Mobile Communications Chapter 7: Wireless LANs

Mobile Communications Chapter 7: Wireless LANs Characteristics IEEE 802.11 PHY MAC Roaming IEEE 802.11a, b, g, e HIPERLAN Bluetooth Comparisons Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 7.1

Characteristics of Wireless LANs (WLANs) Advantages very flexible within the reception area Ad-hoc networks without previous planning possible (almost) no wiring difficulties (e.g. historic buildings, firewalls) more robust against disasters like, e.g., earthquakes, fire Disadvantages typically low bandwidth compared to wired networks (50-300 Mbit/s user data) many proprietary solutions, especially for higher bit-rates, standards take their time (e.g. IEEE 802.11). See IEEE802.11g, IEEE802.11n! products have to follow many national restrictions if working wireless, it takes a very long time to establish global solutions like, e.g., IMT-2000 Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 7.2

Design goals for wireless LANs global, seamless operation low power for battery use no special permissions or licenses needed to use the LAN robust transmission technology other electrical devices can interfere! simplified spontaneous cooperation at meetings, i.e. simple set up easy to use for everyone, simple management security (no one should be able to read my data), privacy (no one should be able to collect user profiles), safety (low radiation) Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 7.3

Comparison: infrared vs. radio transmission Infrared Advantages uses IR diodes, diffuse light, multiple reflections (walls, furniture etc.) simple, cheap, available in many mobile devices no licenses needed simple shielding possible Disadvantages Example interference by sunlight, heat sources etc. many things shield or absorb IR light low bandwidth IrDA (Infrared Data Association) interface available everywhere Radio Advantages typically using the license free ISM band at 2.4 GHz experience from wireless WAN and mobile phones can be used coverage of larger areas possible (radio can penetrate walls, furniture etc.) Disadvantages Example very limited license free frequency bands shielding more difficult, interference with other electrical devices IEEE802.11, HIPERLAN, Bluetooth Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 7.4

Comparison: infrastructure vs. ad-hoc networks infrastructure network AP AP wired network AP: Access Point AP Infrastructure networks: provide access to other networks typically communications between the wireless node and the access point, but not directly between the wireless nodes. the access points controls medium access, but also act as a bridge to other wireless or wired networks in the figure three WLANs with different coverage areas are connected most of the network functionality lies within the access points, whereas the wireless clients can remain quite simple. the MAC can be centralised to the access point or be distributed oven the wireless clients. these wireless networks do rely on the access points. Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 7.5

Comparison: infrastructure vs. ad-hoc networks ad-hoc network Ad-hoc networks: no need for any infrastructure to work. each node can communicate directly with other nodes no access point controlling media access is necessary nodes within an ad-hoc network can communicate directly or via other nodes the complexity of each node is higher because each node has - implement MAC-mechanisms - handle hidden or exposed terminal problem - perhaps priority mechanisms to provide a certain QoS Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 7.6

Ad Hoc Networking Self-organizing and adaptive Can be de-formed on the fly No system administration Most often battery operated devices Multiple hops may be needed, i.e. each node may act as a router Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 7.7

WLAN In this chapter we will examine three WLANs IEEE 802.11 HiperLAN2 Bluetooth The first two are typically infrastructure-based networks, which additionally support ad-hoc networking, whereas Bluetooth is a typical ad-hoc network. The IEEE 802.11 family IEEE 802.11b 2.4 GHz, 11 Mbps IEEE 802.11a 5 GHz, 54 Mbps IEEE 802.11g 2.4 GHz, 54 Mbps IEEE 802.11n 2.4 GHz 200 Mbps Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 7.8

