DOCSLIB.ORG

Migrating Backoff to the Frequency Domain

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Migrating Backoff to the Frequency Domain No Time to Countdown: Migrating Backoff to the Frequency Domain Souvik Sen Romit Roy Choudhury Srihari Nelakuditi Duke University Duke University University of South Carolina Durham, NC, USA Durham, NC, USA Columbia, SC, USA [email protected] [email protected] [email protected] ABSTRACT 1. INTRODUCTION Conventional WiFi networks perform channel contention in Access control strategies are designed to arbitrate how mul- time domain. This is known to be wasteful because the chan- tiple entities access a shared resource. Several distributed nel is forced to remain idle while all contending nodes are protocols embrace randomization to achieve arbitration. In backing off for multiple time slots. This paper proposes to WiFi networks, for example, each participating node picks a break away from convention and recreate the backing off op- random number from a specified range and begins counting eration in the frequency domain. Our basic idea leverages the down. The node that finishes first, say N1, wins channel con- observation that OFDM subcarriers can be treated as integer tention and begins transmission. The other nodes freeze their numbers. Thus, instead of picking a random backoff duration countdown temporarily, and revive it only after N1’s trans- in time, a contending node can signal on a randomly cho- mission is complete. Since every node counts down at the sen subcarrier. By employing a second antenna to listen to same pace, this scheme produces an implicit ordering among all the subcarriers, each node can determine whether its cho- nodes. Put differently, the node that picks the smallest ran- sen integer (or subcarrier) is the smallest among all others. dom number transmits first, the one that picks the second- In fact, each node can even determine the rank of its chosen smallest number transmits second, and so on. The overall subcarrier, enabling the feasibility of scheduled transmissions operation is often termed as “backoff”. after every round of contention. We develop these ideas into a Back2F protocol that migrates WiFi backoff to the frequency While backoff arbitrates channel contention, it incurs a per- domain. Experiments on a prototype of 10 USRPs confirm fea- formance cost. Specifically, when multiple nodes are simulta- sibility, along with consistent throughput gains over 802.11. neously backing off, the channel must remain idle, naturally Trace based simulations affirm scalability to larger, real-world leading to under-utilization. Moreover, network congestion network topologies. prompts an exponential increase in the backoff range, intro- ducing the possibility of greater channel wastage. Authors in [16] show more than 30% reduction in throughput due to backing off; [13] shows the severity at higher data rates. This paper attempts to address this problem by migrating the backoff operation to the frequency domain. Categories and Subject Descriptors Our main idea is simple. When a node N has a packet to C.2.1 [Network Architecture and Design]: Wireless commu- 1 transmit, it picks a random value, r , from a specified range nication 1 [0;F ]. Once the channel becomes idle, N1 begins the back- off operation. However, instead of counting down from r1 th 1 General Terms to 0, N1 transmits a symbol on the r1 subcarrier . We as- Design, Experimentation, Performance sume that each node has two antennas; thus, while one an- tenna transmits, the other antenna listens to determine which of the subcarriers are active. Assuming N2 is also contending th Keywords for the channel, and say has transmitted on the r2 subcarrier, Wireless, Contention Resolution, Cross-Layer, Backoff N1 observes activity on both subcarriers r1 and r2. Assuming r1 < r2, N1 immediately infers that it has won channel con- tention, and begins transmission. N2 learns that it has lost, and defers its own transmission until N1 has finished. We call this approach Back2F, as an acronym for migrating backoff to the frequency domain. Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are The advantages of Back2F are two fold. First, one round of not made or distributed for profit or commercial advantage and that copies frequency domain backoff should ideally last for few OFDM bear this notice and the full citation on the first page. To copy otherwise, to republish, to post on servers or to redistribute to lists, requires prior specific symbols, substantially less than the average backoff in proto- permission and/or a fee. 1 MobiCom’11, September 19–23, 2011, Las Vegas, Nevada, USA. Subcarriers are narrowband OFDM channels used by Copyright 2011 ACM 978-1-4503-0492-4/11/09 ...$10.00. 