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wireless networks, mobile computing.pptx

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wireless LAN

Engineering
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Wireless Networks  Characteristics  IEEE 802.11  PHY  MAC  Roaming  HIPERLAN  Standards  PHY  MAC  Ad-hoc networks  Bluetooth
Characteristics of wireless LANs 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 - or users pulling a plug... Disadvantages  typically very low bandwidth compared to wired networks (1-10 Mbit/s)  many proprietary solutions, especially for higher bit- rates, standards take their time (e.g. IEEE 802.11)  products have to follow many national restrictions if working wireless, it takes a vary long time to establish global solutions like, e.g., IMT-2000
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  simplified spontaneous cooperation at meetings  easy to use for everyone, simple management  protection of investment in wired networks  security (no one should be able to read my data), privacy (no one should be able to collect user profiles), safety (low radiation)  transparency concerning applications and higher layer protocols, but also location awareness if necessary
Comparison: infrared vs. radio transmission Infrared  uses IR diodes, diffuse light, multiple reflections (walls, furniture etc.) Advantages  simple, cheap, available in many mobile devices  no licenses needed  simple shielding possible Disadvantages  interference by sunlight, heat sources etc.  many things shield or absorb IR light  low bandwidth Example  IrDA (Infrared Data Association) interface available everywhere Radio  typically using the license free ISM band at 2.4 GHz Advantages  experience from wireless WAN and mobile phones can be used  coverage of larger areas possible (radio can penetrate walls, furniture etc.) Disadvantages  very limited license free frequency bands  shielding more difficult, interference with other electrical devices Example  WaveLAN, HIPERLAN, Bluetooth
Comparison: infrastructure vs. ad-hoc networks infrastructur e network ad-hoc network AP AP AP wired network AP: Access Point
Distribution System Portal 802.x LAN Access Point 802.11 LAN BSS2 802.11 LAN BSS 1 Access Point 802.11 - Architecture of an infrastructure network 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 (EES: Extended Service Set) based on several STA1 STA2 STA3 ESS
802.11 - Architecture of an ad-hoc network Direct communication within a limited range  Station (STA): terminal with access mechanisms to the wireless medium  Basic Service Set (BSS): group of stations using the same radio frequency 802.11 LAN BSS2 802.11 LAN BSS1 STA1 STA4 STA5 STA2 STA3
IEEE standard 802.11 mobile terminal fixed terminal applicatio n TCP 802.11 PHY 802.11 MAC IP 802.3 MAC 802.3 PHY applicatio n TCP 802.3 PHY 802.3 MAC IP 802.11 MAC 802.11 PHY LLC server infrastructure network access point LLC LLC
802.11 - Layers and functions PLCP Physical Layer Convergence Protocol  clear channel assessment signal (carrier sense) PMD Physical Medium Dependent  modulation, coding PHY Management  channel selection, MIB Station Management  coordination of all management function s MAC  access mechanisms, fragmentation, encryption MAC Management  synchronization, roaming, MIB, power management DLC LLC MAC MAC Management
802.11 - Physical layer 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 Mbit/s, 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
FHSS PHY packet format synchronization SFD PLW PSF HEC payload PLCP preamble PLCP header bits Synchronization  synch with 010101... pattern SFD (Start Frame Delimiter)  0000110010111101 start pattern PLW (PLCP_PDU Length Word)  length of payload 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 x16+x12+x5+1 80 16 12 4
DSSS PHY packet format PLCP preamble PLCP header bits synchronization SFD signal service length HEC payload Synchronization  synch., gain setting, energy detection, frequency offset compensation SFD (Start Frame Delimiter)  1111001110100000 Signal  data rate of the payload (0A: 1 Mbit/s DBPSK; 14: 2 Mbit/s DQPSK) Length  length of the payload Service  future use, 00: 802.11 compliant HEC (Header Error Check)  protection of signal, service and length, x16+x12+x5+1 128 16 8 8 16 16 variable
802.11 - MAC layer I - DFWMAC 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)  collision avoidance via randomized „back-off“ mechanism  minimum distance between consecutive packets  ACK packet for acknowledgements (not for broadcasts)  DFWMAC-DCF w/ RTS/CTS (optional)  Distributed Foundation Wireless MAC  avoids hidden terminal problem  DFWMAC- PCF (optional)  access point polls terminals according to a list
802.11 - MAC layer II Priorities  defined through different inter frame spaces  no guaranteed, hard priorities  SIFS (Short Inter Frame Spacing)  highest priority, for ACK, CTS, polling response  PIFS (PCF IFS)  medium priority, for time-bounded service using PCF  DIFS (DCF, Distributed Coordination Function IFS)  lowest priority, for asynchronous data service t medium busy PIFS SIFS DIFS DIFS next frame contention direct access if medium is free DIFS
