Presentation at Wireless LANs course, Tampere University of Technology, Finland 802.11 Physical Layers
DSSS-OFDM extention uses the preambule of of IEEE 802.11b for backwards compatibility. DSSS - direct sequence spread spectrum PBCC - Packet Binary Convolutional Code
IEEE 802.11g is the first standard for wireless data communications in the 2.4 GHz band that is not spread spectrum technology. SIFS – short interframe spacing (RTS – SIFS – CTS – Data, I.e .the distance between clear to send and data transmission To give the equipment for 802.11g the same time for convolutional decoding, the SIFS is followed by 6 us of sielence, so called virtual extension of SIFS MAC – media access control The backward-compatibility with 802.11b CCA – clear channel assessment
ERP – extended rate physical layer A new physical layer for the 802.11 Medium Access Control (MAC) in the 2.4 GHz frequency band, known as the extended rate PHY (ERP). The ERP adds OFDM as a mandatory new coding scheme for 6, 9, 12 and 24 Mbps (mandatory speeds), and 18, 36, 48 and 54 Mbps (optional speeds). The ERP includes the modulation schemes found in 802.11b including CCK for 11 and 5.5 Mbps and Barker code modulation for 2 and 1 Mbps. PBCC - packet binary convolutional coding An ERP-PBCC station may implement 22 Mbit/s alone or 22 and 33 Mbit/s. The radio portion of ERP systems implements all mandatory modes of IEEE 802.11a and IEEE 802.11b, except it uses the 2.4 GHz frequency band and canalization plan specified in IEEE 802.11b standard. The ERP has the capability to decode all previous standards (except IEEE 802.11a) PLCPs and all ERP-OFDM PLCPs. In addition, it is mandatory that all ERP-compliant equipment be capable of sending and receiving the short preamble ( section 3.3) that is -and remains- optional for IEEE 802.11b PHYs. An ERP BSS is capable of operating in any combination of available ERP modes and Non-ERP modes (previous standards PHYs). For example, a BSS could operate in an ERP-OFDM-only mode, a mixed mode of ERP-OFDM and ERP-DSSS/CCK, or a mixed mode of ERP-DSSS/CCK and Non-ERP. When options are enabled, combinations are also allowed.
ERP – extended rate physical layer A new physical layer for the 802.11 Medium Access Control (MAC) in the 2.4 GHz frequency band, known as the extended rate PHY (ERP). The ERP adds OFDM as a mandatory new coding scheme for 6, 9, 12 and 24 Mbps (mandatory speeds), and 18, 36, 48 and 54 Mbps (optional speeds). The ERP includes the modulation schemes found in 802.11b including CCK for 11 and 5.5 Mbps and Barker code modulation for 2 and 1 Mbps. PBCC - packet binary convolutional coding An ERP-PBCC station may implement 22 Mbit/s alone or 22 and 33 Mbit/s.
PLCP - physical layer convergence protocol PPDU - protocol data unit BSS - Basic Service Set
PBCC – Packet Binary convolutional coding The input bits are divided into two. In each pair the first bit is fed to the upper input of the convolutional encoder and the second to the lower input The encoder output is mapped to a 8-PSK constellation => 2 information bits per symbol (rate of 2/3) An advantage PBCC has over CCK is that its 'convolutional coding' is a method of forward error correcting that enables you to reduce the bit-rate error without increasing your transmission power. In real life, this means you can get a higher data transmission rate and expand your range, all while not using any more power than a conventional 802.11b device. As the system is frame (PPDU) based, the encoder shall be in state zero; i.e., all memory elements contain zero, at the beginning of every PPDU. The encoder shall also be placed in a known state at the end of every PPDU to prevent the data bits near the end of the PPDU from being decoded incorrectly. This is achieved by appending one octet containing all zeros to the end of the PPDU prior to transmission and discarding the final octet of each received PPDU.An encoder block diagram is shown in the figure. It consists of two paths of four memory elements each. For every pair of data bits input, three output bits are generated. The output of the convolutional code is mapped to an 8-PSK constellation; each 3-bit output sequence from the packet binary convolutional encoder is used to produce one symbol. This yields a throughput of two information bits per symbol. In ERP-PBCC-22 and ERP-PBCC-33, the input data stream is divided into pairs of adjacent bits. In each pair, the first bit is fed to the upper input of the convolutional encoder, and the second is fed to the lower input of the convolutional encoder. An illustration of the mapping for the jth (j≥0) pair of input bits (b2j, b2j+1) is given in the figure The mapping from the outputs to 8-PSK constellation points is determined by a pseudo-random cover sequence. The cover sequence is the same one as described in 802.11b. The current binary value of this sequence at every given point in time is taken as shown in figure with the constellation.
