Mobile Systems

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

2. Synchronization and Frequency Hopping Synchronzation: The process of the stations in a BSS (Basic Service Set) get- ting in step with each other, for reliable communication. The process involves beaconing for • announcing the presence of a BSS, and • distribute a common timeframe, provided by a • Timer Synchronization Function (TSF), which is a 64 bit timer, running at 1MHz. Mobile Systems ITU

3. Synchronization and Frequency Hopping Examples of Infra Structure Basic Service Sets (BSS’s): One Basic Service Set: One Basic Service Set: AP (Access Point) AP M2 M1 M3 M1 M2 AP: Station which has access to a Distribution System Mobile Systems ITU

4. Synchronization and Frequency HoppingExample of Basic Service Set The simplest nontrivial Independent Basic Service Set (No Access Point with access to a Distribution System): M1 M2 Mobile Systems ITU

5. Synchronization and Frequency HoppingSynchronization in Infrastructure BSS (I) The AP transmits a Beacon frame containing (among others): • Timestamp (64 bits) with • value of TSFTIMER (Timing Synchronzation Function) of the AP; the unit = 1 msec. • Beacon Interval (16 bit) with • the period mesured in ”Time units” of 1024 msec. • Capability Information (16 bits) among others with • ESS, IBSS, CF Pollable, CF Poll Request, …. • Supported rates. • Frequency Hopping (FH) parameter set. Mobile Systems ITU

6. Synchronization and Frequency HoppingSynchronization in Infrastructure BSS (II) The AP Beacon frame is transmitted as any other frame: • thus it can be delayed. The mobile station (STA) receiving a Beacon frame, updates its TSF timer (Timer Synchronization Function), with the re- ceived value + time for update in STA. Mobile Systems ITU

7. Synchronization and Frequency HoppingSynchronization in Independent BSS (I) No AP in an Independent Basic Service Set (IBSS). The timer synchronization is completely distributed among the mobile stations. The basic beaconing process is estab- lished by: STA 1, that starts the IBSS, sets TSF Timer = 0, and emits a Beacon, choosing a Beacon period. Then the other STA’s will attempt to send a Beacon, at the next Target Beacon Transmission Time (TBTT). STA 1 STA 3 STA 2 Mobile Systems ITU

8. Synchronization and Frequency HoppingSynchronization in Independent BSS (II) To ensure that • a Beacon frame is sent at each TBTT (Target Beacon Transmission Time), and • collisions are minimized, each station in the IBSS, choses a random delay, which it will expire after the TBTT, before it attempts a Beacon transmission. If a Beacon is received before delay expires, no Beacon is send. Maybe more than one Beacon recieved; allowed operation. Mobile Systems ITU

9. Synchronization and Frequency HoppingSynchronization in Independent BSS (III) Each station sends its own TSF (Timing System Function) timer, in the Beacon it transmits. A STA will update its TSF timer, with the value received from a Beacon frame if • if it is > value currently in the timer, after modification for processing time. Spread the value of the TSF timer with the fastest running clock. Time for spreading. If no hops; if hops. Mobile Systems ITU

10. Synchronization and Frequency HoppingFrequency Hopping (I) Timing synchronization of STA’s for Frequency Hopping (FH). What is FH? T: aCurrentDwellTime (FH period) T time T T T T 2 2.402 BSS 3 2.403 . . BSS 2 2.479 80 2.480 BSS 1 frequency GHz Mobile Systems ITU

11. Synchronization and Frequency HoppingFrequency Hopping (II) Why Frequency Hopping (FH) • Reduces interference from foreign sources, because not staying on a single frequency. • Co-locate different Basic Service Sets within the same geographical area. • Reduces the amount of interference between different Basic Service Sets, which follows different hopping sequences. • Utilizes the spectrum awailable. Mobile Systems ITU

