Communication Networks 629. IEEE Standards and Developments

Transcription

1 Communication Networks IEEE Standards and Developments Prof. Jochen Seitz Overview Organization of a WLAN according to IEEE Current IEEE standards IEEE layers IEEE MAC MAC synchronization, power saving and roaming WLAN security concerns and options WLAN and ad hoc networks Communication Networks 629 Prof. Jochen Seitz 1

2 LAN WLAN in Infrastructure Mode STA 1 ESS BSS 1 Access Point BSS x LAN Portal Distribution System Access Point STA LAN STA 3 Station (STA) Device with access to the wireless medium and connectivity to the access point Basic Service Set (BSS) Group of devices working on the same radio frequency Access Point Device that allows communication between stations and integrates them into the distribution system Portal Gateway to some other network Distribution System Connection of different WLAN cells to build an Extended Service Set EES Communication Networks LAN WLAN in Ad-hoc Mode STA 1 BSS 1 STA 3 STA 2 BSS 2 STA 5 STA LAN Direct communication with limited range Station (STA): Device with access to the wireless medium Basic Service Set (BSS): Group of devices working on the same radio frequency (Independent Basic Service Set IBSS) Communication Networks 631 Prof. Jochen Seitz 2

3 Hidden Node Problem A B C A would like to communicate with B C is already transmitting information to B using the same channel There are collisions in B Both A and C cannot detect these collisions Communication Networks 632 Exposed Node Problem A B C D B is already transmitting information to A C would like to communicate with D C finds the channel occupied and refrains from sending Free capacities are unused Communication Networks 633 Prof. Jochen Seitz 3

4 Mobile Terminal IEEE-Standard Fixed Terminal Server Application TCP IP MAC PHY MAC PHY Infrastructure Network Access Point Application MAC PHY TCP IP MAC PHY Communication Networks 634 IEEE Important Substandards n original standard from 1997 proposed amendment adding multiple-input multipleoutput data rate 1 or 2 MBit/s (MIMO) and many other newer features frequency range to GHz publication by the IEEE in September 2009 (ISM band) data rate at about 600 MBit/s a frequency range around 2,5 GHz or 5 GHz extended physical layer, published ac data rate 54 MBit/s Theoretical max. data rate 6.936Gbit/s with 8x8 MIMO frequency Range around 5 GHz Frequency: 5 GHz b p extended physical layer, published 1999 draft amendment to the IEEE standard to add wireless access in vehicular environments (WAVE) data Rate 11 MBit/s data exchange between high-speed vehicles and same frequency range as original between the vehicles and the roadside infrastructure in g the licensed ITS band of 5.9 GHz ( GHz) extended physical layer, published 2003 used as the groundwork for Dedicated Short Range data rate 54 MBit/s Communications (DSRC) frequency range to GHz approved p amendment was published in November 2010 Communication Networks 635 Prof. Jochen Seitz 4

5 Station Management Layers LLC Logical Link Control MAC Medium Access Control MAC Management PLCP Physical Layer Convergence Protocol PMD Physical Medium Dependent PHY Management Communication Networks Functions MAC Medium Access Segmentation/Reassembly Ciphering MAC Management Synchronization Roaming MIB Power Control PLCP Clear Channel Assessment Signal (Carrier Sense) PMD Modulation Coding PHY Management Channel Selection MIB Station Management Coordination of Management Functions Communication Networks 637 Prof. Jochen Seitz 5

6 images/80211fh.gif Radio Transmission Infrared (only for the first IEEE standard) nm, diffuse infrared operating at 1 Mbit/s, typically at a distance of max.10m carrier detection, energy detection and synchronization Frequency Hopping Spread Spectrum (FHSS) operating at 1 Mbit/s or 2 Mbit/s at least 2,5 frequency hops / s in the USA Direct Sequence Spread Spectrum (DSSS) originally DBPSK modulation for 1Mbit/s (Differential Binary Phase Shift Keying) and DQPSK for 2MBit/s (Differential Quadrature PSK) preamble of a frame is always transmitted at 1Mbit/s, remainder of the frame might be transmitted at higher speed chipping sequence: +1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1 (Barker Code) max. transmission power 1W (USA), 100mW (EU), min. 1mW Orthogonal Frequency Division Multiplex (OFDM) a frequency-division multiplexing (FDM) scheme utilized as a digital multi-carrier modulation method Communication Networks MAC Frame Format Communication Networks 639 Prof. Jochen Seitz 6

