IEEE 802.11 based WLANs

IEEE 802.11 based WLANs

Wireless Local Area Wireless? A wireless LAN or WLAN is a wireless local area network that uses radio waves as its carrier. The last link with the users is wireless, to give a network connection to all users in a building or campus. The backbone network usually uses cables Common Topologies The wireless LAN connects to a wired LAN There is a need of an access point that bridges wireless LAN traffic into the wired LAN. The access point (AP) can also act as a repeater for wireless nodes, effectively doubling the maximum possible distance between nodes.

Integration With Existing Networks Wireless Access Points (APs) - a small device that bridges wireless traffic to your network. Most access points bridge wireless LANs into Ethernet networks, but Token-Ring options are available as well. How are WLANs Different? They use specialized physical and data link protocols They integrate into existing networks through access points which provide a bridging function They let you stay connected as you roam from one coverage area to another They have unique security considerations They have specific interoperability requirements They require different hardware They offer performance that differs from wired LANs.

Physical and Data Link Layers Physical Layer: The wireless NIC takes frames of data from the link layer, scrambles the data in a predetermined way, then uses the modified data stream to modulate a radio carrier signal. Data Link Layer: Uses Carriers-Sense-Multiple-Access with Collision Avoidance (CSMA/CA). 802.11 WLANs - Outline

801.11 bands and layers Link layer Media access layer frames and headers CSMA/CD Physical layer frames modulation Frequency hopping Direct sequence Infrared Security Implementation Based on: Jim Geier: Wireless LANs, SAMS publishing and IEEE 802 - standards 802.11 WLAN technologies

IEEE 802.11 standards and rates IEEE 802.11 (1997) 1 Mbps and 2 Mbps (2.4 GHz band ) IEEE 802.11b (1999) 11 Mbps (2.4 GHz band) = Wi-Fi IEEE 802.11a (1999) 6, 9, 12, 18, 24, 36, 48, 54 Mbps (5 GHz band) IEEE 802.11g (2001 ... 2003) up to 54 Mbps (2.4 GHz) backward compatible to 802.11b IEEE 802.11 networks work on license free industrial, science, medicine (ISM) bands: 26 MHz 902 EIRP power in Finland 928 83.5 MHz

2400 2484 100 mW 200 MHz 5150 5350 255 MHz 5470 200 mW indoors only

5725 f/MHz 1W EIRP: Effective Isotropically Radiated Power - radiated power measured immediately after antenna Equipment technical requirements for radio frequency usage defined in ETS 300 328 Other WLAN technologies High performance LAN or HiperLAN (ETSI-BRAN EN 300 652) in the 5 GHz ISM version 1 up to 24 Mbps version 2 up to 54 Mbps HiperLAN provides also QoS for data, video, voice and images Bluetooth

range up to 100 meters only (cable replacement tech.) Bluetooth Special Interest Group (SIG) Operates at max of 740 kbps at 2.4 GHz ISM band Applies fast frequency hopping 1600 hops/second Can have serious interference with 802.11 2.4 GHz range network IEEE 802.11a rates and modulation formats Data Rate Coded bits per Code bits per Data bits per sub-carrier

OFDM symbol OFDM symbol 1/2 1 48 24 BPSK 3/4 1 48

36 12 QPSK 1/2 2 96 48 18 QPSK 3/4

2 96 72 24 16QAM 1/2 4 192 96 36

16QAM 3/4 4 192 144 48 64QAM 2/3 6 288

192 54 64QAM 3/4 6 288 216 Modulation Coding Rate 6

BPSK 9 (Mbps) The IEEE 802.11 and supporting LAN Standards IEEE 802.2 Logical Link Control (LLC) OSI Layer 2 (data link) IEEE 802.3 IEEE 802.4 IEEE 802.5 IEEE 802.11 Carrier Token Token

Wireless Sense Bus Ring a b g bus star MAC PHY OSI Layer 1 (physical) ring

See also IEEE LAN/MAN Standards Committee Web site www.manta.ieee.org/groups/802/ Figure 14.1 Basic service sets (BSSs) 14.12 Figure 14.2 Extended service sets (ESSs) 14.13 IEEE 802.11 defines the physical (PHY), logical link (LLC) and media access control (MAC) layers for a wireless local area network Network

