Wireless Local Area Networks (WLAN)

Transcription

1 Department of Computer Science Institute for System Architecture, Chair for Computer Networks Wireless Local Area Networks (WLAN) Mobile Communication and Mobile Computing Prof. Dr. Alexander Schill

2 Network Types classification of networks via dimension: Private Area Network (PAN) Local Area Network (LAN) Metropolitan Area Network (MAN) Wide Area Network (WAN) Global Area Network (GAN) wireless versions: WPAN, e.g. Bluetooth WLAN, e.g WMAN, WWAN, e.g and , and UMTS / LTE 2

3 Wireless LANs: Motivation / Application areas Advantages flexibility widely available Internet access at low cost ad-hoc-networks no problems with cables Problems higher error rate on the transmission link in comparison to Standard-LANs security aspects shared medium Some national restrictions concerning the used frequency bands (Industrial Scientific Medical (ISM)- Band) Applications Internet access networks in exhibition halls hospitals warehouses airports structure of networks in historic buildings extension of existing wired local area networks in offices, universities etc. 3

4 Wireless Standards in general Common Name: Wi-Fi (Wireless Fidelity) WiMAX Bluetooth, ZigBee Standard: IEEE IEEE / a IEEE Frequency Band: [GHz] Data rate: [MBit/s] 2,4 and ,4 300 and more Up to Reach: about 100 m coverage up to 50 km (cellular) Specifics cheap and flexible, improved security wireless stationary internet access about 5 m, sometimes up to 50 m voice channel, peripheral devices, sensor networks Used for WLAN WMAN Remote Devices 4

5 Frequency Bands 2,4 GHz Band 2,4 to 2,4835 GHz ISM-Band License-free transmitted power max. 100 mw 5 GHz Band 5,15-5,725 GHz in Europe License-free transmitted power max mw with special power control 5

6 Network Topologies (1) infrastructure mode like a star-network Access-Point (AP) is a central point AP coordinates the network nodes and communicates with other networks AP AP Three infrastructure APs in one fixed network AP Network 6

7 Network Topologies (2) Ad-hoc Mode Like Peer-to-Peer Network no central Station or higher-level infrastructure available All network nodes are equivalent Direct connection the nodes see each other and can communicate one with each other Beaconing-Mechanism every node sends a Beacon - Signal in certain intervals. Via this signal every node knows its direct neighbors. ad-hoc-nets appear spontaneously and organize and administrate themselves Indirect connection no direct communication possible special routing methods for transmission of the data (e.g. OLSR Optimized Link State Routing) 7

8 System Architecture STA x LAN Distribution System ESS BSS 1 AP STA 2 AP BSS 2 Portal STA 3 Station (STA) device with interface Access Point (AP) Allow access for registered stations to the distribution system Basic Service Set (BSS) AP and associated stations Independent BSS (IBSS) in Ad-hoc-Mode Distribution System Connects more than one BSS via the access points to form a larger logical network Extended Service Set (ESS) Multiple connected wireless networks Portal Allows entering of other networks 8

9 WLAN standards Standard Frequency Bandwidth Max. data rate DRmax Normal Data rate DR ,4 GHz 2 MBit/s 1,2 MBit/s Trensmission DSSS (FHSS, Infrared) Range R (indoor/ outdoor) Remarks 30/300 outdated a 5 GHz 54 MBit/s 32 MBit/s OFDM 10/100 high data rate, but incompatible to other standards, low range b 2,4 GHz 11 MBit/s 7 MBit/s DSSS 30/300 higher range, but lower data rate g 2,4 GHz 54 MBit/s 32 MBit/s OFDM 30/300 higher data rate and range, but sensitive to noise n ac 2,4 GHz and 5 GHz 300 MBit/s (and more) ~ 100 MBit/s DSSS... Direct Sequence Spread Spectrum FHSS Frequency Hopping Spread Spectrum OFDM... Orthogonal Frequency Division Multiplexing OFDM 10/100 very high data rate, but also sensitive to noise 9

