1. Wireless Networking IEEE 802.11 In Depth Module-05 Jerry Bernardini Community College of Rhode Island 6/26/20151Wireless Networking J. Bernardini

2. Presentation Reference Material CWNA Certified Wireless Network Administration Official Study Guide, Fourth Edition, Tom Carpenter, Joel Barrett – Chapter-4 Pages 153-200 The California Regional Consortium for Engineering Advances in Technological Education (CREATE) project 6/26/2015Wireless Networking J. Bernardini2

3. Bits, Bytes, Octets, Frames, Packets Bits =1 or 0 Bytes = 8 bits Octets = 8 bits = Byte – Octet is used by telecommunication people – Byte is used by IT people Frames = grouping of bits at layer-2 Packets = grouping of bits at layer-3 Datagrams = another term for packets 6/26/2015Wireless Networking J. Bernardini3

4. Coding – ASCII Table 6/26/2015Wireless Networking J. Bernardini4

5. CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini 5 OSI data flow

6. CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini 6 IEEE 802.11 Physical Layer Standards IEEE wireless standards follow OSI model, with some modifications Data Link layer divided into two sublayers: – Logical Link Control (LLC) sublayer: Provides common interface, reliability, and flow control – Media Access Control (MAC) sublayer: Appends physical addresses to frames Physical layer divided into two sublayers: – Physical Medium Dependent (PMD) sublayer: Makes up standards for characteristics of wireless medium (such as DSSS or FHSS) and defines method for transmitting and receiving data – Physical Layer Convergence Procedure (PLCP) sublayer: Performs two basic functions Reformats data received from MAC layer into frame that PMD sublayer can transmit “Listens” to determine when data can be sent

7. Data Link Layer - Physical Layer- Data Units Data Link Layer (Layer-2) Physical Layer (Layer-1) 802.2 Logical Link Control 802.11 Media Access Control PHY Layer Convergence Protocol Physical Medium Dependent LLC MAC PLCP PMD PHY = Physical Layer MSDU (MAC Service Data Unit) MPDU (MAC Protocol Data Unit) PSDU (PLCP Service Data Unit) PPDU (PLCP Protocol Data Unit) Modulated Radio Signal ( MPDU = PSDU name change to indicated service needed) (From upper layers 2304 bytes max)

8. 8 Where the IEEE 802.11 Standard Fits

9. 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

10. 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.

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

12. 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

13. CSMA/CA and ACK 13 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

14. CSMA/CA Request to Send/Clear to Send 14 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

15. Interframe Spacing ensures no frame overlap and proper frame processing sequence 15 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

16. 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 6/26/2015Wireless Networking J. Bernardini16

17. Ethernet and 802.11 Frames Ethernet Frame Wireless Frame Preamble 7166246 - 15004 SourceDestinationDataFCS Start Of Frame Type or Length Field Frame Cntrl 26620 - 23044 SourceDestinationData FCS 2 Rec. Adr Sequence Cntrl 66 Trans. Adr Duration ID Sync 10 or 18 Start Of Frame 2 4 or 6 PLCP Header MAC Packet DATA Unit, (MPDU) 1518

18. Frame Categories / Types Management Frames oBeacon Frame oProbe Frames oAssociation Frames… more Control Frames oRTS and CTS Frames oACK – Acknowledgement Frames… more Data Frames oData Payload Frames

19. Twelve Management Frame Types 6/26/2015Wireless Networking J. Bernardini19

20. Eight Control Frames Used to assist with the delivery of data frames 6/26/2015Wireless Networking J. Bernardini20

21. Fifteen Data Frames The frames that actually carry application data 6/26/2015Wireless Networking J. Bernardini21

22. IEEE 802.11 Frame Formats CWNA Guide to Wireless LANs, Second Edition22 Management Frame Control (2) Duration (2) Des. Address (6) Source Address (6) BSSID (6) Seq. Control (2) Frame Body ( 1 to 2311) Frame Check Seq. (6) Control Frame Control (2) Duration (2) Receiver Address (6) Transmit Address ( 6) Frame Check Seq. (6) Data Frame Control (2) Duration (2) Address 1 (6) Address 2 (6) Address 3 (6) Seq. Control (2) Address 4 (6) Data ( 1 to 2311) Frame Check Seq. (6) (Bytes per field)

