Mobile Systems

1. Mobile Systems The IEEE 802.11 WLAN Part I Ver 1.1 Mobile Systems ITU

2. IEEE Std. 802.11-1997, 1-2 Mbit/sec. Information Technology- Telecommunications and Information exchange between systems- Local and Metropolitian area networks- Specific requirements- Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications. Sponsor: LAN MAN Standards Committee of the IEEE Computer Soc. Approved 26 June 1997. Mobile Systems ITU

3. ABSTRACT (I) The medium access control (MAC) and physical characteris- tics for wireless local area networks (LAN’s) are specified in this standard, part of a series of standards for local and me- tropolitian area networks. The medium access control unit in this standard is designed to support physical layer units as they may be adopted dependent on the availablity of the spectrum. This standard contains three physical layer units: Mobile Systems ITU

4. ABSTRACT (II) • two radio units, both operating in the 2400-2500 MHz band, and • one baseband infrared unit. One radio unit employs the frequency-hopping spread spec- trum technique, and the other emplys the direct sequence spread spectrum technique. KEYWORDS: ad hoc network, infrared, LAN, local area net- work, mobility, radio frequency, wireless Mobile Systems ITU

5. The IEEE Standard 802.11 WLAN • Similaries between WLAN (Wireless Local Area Networks) • and Wired Local Area Networks. • The IEEE 802.11 WLAN is designed to look like any • IEEE 802 wired LAN. • The 802.11 must support all protocols and LAN • management tools, that operate on a wired network. • The IEEE 802.11 is designed to the same interface as • IEEE 802.3 Mobile Systems ITU

6. The IEEE Standard 802.11 WLAN • Differences between WLAN (Wireless Local Area Networks) • and Wired Local Area Networks. • No wires (because air link), and mobility. • The air link: Radio or infrared. • Data carried by a WLAN is not private or protected. • Data is broadcast to all. • IEEE 802.11 Wired Equivalent Privacy (WEP), protection • at the same level as wired privacy. • Electromagnetic Propagation. • Reflection and/or attenuation (dæmpning) of the signal carrying • LAN data. • Small changes in physical position => large changes in recieved • signal strength. Mobile Systems ITU

7. The IEEE Standard 802.11 WLAN Differences between WLAN (Wireless Local Area Networks) and Wired Local Area Networks. Problems introduced by mobility: • The location-based services lose the ”hook” to a user location, when network addresses are not on a physical site. The notion of ”the nearest network printer” must be re- defined. Increases the complexity of the service location provider. Mobile Systems ITU

8. The IEEE 802.11 WLAN family First standard for WLAN: IEEE Std. 802.11-1997, 1-2 Mbit/sec Defines: • MAC: Medium Access Control layer, management protocols and services. • PHY: PHysical Layer, consisting of: • PLCP: Physical Layer Convergence Procedure sublayer. • PMD: Physical Medium Dependent sublayer. • Three different physical layers: • Infrared (IR) baseband PHY • frequency hopping spread spectrum (FHSS) radio in the • 2.4 GHz band. • direct sequence spread spectrum (DSSS) radio in the • 2.4 GHz band. MAC layer PLCP sublayer PMD sublayer Wireless Media Mobile Systems ITU

9. IEEE Std. 802.11-1997, 1-2 Mbit/sec. Information technology- Telecommunications and information exchange between sy- stems- Local and metropolitian area networks- Specific requirements- Part11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications. Sponsor: LAN MAN Standards Committee of the IEEE Computer Society Approved 26 June 1997. Mobile Systems ITU

10. The IEEE 802.11 WLAN family In 1999 two new physical layers are approved: • IEEE Std 802.11a is an Orthogonal Frequency Domaine Multiplexing (OFDM) radio in the UNII bands delivering up till 54 Mbit/sec. U-NII: Unlicensed national information structure (US) at 5 GHz. • IEEE Std 802.11b is an extension of the DSSS (Direct Sequence Spread Spectrum) PHY (Physical layer) in the 2.4 GHz ISM band, delivering up till 11Mbit/sec. data rates. ISM: Industrial, Scientific, and Medical band at 2.4 GHz. Mobile Systems ITU

