1. Wireless Local Area Networks

2. Wireless Local Area Networks
The proliferation of laptop computers and other mobile devices (PDAs and cell phones) created an obvious application level demand for wireless local area networking.
Companies jumped in, quickly developing incompatible wireless products in the 1990’s.
Industry decided to entrust standardization to IEEE committee that dealt with wired LANS – namely, the IEEE 802 committee!!

3. Wireless MAC Protocol Outline: design challenges for wireless MAC
hidden/exposed stations
flexible control for QoS support
two design paradigms
multiple access based
token based
rationale for design choices

4. Wireless Networking Environment
A simple model:
A single shared physical channel among users
Omni-directional antenna, limited transmission range
Same transmission rate for all users
Channel characteristics(illustrated with examples)
wireless transmission is spatial and local
sender & receiver: different views of the world
relevant contention is at the receiver side
contention may induce collisions
contention/collision/congestion is location dependent
channel access is a collective behavior from the fairness perspective: the notion of “local” is misnomer
Wireless MAC: how to address channel access in a wireless environment

5. Design Goals for Wireless MAC
Requirements for a wireless MAC protocol:
robustness
efficiency
fairness
support for priority and QoS
support for multicast

6. Hidden Station Problem
Hidden Stations: within the range of the intended receiver, but out of range of the transmitter
hidden sender C A B C D
Problem: A transmits to B, if C transmits (to D), collision at B
Solution: hidden sender C needs to defer (Question: who tells C, A or B?)
hidden receiver C A B C D
Problem: A transmits to B, if D xmits to C, C cannot reply. D confuses (4 cases)
Solution: D needs to be notified that its receiver C is hidden

7. Exposed Station Problem
Exposed Stations: within the range of the intended sender, but out of range of the receiver
exposed sender B A B C D
Problem: C transmits to D, if B transmits (to A), B cannot hear from A
Solution: exposed sender B needs to defer
exposed receiver B A B C D
Problem: C transmits to D, if A xmits to B, B cannot reply. A confuses (4 cases)
Solution: A needs to be notified that its receiver B is exposed (how can B hears A?)

8. Summary of hidden and exposed station problem
Receiver’s perception of a clean/collided packet is critical
Hidden/exposed senders need to defer their transmissions
Hidden/exposed receivers need to notify their senders about their status

9. MAC Protocol Resolve channel contention & access:
Channel access arbitration
know who are there
allocate the channel among multiple senders & receivers who share the channel
Collision avoidance
multiple access based
token based
Collision resolution
backoff based

10. Solution Space for channel contention
Multiple access approach
with carrier sensing
carrier sensing: provides collision information at the sender, NOT the receiver
FAMA,
without carrier sensing
MACA, MACAW
cons and pros: robust, solves hidden/exposed station problem, hard to provide QoS
Token based approach
TDMA, DQRUMA
cons and pros: easy to provide QoS, less robust, hard to handle hidden/exposed stations

11. IEEE 802 Standards Working Groups
The important ones are marked with *. The ones marked with  are hibernating. The one marked with † gave up.

12. IEEE Standards for Wireless Networks
Wireless LANs
IEEE
Wireless Personal Area Networks (WPAN)
IEEE
Broadband Wireless Access (BBWA)
IEEE
Mobile Broadband Wireless Access (MBWA)
IEEE
Media Independent Handover (MIH)
IEEE
Wireless Regional Area Networks

13. IEEE 802.11 (WLAN) 802.11a 5 GHz, up to 54 Mbps 802.11b
802.11d
Enables to work in various countries where it can't today
802.11e
QoS Enhancement
802.11f
Adds Access Point Interoperability
802.11g
2.4 GHz, up to 54 GHz, compatible with b
802.11h
Resolves interference issues
802.11i
Security Enhancement
802.11j
Japanese regulatory extensions
802.11k
Radio resource measurement
802.11m
Enhanced maintenance features, improvements, and amendments
802.11n
Next generation of with throughput in excess of 100Mbps
802.11r
Enhancements for fast roaming of WLAN units
802.11s
Wireless mesh networks

14. Common Aliases of Wireless Standards
802.11b/g
Wi-Fi
Bluetooth
Ultra Wideband
ZigBee
802.16
WiMAX

15. Categories of Wireless Networks
Base Station :: all communication through an access point {note hub topology}. Other nodes can be fixed or mobile.
Infrastructure Wireless :: base station network is connected to the wired Internet.
Ad hoc Wireless :: wireless nodes communicate directly with one another.
MANETs (Mobile Ad Hoc Networks) :: ad hoc nodes are mobile.