12. Synchronization and Frequency HoppingFrequency Hopping (III) Computing of a Frequency Hopping (FH) sequence: An FH pattern Fx consists of a permutation of all frequency channels used. The hopping patterns are devided into 3 sets: Set 1: x={0,3,6,9,12,15,18,21,24,27,30,33,36,39,42,45,48,51,54,57,60,63,66,69,72,75} Set 2: x={1,4,7,10,13,16,19,22,25,28,31,34,37,40,43,46,49,52,55,58,61,64,67,70,73,76} Set 3: x={2,5,8,11,14,17,20,23,26,29,32,35,38,41,44,47,50,53,56,59,62,65,68,71,74,77} Mobile Systems ITU

13. Synchronization and Frequency HoppingFrequency Hopping (IV) fx(i) is the channel number for the i’th frequency in the x’th hopping pattern. p is the number of frequency channels in the hopping pattern. p=79 for most of Europe (execpt France and Spain). fx(i)=[b(i)+x]mod(79)+2 where b(i) is the Base-Hopping sequence with p=79 entries. Mobile Systems ITU

14. Synchronization and Frequency HoppingFrequency Hopping (V); the Base-Hopping sequence. i 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 b(i) 0 23 62 8 43 16 71 47 19 61 76 29 59 22 52 63 26 77 31 2 i 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 b(i) 18 11 36 72 54 69 21 3 37 10 34 66 7 68 75 4 60 27 12 25 i 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 b(i) 14 57 41 74 32 70 9 58 78 45 20 73 64 39 13 33 65 50 56 42 i 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 b(i) 48 15 5 17 6 67 49 40 1 28 55 35 53 24 44 51 38 30 46 Mobile Systems ITU

15. Synchronization and Frequency HoppingFrequency Hopping (VI) From channel number to frequency: Ch: Channel number ; 2 <= Ch <= 80. Freq: Channel Center Frequency in GHz Freq=2.400+Ch The frequency hopping sequences, are designed to ensure a minimum distance in frequency between contiguous hops. The minimum hop size is 6 MHz in North America and Europe. 3 6 9 1 GHz = 10 MHz = 10 KHz = 10 Hz Mobile Systems ITU

16. The General Frame Format (I) MAC Header 2 2 6 6 6 2 6 0-2312 4 Bytes Address 3 Frame Check Seq. Address 2 Frame Body (data) Address 1 Address 4 Duration/ID Seqence Control Mobile Systems ITU Frame Control

17. The Frame Control Field (I) 16 bit field, necessary for interpretation of all fields in the MAC header. bits 2 2 4 Protocol version Type Subtype bits 1 1 1 1 1 1 1 1 To DS From DS Frags Retry Power More data WEP Order Mobile Systems ITU

18. The Frame Control Field (II) • Protocol Version (2 bits). • Frame Type (2 bits) and Subtype (4 bits): • Control, 6 different types: F.inst. Power Save; RTS; CTS; ACK; … • Data, 8 different types: • Management, 11 different types: F.Inst. assoc. request and • response; Beacon; Disassociation; Authentication; … Mobile Systems ITU

19. The Frame Control Field (III) • To DS (1 bit) and From DS (1 bit) (Distribution System). • (To DS=1) and (From DS=0) in frames sent from the Mobile Sta- • tion to the Acess Point (AP), targeted to be received by the Dis- • tribution System. • (To DS=0) and (From DS=1), from the Access Point (AP) a Mobile • Station. • (To DS=0) and (From DS=0) then direct communication between • two Mobile Stations are carried out. Mobile Systems ITU

20. The Frame Control Field (IV) • Frags (1 bit): More Fragments Subfield. • Frags =1 indicates that this frame is not the last fragment of a • data or a management frame, that thas been fragmented. • Frags =0 in the last frame. • Retry (1 bit): 0 Indicates if a frame is transmitted for the • first time. • Retry =1 when it is a retransmission (because of noise or • collision of frames). Mobile Systems ITU

