CSE 5345 – Fundamentals of Wireless Networks

Today Wireless LAN

What is IEEE 802.11? IEEE: Institute of Electrical and Electronics Engineers 802.11: Family of standards set forth by the IEEE to define the specifications for wireless LANs Defines: Medium Access Control (MAC) Physical Layer (PHY) 3

IEEE 802.11 and the ISO stack 4

IEEE 802.11 (1997, MAC+PHY) Local, high-speed wireless connectivity for fixed, portable and moving stations stations can be moving at pedestrian and vehicular speeds Standard promises interoperability vendors products on the same physical layer should interoperate Targeted for use in inside buildings, outdoor areas, anywhere! 5

IEEE 802.11 Uses Direct Sequence spread spectrum (DSSS) technology Frequency-Hopping spread spectrum (FHSS) can only be used for 1 or 2Mbps in US due to FCC regulations Operates in unlicensed 2.4 GHz ISM band ISM: Industrial, Scientific and Medical ISM regulatory range: 2.4 GHz to 2.4835 GHz for North America 6

IEEE 802.11 Supported Speeds and Distances 1, 2 Mbps at distances of 150-2000 feet without special antenna Greater distances can be achieved by using special antennas Distance (or signal strength) greatly depends on obstructions such as buildings and other objects Maximum speed obtained depends on signal strength 7

IEEE 802.11b (1999, PHY) ‘b’ in IEEE 802.11b September 1999, 802.11b “High Rate” amendment was ratified by the IEEE 802.11b amendment to 802.11 only affects the physical layer, basic architecture is the same Added two higher speeds 5.5 and 11 Mbps More robust connectivity 802.11b was the‘favorite’ in 802.11 also known as Wi-Fi (Wireless Fidelity) 8

IEEE 802.11a (1999, PHY) “Fast Ethernet” standard of wireless LANs Speeds of up to 54 Mbps 5 GHz (U-NII band) instead of 2.4 GHz Unlicensed National Information Infrastructure OFDM instead of DSSS for encoding Orthogonal Frequency Division Multiplexing 9

IEEE 802.11a Advantages Disadvantages higher speed less RF interference than 2.4 GHz 2.4 GHz used by Bluetooth, cordless/cellular phones, etc. Disadvantages shorter range, need to increase AP density or power 4X to compensate 10

IEEE 802.11g (2003, PHY) Another high-speed standard Speeds of up to 54 Mbps Still works at 2.4 GHz not in the 5 GHz range like 802.11a Advantages compatible with 802.11b, ?? better range than 802.11a a/b/g combo available now 11

IEEE 802.11e Another standard for WLANs Wireless Multimedia Extension Quality-of-Service enhancement to MAC layer Backward compatible Higher throughput Wireless Multimedia Extension A subset of 802.11e Service differentiation Product available now 12

IEEE 802.11i Security enhancement Product available WEP broken Backward compatible approach (TKIP) Software update only for legacy devices New: CCMP Product available And a bunch of others .11n, .11r, .11f……. 13

ISM Bands  Industrial, Scientific, and Medical bands. License exempt From To Bandwidth Availability 6.765 MHz 6.795 MHz 30 kHz 13.553 MHz 13.567 MHz 14 kHz Worldwide 26.957 MHz 27.283 MHz 326 kHz Worldwide 40.660 MHz 40.700 MHz 40 kHz Worldwide 433.050 MHz 434.790 MHz 1.74 MHz Europe, Africa, Middle east, Former Soviet Union 902.000 MHz 928.000 MHz 26 MHz America, Greenland 2.400 GHz 2.500 GHz 100 MHz Worldwide 5.725 GHz 5.875 GHz 150 MHz Worldwide 24.000 GHz 24.250 GHz 250 MHz Worldwide 61.000 GHz 61.500 GHz 500 MHz 122.000 GHz 123.000 GHz 1 GHz 244 GHz 246 GHz 2 GHz Ref: http://en.wikipedia.org/wiki/ISM_band