7 Fields in the IEEE MAC Frame Synch Preamble: for FH PHY 80 bits for DSSS PHY 128 bits alternating '0's and '1's SFD Start Frame Delimiter: 16 bits PLW PLCP-PDU Length Word: 12 bits indicating the number of bytes in the packet first portion of the PLCP header PLCP header is transmitted only at 1 Mbps! PSF PLCP Signaling Field: 4 bits to show the rate of the MAC payload transmission Bit 0 is reserved and is always '0' Bits 1 to 3 indicate the data rates Communication Networks MAC Distributed Foundation Wireless MAC (DFWMAC) Different traffic types Asynchronous data transmission (standard) Exchange of MAC frames without Quality of Service ( best-effort ) Broadcast and multicast Time-limited transmission (optional) Point Coordination Function (PCF) only in infrastructure mode Communication Networks 641 Prof. Jochen Seitz 7

8 MAC Procedures Distributed Coordination Function: Carrier Sense Multiple Access with Collision Avoidance (DFWMAC-DCF CSMA/CA) Collision avoidance based on arbitrary backoff algorithm Minimum time span between two MAC frames (so called inter frame spacing) Correct transmission signaled with ACK-frame (except Broadcast or Multicast) Distributed Coordination Function with Request to Send / Clear To Send Frames (DFWMAC-DCF with RTS/CTS) Avoidance of hidden node problem Point Coordination Function (DFWMAC-PCF) List-based polling done in the Access Point (standard) (optional) (optional) Communication Networks MAC: Inter Frame Spacing Implementation of Priorities No guarantees Shorter inter frame spacing allows earlier sending time for the frame: (Short Inter Frame Spacing) Highest priority, for ACK, CTS, response to polling PIFS (PCF IFS) Medium priority, for time limited services in PCF DIFS (DCF, Distributed Coordination Function IFS) Lowest priority, for asynchronous data transmission DIFS medium busy DIFS PIFS Communication Networks 643 competition direct access, if medium is free DIFS next frame t Prof. Jochen Seitz 8

9 CSMA/CA I DIFS medium busy waiting time DIFS PIFS Competition Window (arbitrary backoff time) next frame time slot Carrier sense based on clear channel assessment signal Station may send, if medium is free for the appropriate IFS If medium is busy, station sets backoff time to an arbitrary number of time slots After the medium is free again, station waits the appropriate IFS and the backoff time If medium gets busy during backoff time, backoff time is frozen t Communication Networks CSMA/CA II DIFS DIFS DIFS DIFS bo e bo r bo e bo r bo e busy Station 1 bo e busy Station 2 Station 3 busy bo e busy bo e bo r Station 4 bo e bo r bo e busy bo e bo r Station 5 t busy medium busy (frame, ack etc.) bo e elapsed backoff time data arrival at MAC-SAP bo r residual backoff time Communication Networks 645 Prof. Jochen Seitz 9

10 CSMA/CA III Sending unicast frames Frames can be sent after DIFS (plus backoff time as described before) Receiver responds immediately (after ), if frame has been correctly received (CRC) If an error occurs, the frame is automatically repeated Sender Receiver Further Stations DIFS Data Communication Networks 646 Ack DIFS waiting time competition Data t RTS / CTS Sending unicast frames using RTS/CTS Before transmitting a data frame, an RTS frame has to be sent including the duration of the data frame (after DIFS) Receiver acknowledges RTS frame with a CTS frame (after ) Sender may then send the data frame after, which is acknowledged as usual Other stations store the time the medium is busy (as contained in the RTS and CTS frames) Sender Receiver DIFS RTS CTS data ACK Further Stations NAV = Network Allocation Vector Communication Networks 647 NAV (RTS) NAV (CTS) waiting time DIFS competition data t Prof. Jochen Seitz 10

11 RTS / CTS: Fragmentation Sender Receiver DIFS RTS CTS frag 1 ACK 1 frag 2 ACK2 Further Stations NAV (RTS) NAV (CTS) NAV (frag 1 ) NAV (ACK 1 ) DIFS competition data t Communication Networks 648 DFWMAC-PCF I t 0 t 1 Super Frame Medium busy Point Coordinator PIFS D 1 D 2 Stations U 1 U 2 NAV of the stations NAV Point Coordination Function Communication Networks 649 Prof. Jochen Seitz 11

