Download Lecture note 13

IEEE 802.11 Wireless LAN
Part I
Introduction, Physical layer, basic MAC
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Why Wireless Networking?
• Mobility
– Enable users to physically move while using an appliance.
• Cost Saving
– Easy to install in difficult-to-wire areas (E.g., a historical
Museum building)
– Quick to set up (no need to pull cables)
– Increased reliability due to less used cables (the probability
of cable down time thus becomes less)
– Get rid of messy cables in office, home, and other places.
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Wireless Signal Property
• Two type of antennas:
– Omni-directional
– Directional
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Signal Propagation and Quality
Is Often Unpredictable
• A small change in
position or direction
may result in dramatic
differences in signal
strength -- whether a
station is stationary or
mobile!
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Wireless LAN Usages
Ad hoc mode
Infrastructure mode
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IEEE 802.11 Equipments
Lucent’s Orinoco (formly Wavelan)
PCMCIA card
Lucent’s USB
interface card
Lucent’s access point
Lucent’s Orinoco PCI card
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Physical Technology
• Three physical technologies are defined and
used in IEEE 802.11:
– Direct-sequence spread spectrum
• Can support 1, 2, 5.5 and 11 Mbps bandwidth
• The most popular one in the current market.
– Frequency-hopping spread spectrum
• Can support only 1 and 2 Mbps bandwidth
• Popular once but now seldom used.
– Infrared light (diffused or point-to-point)
• Can support up to 16 Mbps bandwidth
• Rarely used.
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Why Direct Sequence Scheme?
• Spread a signal’s power
over a wider band of
frequencies.
• Thus this scheme requires
higher bandwidth to
transmit data signal.
• Because most interference
noise signal is typically
narrow in bandwidth,
• This will result in much
less interference and bit
errors.
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Frequency Hopping SS
• Must hop to another
channel in 400 ms.
• Sender and receiver
must use the same
frequency hopping
sequence to correctly
receive data.
Each channel uses 1
MHZ spectrum.
– Good for military
purpose because now
eavesdropping becomes
hard and signal jamming
also becomes hard.
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Direct Sequence SS
• Convert a data signal to a higher data rate bit
sequence (referred to as a chipping code).
• The code used in 802.11 is a 11-bit Barker
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sequence 10110111000.
802.11 Uses Unlicensed ISM Band
Nowadays
most 802.11
NICs use this
area.
Microwaves also
use this area. So
your 802.11
transfer may be
interfered by your
microwave!
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Modulation Techniques
• DQPSK
modulation is
used when the
transmission
rate is 2 Mbps.
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DSSS Physical Layer Frame Format
MAC Frame
• The physical layer header (23 bytes) and the MAC layer
header(34 bytes) together cause a lot of overhead.
• The SYNC field consists of alternating 0s and 1s. That is,
010101010101 …. A receiver will begin to synchronize
with the incoming signal after detecting the sync.
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DSSS Physical Layer Frame Format
• Start frame delimiter: always 1111001110100000.
• Signal: identifies the type of modulation that the receiver
must use to demodulate the signal. The value of this field is
equal to the data rate divided by 100 Kbps.
– In 802.11, a sender can send its frame at either 1, 2, 5.5, or 11 Mbps
rate.
– To enable the receiver to receive frames that are sent at different rates,
the sender always sends the PLCP preamble and header at the fixed 1
Mbps and the receive always listens for frames at 1 Mbps.
– Thus the receiver can correctly decode the signal field at 1 Mbps and
then switch to a different and higher rate to receive MPDU.
• Service: always 0.
• Length: define the number of microsecond to transmit the
MPDU.
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Lucent Orinoco(WaveLAN) Spec.
Although you have 11
channels to choose from,
selected channels should
be separated at least by 30
MHZ to avoid
interference. So, actually
there are only 3 nonoverlapping channels!
11 Mbps
5.5 Mbps
2 Mbps
1 Mbps
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Application 1
No access point is needed. PC can communicate easily
and directly among themselves. But no QoS can be achieved. 13- 16
Application 2
Access points are needed. But now Information on wired networks can
be accessed and the coverage area is larger. Also QoS now is possible
Although two PCs are in the same cell and can communicate directly,
their communication still needs to go through the access point. 13- 17
Application 2 (cont.)