10 Sub-Standards Standard Characteristics a Enhancement of the physical layer b Enhancement of the physical layer c Define Wireless Bridging between Access Points d Country specific regulations for b e Enhanced WLAN for QoS (Quality of Service) f Roaming and inter-operability between base station different vendors g Enhancement of the physical layer h Optional for a for Europe i Enhancement of security and authentication j Model of a in Japan n, ac Enhancements for higher data rates o Prioritizing of voice data in WLAN in opposite to the data traffic p Enhancement to a for use in vehicular networks r Fast Roaming at the swapping between Access Points u Inter-operability with other non 802 networks 10

11 OFDM: Example a 64 sub-bearer signals per bearer, use of 64QAM (Quadrature Amplitude Modulation) 48 sub-bearers for data modulation, 4 as phase reference, and 12 as distance to next bearer 11

12 OFDM with b ISM frequency band of 2,4 GHz Bandwidth per channel: 22 MHz, 14 overlapping channels Channel allocation slightly different in various countries 12

13 802.11n, ac (1) Focus: Higher end applications in WLANs Wireless Streaming Media Videoconferencing Technical aspects Actual data rate should reach in the area of 100 MBit/s Possible gross data rate up to 300 MBit/s MIMO-techniques (Multiple Input/Multiple Output) use of several sender-/receiver channels (max. 4) SISO Transmitter Receiver MIMO Transmitter Receiver 13

14 802.11n, ac (2) OFDM, adaptive modulation with BPSK, QPSK, 16QAM, 64QAM, 256QAM 2,4 GHz-band with downward-compatibility to b/g, 5 GHz band also a possible option optional extension of the radio channel from 20 MHz up to 40 MHz radio interface regular sending packet aggregation (more efficient) Backoff: Jam signal BPSK: Binary Phase Shift Keying SIFS: Short Inter Frame Spacing for ACK and response to polling ACK: Acknowledgement signal DIFS: Distributed Coordination Function Inter Frame Spacing for asynchronous services -> Overhead due to packet aggregation reduced 14

15 Specific Problems and Solutions Physical problems interference: spread spectrum techniques, frequency hopping hidden Terminal problem: use of CSMA/CA (see later) Data security Wired Equivalent Privacy (WEP) service Current improvements WPA (WiFi Protected Access), WPA i 15

16 Medium Access (1) Carrier Sense Multiple Access with collision avoidance (CSMA/CA) CSMA/CD in wireless networks not possible No collision detection principle: listen before talking (only CSMA without CD) Procedure: 1. a station intends to send, is listening before sending 2. if medium is free then send 3. if medium is busy, wait until the back off interval is over and begin at 1 (repeat until maximum number of attempts) 16

17 Medium Access (2) Problem of CSMA in the wireless case: hidden terminal A and C cannot communicate directly A sends to B and occupies the medium C also wants to send to B, but does not recognize the occupied medium Collision at B, so B cannot reconstruct data A also does not detect the collision Ú A is hidden for C and C is hidden for A A B C 17

18 Medium Access (3) Further problem: exposed terminal B sends to A, and C wants to send to an other station (not A and not B) C recognizes the signals from B and is waiting until B will finish its transmission unnecessary waiting, because signals from C cause no collision at A Ú exposed terminal: C is exposed to other stations A B C 18

19 Medium Access (4) solution: RTS/CTS-Mechanism (Request To Send/Clear To Send) solution: hidden terminal A sends a RTS-Signal to B and B sends CTS after that (ready for transmission) All other possible senders (C) also get the CTS-signal and will wait and reschedule their transmission RTS CTS CTS A B C solution: exposed terminal C sends RTS to the receiver and gets CTS, so it can transmit B hears RTS, too. But B does not receive any CTS (too far away). So B assumes the channel to be free (which is ok) and can also send. A B C RTS CTS 19

20 802.11: Data Security in WLANs WEP (Wired Equivalent Privacy) - symmetrical cryptography, e.g. using RC4 However: Small key lengths ( bit), same key for multiple clients, low security! clear text (in frame body) integrityalgorithm Integrity check value (ICV) + clear text of frame body + ICV secret key + Generator for Pseudo-random number key sequence Encryption encoded text in frame body 20

