4/27/2020

Introduction to Wireless Networking 802.11 Medium Access Module-06 Jerry Bernardini Community College of Rhode Island

Wireless Networking J. Bernardini 1

Presentation Reference Material

• CWNA Certified Wireless Network Administration Official Study Guide (PWO-104), David Coleman, David Westcott, 2009, Chapter-8 4/27/2020 Wireless Networking J. Bernardini 2

IEEE 802.3 CSMA/CD vs. IEEE 802.11 CSMA/CA • • • • • • • • • CSMA/CD is for wired collision handling CSMA/CA is for wireless collision handling CSMA = Carrier Sense Multiple Access CD = Collision Detection CA = Collision Avoidance – Why do collisions occur?

Answer = Two or more stations transmit at the same time – Why is it important to detect or avoid collisions?

Answer = Because there is data loss and retransmission is necessary Wired networks are designed for the transmitting station to detect most collisions Many collisions will not be detected by Wireless networks – therefore avoid collisions

• IEEE 802.11 Collision Handling CSMA/CA In CSMA/CA a Wireless node that wants to transmit performs the following sequence: 1. Listen on the desired channel. 2. If channel is idle (no active transmitters) it sends a packet. 3. If channel is busy the node waits random time until transmission stops and then waits an additional time period. • 4. If the channel is still idle at the end of the time period the node transmits its packet otherwise it repeats the process defined in 3 above until it gets a free channel. Additional support mechanisms such as ACK, RTS/CTS can be used but increase overhead noticeably.

CSMA/CA and ACK •CSMA/CA also reduces collisions via explicit

frame acknowledgment

•

Acknowledgment frame (ACK):

Sent by receiving device to sending device to confirm data frame arrived intact •If ACK not returned, transmission error assumed •CSMA/CA does not eliminate collisions and does not solve hidden node problem 5

Two Kinds of Carrier Sensing Mechanisms • Physical Carrier Sense – – – – – Uses Clear Channel Assessment (CCA) Is the RF energy on the channel above a threshold? If CCA>threshold --->wait for CCA< threshold before trasmitting Checks received signal strength using RSSI RF energy from a hidden node could be missed • Virtual Carrier Sense – Uses the Network Allocation Vector (NAV) in each station – – NAV is a timer that determines if station can contend for RF medium NAV >0 --->wait for count down to NAV=0 – – NAV=0 --->use CCA to check for RF energy on medium IF NAV=0 and CCA > threshold --->station resets NAV>0 and waits

CSMA/CA Request to Send/Clear to Send • Request to Send/Clear to Send (RTS/CTS) protocol: Option used to solve hidden node problem – Significant overhead upon the WLAN with transmission of RTS and CTS frames • Especially with short data packets – RTS threshold: Only packets that longer than RTS threshold transmitted using RTS/CTS 7

CSMA/CA Collision Handling • 802.11 standard employs half-duplex radios-radios capable of transmission or reception-but not both simultaneously Wired LAN 1 Listening 2 Wireless Client Transmitting Transmitting Data Frames AP-1 Access Points Listening

• • • • • Interframe Spacing ensures no frame overlap and proper frame processing sequence Interframe spaces (IFS): Intervals between transmissions of data frames Short IFS (SIFS): For immediate response actions such as ACK, CTS, RTS, fragmented frames • SIFS times vary based upon PHY modulation • FHSS-28us, DHSS-10us, OFDM-16us, HR/DSS-10us, ERP-10us Point Coordination Function IFS (PIFS): Time used by a device to access medium after it has been asked and then given approval to transmit • PIFS times = SIFS time + PHY slot time Distributed Coordination Function IFS (DIFS): Standard interval between transmission of data frames • DIFS times = SIFS time + 2x PHY slot time Extended IFS (EIFS): used when frame reception is incomplete or corrupted • EIFS longest time • EIFS time = SIFS + 8x ACK + Preamble + PLCP header length + DIFS 9

Contention Window and Backoff Time • • Contention Window is a range of integers which is chosen at random to become the backoff time Backoff time is a random time used to establish a frame-to transmit – Random Backoff Time = Random Integer x Slot Time – – Slot time varies for PHY modulation FHSS-50us, DHSS-20us, OFDM-9us, HR/DSS-20us, ERP Long Slot-20us, ERP Short Slot-9us, 802.1n-9us 4/27/2020 Wireless Networking J. Bernardini 10

Transmitting on the WLAN: Fragmentation • • • Fragmentation: Divide data to be transmitted from one large frame into several smaller ones – Reduces probability of collisions – Reduces amount of time medium is in use If data frame length exceeds specific value, MAC layer fragments it – Receiving station reassembles fragments Alternative to RTS/CTS – High overhead • ACKs and additional SIFS time gaps 11

IEEE 802.11 MAC Functions • • • • • • • • • • Scanning- discover AP or BSS Synchronization- all stations have the same clock Frame Transmission- rules for frame transfer Authentication-allow device in network Association-after authentication associate with AP Reassociation-roaming and association with new AP Data Protection-data encryption protects data Power Management-save power by sleeping transceiver Fragmentation-breakup frame for efficiency and interfer.