802.11 networks can work as LLC basic service set (BSS) MAC extended service set (ESS) FHSS DSSS IR PHY BSS can also be used in ad-hoc networking 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 AP: Access Point ad-hoc network

DS: Distribution System DS, ESS 802.11 IEEE 802.11 Architecture BSS and ESS Basic (independent) service set (BSS) Extended service set (ESS) In ESS multiple access points connected by access points and a distribution system as Ethernet BSSs partially overlap

Physically disjoint BSSs Physically collocated BSSs (several antennas) 802.11 Logical architecture LLC provides addressing and data link control MAC provides access to wireless medium CSMA/CA Priority based access (802.12) joining the network authentication & privacy Services Station service: Authentication, privacy, MSDU* delivery Distributed system: Association** and participates to data distribution Three physical layers (PHY)

FHSS: Frequency Hopping Spread LLC: Logical Link Control Layer Spectrum (SS) MAC: Medium Access Control Layer DSSS: Direct Sequence SS PHY: Physical Layer FH: Frequency hopping IR: Infrared transmission *MSDU: MAC service data unit ** with an access point in ESS or BSS DS: Direct sequence IR: Infrared light 802.11 DSSS DS-transmitter

Supports 1 and 2 Mbps data transport, uses BPSK and QPSK modulation Uses 11 chips Barker code for spreading - 10.4 dB processing gain Defines 14 overlapping channels, each having 22 MHz channel bandwidth, from 2.401 to 2.483 GHz Power limits 1000mW in US, 100mW in EU, 200mW in Japan Immune to narrow-band interference, cheaper hardware PPDU:baseband data frame 802.11 FHSS

Supports 1 and 2 Mbps data transport and applies two level - GFSK modulation* (Gaussian Frequency Shift Keying) 79 channels from 2.402 to 2.480 GHz ( in U.S. and most of EU countries) with 1 MHz channel space 78 hopping sequences with minimum 6 MHz hopping space, each sequence uses every 79 frequency elements once Minimum hopping rate 2.5 hops/second Tolerance to multi-path, narrow band interference, security Low speed, small range due to FCC TX power regulation (10mW) * f f c f , f nom 160 kHz How ring-network works A node functions as a repeater

A only destination copies C B frame to it, all other nodes B transmits frame addressed to A have to discarded A the frame B Unidirectional link C A A A copies frame A

C B A C ignores frame A C A C absorbs returning frame B Token ring

A ring consists of a single or dual (FDDI) cable in the shape of a loop Each station is only connected to each of its two nearest neighbors. Data in the form of packets pass around the ring from one station to another in uni-directional way. Advantages : (1) Access method supports heavy load without degradation of performance because the medium is not shared. (2) Several packets can simultaneous circulate between different pairs of stations. Disadvantages: (1) Complex management (2) Re-initialization of the ring whenever a failure occurs How bus-network works

In a bus network, one nodes 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) (2) Round robin scheme : a token is passed between nodes; node holds the token can use the bus (e.g.Token bus) Advantages: (1) Simple access method (2) Easy to add or remove stations C D A B

Disadvantages: D (1) Poor efficiency with high term term network load (2) Relatively insecure, due to the shared medium term: terminator impedance 802.11 LAN architecture wireless host communicates Internet AP BSS 1

hub, switch or router AP with base station base station = access point (AP) Basic Service Set (BSS) (aka cell) in infrastructure mode contains: wireless hosts access point (AP): base station ad hoc mode: hosts only BSS 2 and Mobile Networks 6: Wireless 6-22

802.11: Channels, association 802.11b: 2.4GHz-2.485GHz spectrum divided into 11 channels at different frequencies AP admin chooses frequency for AP interference possible: channel can be same as that chosen by neighboring AP! host: must associate with an AP scans channels, listening for beacon frames containing APs name (SSID) and MAC address selects AP to associate with may perform authentication will typically run DHCP to get IP address in APs subnet 6: Wireless and Mobile Networks 6-23 802.11: passive/active scanning