t medium busy DIFS DIFS next frame contention window (randomized back- off mechanism ) 802.11 - CSMA/CA access method I slot time  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)  if another station occupies the medium during the back-off time of the station, the back-off timer stops (fairness) direct access if medium is free DIFS
802.11 - competing stations - simple version t station1 station2 busy station3 station4 station5 packet arrival at MAC boe busy boe elapsed backoff time bor residual backoff time busy medium not idle (frame, ack etc.) boe bor boe busy boe busy DIFS DIFS DIFS DIFS boe bor boe bor boe busy boe bor boe bor
802.11 - CSMA/CA access method II t SIFS 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 DIFS data ACK waiting time other station s receiver sende r data DIFS contentio n
Mobile Communications: Wireless LANs 802.11 - DFWMAC Sending unicast packets  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 t SIFS DIFS data ACK defer access other station s receiver sende r data DIFS contentio n RTS CTS SIFS 7.17.1 SIFS NAV (RTS) NAV (CTS)
Fragmentation t data other station s receiver sende r frag1 DIFS contentio n DIFS RTS SIFS SIFS CTS NAV (RTS) NAV (CTS) NAV (frag1) NAV (ACK1) SIFS ACK2 frag2 SIFS SIFS ACK1
DFWMAC-PCF I wireless stations stations ‘ NAV poin t coordinato r D1 U1 SIFS NAV SIFS D2 U2 SIFS SIFS SuperFrame medium busy PIFS t0 t1
DFWMAC-PCF II t NAV wireless stations stations‘ point coordinator D3 NAV PIFS D4 U4 SIFS SIFS CFend contentio n perio d contention free period t2 t3 t4
802.11 - Frame format Types  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  sending time, checksum, frame control, data Frame Contro l Duratio n ID Address 1 Address 2 Address 3 Sequence Control Address 4 Data CRC 2 2 6 6 6 6 2 4 0-2312 bytes version, type, fragmentation, security, ...
MAC address format scenario to DS from DS address 1 address 2 address 3 address 4 ad-hoc network 0 0 DA SA BSSID - infrastructure network, from AP 0 1 DA BSSID SA - infrastructure network, to AP 1 0 BSSID SA DA - infrastructure network, within DS 1 1 RA TA DA SA DS: Distribution System AP: Access Point DA: Destination Address SA: Source Address BSSID: Basic Service Set Identifier RA: Receiver Address TA: Transmitter Address
802.11 - MAC management 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  managing, read, write
Synchronization using a Beacon (infrastructure) beacon interval t access point mediu m busy B busy busy busy B B B value of the timestamp B beacon frame
Synchronization using a Beacon (ad-hoc) mediu m station1 busy B1 beacon interval busy busy busy B1 value of the timestamp B beacon frame station2 B2 B2 t random delay
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  Traffic Indication Map (TIM)  list of unicast receivers transmitted by AP  Delivery Traffic Indication Map (DTIM)  list of broadcast/multicast receivers transmitted by AP Ad-hoc  Ad-hoc Traffic Indication Map (ATIM)  announcement of receivers by stations buffering frames  more complicated - no central AP  collision of ATIMs possible (scalability?)
Power saving with wake-up patterns (infrastructure) TIM interval t access point mediu m busy busy busy busy T T TIM D DTIM DTIM interval D B D B B broadcast/ multicast statio n awake p PS poll T d p d d data transmission to/from the station
Power saving with wake-up patterns (ad-hoc) awake A transmit ATIM D transmit data t station1 B1 B1 B beacon frame station2 B2 random delay A D B2 a d ATIM windo w beacon interval a acknowledge ATIM d acknowledge data
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
Future developments IEEE 802.11a  compatible MAC, but now 5 GHz band  transmission rates up to 20 Mbit/s  close cooperation with BRAN (ETSI Broadband Radio Access Network) IEEE 802.11b  higher data rates at 2.4 GHz  proprietary solutions already offer 10 Mbit/s IEEE WPAN (Wireless Personal Area Networks)  market potential  compatibility  low cost/power, small form factor  technical/economic feasibility  Bluetooth
ETSI - HIPERLAN ETSI standard  European standard, cf. GSM, DECT, ...  Enhancement of local Networks and interworking with fixed networks  integration of time-sensitive services from the early beginning HIPERLAN family  one standard cannot satisfy all requirements  range, bandwidth, QoS support  commercial constraints  HIPERLAN 1 standardized since 1996 physical layer channel access control layer medium access control layer physical layer data link layer HIPERLAN layers OSI layers network layer higher layers physical layer medium access control layer logical link control layer IEEE 802.x layers
Overview: original HIPERLAN protocol family HIPERLAN 1 HIPERLAN 2 HIPERLAN 3 HIPERLAN 4 Application wireless LAN access to ATM fixed networks wireless local loop point-to- point wireless ATM connections Frequency 5.1-5.3GHz 17.2-17.3GHz Topology decentralized ad- hoc/infrastructur e cellular, centralize d point-to- multipoi nt point-to-point Antenna omni-directional directional Range 50 m 50-100 m 5000 m 150 m QoS statistical ATM traffic classes (VBR, CBR, ABR, UBR) Mobility <10m/s stationary Interface conventional LAN ATM networks Data rate 23.5 Mbit/s >20 Mbit/s 155 Mbit/s Power conservatio n yes not necessary Check out Wireless ATM for new names!
