High Rate Direct Sequence PHY

To distinguish it from the original direct sequence PHY, the high rate PHY that runs at 11 Mbps is abbreviated as HR/DSSS. Like its predecessor, it is split into a convergence procedure that prepares frames for radio transmission, and a medium-dependent layer that turns the bits into radio waves in the air.

PLCP Framing and Scrambling

The long headers required by the original PHY greatly reduce performance. The 802.11 MAC requires an acknowledgment for every data frame, and the 192 microsecond preamble is much, much longer than the MAC acknowledgment. At the 11 Mbps data rate, the preamble and PLCP framing header sucks up 25% of the time used to transmit a 1,500 byte frame and its corresponding MAC acknowledgment. As long as a new PHY was being developed, the designers of 802.11b came up with a new "short" framing format that improves protocol efficiency and, hence, throughput. Using the short headers cuts the preamble and PLCP framing overhead cuts the preamble and framing overhead to 14%. While still substantial, it is a dramatic improvement. Figure 12-17 shows the PLCP framing specified in 802.11b. When 802.11b was first released, short headers were not supported by all devices because of the large installed base of 2 Mbps direct sequence equipment. At this point, almost every card can safely support the short preamble, and most access point vendors use it by default.

Naturally, the optional short format may be used only if all stations support it. To prevent networks configured for the short format from disappearing, 802.11b requires that stations answering Probe Requests from an active scan return a response using the same PLCP header that was received. If a station that supports

Figure 12-17. HR/DSSS PLCP framing

only the long PLCP header sends a Probe Response, an access point returns a response using the long header, even if the BSS is configured for the short header.

Preamble

Frames begin with the preamble, which is composed of the Sync field and the SFD field. The preamble is transmitted at 1.0 Mbps using DBPSK.

Long Sync

The Long Sync field is composed of 128 one bits. It is processed by the scrambler before transmission, though, so the data content varies. High-rate systems use a specified seed for the scrambling function but support backwards compatibility with older systems that do not specify a seed.

Short Sync

The Short Sync field is composed of 56 zero bits. Like the Long Sync, it is also processed by the scrambler.

Long SFD

To indicate the end of the Sync field, the long preamble concludes with a Start of Frame Delimiter (SFD). In the long PLCP, the SFD is the sequence 1111 0011 1010 0000. As with all IEEE specifications, the order of transmission from the physical interface is least-significant bit first, so the string is transmitted right to left.

Short SFD

To avoid confusion with the Long SFD, the Short SFD is the reverse value, 0000 0101 1100 1111.

The PLCP header follows the preamble. It is composed of the Signal, Service, Length, and CRC fields. The long header is transmitted at 1.0 Mbps using DBPSK. However, the short header's purpose is to reduce the time required for overhead transmission so it is transmitted at 2.0 Mbps using DQPSK.

Long Signal

The Long Signal field indicates the speed and transmission method of the enclosed MAC frame. Four values for the 8-bit code are currently defined and are shown in Table 12-5.

Table 12-5. Signal field values

Speed	Value (msb to lsb)	Hex value
1 Mbps	0000 1010	0x0A
2 Mbps	0001 0100	0x14
5.5 Mbps	0011 0111	0x37
11 Mbps	0110 1110	0x6E

 

Short Signal

The Short Signal field indicates the speed and transmission method of the enclosed frame, but only three values are defined. Short preambles can be used only with 2 Mbps, 5.5 Mbps, and 11 Mbps networks.

Service

The Service field, which is shown in Figure 12-18, was reserved for future use by the first version of 802.11, and bits were promptly used for the high-rate extensions in 802.11b. First of all, the Length field describes the amount of time used for the enclosed frame in microseconds. Above 8 Mbps, the value becomes ambiguous. Therefore, the eighth bit of the service field is used to extend the Length field to 17 bits. The third bit indicates whether the 802.11b implementation uses locked clocks; clock locking means that transmit frequency and symbol clock use the same oscillator. The fourth bit indicates the type of coding used for the packet, which is either 0 for CCK or 1 for PBCC. All reserved bits must be set to 0. The Service field is transmitted from left to right (b0 to b7), which is the same in both the short and long PLCP frame formats. (Further changes are made by 802.11g, which will be discussed in Chapter 14.)

