Transcript Short Slot Times - Community College of Rhode Island

Wireless Networking
IEEE 802.11 In Depth
Module-05
Jerry Bernardini
Community College of Rhode Island
4/11/2016
Wireless Networking J. Bernardini
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Presentation Reference Material
• CWNA Certified Wireless Network
Administration Official Study Guide, Fourth
Edition, Tom Carpenter, Joel Barrett
– Chapter-4 Pages 153-200
• The California Regional Consortium for
Engineering Advances in Technological
Education (CREATE) project
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Bits, Bytes, Octets, Frames, Packets
• Bits =1 or 0
• Bytes = 8 bits
• Octets = 8 bits = Byte
– Octet is used by telecommunication people
– Byte is used by IT people
• Frames = grouping of bits at layer-2
• Packets = grouping of bits at layer-3
• Datagrams = another term for packets
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Coding – ASCII Table
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OSI data flow
CWNA Guide to Wireless LANs, Second
EditionCCRI J. Bernardini
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IEEE 802.11 Physical Layer Standards
• IEEE wireless standards follow OSI model, with some modifications
• Data Link layer divided into two sublayers:
– Logical Link Control (LLC) sublayer: Provides common interface, reliability, and flow
control
– Media Access Control (MAC) sublayer: Appends physical addresses to frames
• Physical layer divided into two sublayers:
– Physical Medium Dependent (PMD) sublayer: Makes up standards for
characteristics of wireless medium (such as DSSS or FHSS) and defines method for
transmitting and receiving data
– Physical Layer Convergence Procedure (PLCP) sublayer: Performs two basic
functions
• Reformats data received from MAC layer into frame that PMD sublayer can transmit
• “Listens” to determine when data can be sent
CWNA Guide to Wireless LANs, Second
EditionCCRI J. Bernardini
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Data Link Layer - Physical Layer- Data Units
MSDU (MAC Service Data Unit)
(From upper layers 2304 bytes max)
LLC
MAC
802.2 Logical Link Control
Data Link Layer (Layer-2)
802.11 Media Access Control
MPDU (MAC Protocol Data Unit)
PSDU (PLCP Service Data Unit)
(MPDU = PSDU name change to
PLCP
PHY Layer Convergence Protocol
indicated service needed)
Physical Layer (Layer-1)
PPDU (PLCP Protocol Data Unit)
PMD
Physical Medium Dependent
PHY = Physical Layer
Modulated Radio Signal
Where the IEEE 802.11 Standard Fits
8
IEEE 802.3 CSMA/CD vs. IEEE 802.11 CSMA/CA
•
•
•
•
•
•
CSMA/CD is for wired collision handling
CSMA/CA is for wireless collision handling
CSMA = Carrier Sense Multiple Access
CD = Collision Detection
CA = Collision Avoidance
Why do collisions occur?
–
Answer = Two or more stations transmit at the same time
• Why is it important to detect or avoid collisions?
–
Answer = Because there is data loss and retransmission is necessary
• Wired networks are designed for the transmitting
station to detect most collisions
• Many collisions will not be detected by Wireless
networks – therefore avoid collisions
IEEE 802.11 Collision Handling CSMA/CA
•
In CSMA/CA a Wireless node that wants to transmit performs
the following sequence:
1. Listen on the desired channel.
2. If channel is idle (no active transmitters) it sends a packet.
3. If channel is busy the node waits random time until
transmission stops and then waits an additional time period.
4. If the channel is still idle at the end of the time period the
node transmits its packet otherwise it repeats the process
defined in 3 above until it gets a free channel.
• Additional support mechanisms such as ACK, RTS/CTS can be
used but increase overhead noticeably.
CSMA/CA Collision Handling
• 802.11 standard employs half-duplex radios-radios
capable of transmission or reception-but not both
simultaneously
Wired LAN
1
Listening
Transmitting Data Frames
2
Wireless Client
Transmitting
AP-1
Access
Points
Listening
Two Kinds of Carrier Sensing Mechanisms
• Physical Carrier Sense
–
–
–
–
–
Uses Clear Channel Assessment (CCA)
Is the RF energy on the channel above a threshold?