HIPERLAN 1 - Characteristics Data transmission  point-to-point, point-to-multipoint, connectionless  23.5 Mbit/s, 1 W power, 2383 byte max. packet size Services  asynchronous and time-bounded services with hierarchical priorities  compatible with ISO MAC Topology  infrastructure or ad-hoc networks  transmission range can be larger then coverage of a single node („forwarding“ integrated in mobile terminals) Further mechanisms  power saving, encryption, checksums
HIPERLAN 1 - Services and protocols CAC service  definition of communication services over a shared medium  specification of access priorities  abstraction of media characteristics MAC protocol  MAC service, compatible with ISO MAC and ISO MAC bridges  uses HIPERLAN CAC CAC protocol  provides a CAC service, uses the PHY layer, specifies hierarchical access mechanisms for one or several channels Physical protocol  send and receive mechanisms, synchronization, FEC, modulation, signal strength
HIPERLAN layers, services, and protocols HM-entity HC-entity HM-entity HC-entity MAC layer CAC layer PHY layer HP-entity HP-entity LLC layer HMPDU HCPDU data bursts MAC protocol CAC protocol PHY protocol MAC service CAC service PHY service MSDU MSAP MSDU MSAP HCSDU HCSAP HCSDU HCSAP
HIPERLAN 1 - Physical layer Scope  modulation, demodulation, bit and frame synchronization  forward error correction mechanisms  measurements of signal strength  channel sensing Channels  3 mandatory and 2 optional channels (with their carrier frequencies)  mandatory  channel 0: 5.1764680 GHz  channel 1: 5.1999974 GHz  channel 2: 5.2235268 GHz  optional (not allowed in all countries)  channel 3: 5.2470562 GHz  channel 4: 5.2705856 GHz
HIPERLAN 1 - Physical layer frames Modulation  GMSK for high bit-rate, FSK for LBR header LBR synchronization data0 data1 datam-1 . . . Maintaining a high data-rate (23.5 Mbit/s) is power consuming - problematic for mobile terminals  packet header with low bit-rate comprising receiver information  only receiver(s) address by a packet continue receiving Frame structure  LBR (Low Bit-Rate) header with 1.4 Mbit/s  450 bit synchronization  minimum 1, maximum 47 frames with 496 bit each  for higher velocities of the mobile terminal (> 1.4 m/s) the maximum number of frames has to be reduced HBR
HIPERLAN 1 - CAC sublayer Channel Access Control (CAC)  assure that terminal does not access forbidden channels  priority scheme, access with EY-NPMA Priorities  5 priority levels for QoS support  QoS is mapped onto a priority level with the help of the packet lifetime (set by an application)  if packet lifetime = 0 it makes no sense to forward the packet to the receiver any longer  standard start value 500ms, maximum 16000ms  if a terminal cannot send the packet due to its current priority, waiting time is permanently subtracted from lifetime  based on packet lifetime, waiting time in a sender and number of hops to the receiver, the packet is assigned to one out of five priorities  the priority of waiting packets, therefore, rises automatically
prioritization HIPERLAN 1 - EY-NPMA I EY-NPMA (Elimination Yield Non-preemptive Priority Multiple Access)  3 phases: priority resolution, contention resolution, transmission  finding the highest priority  every priority corresponds to a time-slot to send in the first phase, the higher the priority the earlier the time-slot to send  higher priorities can not be preempted  if an earlier time-slot for a higher priority remains empty, stations with the next lower priority might send  after this first phase the highest current priority has been determined contention transmission transmissio n synchronizatio n priority detection priority assertion t user data elimination burst elimination survival verifivcation yield listening IYS IPS IPA IES IESV
HIPERLAN 1 - EY-NPMA II Several terminals can now have the same priority and wish to send  contention phase  Elimination Burst: all remaining terminals send a burst to eliminate contenders (11111010100010011100000110010110, high bit- rate)  Elimination Survival Verification: contenders now sense the channel, if the channel is free they can continue, otherwise they have been eliminated  Yield Listening: contenders again listen in slots with a nonzero probability, if the terminal senses its slot idle it is free to transmit at the end of the contention phase  the important part is now to set the parameters for burst duration and channel sensing (slot-based, exponentially distributed)  data transmission  the winner can now send its data (however, a small chance of collision remains)  if the channel was idle for a longer time (min. for a duration of 1700 bit) a terminal can send at once without using EY-NPMA  synchronization using the last data transmission
HIPERLAN 1 - DT-HCPDU/AK-HCPDU 1 0 1 0 1 0 1 0 0 1 HI HDA HDA HDACS BLIR = n BL- IRCS 1 0 1 2 3 4 5 6 7 bit LBR HBR bit 0 1 2 3 4 5 6 7 byte TI BLI = n PLI = m HID DA SA UD PAD CS 1 2 3 - 6 7 - 12 13 - 18 19 - (52n-m-4) (52n-m-3) - (52n-4) (52n-3) - 52n 1 0 1 0 1 0 1 0 0 1 HI AID AID AIDCS 0 1 2 3 4 5 6 7 bit LBR Data HCPDU Acknowledgement HCPDU HI: HBR-part Indicator HDA: Hashed Destination HCSAP Address HDACS: HDA CheckSum BLIR: Block Length Indicator BLIRCS: BLIR CheckSum TI: Type Indicator BLI: Block Length Indicator HID: HIPERLAN IDentifier DA: Destination Address SA: Source Address UD: User Data (1-2422 byte) PAD: PADding CS: CheckSum AID: Acknowledgement IDentifier AIDS: AID CheckSum
