Chapter 6: IEEE 802.11b and IEEE 802.11a

This chapter is taken directly from IEEE Standard 802.11b-1999 and IEEE Standard 802.11a-1999. It is intended for pedagogical, advocacy, and educational purposes, so that readers have the core concepts at their command before making planning or implementation decisions. Designers and read- ers requiring detailed information are advised to consult the standards cited in their entirety.

IEEE 802.11b

IEEE Std 802.11b-1999 (a supplement to ANSI/IEEE Std 802.11, 1999 Edition) Supplement to IEEE Standard for Information technology—Telecommunications and information exchange between systems—Local and metropolitan area networks—Specific requirements—Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications: Higher-Speed Physical Layer Extension in the 2.4 GHz Band.

Figure 6-1 shows the reference model used in the protocol specification while Table 6-1 lists the modulation schemes supported by IEEE 802.11b.

Figure 6-1: Layer reference model

Table 6-1. Modulation schemes supported by IEEE 802.11b

CCK	Complementary code keying
HR/DSSS	High Rate direct sequence spread spectrum using the Long Preamble and header
HR/DSSS/short	High Rate direct sequence spread spectrum using the optional Short Preamble and header mode
HR/DSSS/PBCC	High Rate direct sequence spread spectrum using the optional packet binary convolutional coding mode and the optional Short Preamble and header

Overview of the High Rate, Direct Sequence Spread Spectrum PHY Specification

The standard specifies the High Rate extension of the PHY for the Direct Sequence Spread Spectrum (DSSS) system. This is known as the High Rate PHY for the 2.4 GHz band designated for ISM applications. This extension of the DSSS system builds on the data rate capabilities, as described in Clause 15 of IEEE 802.11, 1999 Edition, to provide 5.5 Mbps and 11 Mbps payload data rates in addition to the 1 Mbps and 2 Mbps rates. To provide the higher rates, 8-chip complementary code keying (CCK) is employed as the modulation scheme. The chipping rate is 11 MHz, which is the same as the DSSS system described in Clause 15, thus providing the same occupied channel bandwidth. The basic new capability described in 802.11b is called High Rate Direct Sequence Spread Spectrum (HR/DSSS). The basic High Rate PHY uses the same PLCP preamble and header as the DSSS PHY, so that both PHYs can coexist in the same BSS and can use the rate switching mechanism as provided.

In addition to providing higher speed extensions to the DSSS system, a number of optional features included in IEEE 802.11b allow the performance of the radio frequency LAN system to be improved as technology allows the implementation of these options to become cost effective. An optional mode replacing the CCK modulation with packet binary convolutional coding (HR/DSSS/PBCC) is provided. Another optional mode is provided that allows data throughput at the higher rates (2, 5.5, and 11 Mbps) to be significantly increased by using a shorter PLCP preamble. This mode is called HR/DSSS/short, or HR/DSSS/PBCC/short. The Short Preamble mode can coexist with DSSS, HR/DSSS, or HR/DSSS/PBCC under limited circumstances, such as on different channels or with appropriate CCA mechanisms. An optional capability for Channel Agility is also provided. This option allows an implementation to overcome some inherent difficulty with static channel assignments (a tone jammer), without burdening all implementations with the added cost of this capability. This option can also be used to implement IEEE 802.11-compliant systems that are interoperable with both FH and DS modulations.

Scope of the Standard A clause in IEEE 802.11b specifies the PHY entity for the HR/DSSS extension and the changes that have to be made to the base standard (IEEE 802.11) to accommodate the High Rate PHY. The High Rate PHY layer consists of the following two protocol functions:

• A PHY convergence function, which adapts the capabilities of the physical medium dependent (PMD) system to the PHY service. This function is supported by the PHY convergence procedure (PLCP), which defines a method for mapping the MAC sublayer protocol data units (MPDU) into a framing format suitable for sending and receiving user data and management information between two or more STAs using the associated PMD system. The PHY exchanges PHY protocol data units (PPDU) that contain PLCP service data units (PSDU). The MAC uses the PHY service, so each MPDU corresponds to a PSDU that is carried in a PPDU.

• A PMD system, whose function defines the characteristics of, and method of transmitting and receiving data through, a wireless medium between two or more STAs, each using the High Rate PHY system.

High Rate PHY Functions The 2.4 GHz High Rate PHY architecture is depicted in the ISO/IEC basic reference model shown in Figure 6-1. The High Rate PHY contains three functional entities: (i) the PMD function, (ii) the PHY convergence function, and (iii) the layer management function.

