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Technology Stocks : SectorTalk - Wireless Broadband - MMDS, Satellite, LMDS -- Ignore unavailable to you. Want to Upgrade?

To: SpecialK who wrote (2)	11/1/2000 5:48:59 PM
From: SpecialK	Respond to of 7

Making a Case for MMDS telecoms-mag.com Focus On Siddhartha Shankar Menon MMDS (multichannel multipoint distribution service) provides several advantages over competitive terrestrial Internet access methods because it effectively utilizes full duplex service provisioning. Despite its flexibility, however, the technology, also known as wireless cable, has some important technical drawbacks compared to DSL and cable modems. MMDS comprises eight channels in the E and F Group at 2596 MHz to 2644 MHz. There is also a bank of 125-KHz channels assigned for upstream voice and data applications as well as 28 downstream channels in the A through G Group. The downstream spectrum also includes 2686.0 MHz to 2689.875 MHz. The technology's upstream and downstream specifications reveal some technical advantages over competitive technologies. MMDS systems broadcast downstream data in a TDM format to subscriber modems, a plus given the asymmetric nature of Internet traffic. Multiplexing techniques therefore play an indispensable role in handling Internet traffic given its bursty nature; the average downstream and upstream rates can vary as much as 5:1 to 20:1. MMDS's ability to broadcast this traffic directly to subscribers via RF (radio frequency) waves instead of passing it through the switched network is particularly advantageous. While cable modem systems are also able to dedicate transmission to Internet data, many have to be built from scratch to provide full duplex usage and are more costly to implement. MMDS systems have distinctive data rate and bandwidth specifications. These systems provide a downstream raw data rate of 30 Mbps and an attainable speed of 27 Mbps, which factors in FEC (forward error correction). MMDS's downstream bandwidth capability utilizes 6-MHz channels that allow 540 users to access the system simultaneously at 50 kbps (see Figure 1). If the Internet traffic is particularly bursty and there is a sudden upsurge in demand, the system can handle up to 2700 subscribers, a loading factor of five times the typical capacity. The ratio of upstream to downstream subscribers in typical 6-MHz channels is approximately 3:1. Upstream, MMDS consists of 48 channels that are each 125 MHz wide. These transmissions have a data rate of 200 kbps with no FEC. The capacity is at least at 36 percent of the MAC (media access control) efficiency of 72 kbps, which translates into 38 active simultaneous users uploading data at an average rate of 38.4 kbps. Upstream transmissions can accommodate up to 187.5 subscribers at a five times loading factor in heavy traffic (see Figure 1). Upstream 48 channels each 125 KHz wide Raw data rate 200 kbps Capacity 36% MAC efficiency 37.5 active users, 38.4 kbps averate rate with burstiness of 20x 187.5 subscribers @ 5x loading factor For 48 channels accommodates 9,000 subscribers per 6 MHz TV channel Downstream 64-QAM 5 MHz channel Raw digital rate 30 Mbps 540 simultaneous users @ 50 kbps throughput rate 27000 subscribers @ 5x load factor The ratio of U/S to D/S in a 6 MHz channel is approximately 3:1 Fig.1 Two-way MMDS service specifications. MMDS systems utilize innovative technologies to compensate for their shared bandwidth infrastructure. MMDS data is sent as packets of payload and address data that requires each subscriber's modem to monitor the downstream flow and screen for information specifically intended for that particular user. The shared nature of the downstream flow requires an algorithm--most commonly MAC--to separate the upstream and downstream bandwidth resources among subscribers, especially when Internet traffic is heavy. The upstream traffic is sent to the POP, which typically constitutes the software and hardware used by the local ISP (servers, modems, gateways) and connects to the headend and consumer via RF wireless broadcast signals (see Figure 2). This RF upstream and downstream MMDS system is preferable to competing technologies, especially SDSL (symmetric DSL), in that its asymmetric broadcast capabilities allow it to be more responsive to two-way technical requirements and usage patterns. Perhaps the most compelling reason is that two-way RF systems specifically deployed to handle Internet traffic are the most flexible in meeting the distinct patterns of Internet users because they do not have to accommodate telephony-related traffic. In addition, these systems can operate entirely from the headend without feedback from the subscribers to prevent heavy data users from monopolizing the downstream channel capacity. MMDS is taking advantage of spectral efficiencies of digital video compression to increase the number of RF channels per system and is closing the capacity and speed gap with DSL and cable. For example, cellularization techniques utilize multiple hub sites to offer signals to geographically dispersed groups of subscribers. This augments capacity by sending different information from different cell sites using the same RF channels. Because the frequency is reused, users can send more bandwidth-intensive graphical and audio files while simultaneously conserving bandwidth. While the limitations of DSL and cable modems have been well documented, MMDS has its own technological disadvantages. One is a line-of-sight constraint, which can pose difficulties for MMDS users in some geographic terrain. Once the signal comes into contact with a physical barrier it rapidly diffuses or attenuates, and the data is lost. A second disadvantage is that xDSL generally offers faster data rates. While MMDS might have a comparatively impressive downstream rate of 27 Mbps, the upstream is relatively inadequate at 200 kbps. Coaxial cable systems offer other advantages over MMDS, including greater user capacity and available bandwidth as well as the absence of the line-of-sight constraint. Cable provides more bandwidth, especially in terms of downstream spectrum, which is in the 50-MHz to 550-MHz range. MMDS, on the other hand, only offers 6-MHz channels in comparison. Even in terms of upstream capability some cable modem systems can far exceed MMDS. For example, while optimal coaxial systems offer upstream transmissions in the 5-MHz to 35-MHz range, wireless cable is limited to channels 125 KHz wide. Cable systems were better designed to accommodate video capacity requirements, which is particularly useful for innovative high-end Internet use such as transmitting high-definition pictures and video files. However, cable systems were not designed to carry full duplex traffic, and much of the existing cable plant must be upgraded or replaced to offer this functionality. MMDS that uses two-way RF broadcasting to route Internet data proves to be more flexible than DSL or cable because the systems are dedicated to the specifications unique to Internet use. The technology is more effective and efficient than xDSL, which often funnels its traffic, at least part of the way, through the PSTN. MMDS does not burden networked corporate users with high infrastructure costs associated with updating cable systems. While the technological specifications and factors involved with MMDS are highly dynamic and complex, MMDS is the most appropriate emerging technology for two-way Internet applications. * Siddhartha Menon is a new media consultant and marketing executive at IMI Inc., a media consultancy firm. He holds a graduate degree in communication, culture and technology from Georgetown University and is a member of the International Communication Association