HIPERLAN 1 - MAC layer Compatible to ISO MAC Supports time-bounded services via a priority scheme Packet forwarding  support of directed (point-to-point) forwarding and broadcast forwarding (if no path information is available)  support of QoS while forwarding Encryption mechanisms  mechanisms integrated, but without key management Power conservation mechanisms  mobile terminals can agree upon awake patterns (e.g., periodic wake-ups to receive data)  additionally, some nodes in the networks must be able to buffer data for sleeping terminals and to forward them at the right time (so called stores)
HIPERLAN 1 - DT-HMPDU LI: Length Indicator TI: Type Indicator RL: Residual Lifetime PSN: Sequence Number DA: Destination Address SA: Source Address ADA: Alias Destination Address ASA: Alias Source Address UP: User Priority ML: MSDU Lifetime KID: Key Identifier IV: Initialization Vector UD: User Data, 1–2383 n= 40–2422 bit 0 1 2 3 4 5 6 7 byte LI = n TI = 1 RL PSN DA SA ADA ASA UP ML ML KID IV IV UD SC 1 - 2 3 4 - 5 6 - 7 8 - 13 14 - 19 20 - 25 26 - 31 32 33 34 35 - 37 38 - (n-2) (n-1) - n Data HMPDU
Information bases Route Information Base (RIB) - how to reach a destination  [destination, next hop, distance] Neighbor Information Base (NIB) - status of direct neighbors  [neighbor, status] Hello Information Base (HIB) - status of destination (via next hop)  [destination, status, next hop] Alias Information Base (AIB) - address of nodes outside the net  [original MSAP address, alias MSAP address] Source Multipoint Relay Information Base (SMRIB) - current MP status  [local multipoint forwarder, multipoint relay set] Topology Information Base (TIB) - current HIPERLAN topology  [destination, forwarder, sequence] Duplicate Detection Information Base (DDIB) - remove duplicates  [source, sequence]
Ad-hoc networks using HIPERLAN 1 neighborhood (i.e., within radio range) Information Bases (IB): RIB: Route NIB: Neighbor HIB: Hello AIB: Alias SMRIB: Source Multipoint Relay TIB: Topology DDIB: Duplicate Detection RIB NIB HIB AIB SMRIB TIB DDIB RIB NIB HIB AIB SMRIB TIB DDIB RIB NIB HIB AIB SMRIB TIB DDIB RIB NIB HIB AIB DDIB RIB NIB HIB AIB DDIB RIB NIB HIB AIB DDIB 2 3 4 5 6 1 Forwarder Forwarde r Forwarde r
Bluetooth Consortium: Ericsson, Intel, IBM, Nokia, Toshiba - many members Scenarios  connection of peripheral devices  loudspeaker, joystick, headset  support of ad-hoc networking  small devices, low-cost  bridging of networks  e.g., GSM via mobile phone - Bluetooth - laptop Simple, cheap, replacement of IrDA, low range, lower data rates  2.4 GHz, FHSS, TDD, CDMA
States of a Bluetooth device (PHY layer) STANDBY inquiry page connected transmit PARK HOLD SNIFF unconnecte d connectin g active low power
Bluetooth MAC layer access code packet header payload MAC address type flow ARQN SEQN HEC 3 4 1 1 1 8 bits Synchronous Connection-Oriented link (SCO)  symmetrical, circuit switched, point-to-point Asynchronous Connectionless Link (ACL)  packet switched, point-to-multipoint, master polls Access code  synchronization, derived from master, unique per channel Packet header  1/3-FEC, MAC address (1 master, 7 slaves), link type, alternating bit ARQ/SEQ, checksum 72 54 0-2745 bits
Scatternets piconet s Each piconet has one master and up to 7 slaves Master determines hopping sequence, slaves have to synchronize Participation in a piconet = synchronization to hopping sequence Communication between piconets = devices jumping back and forth between the piconets

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wireless networks, mobile computing.pptx

  • 1. Wireless Networks  Characteristics  IEEE 802.11  PHY  MAC  Roaming  HIPERLAN  Standards  PHY  MAC  Ad-hoc networks  Bluetooth
  • 2. Characteristics of wireless LANs 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 - or users pulling a plug... Disadvantages  typically very low bandwidth compared to wired networks (1-10 Mbit/s)  many proprietary solutions, especially for higher bit- rates, standards take their time (e.g. IEEE 802.11)  products have to follow many national restrictions if working wireless, it takes a vary long time to establish global solutions like, e.g., IMT-2000
  • 3. 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  simplified spontaneous cooperation at meetings  easy to use for everyone, simple management  protection of investment in wired networks  security (no one should be able to read my data), privacy (no one should be able to collect user profiles), safety (low radiation)  transparency concerning applications and higher layer protocols, but also location awareness if necessary
