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S-72.1130 Telecommunication Systems

S-72.1130 Telecommunication Systems. Wireless Local Area Networks. Outline. LAN basics Structure/properties of LANs WLANs Link layer services Media access layer frames and headers CSMA/CA Physical layer frames modulation Frequency hopping Direct sequence Infrared Installation

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S-72.1130 Telecommunication Systems

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  1. S-72.1130 TelecommunicationSystems Wireless Local Area Networks

  2. Outline • LAN basics • Structure/properties of LANs • WLANs • Link layer services • Media access layer • frames and headers • CSMA/CA • Physical layer • frames • modulation • Frequency hopping • Direct sequence • Infrared • Installation • Security

  3. S-72.1130 TelecommunicationSystems LAN Basics

  4. What is a LAN? Local area means: • Freedom from regulatory constraints at ISM Band (Industrial, Science and Medical) • Short distance (~1km) between computers • Low cost • High-speed (10 Mb/s.. 10 Gb/s); support for TCP or UDP type of communications • Flexible error control: in MAC and in upper levels • Computers move, machines have unique MAC address • MAC protocol takes care of optimizing throughput for the expected services • Physical level takes care of physical transmission of packets over a medium

  5. 3 2 4 1 Shared multiple access medium 5 M  Multiple Access Communications • Shared media basis for broadcast networks • Inexpensive: radio over air; copper or coaxial cable • M users communicate by broadcasting into medium • Key issue: How to share the medium?

  6. Approaches to Media Sharing Medium sharing techniques Static channelization Dynamic medium access control • Partition medium • Dedicated allocation to users • Satellite transmission • Cellular Telephone Scheduling Random access • Polling: take turns • Request for slot in transmission schedule • Token ring • Wireless LANs • Loose coordination • Send, wait, retry if necessary • Aloha • Ethernet

  7. Bus Network • In a bus network, one node’s transmission traverses the entire network and is received and examined by every node. The access method can be : • (1) Contention scheme : multiple nodes attempt to access bus; only one node succeed at a time (e.g. CSMA/CD in Ethernet 802.3) • (2) Round robin scheme : a token is passed between nodes; node holding the token can use the bus (e.g.Token bus 802.4) • Advantages: • (1) Simple access method • (2) Easy to add or remove stations • Disadvantages: • (1) Poor efficiency with high network load • (2) Relatively insecure, due to the shared medium C D A B D term term - Line coded, serial data - twisted pair or coaxial cable term: terminator impedance

  8. RAM RAM Typical Wired LAN • Transmission Medium • Network Interface Card (NIC) • Unique MAC “physical” address Serial format Ethernet Processor ROM NIC implements MAC protocol & physical port. Parallel interface to PC Reference: A. Leon-Garcia, I. Widjaja, Communication Networks , Instructor's Slide Set

  9. hub stations hub stations hub stations hub router server IEEE 802-series of LAN Standards • 802 standards free to download from http://standards.ieee.org/getieee802/ WiMAX Demand priority: A round-robin (token ring) arbitration method to provide LAN access based on message priority level DQDB: Distributed queue dual buss, a ring network

  10. S-72.1130 TelecommunicationSystems IEEE 802 LAN Standard

  11. The IEEE 802 LAN Standards (http://www.ieee802.org/) OSI Layer 3 Network IEEE 802.2 Logical Link Control (LLC) LLC OSI Layer 2 (data link) IEEE 802.11 Wireless IEEE 802.3 Carrier Sense IEEE 802.4 Token Bus IEEE 802.5 Token Ring b: Wi-Fi MAC a b g Ethernet Physical Layers - options: twisted pair, coaxial, optical, radio paths; (not for all MACs above!) OSI Layer 1 (physical) Bus (802.3…) Star (802.3u…) Ring (802.5…)

