PART 6 IEEE 802.11 Wireless LANs

Transcript

PART 6
IEEE 802.11 Wireless LANs
Lecture 6.1
Introduction
Giuseppe Bianchi, Ilenia Tinnirello
WLAN History
Ł Original goal:
Deploy “wireless Ethernet”
First generation proprietary solutions (end ’80, begin ’90)
WaveLAN (AT&T))
HomeRF (Proxim)
Abandoned by major chip makers (e.g. Intel: dismissed in april 2001)
Ł IEEE 802.11 Committee formed in 1990
Charter: specification of MAC and PHY for WLAN
First standard: june 1997
1 and 2 Mbps operation
Reference standard: september 1999
Multiple Physical Layers
Two operative Industrial, Scientific & Medical unlicensed bands
» 2.4 GHz: Legacy; 802.11b/g
» 5 GHz : 802.11a
Ł 1999: Wireless Ethernet Compatibility Alliance (WECA)
certification
Later on named Wi-Fi
Boosted 802.11 deployment!!
WLAN data rates
Ł
Ł
Legacy 802.11
Work started in 1990; standardized
in 1997
1 mbps & 2 mbps
The 1999 revolution: PHY
layer impressive
achievements
802.11a: PHY for 5 GHz
Published in 1999
Products available since early
2003
802.11b: higher rated PHY for 2.4
GHz
Published in 1999
Products available since 1999
Interoperability tested (wifi)
Ł
2003: extend 802.11b
802.11g: OFDM for 2.4 GHz
Published in june 2003
Products available, though no
extensive interoperability
testing yer
Backward compatiblity with
802.11b Wi-Fi
Standard
Transfer
Method
Frequenc
y Band
Data
Rates
Mbps
802.11 legacy
FHSS,
DSSS, IR
2.4 GHz,
IR
1, 2
802.11b
DSSS,
HR-DSSS
2.4 GHz
1, 2, 5.5,
11
"802.11b+"
non-standard
DSSS,
HRDSSS,
(PBCC)
2.4 GHz
1, 2, 5.5,
11,
22, 33, 44
802.11a
OFDM
5.2, 5.5
GHz
6, 9, 12,
18, 24, 36,
48, 54
802.11g
DSSS,
HRDSSS,
OFDM
2.4 GHz
1, 2, 5.5,
11; 6, 9,
12, 18, 24,
36, 48, 54
Why multiple rates?
“Adaptive” coding/modulation
Example: 802.11a case
PHY distance/rate tradeoffs
(open office)
140.0
5 GHz OFDM (.11a)
120.0
Distance (m)
2.4 GHz OFDM (.11g)
100.0
2.4 GHz (.11b)
80.0
60.0
40.0
20.0
0.0
1Mbps
5.5Mbps 6Mbps
11Mbps 12Mbps
24Mbps
36Mbps
54Mbps
Coverage performance
Cisco Aironet 350 Access Point
11 Mb/s DSS
da ~30 a ~45 metri
5.5 Mb/s DSS
da ~45 a ~76 metri
Configurable TX power:
50, 30, 20, 5, 1 mW
(100 mW outside Europe)
2 Mb/s DSS
da ~76 a ~107 metri
Greater TX power, faster battery consumptions!
!
WLAN NIC addresses
Ł Same as Ethernet NIC
48 bits = 2 + 46
Ł Ethernet & WLAN addresses do coexist
Undistinguishable, in a same (Layer-2) network
Role of typical AP = bridge
» To be precise: when the AP acts as “portal” in 802.11
nomenclature
802 IEEE
48 bit address
1 bit = individual/group
1 bit = universal/local
46 bit adress
C:> arp –a
192.168.1.32 00.0a.e6.f7.03.ad dinamico
192.168.1.43 00.06.00.32.1a dinamico
192.168.1.52 00.82.00.11.22.33
Protocol stack
Ł 802.11: “just” another 802 link layer J
On which bandwidths?
Ł Classical approach: narrow frequency band, much power
as possible into the signal (noise reduction based on
brute force)
Authority must impose rulse on how RF spectrum is used.