North American Channels 2.4 GHz Band: 5-MHz Channels. Only 12 in USA. 20 MHz  Only 3 non-overlapping channels Channel 5 Channel 9 Channel 3 Channel 7 2400 Channel 1 Channel 6 Channel 11 2483.5 2402 2412 2422 2432 2442 2452 2462 2472 2482 5 GHz Band: 12 non-overlapping channels 36 40 44 48 52 56 60 64 5150 5180 5200 5220 5240 5260 5280 5300 5320 5350 149 153 157 161 5725 5745 5765 5785 5805 5825

IEEE 802.11 Physical Layers First version in 1997: IEEE 802.11  Issued in four stages First version in 1997: IEEE 802.11  Includes MAC layer and three physical layer specifications Two in 2.4-GHz band and one infrared All operating at 1 and 2 Mbps No longer used Two additional amendments in 1999:  IEEE 802.11a-1999: 5-GHz band, 54 Mbps/20 MHz, OFDM  IEEE 802.11b-1999: 2.4 GHz band, 11 Mbps/20 MHz Fourth amendment: New band new circuitry for analog equipments  IEEE 802.11g-2003 : 2.4 GHz band, 54 Mbps/20 MHz, OFDM

Media Access Control Wireless channel is a shared medium Need access control mechanism To avoid interference, collision Active research area 17

Carrier Sense Multiple Access (CSMA) Sense the channel before transmission Idle -> transmit, busy->wait 18

Hidden Terminal Problem Node B can communicate with A and C both A and C cannot hear each other When A transmits to B, C cannot detect the transmission using carrier sense If C transmits, collision will occur at node B A B C 19

Where is the problem Signal fades with distance from transmitter Receiver’s vicinity not cleared No signal A B C 20

802.11 Solution When node A wants to send a packet to node B, node A first sends a Request-to-Send (RTS) to B On receiving RTS, node B responds by sending Clear-to-Send (CTS) to A When a node (such as C) overhears a CTS, it keeps quiet for the duration of the transfer Transfer duration is included in RTS and CTS both RTS A B C CTS 21

RTS/CTS Optional RTS can collide with each other But short frames Still efficient Discussion: when to use RTS/CTS RTS A B C CTS 22

Exposed Terminal Problem A transmits to D, B transmits to C D cannot hear B, C cannot hear A A and B can hear each other Concurrent transmission is ok Carrier sense prevents it D A B C 23

Collision Detection Immediately stop transmission if collision detected Trashed anyway CSMA/CD 24

802.11 Solution Acknowledgement A way to achieve reliability Upon reception of a data frame, receiver sends Ack If no Ack received in time, transmitter assumes data lost Due to collision or error on channel A way to achieve reliability Error prone transmission Local link retransmission possible ARQ 25

Collision Avoidance p persistent Random backoff transmit with probability p (<=1) in an idle slot Random backoff Wait a random period before transmit 26

4-Way Handshake Access Mobile Point Node Ready to send Clear to send Data Ack

IEEE 802.11 MAC  Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)  Listen before you talk. If the medium is busy, the transmitter backs off for a random period.  Avoids collision by sending a short message: Ready to send (RTS) RTS contains dest. address and duration of message. Tells everyone to backoff for the duration.  Destination sends: Clear to send (CTS) Other stations set their network allocation vector (NAV) and wait for that duration  Can not detect collision  Each packet is acked. MAC-level retransmission if not acked. 