  12. IEEE 802 Layers Logical Link Control (LLC) Sublayer • Utilizes services of HDLC (High-level Data Link Control) • Therefore, LLC SAPs separate upper layer data exchanges => NIC applies different buffer segments for each SAP (port) • LLC provides means to exchange frames between LANs using different MACs Medium Access Control Sublayer • Coordinates access to medium • Connectionless/Connection oriented frame transfer service • Machines identified by MAC/physical address (in NIC) • Broadcasts frames with MAC addresses • Examples: CSMA/CD, CSMA/CA IEEE 802.2 Logical Link Control (LLC) LLC b: Wi-Fi IEEE 802.11 Wireless IEEE 802.3 Carrier Sense IEEE 802.4 Token Bus IEEE 802.5 Token Ring MAC Ethernet a b g Physical layers PHY Physical level • Star, bus or ring topology • Cabling and electrical interfaces • Twisted pair, coaxial, fiber • Line coding (wired LANs) or modulation (WLANs) (More of HDLC in supplementary…)

  13. S-72.1130 TelecommunicationSystems IEEE 802 LAN Standard: Logical Link Layer (LLC)

  14. Logical Link Control Layer (LLC) • Specified by ISO/IEC 8802-2 (ANSI/IEEE 802.2) • Objective: exchange data between users across LAN using 802-based MAC controlled link • Provides addressing and data link control (routing) • Independent of topology, medium, and chosen MAC access method Data to higher level protocols Info: carries user data Supervisory: carries flow/error control Unnumbered: carries protocol control data Source SAP LLC’s Protocol Data Unit (PDU) (SAP: Service Access Point)

  15. HDLC Frame types • Information frames, or I-frames, transport user data from the network layer. In addition they can also include flow and error control information piggybacked on data. • Supervisory Frames, or S-frames, are used for flow and error control whenever piggybacking is impossible or inappropriate, such as when a station does not have data to send. S-frames do not have information fields. • Unnumbered frames, or U-frames, are used for various miscellaneous purposes, including link management. Some U-frames contain an information field, depending on the type. http://en.wikipedia.org/wiki/High-Level_ Data_Link_Control#I-Frames_.28user_data.29

  16. LLC Services • A Unacknowledged connectionless service • no error or flow control - no ack-signal usage • unicast (individual), multicast, broadcast addressing • higher levels take care or reliability - thus fast for instance for TCP • B Connection oriented service • supports unicast only • error and flow control for lost/damaged data packets by cyclic redundancy check (CRC) • Asynchronous balanced mode of HDLC: error control, sequencing, flow control • Phases: Connection setup, data exchange, and release • C Acknowledged connectionless service • Problem: A workstation has a single, physical MAC address, how to separate network or higher level service access? Ans: HDLC SAP addressing: • Can handle several logical connections, distinguished by their SAP (service access points, next slides). • ack-signal used • error and flow control by stop-and-wait ARQ • faster setup than for B HDLC : High-level Data Link Control

  17. SAP Addressing IEE802.11 (CSMA/CA)... ATM... IEE802.11 (CDMA)... Reference: W. Stallings: Data and Computer Communications, 7th ed

  18. HTTP Request HTTP Request HTTP Request HTTP Request TCP header TCP header TCP header IP header IP header Ethernet header FCS remember encapsulation…. TCP Header contains source & destination port numbers IP Header contains source and destination IP addresses; transport protocol type data link Ethernet Header contains source & destination MAC addresses; network protocol type Traffic to the target BSS / ESS PHY layer transmits packet using a modulation method (DSSS, OFDM, IR, FHSS) MAC IP port

  19. S-72.1130 TelecommunicationSystems IEEE 802 LAN Standard: Media Access Control (MAC) Layer

  20. Media Access Control: Ways to Share a Medium • Medium sharing required for multiple users to access the channel • Communications by • unicasting • multicasting • broadcasting Medium sharing techniques Static channelization Dynamic medium access control • FDMA,TDMA, CDMA • Uses partition medium • Dedicated allocation to users • Examples: • Satellite transmission • Cellular Telephone Scheduling Random access (contention) • Polling (take turns): Token ring 802.5 • Reservation systems: Request for slot in transmission schedule 802.4 • Loose coordination • Send, wait, retry if necessary • Aloha • CSMA/CD (Ethernet) • CSMA/CA (802.11 WLAN)

