How does a Wireless LAN Work?

 
The standards used for Wireless LAN (WLAN) communications are based on the
Institute of Electrical and Electronic Engineers (IEEE’s) 802.11 series.The
802.11 standards are responsible for defining the Physical and Media Access
Control (MAC) layers of operation in a WLAN.The primary standard used in
this chapter is 802.11b, which is an extension of the original 802.11 standard.
802.11b’s primary objective defines the use of the 2.4 Gigahertz (GHz) band in
radio frequency (RF) for high-speed data communications. 802.11b supports the
original 802.11 data rate of 2 Mbps up to 11 Mbps.There is also an 802.11g
standard that supports speeds up to 54 Mbps and also uses the 2.4 GHz frequency
band.
The frames generated by a WLAN device differ in many ways from the
frames generated by an Ethernet device. WLANs are not physically connected by
cables like an Ethernet LAN is, so new fields in the frames must be created to
describe aspects of the WLAN.The following section examines a typical 802.3
Ethernet frame and compares it to an 802.11b frame.
An 802.3 Ethernet frame is comprised of seven fields each with a specific
function (Figure 1.1 illustrates an 802.3 Ethernet frame):  Preamble The Preamble field is a 7-byte long alternating pattern of 0s
and 1s that tells receiving devices that a new frame is arriving.
 Start of Frame Delimiter The byte before the destination address in
both an Ethernet and an IEEE 802.3 frame is a Start of Frame (SOF)
Delimiter.This byte ends with two consecutive 1 bits, which serve to
synchronize the frame reception portions of all stations on the LAN.
 Destination Address and Source Address The Destination Address
(DA) and Source Address (SA) fields are 2 or 6 bytes long and contain
the MAC address of the source device on the network and the DA.The
DA may be a single MAC address in the case of a unicast, a broadcast to
all nodes on the network, or a multicast to a group of nodes on the network.
 Length This field is 2 bytes long and describes the number of bytes of
data following this field.
 Data Unit The Data Unit field contains the user data of the frame and
is 46 to 1500 bytes long.This is where the data being encapsulated into
the frame is located (for example, a graphic in a Web page requested by
your system).This field will vary in length based on the data encapsulated.
 Frame Check Sequence The Frame Check Sequence (FCS) field is 4
bytes long.The FCS is a cyclic redundancy check (CRC) that allows the
receiver of a frame to perform basic error control on the frame. If a
frame fails the CRC check, it is discarded and the upper layer protocol
is typically responsible for retransmission.An 802.11b frame (illustrated in Figure 1.2) is comprised of nine fields:
 The first field in an 802.11b frame is the Frame Control (FC) field,
which is 2 bytes long.The FC field contains the following 11 subfields,
which are some of the prime differentiators in an 802.11b frame:
 Protocol Version The Protocol Version field is the first field
within the FC field and is 2 bits long.The default value for this field
is 0 with all other values being reserved.
 Type The Type field is 2 bits long and works in conjunction with the
4-bit Subtype field to identify the function of the frame.The possible
combinations and their descriptions are illustrated in Table 1.1.
 Subtype The Subtype field is 4 bits long and works in conjunction
with the 2-bit Type field to identify the function of the frame.The
possible combinations and their descriptions are illustrated in Table 1.1.
 To DS The To DS field is 1 bit long and is set to 1 in all frames
sent by an associated station with an access point (AP) to signify that
the frame is destined for the network behind the AP, such as a server
connected to the same Ethernet network as the AP. All other frames
have the To DS bit set to 0.
 From DS The From DS field is 1 bit long and is set to 1 on all
frames exiting the DS. All other frames have the From DS bit set to 0.  More Fragments The More Fragments (MF) field is 1 bit long and
is set to 1 in all frames that contain another fragment of the current
MAC Service Data Unit (MSDU) or MAC Management Protocol
Data Unit (MMPDU).All other frames have the MF bit set to 0.