  • 4. Comparison: infrared vs. radio transmission Infrared  uses IR diodes, diffuse light, multiple reflections (walls, furniture etc.) Advantages  simple, cheap, available in many mobile devices  no licenses needed  simple shielding possible Disadvantages  interference by sunlight, heat sources etc.  many things shield or absorb IR light  low bandwidth Example  IrDA (Infrared Data Association) interface available everywhere Radio  typically using the license free ISM band at 2.4 GHz Advantages  experience from wireless WAN and mobile phones can be used  coverage of larger areas possible (radio can penetrate walls, furniture etc.) Disadvantages  very limited license free frequency bands  shielding more difficult, interference with other electrical devices Example  WaveLAN, HIPERLAN, Bluetooth
  • 5. Comparison: infrastructure vs. ad-hoc networks infrastructur e network ad-hoc network AP AP AP wired network AP: Access Point
  • 6. Distribution System Portal 802.x LAN Access Point 802.11 LAN BSS2 802.11 LAN BSS 1 Access Point 802.11 - Architecture of an infrastructure network 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 (EES: Extended Service Set) based on several STA1 STA2 STA3 ESS
  • 7. 802.11 - Architecture of an ad-hoc network Direct communication within a limited range  Station (STA): terminal with access mechanisms to the wireless medium  Basic Service Set (BSS): group of stations using the same radio frequency 802.11 LAN BSS2 802.11 LAN BSS1 STA1 STA4 STA5 STA2 STA3
  • 8. IEEE standard 802.11 mobile terminal fixed terminal applicatio n TCP 802.11 PHY 802.11 MAC IP 802.3 MAC 802.3 PHY applicatio n TCP 802.3 PHY 802.3 MAC IP 802.11 MAC 802.11 PHY LLC server infrastructure network access point LLC LLC
  • 9. 802.11 - Layers and functions PLCP Physical Layer Convergence Protocol  clear channel assessment signal (carrier sense) PMD Physical Medium Dependent  modulation, coding PHY Management  channel selection, MIB Station Management  coordination of all management function s MAC  access mechanisms, fragmentation, encryption MAC Management  synchronization, roaming, MIB, power management DLC LLC MAC MAC Management
  • 10. 802.11 - Physical layer 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 Mbit/s, 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
  • 11. FHSS PHY packet format synchronization SFD PLW PSF HEC payload PLCP preamble PLCP header bits Synchronization  synch with 010101... pattern SFD (Start Frame Delimiter)  0000110010111101 start pattern PLW (PLCP_PDU Length Word)  length of payload 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 x16+x12+x5+1 80 16 12 4
  • 12. DSSS PHY packet format PLCP preamble PLCP header bits synchronization SFD signal service length HEC payload Synchronization  synch., gain setting, energy detection, frequency offset compensation SFD (Start Frame Delimiter)  1111001110100000 Signal  data rate of the payload (0A: 1 Mbit/s DBPSK; 14: 2 Mbit/s DQPSK) Length  length of the payload Service  future use, 00: 802.11 compliant HEC (Header Error Check)  protection of signal, service and length, x16+x12+x5+1 128 16 8 8 16 16 variable
  • 13. 802.11 - MAC layer I - DFWMAC 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)  collision avoidance via randomized „back-off“ mechanism  minimum distance between consecutive packets  ACK packet for acknowledgements (not for broadcasts)  DFWMAC-DCF w/ RTS/CTS (optional)  Distributed Foundation Wireless MAC  avoids hidden terminal problem  DFWMAC- PCF (optional)  access point polls terminals according to a list
  • 14. 802.11 - MAC layer II Priorities  defined through different inter frame spaces  no guaranteed, hard priorities  SIFS (Short Inter Frame Spacing)  highest priority, for ACK, CTS, polling response  PIFS (PCF IFS)  medium priority, for time-bounded service using PCF  DIFS (DCF, Distributed Coordination Function IFS)  lowest priority, for asynchronous data service t medium busy PIFS SIFS DIFS DIFS next frame contention direct access if medium is free DIFS
  • 15. t medium busy DIFS DIFS next frame contention window (randomized back- off mechanism ) 802.11 - CSMA/CA access method I slot time  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)  if another station occupies the medium during the back-off time of the station, the back-off timer stops (fairness) direct access if medium is free DIFS
  • 16. 802.11 - competing stations - simple version t station1 station2 busy station3 station4 station5 packet arrival at MAC boe busy boe elapsed backoff time bor residual backoff time busy medium not idle (frame, ack etc.) boe bor boe busy boe busy DIFS DIFS DIFS DIFS boe bor boe bor boe busy boe bor boe bor
  • 17. 802.11 - CSMA/CA access method II t SIFS 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 DIFS data ACK waiting time other station s receiver sende r data DIFS contentio n
  • 18. Mobile Communications: Wireless LANs 802.11 - DFWMAC Sending unicast packets  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 t SIFS DIFS data ACK defer access other station s receiver sende r data DIFS contentio n RTS CTS SIFS 7.17.1 SIFS NAV (RTS) NAV (CTS)
  • 22. 802.11 - Frame format Types  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  sending time, checksum, frame control, data Frame Contro l Duratio n ID Address 1 Address 2 Address 3 Sequence Control Address 4 Data CRC 2 2 6 6 6 6 2 4 0-2312 bytes version, type, fragmentation, security, ...
