Now that you have an understanding of the OSI system model, we can relate the different technologies used in WLANs to the OSI system model. As the name wire- less LANimplies, it is networking without wires.The wires you are accustomed to using are replaced by radio signals. A number of various techniques are available for sending data over radio signals—these are covered in greater detail in Chapter 2.
The standards covered by the Cisco WLAN products detailed in this book are based on the IEEE’s 802.11 series.The 802.11 standards are responsible for defining the Physical and MAC layers of operation in a WLAN.The primary standard we focus on in the 802.11b standard, which is an extension to the original 802.11 standard. 802.11b’s primary objective defines the use of the 2.4 GHz band in radio frequency (RF) for high-speed data communications. 802.11b supports the original 802.11 data rate of 2 Mbps as well as higher speeds up to 11 Mbps.
The frames generated by a WLAN device differ from the frames generated by an Ethernet device in many ways.WLANs are not physically connected by cables like an Ethernet LAN, so new fields in the frames must be created to describe
aspects of the WLAN.We first examine a typical 802.2 Ethernet frame and com- pare it to a 802.11b frame.
An 802.2 Ethernet frame is comprised of six fields each with a specific func- tion. Figure 1.6 illustrates an Ethernet frame.
■ Preamble The first field in an Ethernet frame is the preamble.The
preamble is an 8-byte long alternating pattern of 0’s and 1’s telling receiving devices that a new frame is arriving.
■ Destination Address and Source Address The next fields are the
destination address (DA) and source address (SA).The fields are 2 or 6 bytes long and contain the MAC address of the source device on the network and the destination address.The destination address 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 The next field is the length and is 2 bytes long describing the
number of bytes of data following this field.
■ Data Unit The next field is the data unit containing the user data of
the frame and is 46–1500 bytes long.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.
■ Frame Check Sequence The last field in an Ethernet frame is the
Frame Check Sequence (FCS) field and is 4 bytes long.The FCS is a cyclic redundancy check (CRC) on the frame allowing the receiver of the frame to perform basic error controls on the frame. If a frame fails the CRC check, it is discarded and the upper layer protocol is typically responsible for retransmission.
Figure 1.6Ethernet Frame Format
P DA SA L DU FCS
P=Preamble 8 bytes DA = Destination Address 2 or 6 bytes SA = Source Address 2 or 6 bytes L = Length 2 bytes DU = Data Unit 46–1500 bytes FCS = Frame Check Sequence 4 bytes
A 802.11b frame (illustrated in Figure 1.7) is comprised of nine fields.
■ The first field in an 802.11b frame is the frame control(FC) field and is 2
bytes long.The FC field contains ten subfields including protocol ver- sion, type, subtype, to Distribution System (DS), from DS, more frag- ments, retry, power management, more data,Wired Equivalent Protocol (WEP), and order.These fields are some of the prime differentiators in an 802.11b frame and are described in greater detail here:
■ Protocol Version The protocol version field is the first field within
the frame control field and is 2 bits long.The default value for this field is 0 with all other values being reserved at this time.
■ 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 pos- sible combinations and their descriptions are illustrated in Table 1.1.
■ To Distribution System The To DS field is 1 bit long and is set
to 1 in all frames sent by an associated station with an 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 Distribution System 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.
Figure 1.7802.11b Frame Format
A2
FC = Frame Control 2 bytes D/ID = Duration/ID 2 bytes A1 = Address 1 6 bytes A2 = Address 2 6 bytes A3 = Address 3 6 bytes SC = Sequence Control 2 bytes A4 = Address 4 6 bytes FB = Frame Body 0–2312 bytes FCS = Frame Check Sequence 4 bytes
FCS FC D/ID A1 A3 SC A4 FB
■ Retry The retry field is 1 bit long and is set to 1 in all frames, data
or management, that 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 sta- tion.The value is used to indicate the state in which 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.
N
OTEThe PM field in frames transmitted by a wireless Access Point 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 field (MD) 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.
N
OTEThe StrictlyOrdered service class is a mechanism built into the 802.11 standard that provides additional protection against out of order frames. This is accomplished by holding any multicast or broadcast traffic that matches addresses for frames that are already queued. Without this mechanism, it would be possible for broadcast or multicast traffic to reach a recipient out of order and create communications problems.
Table 1.1802.11 Type and Subtype Combinations in the Frame Control (FC) Field
Type Type Subtype
Value Description Value Subtype Description b3 b2 b7 b6 b5 b4
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
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 1000-1111 Reserved
11 Reserved 0000-1111 Reserved
■ The next field in an 802.11b frame is the Duration/ID field and is 16
bits long. It is used to carry the association ID of a station with an Access Point.
■ 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: desti- nation and source. In 802.11b frames, there may be up to four, the basic service set identifier (BSSID), destination address (DA), source address (SA), receiver address (RA), and transmitter address (TA).
■ The BSSID is the MAC address of the Access Point. ■ The DA is the MAC address of the final recipient.
■ The SA is the MAC address of the sending station on the WLAN. ■ The RA is the MAC address of the intended immediate recipient
stations on the WLAN.
■ The TA is the MAC address of the sending station on the WLAN. ■ The next field in an 802.11b frame is the frame body and is 0–2312 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, just as in the Ethernet
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 communica- tions on a physical medium of radio waves rather than standard copper or fiber media.
Table 1.1Continued
Type Type Subtype