Figure 12-18. Service field in the HR/DSSS PLCP header

 

Length

The Length field is the same in both the short and long PLCP frame formats and is the number of microseconds required to transmit the enclosed MAC frame. Approximately two pages of the 802.11b standard are devoted to calculating the value of the Length frame, but the details are beyond the scope of this book.

CRC

The CRC field is the same in both the short and the long PLCP frames. Senders calculate a CRC checksum using the Signal, Service, and Length fields. Receivers can use the CRC value to ensure that the header was received intact and was not damaged during transmission. CRC calculations take place before data scrambling.

The data scrambling procedure for the HR/DSSS PHY is nearly identical to the data scrambling procedure used with the original DS PHY. The only difference is that the scrambling function is seeded to specified values in the HR/DSSS PHY. Different seeds are used for short and long PLCP frames.

HR/DSSS PMD

Like the DS PHY, the 802.11b PHY uses a single PMD specification. The general transceiver design is shown in Figure 12-19.

Figure 12-19. HR/DSSS transceiver

 

Transmission at 1.0 Mbps or 2.0 Mbps

To ensure backwards compatibility with the installed base of 802.11-based, direct-sequence hardware, the HR/DSSS PHY can transmit and receive at 1.0 Mbps or 2.0 Mbps. Slower transmissions are supported in the same manner as the lower-rate, direct-sequence layers described in previously in this chapter. Any transmissions at the slower rates must use long headers.

Transmission at 5.5 Mbps with CCK

Higher-rate transmission is accomplished by building on the DQPSK-based phase shift keying techniques. DQPSK transmits two bits per symbol period, encoded as one of four different phase shifts. By using CCK, the symbol words themselves carry additional information. 5.5-Mbps transmission encodes four data bits into a symbol. Two bits are carried using conventional DQPSK, and the other two are carried through the content of the code words. Figure 12-20 illustrates the overall process.

Figure 12-20. 802.11b transmission at 5.5 Mbps

1. The MAC frame embedded in the PCLP frame is divided into a string of 4-bit blocks. Each 4-bit block is further divided into two 2-bit segments.
2. The first 2-bit segment is encoded by means of a DQPSK-type phase shift between the current symbol and the previous symbol (Table 12-6). Even and odd symbols use a different phase shift for technical reasons. Symbol numbering starts with 0 for the first 4- bit block.

   Table 12-6. Inter-symbol DQPSK phase shifts

   Bit pattern	Phase angle (even symbols)	Phase angle (odd symbols)
   00	0	P
   01	p/2	3p/2
   11	p	0
   10	3p/2	p/2

    
3. The second 2-bit segment is used to select one of four code words for the current symbol (Table 12-7). The four code words can be derived using the mathematics laid out in clause 18.4.6.5 of the 802.11 standard.

Table 12-7. Mbps code words

Bit sequence	Code word
00	i,1,i,-1,i,1,-i,1
01	-i,-1,-i,1,1,1,-i,1
10	-i,1,-i,-1,-i,1,i,1
11	i,-1,i,1,-i,1,i,1

 

Transmission at 11 Mbps with CCK

To move to a full 11 Mbps, 8 bits must be encoded with each symbol. As with other techniques, the first two bits are encoded by the phase shift of the transmitted symbol relative to the previous symbol. Six bits are encoded using CCK. Figure 12-21 illustrates the process.

Figure 12-21. 802.11b transmission at 11 Mbps

1. The MAC frame embedded in the PCLP frame is divided into a string of 8-bit blocks. Each 8-bit block is further divided into four 2-bit segments.
2. The first 2-bit segment is encoded by means of a DQPSK-type phase shift between the current symbol and the previous symbol. As with the 5.5-Mbps rate, even and odd symbols use a different phase shift for technical reasons. Symbol numbering starts with 0 for the first 8-bit block. The phase shifts are identical to the phase shifts used in 5.5-Mbps transmission.
3. The remaining six bits are grouped into three successive pairs. Each pair is associated with the phase angle in Table 12-8 and is used to derive a code word.

