Wireless LANs By Waheed ur Rehman wahrehmangmail com
Wireless LANs By Waheed ur Rehman wahrehman@gmail. com
Agenda l l l Introduction Components and station types Physical Layer specification l l MAC layer specification l l PMD and PLCL DCF and PCF Frame format Association and reassociation Power Management
Wireless Market Segments
Wireless Ethernet (Introduction) Ø IEEE standard is 802. 11 Ø proposal for 1, 2 Mbps was asked in 1990 ØFinal draft approved on 26 June 1997 Ø There are two architecture Ø Adhoc based and Infrastructure based Ø In adhoc based there is no access point Ø Infrastructure based have an AP Ø three popular variations are 802. 11 a, 802. 11 b, 802. 11 g
Figure 15. 1 BSSs
Wireless Ethernet Standard Frequency Data Rate Aval. BW 1 802. 11 2. 4 -2. 4835 GHz 1, 2 Mbps 83. 5 MHz 2 802. 11 a 5. 15 -5. 35 GHz 6, 9, 12, 18, 24, 3 6, 48, 54 Mbps 300 MHz 3 802. 11 b 2. 4 -2. 4835 GHz 1, 2, 5. 5, 11 Mbps 83. 5 MHz 4 802. 11 g 2. 4 -2. 4835 GHz 1, 2, 5. 5, 6, 9, 12, 83. 5 MHz 18, 24, 36, 48, 54 Mbps
ESS
Wireless Ethernet
Wireless Ethernet Basic Service Set (BSS) Ø consist of set of stationary or mobile stations Ø an optional AP Extended Service Set (ESS) Ø combination of 2 or more BSS Ø BSS are connected through distribution system Ø usually a wired LAN
Wireless Ethernet Station Types Ø No-transition mobility stationary or move only within BSS Ø BSS-Transition M one BSS to another Ø ESS-Transition M one ESS to another
Figure 15. 3 Physical layer specifications 1 to 2 Mbps Some similarities with CDMA Each bit is transmitted as 11 chips, called barker sequence Uses phase shift modulation 1 bit per baud 1 Mbaud when operating at 1 Mbps 2 Mbaud when operating at 2 Mbps Uses 79 channels, 1 MHz each Pseudorandom number generator is used Dwell time: amount of time on each freq. Adjustable but muss be less than 400 msec Security and interference management 802. 11 b, High rate DSSS 11 Mbps in 2. 4 GHz Data rates supported are 1, 2, 5. 5 and 11 Mbps 1, 2 Mbps uses phase shift modulation: Backward compatibility 5. 5 and 11 Mbps uses walsh/Hadamard codes Range of 802. 11 b is 7 times greater than 802. 11 a Uses OFDM of 802. 11 a But operates in 2. 4 GHz Upto 54 Mbps theoretically Backward compatibility 802. 11 a uses OFDM Deliver up to 54 bps 5 GHz band Split the signal into different narrow bands Better immunity to interference And possibility of using non contiguous bands
Physical Layer (802. 11 b) l Divided into 2 parts l l l Physical Medium Dependent (PMD) Physical Layer Convergence Procedure (PLCP) Note that 802. 11 b make changes to the physical layer of the original 802. 11
PMD Standard l l l Translate binary 0 s and 1 s into radio signals Uses ISM band @ 2. 4 GHz 1, 2, 5. 5, 11 Mbps For 1 Mbps – PSK is used For 2, 5. 5, 11 Mbps – 4 PSK is used
PLCP Standard l l l l PLCP frame has 3 parts Preambles, header and data Preambles allows receiver to get ready Header provides information about the frame itself Data is actual data being transmitted. . 1 to 16384 bits PLCP header and preambles are always transmission at 1 Mbps Advantages / Disadvantages ? ?
Figure 15. 6 MAC layers in IEEE 802. 11 standard
Wireless Ethernet MAC Sublayer Ø Distribution Coordinate Function (DCF) Ø Point Coordination Function (PCF)
Wireless Ethernet Distributed Coordination Function Access Method CSMA/CA ØCSMA/CD cannot be used because increased bandwidth required (sending and receiving simultaneously Hidden station problem A B C Signal fading for distant stations can prevent a station at one end from hearing a collision at the other end.