802.11 - Architecture of an infrastructure network STA 1 ESS 802.11 LAN BSS 1 Access Point BSS 2 Distribution System Access Point 802.x LAN Portal STA 2 802.11 LAN STA 3 Station (STA) terminal with access mechanisms to the wireless medium and radio contact to the access point Basic Service Set (BSS) group of stations using the same radio frequency Access Point station integrated into the wireless LAN and the distribution system Portal bridge to other (wired) networks Distribution System interconnection network to form one logical network (ESS: Extended Service Set) based on several BSS. The architecture of the distributing system is not specified furher in the IEEE 802.11. Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 7.9

802.11 - Architecture of an infrastructure network STA 1 ESS 802.11 LAN BSS 1 Access Point BSS 2 Portal Distribution System Access Point 802.x LAN Stations can select an AP and associate with it. The APs support roaming, (i.e. changing access points) The distribution system handles data transfer between the different APs. APs provide synchronization within a BSS (Basic Service Set) APs support power management STA 2 802.11 LAN STA 3 APs can control medium access to support time-bounded services. Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 7.10

802.11 - Architecture of an ad-hoc network 802.11 LAN Direct communication within a limited range STA 1 IBSS 1 STA 3 Station (STA): terminal with access mechanisms to the wireless medium Independent Basic Service Set (IBSS): STA 2 group of stations using the same radio frequency Here two IBSS are shown. They could be separated by either IBSS 2 - distance STA 5 - using different frequencies or codes STA 4 802.11 LAN Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 7.11

IEEE standard 802.11 Protocol architecture The basic IEEE 802.11 standard!! mobile terminal fixed terminal application TCP IP LLC access point LLC infrastructure network application TCP IP 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 Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 7.12

PHY DLC Station Management 802.11 - Layers and functions MAC access mechanisms, fragmentation, encryption PLCP Physical Layer Convergence Protocol Provides a carrier sense signal, called clear channel assessment signal PMD Physical Medium Dependent modulation, encoding /decoding The standard also specifies management layers and the station management: MAC Management synchronization, roaming, MIB (management information base), power management PHY Management channel selection, MIB Station Management coordination of all management functions LLC MAC PLCP PMD MAC Management PHY Management Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 7.13

802.11 - Physical layer Note: Original 802.11! 3 versions: 2 radio (typ. 2.4 GHz), 1 IR data rates 1 or 2 Mbit/s FHSS (Frequency Hopping Spread Spectrum) spreading, despreading, signal strength, typ. 1 Mbit/s min. 2.5 frequency hops/s (USA), two-level GFSK modulation DSSS (Direct Sequence Spread Spectrum) DBPSK modulation for 1 Mbit/s (Differential Binary Phase Shift Keying), DQPSK for 2 Mbit/s (Differential Quadrature PSK) preamble and header of a frame is always transmitted with 1 Mbps, rest of transmission 1 or 2 Mbit/s chipping sequence: +1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1 (Barker code) max. radiated power 1 W (USA), 100 mw (EU), min. 1mW Infrared 850-950 nm, diffuse light, typ. 10 m range carrier detection, energy detection, synchonization Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 7.14

PHY DLC FHSS (Frequency Hop Spread Spectrum) PHY packet format Synchronization 010101... pattern used for synchronization and signal detection by the CCA (Clear Channel Assessment) SFD (Start Frame Delimiter) 0000110010111101 start pattern indicates the start of the frame PLW (PLCP_PDU Length Word) length of payload in bytes incl. 32 bit CRC of payload, PLW < 4096 PSF (PLCP Signaling Field) data of payload (1 or 2 Mbit/s) HEC (Header Error Check) CRC with x 16 +x 12 +x 5 +1 80 16 12 4 16 variable synchronization SFD PLW PSF HEC payload LLC MAC PLCP PMD PLCP preamble PLCP header PLCP: Physical Layer Convergence Protocol Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 7.15

Synchronization DSSS PHY packet format (More important!!!) synch., gain setting, energy detection, frequency offset compensation SFD (Start Frame Delimiter) Signal 1111001110100000 indicates the start of a frame data rate of the payload (0A: 1 Mbit/s DBPSK; 14: 2 Mbit/s DQPSK) Service Length future use, 00: 802.11 compliant length of the payload HEC (Header Error Check) protection of signal, service and length, x 16 +x 12 +x 5 +1 128 16 8 8 16 16 variable bits synchronization SFD signal service length HEC payload PLCP preamble PLCP header Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 7.16

