1. CSE 5345 – Fundamentals of Wireless Networks

2. Today
Wireless LAN

3. What is IEEE ?
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

4. IEEE and the ISO stack 4

5. IEEE (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

6. 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 GHz for North America 6

7. IEEE 802.11 Supported Speeds and Distances
1, 2 Mbps at distances of 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

8. IEEE 802.11b (1999, PHY) ‘b’ in IEEE 802.11b
September 1999, b “High Rate” amendment was ratified by the IEEE
802.11b amendment to 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
also known as Wi-Fi (Wireless Fidelity) 8

9. 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

10. 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

11. 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 a
Advantages
compatible with b, ??
better range than a
a/b/g combo available now 11

12. 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 e
Service differentiation
Product available now 12

13. 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

14. ISM Bands  Industrial, Scientific, and Medical bands. License exempt
From To
Bandwidth
Availability
6.765 MHz
6.795 MHz
30 kHz
MHz
MHz
14 kHz
Worldwide
MHz
MHz
326 kHz
Worldwide
MHz
MHz
40 kHz
Worldwide
MHz
MHz
1.74 MHz
Europe, Africa, Middle east,
Former Soviet Union
MHz
MHz
26 MHz
America, Greenland
2.400 GHz
2.500 GHz
100 MHz
Worldwide
5.725 GHz
5.875 GHz
150 MHz
Worldwide
GHz
GHz
250 MHz
Worldwide
GHz
GHz
500 MHz
GHz
GHz
1 GHz
244 GHz
246 GHz
2 GHz
Ref:

15. 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

16. IEEE 802.11 Physical Layers First version in 1997: IEEE 802.11
 Issued in four stages
First version in 1997: IEEE 
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 a-1999: 5-GHz band, 54 Mbps/20 MHz,
OFDM
 IEEE b-1999: 2.4 GHz band, 11 Mbps/20 MHz Fourth amendment:
New band new circuitry for analog equipments 
IEEE g-2003 : 2.4 GHz band, 54 Mbps/20 MHz,
OFDM

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

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

19. 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

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

21. 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

22. 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

23. 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

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

25. 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

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

27. 4-Way Handshake Access Mobile Point Node Ready to send
Clear to send Data
Ack

28. IEEE 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. 

29. 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

30. 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.

31. 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

32. 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

33. 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 

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

35. 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

36. Basic Service Set
BSS 36

37. 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

38. Independent Basic Service Set (IBSS)38

39. 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

40. 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

41. Extended Service Set
BSS
Distribution
System
BSS 41

42. Extended Service Set
BSS
Distribution
System
BSS 42