The tail is 3 clock cycles at 11 Mchip/s and the head is 3 clock cycles at 16.5 Msymbol/s (QPSK). The resync is 9 clock cycles at 16.5 Msymbol/s. The total clock switching time (tail and head and resync) is 1 µs. The tail bits are 1 1 1, the head bits are 0 0 0, and the resync bits are 1 0 0 0 1 1 1 0 1. The modulation is BPSK, which is phase synchronous with the previous symbol.
PSDU - Physical Layer Service Data Unit. It represents the contents of the PPDU (i.e., the actual 802.11 frame being sent).
PPDU – Physical protocol data units SFD – The Start Frame Delimiter PLCP - physical layer convergence protocol PLCP is transmitted with 3 Mb/s. It is the same as the one in 802.11b The PSDU format is similar to the one in 802.11a. The scrambler in 802.11b is used to scramble the PLCP header and the scrambler of 802.11a is used to scramble the data symbols in the OFDM segment.
The short format is used to maximize the throughput by reducing the overhead associated with the preamble and header.
Forming the PSDU portion of the DSSS-OFDM PLCP. The figure shows an expanded view of the DSSS-OFDM PSDU. The PSDU is composed of four major sections. The first is the long sync training sequence that is used for acquisition of receiver parameters by the OFDM demodulator. The long sync training sequence for DSSS-OFDM is identical to the long training symbols described in 802.11a. The second section is the OFDM SIGNAL field that provides the demodulator information on the OFDM data rate and length of the OFDM data section. The SIGNAL field for DSSS-OFDM is identical to the SIGNAL field described in 802.11a After the SIGNAL field is the data section of the PSDU. This is identical to the modulation procedure described in 802.11a. After the data section, the PSDU for DSSS-OFDM appends a signal extension section to provide additional processing time for the OFDM demodulator. This signal extension is a period of no transmission.
There should be coherent relationship between the single carrier segment and the OFDM segment, so that the receiver has the opportunity to track through the transition without any forced parameter reacquisition. The single carrier preamble and header provide all parametric information required for demodulation of the OFDM segment to within conventional estimation-in-noise accuracy. Although multicarrier sync features are provided for convenience at OFDM segment onset, if and how to use the multicarrier sync for reacquisition is an implementer’s decision. Multicarrier sync is not necessary. The packet is coherent throughout.
Constant in this context means that the same clock crystal that sets the frequencies and timing of each part is the same through the transition. This allows the frequency and timing tracking loops to work undisturbed through the transition.
