2
Outlines
๏Last Lecture Review
๏Problems with DCF
๏Virtual Carrier Sensing
๏RTC/CTS Protocol
๏Interframe Spacing
๏PCF
๏Fragmentation / Reassembly
๏MAC Frame Format
๏Frame Types
๏Physical Media in Original IEEE 802.11
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Problems With DCF-Wireless Networks-Lecture Slides, Slides of Wireless Networking
This course consist on introduction to wireless communication, evolution of wireless communication systems, medium access techniques, propagation models, error control techniques, cellular systems, emerging networks. This lecture inlcudes: Problem, Rts, Cts, Virtual, Carrier, Source, Destination, Clear, Send, Ready, Access
Typology: Slides
2011/2012
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2
Outlines
Last Lecture Review
Problems with DCF
Virtual Carrier Sensing
RTC/CTS Protocol
Interframe Spacing
PCF
Fragmentation / Reassembly
MAC Frame Format
Frame Types
Physical Media in Original IEEE 802.
3
Last Lecture
Overview of IEEE 802.
IEEE 802.11 Protocols
Architecture
Services
MAC Protocols
DCF
PCF
5
Exposed Node problem
A B D C
6
RTS/CTS Protocol
RTS CTS MDU ACK Source Destination ๏ Virtual Carrier Sensetechnique. ๏ Source sends Request-to-Send beacon ๏ Destination, if free, sendsClear-to-Send beacon. ๏ Source transmits data packet. ๏ Destination ACKs if receivessuccessfully
8
Interframe Spacing
Short interframe space (SIFS)
The SIFS is used for the highest-priority transmissions, such asRTS/CTS frames and positive acknowledgments.
PCF interframe space (PIFS)
The PIFS is used by the PCF during contention-free operation.Stations with data to transmit in the contention-free period cantransmit after the PIFS has elapsed and pre-empt anycontention-based traffic
DCF interframe space (DIFS)
The DIFS is the minimum medium idle time for contention-based services. Stations may have immediate access to themedium if it has been free for a period longer than the DIFS.
Extended interframe space (EIFS)
The EIFS is not a fixed interval. It is used only when there is anerror in frame transmission.
9 CTS RTS DIFS SIFS MPDU SIFS ACK SIFS DIFS Next MPDU CW Defer Access Backoff after defer NAV(RTS) NAV(CTS) Sender Receiver Other
11
12
Fragmentation and Reassembly
In Ethernet, MAC frame can be upto 1518 bytes long.
Not possible to support such larger size of framebecause of:
Higher bit error rate โบ If it is corrupted, large size would incur high overheads. โบ On FH, medium is interrupted periodically (20ms), smallerpacket would result in smaller chance of postponingtransmission.
14
MAC Frame Format
15
MAC Frame Fields
Frame Control โ frame type, control information
Duration/connection ID โ channel allocation time
Addresses โ context dependant, types include source anddestination
Sequence control โ numbering and reassembly
Frame body โ MSDU or fragment of MSDU
Frame check sequence โ 32-bit CRC
17
Frame Control Fields
Protocol version โ 802.11 version
Type โ control, management, or data
Subtype โ identifies function of frame
To DS โ 1 if destined for DS
From DS โ 1 if leaving DS
More fragments โ 1 if fragments follow
Retry โ 1 if retransmission of previous frame
Power management โ 1 if transmitting station is in sleepmode
More data โ Indicates that station has more data to send
WEP โ 1 if wired equivalent protocol is implemented
Order โ 1 if any data frame is sent using the Strictly Orderedservice
18
Control Frame Subtypes (Type 01)
Power save โ poll (PS-Poll)
Request to send (RTS)
Clear to send (CTS)
Acknowledgment
Contention-free (CF)-end
CF-end + CF-ack
20
Management Frame Subtypes (Type 00)
Association request
Association response
Reassociation request
Reassociation response
Probe request
Probe response
Beacon
Announcement traffic indication message
Dissociation
Authentication
Deauthentication
21
Physical Media Defined by Original 802.
Standard
Direct-sequence spread spectrum
Operating in 2.4 GHz ISM band โบ Data rates of 1 and 2 Mbps
Frequency-hopping spread spectrum
Operating in 2.4 GHz ISM band โบ Data rates of 1 and 2 Mbps
Infrared
1 and 2 Mbps โบ Wavelength between 850 and 950 nm