 link, and network layers. The PHY layer, also known as the pipe, deals with the physical means of sending data over a communi- cations medium. The MAC layer is responsible for efficiently controlling access to the pipe and efficiently sharing it among many users. The data link layer employs procedures and protocols to carry data across the link and ensures reliability by detecting and correcting transmission errors. The network layer is responsible for routing within the wireless network and for determining how data packets are transferred between modems. Flash-OFDM provides one such solution. Flash-OFDM has a spectrally efficient, high-capacity PHY layer and uses a packet-switched air interface. It also has a contention-free, QoS-aware MAC layer. It provides support for interactive data applications, including voice, and efficient operation using all existing Internet protocols (such as TCP/IP). It also offers full vehicular mobility, and it offers low costs for subscribers. The Flash-OFDM design, shown in Figure 15.1, produces a business model that might attract current and future users of 2.5G and 3G systems. For example, while most 3G services require at least 5MHz of bandwidth, Flash-OFDM's signals need only 1.25MHz, which translates into a lower cost per subscriber. From a PHY- layer perspective, Flash-OFDM is orthogonal and has comparatively fewer critical requirements for power control in the multiuser mobile environment. In addition, Flash-OFDM subscribers can be "active" (i.e., with always-on IP) without con- stantly communicating with base stations simply to maintain the communication link, which reduces both interference and power requirements. Flash-OFDM's MAC layer is extremely efficient in allocating bandwidth among subscribers, thereby increasing bandwidth utilization and providing built-in hooks for QoS WMANs Public Internet Radio Router Base Station Gateway IP Router All-IP Converged Voice and Data Network IP Router Flashview Element Manager 1.25MHz Paired Radio Router 1.25MHz Paired 1.25MHz Pa