Recently, next-generation WLAN has been proposed to offer a throughput of at least 100 Mbps measured at the MAC service access point (SAP). Although the IEEE standard 802.11n is not expected to be approved until March 2009, devices built to the current 802.11n draft are supposed to require only software upgrades to be compliant with the ratified standard. The standard draft provides not only PHY but also MAC enhancements.
PHY enhancements
Orthogonal frequency division multiplexing (OFDM) and multiple input multiple output (MIMO) antennas are proposed to be used in the PHY layer of IEEE 802.11n which are able to provide various capabilities, such as antenna diversity (selection) and spatial multiplexing. By using MIMO-OFDM and channel bonding, next-generation WLANs offer raw data rates of about 600 Mbps at the PHY layer. Moreover, using multiple antennas provides multipath capability which increases both throughput and transmission range. The enhanced PHY layer applies two adaptive coding schemes: space time block coding (STBC) and low density parity check coding (LDPC). IEEE 802.11n is able to co-exist with IEEE 802.11 legacy standards. However, in greenfield deployments (i.e., only 802.11n devices) of next-generation WLANs it is possible to increase the channel bandwidth from 20 MHz to 40 MHz via channel bonding, resulting in significantly increased raw data rates.
MAC enhancements
Similar to IEEE 802.11e , the MAC layer of IEEE 802.11n uses the hybrid coordination function (HCF) for providing absolute and relative QoS. HCF uses a contention based channel access method, known as enhanced distributed channel access (EDCA), and a polling-based HCF-controlled channel access (HCCA) method concurrently. To achieve a MAC throughput of 100 Mbps and higher, next-generation WLANs allow for the truncation of transmission opportunities (TXOPs), reverse direction (i.e., bidirectional TXOP), and use of a reduced interframe space (RIFS) to decrease the dead time between frames (RIFS may be used only in greenfield deployments). The most important MAC enhancement of next-generation WLANs is frame aggregation. As shown in Fig. 1, the following two methods may be deployed for frame aggregation in next-generation WLANs: (i) aggregate MAC protocol data unit (A-MPDU), and (ii) aggregate MAC service data unit (A-MSDU). A-MPDU concatenates up to 64 MPDU subframes into a single physical layer SDU, provided all constituent MPDUs are destined to the same receiver. A-MSDU concatenates multiple MSDU subframes into a single MPDU, whereby all constituent MSDUs not only have to be destined to the same receiver but also must have the same traffic identifier (TID), i.e., the same QoS level. A-MPDU and A-MSDU can be used separately or jointly to increase the MAC throughput of next-generation WLANs.
 Fig. 1. Frame aggregation schemes in next-generation WLAN: (a) A-MSDU, and
(b) A-MPDU.
In IEEE 802.11n, high-throughput (HT) STA and HT AP are able to provide QoS. If a HT STA, which gains access to the channel using EDCA, runs out of frames to transmit, a TXOP holding HT STA can transmit a contention free end (CF-End) frame to truncate the TXOP and thereby improve bandwidth-efficiency. The reverse direction (RD) protocol provides bidirectional TXOP connections. More precisely, during an RD exchange sequence, the RD initiator can transmit PHY protocol data units (PPDUs) and obtain response PPDUs from the RD responder in a single TXOP. In the HT control field of an IEEE 802.11n PPDU, the RD grant (RDG)/More PPDU field is used to indicate the RD permission and the last transferred PPDU. In EDCA, an RD responder must transmit the same type of AC data frames as received, while in HCCA an RD responder is allowed to transmit data frames of any TID
Further reading
- IEEE P802.11n, Draft 5.0, “Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications: Enhancements for Higher Throughput,” May 2008.
- IEEE 802.11n Task Group, http://www.ieee802.org/11/Reports/tgn_update.htm
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