16. Wireless LANs
a) Wireless networking with a base station. (b) Ad hoc networking.

17. Part of the 802.11 protocol stack.

18. IEEE standard: 802.11 fixed terminal mobile terminal server
infrastructure network
access point
application
application
TCP
TCP IP IP
LLC
LLC
LLC
MAC
MAC
802.3 MAC
802.3 MAC
PHY
PHY
802.3 PHY
802.3 PHY

19. Wireless Physical Layer
Physical layer conforms to OSI (five options)
1997: infrared, FHSS, DHSS
1999: a OFDM and b HR-DSSS
2001: g OFDM
Infrared
Two capacities 1 Mbps or 2 Mbps.
Range is 10 to 20 meters and cannot penetrate walls.
Does not work outdoors.
FHSS (Frequence Hopping Spread Spectrum)
The main issue is multipath fading.
79 non-overlapping channels, each 1 Mhz wide at low end of 2.4 GHz ISM band.
Same pseudo-random number generator used by all stations.
Dwell time: min. time on channel before hopping (400msec).

20. Wireless Physical Layer
DSSS (Direct Sequence Spread Spectrum)
Spreads signal over entire spectrum using pseudo-random sequence (similar to CDMA).
Each bit transmitted using an 11 chips Barker sequence, PSK at 1Mbaud.
1 or 2 Mbps.
802.11a OFDM (Orthogonal Frequency Divisional Multiplexing)
Compatible with European HiperLan2.
54Mbps in wider 5.5 GHz band  transmission range is limited.
Uses 52 FDM channels (48 for data; 4 for synchronization).
Encoding is complex ( PSM up to 18 Mbps and QAM above this capacity).
E.g., at 54Mbps 216 data bits encoded into into 288-bit symbols.
More difficulty penetrating walls.

21. Wireless Physical Layer
802.11b HR-DSSS (High Rate Direct Sequence Spread Spectrum)
11a and 11b shows a split in the standards committee.
11b approved and hit the market before 11a.
Up to 11 Mbps in 2.4 GHz band using 11 million chips/sec.
Note in this bandwidth all these protocols have to deal with interference from microwave ovens, cordless phones and garage door openers.
Range is 7 times greater than 11a.
11b and 11a are incompatible!!

22. Wireless Physical Layer
802.11g OFDM(Orthogonal Frequency Division Multiplexing)
An attempt to combine the best of both a and b.
Supports bandwidths up to 54 Mbps.
Uses 2.4 GHz frequency for greater range.
Is backward compatible with b.

23. 802.11 - MAC layer Access methods MAC-DCF CSMA/CA (mandatory)
collision avoidance via randomized “back-off” mechanism
minimum distance between consecutive packets
ACK packet for acknowledgements (not for broadcasts)
MAC-DCF w/ RTS/CTS (optional)
Distributed Foundation Wireless MAC
avoids hidden terminal problem
MAC- PCF (optional)
access point polls terminals according to a list

24. Distribute Coordination Function (DCF)
Uses CSMA/ CA (CSMA with Collision Avoidance).
Uses both physical and virtual carrier sensing.
Two methods are supported:
based on MACAW(Multiple Access with Collision Avoidance for Wireless) with virtual carrier sensing.
1-persistent physical carrier sensing.

25. Virtual Channel Sensing in CSMA/CA
“virtual” implies source station sets duration field in data frame or in Ready-to-Send (RTS) and Clear-to-Send (CTS) frames.
Stations then adjust their NAV (Network Allocation Vector) accordingly!