802.11 - MAC layer I DFWMAC Distributed Foundation Wireless Medium Access Control PHY DLC Traffic services Asynchronous Data Service (mandatory) exchange of data packets based on best-effort support of broadcast and multicast Time-Bounded Service (optional) implemented using PCF (Point Coordination Function) Access methods DFWMAC-DCF CSMA/CA (mandatory) (DCF: distributed coordination function)* collision avoidance via randomized back-off mechanism ACK packet for acknowledgements (not for broadcasts) DFWMAC-DCF w/ RTS/CTS (optional) Distributed Foundation Wireless MAC avoids hidden terminal problem DFWMAC- PCF (optional) (PCF: point coordination function) access point polls terminals according to a list LLC MAC PLCP PMD Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 7.17

802.11 - MAC layer II General! Priorities defined through different inter frame spaces (IFS) no guaranteed hard priorities SIFS (Short Inter Frame Spacing) highest priority, for ACK, CTS, polling response PIFS (PCF, Point Coordination Function IFS) medium priority, for time-bounded service using PCF DIFS (DCF, Distributed Coordination Function IFS) lowest priority, for asynchronous data service DIFS DIFS medium busy PIFS SIFS contention next frame direct access if medium is free DIFS t Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 7.18

CSMA A station who wish to send starts listening Carrier Sense Carrier sense multiple access If the channel is free, it starts to send If the channel is busy the station waits to send, the station might be persistent Acknowledgement The backoff algorithm: after the media is free again each station waits a random time until sensing the media again. The time is chosen randomly within the interval of length L Backoff time interval initially set to L If a collision => L=L*2 Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 7.19

DFWMAC-DCF CSMA/CA (DCF: distributed coordination function) 802.11 - CSMA/CA access method the first DIFS medium busy DIFS contention window (randomized back-off mechanism) next frame direct access if medium is free DIFS slot time t station ready to send starts sensing the medium (Carrier Sense based on CCA, Clear Channel Assessment) if the medium is free for the duration of an Inter-Frame Space (IFS), the station can start sending (IFS depends on service type) if the medium is busy, the station has to wait for a free IFS, then the station must additionally wait a random back-off time (collision avoidance, multiple of slot-time) To provide fainess an backoff timer is added! if another station occupies the medium during the back-off time of the station, the back-off timer stops (fairness) Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 7.20

802.11 - CSMA/CA access method the first Sending unicast packets station has to wait for DIFS before sending data receivers acknowledge at once (after waiting for SIFS) if the packet was received correctly (CRC) automatic retransmission of data packets in case of transmission errors sender DIFS data receiver SIFS ACK other stations waiting time DIFS contention data t Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 7.21

802.11 - competing stations - simple version DIFS DIFS bo e bo r DIFS bo e bo r DIFS 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 not idle (frame, ack etc.) bo e elapsed backoff time packet arrival at MAC bo r residual backoff time Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 7.22

Motivation - hidden and exposed terminals Hidden terminals A sends to B, C cannot receive A C wants to send to B, C senses a free medium (CS fails) collision at B, A cannot receive the collision (CD fails) A is hidden for C Exposed terminals B sends to A, C wants to send to another terminal (not A or B) C has to wait, CS signals a medium in use but A is outside the radio range of C, therefore waiting is not necessary C is exposed to B A B C Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 7.23