802.11. cols like 802.11. Second, Back2F creates a logical ordering We make three observations that are not necessarily new. (1) among contending nodes, and each node learns its own rank Fundamentally, backing off is not a time domain operation. in this order. This ranking among nodes creates the possi- Its implementation is in the time domain, forcing the chan- bility of batched transmissions, eliminating the need for per- nel to be idle before each packet transmission. (2) The du- packet backoff. Since 802.11 currently backs off before every ration of each backoff slot is fixed, so the channel wastage packet, Back2F helps improve the channel usage and network grows with smaller packets and higher bitrates. A packet’s throughput. airtime is shorter at higher rates, and hence, the fraction of channel-time occupied by idle slots is larger. (3) Finally, Of course, extracting these gains entail a number of research although channel utilization may improve with few nodes challenges: (1) Active subcarriers need to be detected accu- (backing off in parallel), just a few more nodes can cause col- rately in face of loose time synchronization among transmit- lisions (802.11 experiences 18% collisions with 3 nodes and ters, energy leakage between narrow-band subcarriers, and CW =16). A collision forces nodes to exponentially increase channel fading. (2) Collision among nodes – which hap- their backoff, pushing the system back to under-utilization. pens when multiple nodes choose the same subcarrier – needs Fig. 1 shows the channel under-utilization due to 802.11’s to be mitigated successfully. (3) Finally, since nodes are lo- backoff, under varying bitrates and network densities. Au- cated in different contention neighborhoods, the node rank- thors in [16, 29] corroborate these findings with extensive ings do not obey any global order. Back2F needs to cope analysis and measurements, emphasizing the need to improve with relative ranking among nodes, while maintaining spatial wireless contention resolution. reuse and fairness comparable to 802.11. We address these challenges through activation of diverse subcarriers, multiple rounds of contention, and virtual countdown. We consolidate 0.45 these ideas into a protocol, and implement a prototype on the 0.4 USRP/ GNURadio platform. Experimental results show 95% accuracy in subcarrier detection, less than 2% probability of 0.35 collision, and throughput gains of more than 35% at high bit- 0.3 rates. Trace driven simulations (with topologies and channel 0.25 conditions drawn from our university network) confirms sta- bility and scalability of Back2F to real-world scenarios. 0.2 2 APs 512 bytes 0.15 Our contributions in this paper may be summarized as: 6 APs 512 bytes 0.1 10 APs 512 bytes • We identify an opportunity to migrate protocol operations 2 APs 1024 bytes from the time to the frequency domain. Although we in- 0.05 6 APs 1024 bytes 10 APs 1024 bytes stantiate our ideas through a WiFi based MAC, they may 0 be generalized to other arbitration strategies. Channel waste due to backoff(%) 10 15 20 25 30 35 40 45 50 55 • We design an OFDM based system where random backoff is Bitrate in Mbps realized by selectively transmitting on a subcarrier. A log- ical order among senders is enforced in a decentralized Figure 1: Overhead of 802.11 backoff. Larger fraction of manner, for improved channel usage. channel wasted with smaller packets at higher bitrates. • We address the challenges behind such a scheme, and pro- totype it on the USRP/GNURadio platform. Promising re- Orthogonal Frequency Division Multiplexing: OFDM can sults, in terms of throughput, fairness, and scalability, be abstracted as a PHY scheme that divides the wireless spec- give us confidence to build a larger system. trum into multiple narrow band channels, called subcarriers. The subcarriers carry modulated data streams in parallel, but 2. 802.11 AND OFDM at a lower rate per-subcarrier. The benefit of OFDM emerges This section highlights the limitations of 802.11’s backoff (in from its ability to cope with channel adversities, including time domain), and presents a simple abstraction of OFDM (to narrowband interference and frequency-selective fading due better explain the shift to frequency domain). to multipath. The 802.11a/g implementation of OFDM has 52 subcarriers, of which 48 are used for data transmission, 802.11 Channel Access: WiFi prescribes each transmitter to and 4 for equalization. A transmitter stripes bits across all backoff for a random number of slots, chosen from the range subcarriers, however, it is possible to transmit/receive only [0;CW − 1], where CW is the contention window. Each time on a subset of them. slot corresponds to 9µs. The node counts down only if the channel is idle – if the node senses a busy channel, the count- As we will see later, a Back2F node picks a random number, down is frozen, and revived only after the channel is idle. say 11, and transmits a short signal only on the 11th subcar- Whichever node completes the countdown first begins trans- rier. The node’s second antenna detects a strong signal on the mission.