BBS 1 AP 1 BBS 1 BBS 2 1 1 2 AP 2 AP 1 BBS 2 1

2 3 2 3 AP 2 4 H1 H1 Passive Scanning: Active Scanning: (1) beacon frames sent from APs (2) association Request frame

sent: H1 to selected AP (3) association Response frame sent: H1 to selected AP (1) Probe Request frame broadcast from H1 (2) Probes response frame sent from APs (3) Association Request frame sent: H1 to selected AP (4) Association Response frame sent: H1 to selected AP 6: Wireless and Mobile Networks 6-24 IEEE 802.11: multiple access avoid collisions: 2+ nodes transmitting at same time 802.11: CSMA - sense before transmitting

dont collide with ongoing transmission by other node 802.11: no collision detection! difficult to receive (sense collisions) when transmitting due to weak received signals (fading) cant sense all collisions in any case: hidden terminal, fading goal: avoid collisions: CSMA/C(ollision)A(voidance) A C A B B C Cs signal strength

As signal strength space 6: Wireless and Mobile Networks 6-25 IEEE 802.11 MAC Protocol: CSMA/CA 802.11 sender 1 if sense channel idle for DIFS then transmit entire frame (no CD) 2 if sense channel busy then start random backoff time timer counts down while channel idle transmit when timer expires if no ACK, increase random backoff interval, repeat 2

802.11 receiver - if frame received OK sender receiver DIFS data SIFS ACK return ACK after SIFS (ACK needed due to hidden terminal problem) 6: Wireless and Mobile Networks 6-26

Avoiding collisions (more) idea: allow sender to reserve channel rather than random access of data frames: avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets to BS using CSMA RTSs may still collide with each other (but theyre short) BS broadcasts clear-to-send CTS in response to RTS CTS heard by all nodes sender transmits data frame other stations defer transmissions avoid data frame collisions completely using small reservation packets! 6: Wireless and Mobile Networks

6-27 Collision Avoidance: RTS-CTS exchange A B AP RTS(B) RTS(A) reservation collision RTS(A) CTS(A)

CTS(A) DATA (A) time ACK(A) 6: Wireless and Mobile Networks defer ACK(A) 6-28 802.11 frame: addressing 2 2 6 6

6 frame address address address duration control 1 2 3 Address 1: MAC address of wireless host or AP to receive this frame Address 2: MAC address of wireless host or AP transmitting this frame 2

6 seq address 4 control 0 - 2312 4 payload CRC Address 4: used only in ad hoc mode Address 3: MAC address of router interface to which AP is attached

6: Wireless and Mobile Networks 6-29 802.11 frame: addressing R1 router H1 Internet AP R1 MAC addr H1 MAC addr dest. address source address 802.3 frame

AP MAC addr address 1 H1 MAC addr R1 MAC addr address 2 address 3 802.11 frame 6: Wireless and Mobile Networks 6-30 802.11 frame: more frame seq # (for RDT) duration of reserved transmission time (RTS/CTS) 2

frame control 2 Protocol version 2 6 duration address 1 6 6

6 2 address 2 address 3 seq control address 4 0 - 2312 4

payload CRC 2 4 1 1 1 1 1 1

1 1 Type Subtype To AP From AP More frag Retry Power

mgt More data WEP Rsvd frame type (RTS, CTS, ACK, data) 6: Wireless and Mobile Networks 6-31 802.11: mobility within same subnet

H1 remains in same IP subnet: IP address can remain same switch: which AP is associated with H1? self-learning (Ch. 5): switch will see frame from H1 and remember which switch port can be used to reach H1 router hub or switch BBS 1 AP 1 AP 2 H1 6: Wireless and Mobile Networks

BBS 2 6-32 802.11: advanced capabilities 10-1 10-2 10-3 BER Rate Adaptation base station, mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves, SNR varies 10-4 10-5

10-6 10-7 10 QAM256 (8 Mbps) QAM16 (4 Mbps) BPSK (1 Mbps) operating point 20 SNR(dB) 30 40 1. SNR decreases, BER increase as node moves away from base station

2. When BER becomes too high, switch to lower transmission rate but with lower BER 6: Wireless and Mobile Networks 6-33 802.11: advanced capabilities Power Management node-to-AP: I am going to sleep until next beacon frame AP knows not to transmit frames to this node node wakes up before next beacon frame beacon frame: contains list of mobiles with AP-tomobile frames waiting to be sent node will stay awake if AP-to-mobile frames to be sent; otherwise sleep again until next beacon frame 6: Wireless and Mobile Networks