802.11 DFWMAC-DCF with RTS/CTS extension To avoid the hidden terminal problem!! Sending unicast packets The second! station can send RTS with reservation parameter after waiting for DIFS (reservation determines amount of time the data packet needs the medium) acknowledgement via CTS after SIFS by receiver (if ready to receive) sender can now send data at once, acknowledgement via ACK other stations store medium reservations distributed via RTS and CTS sender receiver DIFS RTS SIFS CTS SIFS data SIFS ACK NAV: Net Allocation Vector other stations NAV (RTS) NAV (CTS) defer access DIFS contention data t Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 7.24

Fragmentation If a node senses the wireless channel to be very disturbed, the node may choose to fragment its sending frame into small sub frames (frags). sender receiver DIFS RTS SIFS CTS SIFS frag 1 SIFS ACK SIFS 1 frag 2 SIFS ACK2 other stations NAV (RTS) NAV (CTS) NAV (frag 1 ) NAV (ACK 1 ) DIFS contention data t In the frags, the control field gives information to other stations of how to set their NAV. Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 7.25

DFWMAC-PCF I The third! t 0 t 1 SuperFrame medium busy point coordinator PIFS D 1 SIFS SIFS D 2 SIFS SIFS wireless stations U 1 U 2 stations NAV NAV t 2 t 3 t 4 point coordinator wireless stations D 3 PIFS D 4 SIFS stations NAV NAV contention free period contention period Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 7.26 U 4 SIFS CFend t

Interframe spacing (IFS) Interframe intervals used as a priority mechanism SIFS PIFS DIFS EIFS CTS, ACK, fragments PCF mode DCF before sending RTS After error in packet reception DIFS RTS SIFS CTS SIFS DATA SIFS ACK Why SIFS must be shorter than DIFS? DIFS = 2*slots + SIFS In PCF, why PIFS < DIFS? DIFS PIFS POLL 27

Types 802.11 - Frame format MAC frames control frames, management frames, data frames Sequence numbers important against duplicated frames due to lost ACKs Addresses receiver, transmitter (physical), BSS identifier, sender (logical) Miscellaneous bytes sending time, checksum, frame control, data 2 2 6 6 6 6 Duration/ ID Address 1 Address 2 Address 3 Sequence Control Frame Control Protocol version Type Subtype To DS More Frag Power Retry Mgmt 2 0-2312 4 Address Data 4 bits 2 2 4 1 1 1 1 1 1 1 1 From DS More Data WEP Order CRC Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 7.28

Frame control: 802.11 - Frame format see next slide Duration/ID: indicating the time the medium is occupied (in micro s) Address 1 to 4: IEEE 802 MAC addresses (48 bits each), see coming slide Sequence ctrl: Data: CRC: bytes Frame Control sequence number to filter duplicates The MAC frame contains arbitrary data (max 2312 bytes), which is transferred transparently from sender to receivers 32 bit checksum is used to protect the frame as is common practice in all 802.x networks 2 2 6 6 6 6 Duration/ ID Address 1 Address 2 Address 3 Sequence Control Protocol version Type Subtype To DS More Frag Power Retry Mgmt 2 0-2312 4 Address Data 4 bits 2 2 4 1 1 1 1 1 1 1 1 From DS More Data WEP Order CRC Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 7.29

802.11 - Frame format Protocol version: the current protocol version, fixed to 0 now. If major future revisions, this value will be increased Type: Subtype: To DS/From DS: More Frag type of frame: management, control, data several subtypes exist, like for different control frames see coming slides If more fragments to follow Retry: if the current frame is a retransmission the bit is set to 1 Power man: More Data: WEP: Order: bytes Frame Control the mode of the station after a successful transmission of a frame, set to 1 if the station goes into power-save mode if the station has more data to send Wireless equivalent privacy. Security if this bit is set to 1 the received frames must be processed in order 2 2 6 6 6 2 6 0-2312 4 Duration/ Address Address Address Sequence Address Data ID 1 2 3 Control 4 bits 2 2 4 1 1 1 1 1 1 1 1 Protocol version Type Subtype To DS From DS More Frag Power Retry Mgmt More Data WEP Order CRC Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 7.30

