Network Design - Chapter 9: Wireless Technology in Access Networks - University of Pittsburgh

Nội dung text: Network Design - Chapter 9: Wireless Technology in Access Networks - University of Pittsburgh

1. Wireless Technology in Access Networks David Tipper Associate Professor Department of Information Science and Telecommunications University of Pittsburgh Slides 9 Wireless in Access Networks • Increasing use of wireless technology in access networks – IEEE 802.11 technology for Wireless LANS – WiMAX for last mile – Free Space Optical for short line of sight high bandwidth connections – Proprietary solutions for wireless multi- hop mesh networks (based on 802.11 or WiMAX) – Variety of Speeds,Cost, Coverage Range, etc. 2 Telcom 2110 Spring 06
2. Wireless Networks • Proliferation of wireless technology and increasing popularity • Why Wireless in Access Networks? – Provide Mobility • WLAN in a building, campus environment, hotel, hospital, etc. – Cost Advantages • Cheaper than wiring – Flexibility – ease of deployment • Increased data rates 3 Telcom 2110 Spring 06 Wireless Issues • Wireless link implications – communications channel is the air • poor quality: fading, shadowing, weather, etc. – regulated by governments • frequency allocated, licensing, etc. – limited bandwidth • Low bit rate, frequency planning and reuse, interference – power issues • Power levels regulated (safety issues), conserve mobile terminal battery life – security issues • wireless channel is a broadcast medium! • Focus on Wireless LANs first 4 Telcom 2110 Spring 06
3. Wireless LANs • Wireless LAN market – Medical: hospitals doctors and nurses have PDA’s – Education: many universities/colleges have campus wide network and require laptops for students – Manufacturing – factories, storage, etc – Retail/Small Business – Superstores, grocery stores, GAP, Walmart etc. – Public Access (Hotels, airports, coffee shops) • Large growth in this segment (T-Mobile has 2300 in U.S. coffee shops and bookstores, Wayport > 500 hotels, BT 5000 in U.K.) – Wireless ISPs in many cities and housing developments – Homes – mobility in and around house – Market over $10 billion in 2005 5 Telcom 2110 Spring 06 Wireless LANs • Wireless Local Area Networks – Support communication to mobile data users via wireless channel – Types of WLAN 1. Infrastructure based (most popular) Connect users to a wired infrastructure network Wireless access network like cellular phone system IEEE 802.11, a, b, g , etc. 2. Ad-Hoc based networks – Provide peer to peer communication – mobiles communicate between each other directly – Rapid Deployment (conference room) – Bluetooth, IEEE 802.11, a, b, g Proprietary 3. Point – to –Point (cable replacement!) 6 Telcom 2110 Spring 06
4. WLAN Topologies ad-hoc based architecture Point-to-point Infrastructure based architecture ESS BSS 1 BSS 2 WT 1 WT 2 WT 3 AP 1 AP 2 Wired-distribution network BSS = Basic Service Set WT 4 ESS = Extended Service Set AP 3 AP = Access Point WT 5 WT = Wireless Terminal BSS 3 Basic Service Area (BSA) 7 Telcom 2110 Spring 06 Communication link Spectrum for Wireless LANS • Licensed Vs. Unlicensed – Private yard Vs. Public park • Industrial Scientific and Medical bands – 902-928 MHz – 2.4 – 2.4835 GHz – 5.725 – 5.875 GHz • (Unlicensed - National Information Infrastructure Bands) U-NII bands (5-6 GHz) region – Three bands of 100 MHz each • Band 1: 5.15 - 5.25 GHz • Band 2: 5.25 - 5.35 GHz • Band 3: 5.725 - 5.825 GHz • 18-19 GHz licensed available in U.S. • 17 GHz, 40 GHz and 60 GHz under study 8 Telcom 2110 Spring 06
5. IEEE 802.11 Standard • The project was initiated in 1990 • The first complete standard was released in 1997 • Supports two topologies: Infrastructure and Ad hoc • Set of standards for MAC layer and below now studying higher levels especially security • Main standard IEEE 802.11, 802.11a, 802.11b, 802.11g • Common MAC layer for all sub-standards • Supports different physical layers at various data rates and frequencies – Diffused infrared (802.11) – Frequency hopping spread spectrum (802.11) – Direct sequence spread spectrum (802.11b) – Orthogonal Frequency Division Multiplexing (OFDM) (802.11a, g) 9 Telcom 2110 Spring 06 IEEE 802.11 Terminology • Access Point (AP) – Acts as a base station for the wireless LAN and is a bridge between the wirless and wired network • Basic Service Area (BSA) – The coverage area of one access point • Basic Service Set (BSS) – A set of stations controlled by one access point • Distribution system – The fixed (wired) infrastructure used to connect a set of BSS to create an extended service set (ESS) •Portal(s) – The logical point(s) at which non-802.11 packets enter an ESS 10 Telcom 2110 Spring 06