  • 23. MAC address format scenario to DS from DS address 1 address 2 address 3 address 4 ad-hoc network 0 0 DA SA BSSID - infrastructure network, from AP 0 1 DA BSSID SA - infrastructure network, to AP 1 0 BSSID SA DA - infrastructure network, within DS 1 1 RA TA DA SA DS: Distribution System AP: Access Point DA: Destination Address SA: Source Address BSSID: Basic Service Set Identifier RA: Receiver Address TA: Transmitter Address
  • 24. 802.11 - MAC management 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  managing, read, write
  • 25. Synchronization using a Beacon (infrastructure) beacon interval t access point mediu m busy B busy busy busy B B B value of the timestamp B beacon frame
  • 26. Synchronization using a Beacon (ad-hoc) mediu m station1 busy B1 beacon interval busy busy busy B1 value of the timestamp B beacon frame station2 B2 B2 t random delay
  • 27. 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  Traffic Indication Map (TIM)  list of unicast receivers transmitted by AP  Delivery Traffic Indication Map (DTIM)  list of broadcast/multicast receivers transmitted by AP Ad-hoc  Ad-hoc Traffic Indication Map (ATIM)  announcement of receivers by stations buffering frames  more complicated - no central AP  collision of ATIMs possible (scalability?)
  • 28. Power saving with wake-up patterns (infrastructure) TIM interval t access point mediu m busy busy busy busy T T TIM D DTIM DTIM interval D B D B B broadcast/ multicast statio n awake p PS poll T d p d d data transmission to/from the station
  • 29. Power saving with wake-up patterns (ad-hoc) awake A transmit ATIM D transmit data t station1 B1 B1 B beacon frame station2 B2 random delay A D B2 a d ATIM windo w beacon interval a acknowledge ATIM d acknowledge data
  • 30. 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
  • 31. Future developments IEEE 802.11a  compatible MAC, but now 5 GHz band  transmission rates up to 20 Mbit/s  close cooperation with BRAN (ETSI Broadband Radio Access Network) IEEE 802.11b  higher data rates at 2.4 GHz  proprietary solutions already offer 10 Mbit/s IEEE WPAN (Wireless Personal Area Networks)  market potential  compatibility  low cost/power, small form factor  technical/economic feasibility  Bluetooth
  • 32. ETSI - HIPERLAN ETSI standard  European standard, cf. GSM, DECT, ...  Enhancement of local Networks and interworking with fixed networks  integration of time-sensitive services from the early beginning HIPERLAN family  one standard cannot satisfy all requirements  range, bandwidth, QoS support  commercial constraints  HIPERLAN 1 standardized since 1996 physical layer channel access control layer medium access control layer physical layer data link layer HIPERLAN layers OSI layers network layer higher layers physical layer medium access control layer logical link control layer IEEE 802.x layers
  • 33. Overview: original HIPERLAN protocol family HIPERLAN 1 HIPERLAN 2 HIPERLAN 3 HIPERLAN 4 Application wireless LAN access to ATM fixed networks wireless local loop point-to- point wireless ATM connections Frequency 5.1-5.3GHz 17.2-17.3GHz Topology decentralized ad- hoc/infrastructur e cellular, centralize d point-to- multipoi nt point-to-point Antenna omni-directional directional Range 50 m 50-100 m 5000 m 150 m QoS statistical ATM traffic classes (VBR, CBR, ABR, UBR) Mobility <10m/s stationary Interface conventional LAN ATM networks Data rate 23.5 Mbit/s >20 Mbit/s 155 Mbit/s Power conservatio n yes not necessary Check out Wireless ATM for new names!