MAC address format MAC frames can be transmitted between mobile stations between mobile stations and an access point between access points over a DS 2 bits within the Frame Control Field to DS and from DS, differentiate these cases and control the meaning of the four addresses used. DS: Distribution System AP: Access Point DA: Destination Address SA: Source Address BSSID: Basic Service Set Identifier RA: Receiver Address (AP) TA: Transmitter Address (AP) scenario to DS from address 1 address 2 address 3 address 4 DS ad-hoc network 0 0 DA SA BSSID - infrastructure 0 1 DA BSSID SA - network, from AP infrastructure 1 0 BSSID SA DA - network, to AP infrastructure network, within DS 1 1 RA TA DA SA Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 7.31

Special Frames: ACK, RTS, CTS Acknowledgement Request To Send Clear To Send ACK RTS CTS bytes 2 2 6 4 Frame Duration Receiver CRC Control Address bytes 2 2 6 6 4 Frame Duration Receiver Transmitter CRC Control Address Address bytes 2 2 6 4 Frame Duration Receiver CRC Control Address Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 7.32

802.11 - MAC management MAC management plays a central role in an IEEE 802.11 station. It more or less controls all functions related to system integration! Synchronization try to find a LAN, try to stay within a LAN timer etc. Power management sleep-mode without missing a message periodic sleep, frame buffering, traffic measurements Association/Reassociation integration into a LAN roaming, i.e. change networks by changing access points scanning, i.e. active search for a network MIB - Management Information Base All parameters representing the current state of a wireless station and an access point are stored within a MIB for internal and external access Managing, read, write Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 7.33

Synchronization using a Beacon (infrastructure) Each node of an 802.11 network maintains an internal clock! To synchronize the clocks of all nodes, IEEE 802.11 specifies a Timer synchronization function (TSF) Within a BSS (basic server set), timing is conveyed by the (quasi)periodic transmission of a beacon. A beacon contains a timestamp and other management information used for power management and roaming (e.g. identification of the BSS). Within infrastructure-base networks, the access point performs synch by transmitting beacons beacon interval access point medium B B B B busy busy busy busy value of the timestamp B beacon frame t Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 7.34

Synchronization using a Beacon (ad-hoc) For ad-hoc networks, the situation is slightly more complicated as they do not have an access point for beacon transmission. In this case, each node maintains its own synchronization timer and starts transmission of a beacon frame after the beacon interval. However, the standard random backoff algorithm is also applied to the beacon frames so only one beacon wins. All other stations now adjust their internal clocks according to the received beacon and suppress their beacons for this cycle. 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 Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 7.35

Power management 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 Ad-hoc Traffic Indication Map (TIM), sent with the beacons list of unicast receivers transmitted by AP Delivery Traffic Indication Map (DTIM) list of broadcast/multicast receivers transmitted by AP Ad-hoc Traffic Indication Map (ATIM) announcement of receivers by stations buffering frames more complicated - no central AP collision of ATIMs possible (scalability?) Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 7.36

Power saving with wake-up patterns (infrastructure) Example: one access point and one station The AP transmits a beacon frame each beacon interval. This interval is equal to the TIM interval. The AP maintains a delivery traffic indication map (DTIM) interval. TIM interval DTIM interval access point medium D B busy T T d D busy busy busy B station T TIM D DTIM p awake d t B broadcast/multicast p PS poll d data transmission to/from the station Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 7.37

Power saving with wake-up patterns (ad-hoc) ATIM window beacon interval B station 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 Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 7.38

802.11 - 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 Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 7.39

IEEE 802.11b (IEEE 1999) Supplement to the original standard (Higher speed physical layer extension in the 2.4 GHz band) A new physical layer (JUST A NEW PHYSICAL LAYER!) All the MAC schemes, management procedures, etc are still used. Depending on the current interface and the distance between sender and reciever 802.11 systems offer 11, 5.5, 2 or 1 Mbps Next slide shows the mandatory packet format for 802.11b which is similar to the original. There are others as well. However all control packets follow the mandatory for all stations to understand. Operates on certain frequencies at the 2.4 GHz ISM band. These frequences depend on national regulations. Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 7.40

