (such as satellite communication Figure 5. SDN architecture of terrestrial network in site networks). These smart entities are used for various purposes, diversity context. such as improving performance, security, and address translation. This section analyzes how NFV improves classic SDN Opportunities for Site Diversity — PEP functionality in satellite networks. Implementing SDN principles in the context of site diversity TCP Performance Optimization — can help design effective delivery decision algorithms and On some WANs, especially in restricted environments (such as simplify the delivery process. This can be achieved through the satellite networks), the TCP / IP model is not optimal in following improvements shown in Figure 5: performance. Various versions of the TCP are proposed for satellite networks with the aim of improving the performance • Switches that support SDN can replace GWs in Hubs. • The SDN (OpenFlow) main monitor located on the hub site running network applications is responsible for switching management between hubs. For illustration, the SDN controller is described in Figure 5. However, for purposes of scalability and reliability, multiple control entities should be considered. • NCC and NMC interfaces exposed to switch applications, which collect monitoring information and enable the operation of certain ST configurations. •Optional: Basic network supports SDN technology. The Switch Manager app determines when a handover is required (and the stream or ST included in the switch) based on the following conditions: •Traffic limit: QoS requirements, specific service level agreement for a specific user. of the TCP. However, they face publishing problems on user stations. The solution that was found and used was to introduce IEEE COMMUNICATIONS SURVEYS & TUTORIALS, VOL. 22, NO. 2, SECOND QUARTER 2020 equipment at the boundaries of the satellite network to convert The demand for VNO services is clear, not new. VNO service the operation of TCP into a compatible version with satellites. allows SNO to effectively divide satellite resources among These devices, called performance improvement factors (PEPs), multiple SVNOs by providing dedicated satellite capacity with are distributed in the satellite network and provide advanced different QoS levels. Usually, SVNO will reassemble these services such as web caching. services, in turn, to provide comprehensive value-added The protocol optimization provided by PEP is incompatible services to its customers. However, SVNO has limited control with many solutions, especially in military or space over the services (and basic resources) they purchase, mainly deployments where security and mobility restrictions exist. For due to the closed nature of satellite equipment and the example, implementing mobile phone architectures such as management interface between SNO and SVNO. mobile IP provides a solution to the complex problems of PEP. Figure 7 shows the network management system (NMS) used The most problematic situation occurs during mixed delivery, by SVNO on the one hand, and SNO NMC running NCC, GW, that is, from a satellite network requiring improvement of the and all STs on the other hand. Even if some management PEP to a network that is no longer needed (which may be functions (for example, routing, etc.) can be completed directly counter-productive). In this case, the managed and accelerated from NMS for SVNO to ST, most functions must also pass TCP connection by PEP should still exist after the PEP is through NMC (for example, to obtain status and statistical deactivated (or the PEP changed generally). However, PEP is information for ST). To this end, SNO provides a management physically confined to the infrastructure and cannot follow the interface (I.SNO-SVNO) as part of the VNO service to enable end-user. For the proposed mixed satellite / terrestrial solutions NMS to manage SVNO satellite terminals. This interface is to solve this problem, it requires the exchange of context usually SNMP dependent and complementary to some vendor- between potential people. Other medium boxes that provide specific solutions. advanced services in satellite networks (NAT, firewalls, security, etc.) suffer from the same problems. The VNO service offered to SVNO depends on the level of visibility and control function offered by the I.SNO-SVNO PEPs and Network Function Virtualization — management interface, which is far from universal. Besides, The network function virtualization model aims to perform data some control functions require manual intervention by SNO to level processing or control level functions in large capacity data verify or implement the required configuration. From SVNO centers or network elements. This opens up a new era for perspective, this requires the development of new services and thinking about middleboxes because middleboxes can be easily the complexity of the process of creating your services. deployed on-demand and provide advanced services under Virtualization of VNO Network Services and Network operator control. Moreover, these mid boxes may be mobile Programming Opportunities — because they only depend on programs that can be migrated SVNO requires more control over its resources by reducing (or from one standard server to another. Given the use case 1 (field not using) SNO intervention. The problem is: diversity) presented above, PEP is usually implemented in a 1. The process of supplying an automated service faster. satellite center. When the satellite station is delivered to the 2 enrich their service catalog. new hub, the TCP connection will be disconnected via PEP 3 Enable satellite communication as a service consumption because the new PEP will not know the connection context. model. With the NFV model, PEP will not be executed as a dedicated medium box, but as a program that can be run on different devices. Besides, the PEP functionality can be customized to the connection context (for example, ST), and can be modified according to application requirements (security, mobility, performance, etc.). If the ST switches from one satellite hub to another, its "dedicated virtual PEP" will be carried over to the new hub and will continue to implement appropriate TCP improvements. Some cloud computing systems support NFV and have already provided solutions for deploying virtual network functions (VNF). Some vendors have suggested default functionality to improve and accelerate the TCP protocol for web application servers. From a technical point of view, PEP virtualization will become a reality. Figure 6. SDN-enabled satellite/ADSL hybrid architecture. USE CASE 3: ENHANCING VNO SERVICES Description and State of the Art — IEEE COMMUNICATIONS SURVEYS & TUTORIALS, VOL. 22, NO. 2, SECOND QUARTER 2020 Opening exposed satellite equipment to Layer 2 operators CONCLUSION through the program interface (a rich instruction set that By describing four practical use cases, this article bypasses SNMP), as well as network virtualization, is the way illustrates some of the opportunities that these emerging to achieve these goals. By applying a virtualization device (i.e. models of satellite broadband networks provide and their server virtualization on network devices and applicable to impact on typical satellite system constructs. SDN and network devices) on the SNO satellite hubs, a virtual hub can NFV are complementary solutions. SDN provides be distributed based on SVNO (Figure 8). Insulation resulting flexibility, automation, and network customization. NFV from isolation is a major function of virtual network simulation, brings agility in providing services and shortens the time it is suitable for data and control, management plans, to market new services. There is no doubt that they will performance and safety, and SNO can delegate complete occupy a central place in future satellite communication control and management of the virtual axis to the customer systems. SVNO. Therefore, SVNO can independently execute its strategy on its virtual satellite network. By controlling the management interface (scope of functions) from NMC to NMS, REFERENCES SVNO can fully automate the process of supplying services provided to its clients. In fact, the throttle engine can be used to [1] D. Kreutz, F. M. V. Ramos, P. E. 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Shayan, "Towards virtualisation and secured writing a best-selling algorithms textbook, and developing courses on software defined networking for wireless and cellular networks," 2016 algorithms and parallel programming.” He received the ACM 2013 IEEE Canadian Conference on Electrical and Computer Engineering Paris Kanellakis Theory and Practice Award “for contributions to (CCECE), Vancouver, BC, 2016, pp. 1-5, doi: efficient and robust parallel computation through both provably 10.1109/CCECE.2016.7726826. efficient randomized scheduling protocols and a set of parallel-language [15] Z. Wang, D. Tao and Z. Lin, "Dynamic Virtualization Security Service primitives constituting the Cilk framework.” He has received several Construction Strategy for Software Defined Networks," 2016 12th “best paper” awards and the ACM SIGPLAN ten-year retrospective International Conference on Mobile Ad-Hoc and Sensor Networks award for most influential 1998 PLDI paper. 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Zhang, "Toward a software-based network: integrating software defined networking and network function virtualization," in IEEE Network, vol. 29, no. 3, pp. 36-41, May-June 2015, doi: 10.1109/MNET.2015.7113223. Noureldin Mohamed is currently pursuing a B.Sc. degree in computer science from the Beijing Institute of Technology, Beijing, China. In 2017 He received The Gates Cambridge Scholarship to pursue a full-time postgraduate degree in Computer Science and technology at the University of Cambridge. he was a recipient of the Best scientist in Egypt and the Beat Arab youngest innovator in the world Awards, His major research interests are connected with Programming methodology, programming languages and systems, wireless systems, network programmability, information secutity and privacy, software-defined networking, network function virtualization, He holds a patent, He participated several joint research projects with top technology companies and collaborative projects (e.g., International Business Machines Corporation (IBM), Alphabet Inc., Microsoft Corporation). Charles E. Leiserson received a B.S. from Yale University in 1975 and a Ph.D. from Carnegie Mellon University in 1981. He joined the MIT faculty in 1981, where he is now Professor of Computer Science and Engineering in the MIT Department of Electrical Engineering and Computer Science (EECS) and head of the Supertech research group in the MIT Computer Science and Artificial Intelligence Laboratory (CSAIL). Professor Leiserson’s research centers on the theory of parallel computing, especially as it relates to engineering reality. He co-authored the first paper on systolic architectures. He invented the retiming method of digital-circuit optimization and developed the algorithmic theory behind it. Retiming is now a foundational optimization method in all major electronic-design systems. On leave from MIT at Thinking Machines Corporation, he designed and led the implementation of the network architecture for the Connection Machine Model CM-5 Supercomputer, which in- corporated the “universal” fat-tree interconnection network he developed at MIT. Fat-trees are now the preferred interconnect strategy for InfiniBand technology. Professor Leiserson has made numerous contributes to computer-science education. He is well known as co- author of the textbook Introduction to Algorithms (The MIT Press), which was named “Best 1990 Professional and Scholarly Book in Computer Science and Data Processing” by the Association of American Publishers. Currently, in its third edition, it is the leading textbook on computer algorithms, He developed the MIT undergraduate courses on algorithms and on dis-crete mathematics for computer