Licenses restrict frequencies and transmission power
Ł However.. Several bands have been reserved for
unlicensed use
To allow consumer market for home use, bands for
“industrial, scientific and medical” equipment (ISM
bands): e.g. 2.4 Ghz, 5 Ghz
Limitations only on transmitted power
“unlicensed” does not mean “plays well with others
The 2.4 (wifi) GHz spectrum
Ł 2.4 GHz bandwidth: 2.400-2.483,5
Ł Europe (including Italy):
13 channels (each 5 MHz)
- 2.412 – 2.417 –
- 2.422 – 2.427 –
…… – 2.472
Channel spectrum: (11 MHz chip clock)
2412
2437
2462
DSSS Frequency Channel Plan
The 5.0 (OFDM) GHz spectrum
Ł US regulation:
3 x 100 MHz channels
5.150-5.250 (40 mW)
5.250-5-350 (200 mW)
5.725-5.825 (800 mW)
Operating 20 MHz channels
5.180, 5.200, 5.220, 5.240
5.260, 5.280, 5.300, 5.320
5.745, 5.765, 5.785, 5.805
20 MHz / 64 subcarriers = 0.3125 MHz per subcarriers (52 used)
802.11 MAC Data Frame
MAC header:
-28 bytes (24 header + 4 FCS) or
- 34 bytes (30 header + 4 FCS)
PHY
Frame
Control
IEEE 802.11
Duration
/ ID
2
2
Data 0 - 2312
Address 1
Address 2
6
6
Protocol
version
Type
Sub Type info
2
2
12
Sub Type
To
DS
4
1
FCS
Address 3
Sequence
Control
6
Address 4
2
Fragment
number
4
From More
Pwr More
Retry
WEP Order
DS Frag
MNG Data
1
1
1
1
1
1
1
Details and explanation later on!
6
Data
0-2312
Sequence number
12
Frame
check
sequence
4
PART 6
IEEE 802.11 WLAN
Lecture 6.2
Wireless LAN Networks
and related Addressing
Basic Service Set (BSS)
group of stations that can communicate with each other
Ł Infrastructure BSS
or, simply, BSS
Stations connected
through AP
Ł Independent BSS
or IBSS
Stations connected in
ad-hoc mode
Network infrastructure
AP
Frame Forwarding in a BSS
Network infrastructure
AP
BSS: AP = relay function
No direct communication allowed!
IBSS: direct communication
between all pairs of STAs
Why AP = relay function?
Ł Management:
Mobile stations do NOT neet to maintain neighbohr relationship with
other MS in the area
But only need to make sure they remain properly associated to the
AP
Association = get connected to (equivalent to plug-in in a wire to a
bridge J )
Ł Power Saving:
APs may assist MS in their power saving functions
by buffering frames dedicated to a (sleeping) MS when it is in PS
mode
Ł Obvious disadvantage: use channel bandwidth twice…
Extended Service Set
BSS1
AP1
BSS2
AP2
BSS3
AP3
BSS4
AP4
ESS: created by merging different BSS through a network infrastructure
(possibly overlapping BSS – to offer a continuous coverage area)
Stations within ESS MAY communicate each other via Layer 2 procedures
APs acting as bridges
MUST be on a same LAN or switched LAN or VLAN (no routers in between)
The concept of Distribution
System
Distribution system (physical connectivity + logical service support)
AP1
AP2
AP3
MSs in a same ESS need to
1) communicate each other
2) move through the ESS
No standard implementation, but set of services:
Association, disassociation, reassociation,
Integration, distribution
Basically. DS role:
- track where an MS is registrered within an ESS area
- deliver frame to MS
Association and DS
AP1
AP2
AP3
IAPP
IAPP
Association
Ł
Typical implementation (media)
Switched ethernet backbone
But alternative “distribution medium” are
possible
E.g. wireless distribution system (WDS)
DS implementation:
- an AP must inform other APs
of associated MSs MAC
addresses
- proprietary implementation
no interoperability
- standardized protocol IAPP
(802.11f) in june 2003
Current trend:
- Centralized solutions (Cisco,
Aruba)- CAPWAP?
Wireless Distribution System
AP1
AP2
DS medium:
- not necessarily an ethernet backbone!