IEEE 802.11b PHY frame formats Long PLCP PPDU format (PLCP: Physical Layer Convergence Protocol) 128 16 8 8 16 16 variable bits synchronization SFD signal service length HEC payload PLCP preamble PLCP header 192 µs at 1 Mbit/s DBPSK 1, 2, 5.5 or 11 Mbit/s The format is similar to the original 802.11 format. One difference is the rate encoded in the signal field in multiples of 100 kbps. Signal+Service = 16 bits! => 0x0A: 1 Mbps; 0x14: 2 Mbps; 0x37: 5.5 Mbps; 0x6E: 11 Mbps Note that the preamble and the header are always transmitted at 1 Mbps! Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 7.41

IEEE 802.11b PHY frame formats Long PLCP PPDU format (PLCP: Physical Layer Convergence Protocol) 128 16 8 8 16 16 variable bits synchronization SFD signal service length HEC payload PLCP preamble PLCP header 192 µs at 1 Mbit/s DBPSK 1, 2, 5.5 or 11 Mbit/s Short PLCP PPDU format (optional) 56 16 8 8 16 16 variable bits short synch. SFD signal service length HEC payload PLCP preamble (1 Mbit/s, DBPSK) PLCP header (2 Mbit/s, DQPSK) 96 µs 2, 5.5 or 11 Mbit/s Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 7.42

Channel plan for IEEE 802.11b Channel Frequency (MHz) US/Canada Europé Japan 1 2412 x x x 2 2417 x x x 3 2422 x x x 4 2427 x x x 5 2432 x x x 6 2437 x x x 7 2442 x x x 8 2447 x x x 9 2452 x x x 10 2457 x x x 11 2462 x x x 12 2467 - x x 13 2472 - x x 14 2484 - - x Altogether 14 channels have been defined as the table shows. For each channel a center frequency is given. Depending on national restrictions 11 (US/Canada), 13 (Europe) or 14 channels (Japan) can be used. Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 7.43

Channel selection (non-overlapping) Non-overlapping usage of channels for an IEEE 802.11b installation with minimum interference. The spacing between center frequencies should be at least 25 MHz. The occupied bandwidth of the main lobe of the signal is 22 MHz. Europe (ETSI) Users can install overlapping cells for WLANs using three non-overlapping chanells. channel 1 channel 7 channel 13 2400 2412 2442 2472 2483.5 [MHz] US (FCC)/Canada (IC) 22 MHz channel 1 channel 6 channel 11 2400 2412 2437 2462 2483.5 [MHz] 22 MHz Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 7.44

WLAN: IEEE 802.11b Data rate 1, 2, 5.5, 11 Mbit/s, depending on SNR User data rate max. approx. 6 Mbit/s Transmission range Frequency Security Availability 300m outdoor, 30m indoor Max. data rate ~10m indoor Free 2.4 GHz ISM-band Limited, WEP insecure, Many products, many vendors Quality of Service Typ. Best effort, no guarantees (unless polling is used, limited support in products) Special Advantages/Disadvantages Advantage: many installed systems, lot of experience, available worldwide, free ISM-band, many vendors, integrated in laptops, simple system Disadvantage: heavy interference on ISM-band, no service guarantees, slow relative speed only Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 7.45

IEEE 802.11a Initially aimed at the US 5 GHz U-NII (Unlicenced National Information Infrastucture) bands. Offering up to 56 Mbps using OFDM (IEEE, 1999). The first products were offered in 2001, but harmonization between IEEE and ETSI took some time. ETSI (in Europe) defines different frequecy bands: 5.15-5.35 and 5.47-5.725 GHz. ETSI also requires: Dynamic frequency selection (DFS) and transmitt power control (TPC). DFC and TPC are not necessary, if the transmission power stays below 50 mw The physical layer of 802.11a and the ETSI standard HiperLAN2 has been jointly developed, so both physical layers are almost identical. However, 802.11a products were available first. Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 7.46