High Rate PLCP Sublayer

Overview A convergence procedure is defined in the standard for the 2, 5.5, and 11 Mbps specification, in which PSDUs are converted to and from PPDUs. During transmission, the PSDU will be appended to a PLCP preamble and header to create the PPDU. Two different preambles and head- ers are defined: the mandatory supported Long Preamble and header, which interoperates with the original 1 Mbps and 2 Mbps DSSS specification (as described in IEEE Std 802.11, 1999 Edition), and an optional Short Preamble and header. At the receiver, the PLCP preamble and header are processed to aid in demodulation and delivery of the PSDU.

The optional Short Preamble and header is intended for applications where maximum throughput is desired and interoperability with legacy and nonshort-preamble capable equipment is not a consideration. That is, it is expected to be used only in networks of like equipment, which can all handle the optional mode.

PPDU Format Two different preambles and headers are defined: the mandatory supported Long Preamble and header, which is interoperable with the current 1 Mbps and 2 Mbps DSSS specification (as described in IEEE Std 802.11, 1999 Edition) and an optional Short Preamble and header. Below we only cover the Long PLCP PPDU format.

Figure 6-2 shows the format for the interoperable (long) PPDU, including the High Rate PLCP preamble, the High Rate PLCP header, and the PSDU. The PLCP preamble contains the following fields: synchronization (sync) and start frame delimiter (SFD). The PLCP header contains the following fields: signaling (SIGNAL), service (SERVICE), length (LENGTH), and CCITT/ITU CRC-16. The format for the PPDU, including the long High Rate PLCP preamble, the long High Rate PLCP header, and the PSDU, do not differ from 802.11, 1999 Edition for 1 Mbps and 2 Mbps. The only exceptions are

• The encoding of the rate in the SIGNAL field

• The use of a bit in the SERVICE field to resolve an ambiguity in PSDU length in octets, when the length is expressed in whole microseconds

• The use of a bit in the SERVICE field to indicate if the optional PBCC mode is being used

• The use of a bit in the SERVICE field to indicate that the transit frequency and bit clocks are locked

  
  Figure 6-2: Long PLCP PDU format

PLCP PPDU Field Definitions

Long PLCP SYNC Field The SYNC field consists of 128 bits of scrambled “1” bits. This field is provided so the receiver can perform the necessary synchronization operations.

Long PLCP SFD The SFD is provided to indicate the start of PHY-dependent parameters within the PLCP preamble. The SFD will be a 16-bit field, [1111 0011 1010 0000], where the rightmost bit will be transmitted first in time.

Long PLCP SIGNAL Field The 8-bit SIGNAL field indicates to the PHY the modulation that is used for transmission (and reception) of the PSDU. The data rate will be equal to the SIGNAL field value multiplied by 100 kbps. The High Rate PHY supports four mandatory rates given by the following 8-bit words, which represent the rate in units of 100 kbps, where the lsb will be transmitted first in time:

• X’0A’ (msb to lsb) for 1 Mbps

• X’14’ (msb to lsb) for 2 Mbps

• X’37’ (msb to lsb) for 5.5 Mbps

• X’6E’ (msb to lsb) for 11 Mbps

This field will be protected by the CCITT CRC-16 frame check sequence.

Long PLCP SERVICE Field Three bits have been defined in the SERVICE field to support the High Rate extension. The rightmost bit (bit 7) will be used to supplement the LENGTH field. Bit 3 will be used to indicate whether the modulation method is CCK <0> or PBCC <1>, as shown in Table 6-2. Bit 2 will be used to indicate that the transmit frequency and symbol clocks are derived from the same oscillator. This locked clocks bit will be set by the PHY layer based on its implementation configuration. The SERVICE field will be transmitted b0 first in time, and shall be protected by the CCITT CRC-16 frame check sequence. An IEEE 802.11-compliant device will set the values of the bits b0, b1, b4, b5, and b6 to 0.

Table 6-2. SERVICE field definitions

Long PLCP LENGTH Field The PLCP length field is an unsigned 16-bit integer that indicates the number of microseconds required to transmit the PSDU. The transmitted value is determined from the LENGTH and DataRate parameters in the TXVECTOR issued with the PHY-TXSTART.request primitive. The length field provided in the TXVECTOR is in octets and is converted to microseconds for inclusion in the PLCP LENGTH field. The LENGTH field is calculated as follows. Since there is an ambiguity in the number of octets that is described by a length in integer microseconds for any data rate over 8 Mbps, a length extension bit will be placed at bit position b7 in the SERVICE field to indicate when the smaller potential number of octets is correct.

• 5.5 Mbps CCK Length = number of octets x 8/5.5, rounded up to the next integer.

• 11 Mbps CCK Length = number of octets x 8/11, rounded up to the next integer; the service field (b7) bit will indicate a “0” if the rounding took less than 8/11 or a “1” if the rounding took more than or equal to 8/11.

• 5.5 Mbps PBCC Length = (number of octets + 1) x 8/5.5, rounded up to the next integer.

