Elements of a wireless network
network
infrastructure
Elements of a wireless network
wireless hosts
r laptop, PDA, IP phone
r run applications
r may be stationary
(non-mobile) or mobile
m
network
infrastructure
wireless does not
always mean mobility
Csci 183/183W/232: Wireless LAN
Csci 183/183W/232: Wireless LAN
1
Elements of a wireless network
wireless link
typically used to
connect mobile(s) to
base station
r also used as backbone
link (WiMAX)
r multiple access
protocol coordinates
link access
r various data rates,
transmission distance
Csci 183/183W/232: Wireless LAN
infrastructure mode
base station connects
mobiles into wired
network
r handoff: mobile
changes base station
providing
p
g connection
into wired network
r
network
infrastructure
Csci 183/183W/232: Wireless LAN
3
4
Wireless Link Characteristics
Elements of a wireless network
Ad hoc mode
r no base stations
r nodes can only
transmit to other
nodes within link
coverage
r nodes organize
themselves into a
network: route among
themselves
Differences from wired link ….
m
m
m
Csci 183/183W/232: Wireless LAN
2
Elements of a wireless network
r
network
infrastructure
base station
typically connected to
wired network
r relay - responsible
for sending packets
between wired
network and wireless
host(s) in its “area”
m e.g., cell towers
for cellular
networks, 802.11
access points
r
decreased signal strength: radio signal
attenuates as it propagates through media
(path loss)
interference from other sources: standardized
wireless
l
network
k frequencies
f
((e.g., 2.4
4 GH
GHz))
shared by other devices (e.g., phone); devices
(motors) interfere as well
multipath propagation: radio signal reflects off
objects, ground, arriving at destination at
slightly different times
…. make communication across (even a point to point)
wireless link much more “difficult”
5
Csci 183/183W/232: Wireless LAN
6
1
Wireless network characteristics
Special challenge for Wireless MAC
Multiple wireless senders and receivers create
additional problems (beyond multiple access):
C
A
B
A
C’s signal
strength
A’s signal
strength
t
th
B
Hidden terminal problem
B, A hear each other
B, C hear each other
r A, C can not hear each other
means A, C unaware of their
interference at B
r
r
C
Collision detection is difficult
r
Hidden station problem
r
Exposed station problem
• Transmission power is much higher than receiving power
• A station is not able to detect a potential competitor for the
medium because the competitor is too far away
• Example: A is transmitting to B. C cannot hear the transmission,
y concludes that it can transmit.
thus falsely
• A station hears the on-going transmission, and falsely assume it
cannot transmit
• Example: B tis transmitting to A. C hears the transmission, thus
falsely concludes that it cannot transmit o D.
space
Signal fading:
B, A hear each other
B, C hear each other
r A, C can not hear each other
interferring at B
r
r
Csci 183/183W/232: Wireless LAN
r Idea
m spread signal over wider frequency band than
required
m originally deigned to thwart jamming
r 802.11g
m 2.4-2.485 GHz range
m up to 54 Mbps
m DSSS
r Frequency Hopping
m transmit over random sequence of frequencies
m sender and receiver share…
r All use CSMA/CA for
• pseudorandom number generator
• seed
multiple access
r All have base-station
and ad-hoc network
versions
m
Data stream: 1010
Random sequence: 0100101101011001
1
0
XOR of the two: 1011101110101001
Csci 183/183W/232: Wireless LAN
Csci 183/183W/232: Wireless LAN
10
OFDM
r Direct Sequence
m for each bit, send XOR of that bit and n
random bits
m random sequence known to both sender and
receiver
m called n-bit
bit chipping code
m 802.11 defines an 11-bit chipping code
1
0
802.11 uses 79 x 1MHz-wide frequency bands
9
Spread Spectrum (cont)
1
0
8
Spread Spectrum
r 802.11a
m 5.1-5.8 GHz range
m up to 54 Mbps
m OFDM
Csci 183/183W/232: Wireless LAN
Csci 183/183W/232: Wireless LAN
7
IEEE 802.11 Wireless LAN
r 802.11b
m 2.4-2.485 GHz
unlicensed radio
spectrum
m up to 11 Mbps
m direct sequence spread
spectrum (DSSS) in
physical layer
• all hosts use same
chipping code
(insensitive to
multipath fading)
m widely deployed, using
base stations
r
r A large number of closely-spaced
orthogonalsub-carriers are used to carry
data. The data are divided into several
parallel data streams or channels, one for
each sub
sub-carrier
carrier. Each sub
sub-carrier
carrier is
modulated with a conventional modulation
scheme (such as quadrature amplitude
modulation or phase shift keying) at a low
symbol rate, maintaining total data rates
similar to conventional single-carrier
modulation schemes in the same bandwidth.