21. The Frame Control Field (V) • Power Management (1 bit). • If 0 the station is ready for communication. • If 1, the station will be entering the power management mode, • and not available for future communication. • More data (1 bit). • If 1 there is at least one frame buffered in the Access point (AP) • for the mobile station. Mobile Systems ITU

22. The Frame Control Field (VI) • WEP (1 bit) (Wired Equivalent Protection). • WEP =1 indicates that the frame body of this MAC frame has been • encrypted using the WEP agorithm. • Order (1 bit). This subfield indicates that the content of the • frame is sent to the MAC, with a request for strictly orde- • red service. Mobile Systems ITU

23. The General Frame Format (II) • Duration/ID field (16 bits). • Duration information for the NAV (Network Allocation Vector). • The NAV is a value (15 bits), that indicates to a station the • amount of time that remains, until the medium becomes • available, thus avoiding collision. • or a • short ID, called Association ID (AID), used by a mobile station • to retrieve frames, buffered in the Access Point (AP). Mobile Systems ITU

24. The General Frame Format (III) Addressing. • The Medium Access Control (MAC) frame format, contains • 4 address fields, and five different addresses are generated: • The source address (SA). Address of MAC generating the frame. • The destination address (DA). Address of MAC to which the frame • is sent. • The transmitter address (TA) of the sending MAC. • The receiver address (RA). Address of the MAC to which the frame • is sent. • the Basic Service Set identifier (BSSID). Mobile Systems ITU

25. The General Frame Format (IV) Addressing. • Note that • SA (Source Address) might be different from • TA (Transmission Address) because of • Distribution System operations. Mobile Systems ITU

26. The General Frame Format (VI) Addressing. • A basic IEEE 48 bit address consists of three fields: • Individual/Group field (1 bit). • Is it a single MAC address or a group of MAC’s. • Specially broadcast to all stations. when the Individual/Group • field=1, and all other bits in the address are 1 also. • Universal/Local field (1 bit). • Is the address administered globally by the IEEE network, • or is it local? • Address field on 46 bits. Mobile Systems ITU

27. The General Frame Format (VII) Addressing. • The Basic Service Set Identifier (BSSID), is a unique • identifier for a particular Basic Service Set (BSS) of an • IEEE 802.11 Wireless Local Area Network. • The identifier is generated for two different situations: • In an infrastructure BSS, • In an independent BSS (ad-hoc network). Mobile Systems ITU

28. The General Frame Format (VIII) Addressing. • The identifier is generated for two different situations: • In an infrastructure BSS, the Basic Service Set • Identifier is the MAC address of the Access Point (AP). • In an independent BSS (ad-hoc network), the BSSID is • locally administered, and • generated randomly by the station, that starts the • Independent Basic Service Set (IBSS). Mobile Systems ITU

29. The General Frame Format (IX). • The sequence control field (16 bits), consists of two subfields: • A sequence number subfield (12 bits), assigned by the transmitting • station to each frame sent. • This number is incremented by one after each transmision, and covers • the range 0 - 4095. • A 12 bit addressing space: 2^12 = 4095 positions. • A fragment number (4 bits), when the data is partitioned into fragments. • The fragment number is constant in all transmissions or retransmissions • of a particular fragment. Mobile Systems ITU

30. The General Frame Format (X). • The Frame Body Field contains the actual data or • management information to be transferred. • The maximum length is 2304 bytes. • Frame Check Sequence Field. • Generation of a checksum for the frame transmitted • This is then checked at the receiving side. Mobile Systems ITU

31. Transmission of Data Between Two Stations (I) • The transmission is controlled by 6 subtypes of the Control Frame. • They are used for all exchange of information between • a Mobile Station or • an Access Point (AP) or • another Mobile Station. Mobile Systems ITU

32. Transmission of Data Between Two Stations (II) • The transmission from a source station to a destination station is carried out as follows: • The source sends a request to send (RTS). • The destination returns a clear to send (CTS). • The source sends the data. • The destination sends an acknowledgement. Mobile Systems ITU