Load more

Recommended publications
  • MAC Protocols for IEEE 802.11Ax: Avoiding Collisions on Dense Networks Rafael A
    MAC Protocols for IEEE 802.11ax: Avoiding Collisions on Dense Networks Rafael A. da Silva1 and Michele Nogueira1;2 1Department of Informatics, Federal University of Parana,´ Curitiba, PR, Brazil 2Electrical and Computer Engineering, Carnegie Mellon University, Pittsburgh, PA, USA E-mails: [email protected] and [email protected] Wireless networks have become the main form of Internet access. Statistics show that the global mobile Internet penetration should exceed 70% until 2019. Wi-Fi is an important player in this change. Founded on IEEE 802.11, this technology has a crucial impact in how we share broadband access both in domestic and corporate networks. However, recent works have indicated performance issues in Wi-Fi networks, mainly when they have been deployed without planning and under high user density. Hence, different collision avoidance techniques and Medium Access Control protocols have been designed in order to improve Wi-Fi performance. Analyzing the collision problem, this work strengthens the claims found in the literature about the low Wi-Fi performance under dense scenarios. Then, in particular, this article overviews the MAC protocols used in the IEEE 802.11 standard and discusses solutions to mitigate collisions. Finally, it contributes presenting future trends arXiv:1611.06609v1 [cs.NI] 20 Nov 2016 in MAC protocols. This assists in foreseeing expected improvements for the next generation of Wi-Fi devices. I. INTRODUCTION Statistics show that wireless networks have become the main form of Internet access with an expected global mobile Internet penetration exceeding 70% until 2019 [1]. Wi-Fi Notice: This work has been submitted to the IEEE for possible publication.
    [Show full text]
  • Collision Detection
    Computer Networking Michaelmas/Lent Term M/W/F 11:00-12:00 LT1 in Gates Building Slide Set 3 Evangelia Kalyvianaki [email protected] 2017-2018 1 Topic 3: The Data Link Layer Our goals: • understand principles behind data link layer services: (these are methods & mechanisms in your networking toolbox) – error detection, correction – sharing a broadcast channel: multiple access – link layer addressing – reliable data transfer, flow control • instantiation and implementation of various link layer technologies – Wired Ethernet (aka 802.3) – Wireless Ethernet (aka 802.11 WiFi) • Algorithms – Binary Exponential Backoff – Spanning Tree 2 Link Layer: Introduction Some terminology: • hosts and routers are nodes • communication channels that connect adjacent nodes along communication path are links – wired links – wireless links – LANs • layer-2 packet is a frame, encapsulates datagram data-link layer has responsibility of transferring datagram from one node to adjacent node over a link 3 Link Layer (Channel) Services • framing, physical addressing: – encapsulate datagram into frame, adding header, trailer – channel access if shared medium – “MAC” addresses used in frame headers to identify source, dest • different from IP address! • reliable delivery between adjacent nodes – we see some of this again in the Transport Topic – seldom used on low bit-error link (fiber, some twisted pair) – wireless links: high error rates 4 Link Layer (Channel) Services - 2 • flow control: – pacing between adjacent sending and receiving nodes • error control: – error
    [Show full text]
  • How Computers Share Data
    06 6086 ch03 4/6/04 2:51 PM Page 41 HOUR 3 Getting Data from Here to There: How Computers Share Data In this hour, we will first discuss four common logi- cal topologies, starting with the most common and ending with the most esoteric: . Ethernet . Token Ring . FDDI . ATM In the preceding hour, you read a brief definition of packet-switching and an expla- nation of why packet switching is so important to data networking. In this hour, you learn more about how networks pass data between computers. This process will be discussed from two separate vantage points: logical topologies, such as ethernet, token ring, and ATM; and network protocols, which we have not yet discussed. Why is packet switching so important? Recall that it enables multiple computers to send multiple messages down a single piece of wire, a technical choice that is both efficient and an elegant solution. Packet switching is intrinsic to computer network- ing—without packet switching, no network. In the first hour, you learned about the physical layouts of networks, such as star, bus, and ring technologies, which create the highways over which data travels. In the next hour, you learn about these topologies in more depth. But before we get to them, you have to know the rules of the road that determine how data travels over a network. In this hour, then, we’ll review logical topologies. 06 6086 ch03 4/6/04 2:51 PM Page 42 42 Hour 3 Logical Topologies Before discussing topologies again, let’s revisit the definition of a topology.