Wireless Ethernet Network Allocation Vector (NAV) Ø RTS (req. to send) contain duration of time it needs to occupy the channel. Ø stations that are affected by this transmission creates a timer called NAV ØShows that how much time must pass before these stations are allowed to check the channel for idleness.
Figure 15. 8 CSMA/CA and NAV
Wireless Ethernet Collision during Handshaking Ø two or more stations may try to send frames simultaneously, resulting in collision Ø There is no collision detection mechanism, so the sender, upon not receiving the CTS frame, assume collision.
Figure 15. 7 CSMA/CA flowchart
Wireless Ethernet Point Coordination Function (PCF) Ø optional access method that can be implemented in infrastructure network Ø implemented on top of DCF Ø PCF has centralized polling access method ØAP perform polling for stations that are capable of being polled using PCF interframe space(PIFS)
Figure 15. 9 Frame format
Wireless Ethernet Frame Format 2 bytes FC 2 bytes 6 bytes D Add 1 Add 2 Add 3 2 bytes 6 bytes 0 -2312 bytes 4 bytes SC Add 4 Frame body FCS FC : Frame Control D : duration of transmission Add 1. . 4 : source, destination and AP addresses SC : Sequence number of the frame Frame Body : Info based on type and subtype FCS : CRC error detection Seq
Subfields in FC field Field Explanation Version The current version is 0. Type of information: management (00), control (01), or data (10). Subtype Defines the subtype of each type (see ). To DS Defined later. From DS Defined later. Used for initial comm. For accessing Used channel to carry data and More flag When set to 1, Bw means moreand fragments. stations AP And ack control info Retry When set to 1, means retransmitted frame. Pwr mgt When set to 1, means station is in power management mode. More data When set to 1, means station has more data to send. WEP Wired equivalent privacy. When set to 1, means encryption implemented. Rsvd Reserved.
Control frames
Values of subfields in control frames Subtype Meaning 1011 Request to send (RTS) 1100 Clear to send (CTS) 1101 Acknowledgment (ACK)
Subfields in FC field To DS From DS Address 1 Address 2 Address 3 Address 4 0 0 Destination station Source station BSS ID N/A 0 1 Destination station Sending AP Source station N/A 1 0 Receiving AP Source station Destination station N/A 1 1 Receiving AP Sending AP Destination station Source station
Addressing mechanism: case 1
Addressing mechanism: case 2
Addressing mechanism: case 3
Addressing mechanism: case 4
Association and Reassociation l l Association is the process of becoming a part of a network Association is accomplished by scanning Two types of scanning, passive and active scanning Passive Scanning l l Client listening to channel for set period of time (usually 10 sec) Client listen to the beacon frame sent by all APs
Association and Reassociation l Active Scanning l l Client send a special frame on each available channel called Probe frame Then wait for a Probe response from all the APs Client then send associate request frame to AP AP send associate response frame
Power Management l l AP keeps a list of those clients in sleeping mode If some data is arrived for such station, its buffered. AP sends Traffic Indication Message (TIM) to the client for station whose data is buffered The amount of sleep mode for mobile client is generally 100 msec
Any Questions?
Quiz 1. The two architectures of wireless Ethernet based Adhoc based are Infrastructure _______ and ____.
Quiz 2. 802. 11 originally operated at 2. 4 ____ GHz frequency.
Quiz 3. The highest data rate of 802. 11 g is 54 Mbps ______.
Quiz 4. The three types of station according to transition mobility are No ______ Transition , _______, and ESS ______. BSS Transition Mobility
Quiz 5. The physical layer of 802. 11 b is divided into PLCP PMD ______ and _______.
Quiz 6. PLCP frame has three parts namely preamble header and _____. data ____, _____
Quiz 7. PLCP header and preambles are or 2 Mbps always transmission at 1_______.
Quiz 8. The MAC sublayer is divided into ______ and _____. DCF PCF
Quiz 9. Client listening to channel for set period of time which is usually 10 _______ sec.
Quiz 10. The amount of sleep mode for mobile client is generally _____ 100 msec
Assignment. l l What is hidden station problem and exposed station problem? How can we solve these problems? Explain 802. 11 n, 802. 11 ac, and 802. 11 ad. Deadline : (in class) Proper referencing and properly formatted.
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