## 3 Wireless Access-Metro Networks
### WiFi  
Another wireless network standard, known as WiFi, is IEEE 802.11 [20]. Due to the use of unlicensed frequency bands (2.4 GHz with 14 distinct channels) in IEEE 802.11b/g, providing up to 11/54 Mbps data rate, WiFi networks have gained much attention.  
Next-generation WLANs offer a throughput of at least 100 Mb/s measured at the MAC service access point (SAP). The IEEE 802.11n draft 5.0 provides both PHY and MAC enhancements [22]. As shown in Fig. 3, the main MAC enhancement of 802.11n is frame aggregation which comes in two flavors [23]:  
* **A-MSDU:** Aggregate MAC service data unit (A-MSDU) is used to join multiple MSDU sub-frames (see Fig. 3(a)). Specifically, a high-throughput (HT) wireless station (STA) packs multiple MSDUs with possibly different destination addresses into one MAC protocol data unit (MPDU) and sends it to the HT access point (AP). An HT AP is allowed to pack multiple MSDUs into one MPDU, whereby all constituent MSDU subframes must be destined to the same HT STA. In either direction, all constituent MSDU subframes must have the same traffic identifier (TID) value (_i.e._, same QoS level).
* **A-MPDU:** Aggregate MAC protocol data unit (A-MPDU) is used to join multiple MPDU sub-frames (see Fig. 3(b)). Specifically, multiple MPDUs with the same receiver address are packed into one PHY service data unit (PSDU).  
It is important to note that both A-MSDU and A-MPDU require only a single PHY preamble and PHY header. In A-MSDU, the PSDU includes a single MAC header and frame check sequence (FCS),  
Figure 3: Frame aggregation schemes in next-generation WLAN: (a) A-MSDU, and (b) A-MPDU.  
as opposed to A-MPDU where each MPDU subframe contains its own MAC header and FCS. Due to the resultant lower overhead, A-MSDU is able to achieve a higher throughput than A-MPDU for error-free channels. For error-prone channels, however, the throughput of A-MSDU decreases quickly which indicates the lower robustness of A-MSDU. Thus, adaptively using A-MSDU under good channel conditions and A-MPDU under bad channel conditions yields better performance [24]. Note that A-MPDU and A-MSDU can be used separately or jointly. It was shown in [23] that joint two-level aggregation is able to achieve a higher throughput efficiency. In this approach, the maximum size of A-MSDU is restricted to 4095 octets. Finally, it is worthwhile to note that fragmentation of MSDUs and A-MSDUs with the same receiver address is allowed.  
Although most deployed WLANs are based on the IEEE standard 802.11a/b/g or the emerging IEEE standard 802.11n, the recent amendment IEEE 802.11s provides mesh capability by converging the advantages of different routing protocols [25]. IEEE 802.11s, also known as wireless mesh network (WMN) defines the hybrid wireless mesh protocol (HWMP) which is inspired by a combination of on-demand and tree-based pro-active routing algorithms [26]. While ad-hoc on-demand distance vector (AODV) is used for peer-to-peer transmissions between fixed or mobile mesh nodes, called mesh points (MPs), the tree-based routing protocol is applied to provide MPs with access to the wired distribution system (DS) through fixed gateway nodes, called mesh portal points (MPPs) [27]. In WMNs, the mesh access point (MAP) is a special type of MP equipped with the additional capability of an AP to provide service to STAs. In addition to routing, WMNs face several challenges such as security and QoS, which must be considered and addressed in IEEE 802.11s [28]. Recently, IEEE 802.11s and its challenges have been discussed in greater detail in [29] and some experimental work has been reported in [30], [31], and [32].