RTS/CTS- solution to hidden node problem 4/27/2020 Wireless Networking J. Bernardini 12

Beacon Management Frame

• • • A special management frame that is used by a client stations seeking a wireless network to join.

Instead of beacon frames a station could use probe request and probe response frames In an ad hoc (IBSS ) wireless network all stations take turns broadcasting the beacon frame Beacon Beacon S2 S1 AP Control Point Wireless Networking J. Bernardini 4/27/2020 13

Active Scanning (Probes) • A station could use probe request and probe response frames Instead of beacon frames 1. Station is configured with SSID and switched to a channel 2. Probe request sent by requesting station • 3. All stations that have the same SSID and have normal configurations respond with a Probe Response frame The process also involves waiting for ProbeDelay and MinChannel Timers 4/27/2020 S1 Probe Request Probe Response AP Control Point Wireless Networking J. Bernardini S2 14

Passive Scanning (Beacons) 1. Client stations listens for a beacon from an access point (AP) 2. If multiple beacons are received the strongest one is selected 3. The listening station then requests authentication and association 4/27/2020 S1 Beacons Beacons AP Control Point Wireless Networking J. Bernardini S2 15

Slot Times

• • The amount of time a device waits after a collision before retransmitting a packet. Radio defined time interval or clock tick.

– FHSS Slot Time = 50  S – – DSSS Slot Time = 20  S Infrared Slot Time = 8  S – For DSSS: SIFS = 10  S PIFS = SIFS + 1 Slot Time = 10  S + 20  S = 30  S DIFS = PIFS + 1 Slot Time = 30  S + 20  S = 50  S – Time Unit = TU = 1,024  S  1 mS Beacon interval = 100 TU or 100 mS.

Slot Time Notes

• • • • • • Short Slot Times - The amount of time a device waits after a collision before retransmitting a packet. You can increase throughput on 802.11g, 2.4-GHz radios by enabling short slot time (most .11g radios enable this by default). Reducing the slot time from the standard 20 microseconds to the 9-microsecond short slot time decreases the overall backoff, which increases throughput.

Backoff, which is a multiple of the slot time, is the random length of time a station waits before sending a packet on the LAN. Many 802.11g radios support short slot time, but some do not. When short slot time is enabled, the wireless device uses the short slot time only when all clients associated to the 802.11g, 2.4-GHz radio support short slot time.

Short slot time is an 802.11g-only feature and does not apply to 802.11a radios. 4/27/2020 Wireless Networking J. Bernardini 17

Communications Options

• MAC Layer – Access Methods • DCF – RTS/CTS (optional) Distributed function Wireless MAC Avoids hidden node problem • DCF – PCF (optional) AP polls stations Superframes to allow station to eventually get access Superframe = Beacon + CFP + CP CFP = Contention-Free Period CP = Contention Period

RTS/CTS

• 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

Fragmentation • • Every network has an MTU (Maximum Transmission Unit) size. Packets larger than the allowable MTU size must be broken down into multiple smaller packets, or fragments, to enable them to traverse the network with lower bit error rates, (BER).

Fragment size can typically be set by the user using a threshold setting between 256 and 2,048 bytes.

CRC Header Header Header Data Data -1 Threshold CRC Data -2 CRC Drawing not to scale.

Dynamic Rate Switching

• • • • • Dynamic Rate Switching =Dynamic Rate Selection =Automatic Rate Shifting----All mean the same thing Process of reducing or increasing the data rate based upon RF signal levels RF signals attenuate over distance or by absorption AP will reduce data rate for weaker signals AP will increase data rate for higher signals 4/27/2020 Wireless Networking J. Bernardini 21

Dynamic Rate Selection (DRS)

• Dynamic Rate Selection or Adaptive Rate Selection/shifting.