11. IEEE 802.11 Architecture • Support networks with decisions in mobile stations, • thus eliminating bottlenecks of a centralized structure. • Error tolerant in WLAN equipment. • Flexible: Supporting • small transient networks, and • large semipermanent or permanent networks. • Deeppower saving modes to prolong battery life without • losing network connectivity. Mobile Systems ITU

12. IEEE 802.11 Architecture • Architectural Elements: • The Station, • the Access Point (AP, which is a Station), the wireless medium. • the Basic Service Set (BSS). Stations that communicate. • The Distribution System (DS), and • the Extended Service Set (ESS). Mobile Systems ITU

13. The Station and The Basic Service Set • Station: Mobile, portable or stationary. • Other names might be: Network adapter or network interface card. • It always consists of a • MAC: Medium Access Control • PHY: Physical Layer (Antenna, Radio, etc.) • Station services: • Authentication: Prove the identity of one station to another. • Deauthentication: Eliminate a previously authorized user from acces. • Privacy: Equivalent level of protection, compared to a wired network. • Data delivery: Reliable delivery of data frames from the MAC in • one station to the MAC in one or more stations. Mobile Systems ITU

14. The Station and The Basic Service Set • Station services: • Authentication: Prove the identity of one station to another. • Deauthentication: Eliminate a previously authorized user from acces. • Privacy: Equivalent level of protection, compared to a wired network. • Data delivery: Reliable delivery of data frames from the MAC in • one station to the MAC in one or more stations. Mobile Systems ITU

15. The Station and The Basic Service Set • Basic Service Set (BSS): Set of stations communcating with one another. • Independent Basic Service Set (IBSS): All stations communicating direct- • ly with one another. Also often denoted an ad hoc network. Mobile Systems ITU

16. The Extended Service Set (ESS) • The access points (AP) communicate among themselves to forward • traffic from one BSS to another. • The AP’s perform this communication via an abstract medium called • the Distribution System (DS). An Access Point is a station, with access to a distribution sys- tem. Mobile Systems ITU

17. Distribution System and Services Distribution System: • One AP Communicating with another to exchange frames for • stations in their BSS’s, • forward frames to follow mobile stations from one BSS to another, • exchange frames with wired networks. Services: • Station Services: Authentication, deauthentication, privacy, delivery • of data. • Distribution Services: Association, disassociaton, reassociation, • distribution, integration. Mobile Systems ITU

18. Station Services (I) • Authentication: • Prove the identity of one station to another. • Without this, the station is not allowed to use the WLAN for • data delivery. • Deauthentication: • Eliminate a previously authorized user from any further use • of the network. Mobile Systems ITU

19. Station Services (II) • Privacy: • Equivalent level of protection as that provided by a wired network • with restricted physical access to the network plant. • Delivery: • Data delivery similar to that provided by other IEEE 802 LAN’s. • Reliable delivery of dataframes from one MAC in one station to • the MAC in one or more other stations, with minimal dublication • and minimal reordering. Mobile Systems ITU

20. Distribution Services (I) • Association: • Establish a logical connection between a mobile station and an AP. • Necessary for the DS to deliver data to the mobile station. • Invoked once, when the station enters the WLAN for the first time, • after power on, or when rediscovering the WLAN after some time. • Reassociation: • As association, but include information about the AP, with which • the mobil station was previously associated. Mobile Systems ITU

21. Distribution Services (II) • Disassociation: • Mobile Station (MS) inform AP that it does not need service. • AP inform one or more MS that logical connection can no longer • be provided. • Distribution: • Frame sent to its own basic service set (BSS) or • to another mobile station associated with another AP or • to a network outside the IEEE 802.11 WLAN. Mobile Systems ITU

22. Distribution Services (III) • Integration Service: • Connect the IEEE 802.11 WLAN to other LAN’s, including • one or more wired LAN’s, or other IEEE 802.11 WLAN’s. • The integration is performed by a portal, which is an abstract • architectural concept. • The integration service translates IEEE 802.11 frames to • frames that may traverse another network, and • translate frames from other networks to frames that may be • delivered by an IEEE 802.11 WLAN. Mobile Systems ITU

23. Interaction Between Some Services (I) The IEEE 802.11 std. requires that each station must maintain two variables that are dependent on • the authentication/deauthentication service, and • association/reassociation/disassociation services. These two Boolean variables are • authentication state • association state. Mobile Systems ITU

