4.3 IEEE Physical Layer IEEE IEEE b IEEE a IEEE g IEEE n IEEE 802.

Transcription

1 4.3 IEEE Physical Layer IEEE IEEE b IEEE a IEEE g IEEE n IEEE ac,ad Andreas Könsgen Summer Term 2012

2 4.3.3 IEEE a Data rate 6, 9, 12, 18, 24, 36, 48, 54 Mbit/s, depending on SNR User throughput (1500 byte packets): 5.3 (6), 18 (24), 24 (36), 32 (54) (numbers in parentheses specify the respective physical bit rate) 6, 12, 24 Mbit/s mandatory Frequency Uses less crowded 5 GHz band: , , GHz But: stronger shading due to higher frequency - 2 -

3 Operating channels US: U-NII U-NII Unlicensed National Information Infrastructure channel [MHz] 16.6 MHz channel center frequency = channel number [MHz] [MHz] 16.6 MHz TX power: USA: 40 mw 800 mw (dependent on channel) - 3 -

4 Operating channels Europe: ETSI ETSI has assigned slightly different frequency bands for Europe: GHz and GHz. 12 channels are defined for the 5 GHz Band, each 20 MHz wide; channels are non-overlapping Additionally ETSI demands the use of Dynamic Frequency Selection (DFS) and Transmit Power Control (TPC); this has led to the development of IEEE h standard Max. 200 mw EIRP DFS and TPC are not required if transmit power is always below 50mW EIRP and only frequency band Ghz is used - 4 -

5 OFDM in IEEE a OFDM with 52 used subcarriers (64 in total) 48 data + 4 pilot (plus 12 virtual subcarriers) khz spacing pilot khz channel center frequency subcarrier number - 5 -

6 PHY transmission modes: a Bit rate Mbit/s Subcarrier modulation Coding rate Data bits per OFDM symbol Coded Bits Per OFDM symbol Coded Bits per subcarrier RF bandwidth MHz 6 9 BPSK 1/2 3/ QPSK 16QAM 1/2 3/4 1/2 3/ sensitivity against interference QAM 2/3 3/

7 PHY Frame Format (1) variable 6 variable bits rate reserved length parity tail service payload tail pad PLCP header PLCP preamble signal data 12 1 variable symbols 6 Mbit/s 6, 9, 12, 18, 24, 36, 48, 54 Mbit/s Preamble: for frequency alighment, channel estimation, synchronization PLCP Physical Layer Convergence Protocol Tail: for FEC coder/decoder Pad: for matching OFDM symbol length Service: synchronise scrambler - 7 -

8 PHY Frame Format (2) PLCP Preamble 12 symbols frequency alignment, channel estimation, synchronisation Signal BPSK modulated; contains following fields: 4 bit rate, 12 bit length, 1 bit parity, 6 bit tail (zeros) Data field sent with rate determined in header, it contains: 16 bit service for synchronisation of scrambler, payload, 6 tail bits for decoder, pad field for matching length to OFDM symbols - 8 -

9 IEEE a Pros & Cons Advantages: More bandwidth in 5 GHz band Less interference by other equipment Disadvantages Adverse propagation characteristics of 5 GHz band Þ low range - 9 -

10 Frequency channels like b Modes IEEE g OFDM Mode (mandatory) Pure OFDM like in a, supports same data rates as in a No downward compatibility DSSS-OFDM Preamble and header are transmitted using DBPSK with 1 Mbit/s, only data is using OFDM with higher rates /802.11b equipment can decode preamble and header

11 4.3.5 IEEE n Multiple Input Multiple Output (MIMO): AP and stations have more than one antenna signals propagate simultaneously along different paths Extended-bandwidth frequency channels: 40 MHz Modulation/FEC schemes like g or a

12 4.3.6 IEEE ac Gigabit Wi-Fi Extra-wide channels: 80 and 160 MHz Multi-user MIMO: parallel supply for multiple users with SDMA, e.g. beamforming High-order modulation up to 256QAM with 3/4 and 5/6 coding rate Frequency band 2.4 or 5 GHz Speed: 3.47 Gbit/s for a station; 6.93 Gbit/s aggregated

13 IEEE ac: Multi-user MIMO Separation of stations by SDMA, e.g. by beamforming

14 IEEE ad Very High Throughput in 60 GHz Directional Multi-Gigabit transmission Supports beamforming Single-carrier transmission scheme (mandatory) Modulation: BPSK, QPSK, 16QAM Convolutional coding with rates between 1/2 and 13/16 Up to 4.6 Gbit/s ODFM transmission (optional) Modulation: QPSK to 64QAM Convolutional coding with rates between 1/2 and 13/16 Up to 6.7 Gbit/s

15 4.4 IEEE MAC Layer Access Methods MAC Management Extensions Andreas Könsgen Summer Term 2012

16 MAC what does it do? Primarily: Media Access But also: Roaming Authentication Power Saving

17 Medium Access Control - Schemes CSMA Carrier Sense Multiple Access Check channel before start transmitting CSMA/CA CSMA with Collision Avoidance Avoid collisions by back-off CSMA/CD CSMA with Collision Detection Communication aborted, when collision detected; e.g. LAN

18 IEEE Medium Access Control DCF Distributed Coordination Function No central control Optionally with RTS/CTS avoids hidden terminal problem Best effort PCF Point Coordination Function Base station (Access Point) for control of all activities of the cell polls terminals Guarantees Quality-of-Service (QoS) optional

19 Transmission 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 EIFS (Enhanced Inter Frame Spacing) After a corrupted frame, medium must be free at least for EIFS DIFS medium busy PIFS SIFS contention next frame t