If these access points are on the same subnet, when a mobile
node move from one cell into another cell, its IP address need
not be changed. Otherwise, its IP address needs to be changed
and either DHCP or Mobile-IP is needed.
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Application 2 (cont.)
f1
f1
f1
f2
f1
f1
If these access points use the same frequency channel, when a mobile
node move from one cell into another cell, its used frequency channel
need not be changed. However, it packet transmission or reception
may be interfered by neighboring access points when it is at the cell
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boundary.
Application 3
Wireless local bridges can be used to wirelessly connect
two remote wired networks (e.g., two buildings). The
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range of these bridges can be up to 30 miles.
Application 4
Wireless relay nodes can act as routers to wirelessly forward
packets. (This type of network is called “packet radio
network.”) Some big cities in US have deployed this type 13of21
networks.
IEEE 802.11 Protocol Architecture
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Components of the IEEE 802.11
Architecture
• Basic service set: the basic building block
of an 802.11 LAN.
– Independent BSS: Also called “ad hoc
network”. Stations in the same IBSS can
communicate directly with each other.
– Infrastructure BSS: a station in the BSS needs
to become an access point. All other stations
need to send packets to the AP, which then
forwards the packet to the destination station.
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Example: BSS
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Distribution System
• To extend the coverage area of a wireless network,
sometimes we want to connect multiple BSSs.
• The component used to interconnect BSSs is
called “distribution system (DS).”
• The medium used for the DS can be different from
the wireless medium. (Normally most vendors just
use Ethernet as the DS.)
• The DS can be considered as a layer-2 network.
• An access point (AP) is a station that provides
access to the DS by providing DS services in
addition to acting as a station.
• Data move between a BSS and the DS via an AP. 13- 25
Example: Distribution System
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Extended Service Set
• A network that is formed by BSSs and DS
is called the “extended service set.”
• ESS network appears the same to an LLC
layer as an IBSS network.
• Stations within an ESS may communicate
and mobile stations may move from one
BSS to another transparently to LLC.
• (So, basically, an extended service set is a
layer-2 network.)
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Example: Extended Service Set
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Wired and Wireless Networks
Integration
• A portal is the logical point at which packets from
a non-802.11 LAN (e.g., 802.3 Ethernet) enter the
802.11 DS.
• It is possible for one device to offer both the
functions of an AP and a portal. (E.g., if the DS
itself is implemented from 802.2 Ethernet).
• Nowadays, what most people say “access point”
actually is a portal. This is because access points
in today’s markets all connect to an Ethernet.
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Example: Portal
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802.11 Architecture Services
• Station services
–
–
–
–
Authentication
Deauthentication
Privacy (WEP: wired equivalent privacy)
MSDU delivery
• Distribution system services
–
–
–
–
–
Association (always initiated by the station, not AP)
Disassociation (can be initiated by either AP or station)
Distribution
Integration
Reassociation (always initiated by the station, not AP)
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Station States
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Frame Types
• Class 1 Frames
– Control Frames
•
•
•
•
Request to send (RTS)
Clear to send (CTS)
Acknowledgement (ACK)
Contention-Free (CF)
– Management Frames
•
•
•
•
•
Probe request/response
Beacon
Authentication
Deauthentication
Announcement traffic indication message (ATIM)
– Data Frame
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Frame Types (cont.)
• Class 2 Frames
– Management frames
• Association request/response
• Reassociation request/response
• Disassociation
• Class 3 Frames
– Data frames
– Management frames
• Deauthentication
– Control Frames
• Power save poll
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MAC Generic Frame Format
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Frame Types
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Frame Subtypes
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Other fields in the Frame Control Field
• To DS: set to 1 if the frame is destined for the DS.
(I.e., the frame is from a 802.11 station to a host
on an Ethernet)
• From DS: set to 1 if this frame is from the DS.
(I.e., the frame is from a host on an Ethernet to a
802.11 station)
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Other fields in the Frame Control Field
• More fragment: set to 1 if another fragment of the
same frame will follow in a subsequent frame.
• Retry: set to 1 if this frame is a retransmission of an
earlier frame.