24. Interaction Between Some Services (II) • A station may be authenticated with many different stations simultaneously. • A station may be associated with only one other station at a time. Mobile Systems ITU

25. Interaction Between Some Services (III)The General Frame Format MAC Header 2 2 6 6 6 2 6 0-2312 4 FCS Address 2 Frame Body Address 4 Address 1 Sequence Control Duration/ID FrameControl (2 bytes) Address 3 Frame body max. 18496 bits Mobile Systems ITU

26. Interaction Between Some Services (IV) Class 1 Frames. State 1: Unauthenticated, Unassociated DeAuthentication Notfication Successful authentication Class 1 & 2 Frames State 2: Authenticated, Unassociated Successful Association or reassociation Disassociation, Notification State 3: Authenticated, and Associated. Class 1, 2 & 3 Frames Mobile Systems ITU

27. Interaction Between Some Services (V) A Station movingbetween Access Points. a: Find AP1 and authenticate and associate. e: Disassociate stations. b: Preauthenticate with AP2, when moving. f: Find another AP3 for authen- c: Reassociate with AP2. tication and association. d: Terminate the association with AP1. Mobile Systems ITU

28. Medium Access Control (MAC) • The IEEE 802.11 medium access control supplies the • required function for • reliable delivery mechanism for • user data over • noisy, unreliable wireless media, • while providing advanced LAN services, beyond those • of existing wired LAN’s. Mobile Systems ITU

29. Medium Access Control (MAC) • MAC Functionality: • Reliable data delivery service, through a frame exchange protocol. • Fairly control access to the shared wireless medium, through two • different access mechanisms: • The basic access mechanism, called the Distributed Coordination • Function (DCF), and a • centrally controlled access mechanism, called the Point Coordination • Function (PCF). • Privacy service called Wired Equivalent Privacy (WEP) for encryption. Mobile Systems ITU

30. MAC Frame Exchange Protocol (I) • The media used by IEEE 802.11 WLAN are very noisy and unreliable. • Thus the MAC implements a frame exchange protocol, which allows the • source of a frame to determine, • when the frame has sucessfully been received at the • destination. • The frame exchange protocol requires the participation of all stations in the WLAN. Every station decodes and reacts to information in the MAC header of every frame it receives. Mobile Systems ITU

31. MAC Frame Exchange Protocol (II) • The minimal MAC frame exchange protocol consists of two frames: • a frame sent from the source to the destination, and • an acknowledgement from the destination of correctly received frame. • If the source did not receive an acknowledgement it will retransmit. • A frame and its acknowledgement, constitute an atomic • unit of the MAC protocol, and cannot be interripted by • transmission from any other station. Mobile Systems ITU

32. MAC Frame Exchange Protocol (III) • If the source does not receive the acknowledgement because • the destination did not send one, due to error in the ori- • ginal frame, or because • the acknwledgement was corrupted, • the source will attempt to transmit the frame again. • Reduces the error rate of the medium, at the cost of • reduced bandwidth. Mobile Systems ITU

33. The Hidden Node Problem (I) Fact: Not every WLAN station can be expected to commu- nicate directly with every other WLAN station. Station A communicates directly with B. Station B communicates directly with C. Station A cannot communicate with station C. Mobile Systems ITU

34. The Hidden Node Problem (II) If station A was sending a frame to station B, the frame could be corrupted by a transmission initia- ted by station C ! Mobile Systems ITU

35. Hidden The Node Problem (III) • Announce to all station in the neighborhood of both the • source and destination, the impending transmision. • Source sends: Request to Send (RTS). • Destination answers: Clear to Send (CTS). • Then the Source sends • the dataframe(s). • The Destination sends • acknowledgement. Mobile Systems ITU

36. The Hidden Node Problem (IV) Four frame exchange protocol, which is atomic in the MAC (Medium Access Control) protocol. They cannot be interrupted by transmissions of other stations. Mobile Systems ITU

37. MAC Frame Exchange Protocol (IV) • Now extend into a four frame protocol for communicating • between different MAC’s: • The source sends a Request To Send (RTS) to the destination. • This is also received by other stations too. • The destination returns a Clear To Send (CTS) to the source. • This is also received by other stations too. • The source sends the Data Frame to the destination. • The destination sends an Acknowledgement to the source, thus • completing the data transfer. Mobile Systems ITU