11
Csci 183/183W/232: Wireless LAN
12
2
Base Station Approach
Ad Hoc Network Approach
r Wireless host communicates with a base station
m base station = access point (AP)
r No access point (i.e., base station)
m “peer-to-peer” mode
r Basic Service Set (BSS) (a.k.a. “cell”) contains:
r wireless hosts communicate with each other
wireless hosts
m access point (AP): base station
r BSS
BSS’ss combined via a distribution system (DS)
to get packet from wireless host A to B may
need to route through wireless hosts X,Y,Z
r Applications:
m “laptop” meeting in conference room, car
m interconnection of “personal” devices
m battlefield
m
m
m
The DS runs at layer 2 of the ISO architecture!
r
Csci 183/183W/232: Wireless LAN
r Definitions
m MAC-level Acknowledgement
into 11 channels at different frequencies
m AP admin chooses frequency for AP
m interference possible: channel can be same as
that chosen by
y neighboring
g
g AP!
• Indicate the received frame is correct
• The source should wait ACKTimeout amount of time for ACK
m
r host: must associate with an AP
m scans channels, listening for beacon frames
containing AP’s name (SSID) and MAC address
m selects AP to associate with
m may perform authentication
m will typically run DHCP to get IP address in AP’s
subnet
802.11 sender
1 if sense channel idle for DIFS then
transmit entire frame (no CD)
2 if sense channel busy then
start random backoff timer;
timer
i
counts d
down while
hil channel
h
l idl
idle;
transmits when timer expires;
if no ACK, increase random backoff
interval, repeat 2
r 802.11: no collision detection!
m difficult to receive (sense collisions) when transmitting due
to weak rece
received
ved ssignals
gnals (fading)
(fad ng)
m can’t sense all collisions in any case: hidden terminal, fading
m goal: avoid collisions: CSMA/C(ollision)A(voidance)
C
A
B
C
sender
receiver
data
SIFS
ACK
return ACK after SIFS (ACK needed due
to hidden terminal problem)
space
Csci 183/183W/232: Wireless LAN
16
DIFS
802.11 receiver
- if frame received OK
C’s signal
strength
A’s signal
strength
Csci 183/183W/232: Wireless LAN
IEEE 802.11 MAC Protocol: CSMA/CA
nodes transmitting at same time
B
SIFS: short IFS
PIFS: PCF IFS
DIFS: DCF IFS
DIFS>PIFS>SIFS
15
r 802.11: CSMA - sense before transmitting
m don’t collide with ongoing transmission by other node
A
Inter-frame
Inter
frame Space (IFS)
•
•
•
•
IEEE 802.11: multiple access
r avoid collisions:
14
Definitions
r 802.11b/g: 2.4GHz-2.485GHz spectrum divided
2+
Csci 183/183W/232: Wireless LAN
13
802.11: Channels, association
Csci 183/183W/232: Wireless LAN
IETF MANET
(Mobile Ad hoc Networks)
working group
17
Csci 183/183W/232: Wireless LAN
18
3
Collision Avoidance: RTS-CTS exchange
Avoiding collisions – Option I
A
idea:
allow sender to “reserve” channel rather than random
access of data frames: avoid collisions of long data frames
r sender first transmits small request-to-send (RTS) packets
to BS using CSMA
m RTSs may still collide with each other (but they’re short)
r BS broadcasts
b
d sts clear-to-send
l
t s d CTS in
i response
s
s to
t RTS
r RTS heard by all nodes
m sender transmits data frame
m other stations defer transmissions
B
reservation collision
DATA (A)
Avoid data frame collisions completely
using small reservation packets!