12 DFWMAC-PCF II t 2 t 3 t 4 Point Coordinator D 3 PIFS D 4 CFend Stations U 4 NAV of the stations NAV period without competition competition t Communication Networks 650 MAC Address Format Frame Type to DS from DS Address 1 Address 2 Address 3 Address 4 Ad-hoc Network 0 0 DA SA BSSID - Infrastructure Network, from AP 0 1 DA BSSID SA - Infrastructure Network, to AP 1 0 BSSID SA DA - Infrastructure Network, in DS 1 1 RA TA DA SA DS : Distribution System AP : Access Point DA : Destination Address SA : Source Address BSSID : Basic Service Set Identifier RA : Receiver Address TA : Transmitter Address Communication Networks 651 Prof. Jochen Seitz 12

13 MAC Management Synchronization Finding and staying in a WLAN Timer etc. Power Management Sleep modus without loosing a frame Periodically sleeping, buffering of frames, traffic map Association / Reassociation Associating with a distribution system Roaming, i.e. changing networks when changing access points Scanning, i.e. actively looking for a WLAN MIB - Management Information Base Administering, reading, writing Communication Networks 652 MAC Synchronization in Infrastructure Mode beacon interval (20ms 1s) access point medium B B B B busy busy busy busy value of the timestamp B beacon frame t Communication Networks 653 Prof. Jochen Seitz 13

14 MAC Synchronization in Ad hoc Mode beacon interval station 1 B 1 B 1 station 2 B 2 B 2 medium busy busy busy busy t value of the timestamp B beacon frame random delay Communication Networks 654 Power Saving Idea: switch the transceiver off if not needed States of a station: sleep and awake Timing Synchronization Function (TSF) stations wake up at the same time Infrastructure Traffic Indication Map (TIM) list of unicast receivers transmitted by AP Delivery Traffic Indication Map (DTIM) list of broadcast/multicast receivers transmitted by AP Ad-hoc Ad-hoc Traffic Indication Map (ATIM) announcement of receivers by stations buffering frames more complicated - no central AP collision of ATIMs possible (scalability?) APSD (Automatic Power Save Delivery) new method in e replacing above scheme Communication Networks 655 Prof. Jochen Seitz 14

15 Power Saving in Infrastructure Mode TIM interval DTIM interval access point D B T T d D B medium busy busy busy busy station T TIM D DTIM p awake d t B broadcast/multicast p PS poll d data transmission to/from the station Communication Networks 656 Power Saving in Ad hoc Mode ATIM window beacon interval station B 1 A D B 1 1 station 2 B 2 B 2 a d B beacon frame random delay A transmit ATIM D transmit data t awake a acknowledge ATIM d acknowledge data Communication Networks 657 Prof. Jochen Seitz 15

16 Roaming No or bad connection? Then perform: Scanning scan the environment, i.e., listen into the medium for beacon signals or send probes into the medium and wait for an answer Reassociation Request station sends a request to one or several AP(s) Reassociation Response success: AP has answered, station can now participate failure: continue scanning AP accepts Reassociation Request signal the new station to the distribution system the distribution system updates its data base (i.e., location information) typically, the distribution system now informs the old AP so it can release resources Fast roaming r e.g. for vehicle-to-roadside networks Communication Networks 658 WLAN Security Concerns Anyone within the geographical network range of an open, unencrypted wireless network can 'sniff' the traffic, gain unauthorized access to internal network resources as well as to the Internet, and then possibly send spam or do other illegal actions using the wireless network's IP address. If router security is not activated or if the owner deactivates it for convenience, it creates a free hotspot. Modern operating systems such as Mac OS, or Microsoft Windows make it fairly easy to set up a PC as a wireless LAN 'base station' using Internet Connection Sharing, thus allowing all the PCs in the home to access the Internet via the 'base' PC. Communication Networks 659 Prof. Jochen Seitz 16

