Metro Area Networks (MANs)

The WMAN encompasses a range of radio- and laser-based technologies targeted at providing wireless networking over distances of a few hundred meters to several miles. Wireless broadband, broadband wireless access (BWA), wireless local loop (WLL), fixed wireless, and wireless cable all refer to technologies that can be used to deliver telecommunications services over the last few miles of the network. Wireless broadband and BWA are general terms referring to highspeed wireless networking systems. WLL is derived from the wired telephony term local loop, which refers to the connection between a local telephone switch and a subscriber. WLL and fixed wireless generally refer to the delivery of voice and data services between fixed locations over a high-speed wireless medium. Some new market entrants offer mobile applications of this technology. Fixed wireless includes Local Multipoint Distribution Service (LMDS), Multichannel Multipoint Distribution Service (MMDS), Unlicensed National Information Infrastructure (U-NII) systems, and similar networks. Wireless cable usually refers to MMDS systems used to deliver television signals such as the Instructional Television Fixed Service (ITFS).

Two basic network topologies are supported by these systems. The simplest is a point-to-point system providing a high-speed wireless connection between two fixed locations. Bandwidth is not shared, but links typically require line of sight between the two antennas. The second topology is a point-to-multipoint network where a signal is broadcast over an area (called a cell) and communicates with fixed subscriber antennas in the cell. Because bandwidth in the cell is finite and is shared among all users, performance may be a concern in high-density cells. Systems of different frequencies may be combined to cover an area where terrain or other obstructions prevent full coverage.

Other than frequency, the main difference between fixed wireless systems, and cellular, WLAN, and WPAN networks is the mobility of subscriber equipment. There has been some discussion about adding support for mobile subscriber equipment to fixed wireless systems. The addition of mobility support would enable these BWA systems to potentially function as fourth-generation (4G) cellular networks, delivering subscriber speeds of several megabits. Several technical, regulatory, and commercial hurdles must still be overcome before this could become a reality, but companies such as Wi-Fi have already started examining products targeted at this potential application.^[4]

Local Multipoint Distribution Service (LMDS)

LMDS is a fixed wireless, radio-based technology. In North America, LMDS operates in the 28 to 31 GHz frequency range, but it may operate anywhere from 2 to 40 GHz in other regions. In 1998, the Federal Communications Commission (FCC) held an auction for this spectrum, dividing each geographical area into the A Block and B Block. The A Block had a bandwidth of 1.5 GHz and the B Block had a bandwidth of 150 MHz. The intent was for the auction winners to deploy high-speed voice and data communications services in the last mile. The realities of deployment have not yet lived up to that vision.

The network topology of LMDS uses a central transmitter that sends its signal over a cell with a radius of 5 km or less. Antennas are usually placed on rooftops for line of sight to the central transmitter. This is because first-generation (1G) LMDS equipment uses radio technology that is affected by hills, walls, trees, and other physical barriers. This limitation may be lessened as equipment starts to adopt more advanced spectrum utilization techniques such as orthogonal frequency division multiplexing (OFDM).

As a high-frequency outdoor radio technology, LMDS performance and range vary depending on weather conditions. LMDS has a range of less than 5 km and supports gigabit speeds, although services are usually offered at a much lower rate. The physics of the 30 GHz signal make it about a millimeter in length; this spectrum is sometimes referred to as millimeter wave spectrum. One effect of having such a small wavelength is that rain can effectively block the signal. In areas where rain is a factor, a lower frequency is required. A higher frequency allows faster data rates, but it also limits range, requiring more equipment to cover the same area as a lower-frequency technology. LMDS bandwidth in a specific area is shared among all the users like cable. To ensure end-user performance, networks must be built with enough capacity to handle sporadic peak loads and unexpected growth in the subscriber base. In addition, there are no standards governing LMDS implementations, leading to a number of incompatible proprietary solutions. Higher network deployment costs make 1G LMDS networks more suitable for high-margin business applications rather than residential use.^[5]

802.16: Protocol for WMANs

An 802.16 wireless service provides a communications path between a subscriber site and a core network (the network to which 802.16 is providing access). Examples of a core network are the public telephone network and the Internet. IEEE 802.16 standards are concerned with the air interface between a subscriber's transceiver station and a base transceiver station.