- could be the 802.11 technology itself
Resulting AP = wireless bridge
AP3
Addresses
Ł At least three addresses
Receiving station
Transmitting station
BSS address
To make sure a frame is valid within the
considered BSS
For filtering purpose (filter frame within a
BSS)
BSSID
Ł Address of a BSS
Infrastructure mode:
AP MAC address
Ad-hoc mode:
802 IEEE
48 bit addresses
1 bit = individual/group
1 bit = universal/local
46 bit address
Random value
» With universal/local bit set to 1
Generated by STA initiating the IBSS
Addressing in an IBSS
SA
DA
Frame
Control
Duration
/ ID
Address 1
DA
Address 2
SA
Address 3
BSSID
SA = Source Address
DA = Destination Address
Sequence
Control
Address 4
---
Data
FCS
Addressing in a BSS?
AP
SA
X
DA
Addressing in a BSS!
Distribution system
AP
SA
Frame must carry following info:
1) Destined to DA
2) But through the AP
What is the most general addressing structure?
DA
Addressing in a BSS (to AP)
Distribution system
Address 2 = wireless Tx
Address 1 = wireless Rx
Address 3 = dest
AP
BSSID
SA
Frame
Control
Duration
/ ID
DA
Address 1
BSSID
Address 2
SA
1
Protocol
version
Type
Sub Type
To
DS
2
2
4
1
Address 3
DA
Sequence
Control
Address 4
---
0
From More
Pwr More
Retry
WEP Order
DS Frag
MNG Data
1
1
1
1
1
1
1
Data
FCS
Addressing in an ESS
Distribution System
AP
AP
BSSID
DA
DA
Idea: DS will be able to forward frame to dest
(either if fixed or wireless MAC)
SA
Frame
Control
Duration
/ ID
Address 1
BSSID
Address 2
SA
1
Protocol
version
Type
Sub Type
To
DS
2
2
4
1
Address 3
DA
Sequence
Control
Address 4
---
0
From More
Pwr More
Retry
WEP Order
DS Frag
MNG Data
1
1
1
1
1
1
1
Data
FCS
Addressing in a BSS (from AP)
Distribution system
Address 2 = wireless Tx
Address 1 = wireless Rx
Address 3 = src
AP
BSSID
SA
Frame
Control
Duration
/ ID
DA
Address 1
DA
Address 2
BSSID
0
Protocol
version
Type
Sub Type
To
DS
2
2
4
1
Address 3
SA
Sequence
Control
Address 4
---
1
From More
Pwr More
Retry
WEP Order
DS Frag
MNG Data
1
1
1
1
1
1
1
Data
FCS
From AP: do we really need 3
addresses?
Distribution system
AP
BSSID
SA
DA
DA correctly receives frame, and send 802.11 ACK to … BSSID (wireless transmitted)
DA correctly receives frame, and send higher level ACK to … SA (actual transmitter)
Addressing within a WDS
Wireless Distribution System
AP
AP
RA
TA
Address 4:
initially forgotten…
DA
SA
Frame
Control
Duration
/ ID
Address 1
RA
Address 2
TA
1
Protocol
version
Type
Sub Type
To
DS
2
2
4
1
Address 3
DA
Sequence
Control
Address 4
SA
1
From More
Pwr More
Retry
WEP Order
DS Frag
MNG Data
1
1
1
1
1
1
1
Data
FCS
Addressing: summary
Receiver
Transmitter
Function
To DS
From DS
Address 1
Address 2
Address 3
Address 4
IBSS
0
0
RA = DA
SA
BSSID
N/A
From AP
0
1
RA = DA
BSSID
SA
N/A
To AP
1
0
RA = BSSID
SA
DA
N/A
Wireless DS
1
1
RA
TA
DA
SA
Ł
Ł
Ł
Ł
Ł
BSS Identifier (BSSID)
unique identifier for a particular BSS. In an infrastructure BSSID it is the MAC address of the AP. In IBSS, it is
random and locally administered by the starting station. (uniqueness)
Transmitter Address (TA)
MAC address of the station that transmit the frame to the wireless medium. Always an individual address.
Receiver Address (RA)
to which the frame is sent over wireless medium. Individual or Group.
Source Address (SA)
MAC address of the station who originated the frame. Always individual address.
May not match TA because of the indirection performed by DS of an IEEE 802.11 WLAN. SA field is considered
by higher layers.
Destination Address (DA)
Final destination . Individual or Group.
May not match RA because of the indirection.