 Retry The Retry field is 1 bit long and is set to 1 in all frames, data
or management, which are retransmissions of earlier frames. Frames
that are not retransmissions of a previous frame are set to 0.
 Power Management The Power Management (PM) field is 1 bit
long and is used to indicate the power management mode of a station.
The value is used to indicate the state that the station will be in
after the successful completion of the frame exchange sequence.A
value of 1 is used to indicate that the station will be in power-save
mode, whereas 0 indicates that the station is in active mode.
NOTE
The PM field in frames transmitted by a wireless AP will always be set to
0, indicating active mode. It would not be desirable for an AP on your
network to go into power-save mode.
 More Data The More Data (MD) field is 1 bit long and used to
tell an associated station in power-save mode that one or more
frames are buffered for the station on the AP.The MD field is set to
0 for all other directed frames.
 WEP The WEP field is 1 bit long and is set to 1 if the frame body
contains data that has been processed by the WEP algorithm. Frames
that have not been processed by WEP have a WEP field value of 0.
 Order The Order field is 1 bit long and is set to 1 in any data
frame that contains data using the StrictlyOrdered service class. All
other frames have a value of 0 in the Order field. Table 1.1 802.11 Type and Subtype Combinations in the FC Field
Type Subtype
Type Value Description Value Subtype Description
00 Management 0000 Association Request
00 Management 0001 Association Response
00 Management 0010 Reassociation Request
00 Management 0011 Reassociation Response
00 Management 0100 Probe Request
00 Management 0101 Probe Response
00 Management 0110-0111 Reserved
00 Management 1000 Beacon
00 Management 1001 Announcement traffic indication
message (ATIM)
00 Management 1010 Disassociation
00 Management 1011 Authentication
00 Management 1100 Deauthentication
00 Management 1101-1111 Reserved
01 Control 0000-1001 Reserved
01 Control 1010 Power Save (PS) Poll
01 Control 1011 Request To Send (RTS)
01 Control 1100 Clear To Send (CTS)
01 Control 1101 Acknowledgement (ACK)
01 Control 1110 Contention-Free (CF) End
01 Control 1111 CF-End + CF-ACK Table 1.1 802.11 Type and Subtype Combinations in the FC Field
Type Subtype
Type Value Description Value Subtype Description
10 Data 0000 Data
10 Data 0001 Data + CF-ACK
10 Data 0010 Data + CF-Poll
10 Data 0011 Data + CF-ACK + CF-Poll
10 Data 0100 Null function (no data)
10 Data 0101 CF-ACK (no data)
10 Data 0110 CF-Poll (no data)
10 Data 0111 CF-ACK + CF-Poll (no data)
10 Data 1000-1111 Reserved
11 Reserved 0000-1111 Reserved
 The next field in an 802.11b frame is the Duration/ID field, which is 16
bits long and is used to carry the association ID of a station with an AP.
 The next fields in the 802.11b frames are address fields. If you review an
Ethernet frame, you see that there are only two fields for addresses: destination
and source. In 802.11b frames, there may be up to four, which
include the following:
 The basic service set identifier (BSSID) is the MAC address of the
AP.
 The DA is the MAC address of the final recipient.
 The SA is the MAC address of the sending station on the WLAN.
 The receiver address (RA) is the MAC address of the intended
immediate recipient station on the WLAN.
 The transmitter address (TA) is the MAC address of the sending station
on the WLAN.
 The next field in an 802.11b frame is the Frame Body field, which is 0 to
2312 bytes long.The frame body is the payload, or data contained within
the frame.This is where the data being encapsulated into the frame is
located (for example, the graphic in a Web page requested by your
system).This field will vary in length based on the data encapsulated.  The final field in the 802.11b frame format is the FCS.
As you can see, there are a number of differences between Ethernet and
802.11b frames.These differences are required to enable high-speed communications
on a physical medium of radio waves rather than standard copper or fiber
media.