20 DIFS Plain CSMA/CA medium busy DIFS contention window (randomized back-off mechanism) next frame direct access if medium was free DIFS slot time station ready to send starts sensing the medium (Carrier Sense based on CCA, Clear Channel Assessment) when a new packet becomes ready to send: if the medium has been free for at least a DIFS, the station can start sending directly if the medium is busy, the station has to wait for a free DIFS, 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) t

21 Plain CSMA/CA: competing stations 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

22 CSMA/CA using ACKs Sending broadcast packets: as described before 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 (retransmissions with back-off) sender receiver other stations DIFS data SIFS waiting time ACK DIFS contention data t

23 Problems of CSMA/CA With low load: depending on Contention Window (CW) back-off time can be relatively long With high load: depending on CW relatively high number of collisions are possible System needs to adapt to load Start with a relatively small CW, e.g. 7 With each collision CW is doubled until max. of 255. Leads to start with small CW and higher CW with higher load Called exponential back-off

24 CSMA/CA with RTS/CTS (1) Same as CSMA/CA but additionally use of RTS/CTS to reduce hidden terminal problem RTS/CTS is optional, but each station needs to have it implemented to be able to react on RTS/CTS signalling

25 CSMA/CA with RTS/CTS (2) suitable for 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 sender receiver DIFS RTS SIFS CTS SIFS data SIFS ACK other stations NAV (RTS) NAV (CTS) defer access DIFS contention data t NAV Network Allocation Vector (internal)

26 CSMA/CA with RTS/CTS (3) Advantages Collisions can only occur from RTS to CTS (or caused by mobility) Disadvantages: RTS/CTS can lead to significant increase of traffic (overhead) RTS Threshold determining from which frame size on the RTS/CTS mechanism is applied

27 Fragmentation In wireless networks error rate high A method to reduce error rate is to use shorter frames (fragmentation) sender receiver DIFS RTS SIFS CTS SIFS frag 1 SIFS ACK 1 SIFS frag 2 SIFS ACK2 other stations NAV (RTS) NAV (CTS) NAV (frag 1 ) NAV (ACK 1 ) DIFS contention data t

28 PCF (1) PCF Point Coordinated Function DCF cannot guarantee delay or bandwidth Access Point controls medium access and polls individual stations Super Frame contains contention free and contention periods

29 PCF (2) t 0 SuperFrame point coordinator B SIFS P 1 SIFS SIFS P 2 SIFS SIFS wireless stations NAV D 1 NAV contention free period D 2 B beacon P Polling packet (downlink) D Data packet (uplink)

30 PCF (3) SuperFrame t 1 t 2 t 3 point coordinator P 3 PIFS P 4 SIFS SIFS CFend wireless stations D 4 NAV NAV contention free period contention period t t 1 : actual end of CF period t 2 : latest possible end of CF end t 3 : duration of superframe

31 Advantages PCF pros & cons QoS guarantee is possible Disadvantages Depending on traffic pattern and frame size high overhead/waiting time

32 bytes MAC Frame Format (1) 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 Duration/ ID Address 1 Address 2 Address 3 Sequence Control Frame Control Address Data 4 bits Protocol version Type Subtype To DS From DS More Frag Power Retry Mgmt More Data WEP Order CRC

33 MAC Frame Format (2) Frame Control (2 Byte) (see next slide) Duration / ID Values < duration in µs. Used for virtual reserving with RTS and CTS and for fragmentation Values > Identification Adress address fields contain IEEE 802 MAC Addresses (each 48 bit) Sequence Control to be able to identify duplicates a sequence number is required Data Up to 2312 byte of data which are transparently transfered from sender to receiver Checksum 32 bit checksum (CRC), same as in other 802.x networks

34 MAC Frame Frame Control Protocol Version 2 bit, indicates protocol version; fixed to 0 at the moment Type 00: Management, 01: control, 10: data, 11: for future use Subtype 0000 Association Request, 1000 Beacon, 1011 RTS, 1100 CTS, 1111 user data From/To DS Explained next slide More Fragments 1: further data or control frames or fragments of a MDSU are expected Retry 1: frame is a retransmission of an earlier frame Power Management State of station after successful transmission; 1: station goes to power saving mode, 0 station stays active More Data: Transmitter has more to tranmsit after active frame. Security Indicates if security mechanism is used Order 1: following frames need to be processed in strict order

35 Addressing Scheme (1) MAC Frames can be transmitted between wireless stations between wireless stations and Access Points between Access Points over the Distributed System (DS) STA 1 ESS LAN BSS 1 Access Point BSS 2 Portal Distribution System Access Point 802.x LAN STA LAN STA

36 Addressing Scheme (2) To DS From DS Address1 Address2 Address3 Address4 Address1: Address2: 0 0 DA SA BSSID 0 1 DA BSSID SA 1 0 BSSID SA DA 1 1 RA TA DA SA physical address of recipient physical address of sender of frame (receiver of MAC ack.) Address3/Address4 logical assignment of frames Row1 Row2 Row3 Row4 DS DA SA BSSID RA TA Distributed System Destination Address Source Address Basic Service Set ID Receiver Address Transmitter Address Adhoc Network: physical and logical address are same Infrastructure Network FROM Access Point (AP), address 3 logic sender Infrastructure Network TO Access Point, address 4 logic receiver Infrastructure Network, From AP to AP via Distribution System (for mesh networks)

37 Example Control Frames: ACK, RTS, CTS Acknowledgement ACK bytes Frame Duration Receiver CRC Control Address Request To Send RTS bytes Frame Duration Receiver Transmitter CRC Control Address Address Clear To Send CTS bytes Frame Duration Receiver CRC Control Address