38. MAC Frame Exchange Protocol (V) • Reduce the four-way frame exchange protocol to a two-way • protocol if risk of contention is low. • The management information base (MIB): • If length of frame > dotRTSThreshold attribute • Then use the four way protocol, • Else use the two way protocol. • Allow for tuning of the network, by reducing the communi-cation overhead. Mobile Systems ITU

39. Retry Counters • Two Retry counters, when transmission fails. • Short retry counter for frames <= dotShort11RTSThreshold, • Long retry counter for frames <= dotLong11RTSThreshold. • There is also a lifetime timer associated with each frame • transmitted from a MAC. Mobile Systems ITU

40. Basic Access Mechanism (I) • The basic access mechanism from • an Access Point (AP) to the • Physical medium (Radio or Infrared carriers) is • Carrier Sense Multiple Access (CSMA) with • Collision Avoidance (CA) with • binary exponential backoff. • This is also denoted CSMA/CA. Mobile Systems ITU

41. Basic Access Mechanism (II) • A station will listen (Carrier Sense, CS) before beginning a transmission. • If the medium is already carrying a transmission, • Then the station that is listening will not begin its own • transmission, and • the station enters a deferal period. • The duration is determined by a random number which • represents the amount of time, that must elapse while • there are not any transmissions. Mobile Systems ITU

42. Basic Access Mechanism (III) • The waiting time is called the Contention Window the size • of which doubles with every attempt to access the medium • which is deferred. • The random number for the exponential backoff algorithm • is uniformly distributed in the Contention Window interval. • The window is re-initialized when a transmission is sucess- • fully completed. Mobile Systems ITU

43. Basic Access Mechanism (IV) • The Network Allocation Vector (NAV): • The amount of time, that remains, before the medium will • become available. • The NAV is updated through duration values, transmtted • in all frames. • The NAV is virtual carrier sense mechanism, which is com- • bined with the physical carrier sense, into the MAC colli- • sion avoidance part of the CSMA/CD access mechanism. Mobile Systems ITU

44. Timing Intervals (I) The 5 timing intervals needed for control of transmission. Time Short Interframe Space (SIFS), determined by PHY. The Slot Time (ST), determind by PHY. The Priority Interframe Space (PIFS) = SIFS + ST. The Distributed Interframe Space (DIFS)= SIFS + 2 x ST. The Extended Interframe Space (EIFS) >> DIFS. Used for error correc. Mobile Systems ITU

45. Timing Intervals (II) • The 5 timing intervals is used for implementing two diffe- • rent control mechanisms for transmission: • The Distributed Coordinating Function (DCF), used in • Independent Basic Service Sets (IBSS) also denoted ad • hoc networks. • The Centrally Controlled Access Mechanism, used in a • Poll and Response protocol to eliminate contention for • the medium, thus obtaining higher bandwidth than the • DCF. Mobile Systems ITU

46. The Distributed Coordination Function (DCF) The DCF is used in an Independent Basic Service Set (IBSS), also denoted an ad-hoc network: Mobile Systems ITU

47. The Distributed Coordination Function (DCF) • When the MAC is requested to transmit a frame, the follow- • ing is carried out: • Check the physical and virtual carrier sense mechanism • (NAV) if the medium is not in use for an interval of • DIFS: Distributed Interframe Space. • If the medium is in use, apply the back-off mechanism, • and increment the retry counter. • Decrement the back-off value if the medium is idle, by • on slot time interval. When expire then transmit. Mobile Systems ITU

48. The Distributed Coordination Function (DCF) Contention Window End of previous transmission Next Transmission DIFS time Slots DIFS: Distributed Interframe Space Mobile Systems ITU

49. Centrally Controlled Access Mechanism (I) The Centrally Controlled Access is used when • there is an Access Point (AP) Method: Use a poll and response protocol, to eliminate contention for the medium. Point Coordination Function (PCF). Mobile Systems ITU

50. Centrally Controlled Access Mechanism (II) A point coordinator (PC) controls the PCF. A PC is always located in an Access Point (AP). The PCF operation: • A mobile requests the PC to register it on a polling list, residing in the Access Point (AP). • The PC regularly polls the stations for traffic, and • the PC delivers traffic to mobile stations. Near-isochronous service to the stations on the polling list. www.webster.com: Isochronous: Uniform in time. Mobile Systems ITU