Wireless Ethernet IEEE 802.11 Standard Overview

1. Wireless EthernetIEEE 802.11 StandardOverview Dirk GrunwaldAssoc. ProfessorDept. of Computer ScienceUniversity of Colorado, Boulder

2. What’s Covered • High level overview of the 802.11 standard • Motivation • Organization • Station Services • MAC-level protocol • Power Saving

3. Resources

4. Resources • IEEE 802 standards group • http://grouper.ieee.org/groups/802/ • 802.11 Specification is ~$450 or so We have some copies

5. Differences Between WLANs and Wired LANs • Obviously, no wires • Data is broadcast through free space, people can snoop • Vagaries of electromagnetic propagation • Signals are not consistent • Multipath and blocking may cause dead spots, even within open areas • Mobility • The problems that mobility brings

6. Problems Of Mobility • Orders of magnitude slower than fixed network • Higher transmission bit error rates (BER) • Uncontrolled cell population • Difficult to ensure Quality of Service (QoS) • Asymmetric duplex bandwidth • Limited communication bandwidth exacerbates the limitation of battery lifetime.

7. Limitations Imposed by Mobility • Lack of mobility-awareness by applications • Inherently transparent programming model (object-, components-oriented, but not aspect-oriented) • Lack of environment test and set API support • Lack of mobility-awareness by the system • network: existing transport protocols are inefficient to use across heterogeneous mix of fixed/wireless networks • session and presentation: inappropriate for the wireless environment and for mobility • operating systems: lack of env. related conditions and signals • “you’ve lost your network” • client/server: unless changed, inappropriate and inefficient

8. 802.11 Design • Seemingly complex, but designed to overcome some of the complications of mobility • Components • The station • Access Point (AP) • Wireless medium • Basic Service Set (BSS) • The Distribution System (DS) • Extended Service Set (ESS) • Station and Distribution Services

9. 802.11 Design • Based on 802.2 LLC protocol • Many similarities with other protocols • Allows “seamless bridging” with other protocols using 802.2 LLC protocols • Supports variety of PHY • IRDA • 2.4 Ghz band • 2 Mbit/s FHSS (802.11) • 1 / 2 / 5.5 / 11 Mbit/s DSSS (802.11b) • 5.4 Ghz band • 24, 54 Mbit/s

10. The station • The station is a component that connects to the wireless medium • Consists of MAC and PHY • Generally represented by a network interface card (NIC) • Station can be mobile, portable or stationary • Each station supports station services such as • Authentication • Deauthentication • Privacy • Deliver of Data • Describe later..

11. The Basic Service Set • A BSS is a set of stations that communicate with one another • Does not refer to a physical area / region • If all the stations are mobile and there is no connection to a wired network, the BSS is called an independent BSS or IBSS • Also called “adhoc mode” • IBSS is typically short-lived (“adhoc”)

12. IBSS Organization

13. IBSS / Adhoc Organization • Stations send messages directly to other stations • Only stations within the transmission range are “in the IBSS” • There’s still an association / disassociation service • There is no centralized coordination for transmission

14. Infrastructure BSS (or just BSS) • When a BSS contains an access point, it’s a BSS • Also called “Infrastructure mode”

15. Infrastructure BSS • In Infrastructure mode, each station sends messages only to the access point • The access point redistributes or retransmits the messages • Both on the wireless network and an associated wired network • Stations must associate with an access point and possibly authenticate themselves • The access point can control when stations transmit • Power savings mode only possible when using an access point

16. Extended Service Set (ESS)

17. Extended Service Set • An ESS is a set of infrastructure BSSs where the AP’s communicate amoung themselves to forward traffic from one BSS to another • Allows movement of stations from one BSS to another • The AP’s communicate via a “distribution system” (DS) • DS may be either wired or wireless • E.g., access points can directly form a distribution system or access points can be connected via a wired network • Access points can “hand-off”

