Structure of IEEE 802.11 Packets at Various Physical Layers
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Appendix A: Structure of IEEE 802.11 Packets at Various Physical Layers This appendix gives a detailed description of the structure of packets in IEEE 802.11 for the different physical layers. 1. Packet Format of Frequency Hopping Spread-spectrum Physical Layer (FHSS PHY) The packet is made up of the following elements (Figure A.1): 1.1 Preamble It depends on the physical layer and includes: – Synch: a sequence of 80 bits alterning 0 and 1, used by the physical circuits to select the correct antenna (if more than one are in use), and correct offsets of frequency and synchronization. – SFD: start frame delimiter consists of a pattern of 16 bits: 0000 1100 1011 1101, used to define the beginning of the frame. 1.2 Physical Layer Convergence Protocol Header The Physical Layer Convergence Protocol (PLCP) header is always transmitted at 1 Mbps and carries some logical information used by the physical layer to decode the frame: – Length of word of PLCP PDU (PLW): representing the number of bytes in the packet, useful to the physical layer to detect correctly the end of the packet. – Flag of signalization PLCP (PSF): indicating the supported rate going from 1 to 4.5 Mbps with steps of 0.5 Mbps. Even though the standard gives the combinations of bits for PSF (see Table A.1) to support eight different rates, only the modulations for 1 and 2 Mbps have been defined. – Control error field (HEC): CRC field for error detection of 16 bits (or 32 bits). The polynomial generator used is G(x)=x16 + x12 + x5 +1. The number of operating channels must be greater than or equal to 20 and less than 35 slots of 1 MHz in the band indicated in Table A.2. 80 16 124 16 Variable bits Synchronisation SFD PLW PSF HEC Payload PLCP preamble PLCP header Fig. A.1. FHSS 802.11 packet 281 282 Wi-FiTM, BluetoothTM,ZigBeeTM and WiMAXTM Table A.1. Supported rates PSF bit 0 PSF bits 1-2-3 Rate (Mbps) 0 000 1.0 0 001 1.5 0 010 2.0 0 011 2.5 0 100 3.0 0 101 3.5 0 110 4.0 0 111 4.5 Table A.2. FHSS channels Lower limit Upper limit Legal coverage 2.448 GHz 2.482 GHz 2.4465 − 2.4835 GHz 128 16 8 8 16 16 Variable bits Synchronisation SFD Signal Service SizeCRC Payload PLCP preamble PLCP header Fig. A.2. DSSS 802.11 packet 2. Packet Format of Direct-Sequence Spread Spectrum Physical Layer (DSSS PHY) The packet has the following structure (see Figure A.2). 2.1 Preamble It is dependent on the physical layer and includes: – Synch: it is composed of 128 bits set to «1» and cooperates to the synchronization of the receiver. – SFD: start frame delimiter consists of a pattern of 16 bits: 1111 0011 1010 0000, used to define the beginning of the frame. The preamble must be transmitted at 1 Mbps with a DBPSK modulation. 2.2 PLCP Header The PLCP header is always transmitted at 1 Mbps. It contains several fields: – Signal: this field allows to specify the modulation to use for reaching the desired rate, having 0A for 1 Mbps with a DBPSK modulation and 14 for 2 Mbps with a DQPSK modulation. – Service: this field is reserved for future use and must be initialized to 0. A value of 00 signifies that the unit conforms to the IEEE 802.11 standard. Appendix A 283 – Length: this field indicates the size of the frame in bytes, ranging from 4 to 8192 bytes. – Control error field (CRC): CRC field for error detection of 16 bits (or 32 bits). The polynomial generator used is G(x)=x16 + x12 + x5 +1. 3. Packet Format of IEEE 802.11b HR/DSSS PHY The packet structure of IEEE 802.11b DSSS is illustrated in Figure A.3. Notice the presence of two preambles, one similar to that of the IEEE 802.11 standard with 128 bits set to 1 (scrambled bits) and another one shorter with 56 bits set to 0 (scrambled bits) that reduces the overhead. Next we are going to describe just the fields which are different from those we have already seen in section 2: – SFD: it allows the receiver to find the beginning of the frame. This 2-byte field is rep- resented by the sequence 1111 0011 1010 0000 in the case of a long preamble, and its opposite 0000 1100 0101 1111 in the case of a short preamble. – Signal: this field, in the case of a long preamble, allows to specify the modulation to use for reaching the desired rate, having 0x0A for 1 Mbps with a DBPSK modulation, 0x14 for 2 Mbps with a