– – – 802.11a, 802.11g modes: 54, 48, 36, 24, 18, 12, 9, 6 Mbps 802.11b mode: 11, 5.5, 2, 1 Mbps Orinoco 2X mode: 108, 96, 72, 48, 36, 24, 18, 12 Mbps

Example of Sensitivity vs. DR

Data Rate (Mb/sec)

6 9 12 18 24 36 48 54

Received Signal (dBm)

–82 –81 –79 –77 –74 –70 –66 –65

Minimum received signals versus data rate for 802.11a devices.

Data Rate Coverage Areas

-85 dBm -76 dBm -72 dBm 24 Mbps 18 Mbps 9 Mbps

Distributed Coordination Function • • The Distributed Coordination Function (DCF) is the fundamental access mechanism in IEEE 802.11 Medium Access Control (MAC).

DCF can be used in all wireless topologies: IBSS, BSS, and ESS.

DIFS

• DIFS – Is used by default on all 802.11 stations. DIFS is the lowest priority and is used for data and management frames. For DSSS, DIFS is 50  s.

DIFS PIFS  Station Backoff Timers SIFS sender DIFS medium busy contention frame A look at all three: 10  s, 30  s, 50  s, for DSSS.

Point Coordination Function

• • • • An optional polling function.

Provides for limited contention-free service using the access point as a point coordinator.

Supports near real-time services.

In some ways PCF resembles token-based protocols. S2 S1 AP Control Point

Point Coordination Function (PCF) • • • Polling: Channel access method in which each device asked in sequence if it wants to transmit – Effectively prevents collisions Point Coordination Function (PCF): AP serves as polling device or “point coordinator” Point coordinator has to wait only through point coordination function IFS (PIFS) time gap – Shorter than D FIS time gap 28

PIFS

• PIFS –Are used only in Point Coordination Mode by the APs. This mode is enabled by the administrator. It has medium priority and therefore always wins over DIFS, so that the AP can take control in polling. For DSSS, PIFS is 30  S.

DIFS and DCF frames

• • If point coordinator hears no traffic after PIFS time gap, sends out beacon frame – Field to indicate length of time that PCF (polling) will be used instead of DCF (contention) • Receiving stations must stop transmission for that amount of time – Point coordinator then sends frame to specific station, granting permission to transmit one frame 802.11 standard allows WLAN to alternate between PCF (polling) and DCF (contention) 30

Event •

Timing Diagrams

Timing or Sequence Diagrams - A graph showing events/levels as a function of time.

Event tp1 2ms sync 4ms 6ms mxc rst ack flag pd en tp2 data 8ms data A 10ms Time Time Time

•

SIFS

SIFS - Shortest and highest priority time space sent before and/or after RTS, CTS, and ACK frames. For DSSS, SIFS is 10 microseconds or 10  S.

DIFS sender data SIFS ACK receiver others DIFS deferring mode waiting time DIFS data contention

Hybrid Coordination Function (HCF) • • • 802.11e add enhancements to DCF and PCF Two channel-access methods – – Enhanced Distributed Channel Access (EDCA) Hybrid Coordination Function Control Channel ACCESS (HCCA) Standard and HCF sequence: – – – Standard 802.11 ST (radio) transmits a frame ST must contend for media to transmit another frame HCF radio will be given a Transmit Opportunity (TXOP) to send multiple frames.

– – During the TXOP time ST can send a Frame burst Short Interframe Space (SIFS) is used during TXOP 4/27/2020 Wireless Networking J. Bernardini 33

Enhanced Distributed Channel Access (EDCA) • • • • • • • • Access method that allows for eight user priorities(UP) Traffic is prioritized using 802.1D priority tags Priorities Categories include: Voice (AC_VO) Video(AC_VI) Best Effort(AC_BE) Background(AC_BK) All Categories require the use of Enhanced Distributed Channel Access Function (EDCAF) to contend for wireless medium 4/27/2020 Wireless Networking J. Bernardini 34

Block Acknowledgement (BA) • • • • 802.11e defines the aggregation of acknowledges in to one ACK frame Two Block ACK mechanisms: Immediate - for low-latency traffic Delayed - for latency-tolerant traffic 4/27/2020 Wireless Networking J. Bernardini 35

Wi-Fi Multimedia (WMM) • • • • • • 802.11e defines layer-2 methods for time-sensitive applications over 802.11 wireless The Wi-Fi Alliance introduced Wi-Fi Multimedia certification WMM) WMM is based upon EDCA WMM uses 802.1D priority tags Traffic is prioritized for Voice, Video, Best-Effort, Background WMM-PS is for Power Saving or battery operated wireless equipment.

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