802.11b Physical Layer Revealed

The IEEE 802.11 standard includes a common Medium Access Control (MAC) Layer, which defines protocols that govern the operation of the wireless LAN. In addition, 802.11 comprises several alternative physical layers that specify the transmission and reception of 802.11 frames. Let's take a closer look at the 802.11b Physical Layer, which uses direct sequence spread spectrum (DSSS) technology to support operation of up to 11Mbps data rates in the 2.4GHz band.

As with other 802.11 Physical layers, 802.11b includes Physical Layer Convergence Procedure (PLCP) and Physical Medium Dependent (PMD) sub-layers. These are somewhat sophisticated terms that the standard uses to divide the major functions that occur within the Physical Layer. The PLCP prepares 802.11 frames for transmission and directs the PMD to actually transmit signals, change radio channels, receive signals, and so on.

PLCP Frame Fields

The PLCP takes each 802.11 frame that a station wishes to transmit and forms what the 802.11 standard refers to as a PLCP protocol data unit (PPDU). The resulting PPDU includes the following fields in addition to the frame fields imposed by the MAC Layer:

• Sync. This field consists of alternating 0s and 1s, alerting the receiver that a receivable signal is present. The receiver begins synchronizing with the incoming signal after detecting the Sync.
• Start Frame Delimiter. This field is always 1111001110100000 and defines the beginning of a frame.
• Signal. This field identifies the data rate of the 802.11 frame, with its binary value equal to the data rate divided by 100Kbps. For example, the field contains the value of 00001010 for 1Mbps, 00010100 for 2Mbps, and so on. The PLCP fields, however, are always sent at the lowest rate, which is 1Mbps. This ensures that the receiver is initially uses the correct demodulation mechanism, which changes with different data rates.
• Service. This field is always set to 00000000, and the 802.11 standard reserves it for future use.
• Length. This field represents the number of microseconds that it takes to transmit the contents of the PPDU, and the receiver uses this information to determine the end of the frame.
• Frame Check Sequence. In order to detect possible errors in the Physical Layer header, the standard defines this field for containing 16-bit cyclic redundancy check (CRC) result. The MAC Layer also performs error detection functions on the PPDU contents as well.
• PSDU. The PSDU, which stands for Physical Layer Service Data Unit, is a fancy name that represents the contents of the PPDU (i.e., the actual 802.11 frame being sent).

Don't expect to see the physical layer fields with 802.11 analyzers from AirMagnet and Wildpackets, however. The 802.11 radio card removes these fields before the resulting data is processed by the MAC Layer and offered to the analyzer for viewing.

DSSS Spreading Function

802.11b uses DSSS to disperse the data frame signal over a relatively wide (approximately 30MHz) portion of the 2.4GHz frequency band. This results in greater immunity to radio frequency (RF) interference as compared to narrowband signaling, which is why the Federal Communications Commission (FCC) deems the operation of spread spectrum systems as license free.

Because of the relatively wideband DSSS signal, you must set 802.11b access points to specific channels to avoid channel overlap (use channels 1, 6, and 11 in the U.S.), which can cause reductions in performance. Refer to a previous tutorial for more details on setting 802.11b access point channels.

In order to actually spread the signal, an 802.11 transmitter combines the PPDU with a spreading sequence through the use of a binary adder. The spreading sequence is a binary code. For 1Mbps and 2Mbps operation, the spreading code is the 11-chip Barker sequence, which is 10110111000. The binary adder effectively multiplies the length of the binary stream by the length of the sequence, which is 11. This increases the signaling rate and makes the signal span a greater amount of frequency bandwidth.

5.5Mbps and 11Mbps operation of 802.11b doesn't use the Barker sequence. Instead, 802.11b uses complementary code keying (CCK) to provide the spreading sequences at these higher data rates. CCK derives a different spreading code based on fairly complex functions depending on the pattern of bits being sent. The modulator simply refers to a table for the spreading sequence that corresponds to the pattern of data bits being sent. This is necessary to obtain the most efficient processing of the data in order to achieve the higher data rates.

DSSS Modulation

The modulator converts the spread binary signal into an analog waveform through the use of different modulation types, depending on which data rate is chosen. For example with 1Mbps operation, the PMD uses differential binary phase shift keying (DBPSK). This isn't really as complex as it sounds. The modulator merely shifts the phase of the center transmit frequency to distinguish a binary 1 from a binary 0 within the data stream.

For 2Mbps transmission, the PMD uses differential quadrature phase shift keying (DQPSK), which is similar to DBPSK except that there are four possible phase shifts that represents every two data bits. This is a clever process that enables the data stream to be sent at 2Mbps while using the same amount of bandwidth as the one sent at 1Mbps. The modulator uses similar methods for the higher, 5.5Mbps and 11Mbps data rates.

Transmit Frequencies

The transmitter's modulator translates the spread signal into an analog form with a center frequency corresponding to the radio channel chosen by the user. The following identifies the center frequency of each channel:

Channel	Frequency (GHz)
1	2.412
2	2.417
3	2.422
4	2.427
5	2.432
6	2.437
7	2.442
8	2.447
9	2.452
10	2.457
11	2.462
12	2.467
13	2.472
14	2.484

Various countries limit the use of these channels. For example, the U.S. only allows the use of channels 1 through 11, and the U.K. can use channels 1 through 13. Japan, however, authorizes the use all 14 channels. This complicates matters when designing international public wireless LANs. In that case, you need to choose channels with the least common denominator.

After RF amplification takes place based on the transmit power you've chosen (100mW maximum for the U.S.), the transmitter outputs the modulated DSSS signal to the antenna in order to propagate the signal to the destination. The trip in route to the destination will significantly attenuate the signal, but the receiver at the destination will detect the incoming Physical Layer header and reverse (demodulate and despread) the process implemented by the transmitter.

In the future, we'll take a closer look at 802.11a and 802.11g Physical layers as well.

Jim Geier provides independent consulting services to companies developing and deploying wireless network solutions. He is the author of the book, Wireless LANs and offers workshops on deploying wireless LANs.

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