21 Security WPA / WPA2 WiFi protected access, subset of i, resolves the WEP problems authentication Pre-Shared-Key (PSK), 8-64 characters password, used for generation of the session key Extensible Authentication Protocol based on 802.1x (e.g. RADIUS-Server Remote Access Dial-in User Service) encryption Integrity Check TKIP (Temporal Key Integrity Protocol) generates dynamic key per packet (WPA) RC4 (WPA) or AES (WPA2) for encryption remaining security problems Simple PSK allows brute force or dictionary attack 21

22 Security Summary Features WEP WPA WPA2/ IEEE802.11i Encryption RC4 RC4 AES Key length [Bit] 40, or more 128 or more Data integrity CRC-32 Michael CCM Header integrity non Michael CCM Key management non EAP-based EAP-based RC4 R.Rivest Encryption symmetrical method AES Advanced Encryption Standard, a symmetrical cryptosystem, modern DES, RC4 successor CCM Counter Mode with Cipher Block Chaining Message Authentication Code Protocol EAP Extensible Authentication Protocol, used on data link layer, frequently with PPP and SSL/TLS 22

23 WPAN (Wireless Personal Area Networks): Bluetooth Harald Bluetooth was the King of Denmark in the 10th century Initiated by Ericsson, Intel, IBM, Nokia, Toshiba; Open Standard: IEEE Generally for wireless Ad-hoc-piconets (range < 10m); single-chip solution Frequency band in 2,4 GHz area Integrated security (128 bit encryption) Data rates: 433,9 kbit/s asynchronous-symmetrical 723,2 kbit/s / 57,6 kbit/s asynchronous-asymmetrical 64 kbit/s synchronous, voice service Extensions up to 20 Mbit/s (IEEE a UWB (Ultra Wide Band)) Basic setup 2,4-Ghz- HF Bluetooth- Baseband- Controller Host- System 23

24 Bluetooth - Functionality Not connected; Standby Standby t =2 s connection-setup status active states Inquiry with known Address Send data Page for Unknown Address t =0,6 s connected Low-Powerstates PARK Adress resigned HOLD (paused) SNIFF (periodic) Address available 24

25 Possible Configurations Master Slave Piconet (up to 7 slaves per master ) Scatternet 25

26 WPAN: ZigBee Low-energy network for ad-hoc connectivity of sensors and other small devices Standard: IEEE Low Rate WPAN Technical data Frequency range: 2,4-GHz-Band (16 channels), Transmission power: only 1 to 10 mw Reach: up to 50 m Data rate: 250 kbit/s per channel Components ZigBee End Device (ZED) ZigBee Router (ZR) ZigBee Coordinator (ZC) Network topology Star topology with ZEDs P2P-Topology with ZRs Up to devices per cluster (ZC) ZED ZED ZED ZED ZED ZR ZED ZED ZR ZED ZC ZED ZED ZED ZED ZED

27 WPAN: RFID Radio Frequency Identification RFID- Reader Tag Tag Tag Reader signal Tags: - Antenna and RFID-Chip Reflected signal 96-Bit-Identifier, small memory, passive very cheap, universal usage Reader: - active, powerful, own protocols - Sends bearer signal, reflected by Tag Backscatter: Tag creates overlay of bearer signal with own transmission bits à Reader can filter and detect these bits Multiple access handled by collision detection protocol Tag

28 WPAN: NFC Near Field Communication Contactless transmission via very small distance (4 cm) (smartcard is being directly attached to reader) Transmission connectionless: passive RFID-Tags Connection oriented: active transmitters (e.g. smartphone) Data rate up to 424 kbit/s Applications Payment by smartphone or smartcard Smartphone as door key Problems Not really secure (controlled by distance only)

29 WPAN: Applications May replace infrared for peripherals Wireless headsets (e.g. hands-free interaction with mobile phones) Digital image transmission between cameras and archives Control of home appliances by networked sensors (examples: heating control, smart electricity metering, alarm systems) Digital payment systems 29

30 Some further readings IEEE : standards.ieee.org/getieee802/ htm WiFi Alliance (configuration, security): Bluetooth special interest group: 30