IEEE 802.11a IEEE 802.11a uses the same MAC layer as all IEEE 802.11 physical layers do. To be able to offer data rates up to 54 Mbps IEEE 802.11a uses many different technologies. The system uses OFDM (52 subcarriers) that are modulated using BPSK, QPSK, 16-QAM, or 64-QAM. To mitigate transmission errors, FEC is applied using coding rates of 1/2, 2/3 or 3/4 Similar to 802.11b several operating channels have been standardized to minimize interference. Due to the nature of OFDM, the PDU on the physical layer of IEEE 802.11a looks quite different from 802.11b or the original 802.11 physical layers. Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 7.47

Operating channels for 802.11a / US U-NII 36 40 44 48 52 56 60 64 channel 5150 5180 5200 5220 5240 5260 5280 5300 5320 5350 [MHz] 16.6 MHz 149 153 157 161 channel Center frequency = 5000 + 5*channel number [MHz] Channel spacing is 20 MHz 5725 5745 5765 5785 5805 5825 [MHz] The occupied bandwidth is 16.6 MHz 16.6 MHz Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 7.48

IEEE 802.11a PHY frame format IEEE 802.11a uses OFDM (orthogonal frequency division multiplex) coding (2.6.6). Thus the PDU (Packet data unit) on the physical layer looks quite different from 802.11b or the original 802.11 physical layers. 4 1 12 1 6 16 variable 6 variable bits rate reserved length parity tail service payload tail pad PLCP header PLCP preamble signal data 12 1 variable OFDM symbols 6 Mbit/s 6, 9, 12, 18, 24, 36, 48, 54 Mbit/s Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 7.49

WLAN: IEEE 802.11a Data rate 6, 9, 12, 18, 24, 36, 48, 54 Mbit/s, depending on SNR 6, 12, 24 Mbit/s mandatory Transmission range 100m outdoor, 10m indoor 54 Mbit/s up to 5 m, 48 up to 12 m, 36 up to 25 m, 24 up to 30m, 18 up to 40 m, 12 up to 60 m Frequency Free 5.15-5.25, 5.25-5.35, 5.725-5.825 GHz ISM-band Security Limited, WEP insecure, Quality of Service Typ. best effort, no guarantees (same as all 802.11 products) Special Advantages/Disadvantages Advantage: fits into 802.x standards, free ISM-band, available, simple system, uses less crowded 5 GHz band Disadvantage: stronger shading due to higher frequency, no QoS Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 7.50

IEEE 802.11n OFDM MIMO 40 MHz channels 2.4 GHz (in fact also in the 5 GHz band) up to 600 Mbps Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 7.51

WLAN: IEEE 802.11 some examples 802.11-2007: A new release of the standard that includes amendments a, b, d, e, g, h, i & j. (July 2007) 802.11e: MAC Enhancements QoS Enhance the current 802.11 MAC to expand support for applications with Quality of Service requirements, and in the capabilities and efficiency of the protocol. 802.11g: Data Rates at 2.4 GHz; 54 Mbit/s, OFDM, (backwards compatible with b) (2003) 802.11s: Mesh Networking, Extended Service Set (ESS) (June 2011) 802.11ad: Very High Throughput 60 GHz (~Dec 2012) Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 7.52

Ad Hoc Networks Power is an issue All stations are mobile Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 7.53

Broadband Internet access via mesh networks Power is NOT an issue Stations are NOT mobile FL, UniK 4290: Introduction, 28.01.08 54

Mesh networking with Ad hoc networks FL, UniK 4290: Introduction, 28.01.08 55

Sensor networks Power is an issue Stations are NOT mobile Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 7.56

End Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 7.57