26. 1-Persistent Physical Carrier Sensing
Station senses the channel when it wants to send.
If idle, station transmits.
Station does not sense channel while transmitting.
If the channel is busy, station defers until idle and then transmits.
Upon collision, wait a random time using binary exponential backoff.

27. 802.11 - MAC layer (cont) Priorities
defined through different inter frame spaces
no guaranteed, hard priorities
SIFS (Short Inter Frame Spacing)
highest priority, for ACK, CTS, polling response
PIFS (PCF IFS)
medium priority, for time-bounded service using PCF
DIFS (DCF, Distributed Coordination Function IFS)
lowest priority, for asynchronous data service
DIFS
DIFS
PIFS
SIFS
medium busy
contention
next frame t
Access (after CWmin) if medium is free  DIFS

28. 802.11 - CSMA/CA basic access method
contention window
(randomized back-off mechanism)
DIFS
DIFS
medium busy
next frame
direct access if medium is free  DIFS t
slot time
station ready to send starts sensing the medium (Carrier Sense based on CCA, Clear Channel Assessment)
if the medium is free for the duration of an Inter-Frame Space (IFS), the station can start sending after CWmin (IFS depends on packet type)
if the medium is busy, the station has to wait for a free IFS, then the station must additionally wait a random back-off time (collision avoidance, multiple of slot-time)
if another station occupies the medium during the back-off time of the station, the back-off timer stops (fairness) 12

29. 802.11 - CSMA/CA (cont) Sending unicast packets
station has to wait for DIFS (and CWmin) before sending data
receivers acknowledge at once (after waiting for SIFS) if the packet was received correctly (CRC)
automatic retransmission of data packets in case of transmission errors
DIFS
data
sender
SIFS
ACK
receiver
DIFS
data
other
stations t
waiting time
contention

30. IEEE 802.11 MAC Protocol CSMA Version of the Protocol:
sense channel idle for DISF sec (Distributed Inter Frame Space)
transmit frame (no Collision Detection)
receiver returns ACK after SIFS (Short Inter Frame Space)
if channel sensed busy => binary backoff
NAV: Network Allocation Vector (min time of deferral)

31. 802.11 - CSMA/CA with RTS/CTS Sending unicast packets
station can send RTS with reservation parameter after waiting for DIFS (reservation declares amount of time the data packet needs the medium)
acknowledgement via CTS after SIFS by receiver (if ready to receive)
sender can now send data at once, acknowledgement via ACK
other stations store medium reservations distributed via RTS and CTS
DIFS
RTS
data
sender
SIFS
SIFS
SIFS
CTS
ACK
receiver
DIFS
NAV (RTS)
data
other
stations
NAV (CTS) t
defer access
contention

32. Collision Avoidance RTS freezes stations near the transmitter
CTS “freezes” stations within range of receiver (but possibly hidden from transmitter); this prevents collisions by hidden station during data transfer
RTS and CTS are very short: collisions during data phase are thus very unlikely (similar effect as Collision Detection)
Note: IEEE allows CSMA, CSMA/CA and “polling” from AP

33. Fragmentation in
High wireless error rates  long packets have less probability of being successfully transmitted.
Solution: MAC layer fragmentation with stop-and-wait protocol on the fragments.

34. Point Coordinated Function (PCF)
PCF uses a base station to poll other stations to see if they have frames to send.
No collisions occur.
Base station sends beacon frame periodically.
Base station can tell another station to sleep to save on batteries and base stations holds frames for sleeping station.

35. MAC-PCF (Point Coordination Function) like polling
SuperFrame
medium busy
PIFS
SIFS
SIFS D1 D2
point
coordinator
SIFS
SIFS U1 U2
wireless
stations
stations‘
NAV
NAV

36. MAC-PCF (cont) t2 t3 t4 PIFS SIFS D3 D4 CFend point coordinator SIFSU4
wireless
stations
stations‘
NAV
NAV
contention free period t
contention
period

37. DCF and PCF Co-Existence
Distributed and centralized control can co-exist using InterFrame Spacing.
SIFS (Short IFS) :: is the time waited between packets in an ongoing dialog (RTS,CTS,data, ACK, next frame)
PIFS (PCF IFS) :: when no SIFS response, base station can issue beacon or poll.
DIFS (DCF IFS) :: when no PIFS, any station can attempt to acquire the channel.
EIFS (Extended IFS) :: lowest priority interval used to report bad or unknown frame.