Table 12-8. Phase angle encoding for 11-Mbps transmission

Bit pattern	Phase angle
00	0
01	p/2
10	p
11	3p/2

As an example, consider the conversion of the bit sequence 0100 1101 into a complex code for transmission on an 802.11b network. The first two bits, 01, encode a phase shift from the previous symbol. If the symbol is an even symbol in the MAC frame, the phase shift is p/2; otherwise, the shift is 3p/2. (Symbols in the MAC frame are numbered starting from 0, so the first symbol in a frame is even.) The last six bits are divided into three 2-bit groups: 00, 11, and 01. Each of these is used to encode an angle in the code word equation. The next step in transmission is to convert the phase angles into the complex code word for transmission.

Clear channel assessment

Like the original DS PHY, high-rate implementers have three choices for the CS/CCA operation mode. All the direct-sequence CCA modes are considered to be part of the same list. Mode 1 is identical to the DS PHY's CCA Mode 1, and Modes 2 and 3 are used exclusively by the original DS PHY. Modes 4 and 5 are the HR/DSSS-specific CCA modes.

Mode 1

When the energy exceeds the energy detection (ED) threshold, the medium is reported busy. The ED threshold depends on the transmit power used. This mode is also available for classic direct-sequence systems.

Mode 4

Implementations using Mode 4 look for an actual signal. When triggered, a Mode 4 CCA implementation starts a 3.65 ms timer and begins counting down. If no valid HR/DSSS signal is received by the expiration of the timer, the medium is reported idle. 3.65 ms corresponds to the transmission time required for the largest possible frame at 5.5 Mbps.

Mode 5

Mode 5 combines Mode 1 and Mode 4. A signal must be detected with sufficient energy before the channel is reported busy to higher layers.

Once a channel is reported busy, it stays busy for the duration of the intended transmission, even if the signal is lost. The channel is considered busy until the time interval in the Length field has elapsed. Implementations that look for a valid signal may override this requirement if a second PLCP header is detected.

Optional Features of the 802.11b PHY

802.11b includes two optional physical-layer features, neither of which is widely used. Packet Binary Convolutional Coding (PBCC) was proposed as a method of reaching the 11 Mbps data rate, but it was never widely implemented. Proposals for further revisions to wireless LAN technology in the ISM band specified PBCC, but those proposals were rejected in the summer of 2001.

A second optional feature, channel agility, was designed to assist networks in avoiding interference. Channel agility causes the center channel to shift periodically in the hope that interference can be avoided. Channel agility was never widely used because it was not particularly helpful. In the presence of interference, some throughput would be recovered as receivers hopped to another channel, but the additional spectrum required made it much more effective to hunt down and fix the interference, or remap channels around it.

Characteristics of the HR/DSSS PHY

Table 12-9 shows the values of a number of parameters in the HR/DSSS PHY. Like the DS PHY, the HR/DSSS PHY has a number of parameters that can be adjusted to compensate for delays in any part of a real system.

Table 12-9. HR/DSSS PHY parameters

Parameter	Value	Notes
Maximum MAC frame length	4,095 bytes
Slot time	20 ms
SIFS time	10 ms	The SIFS is used to derive the value of the other interframe spaces (DIFS, PIFS, and EIFS).
Contention window size	31 to 1,023 slots
Preamble duration	144 ms	Preamble symbols are transmitted at 1 MHz, so a symbol takes 1 ms to transmit; 96 bits require 96 symbol times.
PLCP header duration	48 bits	The PLCP header transmission time depends on whether the short preamble is used.
Minimum sensitivity	-76 dBm
Adjacent channel rejection	35 dB	See text for measurement notes.

One other item of note is that the total aggregate throughput of all HR/DSSS networks in an area is still lower than the total aggregate throughput of all nonoverlapping frequency-hopping networks in an area. The total aggregate throughput is a function of the number of nonoverlapping channels. In North America and most of Europe, three HR/DSSS networks can be deployed in an area at once. Running each network at the top speed of 11 Mbps, and assuming user payload data throughput of 50%, the total aggregate throughput will be 16.5 Mbps.