    [Show full text]
  • Listen Before You Talk but on the Frequency Domain
    1 Listen Before You Talk But On The Frequency Domain Souvik Sen Romit Roy Choudhury Srihari Nelakuditi Duke University Duke University University of South Carolina Abstract Access control strategies are designed to arbitrate how multiple entities access a shared resource. Several dis- tributed protocols embrace randomization to achieve arbitration. In WiFi networks, for example, each participating node picks a random number from a specified range and begins counting down. The device that reaches zero first wins the contention and initiates transmission. This core idea – called backoff – is known to be inherently wasteful because the channel must remain idle while all contending nodes are simultaneously counting down. This wastage has almost been accepted as a price for decentralization. In this paper, we ask whether the entire backoff operation can be migrated to the frequency domain, thereby eliminating a long-standing source of channel inefficiency. Our core idea draws on OFDM subcarriers in modern WiFi radios, treating each subcarrier as an integer number. By transmitting a signal on a random subcarrier frequency, and using a second antenna to listen for all active frequencies, each node may be able to detect its rank among all contenders. Since signaling on subcarriers is almost instantaneous, the wastage from backoff can become negligible. We design such an unconventional backoff scheme called Back2F, implement it on a software- radio testbed, and demonstrate its feasibility with real-world experiments. A natural next step would be to revisit today’s protocols, and ask what other operations may be similarly migrated to the frequency domain. I. PROBLEM &MOTIVATION Accessing a shared resource in a decentralized manner has been studied for several decades.
    [Show full text]
  • Ethernet Basics Ethernet Basics
    2016-09-24 Ethernet Basics based on Chapter 4 of CompTIA Network+ Exam Guide, 4th ed., Mike Meyers Ethernet Basics • History • Ethernet Frames • CSMA/CD • Obsolete versions • 10Mbps versions • Segments • Spanning Tree Protocol 1 2016-09-24 Ethernet – Early History • 1970: ALOHAnet, first wireless packet-switched network - Norman Abramson, Univ. of Hawaii - Basis for Ethernet’s CSMA/CD protocol - 1972: first external network connected to ARPANET • 1973: Ethernet prototype developed at Xerox PARC - (Palo Alto Research Center) - 2.94 Mbps initially • 1976: "Ethernet: Distributed Packet Switching for Local Computer Networks" published in Communications of the ACM. - Bob Metcalfe and David Boggs - sometimes considered “the beginning of Ethernet” Ethernet goes Mainstream • 1979: DEC, Intel, Xerox collaborate on a commercial Ethernet specification - Ethernet II, a.k.a. “DIX” Ethernet - (Digital Equipment Corporation) • 1983: IEEE 802.3 specification formally approved - Differs from Ethernet II in the interpretation of the third header field • 1987: alternatives to coaxial cables - IEEE 802.3d: FOIRL, Fiber Optic Inter-Repeater Link - IEEE 802.3e: 1 Mbps over Twisted Pair wires (whoopee!) • 1990: Twisted-Pair wiring takes over - IEEE 802.3i: 10 Mbps over Twisted-Pair – 10Base-TX, 10Base-T4 2 2016-09-24 the Future is Now (next chapter) (and Now is so Yesteryear…) 1995 – Now: speed and cabling improvements • 1995: 100Mbps varieties • 1999: 1Gbps on twisted-pair • 2003-2006: 10Gbps on optical fiber and UTP • 2010: 40Gbps, 100Gbps (802.3ba) - optical fiber or twinaxial cable - point-to-point physical topology; for backbones • 2016, September: 2.5GBase-T, 5GBase-T ? - who knows? What Is Ethernet? • Protocols, standards for Local Area Networks » Ethernet II, IEEE 802.3 • Specifies Physical-layer components - Cabling, signaling properties, etc.