18. Extended Service Set (ESS) Router

19. Inter-ESS Coordination • Inter-Access Point Protocol • Not part of 802.11 & no documentation available • But not rocket science – access points know the IP address of different access points. Access points inform other access points when a station associates. Maps are maintained and messages forwarded • Bridging • Must be on same subnet • If a destination is a broadcast orunknown MAC address, the AP sendsit to the wired network • AP records MAC for all stations,grabs all frames with those MACs A B

20. Full IEEE 802.11 Architecture

21. Station Services • Authentication – used to prove identity of one station to another • Deauthentication – eliminate previous authorized user from use of network (security revocation) • Privacy – wired equivalent privacy, similar to that of in-wall wiring • Data Delivery – reliable deliver of data frames from one MAC to another, with minimal duplication or reordering

22. Distribution Services • Association – make a logical connection between a mobile station and an access point. Necessary for the distribution system (DS) to know where and how to deliver data. • Reassociation – as above, but includes information on prior association within the same ESS. • Disassociation – forces a node to associate (again) or to inform an AP that it station longer needs service • Distribution – used by AP to determine if frame should go to current BSS or be send to DS (another AP or portal) • Integration – connects 802.11 to other LAN.

23. Association & Authentication • Each station must maintain two state variables for each other station with which it communicates • Each station may be authenticated with many stations at the same time, but is associated with only one at a time. • There are three classes of frame types – the station must respond to certain classes in different connection states.

24. Relationship Between State Variables and Services State 1UnauthenticatedUnassociated Class 1Frames DeAuthenticationNotification DeAuthenticationNotification SuccessfulAuthentication State 2AuthenticatedUnassociated Class 1 & 2Frames SuccessfulAssociationor Re-association DisassociationNotification I think this should be “associated”, but both text & standard show “authentication”. State 3AuthenticatedAssociated Class 1, 2 & 3Frames

25. Messages • IBSS can only be in State 1, and can thus process data messages. • Normally, data transmission only occurs in State 3. • Stations must be able to react to all message classes in each state, but should only initiate certain message classes according to their state.

26. MAC Layer Functions • Provide reliable data delivery • Hidden node & exposed node problem • Solutions • Fairly control access to shared media • Distributed coordination function • Point coordination function • Protect the data that is delivered • WEP

27. Wireless Ethernet Is Not Ethernet • Can typically on transmit or receive at the same time • Use collision avoidance rather than collision detection • Just because I can hear someone talking doesn’t mean I’ll interfere with the receiver • My signal may be so attenuated by the time it hits them there’s no meaningful interference.

28. A B C A B C Hidden Node Problem In CSMA

29. A B C D A B C D Exposed Node Problem In CSMA

30. 802.11 MACA Protocol • Contention reduction • RTS – Request to send • CTS – Clear to send • Reliability • DATA frame • ACK frame • Reliability frames act as a unit – if you see data, you’re not to transmit until ACK is seen. • Likewise, if you see RTS, you must wait out CTS, DATA and ACK

31. RTS? A B C B C A B A C B A C CTS! CTS! 802.11 MA/CA DATA ACK

32. 802.11 MA/CA Protocol • Not all frames must use RTS / CTS • Lots of overhead for small messages • Control entry in MIB (mandated by 802.11) sets message above which CTS/RTS is used – dot11RTSThreshold • On Aironet driver – See e.g. /proc/aironet/eth1/ConfigRTSThreshold: 2312 • Frames carry state that let stations determine when a CTS/DATA/ACK has been dropped or missed. • A failure of the frame exchange protocol detected at the source is treated as a “collision”, and a randomized exponential back off is used to delay retransmission.