at certain ranges IEEE 802.11g uses the OFDM (802.11a close) and in certain DSSS (802.11b away)
AP – access point Throughput is not the same as data rate for networking systems, because of overhead, environment, and network composition. The throughput of 802.11g products can depend on whether there are 802.11b products nearby. Performance is best in environments where an 802.11g access point (AP) is only communicating with 802.11g clients in a homogeneous WLAN. In these environments, the data rate within 75 feet is 54 Mbps and the throughput is 22–24 Mbps when using Transmission Control Protocol (TCP). In the interest of maximizing performance in the presence of 802.11b products, the 802.11g APs coordinate the use of the transmission medium with protection mechanisms Because the protection mechanisms require overhead communication, compatibility is provided at the expense of throughput. The CTS-to-self protection mechanism lowers the maximum TCP throughput to approximately 15 Mbps, as shown in the table. Protection Mechanism: Air Traffic Control: The 802.11g standard provides protection mechanisms for managing communication in a mixed 802.11b/g environment. The 802.11b radios do not hear when the airspace is busy with 802.11g OFDM signals. Protection mechanisms prevent 802.11b clients from transmitting after improperly assessing that the airspace is empty while 802.11g OFDM signals are being transmitted. The 802.11g products still communicate at the same 802.11g OFDM data rates when protection is in use, but a short 802.11b rate message signals to 802.11b products to not transmit for a specified duration because an 802.11g OFDM message is immediately following. The 802.11b protection messages cause signalling overhead and result in reduced throughput to the user. The AP directs clients to use protection through a signalling mechanism specified within the 802.11g standard. Enterprise-class 802.11g APs may allow users to tune the protection mechanism algorithm to optimize network system performance (some 802.11g APs may allow the administrator to override the use of the protection mechanism for a performance improvement in light traffic networks). The 802.11g standard allows 802.11g clients to use one of several protection mechanisms in a mixed 802.11b/g environment. The Wi-Fi Alliance will test for one of two signalling methods: RTS/CTS and CTS-to-self. Request to send (RTS) is analogous to a pilot’s take off request to an air traffic control tower—the pilot waits to use the airspace until verifying with the control tower that the airspace is clear. The clear to send (CTS) message is like the clearance from the tower. The CTS-to-self protection mechanism method sends a CTS message using an 802.11b rate to clear the air, and then immediately follows with data using an 802.11g date rate. The CTS-to-self protection mechanism provides a maximum TCP throughput of 14.7 Mbps. With any of the protection mechanisms, 802.11g throughput is still greater than 802.11b throughput at the same distance.
802.11g-ONLY When the AP and all clients are 802.11g, communication occurs at the highest possible TCP throughput. The 802.11g AP detects that all of the clients are 802.11g and instructs the network not to use any protection method. Without a protection mechanism engaged, throughput of 24 Mbps or greater is possible. 802.11g AP, MIXED CLIENTS When the AP is 802.11g and there is a mixture of 802.11g clients and 802.11b clients, the AP senses both technologies on the network. The 802.11g AP instructs 802.11g clients to use a protection mechanism. Effectively, 802.11g clients function at reduced 802.11g TCP throughput (up to 15 Mbps), which is faster than the 802.11b client that communicates at a maximum throughput of up to 5.8 Mbps. 802.11b AP, 802.11g CLIENT When the AP is 802.11b and the client is 802.11g, the 802.11g client is able to successfully associate and communicate with the 802.11b AP. Communication between the AP and the 802.11g client uses CCK modulation and achieves typical 802.11b speeds. An 802.11g client can always function as an 802.11b client.