    [Show full text]
  • The Nominal Capacity of Wireless Mesh Networks Jangeun Jun and Mihail L
    MERGING IP AND WIRELESS NETWORKS THE NOMINAL CAPACITY OF WIRELESS MESH NETWORKS JANGEUN JUN AND MIHAIL L. SICHITIU, NORTH CAROLINA STATE UNIVERSITY broadband Internet access using seven gate- GW ABSTRACT ways (the larger red nodes) connected to the 1 Wireless mesh networks are an alternative Internet. In WMNs, each user node operates technology for last-mile broadband Internet not only as a host but also as a wireless router, access. In WMNs, similar to ad hoc networks, forwarding packets on behalf of other nodes 2 each user node operates not only as a host but that may not be within direct wireless transmis- also as a router; user packets are forwarded to sion range of a gateway. The gateways are con- 4 and from an Internet-connected gateway in nected to the Internet (the backhaul multihop fashion. The meshed topology pro- connection itself may also be wireless). The vides good reliability, market coverage, and network is dynamically self-organizing and self- 7 scalability, as well as low upfront investments. configuring, with the nodes in the network 5 Despite the recent startup surge in WMNs, automatically establishing and maintaining much research remains to be done before routes among themselves. Users can be station- Wireless mesh WMNs realize their full potential. This article ary or comparatively mobile [3]. The main dif- tackles the problem of determining the exact ference between a WMN and an ad hoc networks (WMNs) capacity of a WMN. The key concept we intro- network is perhaps the traffic pattern: in duce to enable this calculation is the bottleneck WMNs, practically all traffic is either to or are an alternative collision domain, defined as the geographical from a gateway, while in ad hoc networks the area of the network that bounds from above the traffic flows between arbitrary pairs of nodes.
    [Show full text]
  • MAC Protocols
    CN2 ÉCOLE POLYTECHNIQUE FÉDÉRALE DE LAUSANNE The MAC layer Jean-Yves Le Boudec Fall 2007 1 Contents 1. MAC as Shared Medium 2. MAC as interconnection at small scale 3. MAC and Link layer 2 © Jean-Yves Le Boudec, EPFL CN2 1: Shared Medium Access Why did engineers invent the MAC layer ? Share a cable (Ethernet, Token Ring, 1980s) Use a wireless radio link (GSM, WiFi, WiMax, etc) What is the problem ? If several systems talk together, how can we decode ? Solution 1: joint decoding (decode everyone) used in CDMA cellular networks – complex Solution 2: mutual exclusion protocol Only one system can talk at a time 3 What is the goal of the MAC layer ? MAC = medium access control What does it do ? Implement a mutual exclusion protocol Without any central system – fully distributed How does it work ? there are many solutions, called: Token Passing, Aloha, CSMA, CMSA/CA, CSMA/CD, RMAC, etc We describe here the most basic one, called CSMA/CA, used on Ethernet cables Radio links (WiFi) is similar, with a few complications – see course on mobility for a very detailed explanation 4 © Jean-Yves Le Boudec, EPFL CN2 CSMA/CA derives from Aloha Aloha is the basis of all non-deterministic access methods. The Aloha protocol was originally developped for communications between islands (University of Hawaï) that use radio channels at low bit rates. The Aloha protocol requires acknowledgements and timers. Collisions occur when two packet transmissions overlap, and if a packet is lost, then source has to retransmit; the retransmission strategy is not specified here; many possibilities exist.