Service Set IDentifier (SSID)
Ł Name of the WLAN
network
Plain text (ascii), up to 32 char
Ł Assigned by the network
administrator
All BSS in a same ESS have
same SSID
Ł Typically (but not
necessarily) is transmitted
in periodic management
frames (beacon)
Typical: 1 broadcast beacon
every 100 ms (configurable by
sysadm)
Beacon may transmit a LOT of
other info
Beacon
example
PART 6
IEEE 802.11 WLAN
Lecture 6.3
Network Management
Management Operations
Ł Wireless is not always an advantage
Medium unreliable, lack of physical boundaries,
power consumption
Ł How to join, build, connect, move
nodes, save energies, …
All these issues are faced by the management
procedures, which can be customized
Different vendor offers: very simple products vs.
feature-rich products
Scanning
Ł Before joining a network, you have to find it!! The
discovery operation is called scanning
Ł Several parameters can be specified by the user
BSSType (independent, infrastucture, both)
BSSID (individual, Broadcast)
SSID (network name)
ScanType (active, passive)
Channel List
Probe Delay (to start the active scanning in each channel)
MinChannelTime, MaxChannelTime
Passive Scanning
Ł Saves battery, since it requires listening
only
Ł Scanning report based on beacon frames
heard while sweeping channel to channel
AP2
AP1
AP3
Found
AP1, AP3
Active Scanning
Ł Probe Request Frames used to solicit responses from
a network, when the ProbeDelay time expires
If the medium is detected busy during the MinChannelTime, wait until the
MaxChanneltime expires
One station in each BSS is responsible to send Probe Responses
(station which transmitted the last beacon frame)
Joining
Ł A scan report is generated after each
scan, reporting BSSID, SSID,
BSSType,
Beacon Interval
DTIM period
Timing Parameters (Timestamp)
PHY Parameters, BSSBasicRateSet
Ł Joing: before associating, select a
network (often it requires user action)
Ł and after?
Authentication
Ł What does authentication do?
prove the identity of the new node
Ł On wired network, it is implicitely
provided by phisical access, but
everybody can access wireless
medium!
Ł Two major approaches:
Open-system
Shared-key
Authentication (2)
Ł It can work in both ad-hoc and
infrastucture modes, but it is not bilateral (mutual)
802.11 authentication implicitely assumes that
access points are under the control of the network
administrators and do not have to prove their
identity
The network is no under obligation to authenticate
itself, then man-in-the-middle attacks are possible
Open
Open--System Authentication
Ł Accept Everybody
2way handshake; the station simply informs about its identity
The identity is represented by the MAC address (not by the
user)
Ł Address filtering
Often used with open-system
List with authorized MAC addresses provided by the network
administrator
Better than nothing.. But MAC addresses can be software
programmable!
Moreover, the authorization lists have to be transferred in all
the APs
Shared
Shared--Key Authentication
Ł Can be used only
with some products
implementing the
WEP (Wireless
Encryption Protocol)
Ł Before accepting the
user, the
authenticator has to
verify that he knows
a secret
How? Ask for the secret?
NO, attackers can listen!
Ask for decoding a
challenge text and
compare the answer
(which has to be
encrypted) with the local
decryption.
How does WEP work?
Ł very basic ideas..
Symmetric cipher: the same keystream is used for XOR coding
and XOR decoding
010101010111
000100101001
data
keystream
010001111110
000100101001
coded data
keystream
010101010111
recovered data
Keystream
Ł Both the transmitter and the receiver has to
know the keystream; long data transfer
requires long keystrem
Ł How to distribute and manage long totally
random keystreams?
Keystream = pseudorandom sequence obtained from a short
key + Initialization Vector = f( key, IV)
Function f is called RC4 and it is an intellectual property
no more safe, after 2001!
Standard key length of only 40 bits + 24 bit for the IV
Is cryptography enough?
Ł Not only confidentiality problems, but also integrity
problems
We do not have only to protect our data against not authorized receivers,
but also we have to be sure that nobody changes our data
Even if we do not know the keystream, if we change the encrypted
payload, we destroy the exact information delivery
Ł Integrity-Check-Value: a “summary” of the payload,
appended to the payload for checking the integrity of
the data after decrypting
Ł E.g. payload = “Siamo nel mese di ottobre”
ICV = “snmdo”
xxxxxx = crypt(“siamo nel mese di ottobre snmdo”)
decrypt(xxxxyy) = “siamo inc qwer di ottobre snmdo”
“snmdo: is not the payload summary!