If CCA>threshold --->wait for CCA< threshold before trasmitting
Checks received signal strength using RSSI
RF energy from a hidden node could be missed
• Virtual Carrier Sense
–
–
–
–
–
Uses the Network Allocation Vector (NAV) in each station
NAV is a timer that determines if station can contend for RF medium
NAV >0 --->wait for count down to NAV=0
NAV=0 --->use CCA to check for RF energy on medium
IF NAV=0 and CCA > threshold --->station resets NAV>0 and waits
CSMA/CA and ACK
•CSMA/CA also reduces collisions via explicit frame acknowledgment
•Acknowledgment frame (ACK): Sent by receiving device to sending
device to confirm data frame arrived intact
•If ACK not returned, transmission error assumed
•CSMA/CA does not eliminate collisions and does not solve hidden node
problem
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CSMA/CA Request to Send/Clear to Send
•
Request to Send/Clear to Send (RTS/CTS) protocol: Option used to solve
hidden node problem
– Significant overhead upon the WLAN with transmission of RTS and CTS
frames
• Especially with short data packets
– RTS threshold: Only packets that longer than RTS threshold transmitted
using RTS/CTS
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Interframe Spacing ensures no frame overlap
and proper frame processing sequence
• Interframe spaces (IFS): Intervals between transmissions of data frames
• Short IFS (SIFS): For immediate response actions such as ACK, CTS, RTS,
fragmented frames
• SIFS times vary based upon PHY modulation
• FHSS-28us, DHSS-10us, OFDM-16us, HR/DSS-10us, ERP-10us
• Point Coordination Function IFS (PIFS): Time used by a device to access medium
after it has been asked and then given approval to transmit
• PIFS times = SIFS time + PHY slot time
• Distributed Coordination Function IFS (DIFS): Standard interval between
transmission of data frames
• DIFS times = SIFS time + 2x PHY slot time
• Extended IFS (EIFS): used when frame reception is incomplete or corrupted
• EIFS longest time
• EIFS time = SIFS + 8x ACK + Preamble + PLCP header length + DIFS
15
Contention Window and Backoff Time
• Contention Window is a range of integers which is
chosen at random to become the backoff time
• Backoff time is a random time used to establish a
frame-to-transmit
– Random Backoff Time = Random Integer x Slot Time
– Slot time varies for PHY modulation
– FHSS-50us, DHSS-20us, OFDM-9us, HR/DSS-20us, ERP Long Slot-20us,
ERP Short Slot-9us, 802.1n-9us
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Ethernet and 802.11 Frames
• Ethernet Frame
7
1518
1
Preamble
6
6
2
Source
Destination
46 - 1500
4
Data
FCS
Type or Length
Field
Start Of Frame
• Wireless Frame
10 or 18
2
4 or 6
Sync
PLCP Header
Start Of Frame
2
2
6
6
6
Source
Destination
Rec. Adr
Duration ID
Frame Cntrl
2
6
0 - 2304
4
Trans. Adr
Data
FCS
Sequence Cntrl
MAC Packet DATA Unit, (MPDU)
Frame Categories / Types
• Management Frames
o Beacon Frame
o Probe Frames
o Association Frames… more
• Control Frames
o RTS and CTS Frames
o ACK – Acknowledgement Frames… more
• Data Frames
o Data Payload Frames
Twelve Management Frame Types
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Eight Control Frames
• Used to assist with the delivery of data frames
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Fifteen Data Frames
• The frames that actually carry application data
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IEEE 802.11 Frame Formats
Management
Control
Data
Frame
Control
(2)
Frame
Control
(2)
Duration
(2)
Frame
Control
(2)
Duration
(2)
Des.
Address
(6)
Duration
(2)
Address
1
(6)
Source
Address
(6)
Receiver
Address
(6)
Address
2
(6)
Address
3
(6)
BSSID
(6)
Seq.
Control
(2)
Transmit
Address
( 6)
Frame
Check
Seq.
(6)
Seq.
Control
(2)
Address
4
(6)
Frame
Body
( 1 to
2311)
Data
( 1 to
2311)
Frame
Check
Seq.
(6)
Frame
Check
Seq.
(6)
(Bytes per field)
CWNA Guide to Wireless LANs, Second Edition
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Frame Types and Sizes
Protocol
Data Unit
(PDU)
Frame Type and Vendor Support
MTU
(Bytes)
TCP
Transport maximum segment size
1460
Layer-3 default size
1500
IEEE 802.3 Ethernet default
1500
MPDU
IEEE 802.11 default
1534
MPDU
IEEE 802.11 maximum
2304
MAC
Jumbo Frame
>1500
MAC
Cisco Baby giant
1552
1600
MAC
Cisco Catalyst 4000
9198
9216
MAC
Cisco Catalyst 6000
9216
9234
IP
MAC
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MTU + Overhead
(Bytes)
1518
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Transmitting on the WLAN: Fragmentation
• Fragmentation: Divide data to be transmitted from
one large frame into several smaller ones
– Reduces probability of collisions
– Reduces amount of time medium is in use
• If data frame length exceeds specific value, MAC
layer fragments it
– Receiving station reassembles fragments
• Alternative to RTS/CTS
– High overhead
• ACKs and additional SIFS time gaps
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IEEE 802.11 MAC Functions
•
•
•
•
•
•
•
•
•
•
Scanning- discover AP or BSS
Synchronization- all stations have the same clock
Frame Transmission- rules for frame transfer
Authentication-allow device in network
Association-after authentication associate with AP
Reassociation-roaming and association with new AP
Data Protection-data encryption protects data
Power Management-save power by sleeping transceiver
Fragmentation-breakup frame for efficiency and interfer.