6-34 IEEE 802.11 Media Access Control (MAC) Carrier-sense multiple access protocol with collision avoidance (CSMA/CS) DIFS: Distributed Inter-Frame Spacing SIFS: Short Inter-Frame Spacing ack: Acknowledgement Figure 14.4 CSMA/CA flowchart 14.36 Figure 14.5 CSMA/CA and NAV 14.37

Figure 14.6 Example of repetition interval 14.38 Figure 14.7 Frame format 14.39 Table 14.1 Subfields in FC field 14.40 Figure 14.8 Control frames 14.41 Table 14.2 Values of subfields in control frames 14.42

Table 14.3 Addresses 14.43 Figure 14.9 Addressing mechanisms 14.44 Figure 14.10 Hidden station problem 14.45 Note The CTS frame in CSMA/CA handshake can prevent collision from a hidden station. 14.46 Figure 14.11 Use of handshaking to prevent hidden station problem

14.47 Figure 14.12 Exposed station problem 14.48 Figure 14.13 Use of handshaking in exposed station problem 14.49 Table 14.4 Physical layers 14.50 Figure 14.14 Industrial, scientific, and medical (ISM) band 14.51 Figure 14.15 Physical layer of IEEE 802.11 FHSS

14.52 Figure 14.16 Physical layer of IEEE 802.11 DSSS 14.53 Figure 14.17 Physical layer of IEEE 802.11 infrared 14.54 Figure 14.18 Physical layer of IEEE 802.11b 14.55 Logical Link Control Layer (LLC)

Specified by ISO/IEC 8802-2 (ANSI/IEEE 802.2) purpose: exchange data between users across LAN using 802-based MAC controlled link provides addressing and data link control, 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 LLCs functionalities LLCs protocol data unit (PDU) SAP: service address point

Logical Link Control Layer 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) C Acknowledged connectionless service ack-signal used error and flow control by stop-and-wait ARQ faster setup than for B

A TCP/IP packet in 802.11 TPC/IP send data packet Control header LLC constructs PDU by adding a control header SAP (service access point) MAC frame with new control fields Traffic to the target BSS / ESS *BDU: protocol data unit MAC lines up packets using carrier sense multiple access (CSMA) PHY layer transmits packet using a modulation method

(DSSS, OFDM, IR, FHSS) 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 dont support roaming with GSM! - Address to destination mapping - seamless integration of multiple BSS

ESS 2 ESS 1 Security In theory, spread spectrum radio signals are inherently difficult to decipher without knowing the exact hopping sequences or direct sequence codes used The IEEE 802.11 standard specifies optional security called "Wired Equivalent Privacy" whose goal is that a wireless LAN offer privacy equivalent to that offered by a wired LAN. The standard also specifies optional authentication measures. Authentication and privacy

Goal: to prevent unauthorized access & eavesdropping Realized by authentication service prior access Open system authentication station wanting to authenticate sends authentication management frame receiving station sends back frame for successful authentication Shared key authentication (included in WEP*) Secret, shared key received by all stations by a separate, 802.11 independent channel Stations authenticate by a shared knowledge of the key properties WEPs privacy (blocking out eavesdropping) is based on ciphering: *WEP: Wired Equivalent Privacy 802.11b Security Features Wired Equivalent Privacy (WEP) A protocol to protect link-level data during wireless transmission

between clients and access points. Services: Authentication: provides access control to the network by denying access to client stations that fail to authenticate properly. Confidentiality: intends to prevent information compromise from casual eavesdropping Integrity: prevents messages from being modified while in transit between the wireless client and the access point. Authentication Means: Based on cryptography Non-cryptographic Both are identity-based verification mechanisms (devices request access based on the SSID Service Set Identifier of the wireless network). Authentication

Authentication techniques Privacy Cryptographic techniques WEP Uses RC4 symmetric key, stream cipher algorithm to generate a pseudo random data sequence. The stream is XORed with the data to be transmitted Key sizes: 40bits to 128bits Unfortunately, recent attacks have shown that the WEP approach for privacy is vulnerable to certain attack regardless of key size Data Integrity Data integrity is ensured by a simple encrypted version of CRC (Cyclic Redundant Check) Also vulnerable to some attacks Security Problems