6. Infrastructure Network Topology • A wired infrastructure supports communications between mobile hosts (MHs) and between MHs and fixed hosts • Star topology – The BS or AP is the hub – Any communication from a MH to another has to be sent through the BS or AP – The AP manages user access to the network – APs typically mounted on wall or ceiling, AC power maybe a problem – Power over Ethernet option delivers AC power over UTP Ethernet cable • Designed for multiple APs interconnected to cover larger areas to form ESS 11 Telcom 2110 Spring 06 Infrastructure based Architecture Basic Service Set (BSS) Members of the cell covered by one AP Access Point (AP) Basic Service Area (BSA) a.k.a cell 12 Telcom 2110 Spring 06
7. Infrastructure-based Architecture Portal Extended Service Set (ESS) Distribution System AP1 AP AP3 2 Extended Service Area (ESA): Disjoint or connected 13 Telcom 2110 Spring 06 Ad hoc network topology • Independent Basic Service Set (IBSS) • Distributed topology • MHs communicate between each other directly (like walkie-talkies) • No need for a wired infrastructure • Suitable for rapid deployment • Use in conference rooms • No support for multi-hop ad hoc networking - non standard freeware and proprietary systems available that support multi-hop 14 Telcom 2110 Spring 06
8. IEEE standard 802.11 fixed terminal mobile terminal server infrastructure network access point application application TCP TCP IP IP LLC LLC LLC 802.11 MAC 802.11 MAC 802.3 MAC 802.3 MAC 802.11 PHY 802.11 PHY 802.3 PHY 802.3 PHY 15 Telcom 2110 Spring 06 The MAC Layer • IEEE 802.11 data link layer has two sublayers – Logical Link Layer • determined by wired network interface – Media Access Control (MAC) layer : • security, reliable data delivery, access control • provides coordination among MHs sharing radio channel • MAC Layer has two coordination techniques – Distributed Coordination Function (DCF) • based on CSMA/CA with randomized backoff • Asynchronous, best effort service • DCF with RTS/CTS (optional) avoids hidden terminal problem – Point Coordination Function (PCF) • Optional access mechanism • Provides “time bounded” service based on polling of MSs 17 Telcom 2110 Spring 06
9. Distributed Coordination Function (DCF) • Distributed Coordination Function (DCF) • CSMA/CD can’t be used – because can’t always detect collisions • Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) – MSs listens to channel to see if busy • if busy will backoff random time before checking again • If idle channel for duration of interframe spacing will trasmit – If a collision occurs, clients wait random amount of slot time after medium is clear before retransmitting • CSMA/CA also reduces collisions by using explicit packet acknowledgement (ACK) – Receiving client must send back to sending client an acknowledgement packet showing that packet arrived intact – If ACK frame is not received by sending client, data packet is transmitted again after random waiting time 19 Telcom 2110 Spring 06 Data Transmission And ACKs Medium is idle DIFS Medium is not idle Data Medium is idle DIFS SIFS Medium is not idle ACK MS AP 24 Telcom 2110 Spring 06
10. Busy Channel Contention Window DIFS DIFS Data MS1 Medium is idle Medium is still idle • Each MS has to still wait for a period of DIFS • Each MS chooses a random time of back-off within a contention window • Each MS decrements the back-off. Once the back-off value becomes zero, if the medium is idle, the MS can transmit • The MS with the smallest back-off time will get to transmit • All other MSs freeze their back-off timers that are “decremented” and start decrementing the timer in the next contention window from that point 25 Telcom 2110 Spring 06 When do collisions occur? • MSs have the same value of the back-off timer • MSs are not able to hear each other AP because of the “hidden terminal” effect Communication is • MSs are not able to not possible hear each other Signal is not sensed because of fading • Solution: RTS/CTS – Also avoids excessive collision time due to long packets 28 Telcom 2110 Spring 06
