The local multipoint distribution service (LMDS) and multichannel multipoint distribution service (MMDS) have their historical roots in television. MMDS's pre-cursor, the multipoint distribution service (MDS), was established by the Federal Communications Commission (FCC) in 1972. The Commission originally thought MDS would be used primarily to transmit business data. However, the service became increasingly popular for transmitting entertainment programming. Unlike conventional broadcast stations, whose transmissions are received universally, MDS programming is designed to reach only a subscriber-based audience.
In 1983 the Commission reassigned eight channels from the instructional television fixed service (ITFS) to MDS. These eight channels make up MMDS. MDS and MMDS channels are frequently used in combination with ITFS channels to provide video-entertainment programming to subscribers. This service is known as wireless cable.
LMDS is a fixed broadband line-of-sight, point-to-multipoint, microwave system, which operates at a high frequency (typically within specified bands in the 24-40GHz range) and can deliver at a very high capacity, depending on the associated technologies. Given the complexity of the equipment required (and the power needed to deliver signals) both of these technologies are regarded as prohibitively expensive for the consumer market. Therefore, LMDS operators will initially be targeting enterprises and network operators, although the consumer market is likely to emerge over time as the cost of CPE comes down (partly driven by the take-up of IP). It should be noted that CPE costs $5,000 for LMDS in the 26GHz range.
MMDS allows two-way voice, data and video streaming. It operates at a lower frequency than LMDS (typically within specified bands in the 2-10GHz range) and therefore has a greater range and requires a less powerful signal than LMDS. MMDS is a less complicated, cheaper system to implement. As a consequence, the CPE is cheaper, thus it has a wider potential addressable market. It is also less vulnerable to rain fade - the interference caused by adverse weather conditions that can undermine the quality of the microwave signal. However, the bandwidth offered by LMDS makes this the more viable option.
LMDS and MMDS share a number of common architectural features although they vary from one manufacturer to another according to features and capabilities. The core components are a base-station transceiver (transmitter and receiver), a customer-premise transceiver and some kind of CPE network interface unit (NIU) or card.
For downstream traffic to the customers' premises, the base station converts the digital bitstream containing voice, data and video information into microwaves that are transmitted to a small antenna on the customer's premises. The microwaves are then reconverted back into a digital bitstream by the NIU and delivered to the end-user. The process is reversed for upstream traffic. When the base station receives the microwave signal and has converted it into a digital bitstream, this is routed through, or 'backhauled' to, the wider network, through which the data or call is delivered to its destination.
Unlike the lower frequency cellular systems, LMDS and MMDS both require a line-of-sight between the base station and customer premise transceivers. This is a prerequisite for any system operating above approximately 2-3.5GHz. The base station is connected to the wide-area network switch or internet POP via either a high-capacity wireline (usually fibre optic) or wireless. Similarly, at the customer's premises, the signal can be delivered to the end-user terminals via either of these.
Wireless systems are being deployed to fulfil a number of functions. On a network level they are suitable for both access and backbone infrastructure. It is generally agreed, however, that it is in the access market where the key advantages are held over wireline alternatives. The principal strengths of LMDS/MMDS are:
- Speed of network deployment is much quicker with wireless systems enabling rapid, early market entry
- Entry, deployment and upgrading costs are much lower than for wireline alternatives, for which engineering (cabling and trenching) costs are significantly higher
- The maintenance, management and operation expenditure is lower. Wireless systems can be rolled out much faster, enabling an earlier return on investment
- Scalable architectures enable expanded coverage and services in direct relation to the level of demand
- Only one network architecture is required to provide a full suite of interactive voice, video and data services that can be expanded as and when desired