WEP Features
Frame header
IV
Frame body
ICV
FCS
32 bits
24 bits
encrypted
Ł extra overhead of 24+32 bits
Ł Since the payload often starts with an LLC frame,
whose first byte is known, the first byte of encryption
can be understood..
Ł the ICV should be obtained by an hash unpredictable
function, but in the WEP definition it is cyclic
redundancy check
CRC is not cryptographically secure, because single bit alterations can
be predicted with high probability
Ł Key cannot be considered secure, since they are
statically entered, accessible by users and rarely
changed
Enhanced encryption
Ł WEP limits:
Manual key setting (shared in the whole network)
Limited IV space (24 bits = 224 packets = 16.8 M)
Ł WPA: Still based on RC4, but with dynamic
and automatic key management!
IV long 48 bits
per-packet key
Message Integrity Code (8 bytes) before ICV
Ł WPA2: change RC4 with AES
WPA/TKIP:
Temporal Key Integrity Protocol
Ł Still based on RC4 for compatibility
reasons
WEP: f(Secret Key+IV)=keystream
TKIP: f(Secret Key+IV) = Secret Key Generator
Dynamic keys of 128 bits
Different keys for each session
Other authentication mechanisms
Ł 802.1x: does not define its own security protocols,
but reuse existing protocols
Too many researches with specific standardized functions;
Standardizes the protocol messages and operations, in general terms:
e.g. consider an ICV, without chosing a fiexd a priori hash function;
the hash an cypher functions are negotiated
Ł Three principal roles:
Supplicant – a device requesting access to the controlled port
Authenticator – the point of attachment to the LAN infrastructure (e.g., .11
AP)
Authentication server – verifying the credentials of the supplicant (e.g.,
RADIUS server)
Ł After successful authentication, a controlled port is
opened
Association
Ł After that we choose a network and
we authenticate, we can finally send
an “association” message
The message is required for tracking the host
position in the ESS
Not used for ad hoc networks
Re-association after hand-over
And to be found?
Ł It is required to periodically transmit a beacon!
In infrastructure: beacon transmissions managed by the AP
In ad-hoc: each station contend for the beacon and leave the contention
after the first observed beacon
Ł The beacon frames are periodically scheduled, but
contend as normal frames
Deterministic
scheduling
Random transmission
delays
What information
is carried by beacons?
Ł Timestamp (8 bytes): in Transmission Units TU (=1024 us) to
synchronize stations
Ł Beacon Interval (2 bytes): beacon scheduling interval
Ł Capability Info (2 bytes): bit map to activate/deactivate some
network features
Ł SSID (variable size): network name set by the operators
Element ID, Length, Information for every variable size field
OPTIONAL:
Ł FH ParameterSet (7 bytes)
Ł DS ParameterSet (2 bytes)
Ł CF ParameterSet (8 bytes)
Ł IBSS ParameterSet (4 bytes)
Ł Traffic Indicator Map TIM (variable size): to indicate which
stations have buffered frames; some TIM are Delivery TIM for
broadcast traffic
Why Timestamps for
Synchronization?
" #
&
$
'
%
PART 6
IEEE 802.11 WLAN
Lecture 6.4
Medium Access Control
Protocols: DCF
Wireless Ethernet
Ł 802.3 (Ethernet)
CSMA/CD
Carrier Sense Multiple Access
Collision Detect
A
Ł
Ł 802.11(wireless LAN)
CSMA/CA
(Distributed Coordination Function)
Carrier Sense Multiple Access
Collision Avoidance
Ł
Ł
B
C
Both A and C sense the channel idle at
the same time they send at the same
time.
Collision can be detected at sender in
Ethernet.
Why this is not possible in 802.11?
1. Either TX or RX (no simultaneous
RX/TX)
2. Large amount of power difference in
Tx and Rx (even if simultaneous txrx, no possibility in rx while tx-ing)
3. Wireless medium = additional
problems vs broadcast cable!!
Hidden Terminal Problem
Ł Large difference in signal strength; physical channel
impairments (shadowing)
It may result that two stations in the same area cannot communicate
Ł Hidden terminals
A and C cannot hear each other
A transmits to B
C wants to send to B; C cannot receive A;C senses “idle” medium
(Carrier Sense fails)
Collision occurs at B.