MULTIPLE 802.11g APS, MIXED CLIENTS When there are multiple 802.11g APs and a single 802.11b client on the same channel, all overlapping 802.11g APs signal the use of the protection mechanism. Effectively, 802.11g clients function at a reduced 802.11g TCP throughput (up to 15 Mbps), which is faster than the 802.11b client, which communicates at a typical 802.11b TCP throughput (up to 5.8 Mbps). The APs can also be configured to use different channels for their 802.11g clients so that the 802.11g-only networks do not need to use a protection mechanism. This allows 802.11g clients to have full TCP throughput as if they were in an 802.11g-only network
Introduction <ul><li>After IEEE 802.11a and IEEE 802.11b standards were approved, a work on higher data rate physical layer started. </li></ul><ul><li>Two major competitive proposals: </li></ul><ul><ul><li>Extension of PBCC </li></ul></ul><ul><ul><li>DSSS-OFDM, which uses preamble of IEEE 802.11b for backwards compatibility </li></ul></ul><ul><li>IEEE 802.11g is a compromise - mandatory and optional mechanisms </li></ul>
Introduction (cont.) <ul><li>802.11g is very similar to IEEE 802.11a, especially from PHY point of view but </li></ul><ul><ul><li>IEEE 802.11a 5 GHz </li></ul></ul><ul><ul><li>IEEE 802.11g 2.4 GHz </li></ul></ul><ul><li>Very small differences between the mandatory PHY of 802.11g and 802.11a </li></ul><ul><ul><li>SIFS for 802.11a is 16 us, whereas for 802.11g it is 10us </li></ul></ul><ul><ul><li>An 802.11g packet is followed by 6 us of silence </li></ul></ul><ul><li>Backwards-compatible with 802.11b – compatibility achieved at MAC layer </li></ul>
ERP (Extended Rate PHY) <ul><li>ERP builds on the payload data rates of 1 and 2 Mbit/s, as described in original IEEE 802.11 using DSSS modulation </li></ul><ul><li>DSSS, CCK and optional PBCC modulation is used for building on the payload data rates of 1, 2, 5.5, and 11 Mbit/s, as described in 802.11b </li></ul><ul><li>The ERP draws from IEEE 802.11a to provide additional payload data rates of 6, 9, 12, 18, 24, 36, 48, and 54 Mbit/s. </li></ul><ul><li>Of these rates, transmission and reception capability for 1, 2, 5.5, 11, 6, 12, and 24 Mbit/s data rates is mandatory. </li></ul><ul><li>Two optional ERP-PBCC modulation modes with payload data rates of 22 and 33 Mbit/s are defined. </li></ul><ul><li>An optional modulation mode DSSS-OFDM is also incorporated with payload data rates of 6, 9, 12, 18, 24, 36, 48, and 54 Mbit/s. </li></ul>
ERP (cont.) <ul><li>The 2.4 GHz ISM band is a shared medium, and coexistence with other devices such as IEEE 802.11 and IEEE 802.11b standards is an important issue for maintaining high performance in ERP. </li></ul><ul><li>The ERP modulations (ERP-OFDM, ERP-PBCC, and DSSS-OFDM) have been designed to coexist with existing IEEE 802.11 and IEEE802.11b. </li></ul>
ERP Mandatory operational modes <ul><li>DSSS/CCK – uses the capabilities of 802.11b PHY with the following exceptions: </li></ul><ul><ul><li>Support of short PLCP PPDU header format is mandatory </li></ul></ul><ul><ul><li>Maximum signal input level is –20 dBm </li></ul></ul><ul><ul><li>Locking the Tx central frequency and the symbol clock frequency to the same reference oscillator is mandatory </li></ul></ul><ul><li>OFDM – uses the capabilities of 802.11a PHY with the following exceptions </li></ul><ul><ul><li>Frequency plan is according to 802.11b instead of 802.11a </li></ul></ul><ul><ul><li>The frequency accuracy is 25 ppm </li></ul></ul><ul><ul><li>Maximum input signal level is –20 dBm </li></ul></ul><ul><ul><li>Time slot is 20 us as in 802.11b, except that an optional 9 us slot time might be used when BSS consists only of EPR standards </li></ul></ul><ul><ul><li>SIFS (Short interframe space) time is 10 us in accordance with 802.11b </li></ul></ul>