Protocols defined specifically for wireless transmission address issues related to the transmission of blocks of data over a network. The standards are organized into a three-layer architecture. The lowest layer, the physical layer, specifies the frequency band, the modulation scheme, error-correction techniques, synchronization between transmitter and receiver, the data rate, and the time-division multiplexing (TDM) structure.^[6]

IEEE 802.16 addresses first-mile applications of wireless technology to link commercial and residential buildings to high-rate core networks and thereby provide access to those networks. The 802.16 group's work has been primarily aimed at a point-to-multipoint topology with a cellular deployment of base stations, which are each tied to core networks and in contact with fixed-wireless subscriber stations.

Working Group 802.16 is now completing a draft of the IEEE 802.16 Standard Air Interface for Fixed Broadband Wireless Access Systems. The document includes a flexible Media Access Control (MAC) layer. The accompanying physical (PHY) layer is designed for 10 to 66 GHz, informally known as the LMDS spectrum. The standard is not yet final, but the draft is stable and has passed the Working Group's letter ballot, pending resolution of comments proposed to improve it. Publication is planned for late this year.^[7]

For transmission from subscribers to a base station, the standard uses the Demand Assignment Multiple Access-Time Division Multiple Access (DAMA-TDMA) technique. DAMA is a capacity assignment technique that adapts as needed to respond to demand changes among multiple stations. TDMA is the technique of dividing time on a channel into a sequence of frames, each consisting of a number of slots, and allocating one or more slots per frame to form a logical channel.

With DAMA-TDMA, the assignment of slots to channels varies dynamically. For transmission from a base station to subscribers, the standard specifies two modes of operation: one targeted to support a continuous transmission stream (mode A), such as audio or video, and one targeted to support a burst transmission stream (mode B), such as IP-based traffic. Both are TDM schemes.

Above the physical layer are the functions associated with providing service to subscribers. These functions include transmitting data in frames and controlling access to the shared wireless medium, and are grouped into the MAC layer. The MAC protocol defines how and when a base station or subscriber station may initiate transmission on the channel. Because some of the layers above the MAC layer, such as Asynchronous Transfer Mode (ATM), require quality of service (QoS), the MAC protocol must be able to allocate radio channel capacity to satisfy service demands.

In the downstream direction (base station to subscriber stations), only one transmitter is available and the MAC protocol is relatively simple. In the upstream direction, multiple subscriber stations compete for access, resulting in a more complex MAC protocol. In both directions, a TDMA technique is used in which the data stream is divided into a number of time slots.

The sequence of time slots across multiple TDMA frames that are dedicated to one subscriber forms a logical channel, and MAC frames are transmitted over that logical channel. IEEE 801.16.1 is intended to support individual channel data rates from 2 to 155 Mbps.

Above the MAC layer is a convergence layer that provides functions specific to the service being provided. For IEEE 802.16.1, bearer services include digital audio/video multicast, digital telephony, ATM, Internet access, wireless trunks in telephone networks, and frame relay. Figure 3-4 depicts how the 802.16 protocol works for WMANs.^[8]

Figure 3-4: A WMAN

Consecutive Point Network (CPN)

In a WMAN, the reliability of the network can be ensured by implementing consecutive point network (CPN) technology. Like a Synchronous Optical Network (SONET) fiber ring, the data flow of the network around the wireless ring would reverse flow in the event of a disruption in the network (see Figure 3-5). This ensures that only a limited part of the network is down due to a disruption.

Figure 3-5: CPNs-note that like a SONET ring, data flow reverses itself in case of disruption in the network.

^[4]James and Ruth LaRocca, 802.11 Demystified (New York: McGraw-Hill, 2002), 55-56.

^[5]James and Ruth LaRocca, 802.11 Demystified (New York: McGraw-Hill, 2002), 58-59.

^[6]William Stallings, "IEEE 802.16 for Broadband Wireless," Network World, www.nwfusion.com/news/tech/2001/0903tech.html, September 3, 2001.

^[7]Roger Marks, "Broadband Access: IEEE Takes on Broadband Wireless," EE Times, www.eetimes.com/story/OEG20010606S0008, January 4, 2002.

^[8]William Stallings, IEEE 802.16 for Broadband Wireless, Network World, www.nwfiision.com/news/tech/2001/0903tech.html, September 3, 2001.