A cannot detect the collision (Collision Detection fails).
A is “hidden” to C.
A
B
C
802.11 MAC approach
Ł Still based on Carrier Sense:
Listen before talking
Ł But collisions can only be inferred
afterwards, at the receiver
Receivers see corrupted data through a CRC error
Transmitters fail to get a response
Ł Two-way handshaking mechanism to
infer collisions
DATA-ACK packets
packet
TX
ACK
RX
Channel Access details
Ł A station can transmit only if it senses the
channel IDLE for a DIFS time
DIFS = Distributed Inter Frame Space
Packet
arrival
TX
DIFS
DATA
SIFS
RX
ACK
What about a station arriving in this frame time?
Ł
SIFS = Short Inter Frame Space
DIFS & SIFS in wiwi-fi
Ł DIFS = 50 s
Ł SIFS = 10 s
Why backoff?
DIFS
STA1
DATA
SIFS ACK
DIFS
STA2
STA3
Collision!
RULE: when the channel is initially sensed BUSY, station defers transmission;
But when it is sensed IDLE for a DIFS, defer transmission of a further random time
(BACKOFF TIME)
Slotted Backoff
STA2
DIFS
w=7
Extract random number
in range (0, W-1)
Decrement every slot-time
w=5
STA3
Note: slot times are not physically delimited on the channel!
Rather, they are logically identified by every STA
Slot-time values: 20 s for DSSS (wi-fi)
Accounts for:
1) RX_TX turnaround time
2) busy detect time
3) propagation delay
Backoff freezing
Ł When STA is in backoff stage:
It freezes the backoff counter as long as the channel is
sensed BUSY
It restarts decrementing the backoff as the channel is sensed
IDLE for a DIFS period
STATION 1
DIFS
DATA
DIFS
SIFS ACK
STATION 2
DIFS
SIFS ACK
6
5
BUSY medium
Frozen slot-time 4
DIFS
3
2
1
Backoff rules
Ł First backoff value:
Extract a uniform random number in range (0,CWmin)
Ł If unsuccessful TX:
Extract a uniform random number in range (0,2×(CWmin+1)-1)
Ł If unsuccessful TX:
Extract a uniform random number in range (0,22×(CWmin+1)-1)
Ł Etc up to 2m×(CWmin+1)-1
Exponential Backoff!
CWmin = 31
CWmax = 1023 (m=5)
Throughput vs CWmin
P=1000 bytes
RTS/CTS
Ł Request-To-Send / Clear-To-Send
Ł 4-way handshaking
Versus 2-way handshaking of basic access
mechanism
Ł Introduced for two reasons
Combat hidden terminal
Improve throughput performance with long
packets
RTS/CTS
and
hidden
terminals
Packet
arrival
TX
DIFS
RTS
DATA
SIFS CTS SIFS
RX
others
SIFS
ACK
NAV (RTS)
NAV (CTS)
RX
RTS
CTS
TX
CTS
hidden
(Update NAV)
RTS/CTS: carry the amount of time the channel
will be BUSY. Other stations may update a
Network Allocation Vector, and defer TX
even if they sense the channel idle
(Virtual Carrier Sensing)
RTS/CTS and performance
RTS/CTS cons: larger overhead
RTS/CTS pros: reduced collision duration
RTS/CTS throughput
RTS/CTS convenient with long packets and large number of terminals (collision!);
RTS/CTS more robust to number of users and CWmin settings
Relation between IFS
PIFS used by Point Coordination Function
- Time-bounded services
- Polling scheme
PCF Never deployed
Parameters
SIFS
( sec)
DIFS ( sec)
Slot Time
( sec)
CWmin
CWmax
802.11b
PHY
10
50
20
31
1023
EIFS
Time in
microseconds.
Update the NAV
time in the
neighborhood
PHY
Frame
Control
IEEE 802.11
Duration
/ ID
2
2
Data Frame formats
Data 0 - 2312
Address 1
Address 2
6
6
Protocol
version
Type
Sub Type info
2
2
12
Sub Type
To
DS
4
1
FCS
Address 3
Sequence
Control
6
2
Fragment
number
4
From More
Pwr More
Retry
WEP Order
DS Frag
MNG Data
1
1
1
1
1
1
1
Address 4
6
Data
0-2312
Sequence number
12
Frame
check
sequence
4
Why NAV (i.e. protect ACK)?