RTS/CTS- solution to hidden node problem
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Beacon Management Frame
• A special management frame that is used by a client stations
seeking a wireless network to join.
• Instead of beacon frames a station could use probe request
and probe response frames
• In an ad hoc (IBSS ) wireless network all stations take turns
broadcasting the beacon frame
Beacon
Beacon
S2
S1
AP Control Point
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Active Scanning (Probes)
• A station could use probe request and probe response frames
Instead of beacon frames
1. Station is configured with SSID and switched to a channel
2. Probe request sent by requesting station
3. All stations that have the same SSID and have normal
configurations respond with a Probe Response frame
• The process also involves waiting for ProbeDelay and
MinChannel Timers
Probe Request
S1
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Probe Response
AP Control Point
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S2
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Passive Scanning (Beacons)
1. Client stations listens for a beacon from an access point (AP)
2. If multiple beacons are received the strongest one is selected
3. The listening station then requests authentication and
association
Beacons
S1
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Beacons
AP Control Point
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S2
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Authentication and Association
•
•
1.
2.
3.
•
Using the IEEE 802.11 State Machine
Stations are in one of three states
Unauthenticated / Unassociated
Authenticated / Unassociated
Authenticated / Associated
You cannot transmit data frames for processing until you are
associated
• You cannot transmit associated frames for processing until
you are Authenticated
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IEEE 802.11 State Machine
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Slot Times
• The amount of time a device waits after a collision before
retransmitting a packet.
• Radio defined time interval or clock tick.
–
–
–
–
FHSS Slot Time = 50 S
DSSS Slot Time = 20 S
Infrared Slot Time = 8 S
For DSSS:
SIFS = 10 S
PIFS = SIFS + 1 Slot Time = 10 S + 20 S = 30 S
DIFS = PIFS + 1 Slot Time = 30 S + 20 S = 50 S
– Time Unit = TU = 1,024 S  1 mS
Beacon interval = 100 TU or 100 mS.
Slot Time Notes
•
•
•
•
•
•
Short Slot Times - The amount of time a device waits after a collision before retransmitting a
packet. You can increase throughput on 802.11g, 2.4-GHz radios by enabling short slot time
(most .11g radios enable this by default).
Reducing the slot time from the standard 20 microseconds to the 9-microsecond short slot
time decreases the overall backoff, which increases throughput.
Backoff, which is a multiple of the slot time, is the random length of time a station waits
before sending a packet on the LAN.
Many 802.11g radios support short slot time, but some do not.
When short slot time is enabled, the wireless device uses the short slot time only when all
clients associated to the 802.11g, 2.4-GHz radio support short slot time.
Short slot time is an 802.11g-only feature and does not apply to 802.11a radios.
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Communications Process
• MAC Access Modes
– DCF – CSMA/CA
– DCF/PCF – Point Coordinators and Polling
Contention Free
Delivery
Normal
Delivery
PCF
DCF
Communications Options
• MAC Layer
– Access Methods
• DCF – RTS/CTS (optional)
Distributed function Wireless MAC
Avoids hidden node problem
•
DCF – PCF (optional)
AP polls stations
Superframes to allow station to eventually get access
Superframe = Beacon + CFP + CP
CFP = Contention-Free Period
CP = Contention Period
RTS/CTS
• Sending unicast packets
– Station can send RTS with reservation parameter after waiting for
DIFS (reservation determines 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
RTS/CTS
sender
receiver
others
DIFS
data
RTS
SIFS

CTS
SIFS
ACK
NAV (RTS)
NAV (CTS)

Access to medium deferred
DIFS
contention
NAV – Network Allocation Vector
There are generally three setting in APs for RTS/CTS
Off, On, and On with Threshold
Fragmentation
•
Every network has an MTU (Maximum Transmission Unit) size. Packets
larger than the allowable MTU size must be broken down into multiple
smaller packets, or fragments, to enable them to traverse the network
with lower bit error rates, (BER).
• Fragment size can typically be set by the user using a threshold setting
between 256 and 2,048 bytes.
Header
Data
CRC
Threshold
Header
Data -1
CRC
Header
Data -2
CRC
Drawing not to scale.