38. Interframe Spacing in

39. CSMA/CA Protocol: congestion control

40. Congestion Avoidance: IEEE 802.1 DCF
Before transmitting a packet, randomly choose a backoff interval in the range [0,cw]
cw is the contention window
“Count down” the backoff interval when medium is idle
Count-down is suspended if medium becomes busy
When backoff interval reaches 0, transmit packet (or RTS)

41. DCF Example B1 = 25 B2 = 20 B1 = 5 B2 = 15 data wait data wait B2 = 10
Let cw = 31
B1 = 25
B2 = 20
B1 = 5
B2 = 15
data
wait
data
wait
B2 = 10
B1 and B2 are backoff intervals
at nodes 1 and 2

42. Congestion Avoidance
The time spent counting down backoff intervals contributes to MAC overhead
Choosing a large cw leads to large backoff intervals and can result in larger overhead
Choosing a small cw leads to a larger number of collisions (more likely that two nodes count down to 0 simultaneously)

43. Congestion Control
Since the number of nodes attempting to transmit simultaneously may change with time, some mechanism to manage congestion is needed
IEEE DCF: Congestion control achieved by dynamically adjusting the contention window cw

44. Binary Exponential Backoff in DCF
When a node fails to receive CTS in response to its RTS, it increases the contention window
cw is doubled (up to an upper bound – typically 5 times)
When a node successfully completes a data transfer, it restores cw to CWmin

45. MILD Algorithm in MACAW [Bharghavan94Sigcomm]
When a node fails to receive CTS in response to its RTS, it multiplies cw by 1.5
Less aggressive than , which multiplies by 2
When a node successfully completes a transfer, it reduces cw by 1
More conservative than , where cw is restored to Cwmin
reduces cw much faster than it increases it
MACAW: cw reduction slower than the increase
Exponential Increase Linear Decrease
MACAW can avoid wild oscillations of cw when congestion is high

46. CSMA/CA Protocol: fairness

47. Fairness Issue Many definitions of fairness plausible
Simplest definition: All nodes should receive equal bandwidth A B
Two flows C D

48. Fairness Issue
Assume that initially, A and B both choose a backoff interval in range [0,31] but their RTSs collide
Nodes A and B then choose from range [0,63]
Node A chooses 4 slots and B choose 60 slots
After A transmits a packet, it next chooses from range [0,31]
It is possible that A may transmit several packets before B transmits its first packet A B
Two flows C D

49. Fairness Issue
Observation: unfairness occurs when one node has backed off much more than some other node A B
Two flows C D

50. MACAW Solution for Fairness
When a node transmits a packet, it appends its current cw value to the packet
All nodes hearing that cw value use it for their future transmission attempts
The effect is to reset all competing nodes to the same ground rule

51. Weighted Fair Queueing
Assign a weight to each node
Goal: bandwidth used by each node should be proportional to the weight assigned to the node

52. Distributed Fair Scheduling (DFS) [Vaidya00Mobicom]
A fully distributed algorithm for achieving weighted fair queueing
Key idea: if sender A has weight =1 and sender B has weight = 2, they split the bandwidth 1 to 2
Choose backoff intervals proportional to
(packet size / weight)
DFS attempts to mimic the centralized Self-Clocked Fair Queueing algorithm [Golestani]
Works well on a LAN

53. Distributed Fair Scheduling (DFS)
data
wait
data
wait
Collision !
B2 = 5
Weight of node 1 = 1
Weight of node 2 = 3
Assume equal
packet size
B1 = 15 (DFS actually picks a random value
with mean 15)
B2 = 5 (DFS picks a value with mean 5)