• 11 Mbps PBCC Length = (number of octets + 1) x 8/11, rounded up to the next integer; the service field (b7) bit will indicate a “0” if the rounding took less than 8/11 or a “1” if the rounding took more than or equal to 8/11.

At the receiver, the number of octets in the MPDU is calculated as follows:

• 5.5 Mbps CCK Number of octets = Length x 5.5/8, rounded down to the next integer.

• 11 Mbps CCK Number of octets = Length x 11/8, rounded down to the next integer, minus 1 if the service field (b7) bit is a “1.”

• 5.5 Mbps PBCC Number of octets = (Length x 5.5/8) – 1, rounded down to the next integer.

• 11 Mbps PBCC Number of octets = (Length x 11/8) – 1, rounded down to the next integer, minus 1 if the service field (b7) bit is a “1.”

PLCP CRC (CCITT CRC-16) Field The SIGNAL, SERVICE, and LENGTH fields will be protected with a CCITT CRC-16 frame check sequence (FCS).

Long PLCP Data Modulation and Modulation Rate Change The long PLCP preamble and header is transmitted using the 1 Mbps DBPSK modulation. The SIGNAL and SERVICE fields combined will indicate the modulation that will be used to transmit the PSDU. The SIGNAL field indicates the rate, and the SERVICE field indicates the modulation. The transmitter and receiver initiate the modulation and rate indicated by the SIGNAL and SERVICE fields, starting with the first octet of the PSDU. The PSDU transmission rate will be set by the DATARATE parameter in the TXVECTOR, issued with the PHY-TXSTART.request primitive.

PLCP Transmit Procedure The transmit procedures for a High Rate PHY using the long PLCP preamble and header are the same as those described in IEEE Std 802.11, 1999 Edition, and do not change apart from the ability to transmit 5.5 Mbps and 11 Mbps. The PLCP transmit procedure is shown in Figure 6-3.

Figure 6-3: PLCP transmit procedure

PLCP Receive Procedure The receive procedures for receivers configured to receive the mandatory and optional PLCPs, rates, and modulations are described in this subclause. A receiver that supports this High Rate extension of the standard is capable of receiving 5.5 Mbps and 11 Mbps, in addition to 1 Mbps and 2 Mbps.

If the PHY implements the Short Preamble option, it will detect both Short and Long Preamble formats and indicate which type of preamble was received in the RXVECTOR. If the PHY implements the PBCC Modulation option, it will detect either CCK or PBCC Modulations, as indicated in the SIGNAL field, and will report the type of modulation used in the RXVECTOR.

High Rate PMD Sublayer

Scope and Field of Application Subclause 18.4 of the standard describes the PMD services provided to the PLCP for the High Rate PHY. Also defined here are the functional, electrical, and RF characteristics required for interoperability of implementations conforming to this specification. The relationship of this specification to the entire High Rate PHY is shown in Figure 6-1.

Overview of Service The High Rate PMD sublayer accepts PLCP sublayer service primitives and provides the actual means by which data is transmitted or received from the medium. The combined functions of the High Rate PMD sublayer primitives and parameters for the receive function result in a data stream, timing information, and associated received signal parameters being delivered to the PLCP sublayer. A similar functionality is provided for data transmission.

Interactions The primitives associated with the PLCP sublayer to the High Rate PMD fall into two basic categories:

• Service primitives that support PLCP peer-to-peer interactions

• Service primitives that have local significance and that support sublayer-to-sublayer interactions

PMD_SAP Peer-to-Peer Service Primitives See Table 6-3.

Table 6-3. PMD_SAP peer-to- peer service primitives

Primitive	Request	Indicative	Confirm	Response
PMD_DATA	X	X	—	—

PMD_SAP Sublayer-to-Sublayer Service Primitives See Table 6-4.

Table 6-4. PMD_SAP sublayerto-sublayer service primitives

Primitive	Request	Indicative	Confirm	Response
PMD_TXSTART	X	—	—	—
PMD_TXEND	X	—	—	—
PMD_ANTSEL	X	X	—	—
PMD_TXPWRLVL	X	—	—	—
PMD_MODULATION	X	X	—	—
PMD_PREAMBLE	X	X	—	—
PMD_RATE	X	X	—	—
PMD_RSSI	—	X	—	—
PMD_SQ	—	X	—	—
PMD_CS	—	X	—	—
PMD_ED	X	X	—	—

Refer to the IEEE 802.11b standard for a description of the services provided by each PMD primitive.

PMD Operating Specifications, General Subclauses 18.4.6.1 through 18.4.6.14 in the standard (summarized herewith) provide general specifications for the High Rate PMD sublayer. These specifications apply to both the receive and transmit functions and general operation of a High Rate PHY.

Operating Frequency Range The High Rate PHY will operate in the 2.4— 2.4835 GHz frequency range, as allocated by regulatory bodies in the USA and Europe, or in the 2.471—2.497 GHz frequency range, as allocated by regulatory authority in Japan.