    [Show full text]
  • COMP/ELEC 429/556 Introduction to Computer Networks
    COMP/ELEC 429/556 Introduction to Computer Networks Broadcast network access control Some slides used with permissions from Edward W. Knightly, T. S. Eugene Ng, Ion Stoica, Hui Zhang T. S. Eugene Ng eugeneng at cs.rice.edu Rice University 1 Let’s Begin with the Most Primitive Network T. S. Eugene Ng eugeneng at cs.rice.edu Rice University 2 500 meters Good Old 10BASE5 Ethernet (1976) T. S. Eugene Ng eugeneng at cs.rice.edu Rice University 3 200 meters Then Came 10BASE2 Ethernet (1980s) T. S. Eugene Ng eugeneng at cs.rice.edu Rice University 4 Twisted pair, 100 meters Then Came 10BASE-T Ethernet (1990) More: 100BASE-TX, 1000BASE-T At >= 10Gbit/s, broadcast no longer supported. T. S. Eugene Ng eugeneng at cs.rice.edu Rice University 5 Overview • Ethernet (< 10Gbps) and Wi-Fi are both “multi- access” technologies – Broadcast medium, shared by many nodes/hosts • Simultaneous transmissions will result in collisions • Media Access Control (MAC) protocol required – Rules on how to share medium T. S. Eugene Ng eugeneng at cs.rice.edu Rice University 6 Media Access Control Strategies • Channel partitioning – Divide channel into smaller “pieces” (e.g., time slots, frequencies) – Allocate a piece to each host For exclusive use – E.g. Time-Division-Multi-Access (TDMA) cellular network • Taking-turns – Tightly coordinate shared access to avoid collisions – E.g. Token ring network • Contention – Allow collisions – “Recover” From collisions – E.g. Ethernet, Wi-Fi T. S. Eugene Ng eugeneng at cs.rice.edu Rice University 7 Contention Media Access Control Goals • To share medium – If two hosts send at the same time, collision results in no packet being received – Thus, want to have only one host to send at a time • Want high network utilization • Want simple distributed algorithm • Take a walk through history – ALOHAnet 1971 – Ethernet 1973 – Wi-Fi 1997 T.
    [Show full text]
  • A Practical Approach for Achieving Scheduled Wifi in a Single Collision Domain ∗
    Look Who’s Talking: A Practical Approach for Achieving Scheduled WiFi in a Single Collision Domain ∗ Chao-Fang Shih, Yubing Jian, and Raghupathy Sivakumar Georgia Institute of Technology {cshih,yubing,siva}@ece.gatech.edu ABSTRACT a set of parameters including the maximum contention win- We ask the following question in this paper: Can the goals dow that are adaptively adjusted based on local information. of centralized WiFi scheduling be achieved using purely dis- While the approach is simple and scalable, the goal of DCF tributed operations? We present a solution called Look Who’s is to achieve coarse-level fairness and efficiency in the net- Talking (LWT) that allows for arbitrary schedules to be dis- work. Any finer-level goals are beyond the scope of DCF. tributed to nodes in a WiFi network. The nodes in the net- The IEEE 802.11 point coordination function (PCF) mode work then use purely local and distributed operations to achieve on the other hand relies on centralized scheduling by the the prescribed schedule. The scope of LWT in this paper access-point (AP) [1]. Theoretically, the scheduling algo- is restricted to a single collision domain (single or multiple rithm at the AP can be arbitrarily defined. The problems cells), but we discuss how LWT can be extended to multi- with PCF are two-fold: i) it uses a polling process that incurs ple collision domains. We use both experimental evaluations heavy overheads, especially in dynamic load conditions, and (using a WARP-based testbed) and simulation-based analy- ii) the standard does not specify how APs should coordinate sis (using ns3) to evaluate LWT.