11. RTS/CTS Mechanism • RTS-Request to Send (20 bytes) DIFS • CTS-Clear to Send (14 bytes) RTS • They can be used only prior to transmitting data SIFS • After successful contention for the CTS channel, a MS can send an RTS to SIFS the AP • It gets a CTS in reply after SIFS Data • CTS is received by all MSs in the BSS SIFS • They defer to the addressed MS ACK while it transfers data • If there is a collision, no CTS is received and there is contention again MS AP 29 Telcom 2110 Spring 06 Point Coordination Function (PCF) • Optional capability to provide “time-bounded” services • It sits on top of DCF and needs DCF in order to successfully operate • A point coordinator (the AP) polls each station and enables them to transmit without contention – Ad hoc networks cannot use this function • Time (a super time slot) is divided into two parts – Contention Free Period (CFP) – Contention Period (CP) • A MS must be CFP-aware to access the CFP • Point coordination function IFS (PIFS) – Midlength IFS – Used by centralized controller in PCF scheme when polling MHs • Replies to polling can occur after SIFS 31 Telcom 2110 Spring 06
12. PCF Continued Busy Medium SIFS SIFS SIFS SIFS SIFS SIFS D1 + P1 D2 + P2 D3 + P3 D4 + P4 AP MS1 MS2 MS3 MS4 Data Data Data Data Data Data Data + + + + + + + PIFSPoll ACK ACK ACK ACK PIFS Poll ACK + + Poll Poll 32 Telcom 2110 Spring 06 802.11 Security • Authentication – Establishes identity of mobile stations to APS and vice a versa – Most 802.11 networks don’t use any type of authentication! • APs accept connections from all MSs – Open system authentication • Exchange of identities using Service Set Identifier (SSID) of network • SSID can be advertised by AP or entered manually into mobiles – Shared Key authentication • Uses a version of challenge/response protocol • Either 40 or 104 bit shared key • Keys are static and manually configured – De-authentication • Invoked when existing authentication is terminated 33 Telcom 2110 Spring 06
13. WEP Authentication AP MS • Idea Authentication Request – Allow the AP to know that the MS possesses the Authentication Response right secret key Open Security Authentication • Process – The AP sends a 128 byte AP MS arbitrary challenge text – The MS responds by Authentication Request encrypting the random message with the correct Authentication Challenge key – Algorithm used is RC-4 Authentication Response stream cypher Authentication Success • The authentication is NOT mutual Shared Key Authentication 34 Telcom 2110 Spring 06 802.11 Security •Privacy – Prevents message contents from being read by unintended recipient – Uses Wired Equivalent Privacy (WEP) encryption (optional) • WEP encryption – Each packet is encrypted separately – WEP based on RC4 stream cypher with 40 bit secret key – Secret key is combined with a 24 bit initialization vector (IV) that changes every packet to increase key size from 40 to 64 • Weakness – IV is transmitted in plaintext – IVs are reused too often (pseudorandom generator for IV repeats often (4-5 hours) – May start with same IV after shut down – Publicly available tools to hack key • AIRsnort , WEPcrack, etc. • Most networks don’t even implement WEP! 35 Telcom 2110 Spring 06
14. Improving 802.11 Security • Additional Security Procedures • Wi-Fi Protected Access (WPA) Industry group developing techniques for existing networks – Use access control list with approved MAC addresses – Use 128 bit proprietary implementation of WEP key – Use VPNs (IPSec or SSL) – Security architecture based on 802.1x and EAP (Extensible Authentication Protocol) • Allows many protocols within a common framework –Example • Use a RADIUS server • Authenticate the access point using a variation of SSL • Authenticate the MS using passwords (CHAP) • IEEE 802.11i is coming up with a new standard – Use AES instead of RC4 for better security 36 Telcom 2110 Spring 06 Design Issues in WLAN Compare WLAN with wired LAN Cable length, speed Conventional Wired LAN -Txpower level - Frequency channel - Location of access point Typical Wireless LAN 38 Telcom 2110 Spring 06