IEEE 802.11 Priorities DIFS Contention Window PIFS Busy SIFS Random Backoff Frame Time Carrier Sensed  Interframe space (IFS)  Highest priority frames, e.g., Acks, use short IFS (SIFS)  Medium priority time-critical frames use “Point Coordination Function IFS” (PIFS)  Asynchronous data frames use “Distributed coordination function IFS” (DIFS) – DCF controls access to the physical medium

802.11 MAC Overview Basic Access Mechanism for DCF Consider a WLAN with one AP and 2 nodes. A and B have frozen values of b.o. counter 5 and 8 respectively. After finding the channel idle for DIFS, node A starts decrementing the b.o. counter value of 5 and Node B starts decrementing the b.o. counter value of 8. After node A finishes successful transmission, it waits for SIFS time and receives a MAC ACK from the AP. Assuming that node A has another packet to transmit, it again waits to check medium idle for DIFS time. If so, samples a new b.o. value from [0,31] with equal probability. After inferring collision, the nodes wait for EIFS, the upper limit of CW is doubled and nodes sample a new b.o. value from [0, 63]. The back-off and the channel activity continue this way. Markov decision process.

Typical Parameter Values  For DS PHY: Slot time = 20 us, SIFS = 10 us, CWmin = 31, CWmax = 1023  For FH PHY: Slot time = 50 us, SIFS = 28 us, CWmin = 15, CWmax = 1023  11a: Slot time = 9 us, SIFS= 16 us, CWmin= 15, CWmax=1023  11b: Slot time = 20 us, SIFS = 10 us, CWmin= 31, CWmax=1023  11g: Slot time = 20 us or 9 us, SIFS = 10 us, CWmin= 15 or 31, CWmax=1023  PIFS = SIFS + 1 slot time  D IFS = SIFS + 2 slot times

DCF Backoff Remaining Backoff SIFS Ack CTS Ack DIFS AP Data DIFS C A C  Example: Slot Time = 1, CWmin = 5, DIFS=3, PIFS=2, SIFS=1, Backoff Remaining Backoff SIFS Ack CTS Ack DIFS AP Data DIFS C A C A S2 S3 R D S4 A R D T 8 10 12 14 16 18 20 22 24 26 28 0 2 4 6

DCF: Example (Cont)  T=1 Station 2 wants to transmit but the media is busy T=2 Stations 3 and 4 want to transmit but the media is busy T=3 Station 1 finishes transmission.    T=4 Station 1 receives ack for its transmission (SIFS=1) Stations 2, 3, 4 set their NAV to 1.  T=5 Medium becomes free T=8 DIFS expires.  Stations 2, 3, 4 draw backoff count between 0 and 5. The counts are 3, 1, 2  T=9 Station 3 starts transmitting. Announces a duration of 8 (RTS+SIFS+CTS+SIFS+DATA+SIFS+ACK). Station 2 and 4 pause backoff counter at 2 and 1 resp. and wait till T=17  T=15 Station 3 finishes data transmission  T=16 Station 3 receives Ack. T=17 Medium becomes free 

DCF: Example (Cont)  T=20 DIFS expires Stations 2 and 4 start their backoff counter  T=21 Station 4 starts transmitting RTS

Basic Service Set (BSS): A set of stations controlled by a single “Coordination Function” the logical function that determines when a station can transmit or receive Similar to a “cell” in cellular networks A BSS can have one Access-Point Station with AP Functionality Traffic through AP For now 35

Basic Service Set BSS 36

Independent Basic Service Set (IBSS) A BSS without an Access-Point One station can initiate the IBSS Diameter of the cell determined by coverage distance between two wireless stations Different from ad-hoc 37

Independent Basic Service Set (IBSS) 38

Basic Service Set Identifier (BSSID) “Cell identifier”/Network ID 6 octets long (MAC address format) One BSS has one SSID Value of BSSID is the same as the MAC address of the radio in the Access-Point 39

Extended Service Set (ESS) A set of one or more Basic Service Sets Interconnected by a Distribution System (DS) Traffic always flows via Access-Point Diameter of the cell is double the coverage distance between two wireless stations 40

Extended Service Set BSS Distribution System BSS 41

Extended Service Set BSS Distribution System BSS 42