33. NAV / Virtual Carrier • Unlikely that all nodes can be heard by each other • How do you know how long to wait for a transmit to finish? • Message sender can’t tell you, you can’t here them • Messages contain a Network Allocation Vector (NAV) • Value that indicates how much time remains before the media is availble • Although you don’t hear sender, you do hear receiver • You listen to NAV and know when to try again

34. Timeline of RTS / CTS & NAV

35. Timing Intervals • Collision avoidance is done using timing intervals • Slot time • SIFS – short interframe space • PIFS – priority interframe space • = SIFS + slot time • DIFS – distributed interface space • = PIFS + slot time • EIFS – extended interface space

36. Some IFS Relationships

37. Media Control • Distributed Control Function • Obviously distributed • Uses media access timing & contention • Point Control Function • PC = Point Controller, always located in access point • PCF operates by stations requesting that the PC register them on a polling list • PC then regularly polls the stations for traffic while delivering traffic • Every station is required to be able to respond to operation of PCF.

38. DCF Operations • When MAC gets request to xmit, check of physical and virtual carrier • Medium not in use for interval of DIFS (or EIFS if previous frame had errors), the MAC can begin transmission • If medium is in use • MAC will backoff. • Backoff count is decremented each time that physical & virtual carrier indicate no carrier for one slot time. • Once backoff has expired, MAC begins transmission. • If transmission is not successful, collision has occurred.

39. Model of Contention in DCF Mode

40. PCF Operations • PCF uses PIFS, which is shorter than DIFS to “grab hold” of the media for Contention Free Period (CFP) • Competition for media means that CFP may be delayed from ideal start time. Hence only “near isochronous”. • PC gains access to media and broadcasts a “beacon” frame • Transmitted periodically • Stations can request “contention free service” when a poll request is sent • Each station is capable of receiving frames and ack’ing them. • PC uses NAV to hold onto the media for the needed CFP time

41. PC Polling • PC sends a “contention free poll” (CF-Poll) to stations requesting contention-free service • If station has traffic, it may send one frame for each CF-Poll • Can piggy back both ACK and the CF-Poll on data frames during CFP • “PC Can combine CF-Poll and ACK with data frame as well”hence, PC may be sending a frame to one station, along with a CF-Poll and ACK a frame received from an entirely different station.

42. Holding Media in PCF • NAV is primary mechanism to hold media • Announced in Beacon at beginning of CFP • PIFS is secondary mechanism in case some station did not hear Beacon • During CFP, PC assures no interval on medium less than PIFS. • If response not received within SIFS, PC will send frame before PIFS expires • PC announces end of CF period using a CF-End frame • Once NAV is reset, stations compete using DCF

43. Timeline of PCF & DCF Operations

44. Example of PCF Frame Transfer

45. Station Identifiers • SSID is the service set identify • 32 byte “network name” • Zero length “name” is broadcast (I.e. any network) • BSSID is the Basic Service Set ID • Shorter numeric value, randomly generated

46. FrameControl Durationor ID Addr1 Addr2 Addr3 Seq. Cntl Addr4 Frame FCS 2 2 6 6 6 2 6 0-2313 4 Frame Formats • Data to be delivered is the MSDU == MAC Service Data Unit • Converted into MPDU (MAC Protocol Data Unit) for wire • MAC may fragment an MSDU

47. FrameControl Durationor ID Addr1 Addr2 Addr3 Seq. Cntl Addr4 Frame FCS 2 2 6 6 6 2 6 0-2313 4 Frame Control Identifiesthe frame format Station uses to announce power state after this frame. If “11”, using wireless DS ProtocolVersion Type Subtype ToDS FromDS MoreFrags IsRetry? PwrMgt MoreData WEP Order Strictly ordered service requested Frame is buffered at AP for station

48. Fragment bursts get efficient use of media

49. RTS / CTS with Fragmented MSDU

50. Frame Type & Subtype • Management • (re)Assoc/Auth request / response • Probe request / response • Breacon / Announcment Traffic Indication Message • Control • Power save Poll • RTS / CTS / ACK / CF-End • Data • Various combinations of CF, ACK, Poll, etc • Reserved