Csci 183/183W/232: Wireless LAN
AP
defer
time
Csci 183/183W/232: Wireless LAN
19
IEEE 802.11 MAC Layer – Option II
20
802.11 MAC Layer Carrier Sensing
r Protocol Architecture
m Distributed Coordination Function (DCF)
m Point Coordination Function (PCF)
r Carrier sense at two levels
m Physical carrier sense: done by physical layer
m Virtual carrier sense at MAC layer using Network
Allocation Vector (NAV) set while RTS/CTS/Data/Ack
are overheard: partially solves problem of Hidden and
E
Exposed
d terminal
t
i l
• The Duration field reserves the media!
m
Csci 183/183W/232: Wireless LAN
m
m
r Backoff Procedure
When a STA has data to send, it senses medium
The STA may transmit a MAC Protocol Data Unit (MPDA) when
medium idle time is greater or equal to DIFS
If medium is busy, wait for a random backoff time
m
m
m
m
m
Csci 183/183W/232: Wireless LAN
22
DCF
Basic Access
m
Csci 183/183W/232: Wireless LAN
21
DCF Basic Access
r
Reduces collision by deferring transmission if any of the
carrier sense mechanisms sense the channel busy
23
Backoff procedure is invoked for a STA to transfer a frame but the
medium is busy
Set Backoff Timer to be random backoff time
Backoff Timer start decreasing after an idle time of DIFS following the
medium busyness
Backoff Timer is suspended when medium is busy, and won’t resume until
the medium is idle for DIFS
A frame may be transmitted immediately when Backoff Timer is 0
Csci 183/183W/232: Wireless LAN
24
4
DCF
DCF
r Recovery procedures
m Collision may happen during contention
m When collision happens, retransmission
m STAs maintain a station short retry counter (SSRC) and
long retry counter (SLRC) for each MSDU and MMPDU
• SSRC increases by one for each failed RTS or MPDU whose
length is <= dot11RTSThreshold
• SLRC increases by one for each failed MPDU whose length
is > dot11RTSThreshold
• Both counter is reset upon a successful MPDU
• Retry is aborted when SSRC>=dot11ShortRetryLimt =7 or
SLRC>=aLongRetryLimit
r
Random backoff
r
CW (contention window)
time=random()xaSlotTime
m aSlotTime: the value of the
correspondingly named PHY
characteristic (20s for DSSS)
m Random(): a random integer
uniformly
if
l dist
distributed
ib t d over [0,
[0 CW]
m
m
m
Csci 183/183W/232: Wireless LAN
RTS/CTS Scheme
r
NAV (Network Allocation Vector)
r
m
Four way handshake: RTS-CTS-DATA-ACK
m
An indicator, maintained at each STA, for the period that transmission will
not be initiated
Csci 183/183W/232: Wireless LAN
25
DCF RTS/CTS Scheme
r
Increases exponentially after each
retry fails (so does average backoff
time. Why to do this?)
