CS4/MSc Computer Networking. Lecture 12: Wireless Local Area Networks

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1 CS4/MSc Computer Networking Lecture 12: Wireless Local Area Networks

2 Wireless Networking Motivation Mobility Connect from anywhere, anytime, on the move Wi-fihotspots beginning to proliferate» coffee shops, airports, hotels etc. Flexibility Ad hoc networks whenever and wherever required» meetings, multi-user networked games Home networks» homes increasingly have multiple PCs with one Internet connection Costs No fixed wiring» may be difficult and expensive unless buildings designed for the purpose Cheap wireless interface cards and access points 2

3 Wireless Networking Challenges Radio and infra-red transmissions susceptible to noise and interference not as reliable as wired transmission Strength of radio transmission varies in time and space fading effects from multipath propagation uneven propagation due to physical barriers and geographic topography Radio transmissions can be intercepted by eavesdroppers difficult to restrict transmissions to a specific area Radio spectrum is finite and must be shared with other users your neighbour s home wi-fi network competing WLAN standards e.g. Bluetooth v , in 2.5GHz range Difficult to provide the high transmission speeds that are easy with wires e.g. Gigabit wired ethernet Allocation of spectrum by national and international authorities ITU, FCC etc. agreement often difficult; designing products for a global market difficult 3

4 Wireless Network Types Wide Area Networks (WWAN) Connections maintained over large geographical areas multiple antenna sites and cells or satellite systems automatic hand-off between adjacent cells for mobility international roaming between compatible systems Generations of systems 1G systems (analogue) : TACS (UK), AMPS (USA) 2G systems (digital) : GSM (Europe), TDMA (USA) 2½G systems : GPRS (Europe), EDGE 3G systems : UMTS (Europe), CDMA 2000 (USA), TD-SCDMA (China) Private as well as public networks E.g. GSM-R for railways signalling, control & communications 4

5 Wireless Network Types Metropolitan Area Networks (WMAN) Wireless connections between multiple locations within a metro. area e.g. multiple office buildings, a University campus etc. Backups for wired networks Radio or infra-red transmission Technologies: Multichannel Multipoint Distribution Service (MMDS)» 2 10GHz range, 30 miles radius, line-of-sight Local Multipoint Distribution Services (LMDS)» 24-40GHz range, 2-3 miles radius, line-of-sight IEEE » working group set up to establish standards for broadband wireless access» 10 66GHz range» Demand Assignment Multiple Access-Time Division Multiple Access (DAMA-TDMA) 5

6 Wireless Network Types Local Area Networks (WLAN) Communications within a local area within a corporate or campus building, public spaces coffee shops, airports etc. 25m 250m, farther outside than inside, speed decreasing with distance where wiring would be difficult or expensive to supplement an existing LAN to create possibly temporary ad hoc networks in a meeting room to facilitate mobility laptops ubiquitous for `road warriers IEEE standardisation:

7 Wireless Network Types Personal Area Networks (WPAN) Ad hoc communications within a personal operating space e.g. PDAs, mobile phones, laptops, headsets, GPS navigators, printers etc. A cable replacement technology Technologies: Infra-Red Bluetooth (IEEE ) ZigBee (IEEE ) 7

8 Wireless Local Area Networks IEEE Local Area Networks: Private ownership» freedom from regulatory constraints of WANs Short distance (~1km) between computers» low cost» very high-speed, relatively error-free communication» complex error control unnecessary Machines are constantly moved» Keeping track of location of computers a chore» Simply give each machine a unique address» Flat address structure» Broadcast all messages to all machines in the LAN Need a medium access control protocol is one of the IEEE 802 local area network standards A number of variations exist 8

9 IEEE Building Block Basic Service Set (BSS) A group of stations that coordinate their access to the medium Co-located and unrelated BSS s can co-exist simultaneously via different channels Stations intercommunicate within a Basic Service Area (BSA) analogous to a mobile phone cell size depending on situation and conditions e.g. indoors v. outdoors Two cases Infrastructure mode Independent ad hoc A C B D 9

10 BSS types Independent Basic Service Set A single BSS can form an ad hoc network No fixed infrastructure (access point) Typically temporary» can be formed spontaneously and disbanded after a limited period of time Stations need to be in range of each other to communicate A B AP C D Basic Service Set in infrastructure mode has an Access Point (AP) or Base Station» to provide a local bridge between stations stations communicate via the Access Point in PCF mode» all frames go via the access point» stations do not all need to be in range of each other, just in range of the access point communicate directly with each other in DCF mode A B C 10