Dynamic Rate Switching
• Dynamic Rate Switching =Dynamic Rate Selection
=Automatic Rate Shifting----All mean the same thing
• Process of reducing or increasing the data rate based
upon RF signal levels
• RF signals attenuate over distance or by absorption
• AP will reduce data rate for weaker signals
• AP will increase data rate for higher signals
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Dynamic Rate Selection (DRS)
• Dynamic Rate Selection or Adaptive Rate
Selection/shifting.
– 802.11a, 802.11g modes: 54, 48, 36, 24, 18, 12, 9, 6 Mbps
– 802.11b mode: 11, 5.5, 2, 1 Mbps
– Orinoco 2X mode: 108, 96, 72, 48, 36, 24, 18, 12 Mbps
Example of Sensitivity vs. DR
Data Rate
(Mb/sec)
Received
Signal (dBm)
6
–82
9
–81
12
–79
18
–77
24
–74
36
–70
48
–66
54
–65
Minimum received signals versus data rate for
802.11a devices.
Data Rate Coverage Areas
-85 dBm
-76 dBm
-72 dBm
24 Mbps
18 Mbps
9 Mbps
Chapter-4 Topics
•
Frames,
Packets, and Datagrams .
.
Bits, Bytes, and Octets .
MAC & PHY .
IEEE 802.11 CSMA/CA .
Carrier Sense .
Interframe Spacing .
Contention Window .
Collision Avoidance ..
Frame Types and Formats Compared .
IEEE 802.11 Frame Format Versus
IEEE 802.3 Frame Format .
Frame Types .
Layer 3 Protocol Support by IEEE 802.11
Frames .
Jumbo Frame Support (Layer 2) .
MTU Discovery and Functionality (Layer 3)
IEEE 802.11 Frames and Frame Exchange
Sequences
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MAC Functions .
Beacon Management Frame .
Active Scanning (Probes) ..
Passive Scanning (Beacons.
Authentication and Association
Processes ..
The IEEE 802.11 State Machine .
Authentication .
Association, Reassociation, and
Disassociation .
Regulatory Domain Requirements .
Data Flow Optimization Across the RF
Medium .
DCF PCF
IEEE 802.11e and WMM
RTS/CTS and CTS-to-Self Protocols
Fragmentation
Dynamic Rate Switching
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802.11 Open Association Process
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Association, Reassociation, Disassociation
• Covered next week
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Regulatory Domain Requirements
• Covered next week
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Point Coordination Function (PCF)
• Polling: Channel access method in which each device
asked in sequence if it wants to transmit
– Effectively prevents collisions
• Point Coordination Function (PCF): AP serves as
polling device or “point coordinator”
• Point coordinator has to wait only through point
coordination function IFS (PIFS) time gap
– Shorter than DFIS time gap
49
DIFS and DCF frames
•
•
If point coordinator hears no traffic after PIFS time gap, sends out beacon frame
– Field to indicate length of time that PCF (polling) will be used instead of DCF
(contention)
• Receiving stations must stop transmission for that amount of time
– Point coordinator then sends frame to specific station, granting permission
to transmit one frame
802.11 standard allows WLAN to alternate between PCF (polling) and DCF
(contention)
50
Timing Diagrams
• Timing or Sequence Diagrams - A graph showing
events/levels as a function of time.
Event
2ms
Event
tp1
4ms
sync
6ms
8ms
mxc
rst ack
data
10ms
A
flag
en
pd
tp2
Time
Time
data
Time
SIFS
•
SIFS - Shortest and highest priority time space sent
before and/or after RTS, CTS, and ACK frames. For
DSSS, SIFS is 10 microseconds or 10 S.
sender
receiver
DIFS
data
SIFS
ACK
DIFS
DIFS
others
deferring
mode
waiting time
data
contention
Point Coordination Function
• An optional polling function.
• Provides for limited contention-free service using
the access point as a point coordinator.
• Supports near real-time services.
• In some ways PCF resembles token-based protocols.
S2
S1
AP
Control
Point
PIFS
• PIFS –Are used only in Point Coordination Mode by
the APs. This mode is enabled by the administrator.
It has medium priority and therefore always wins
over DIFS, so that the AP can take control in polling.
For DSSS, PIFS is 30 S.
Distributed Coordination Function
• The Distributed Coordination Function (DCF) is the
fundamental access mechanism in IEEE 802.11
Medium Access Control (MAC).
• DCF can be used in all wireless topologies: IBSS, BSS,
and ESS.
DIFS
• DIFS – Is used by default on all 802.11 stations. DIFS
is the lowest priority and is used for data and
management frames. For DSSS, DIFS is 50 s.
DIFS
PIFS
sender
DIFS
medium busy
SIFS
 Station Backoff Timers
contention
frame
A look at all three: 10 s, 30 s, 50 s, for DSSS.