17 WLAN Security Options For closed networks (like home users and organizations) the most common way is to configure access restrictions in the access points. Encryption Checks on MAC address Disable ESSID broadcasting Wireless Intrusion Prevention Systems For commercial providers, hotspots, and large organizations, the preferred solution is often to have an open and unencrypted, but completely isolated wireless network. Captive portal which provides for payment and/or authorization Connect securely to a privileged network using VPN Finally, a general solution may be end-to-end encryption, with independent authentication on all resources that shouldn't be available to the public. Communication Networks 660 Access Control at the Access Point Level Allow access only from known, approved MAC addresses. No security against sniffing or spoofing MAC addresses Secret ESSID Not really secure. Wired Equivalent Privacy (WEP) U.S. FBI has demonstrated the ability to break WEP protection in only three minutes using tools available to the general public The Wi-Fi Protected Access (WPA and WPA2) security protocols Using a long enough random password (e.g. 14 random letters) or passphrase (e.g. 5 randomly chosen words) makes pre-shared key WPA virtually uncrackable. The second generation of the WPA security protocol (WPA2) is based on the final IEEE i amendment to the standard and is eligible for FIPS compliance (a U.S. government computer security standard used to accredit cryptographic modules). With all those encryption schemes, any client in the network that knows the keys can read all the traffic. Communication Networks 661 Prof. Jochen Seitz 17

18 Wi-Fi Protected Access Operating System Client Supplicant EAP Type Support Access point with support for EAP type a scheme of mutual authentication using either IEEE 802.1X/Extensible Authentication Protocol (EAP) authentication or pre-shared key (PSK) technology AAA server (RADIUS) with support for EAP type Communication Networks 662 Authentication Database Wi-Fi Protected Setup (WPS) Standard for easy and secure establishment of a wireless home network, created by the Wi-Fi Alliance and officially launched on January 8, To add a new device to the Network the user can have up to the following four choices: PIN Method (mandatory baseline model), in which a PIN (Personal Identification Number) has to be read from either a sticker on the new wireless client device (STA) or a display, if there is one, and entered at the "representant" of the network, either the wireless access point (AP) or a Registrar of the Network. PBC Method, in which the user simply has to push a button, either an actual or virtual one, on both the AP (or a Registrar of the Network) and the new wireless client device (STA). Support of this model is mandatory for APs and optional for STAs. NFC Method, in which the user simply has to bring the new STA close to the AP or Registrar of the Network to allow a Near Field Communication between the devices. NFC Forum compliant RFID tags can also be used. Support of this model is optional. USB Method, in which the user uses a USB stick to transfer data between the new STA and the AP or Registrar of the Network. Support of this model is optional. Communication Networks 663 Prof. Jochen Seitz 18

19 Restricted Access Networks IEEE 802.1x promises to enhance security on both wired and wireless networks. Wireless access points that incorporate technologies like these often also have routers built in, thus becoming wireless gateways. Communication Networks 664 End-to-End Encryption For protecting valuable data like passwords and personal s: Encryption and authorization in the application layer, using technologies like SSL, SSH, GnuPG, PGP and similar. The disadvantage with the end to end method is, it may fail to cover all traffic: With encryption on the router level or VPN, a single switch encrypts all traffic, even UDP and DNS lookups. With end-to-end encryption on the other hand, each service to be secured must have its encryption "turned on," and often every connection must also be "turned on" separately. For sending s, every recipient must support the encryption method, and must exchange keys correctly. For Web, not all web sites offer https, and even if they do, the browser sends out IP addresses in clear text. Communication Networks 665 Prof. Jochen Seitz 19

20 Ad hoc Networks (I)... a collection of wireless nodes, all of which may be mobile, dynamically create a wireless network among themselves without using any such infrastructure or administrative support. IEEE Feb S Communication Networks 666 Ad hoc Networks (II) A B C D E F G H Communication Networks 667 Prof. Jochen Seitz 20

21 Examples of Ad hoc Networking Technologies Infrastructure Networks Wireless Networks HIPERLAN 2/3/4 (WATM) Ad hoc Networks GSM UMTS GPRS... HIPERLAN 1 IEEE Bluetooth Communication Networks 668 IEEE s a draft IEEE amendment for mesh networking, defining how wireless devices can interconnect to create a WLAN mesh network, which may be used for static topologies and ad-hoc networks broadcast/multicast and unicast delivery using "radioaware metrics over self-configuring multi-hop topologies default mandatory routing protocol Hybrid Wireless Mesh Protocol (HWMP), inspired by a combination of AODV (RFC 3561) and tree-based routing One Laptop per Child project uses the s draft standard for its OLPC XO laptop and OLPC XS school server networking Communication Networks 669 Prof. Jochen Seitz 21