  • 34. HIPERLAN 1 - Characteristics Data transmission  point-to-point, point-to-multipoint, connectionless  23.5 Mbit/s, 1 W power, 2383 byte max. packet size Services  asynchronous and time-bounded services with hierarchical priorities  compatible with ISO MAC Topology  infrastructure or ad-hoc networks  transmission range can be larger then coverage of a single node („forwarding“ integrated in mobile terminals) Further mechanisms  power saving, encryption, checksums
  • 35. HIPERLAN 1 - Services and protocols CAC service  definition of communication services over a shared medium  specification of access priorities  abstraction of media characteristics MAC protocol  MAC service, compatible with ISO MAC and ISO MAC bridges  uses HIPERLAN CAC CAC protocol  provides a CAC service, uses the PHY layer, specifies hierarchical access mechanisms for one or several channels Physical protocol  send and receive mechanisms, synchronization, FEC, modulation, signal strength
  • 36. HIPERLAN layers, services, and protocols HM-entity HC-entity HM-entity HC-entity MAC layer CAC layer PHY layer HP-entity HP-entity LLC layer HMPDU HCPDU data bursts MAC protocol CAC protocol PHY protocol MAC service CAC service PHY service MSDU MSAP MSDU MSAP HCSDU HCSAP HCSDU HCSAP
  • 37. HIPERLAN 1 - Physical layer Scope  modulation, demodulation, bit and frame synchronization  forward error correction mechanisms  measurements of signal strength  channel sensing Channels  3 mandatory and 2 optional channels (with their carrier frequencies)  mandatory  channel 0: 5.1764680 GHz  channel 1: 5.1999974 GHz  channel 2: 5.2235268 GHz  optional (not allowed in all countries)  channel 3: 5.2470562 GHz  channel 4: 5.2705856 GHz
  • 38. HIPERLAN 1 - Physical layer frames Modulation  GMSK for high bit-rate, FSK for LBR header LBR synchronization data0 data1 datam-1 . . . Maintaining a high data-rate (23.5 Mbit/s) is power consuming - problematic for mobile terminals  packet header with low bit-rate comprising receiver information  only receiver(s) address by a packet continue receiving Frame structure  LBR (Low Bit-Rate) header with 1.4 Mbit/s  450 bit synchronization  minimum 1, maximum 47 frames with 496 bit each  for higher velocities of the mobile terminal (> 1.4 m/s) the maximum number of frames has to be reduced HBR
  • 39. HIPERLAN 1 - CAC sublayer Channel Access Control (CAC)  assure that terminal does not access forbidden channels  priority scheme, access with EY-NPMA Priorities  5 priority levels for QoS support  QoS is mapped onto a priority level with the help of the packet lifetime (set by an application)  if packet lifetime = 0 it makes no sense to forward the packet to the receiver any longer  standard start value 500ms, maximum 16000ms  if a terminal cannot send the packet due to its current priority, waiting time is permanently subtracted from lifetime  based on packet lifetime, waiting time in a sender and number of hops to the receiver, the packet is assigned to one out of five priorities  the priority of waiting packets, therefore, rises automatically
  • 40. prioritization HIPERLAN 1 - EY-NPMA I EY-NPMA (Elimination Yield Non-preemptive Priority Multiple Access)  3 phases: priority resolution, contention resolution, transmission  finding the highest priority  every priority corresponds to a time-slot to send in the first phase, the higher the priority the earlier the time-slot to send  higher priorities can not be preempted  if an earlier time-slot for a higher priority remains empty, stations with the next lower priority might send  after this first phase the highest current priority has been determined contention transmission transmissio n synchronizatio n priority detection priority assertion t user data elimination burst elimination survival verifivcation yield listening IYS IPS IPA IES IESV
  • 41. HIPERLAN 1 - EY-NPMA II Several terminals can now have the same priority and wish to send  contention phase  Elimination Burst: all remaining terminals send a burst to eliminate contenders (11111010100010011100000110010110, high bit- rate)  Elimination Survival Verification: contenders now sense the channel, if the channel is free they can continue, otherwise they have been eliminated  Yield Listening: contenders again listen in slots with a nonzero probability, if the terminal senses its slot idle it is free to transmit at the end of the contention phase  the important part is now to set the parameters for burst duration and channel sensing (slot-based, exponentially distributed)  data transmission  the winner can now send its data (however, a small chance of collision remains)  if the channel was idle for a longer time (min. for a duration of 1700 bit) a terminal can send at once without using EY-NPMA  synchronization using the last data transmission