Keep constant after reaching the
maximum
Reset after a successful
transmission
26
DCF -- Fragmentation
r
Control of the channel
m
Setting and resetting NAV according to “Duration” in MAC header when
receiving a valid frame
Once the STA has contented for the channel, it shall continue to
send fragments until
• All fragments of a MSDU or MMPDU have been sent
• An ACK is not received
• STA is restricted from sending additional fragments by PHY layer
r
Duration field
m
m
RTS/CTS: time till the end of ACK0
Fragments/ACK: time till the end of the ACK for the next
fragment
• Last fragment/ACK: length of ACK/0
Csci 183/183W/232: Wireless LAN
Csci 183/183W/232: Wireless LAN
27
DCF
PCF
r Directed (Unicast) MPDU
m STA uses RTS/CTS for directed MPDU only when the
length of a MPDU >= dot11RTSThreshold
r Point Coordinator
m PCF provides contention free frame transfer
m PC resides in AP; It is an option of AP to become PC
r Fundamental Access
m PC senses the medium
m When medium is idle for PIFS,
PIFS PC transmit a Beacon frame
m After beacon, PC shall wait for SIFS, and then transmit
• Always use RTS/CTS: set dot11RTSThreshold=0
• Don’t use RTS/CTS: set dot11RTSThreshold=maximum
g
octets
MPDU length=2304
r Broadcast and multicast
m Regardless of the length of frame, no RTS/CTS
m No ACK
m No MAC layer recovery
Csci 183/183W/232: Wireless LAN
28
• Data frame
• CF-Poll (contention free poll) frame
• Data + CF-Poll frame
29
Csci 183/183W/232: Wireless LAN
30
5
PCF
r
Polling list
m
m
r
PCF
STA indicates its CF-Pollability via Association and Reassociation
PC shall send a CF-Poll to at least one STA when there are entries in
the polling list
NAV
m
m
Each STA set the NAV to CFPMaxDuration
PC shall transmit a CF-End
CF End frame at the end of CFP
Csci 183/183W/232: Wireless LAN
2
2
6
6
6
frame
address address address
duration
control
1
2
3
2
seq address
4
control
0 - 2312
4
payload
CRC
2
Address 3: MAC address
of router interface to
which AP is attached
2
Protocol
version
2
6
6
6
4
1
Subtype
To
AP
1
1
From More
AP
frag
1
0 - 2312
4
payload
CRC
1
Retry
1
Power More
mgt
data
1
1
WEP
Rsvd
frame type
(management, control, data)
From
AP
Addr 1
Addr 2 Addr 3 Addr 4
0
0
DA
SA
0
1
DA
BSSID SA
BSSID SA
TA
Csci 183/183W/232: Wireless LAN
33
To AP
RA
2
Type
6
2
seq address
4
control
Frame subtype (RTS, CTS, ACK, Beacon, etc.)
DA
R1 router
H1
BSSID -
DA
34
802.11 frame: addressing
Interpretation of the MAC Addrs
1
frame seq #
(for reliable ARQ)
duration of reserved
transmission time (RTS/CTS)
frame
address address address
duration
control
1
2
3
Csci 183/183W/232: Wireless LAN
1
32
802.11 frame: more
Address 2: MAC address
of wireless host or AP
transmitting this frame
0
Csci 183/183W/232: Wireless LAN
Address 4: for Intra
DS communication
Address 1: MAC address
of wireless host or AP
to receive this frame
1
architecture, why?
r PCF is on top of DCF
r Super frame contains a contention-free
contention free
period and a contention period
r Question: how time-bounded service is
provided?
31
802.11 frame: addressing
6
r Only available for infrastructured
Internet
AP
-
R1 MAC addr H1 MAC addr
dest. address
-
source address
802.3 frame
AP MAC addr H1 MAC addr R1 MAC addr
SA
address 1
address 2
address 3
802.11 frame
Csci 183/183W/232: Wireless LAN
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Csci 183/183W/232: Wireless LAN
36
6
Supporting Mobility
Mobility (cont)
ad hoc net working
r Case 2: access points (AP)
r Case 1:
m
m
r Scanning (selecting an AP)
m node sends Probe frame
m all AP’s w/in reach reply with ProbeResponse
frame
m node selects one AP; sends it AssociateRequest
frame
m AP replies with AssociationResponse frame
tethered
each mobile node associates with an AP
Distribution system
AP-1
AP-3
m
F
AP-2
A
B
G
C
new AP informs old AP via tethered network
r When
H
m
E
D
m
Csci 183/183W/232: Wireless LAN
37
active: when join or move
passive: AP periodically sends Beacon frame
Csci 183/183W/232: Wireless LAN
38
7