PBCC <ul><li>In the PBCC encoder, incoming data are first encoded with a packet binary convolutional code. A cover code (as defined in PBCC modes in 802.11b ) is applied to the encoded data prior to transmission through the channel. </li></ul><ul><li>Achieves data rates of 22 and 33 Mb/s </li></ul><ul><li>PBCC-22 </li></ul><ul><ul><li>uses 256 state binary code with rate of 2/3 and a cover sequence </li></ul></ul><ul><ul><li>The input bits are divided into adjacent bits </li></ul></ul><ul><li>PBCC-33 achieves the higher data rate by increasing the clock frequency by 50% from 11 MHz to 16.5 MHz only for the data portion of the packet. </li></ul>22/33 Mbit/s ERP-PBCC convolutional encoder PBCC-22 and PBCC-33 cover code mapping
PBCC (cont.) <ul><li>When the clock is switched from 11 MHz to 16.5 MHz, the clock switching structure in the figure below is used. </li></ul>PBCC 33 Mbit/s clock switching
DSSS-OFDM <ul><li>DSSS-OFDM - Hybrid modulation combining a DSSS preambule and header with an OFDM payload transmission </li></ul><ul><li>As a result, for DSSS-OFDM, the PPDU format described in 802.11b is relatively unchanged. The major change is to the format of the PSDU. </li></ul><ul><li>The 802.11b single carrier PSDU is replaced by a PSDU that is very similar to the PSDUs described in 802.11a. </li></ul><ul><li>In addition, 802.11g specifies the radio and physical layer behavior of the transition from the Barker symbol-modulated preamble and the OFDM-modulated data for PSDU. </li></ul>
DSSS-OFDM PPDU Format <ul><li>Long preamble PPDU format for DSSS-OFDM </li></ul>
DSSS-OFDM PPDU Format <ul><li>Short preamble PPDU format for DSSS-OFDM </li></ul>
DSSS-OFDM PLCP PSDU Encoding process <ul><li>DSSS-OFDM PSDU </li></ul>
Single carrier to multicarrier transition <ul><li>The single carrier signal segment of the packet shall have a coherent relationship with the multicarrier (OFDM) segment of the packet. </li></ul><ul><li>All characteristics of the signal shall be transferable from one symbol to the next, even when transitioning to the OFDM segment. </li></ul><ul><li>This enables high-performance, coherent receiver operation across the whole packet. This requirement is no different in nature than that stated in 802.11, 802.11a, and 802.11b. The distinction being that those clauses use a signalling scheme that is either just single carrier or just multicarrier. In contrast, for this mode, both single carrier and multicarrier signalling are used within the context of a single packet. </li></ul>
Single carrier to multicarrier transition <ul><li>The ideal transition would provide </li></ul><ul><ul><li>a constant carrier frequency and phase, </li></ul></ul><ul><ul><li>constant power </li></ul></ul><ul><ul><li>constant spectrum </li></ul></ul><ul><ul><li>constant timing </li></ul></ul>
Baseband Practical Implementation <ul><li>Example of IEEE 802.11g implementation </li></ul>
802.11(a, b, g) comparison Standards 802.11g 802.11b 802.11a Data Rate Support 54, 48, 36, 24, 18, 12, 9, 6,11, 5.5, 2, 1 Mbps 11, 5.5, 2, 1 Mbps 54, 48, 36, 24, 18, 12, 9, 6 Mbps Max. Data Rate 54 Mbps 11 Mbps 54 Mbps Frequency Band 2.4 GHz (2.4 GHz to 2.4835 GHz) 2.4 GHz (2.4 GHz to 2.4835 GHz) 5 GHz (5.725 GHz to 5.850 GHz) Channels 3 non-overlapping channels, up to 13 overlapping 3 non-overlapping channels, up to 13 overlapping 12 non-overlapping channels Technique OFDM/CCK (6,9,12,18,24,36,48,54) OFDM (6,9,12,18,24,36,48,54) DQPSK/CCK (22, 33, 11, 5.5 Mbps) DQPSK (2 Mbps) DBPSK (1 Mbps) DQPSK/CCK (11, 5.5 Mbps) DQPSK (2 Mbps) DBPSK (1 Mbps) BPSK (6, 9 Mbps) QPSK (12, 18 Mbps) 16-QAM (24, 36 Mbps) 64-QAM (48, 54 Mbps) Max. Range* Up to 1,000 ft Up to 1,000 ft Up to 500 ft Backward Compatibility 802.11b N/A N/A Features Replacement for 802.11b with higher data rate and better security Most widely deployed today Ideal for high-density environments
References <ul><li> “Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications Amendment 4: Further Higher Data Rate Extension in the 2.4 GHz Band”, IEEE std 802.11g, 2003 </li></ul><ul><li> T. Cooklev, ”Wireless Communications Standards, A Study of 802.11, 802.15, and 802.16”, IEEE Press, 2004 </li></ul><ul><li> W. Carney, “IEEE 802.11g New Draft Standard Clarifies Future of Wireless LAN”, Texas Instruments White Paper, May 2002 </li></ul><ul><li> http://www.54g.org </li></ul><ul><li> http://www.vocal.com </li></ul>