  21. Selecting a Medium Access Control • Environment: Wired / Wireless? • Applications: • What type of traffic? • Voice streams? Steady traffic, low delay/jitter • Data? Short messages? Web page downloads? • Enterprise or consumer market? Reliability, cost • Scale: • How much traffic can be carried? • How many users can be supported? • Examples: • Design MAC to provide wireless DSL-equivalent access for rural communities • Design MAC to provide Wireless-LAN-equivalent access to mobile users (user in car travelling at 130 km/hr)

  22. MAC Techniques in LANs • Contention • Medium is free for all • A node senses the free medium and occupies it as long as data packet requires it • Example: Ethernet (IEEE 802.3 CSMA/CD) • Reservation (short term statistical access) • Gives everybody a turn • Reservation time depends on token holding time (set by network operator) • For heavy loaded networks • Example: Token Ring/IEEE 802.5, Token Bus/IEEE 802.4, FDDI • Mixed • Flexible compromise: 802.11 WLANs • Reservation(long term) • Link reservation for multiple packets (whole session) • Example: scheduling a time slot: GSM using TDMA or FDMA (uplink/dowlink)

  23. Delay-Bandwidth Product • Delay-bandwidth product key parameter • Coordination in sharing medium involves using bandwidth (explicitly or implicitly) • Difficulty of coordination comparable to delay-bandwidth product • Simple two-station example • Station with frame to send listens to medium and transmits if medium found idle • Station monitors medium to detect collision and defers frame transmission if collision detection

  24. Two-stations MAC example

  25. B does not transmit before t = tprop & A captures the channel Case 1 A B Case 2 B transmits before t = tprop and detectscollision after receiving ack from A A B A detects collision at t = 2 tprop A B Two-Stations MAC … Two stations are trying to share a common medium Distance d meters tprop = d /  seconds A transmits at t = 0 A B

  26. Efficiency of Two-Station Example • Each frame transmission requires 2tprop of quiet time => number of bits wasted for access coordination: 2tpropR • R transmission bit rate • L bits/frame • Efficiency: Normalized Delay-Bandwidth Product Propagation delay Time to transmit a frame

  27. A bit of history of Ethernet • 1970 ALOHAnet radio network deployed in Hawaiian islands • 1973 Metcalf and Boggs invent Ethernet, random access in wired net • 1979 DIX (DEC + Intel + Xerox) Ethernet II Standard • 1985 IEEE 802.3 LAN Standard (10 Mbps) • 1995 Fast Ethernet (100 Mbps) • 1998 Gigabit Ethernet • 2002 10 Gigabit Ethernet • Ethernet is the dominant LAN standard Metcalf’s Sketch

  28. Ethernet MAC: CSMA/CD (802.3)

  29. 802.3 MAC of Ethernet (CSMA/CD) • CSMA/CD: 1. If the medium is idle, transmit; otherwise, go to step 2 2. If the medium is busy, continue listening (CS: carriersensing) until the channel is idle, then transmit immediately 3. If a collision is detected (CD) during transmission, transmit brief jamming signal to assure all stations know about collision and then cease transmission 4. After transmitting the jamming signal, wait a random time (back-off time), then attempt to transmit again

  30. Typical MAC Efficiencies Two-Station Example: • If a<<1, then efficiency close to 100% • As a approaches 1, the efficiency becomes low CSMA-CD (Ethernet) protocol:

  31. MAC protocol selection criteria summarized • Delay-bandwidth product • Efficiency • Transfer delay • Fairness • Reliability • Capability to carry different types of traffic • Quality of service • Cost

  32. application transport network link physical fiber physical layer copper (twister pair) physical layer 802.3 Ethernet Standards: Link & Physical Layers • many different Ethernet standards • common MAC protocol and frame format • different speeds: 2 Mbps, 10 Mbps, 100 Mbps, 1Gbps, 10G bps • different physical layer media: fiber, cable MAC protocol and frame format 100BASE-T2 100BASE-FX 100BASE-TX 100BASE-BX 100BASE-SX 100BASE-T4 Ref: Kurose, Ross: Computer Networking 5: DataLink Layer

