Figure 310 Frame control field

bytes

0-2,312

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1 1 L 1 1

i i i i i 1 J L L 1

1 1 1 1 1 1 J L L 1

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i i i i i L 1 1 1 J

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v í

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4

f

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1

1 1 Suh Itfp*

Tutti

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Mmt

W[P

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l(i

log

Vqnll

Dïft

o , 1

lu

V ; s

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i

S

10

il

Ü

13

14

In Table 3-1, bit strings are written most-significant bit first, which is the reverse of the order used in Figure 3-10. Therefore, the frame type is the third bit in the frame control field followed by the second bit (b3 b2), and the subtype is the seventh bit, followed by the sixth, fifth, and fourth bits (b7 b6 b5 b4).

Table 3-1. Type and subtype identifiers

Subtype value

Subtype name

Management frames (type=00)Ia

0000

Association request

0001

Association response

0010

Reassociation request

0011

Reassociation response

0100

Probe request

0101

Probe response

1000

Beacon

1001

Announcement traffic indication message (ATIM)

1010

Disassociation

1011

Authentication

1100

Deauthentication

Control frames (type=01)Ib]

1010

Power Save (PS)-Poll

1011

RTS

1100

CTS

1101

Acknowledgment (ACK)

1110

Contention-Free (CF)-End

1111

CF-End+CF-Ack

Data frames (type=10)IC

0000

Data

0001

Data+CF-Ack

0010

Data+CF-Poll

0011

Data+CF-Ack+CF-Poll

Table 3-1. Type and subtype identifiers

Subtype value

Subtype name

OlOO

Null data (no data transmitted)

OlOl

CF-Ack (no data transmitted)

OllO

CF-Poll (no data transmitted)

Olll

Data+CF - Ack+CF -Poll

(Frame type 11 is reserved)

[a] Management subtypes 0110-0111 and 1101-1111 are reserved and not currently used.

[b] Control subtypes 0000-1001 are reserved and not currently used.

[c] Data subtypes 1000-1111 are reserved and not currently used.

ToDS and FromDS bits

These bits indicate whether a frame is destined for the distribution system. All frames on infrastructure networks will have one of the distribution system's bits set. Table 3-2 shows how these bits are interpreted. As Chapter 4 will explain, the interpretation of the address fields depends on the setting of these bits.

Table 3-2. Interpreting the ToDS and FromDS bits

To DS=0

To DS=1

From DS=O

All management and control frames

Data frames within an IBSS (never infrastructure data frames)

Data frames transmitted from a wireless station in an infrastructure network

From DS=l

Data frames received for a wireless station in an infrastructure network

Data frames on a "wireless bridge"

More fragments bit

More fragments bit

This bit functions much like the "more fragments" bit in IP. When a higher-level packet has been fragmented by the MAC, the initial fragment and any following nonfinal fragments set this bit to 1. Some management frames may be large enough to require fragmentation; all other frames set this bit to 0.

Retry bit

From time to time, frames may be retransmitted. Any retransmitted frames set this bit to 1 to aid the receiving station in eliminating duplicate frames.

Power management bit

Network adapters built on 802.11 are often built to the PC Card form factor and used in battery-powered laptop or handheld computers. To conserve battery life, many small devices have the ability to power down parts of the network interface. This bit indicates whether the sender will be in a power-saving mode after the completion of the current atomic frame exchange. One indicates that the station will be in power-save mode, and 0 indicates that the station will be active. Access points perform a number of important management functions and are not allowed to save power, so this bit is always 0 in frames transmitted by an access point.

More data bit

To accommodate stations in a power-saving mode, access points may buffer frames received from the distribution system. An access point sets this bit to indicate that at least one frame is available and is addressed to a dozing station.

WEP bit

Wireless transmissions are inherently easier to intercept than transmissions on a fixed network. 802.11 defines a set of encryption routines called Wired Equivalent Privacy (WEP) to protect and authenticate data. When a frame has been processed by WEP, this bit is set to 1, and the frame changes slightly. WEP is described in more detail in Chapter 5.

Order bit

Frames and fragments can be transmitted in order at the cost of additional processing by both the sending and receiving MACs. When the "strict ordering" delivery is employed, this bit is set to 1.

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