11 Extended Service Set (ESS) A set of infrastructure Basic Service Sets Access Points communicate amongst themselves to forward traffic from one BSS to another Allows movement of stations between BSSs Allows access to other networks through portals which connect to other 802 LANs ESS appears as single BSS to LLC sublayer Server Portal Distribution System Portal Gateway to the Internet A1 BSS A AP1 A2 B1 BSS B AP2 B2 11

12 LAN layers (IEEE 802) Network layer Network layer LLC Logical link control Data link layer MAC CSMA-CD Token Ring Wireless LAN Other LANs Physical layer Various physical layers Physical layer IEEE 802 OSI 12

13 MAC sublayer: why not use wireless Ethernet (CSMA-CD)? Difficult to detect collisions in a radio environment radios normally half-duplex: either transmit or receive not both simultaneously transmit power orders of magnitude greater than receive power therefore not possible to abort transmissions that collide The whole frame time is wasted Collisions may not happen at transmitter the hidden station problem: two stations both within range of an intermediate station but not of each other either one cannot hear the transmissions of the other» so think the channel is idle when the other station is using it» signals may collide at the intermediate station A Data Frame B Data Frame C 13

14 MAC sublayer: Collision Avoidance (CSMA-CA) A station wishing to transmit always senses the medium before it starts If the medium is busy, the station defers its transmission Politeness: no need to destroy other station s transmission When a packet (MPDU) is to be transmitted Wait for medium to go quiet Choose a random extra time to wait When the time expires and medium is idle, transmit Different stations will select different back-off times, so collisions are avoided An ACK is sent for every correctly received packet A received ACK indicates no collision occurred There is no other way of knowing about collisions 14

15 CSMA-CA: Overcoming the hidden station problem RTS A requests to send B C CTS B CTS A C B announces A ok to send A sends Data Frame B C remains quiet 15

16 MAC Services Contention Service: Best effort Contention-Free Service: time-bounded transfer MAC can alternate between Contention Periods (CPs) & Contention-Free Periods (CFPs) MAC also performs fragmentation & reassembly (stop&wait) Contentionfree service MSDUs MSDUs Contention service Point coordination function Distribution coordination function (CSMA-CA) MAC Physical 16

17 Distributed Coordination Function DIFS Busy medium DIFS PIFS SIFS Contention window Next frame Defer access All stations must wait an Interframe Space (IFS) time High-Priority frames wait Short IFS (SIFS) Typically to complete the exchange in progress» Only one station can transmit at this time ACKs, CTS, data frames of segmented MSDU, etc. SIFS calculated to give time for transmitter to switch back to receive mode PCF IFS (PIFS) to initiate Contention-Free Periods used by the base station to gain access to the medium DCF IFS (DIFS) to transmit data Wait for reattempt time Time 17

18 Back-Off Procedure Wait for medium to be idle for a DIFS period Choose a random number 0 n of time-slots as the back-off time For each idle slot, decrement the counter For busy slots, freeze the counter When the counter becomes 0, transmit If transmission fails (ack not received), n is doubled (exponential back-off) Must be executed : after each retransmission after a successful transmission SIFS PIFS DIFS contention window medium busy next frame slot-time slot-time defined so that a station can always determine if another station has access the medium at the beginning of the previous slot 18

19 Virtual Carrier Sensing A MAC field tells other stations how long the medium will be used for Stations receiving data or RTS or CTS set their Virtual Carrier Sense indicator (NAV or Network Allocation Vector) for the given duration use this information with physical carrier sense when sensing the medium C A B D DIFS Data Source Destination Other SIFS NAV A C K A RTS data Defer Access B CTS ACK C NAV D NAV 19

20 Point Coordination Function (PCF) Provides contention-free service through polling Point Coordinator (access point) polls other stations asking if they have any frames to send TBTT Contention-free repetition interval B SIFS D1 + Poll SIFS SIFS D2+Ack+ Poll SIFS SIFS CF End Contention period PIFS U 1 + ACK U 2 + ACK Reset NAV NAV CF_Max_duration D1, D2 = frame sent by point coordinator U1, U2 = frame sent by polled station TBTT = target beacon transmission time B = beacon frame 20