Security features in Wireless products are frequently not enabled. Use of static WEP keys (keys are in use for a very long time). WEP does not provide key management. Cryptographic keys are short. No user authentication occurs only devices are authenticated. A stolen device can access the network. Identity based systems are vulnerable. Packet integrity is poor. Other WLAN Security Mechanisms 3Com Dynamic Security Link CISCO LEAP - Lightweight Extensible Authentication Protocol IEEE 802.1x Port-Based Network Access Control RADIUS Authentication Support EAP-MD5 EAP-TLS EAP-TTLS

PEAP - Protected EAP TKIP - Temporal Key Integrity Protocol IEEE 802.11i WLAN Network Planning Network planning target Maximize system performance with limited resource Including coverage throughput capacity interference roaming security, etc. Planning process Requirements for project management personnel

Site investigation Computer-aided planning practice Testing and verifying planning Field measurements Basic tools: power levels - throughput - error rate Laptop or PDA Utility come with radio card HW (i.e. Lucent client manager) Supports channel scan, station search Indicate signal level, SNR, transport rate Advanced tools: detailed protocol data flows Special designed for field measurement Support PHY and MAC protocol analysis

Integrated with network planning tools Examples Procycle from Softbit, Oulu, Finland SitePlaner from WirelessValley, American Capacity planning 802.11b can have 6.5 Mbps rate throughput due to CSMA/CA MAC protocol PHY and MAC management overhead More user connected, less capacity offered Example of supported users in different application cases: Frequency planning

Interference from other WLAN systems or cells IEEE 802.11 operates at uncontrolled ISM band 14 channels of 802.11 are overlapping, only 3 channels are disjointed. For example Ch1, 6, 11 Throughput decreases with less channel spacing A example of frequency allocation in multi-cell network 6 5 4 Mbit/s

11Mb if/frag 512 2Mb if/frag 512 2Mb if/frag 2346 3 2 1 0 Offset 25MHz Offset 20MHz Offset 15MHz Offset

10MHz Offset 5MHz Offset 0MHz Interference from microwave ovens Microwave oven magnetrons have central frequency at 2450~2458 MHz Burst structure of radiated radio signal, one burst will affect several 802.11 symbols 18 dBm level measured from 3 meter away from oven -> masks all WLAN signals! Solutions

Use unaffected channels Keep certain distance Use RF absorber near microwave oven Interference from Bluetooth The received signal level from two systems are comparable at mobile side In co-existing environment, the probability of frequency collision for one 802.11 frame vary from 48% ~62% Deterioration level is relevant to many factors relative signal levels 802.11 frame length activity in Bluetooth channel Solution Co-existing protocol

IEEE 802.15 (not ready) Limit the usage of BT in 802.11 network WLAN benefits Mobility increases working efficiency and productivity extends the On-line period Installation on difficult-to-wire areas inside buildings road crossings Increased reliability Note: Pay attention to security! Reduced installation time

cabling time and convenient to users and difficult-towire cases WLAN benefits (cont.) Broadband 11 Mbps for 802.11b 54 Mbps for 802.11a/g (GSM:9.6Kbps, HCSCD:~40Kbps, GPRS:~160Kbps, WCDMA:up to 2Mbps) Long-term cost savings O & M cheaper that for wired nets Comes from easy maintenance, cabling cost, working efficiency and accuracy Network can be established in a new location just by moving the PCs! WLAN technology problems

Date Speed IEEE 802.11b support up to 11 MBps, sometimes this is not enough far lower than 100 Mbps fast Ethernet Interference Works in ISM band, share same frequency with microwave oven, Bluetooth, and others Security Current WEP algorithm is weak - usually not ON! Roaming No industry standard is available and propriety solution are not interoperable - especially with GSM Inter-operability Only few basic functionality are interoperable, other vendors features

cant be used in a mixed network WLAN implementation problems Lack of wireless networking experience for most IT engineer No well-recognized operation process on network implementation Selecting access points with Best Guess method Unaware of interference from/to other networks Weak security policy As a result, your WLAN may have Poor performance (coverage, throughput, capacity, security) Unstable service Customer dissatisfaction

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