TX
RX
Station C receives frame from station TX
Station C IS NOT in reach from station RX
But sets NAV and protects RX ACK
C
Why EIFS (i.e. protect ACK)?
TX
C
RX
Station C DOES NOT receive frame from station TX
but still receives enough signal to get a PHY.RXEND.indication error
Station C IS NOT in reach from station RX
But sets EIFS (!!) and protects RX ACK
Frame formats
DATA FRAME (28 bytes excluded address 4)
Frame
Control
Duration
/ ID
Address 1
Address 2
RA
TA
Address 3
RTS (20 bytes)
Frame
Control
Duration
FCS
CTS / ACK (14 bytes)
Frame
Control
Duration
RA
FCS
Sequence
Control
Address 4
Data
FCS
DCF overhead
E[ payload ]
S station
E[TFrame _ Tx ] DIFS
CWmin / 2
TFrame _ Tx
TMPDU
SIFS TACK
TFrame _ Tx
TRTS
TMPDU
TPLCP 8 (28 L) / RMPDU _ Tx
SIFS TCTS
SIFS TMPDU
TACK
TPLCP 8 14 / RACK _ Tx
TRTS
TPLCP 8 20 / RRTS _ Tx
TCTS
TPLCP 8 14 / RCTS _ Tx
SIFS TACK
DCF overhead (802.11b)
RTS/CTS
Basic
RTS/CTS
Basic
0
2000
4000
6000
8000
Tra nsmssion Time (use c )
DIFS
Ave Bac koff
RTS+SIFS
CTS+SIFS
Payload+SIFS
ACK
"
!
#
76 ,- 67 ,0/. 0/. 0/. 0/.
882 88 12 12
2 45 3
45 3
45 3 45 3
$ % &' ( ) * + DCF overhead (802.11b)
Fragmentation
Ł Splits message (MSDU) into
several frames (MPDU)
Same fragment size
except the last one
Ł Fragmentation burst
Fragments separated by SIFS
Channel cannot be captured by
someone else
Ł Each fragment reserves channel
for next one
NAV updated fragment by fragment
Ł Missing ACK for fragment x
Release channel (automatic)
Backoff
Restart from transmission of fragment x
Each fragment individually ACKed
Why Fragmentation?
Ł
High Bit Error Rate (BER)
Increases with distance
The longer the frame, the lower the successful TX probability
High BER = high rts overhead & increased rtx delay
Backoff window increases: cannot distinguish collisions from tx error!
Fragment and sequence numbers
DATA FRAME (28 bytes excluded address 4)
Frame
Control
Duration
/ ID
Address 1
Address 2
Address 3
More
Retry
Frag
1
1
Sequence
Control
Fragment
number
Address 4
Data
FCS
Sequence number
4
12
Ł Fragment number
Increasing integer value 0-15 (max 16 fragments since 4 bits available)
Essential for reassembly
Ł More fragment bit (frame control field) set to:
1 for intermediate fragments
0 for last fragment
Ł Sequence Number
Used to filter out duplicates
Unlike Ethernet, duplicates are quite frequent!
Retransmissions are a main feature of the MAC
Ł Retry bit: helps to distinguish retransmissions
Set to 0 at transmission of a new frame
PART 6
IEEE 802.11 WLAN
Lecture 6.4bis
Medium Access Control
Protocols: PCF
PCF vs DCF
PCF
DCF
PHY
PCF deployed on TOP of DCF
Backward compatibility
PCF
Ł Token-based access mechanism
Polling
Ł Channel arbitration enforced by a “point
Coordinator” (PC)
Typically the AP, but not necessarily
Ł Contention-free access
No collision on channel
Ł PCF deployment: minimal!!
Optional part of the 802.11 specification
As such, almost never deployed
But HCCA (PCF extension in 802.11e) is getting considerable attention…
PCF frame transfer
SIFS
Polling strategy: very elementary!!
- send polling command to stations with increasing Association ID value…
- (regardless whether they might have or not data to transmit)

PART 6 IEEE 802.11 Wireless LANs

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