21 MAC frame format MAC header (bytes) Frame Control Duration/ ID Address 1 Address 2 Address 3 Sequence control Address 4 Frame body CRC 2 2 Protocol version Type Subtype To DS From DS More frag Retry Pwr mgt More data WEP Rsvd Frame types: control, management, data Each type has a number of subtypes, e.g. Control: RTS, CTS, ACK To, From: determine what the 4 address fields stand for More frag: more fragments yet to follow Retry: retransmission of previous failed transmissions WEP (Wired Equivalent Privacy): set when information has been encrypted Sequence: 16 bit sequence number of a fragment» 12 bits to identify the frame» 4 bits to identify the fragment 21

22 Physical Layers LLC PDU LLC MAC layer PLCP PLCP preamble header MAC header MAC SDU PLCP PDU CRC Physical layer convergence procedure Physical medium dependent Physical layer Physical layer split into 2 sub-layers Physical Layer Convergence Procedure (PLCP) sub-layer Physical Medium Dependent (PMD) sub-layer PLCP adds Preample synchronisation, framing Header bit rate and other settings, CRC Different for each physical layer 22

23 Infrared physical layer The only physical layer using (infrared) light 0.85µ to 0.95µ wavelength, diffused Signal contained by walls, windows Not operational outdoors No interference with networks in other rooms Range < 20m Pulse position modulation (PPM) Essentially one-hot encoding Slot time is 250ns 1Mbps : 4 bit group encoded to 16 bits 15 zeroes and 1 one 2Mbps : 2 bit group encoded to 4 bits 3 zeroes and 1 one 23

24 Spread-Spectrum Communication Most of radio spectrum is regulated and licenses are required to use it Some bands are left unregulated 915MHz ( ) 2.4GHz ( ) 5GHz ( ) Equipment using these bands must transmit at low power Spread-spectrum: increase the bandwidth of the transmitted signal by modulating it using a pseudorandom, spreading code This waste of bandwidth has advantages: Immunity from noise, multipath fading, jamming Security Knowledge of the spreading code is essential to decode the signal Several users can use the same frequency without interference Two main types: Frequency hoping and Direct Sequence 24

25 Frequency Hopping Spread Spectrum (FHSS) FHSS hops between frequencies in pseudorandom sequence All stations need to be synchronised, know the (initial) sequence and the dwell time, the time spent at each frequency Limited security eavesdropper needs to know hop sequence and dwell time FHSS physical layer Uses 79 channels at 2.4GHz band, each 1MHz wide Standard defines 78 hopping patterns (3 groups of 26) 26 networks can be collocated and operate simultaneously Available rates: 1Mbps and 2Mbps Header always transmitted at 1Mbps, a bit field indicates the rate of the data part of the frame 25

26 Direct Sequence Spread Spectrum (DSSS) DSSS transmits a sequence of chips for each information bit 11-chip Barker sequence: To transmit +1, send: To transmit -1 (0), send: symbol times The DSSS physical layer operates at the 2.4GHz band A number of overlapping 30MHz channels are defined Up to 3 networks can be collocated on non-overlapping channels Available rates:» 1Mbps uses Binary Phase Shift Keying modulation (BPSK)» 2Mbps uses Quadrature Phase Shift Keying modulation (QPSK) 26

27 802.11a Uses orthogonal frequency division multiplexing (OFDM) Multiple carrier signals at different frequencies Similar to FDM but all subchannels dedicated to single source ADSL also uses OFDM Operates at 5GHz band Shorter range than 2.4GHz Few other systems operate at 5GHz, little interference for now 52 frequencies : 48 for data and 4 for synchronisation Up to 12 non-overlapping channels available (depends on the country) Maximum rate 54Mbps, range of lower rates available phase-shift keying up to 18Mbps quadrature amplitude modulation for higher rates 27

28 802.11b / g Extension of DSSS physical layer of Commonly called WiFi 1Mbps, 2Mbps, 5.5Mbps and 11Mbps rates supported Low speeds (1,2Mbps) use DSSS Higher speeds use Complementary Code Keying (CCK) Up to 3 non-overlapping channels available (depends on the country) 14 overlapping channels defined g Uses OFDM (as a) at the 2.4GHz band Maximum rate is 54Mbps Backwards compatible with b If a single b station is present in the BSS, the rate drops Channel availability as for b 28

