Title: Wireless Local Area Networks
1Wireless Local Area Networks
2Wireless 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 1990s. - Industry decided to entrust standardization to
IEEE committee that dealt with wired LANS
namely, the IEEE 802 committee!!
3Wireless 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
4Wireless 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
5Design Goals for Wireless MAC
- Requirements for a wireless MAC protocol
- robustness
- efficiency
- fairness
- support for priority and QoS
- support for multicast
6Hidden Station Problem
- Hidden Stations within the range of the intended
receiver, but out of range of the transmitter - hidden sender C
A
B
D
C
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?)
A
B
D
C
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
7Exposed Station Problem
- Exposed Stations within the range of the
intended sender, but out of range of the receiver - exposed sender B
A
B
D
C
Problem C transmits to D, if B transmits (to A),
B cannot hear from A Solution exposed sender B
needs to defer
A
B
D
C
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?)
8Summary of hidden and exposed station problem
- Receivers 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
9MAC 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
10Solution Space for channel contention
- Multiple access approach
- with carrier sensing
- carrier sensing provides collision information
at the sender, NOT the receiver - FAMA, 802.11
- 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
11IEEE 802 Standards Working Groups
The important ones are marked with . The ones
marked with ? are hibernating. The one marked
with gave up.
12IEEE Standards for Wireless Networks
IEEE 802.11 Wireless LANs
IEEE 802.15 Wireless Personal Area Networks (WPAN)
IEEE 802.16 Broadband Wireless Access (BBWA)
IEEE 802.20 Mobile Broadband Wireless Access (MBWA)
IEEE 802.21 Media Independent Handover (MIH)
IEEE 802.22 Wireless Regional Area Networks
13IEEE 802.11 (WLAN)
802.11a 5 GHz, up to 54 Mbps
802.11b 2.4 GHz, up to 11 Mbps
802.11d Enables 802.11 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 802.11b
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 802.11 with throughput in excess of 100Mbps
802.11r Enhancements for fast roaming of WLAN units
802.11s Wireless mesh networks
14Common Aliases of Wireless Standards
802.11b/g Wi-Fi
802.15.1 Bluetooth
802.15.3 Ultra Wideband
802.15.4 ZigBee
802.16 WiMAX
15Categories 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.
16Wireless LANs
- a) Wireless networking with a base station. (b)
Ad hoc networking.
17The 802.11 Protocol Stack
Part of the 802.11 protocol stack.
18IEEE standard 802.11
fixed terminal
mobile terminal
server
infrastructure network
access point
application
application
TCP
TCP
IP
IP
LLC
LLC
LLC
802.11 MAC
802.3 MAC
802.3 MAC
802.11 MAC
802.11 PHY
802.3 PHY
802.3 PHY
802.11 PHY
19Wireless Physical Layer
- Physical layer conforms to OSI (five options)
- 1997 802.11 infrared, FHSS, DHSS
- 1999 802.11a OFDM and 802.11b HR-DSSS
- 2001 802.11g OFDM
- 802.11 Infrared
- Two capacities 1 Mbps or 2 Mbps.
- Range is 10 to 20 meters and cannot penetrate
walls. - Does not work outdoors.
- 802.11 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).
20Wireless Physical Layer
- 802.11 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.
21Wireless 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!!
22Wireless Physical Layer
- 802.11g OFDM(Orthogonal Frequency Division
Multiplexing) - An attempt to combine the best of both 802.11a
and 802.11b. - Supports bandwidths up to 54 Mbps.
- Uses 2.4 GHz frequency for greater range.
- Is backward compatible with 802.11b.
23802.11 - 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
24Distribute 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.
25Virtual 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!
261-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.
27802.11 - 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
next frame
contention
t
Access (after CWmin) if medium is free ? DIFS
28802.11 - CSMA/CA basic access method
contention window (randomized back-offmechanism)
DIFS
DIFS
medium busy
next frame
t
direct access if medium is free ? DIFS
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)
29802.11 - 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
30IEEE 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 gt binary backoff
- NAV Network Allocation Vector (min time of
deferral)
31802.11 - 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
data
RTS
sender
SIFS
SIFS
SIFS
ACK
CTS
receiver
DIFS
NAV (RTS)
data
other stations
NAV (CTS)
t
defer access
contention
32Collision 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 802.11 allows CSMA, CSMA/CA and
polling from AP
33Fragmentation in 802.11
- 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.
34Point 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.
35MAC-PCF (Point Coordination Function) like
polling
t0
t1
SuperFrame
medium busy
PIFS
SIFS
SIFS
D1
D2
point coordinator
SIFS
SIFS
U1
U2
wireless stations
stations NAV
NAV
36MAC-PCF (cont)
t2
t3
t4
PIFS
SIFS
D3
D4
CFend
point coordinator
SIFS
U4
wireless stations
stations NAV
NAV
t
contention free period
contention period
37DCF 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.
38Interframe Spacing in 802.11.
39CSMA/CA Protocol congestion control
40Congestion AvoidanceIEEE 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)
41DCF Example
Let cw 31
B1 and B2 are backoff intervals at nodes 1 and 2
42Congestion 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)
43Congestion Control
- Since the number of nodes attempting to transmit
simultaneously may change with time, some
mechanism to manage congestion is needed - IEEE 802.11 DCF Congestion control achieved by
dynamically adjusting the contention window cw
44Binary 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
45MILD 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 802.11, which multiplies by
2 - When a node successfully completes a transfer, it
reduces cw by 1 - More conservative than 802.11, where cw is
restored to Cwmin - 802.11 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
46CSMA/CA Protocol fairness
47Fairness Issue
- Many definitions of fairness plausible
- Simplest definition All nodes should receive
equal bandwidth
A
B
Two flows
C
D
48Fairness 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
49Fairness Issue
- Observation unfairness occurs when one node has
backed off much more than some other node
A
B
Two flows
C
D
50MACAW 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
51Weighted Fair Queueing
- Assign a weight to each node
- Goal bandwidth used by each node should be
proportional to the weight assigned to the node
52Distributed 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
53Distributed Fair Scheduling (DFS)
B1 5
Collision !
B2 5
B1 15 (DFS actually picks a random value
with mean 15) B2 5 (DFS picks a
value with mean 5)
Weight of node 1 1 Weight of node 2 3 Assume
equal packet size