33. Transmission of Data Between Two Stations (III). The Request to Send (RTS) Frame (20 bytes): First frame in a four-way frame exhange handshake, between a trans-mitter and a receiver station. Bytes 2 2 6 6 4 Control Duration Receiver Addr. Transmision Addr. FCS • Duration: • Time to transmit a Clear to Send (CTS) + data frame + acknowled- • gement (ACK) frame, and two Short Interframe Space (SIFS). • The duration is measured in microseconds. Mobile Systems ITU

34. Transmission of Data Between Two Stations (IV). The Request to Send (RTS) Frame (20 bytes): Bytes 2 2 6 6 4 Control Duration Receiver Addr. Transmision Addr. FCS Duration: time Data Frame Acknowledgement Clear to Send Short Interframe Space Mobile Systems ITU

35. Transmission of Data Between Two Stations (V) The Clear to Send (CTS) Frame (14 bytes): Second frame in a four-way frame exchange handshake between a transmitter and a receiver station. Bytes 2 2 6 4 Control Duration Receiver Addr. FCS • Duration (Microseconds): • Time to transmit a data frame + acknowledgement (ACK) frame, and • one Short Interframe Space (SIFS). • The Receiver Address is the Transmitter Address from the RTS frame. Mobile Systems ITU

36. Transmission of Data Between Two Stations (VI) The Clear to Send (CTS) Frame (14 bytes): Bytes 2 2 6 4 Control Duration Receiver Addr. FCS Duration: Data Frame Ack time Short Interframe Space Mobile Systems ITU

37. Transmission of Data Between Two Stations (VII). The Data Frame: Third frame in a four-way frame exchange handshake between a transmitter and a receiver station. 2 2 6 6 6 2 6 0-2312 4 Bytes Fr. Con. Dura. Addr1 Addr2 Addr3 S. Cont. Addr4 F. Body FCS The general data frame format, as shown in the intial format. Mobile Systems ITU

38. Transmission of Data Between Two Stations (VIII) The Acknowledgement Frame (14 bytes): The fourth frame in a four-way frame exchange handshake between a transmitter station and a receiver station. 2 2 6 4 Bytes F. Cont. Dura. Receiver Addr. FCS Transmit ACK to the sender of the immediately received data, management or PS-Poll (Power Save Poll) frame, that the frame was received correctly. Also transmit duration information for a fragment burst, to those stations in the neighborhood of the station, intended to receive the fragments. Mobile Systems ITU

39. The Data Frame Subtypes (I) • There are 8 data frame subtypes, devided into two groups: • Group with data: • Simple data, • data with Contention-Free Acknowledgement (CF-ACK), • data with Contention-Free Poll (CF-Poll), • data with CF-ACK and CF-Poll. • Group without data: Why carry out NULL Functions? • Null function (no data), • null function with Contention-Free Acknowledgement (CF-ACK), • null function with Contention-Free Poll (CF-Poll), • null function with CF-ACK and CF-Poll. Mobile Systems ITU

40. The Data Frame Subtypes (II) Group with Data • The Data + CF-ACK frame is the same as a simple data frame execpt: • It is only sent through a Contention Free Period (CFP). • It is never used in an Independent Basic Service Set (IBSS). • The Acknowledgement carried in this frame is acknowledging the • previously received data frame. • The ACK may not be associated with the address of the data frame, • but with another address. Mobile Systems ITU

41. The Data Frame Subtypes (III) Group with Data • The Data + CF-Poll frame is the same as a simple data • frame execpt: • It is sent by the Point Coordinator (PC), during a Contention Free • Period. • This frame is never sent by a Mobile Station. • It is never used in an Independent Basic Service Set (IBSS). • The Point Coordinator (PC) delivers data to a Mobile Station, and • simultaneously, requests that the Mobile Station sends a data frame, • that it may have buffered, when the current reception is completed. Mobile Systems ITU