23. Frame Types and Sizes Protocol Data Unit (PDU) Frame Type and Vendor SupportMTU (Bytes) MTU + Overhead (Bytes) TCPTransport maximum segment size1460 IPLayer-3 default size1500 MACIEEE 802.3 Ethernet default15001518 MPDUIEEE 802.11 default1534 MPDUIEEE 802.11 maximum2304 MACJumbo Frame>1500 MACCisco Baby giant15521600 MACCisco Catalyst 400091989216 MACCisco Catalyst 600092169234 6/26/2015Wireless Networking J. Bernardini23

24. 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 24

25. 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 6/26/2015Wireless Networking J. Bernardini25

26. 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 6/26/2015Wireless Networking J. Bernardini 26 S1 S2 AP Control Point Beacon

27. 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 6/26/2015Wireless Networking J. Bernardini 27 S1 S2 AP Control Point Probe Request Probe Response

28. 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 6/26/2015Wireless Networking J. Bernardini 28 S1 S2 AP Control Point Beacons

29. Authentication and Association Using the IEEE 802.11 State Machine Stations are in one of three states 1.Unauthenticated / Unassociated 2.Authenticated / Unassociated 3.Authenticated / Associated You cannot transmit data frames for processing until you are associated You cannot transmit associated frames for processing until you are Authenticated 6/26/2015Wireless Networking J. Bernardini29

30. IEEE 802.11 State Machine 6/26/2015Wireless Networking J. Bernardini30

31. 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.

32. 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. 6/26/2015Wireless Networking J. Bernardini32

33. Communications Process MAC Access Modes – DCF – CSMA/CA – DCF/PCF – Point Coordinators and Polling Contention Free Delivery Normal Delivery PCF DCF

34. 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

35. 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

36. RTS/CTS RTS CTS data sender receiver others DIFS SIFS Access to medium deferred contention SIFS ACK NAV (RTS) NAV (CTS) There are generally three setting in APs for RTS/CTS Off, On, and On with Threshold   NAV – Network Allocation Vector

37. 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. HeaderDataCRC HeaderData -1CRC HeaderData -2CRC Threshold Drawing not to scale.

38. 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 6/26/2015Wireless Networking J. Bernardini38

39. 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

40. Example of Sensitivity vs. DR Minimum received signals versus data rate for 802.11a devices. Data Rate (Mb/sec) Received Signal (dBm) 6–82 9–81 12–79 18–77 24–74 36–70 48–66 54–65

41. Data Rate Coverage Areas 24 Mbps 18 Mbps 9 Mbps -72 dBm -76 dBm -85 dBm

42. Chapter-4 Topics. 6/26/2015Wireless Networking J. Bernardini 42 Frames, Packets, and Datagrams. Bits, Bytes, and Octets. MAC & PHY. IEEE 802.11 CSMA/CA. Carrier Sense. Interframe Spacing. Contention Window. Collision Avoidance.. Frame Types and Formats Compared. IEEE 802.11 Frame Format Versus IEEE 802.3 Frame Format. Frame Types. Layer 3 Protocol Support by IEEE 802.11 Frames. Jumbo Frame Support (Layer 2). MTU Discovery and Functionality (Layer 3) IEEE 802.11 Frames and Frame Exchange Sequences MAC Functions. Beacon Management Frame. Active Scanning (Probes).. Passive Scanning (Beacons. Authentication and Association Processes.. The IEEE 802.11 State Machine. Authentication. Association, Reassociation, and Disassociation. Regulatory Domain Requirements. Data Flow Optimization Across the RF Medium. DCF PCF IEEE 802.11e and WMM RTS/CTS and CTS-to-Self Protocols Fragmentation Dynamic Rate Switching

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44. 802.11 Open Association Process 6/26/2015Wireless Networking J. Bernardini44

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47. Association, Reassociation, Disassociation Covered next week 6/26/2015Wireless Networking J. Bernardini47

48. Regulatory Domain Requirements Covered next week 6/26/2015Wireless Networking J. Bernardini48

49. 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 DFIS time gap 49

50. DIFS and DCF frames 50 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)

51. Timing Diagrams Timing or Sequence Diagrams - A graph showing events/levels as a function of time. Time Event 2ms4ms6ms8ms10ms sync rstack Time flagen pd tp2 tp1 A data mxc

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

53. 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. AP Control Point S1 S2

54. 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.

55. 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.

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