Number of Operating Channels The channel center frequencies and CHNL_ID numbers will be as shown in Table 6-5. The FCC (U.S.), IC (Canada), and ETSI (Europe) specify operation from 2.4—2.4835 GHz. For Japan, operation is specified as 2.471—2.497 GHz. France allows operation from 2.4465—2.4835 GHz, and Spain allows operation from 2.445—2.475 GHz. For each supported regulatory domain, all channels in Table 6-5 marked with an “X” will be supported.

Table 6-5. High Rate PHY frequency channel plan

Modulation and Channel Data Rates Four modulation formats and data rates are specified for the High Rate PHY. The basic access rate is based on 1 Mbps DBPSK modulation. The enhanced access rate will be based on 2 Mbps DQPSK. The extended direct sequence specification defines two additional data rates. High Rate access rates are based on the CCK modulation scheme for 5.5 Mbps and 11 Mbps. An optional PBCC mode is also provided for potentially enhanced performance.

Spreading Sequences and Modulation for CCK Modulation at 5.5 Mbps and 11 Mbps For the CCK modulation modes, the spreading code length is 8 and is based on complementary codes. The chipping rate is 11 Mchip/s. The symbol duration will be exactly 8 complex chips long. The following formula is used to derive the CCK code words that will be used for spreading both 5.5 Mbps and 11 Mbps:

           B c = {ejø1+2_3+4), ejø1+3+4), ejø1+2+4),               – ejø1+4), ejø1+2+3), ejø1+3), – ejø1+2), ejø1}

where C is the code word C = {c0 to c7}

CCK 5.5 Mbps Modulation At 5.5 Mbps 4 bits (d0 to d3; d0 first in time) are transmitted per symbol. The data bits d0 and d1 encode ø₁ based on DQPSK. The DQPSK encoder is specified in Table 6-6. (In the table, +jw is defined as counterclockwise rotation.) The phase change for ø₁ is relative to the phase ø₁ of the preceding symbol. For the header to PSDU transition, the phase change for ø₁ is relative to the phase of the preceding DQPSK (2 Mbps) symbol. That is, the phase of the last symbol of the CRC-16 is the reference phase for the first symbol generated from the PSDU octets. A “+1” chip in the Barker code represents the same carrier phase as a “+1” chip in the CCK code.

Table 6-6. DQPSK encoding table

Dibit pattern (d0, d1) (d0 is first in time)	Even symbols phase change (+jw)	Odd symbols phase change (+j w)
00	0	p
01	p/2	3p/2 (-p/2)
11	p	0
10	3p/2 (-p/2)	p/2

All odd-numbered symbols generated from the PSDU octets will be given an extra 180 degree (p) rotation, in addition to the standard DQPSK modulation as shown in Table 6-6. Symbols of the PSDU are numbered from “0” for the first symbol, for the purposes of determining odd and even symbols. That is, the PSDU transmission starts on an even-numbered symbol.

The data dibits d2 and d3 CCK encode the basic symbol, as specified in Table 6-7. This table is derived from the formula above by setting ø₂ = (d2 x p) + p/2, ø₃ = 0, and ø ₄ = d3 x p. In this table, d2 and d3 are in the order shown, and the complex chips are shown c0 to c7 (left to right), with c0 transmitted first in time.

Table 6-7. 5.5 Mbps CCK encoding table

d2, d3	c1	c2	c3	c4	c5	c6	c7	c8
00	1j	-1	1j	-1	1j	1	-1j	1
01	-1j	-1	-1j	1	1j	1	-1j	1
10	-1j	-1	-1j	-1	-1j	1	1j	1
11	1j	-1	1j	1	-1j	1	1j	1

CCK 11 Mbps Modulation At 11 Mbps, 8 bits (d0 to d7; d0 first in time) are transmitted per symbol. The first dibit (d0, d1) encodes f₁ based on DQPSK. The DQPSK encoder is specified in Table 6-6. The phase change for ø₁ is relative to the phase ø₁ of the preceding symbol. In the case of header to PSDU transition, the phase change for ø₁ is relative to the phase of the preceding DQPSK symbol. All odd-numbered symbols of the PSDU are given an extra 180 degree (p) rotation, in accordance with the DQPSK modulation shown in Table 6-6. Symbol numbering starts with “0” for the first symbol of the PSDU.

The data dibits (d2, d3), (d4, d5), and (d6, d7) encode ø₂, ø₃, and ø₄ respectively, based on QPSK as specified in Table 6-8 (this table is binary [not Grey] coded).

Table 6-8. QPSK encoding table

Dibit Patter [di, d(i + l)] (di is first in time)	Phase
00	0
01	p/2
10	p
11	3p/2 (-p/2)