• Power management : set to 1 to indicate that this
station will be in a sleep mode.
• More data: If a station has additional frames to send to
a station that is in sleep mode, then the sending station
will place 1 in this field.
• WEP: set to 1 to indicate that the frame body has been
encrypted using the WEP algorithm.
• Order: set to 1 to indicate the receiving station that
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frames must be processed in order.
The Duration Field
• The unit of duration is microsecond. This field can
be used by RTS/CTS/DATA frames to reserve the
wireless channel for a period of time.
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802.11 Address Types
• Destination address (DA): The final destination address of
the MAC service data unit (MSDU).
• Source address (SA): The address of the MAC entity that
initiated the MSDU transmission.
• Receiver address (RA): The address of the access point
that is to receive the frame next.
• Transmitter address (TA): The address of the immediate
preceding access point sending the frame.
• BSSID: This is the ID of a BSS. In an infrastructure BSS,
this is the MAC address of the station in the access point.
In an IBSS, this is a locally administered address formed
from a random number.
• An address filed may contain DA, SA, RA, TA, BSSID.
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The Sequence Control Field
• Sequence number is used to detect duplicate frames. (There is
a per-source cache (address 2, seq#, frag#) for each source
station.)
• A frame can be fragmented into several fragments to reduce
the frame-error-rate that it may experience over lossy
wireless links.
• The fragment threshold is a parameter and can be configured.13- 42
MAC Layer Functional Descriptions
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MAC Architecture
• DCF is a distributed contention scheme. (called
CSMA/CA, every station must support it.)
• PCF is a centralized polling scheme. (Optional.
Actually no access points currently support it.)
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DCF Provides Priority
Data Frame
PCF Frame
Control Frame
(e.g., RTS/CTS/ACK)
• When the medium becomes idle, Because DCF
asks different types of frames to wait a different
amount of time before they can be sent to the
medium, DCF can provide three priority levels for
different types of frames.
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DCF Basic Access Method
• If a station wants to send a frame and the medium
is currently busy, the station should defer its
transmission until the medium becomes idle. 13- 46
DCF Basic Access Method
• Then it should wait a period of time equal to DIFS.
• If during this DIFS time the medium is idle, then
the station should start a backoff timer.
– Otherwise, the DIFS timer should be canceled and later
restarted when the medium again becomes idle.
• When the station’s backoff timer expires, the
station then can send its frame onto the medium.
– But, the backoff timer should be suspended whenever
the medium is busy.
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Backoff Procedure
When the medium becomes idle, resume the
backoff timer only after DIFS idle time.
Suspend the backoff timers if a transfer is going on.
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ACK Procedure
Retransmit timer expires here
• 802.11 performs a link-level ACK scheme. Depending on
frame sizes, a frame can be retransmitted up to 4 or 7 times.
• If the ACK is not received within a certain time but a valid
frame is received, the data transmission is deemed failed.
However the received valid frame can be processed. 13- 49
Exponential Backoff
The CW will be exponentially increased for successive
failed transmissions. CW will be reset to CWmin
immediately after a successful transmission, or the number
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of retransmission has exceeded a certain value.
Use of Network Allocation
Vector (NAV)
• NAV is used to reserve the medium.
• It is used in RTS/CTS/Data/ACK frames to avoid
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the hidden-terminal problem.
Fragment Transmission without
RTS/CTS
• It is SIFS, not DIFS. Therefore, the transmissions of all of the
fragments of a frame can be finished before another station
transmits a frame onto the medium (because it need to wait DIFS
idle time)
• If a fragment is lost, only the lost fragment is retransmitted, rather
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than the whole frame.
Fragment Transmission with
RTS/CTS
• The duration fields in the fragment and ACK frames are
used to update other stations’ NAVs to reserve the medium
for the transmissions of the next pair of fragment and ACK.
• This duration scheme is always used despite whether the
RTS/CTS scheme is used or not.
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Fragment Lost Handling
idle
seize the medium again
• Because the stations that are close to the sending and
receiving stations may hear the NAV update carried in the
fragment and ACK frames, they will not attempt to use the
medium. The result is that the sending station will seize the
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medium again.
IFS Timing Relationships
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