DQPSK modulation, 0x37 for 5.5 Mbps with a CCK4 modulation and 0x6E for 11 Mbps with a CCK8 modulation. With a short preamble, only three rates are possible (2, 5.5 or 11 Mbps). IEEE 802.11b considers the utilization of a method to rearrange MAC frames after the CRC calculation phase. The preamble must be transmitted at 1 Mbps with a DBPSK modulation. In order to reduce the overhead time contribution, the header in the case of a packet with a short preamble is transmitted at 2 Mbps using a DQPSK modulation. Long packet 128 16 8 816 16 variable bits synchronization SFD signalservicelength CRC PSDU DBPSK, 1 Mbps PLCP preamble PLCP header DQPSK, 2 Mbps CCK, 5.5 / 11 Mbps 144 bits at 1 Mbps 48 bits at 1 Mbps Short packet 56 16 8 816 16 variable bits synchronization SFD signalservicelength CRC PSDU DQPSK, 2 Mbps PLCP preamble PLCP header CCK, 5.5 / 11 Mbps 72 bits at 1 Mbps 48 bits at 2 Mbps Fig. A.3. DSSS 802.11b packet 284 Wi-FiTM, BluetoothTM,ZigBeeTM and WiMAXTM 57-73 slots 4 slots 3 sts 32 sts16 bits 16 bits synchronization SFD DR DCLA length CRC payload PLCP preamble PLCP header Fig. A.4. IEEE 802.11 IR packet 4. Packet Format of Infrared Physical Layer (IR PHY) The name of the modulation in use is pulse position modulation (PPM). It is based on the IEC 60825-1, ANSI Z136 standard (Figure A.4). Different data rates are offered: – Mbps with a 16-PPM modulation – Mbps with a 4-PPM modulation The packet is composed of the fields described afterwards. 4.1 Preamble It contains: – Synch: sequence of presence/absence of pulses in a series of slots. – SFD: start frame delimiter consists of a pattern of 4 bits 1001 (1 pulse in a slot L-PPM and none for 0), used to define the beginning of a frame. A slot corresponds to one of the L positions of the symbol and has a duration of 250 ns. 4.2 Physical Layer Convergence Protocol header It includes the following fields: –DR(data rate): this field indicates the actual rate (000: 1 Mbps; 001: 2 Mbps). In the case of a transmission at 1 Mbps, the 16-PPM modulation is used (basic rate). The 4-PPM modulation is used in the case of a rate of 2 Mbps. –DCLA(DC-level adjustment): this field is needed for letting the receiver establish the DC level after the reception of the fields SYNC, SFD and DR. It contains a sequence of 32 slots (1 Mbps: 00000000 10000000 00000000 10000000, 2 Mbps: 00100010 00100010 00100010 00100010). – Length: this field indicates the number of bytes of the PSDU to transmit. – Control error field (CRC): CRC field for error detection of 16 bits. The polynomial generator used is G(x)=x16 + x12 + x5 +1. The main parameters of the different physical layers are summarized in Table A.3. They can be tuned to reduce time delays and adapt to environment conditions. 5. Packet Format of OFDM PHY (Physical Layer of IEEE 802.11a) The packet structure considered at the physical layer level includes, like the other packets, a synchronization part and a header, indicating the modulation type in use and the transmission characteristics to the MAC layer. Figure A.5 clearly shows this structure. Appendix A 285 Table A.3. Main parameters Parameter FHSS DSSS HR/DSSS IR Slot Time (microsec) 50 20 20 8 CCA Time (microsec) 27 <=15 <=15 5 Preamble (microsec) 96 144 144(**) 16/1 Mbps 20/2 Mbps PLCP header (microsec) 32 48 48 41/1 Mbps 25/2 Mbps MPDU max size (bytes) 4095(*) 8192 4095 2500 Modulation GFSK DBPSK DBPSK PPM DQPSK DQPSK CCK Spectrum Regulation Regulation Regulation No restriction restriction restriction restriction * Recommended value: 400 bytes – 1 Mbps, 800 bytes – 2 Mbps; this corresponds to a frame <3.5 ms. ** 144 is the preamble size for a packet with a long header, a packet with a short header has a preamble of 72 leading to an overhead of 192 µs in the first case and 96 µs in the second case. 12 1 variable symbols synchronization signal MAC frame PLCP preamble PLCP header 4 1 16 16 rate reserved size parity end bits 66 service PSDU end jamming OFDM, BPSK, R=1/2 OFDM according to the rate set in signal field Fig. A.5. IEEE 802.11 OFDM packet 5.1 Preamble It contains 12 OFDM symbols to perform the synchronization between the transmitter and the receiver. The duration of this preamble is of 20 µs. The synchronization is divided into two parts. The first part consists of sending ten OFDM training symbols lasting 0.8 µs. These symbols are sent over 12 carriers.