22 Routing in Ad hoc Networks (I) No loops Stability despite frequent changes in topology Unique addressing of member nodes Load distribution Security Little overhead Little energy consumption Handover (even with regards to quality of service) Localization of services Provision of mobile services and multicast abilities Transparent access to legacy networks / Internet Communication Networks 670 Routing in Ad hoc Networks (II) + compliance with all condition = = eierlegende Wollmilchsau (egg-laying wool-milk-sow) Conflicting goals: Functionality Load Energy Efficiency Thus: different routing protocols for different goals Equal load distribution Minimum number of hops Quality of service (bit rate, response time, etc.) Security Costs Communication Networks 671 Prof. Jochen Seitz 22

23 Table-Driven Routing Based on distance vector respectively link state algorithm Proactively keeping routing information in routing table Demands frequent updates Time between updates too long obsolete routing information if nodes are very mobile Time between updates too short high (unnecessary) traffic load Communication Networks 672 Source-initiated On-demand Driven Routing Reactively determining the route when it is needed For ongoing communication the route has to be kept actual Longer time for initial communication message First message will be flooded higher network load Communication Networks 673 Prof. Jochen Seitz 23

24 Different Ad hoc Routing Protocols WRP DSDV CGSR AODV (Wireless Routing Protocol) (Destination-Sequenced Distance-Vector Routing) (Clusterhead Gateway Switch Routing) (Ad-hoc On-Demand Distance Vector Routing) DSR LMR TORA ABR SSR (Dynamic Source Routing) (Lightweight Mobile Routing) (Temporally-Ordered Routing) (Associativity-Based Routing) (Signal Stability Routing) Communication Networks 674 Features of Different Ad hoc Routing Protocols Protocol Finding of Route QoS Avoidance of Loops Communication Networks 675 Flat or Hierarchical DSDV proactive * flat Support of Multicast CGSR proactive * hierarchical (cluster) With Extension WRP proactive * flat AODV reactive * flat * DSR reactive * flat TORA reactive * flat With Extension ABR reactive * * flat SSR reactive * flat CEDAR reactive * * hierarchical (backbone) Spine Routing reactive * hierarchical (virt. backbone) * RDMAR reactive * flat Prof. Jochen Seitz 24

25 Example Ad-hoc On-Demand Distance Vector Routing RFC 3561 (July 2003) Route discovery Route Request (RREQ) Route Reply (RREP) Route Maintenance Data route update Route Error (RERR) WiFi-Alliance: Deploying Wi-Fi Protected Access (WPA ) and WPA2 in the Enterprise, March Communication Networks 676 Ad hoc Networks and QoS (I) Guarantee of QoS even when topology keeps changing all the time! Normal procedure: Find route with enough resources Reserve the required resources Keep on controlling the achieved QoS For ad hoc networks: Limited range and energy Restricted availability of channels / bit rate (shared medium) Unforeseeable radio problems Vertical and horizontal handover Communication Networks 677 Prof. Jochen Seitz 25

26 Ad hoc Networks and QoS (II) Node cannot supply the required QoS Communication Networks 678 Ad hoc Networks and QoS (III) Trying to guarantee QoS through redundancy If QoS cannot be supplied: best effort transmission or communication breakdown Parallel Routes Simultaneous transmission Backup routes established and reserved Backup routes selected Communication Networks 679 Prof. Jochen Seitz 26

27 Interoperability with Other Networks One of the nodes in the ad hoc network allows access to some other network Problem: different network characteristics Addresses Capacity / QoS Routing and Signaling Example: WLAN-based ad hoc network Access Point Internet Communication Networks 680 Multimedia Transmission in an Ad hoc Network Idea: Transmitting a video stream in an ad hoc network based on AODV Problem: Communication Networks 681 Prof. Jochen Seitz 27

28 Summary on Ad hoc Networks Currently much research in this area Still not very well accepted Security? Benefits? Standards? Well suited to enhance an infrastructure network Communication Networks 682 Material Wikipedia Prof. Jochen Schiller: Mobile Communications Book available in many languages, e.g. German, English, Russian, Chinese Slides (PDF and PPT) can be found at Communication Networks 683 Prof. Jochen Seitz 28