MHz bandwidth providing 108 OFDM data tones. For fewer spatial streams, the data rates are simply proportional to the number of streams. As for 802.11a/g, these data rates are achieved with a symbol period of 4.0 uS. A further data rate increase of 10/9 (e.g. from 540 to 600 Mbps) is achieved in an optional short guard interval mode, which reduces the symbol period to 3.6 uS by halving the inter-symbol guard interval from 800 nS to 400 nS. Wireless LAN Standards Table 6-14: 802.11n OFDM Modulation Methods, Coding and Data Rate Modulation Code bits per Code bits per Coding Data bits Data rate subcarrier symbol per symbol (Mbps) (per stream) (all streams) (all streams) 16-QAM 16-QAM 64-QAM 64-QAM 64-QAM MAC framing and acknowledgement overheads will also need to be reduced in order to increase MAC efficiency (defined as the effective data rate at the MAC SAP as a fraction of the PHY layer data rate). With the current MAC overhead, a PHY layer data rate approaching 500 Mbps would be required to deliver the target 100 Mbps data rate at the MAC SAP. Mesh Networking (802.11s) As described in the Section "The 802.11 WLAN Standards, p. 139", the 802.11 topology relies on a distribution system (DS) to link BSSs together to form an ESS. The DS is commonly a wired Ethernet linking access points (Figure 6-3), but the 802.11 standard also provides for a wireless distribution system between separated Ethernet segments by defining a four-address frame format that contains source and destination station addresses as well as the addresses of the two access points that these stations are connected to, as shown in Figure 6-9. The objective of the 802.11s Task Group, which began working in 2004, is to extend the 802.11 MAC as the basis of a protocol to establish a wireless distribution system (WDS) that will operate over self-configuring multi-hop wireless topologies, in other words an ESS mesh. Chapter Six LAN segment A 802.3 Ethernet Sta 4 Sta 1 frame Four address format 802.11 frame Sta 1 Sta 2 LAN segment B WDS link Sta 3 Sta 4 