29 Forthcoming extensions n Uses multiple antennas for both AP and mobile stations Rates above 100Mbps Up to ~600Mbps with 4 antennas pre-n products already available e Defines QoS mechanisms for wireless networks Not a physical layer extension Extend DCF, PCF to differentiate between traffic types Enhanced DCF (EDCF) has 8 traffic categories with different IFS times 29

30 The 9 services of Distribution services manage BSS membership Association connection to an AP Disassociation disconnection Reassociation change AP Distribution how frames sent to AP are routed to the destination Integration handles connection to other networks Station services Authentication Deauthentication Privacy encryption Data delivery 30

31 Joining a BSS - Association A station needs to know the SSID of the network it wants to join SSID : Service Set Identifier the network s name keeping this private offers security against naïve attacks» SSID broadcast by default in beacon frames and these can be intercepted A station needs to get synchronisation information from the base station by passive scanning: look for beacon frame from the base station by active scanning: transmits a probe request frame and waits for a probe response frame Choice of active or passive up to the station itself Station sends an association request (identity, capabilities) The AP accepts or rejects the request Once associated the station must be authenticated before it can transmit or receive data 31

32 Joining a BSS - Authentication The AP and the station perform mutual authentication Prevent unauthorised users to access the network Prevent users from connecting to rogue APs Authentication process: Station sends an authentication frame MAC address filtering:» if the facility is enabled, the AP looks up MAC address is in its guest list Open Authentication:» Minimal authentication just MAC filtering Shared Key Authentication:» AP creates an authentication frame containing random challenge text» joining station encrypts the frame with its pre-shared key and sends it back» AP decrypts the frame and checks that the text is correct 32

33 Security in Wireless LANs Same security issues face wired LANs as wireless LANs Unauthorised access and eavesdropping Threats to physical security of network e.g. denial of service, sabotage Attacks from within an organisation s authorised user community» e.g. disgruntled current and former employees In wireless LANs passive eavesdropping is very easy Radio waves may be received at ranges beyond the control of the host organisation, no need to be inside a controlled area LAN adapters offer a promiscuous mode every packet can be captured» both wired and wireless Wired Equivalent Privacy (WEP) The original security scheme. Proved easy to break Wi-Fi Protected Access (WPA) WPA2 full implementation of the i security upgrade 33

34 WEP encryption A 40-bit secret key is pre-agreed and pre-shared by the network stations A 24-bit Initialisation Vector (IV) is concatenated with secret key normally a random value but sometimes just successive integer values Resulting key is input into the Pseudo-Random Number Generator using the RC4 algorithm Integrity Check Value (ICV) CRC-32 over the message plaintext Plain text, ICV encrypted with key sequence using bitwise XOR IV communicated to the peer by placing it, in clear, before the cipher text 34

35 Security Weaknesses of WEP Key Management synchronising change of keys is tedious and difficult» keys therefore will tend to be long-lived» probably one single key shared between every station on the network Key Size: 40-bit key size is now vulnerable to brute-force attack Initialisation Vector reuse IV is too small, so it is reused frequently if the key sequence for a given IV is found, an attacker can decrypt subsequent packets that were encrypted with the same IV To discover the key sequence, send packets to the station and observe the wireless transmissions, because the content of the packet is known, the key sequence can be deduced The use of CRC-32 to produce the Integrity Check Value (ICV) is not appropriate Attacker can modify encrypted packet and fix up ICV to be correct biggest weakness is that ICV-based attacks are independent of key size WEP use of RC4 has known weak keys WEP authentication messages can be forged easily 35

36 WiFi Protected Access (WPA) Temporal Key Integrity Protocol (TKIP) for encryption Key size is 128 bits Keys dynamically generated and distributed by the authentication server Key hierarchy and management system, dynamically generates unique data encryption keys to encrypt every data packet Message Integrity Check (MIC) greatly improves ICV Two flavours for authentication: Enterprise: uses 802.1X / Extensible Authentication Protocol (EAP)» Requires the use of an authentication server in the network» E.g. Remote Authentication Dial-In User Service (RADIUS) Personal: uses a pre-shared key (common password) WPA2 improves the encryption engine by replacing RC4 with AES (Advanced Encryption Standard) Implements full i security update Hardware support is needed to handle the higher complexity of the algorithm without suffering a significant speed loss 36

37 Reading LGW 6.10 Tanenbaum 1.1.3, 1.2.4, 4.4 Stallings 17, 9 (spread spectrum) 37

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