15. WLAN Deployment scenarios 1. Small network scenario Ex: - small office, home office (SOHO) - coffee shop 39 Telcom 2110 Spring 06 WLAN Deployment scenarios 2. Large network scenario Ex: - large office, warehouse - university campus, dormitory - corporate multistory buildings - hotels, shopping malls 40 Telcom 2110 Spring 06 Intel
16. Design Issues in Large WLANs • Need to assign each AP: Frequency, Power level – physical location • In the 2.4 GHz bands • For 802.11b there are 11 frequency bands that can be used • There are only three non-overlapping channels • For 802.11g there are 3 frequency bands (non-overlapping) • Coverage roughly 375 feet omni-directional • In the 5 GHz bands, • For 802.11a there are eleven channels • There are 8 non-overlapping channels • Coverage roughly 250 feet omni-directional • Network Planning of large networks requires – Coverage Planning, • 3-D, depends on antenna pattern, building architecture, power level – Frequency Planning • frequency reuse is possible and AP can support multiple channels • Interference concerns 41 Telcom 2110 Spring 06 WLAN design steps • Obtain site and building drawing • Determine coverage goals • Determine user population, location and application requirements • Create WLAN design and installation guidelines • Make field measurements to verify design • Adjust design as required • Finalize design • Complete installation 42 Telcom 2110 Spring 06
17. Network design requirements • Radio signal coverage requirement – Availability Factors: -Tx power level – Received signal strength - Frequency channel assignment – Interference level - location of access points - # access points installed • Average user data rate requirement – Amount of traffic user generated • User activity: passive or active • User applications: heavy or light data transaction – Locations where users gather 43 Telcom 2110 Spring 06 Network design Problem description • Determine the number of APs required for the network service scenario • Determine appropriate APs’ parameters including – locations – power levels – frequency channels 44 Telcom 2110 Spring 06
18. WLAN design approaches 1. Trail and error 2. Simple rules of thumb 3. Signal strength prediction tools 4. Mathematical models 45 Telcom 2110 Spring 06 WLAN design approaches • Trail and error – place APs at the convenient location or based on experience (“worked at other locations like that”) – Adjust APs’ locations, power levels, frequency channels based on signal strength measurement • Measure signal strength • Re-position APs, adjust power levels, freq ch. • Re-measure signal strength • • Time runs out. Æ leave it like that! – Tedious method!! – Based on signal coverage, no capacity consideration 46 Telcom 2110 Spring 06
19. WLAN design approaches • Simple rules of thumb – open 160m /semi-open 50m /closed 25m 1 6 11 11 1 6 – Field measurement 47 Telcom 2110 Spring 06 WLAN design approaches • Signal strength prediction tools – Path loss models 407 409 410 411 401 496 497 406 495 418 493 425 21m UP UP DN DN 402 405 404 403 1m 1m 33m 48 Telcom 2110 Spring 06
20. Radio Wave Behavior Review • Waves may be reflected by stationary or moving objects, diffracted over large objects which block line of sight, scattered by small objects and fade with distance depending on environment and frequency [Wesel Fig. 2.4] 49 Telcom 2110 Spring 06 Path loss prediction models Indoor Propagation models similar to outdoor • Discuss two popular models Partition dependent model Lp = L0 + 20log d + ∑ mtypeWtype + X type mtype = the number of partitions of type wtype = the loss in dB associated with that partition d = distance between transmitter and receiver point in meter X = the shadow fading L0 = the path loss at the first meter, computed by where d = 1 m. 2 0 ⎛⎛ 4πd f ⎞ ⎞ L = 10log⎜⎜ 0 ⎟ ⎟ f = operating frequency of 0 ⎜ 8 ⎟ ⎝⎝ 3×10 ⎠ ⎠ the transmitter 50 Telcom 2110 Spring 06