  33. Throughput Performance of CSMA/CD r (Load) We can see that in Ethernet transfer delays grow very fast as the load approaches the maximum possible value for the given value of a (tprop: one-way delay, R: signaling rate, L: frame length) Reference: A. Leon-Garcia, I. Widjaja, Communication Networks, 2nd ed

  34. IEEE 802.3 MAC: Ethernet MAC Protocol: • CSMA/CD • Slot (frame) duration is the critical system parameter • determines delay – throughput tradeoff • upper bound on time to detect collision • upper bound on time to acquire channel • upper bound on length of frame segment generated by collision • effects retransmission scheduling • Truncated binary exponential back-off • for retransmission n: 0 < r < 2k, where k=min(n,10) • Give up after 16 retransmissions

  35. IEEE 802.3 Original Parameters • Transmission Rate: 10 Mbps • Min Frame: 512 bits = 64 bytes • min. frame duration*: 512 bits/10 Mbps = 51.2 msec • 51.2 msec x 2x105 km/sec =10.24 km, 1 way(= mini slot length) • Max Length: 2500 meters + 4 repeaters • Each x10 increase in bit rate, must be accompanied by x10 decrease in distance (norm. bandwidth-delay product remains thus constant) *mini slot product must remain constant!

  36. Server farm Server Server Server Gigabit Ethernet links Switch/router Switch/router Gigabit Ethernet links Ethernet switch Server Ethernet switch Ethernet switch Server Server 100 Mbps links 100 Mbps links 100 Mbps links Hub Hub Hub 10 Mbps links 10 Mbps links 10 Mbps links Department C Department B Department A Typical Ethernet Deployment

  37. S-72.1130 TelecommunicationSystems IEEE 802.11 Wireless Local Area Networks (WLANs)

  38. Why WLANs? • Mobility • Increases working efficiency and productivity • Roaming support: extended on-line times -> universal access & seamless services • No new wiring and installation on difficult-to-wire areas • Offices, public places, and homes • Factories, vehicles, roads, and railroads • Increased reliability - several networks & nodes secure links • However, AAA (Authentication, Authorization, Accounting) challenging • Reduced installation time • No cabling time • Easy setup

  39. WLAN Technology Challenges • High data rates • IEEE 802.11b supports rates up to 11 Mb/s (in practice 6 Mb/s), and 802.11g reaches up to 54 Mb/s, 802.11n ~ 100-200 Mb/s (600 Mb/s theoretical rate) • Interference • Working in ISM band means sharing the frequency bands with microwave ovens, and Bluetooth. Modulation and MAC design challenge • Security • Original WEP (Wired Equivalent Privacy) algorithm is weak – often not set ON by users, more efficient algorithms developed later as WPA 2 (Wifi Protected Access) • Roaming, especially with GSM and UMTS would be desired

  40. Requirements for 802.11 Wireless LAN Standard • Dynamic network management • Stations movable and may be operated while moved • addressing and association procedures • interconnections (roaming) • License free operation • Wireless channel is unreliable • error control • security/secrecy • Wireless channel is also the reason why access method for 802.11 is CSMA/CA and not CSMA/CD • Difficult to detect collisions in wireless environment • External interference, especially at ISM • Hidden terminal problem CSMA/CA: Carrier Sense Multiple Access/Collision Avoidance CSMA/CD: Carrier Sense Multiple Access/Collision Detection

  41. 802.11 WLAN Architecture Overview • LLC provides addressing and data link control – common to all 802 LANs • 802.11 MAC provides • Access to wireless medium • CSMA/CA (DCF) • Contention-free access (PCF) • Joining the network (NAV, addressing) • Services • Station service: Authentication, privacy, MSDU* delivery • Distributed system: Association**, participates to data distribution • Three physical layers (PHY) • FHSS: Frequency Hopping Spread Spectrum (SS) • DSSS: Direct Sequence SS • IR: Infrared transmission IEEE 802.2 Logical Link Control (LLC) LLC b: Wi-Fi IEEE 802.11 Wireless IEEE 802.3 Carrier Sense IEEE 802.4 Token Bus IEEE 802.5 Token Ring MAC Ethernet a b g Physical layers: DSSS, FHSS, IR ... PHY CSMA/CA: Carrier Sense Multiple Accesswith Collision Avoidance LLC: Logical Link Control Layer MAC: Medium Access Control Layer SS: Spread Spectrum FHSS: Frequency hopping SS DSSS: Direct sequence SS IR: Infrared light NAV: Network Allocation Vector SAP: Service Access Point DCF: Distributed Coordination Function PCF: Point Coordination Function *MSDU: MAC service data unit ** with an access point in ESS or BSS

