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IEEE 802.11 Frame Types
Figure A6-19: 802.11 in the OSI Reference Model
- The IEEE 802.11 architecture resides in the Data Link Media Access Control (MAC) sublayer
and the Physical layer in the OSI reference model.
- The Basic Service Set (BSS) and Extended Services Set (ESS) are the 2 available
infrastructure modes of WLAN. Another mode of WLAN is ad-hoc mode.
- Basic Service Set (BSS) is the basic building block of a WLAN. It is often being referred to as
the coverage area of an access point. An access point acts as a master to control the wireless
stations within a BSS. A BSS is identified by an SSID. The most basic BSS is Independent
Basic Service Set (IBSS) or ad-hoc mode BSS, which comprised of 2 wireless clients; whereas
the most basic infrastructure mode BSS is comprised of an access point and a wireless client.
- The IEEE 802.11 WLAN specification defines various frame types than Ethernet for wireless
communications, as well as managing and controlling wireless connections. The types of frames
in the IEEE 802.11 specification are management, control, and data frames. Understanding
the different IEEE 802.11 frame types is essential for analyzing and troubleshooting the
operation of WLANs.
- Every IEEE 802.11 WLAN frame contains the MAC addresses of the source and destination
wireless stations, a Frame Control field that indicates the 802.11 protocol version, frame type,
and various indicators (eg: whether WEP is enabled, power management is active, etc), a
Sequence Control field, the frame body, and the Frame Check Sequence for error detection.
Frame
Control
Duration /
ID
Data Link
Physical
Address
1
MAC
PHY
Address
2
Figure A6-20: 802.11 Frame Format
287
802.2 Logical Link Control (LLC)
802.3
Ethernet
Address
3
802.5
Token
Ring
Sequence
Control
802.11
Wireless
LAN
2 2 6 6 6 2 6 0-2312 4 (bytes)
Protocol
Version
Type Subtype
To
DS
From
DS
4 bits
Fragment
Number
More
Fragments
12 bits
Sequence
Number
Retry Power
Mgmt.
Address
4
More
Data
Frame
Body
2 bits 2 bits 4 bits 1 bit 1 bit 1 bit 1 bit 1 bit 1 bit 1 bit 1 bit
Flags
FCS
Protected
Order
Copyright © 2008 Yap Chin Hoong
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