  • 42. HIPERLAN 1 - DT-HCPDU/AK-HCPDU 1 0 1 0 1 0 1 0 0 1 HI HDA HDA HDACS BLIR = n BL- IRCS 1 0 1 2 3 4 5 6 7 bit LBR HBR bit 0 1 2 3 4 5 6 7 byte TI BLI = n PLI = m HID DA SA UD PAD CS 1 2 3 - 6 7 - 12 13 - 18 19 - (52n-m-4) (52n-m-3) - (52n-4) (52n-3) - 52n 1 0 1 0 1 0 1 0 0 1 HI AID AID AIDCS 0 1 2 3 4 5 6 7 bit LBR Data HCPDU Acknowledgement HCPDU HI: HBR-part Indicator HDA: Hashed Destination HCSAP Address HDACS: HDA CheckSum BLIR: Block Length Indicator BLIRCS: BLIR CheckSum TI: Type Indicator BLI: Block Length Indicator HID: HIPERLAN IDentifier DA: Destination Address SA: Source Address UD: User Data (1-2422 byte) PAD: PADding CS: CheckSum AID: Acknowledgement IDentifier AIDS: AID CheckSum
  • 43. HIPERLAN 1 - MAC layer Compatible to ISO MAC Supports time-bounded services via a priority scheme Packet forwarding  support of directed (point-to-point) forwarding and broadcast forwarding (if no path information is available)  support of QoS while forwarding Encryption mechanisms  mechanisms integrated, but without key management Power conservation mechanisms  mobile terminals can agree upon awake patterns (e.g., periodic wake-ups to receive data)  additionally, some nodes in the networks must be able to buffer data for sleeping terminals and to forward them at the right time (so called stores)
  • 44. HIPERLAN 1 - DT-HMPDU LI: Length Indicator TI: Type Indicator RL: Residual Lifetime PSN: Sequence Number DA: Destination Address SA: Source Address ADA: Alias Destination Address ASA: Alias Source Address UP: User Priority ML: MSDU Lifetime KID: Key Identifier IV: Initialization Vector UD: User Data, 1–2383 n= 40–2422 bit 0 1 2 3 4 5 6 7 byte LI = n TI = 1 RL PSN DA SA ADA ASA UP ML ML KID IV IV UD SC 1 - 2 3 4 - 5 6 - 7 8 - 13 14 - 19 20 - 25 26 - 31 32 33 34 35 - 37 38 - (n-2) (n-1) - n Data HMPDU
  • 45. Information bases Route Information Base (RIB) - how to reach a destination  [destination, next hop, distance] Neighbor Information Base (NIB) - status of direct neighbors  [neighbor, status] Hello Information Base (HIB) - status of destination (via next hop)  [destination, status, next hop] Alias Information Base (AIB) - address of nodes outside the net  [original MSAP address, alias MSAP address] Source Multipoint Relay Information Base (SMRIB) - current MP status  [local multipoint forwarder, multipoint relay set] Topology Information Base (TIB) - current HIPERLAN topology  [destination, forwarder, sequence] Duplicate Detection Information Base (DDIB) - remove duplicates  [source, sequence]
  • 46. Ad-hoc networks using HIPERLAN 1 neighborhood (i.e., within radio range) Information Bases (IB): RIB: Route NIB: Neighbor HIB: Hello AIB: Alias SMRIB: Source Multipoint Relay TIB: Topology DDIB: Duplicate Detection RIB NIB HIB AIB SMRIB TIB DDIB RIB NIB HIB AIB SMRIB TIB DDIB RIB NIB HIB AIB SMRIB TIB DDIB RIB NIB HIB AIB DDIB RIB NIB HIB AIB DDIB RIB NIB HIB AIB DDIB 2 3 4 5 6 1 Forwarder Forwarde r Forwarde r
  • 47. Bluetooth Consortium: Ericsson, Intel, IBM, Nokia, Toshiba - many members Scenarios  connection of peripheral devices  loudspeaker, joystick, headset  support of ad-hoc networking  small devices, low-cost  bridging of networks  e.g., GSM via mobile phone - Bluetooth - laptop Simple, cheap, replacement of IrDA, low range, lower data rates  2.4 GHz, FHSS, TDD, CDMA
  • 48. States of a Bluetooth device (PHY layer) STANDBY inquiry page connected transmit PARK HOLD SNIFF unconnecte d connectin g active low power
  • 49. Bluetooth MAC layer access code packet header payload MAC address type flow ARQN SEQN HEC 3 4 1 1 1 8 bits Synchronous Connection-Oriented link (SCO)  symmetrical, circuit switched, point-to-point Asynchronous Connectionless Link (ACL)  packet switched, point-to-multipoint, master polls Access code  synchronization, derived from master, unique per channel Packet header  1/3-FEC, MAC address (1 master, 7 slaves), link type, alternating bit ARQ/SEQ, checksum 72 54 0-2745 bits
  • 50. Scatternets piconet s Each piconet has one master and up to 7 slaves Master determines hopping sequence, slaves have to synchronize Participation in a piconet = synchronization to hopping sequence Communication between piconets = devices jumping back and forth between the piconets