21. WLAN Propagation Models 51 Telcom 2110 Spring 06 Example Consider an AP operating at 2.412GHz. The distance from the AP to a receiving terminal is approximately 10 meters. There are two office walls and one metal door in office wall between the AP and the receiver. The AP operates at a power level of 100mW (20dBm). Use the partition dependent model to determine the path loss and received signal strength at the receiver location, consider a shadow fading of 13 dBm L = L + 20log d + m W + X 2 p 0 ∑ type type ⎛⎛ 4πd f ⎞ ⎞ type ⎜ 0 ⎟ L0 = 10log ⎜ 8 ⎟ woffice wall = 6 dB, wmetaldoor in office wall = 6 dB ⎜⎝ 3×10 ⎠ ⎟ X = 13 dBm ⎝ ⎠ 9 8 2 2 L0 = 10 log10((4π x1 x 2.412 x 10 )/(3 x 10 )) = 10log10((101.034) ) = 40.1 Lp = 40.1 +20log(10) + (2*6 +6) +13 = 91.1 dB Power received = Pr = Pt -Lp = 20dBm – 91.1 dB = - 71.1 dBm 52 Telcom 2110 Spring 06
22. The JTC Indoor Path Loss Model LTotal = A + B log 10 (d ) + L f (n) Similar to Okumura –Hata model in cellular (curve fitting to measure values used to set up model • A is an environment dependent fixed loss factor (dB) • B is the distance dependent loss coefficient, • d is separation distance between the base station and portable, in meters • Lf is a floor penetration loss factor (dB) • n is the number of floors between the access point and mobile terminal • May add shadowing to either JTC or Partition model – add a X term which is the shadow margin in dB 53 Telcom 2110 Spring 06 JTC Model (Continued) Environment Residential Office Commercial A (dB) 38 38 38 B 28 30 22 Lf(n) (dB) 4n 15 + 4(n-1) 6 + 3(n-1) Log Normal 8 10 10 Shadowing Std. Dev. (dB) 54 Telcom 2110 Spring 06
23. JTC Model (Continued) •Example Consider an AP on the first floor of a 3 floor house The distance to a third floor home office is approximately 8 meters If the AP operates at a power level of .05 W using the JTC model determine the path loss and received signal strength in the office area Using the JTC model with residential parameter set Ltotal = A + B log10 (d) + Lf (n) = 38 + 28 log10 (8) + 4x2 = 71.28 dB Power received = Pr = Pt -Ltotal = 16.98 dbm – 71.28 dB = -54.29 dBm 55 Telcom 2110 Spring 06 Signal strength prediction software • ABP systems •WiSE •CINDOOR 56 Telcom 2110 Spring 06
24. Site survey - Client manager - NOKIA site survey tool - Ekahau Site Survey - ABP systems 57 Telcom 2110 Spring 06 Overlapping channels in the 802.11 specifications 1 2 3 4 5 6 7 8 9 10 11 2.412 2.462 5 MHz Use three non-overlapping channels 58 Telcom 2110 Spring 06
25. 802.11b vs. 802.11a Max Frequency per AP 59 Telcom 2110 Spring 06 WLAN design issues • Capacity considerations • Depending on # users sharing the AP and the amount of data traffic at the time – Heavy vs light data transfer • Intel suggests rules of thumb for 802.11b – 50 nominal users who are mostly idle and occasionally check email – 25 mainstream users who use a lot of email and download or upload moderately sized files – 10 to 20 power users who are constantly on the network and deal with large files • 802.11a/g can support higher #users and/or traffic volume 61 Telcom 2110 Spring 06
26. WLAN standards 62 Telcom 2110 Spring 06 WLAN design approaches • Mathematically formulate the WLAN design problem – Combine signal strength prediction method and mathematical model to automate the manual adjustment • Objective is to meet the network design requirements: – Radio signal coverage • Received signal strength Node (AP) location problem • Interference level Frequency assignment problem • Installation restriction area Power level assignment problem – Data rate capacity 63 Telcom 2110 Spring• Average 06 data rate available to users
27. • Solution techniques: – Computer programming to enumerate all possible combinations of APs’ configuration Æcan take prohibitively long time to find a feasible solution – Heuristic methods: Simulated annealing algorithm, Tabu search algorithm, etc. • Obtain the potential network configuration without manually adjustment – # APs required – APs’ parameters • Locations, power levels, frequency channels 64 Telcom 2110 Spring 06 Example results 65 Telcom 2110 Spring 06