Subject: Mobile Communication and Computing

Topic: Wireless Local Area Networks

Difficulty: Medium

Physical-Layer Architecture

The physical layer is divided into two sub-layers: the Physical Layer Convergence Procedure (PLCP) sub-layer and the Physical Medium Dependent (PMD) sub-layer. The PLCP Figure is the glue between the frames of the MAC and the radio transmissions in the air. It adds its own header. Normally, frames include a preamble to help synchronize incoming transmissions.

Three physical layers were standardized in the initial revision of 802.11, which was published in 1997:

1. Frequency-hopping (FH) spread-spectrum radio PHY

a. All the physical layers standardized in the first draft of 802.11 in 1997, the frequency-hopping spread spectrum (FH or FHSS) layer was the first layer to see widespread deployment. b. The electronics used to support FH modulation are relatively cheap and do not have high power requirements. c. advantage to using frequency-hopping networks was that a greater number of networks could coexist, and the aggregate throughput of all the networks in a given area was high

Preamble

As in a wired Ethernet, the preamble synchronizes the transmitter and receiver and derives common timing relationships. In the 802.11 FH PHY, the Preamble is composed of the Sync field and the Start Frame Delimiter field.

Sync

The sync field is 80 bits in length and is composed of an alternating zero-one sequence (010101...01). Stations search for the sync pattern to prepare to receive data. In addition to synchronizing the sender and receiver, the Sync field serves three purposes. First of all, the presence of a sync signal indicates that a frame is imminent. Second, stations that have multiple antennas to combat multipath fading or other environmental reception problems can select the antenna with the strongest signal. Finally, the receiver can measure the frequency of the incoming signal relative to its nominal values and perform any corrections needed to the received signal.

Start Frame Delimiter (SFD)

As in Ethernet, the SFD signals the end of the preamble and marks the beginning of the frame. The FH PHY uses a 16-bit SFD: 0000 1100 1011 1101.

Header The PLCP header follows the preamble. The header has PHY-specific parameters used by the PLCP. Three fields comprise the header: a length field, a speed field, and a frame check sequence.

PSDU Length Word (PLW)

The first field in the PLCP header is the PLW. The payload of the PLCP frame is a MAC frame that may be up to 4,095 bytes long. The 12-bit length field informs the receiver of the length of the MAC frame that follows the PLCP header.

PLCP Signaling (PSF)

Bit 0, the first bit transmitted, is reserved and set to 0. Bits 1-3 encode the speed at which the payload MAC frame is transmitted. Several speeds are available, so this field allows the receiver to adjust to the appropriate demodulation scheme. Although the standard allows for data rates in increments of 500 kbps from 1.0 Mbps to 4.5 Mbps, the modulation scheme has been defined only for 1.0 Mbps and 2.0 Mbps.

Header Error Check (HEC)

To protect against errors in the PLCP header, a 16-bit CRC is calculated over the contents of the header and placed in this field. The header does not protect against errors in other parts of the frame.

2. Direct-sequence (DS) spread-spectrum radio PHY

PLCP Framing and Processing

The PLCP for the DS PHY adds a six-field header to the frames it receives from the MAC. In keeping with ISO reference model terminology, frames passed from the MAC are PLCP service data units (PSDUs). The PLCP framing

Preamble

The Preamble synchronizes the transmitter and receiver and allows them to derive common timing relationships. It is composed of the Sync field and the Start Frame Delimiter field. Before transmission, the preamble is scrambled using the direct-sequence scrambling function.

Sync

The Sync field is a 128-bit field composed entirely of 1s. Unlike the FH PHY, the Sync field is scrambled before transmission.

Start Frame Delimiter (SFD)

The SFD allows the receiver to find the start of the frame, even if some of the sync bits were lost in transit. This field is set to 0000 0101 1100 1111, which is different from the SFD used by the FH PHY.

Header

The PLCP header follows the preamble. The header has PHY-specific parameters used by the PLCP. Four fields comprise the header: a signaling field, a service identification field, a Length field, a Signal field used to encode the speed, and a frame-check sequence.

Signal

The Signal field is used by the receiver to identify the transmission rate of the encapsulated MAC frame. It is set to either 0000 1010 (0x0A) for 1-Mbps operation or 0001 0100 (0x14) for 2-Mbps operation.

Service

This field is reserved for future use and must be set to all 0s.

Length

This field is set to the number of microseconds required to transmit the frame as an unsigned 16-bit integer, transmitted least-significant bit to most-significant bit.

CRC

To protect the header against corruption on the radio link, the sender calculates a 16-bit CRC over the contents of the four header fields. Receivers verify the CRC before further frame processing.