    [Show full text]
  • Chapter 6 Medium Access Control Protocols and Local Area Networks
    Chapter 6 Medium Access Control Protocols and Local Area Networks Part I: Medium Access Control Part II: Local Area Networks Chapter Overview l Broadcast Networks l Medium Access Control l All information sent to all l To coordinate access to users shared medium l No routing l Data link layer since direct l Shared media transfer of frames l Radio l Local Area Networks l Cellular telephony l High-speed, low-cost l Wireless LANs communications between co-located computers l Copper & Optical l Typically based on l Ethernet LANs broadcast networks l Cable Modem Access l Simple & cheap l Limited number of users Chapter 6 Medium Access Control Protocols and Local Area Networks Part I: Medium Access Control Multiple Access Communications Random Access Scheduling Channelization Delay Performance Chapter 6 Medium Access Control Protocols and Local Area Networks Part II: Local Area Networks Overview of LANs Ethernet Token Ring and FDDI 802.11 Wireless LAN LAN Bridges Chapter 6 Medium Access Control Protocols and Local Area Networks Multiple Access Communications Multiple Access Communications l Shared media basis for broadcast networks l Inexpensive: radio over air; copper or coaxial cable l M users communicate by broadcasting into medium l Key issue: How to share the medium? 3 2 4 1 Shared multiple access medium M 5 … Approaches to Media Sharing Medium sharing techniques Static Dynamic medium channelization access control l Partition medium Scheduling Random access l Dedicated allocation to users l Polling: take turns l Loose l Satellite l Request
    [Show full text]
  • A Novel Channel Access Scheme for Full-Duplex Wireless Networks Based on Contention in Time and Frequency Domains
    1 RCFD: A Novel Channel Access Scheme for Full-Duplex Wireless Networks Based on Contention in Time and Frequency Domains Michele Luvisotto, Student Member, IEEE, Alireza Sadeghi Student Member, IEEE, Farshad Lahouti Senior Member, IEEE, Stefano Vitturi, Senior Member, IEEE, Michele Zorzi, Fellow, IEEE Abstract—In the last years, the advancements in signal processing and integrated circuits technology allowed several research groups to develop working prototypes of in–band full–duplex wireless systems. The introduction of such a revolutionary concept is promising in terms of increasing network performance, but at the same time poses several new challenges, especially at the MAC layer. Consequently, innovative channel access strategies are needed to exploit the opportunities provided by full–duplex while dealing with the increased complexity derived from its adoption. In this direction, this paper proposes RTS/CTS in the Frequency Domain (RCFD), a MAC layer scheme for full–duplex ad hoc wireless networks, based on the idea of time–frequency channel contention. According to this approach, different OFDM subcarriers are used to coordinate how nodes access the shared medium. The proposed scheme leads to efficient transmission scheduling with the result of avoiding collisions and exploiting full–duplex opportunities. The considerable performance improvements with respect to standard and state–of–the–art MAC protocols for wireless networks are highlighted through both theoretical analysis and network simulations. Index Terms—Full–duplex wireless, time–frequency channel access, orthogonal frequency–division multiplexing (OFDM), medium access control (MAC), IEEE 802.11 F 1 Introduction Innovation in Medium Access Control (MAC) plays a cru- of a reduction in the achievable throughput.
    [Show full text]
  • MODULE IV Computer Network Technologies: Access, Wired and Wireless Lans, Extensions, Bridging, and Layer 2 Switching
    MODULE IV Computer Network Technologies: Access, Wired And Wireless LANs, Extensions, Bridging, And Layer 2 Switching Computer Networks and Internets -- Module 4 1 Spring, 2014 Copyright 2014. All rights reserved. Topics d Access technologies d Interconnection technologies d Local area network packets, frames, and topologies d Media access mechanisms and the IEEE MAC sub-layer d Wired LAN technologies (Ethernet and 802.3) d Wireless Networking Technologies d LAN Extensions d Switches and switched networks Computer Networks and Internets -- Module 4 2 Spring, 2014 Copyright 2014. All rights reserved. Access Technologies De®nition Of Access d Used in the ªlast mileº between a provider and a subscriber d Informally classi®ed as either narrowband or broadband d May not be the bottleneck d Many are asymmetric with higher data rate downstream provider's facility downstream subscriber's location upstream d Note: party that is downstream pays a fee for service Computer Networks and Internets -- Module 4 4 Spring, 2014 Copyright 2014. All rights reserved. Access Technology Types d Narrowband (less than 128 Kbps) ± Dialup ± Integrated Services Digital Network (ISDN) ± Is disappearing d Broadband (more than 128 Kbps) ± Digital Subscriber Line (DSL) ± Cable modems ± Wireless (e.g., Wi-Fi and 4G) Computer Networks and Internets -- Module 4 5 Spring, 2014 Copyright 2014. All rights reserved. Digital Subscriber Line (DSL) Technologies d Use frequency-division multiplexing to share local loop between data and POTS d Head-end equipment is DSL Access Multiplexor (DSLAM) d Asymmetric Digital Subscriber Line (ADSL) ± 255 downstream carrier frequencies, 31 upstream ± Maximum downstream data rate is 8.45 Mbps ± Adaptive selection of carrier frequencies POTS upstream downstream KHz 0 4 26 138 1100 Computer Networks and Internets -- Module 4 6 Spring, 2014 Copyright 2014.
    [Show full text]