  42. S-72.1130 TelecommunicationSystems IEEE 802.11 Wireless Local Area Networks (WLANs): Service Sets

  43. IEEE 802.11 Architecture • 802.11 networks can work in • Basic service set (BSS) • Extended service set (ESS) • BSS can also be used in ad-hocnetworking Network LLC 802.xx MAC FHSS PHY DSSS IR Propagation boundary Internet Distribution system Station B LLC: Logical Link Control Layer MAC: Medium Access Control Layer PHY: Physical Layer FHSS: Frequency hopping SS DSSS: Direct sequence SS SS: Spread spectrum IR: Infrared light BSS: Basic Service Set ESS: Extended Service Set Station A BSS 1 BSS 2 Basic (independent) service set (BSS) Extended service set (ESS) Access Point Portal: gateway access to other networks/Internet

  44. Basic and Extended Service Sets • Basic Service Set (BSS) – tens of meters • Operate in Basic Service Area (BSA) that is much like the area of cell in mobile communications • BSSs may geographically overlap, be physically disjoint, or they may be collocated (one BSS may use several antennas) • Ad-hoc or Infrastructure (nomadic) mode: Access coordinated by the given instance of MAC • Extended Service Set (ESS) • Multiple BSSs interconnected by Distribution System (DS) • Each BSS is like a cell and stations in BSS communicate with an Access Point (AP). • Portals attached to DS provide gateways to access Internet or other ESS

  45. Distribution system (DS) services • DS provides distribution services: • Transfer MAC SDUs between APs in ESS (I) • Transfer MSDUs between portals & BSSs in ESS (II) • Transfer MSDUs between stations in same BSS (III) • Multicast, broadcast, or stations’s preference • ESS looks like a single BSS to LLC layer Propagation boundary Internet II III III LLC: Logical Link Control Layer MAC: Medium Access Control Layer PHY: Physical Layer FHSS: Frequency hopping SS DSSS: Direct sequence SS SS: Spread spectrum IR: Infrared light BSS: Basic Service Set ESS: Extended Service Set MSDU: MAC Service Data Unit AP: Access Point Distribution system Station B IIIb Station A I BSS 1 BSS 2 Basic (independent) service set (BSS) Extended service set (ESS) (Infrastructure mode) Access Point Portal: gateway access to other networks/Internet

  46. B1 A1 Gateway to the Internet Portal Distribution System Server Portal AP1 A2 B2 AP2 BSS A BSS B Infrastructure Network (& ESS)

  47. IEEE 802.11 Mobility • Standard defines the following mobility types: • No-transition: no movement or moving within a local BSS • BSS-transition: station movies from one BSS in one ESS to another BSS within the same ESS • ESS-transition: station moves from a BSS in one ESS to a BSS in a different ESS (continuos roaming not supported) • Especially: 802.11 don’t support roaming with GSM or 3G! - Address to destination mapping - seamless integration of multiple BSS ESS 1 ESS 2

  48. S-72.1130 TelecommunicationSystems IEEE 802.11 Wireless Local Area Networks (WLANs): Media Access Protocol

  49. C A B (b) Data Frame B C Data Frame A C transmits data frame & collides with A at B Hidden Terminal Problem (a) Data Frame A transmits data frame C senses medium, station A is hidden from C RTS: Request to Send CTS: Clear to Send • New MAC: CSMA with Collision Avoidance Reference: A. Leon-Garcia, I. Widjaja, Communication Networks , Instructor's Slide Set

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