42. The Data Frame Subtypes (IV) Group with Data • The Data + CF-ACK+ CF-Poll frame is the same as a simple data frame • execpt: • It can only be sent by the Point Coordinator (PC), during a Conten- • tion Free Period. • This frame is never sent by a Mobile Station. • It is never used in an Independent Basic Service Set (IBSS). • This frame simply combines the functions of • data transfer, • Contention-Free Acknowledgement to another address, and • Contention-Free Poll (buffered data from the Mobile Station next). Mobile Systems ITU

43. The Data Frame Subtypes (V) Group with No Data • The Null Function (No Data) • A data frame that contains no frame body (no data). • The only purpose of this frame is to carry the Power • Management Bit in the Frame Control field, to the Access Point • (AP), when a Mobile Station changes to a low power operating • state. • Thus this data frame subtype, without data, is only used for infor-mation exchange for power saving function, in the Mobile Station! Mobile Systems ITU

44. The Data Frame Subtypes (VI) Group with No Data • The Contention Free Poll (CF-Poll) (No data) • Used by the Point Coordinator (PC) to request that the Mobile • Station sends a pending data frame during the Contention Free • Period (CFP). • The PC will send this frame, instead of Data + CF-Poll, when it has • no data to be sent to the Mobile Station. Mobile Systems ITU

45. The Medium Access Control (MAC) Management (I) • Wireless Local Area Networks (WLAN) • much more complex than LAN’s, because • the medium (the ISM band) is not a wire, cable or optic fiber. • The medium is shared by other users, some not aware of data commu- • nication: • Microwave oven (2.4 GHz is the optimum frequency for heating the • water molecules). • Radio frequency identification (RFID). Small tags on boxes etc. • Other WLAN’s not coordinated with this one. • WLAN’s coordinated with this one. • Bluetooth (Wireless Cable) in the ISM band. Mobile Systems ITU

46. The Medium Access Control (MAC) Management (II) • The medium is intermittent (afbryder, standser for kortere perioder) by • nature. • The MAC management is needed for dealing with, among • others, all the above. Mobile Systems ITU

47. The Medium Access Control (MAC) Management (III) • Why MAC Management: • The medium is intermittent by nature. This requires much manage- • ment. • Anyone can connect to the WLAN, simply by using the right antenna • and receiver, thus this requires management for identification and • authentication of a mobile user. Mobile Systems ITU

48. The Medium Access Control (MAC) Management (IV) • Why MAC Management: • The equipment is mobile, thus management for keeping the service • of a mobile user, is needed. • General demand of reducing the power consumption from mobile • station batteries. Thus • this requires management for controlling the communication • between the mobile stations and the access point (AP) to a mini- • mum, and • thereby conserving energy from the mobile battery. Mobile Systems ITU

49. The Medium Access Control (MAC) Management (V) • MAC Management Tools for operating a reliable WLAN through an • unreliable medium: • Authentication: Prove the identity to another station in WLAN. • Association: Process of a mobile station ”connecting” to an AP • Address Filtering: Use when more than one 802.11 WLAN at location. • Privacy: Wired Equivalent Privacy (WEP). • Power management: Turn on and off of receiver/transmitter in mobile. • Synchronization: Distribute clock among stations. Mobile Systems ITU

50. MAC Management: Authentication • Process of one station to prove its identity to another station. • Example: • Station A asserting “I am A”, and asks “Who are you”. • Station B says “Ok, prove you are station A” and asserts “I am B”. • Station now offers some proof of its identity and requests the same • from station B. • If the proofs exchanged were acceptable to both, each station will • then tell the other that its assertion of identity is believed. The shared key authentication algorithm, where each station have a copy of a shared WEP key. Encrypt and decrypt a “challenge text” as the proof, that the stations share the same key. Mobile Systems ITU