WLL systems come in several architectures: a PSTN-based Direct Connect network, a Mobile Telephone Switching Office/Mobile Switching Center (MTSO/MSC)-based network, and proprietary networks.
There are several key components of the PSTN Direct Connect network:
§ The PSTN-to-Radio Interconnect system, which provides the concentration interface between the WLL and the wireline network
§ The System Controller (SC), which provides radio channel control functions and serves as a performance monitoring concentration point for all cell sites
§ The Base Transceiver Station (BTS), which is the cell site equipment that performs the radio transmit and receive functions
§ The Fixed Wireless Terminal (FWT), which is a fixed radio telephone unit that interfaces to a standard telephone set acting as the transmitter and receiver between the telephone and the base station
§ The Operations and Maintenance Center (OMC), which is responsible for the daily management of the radio network and provides the database and statistics for network management and planning
An MTSO/MSC-based network contains virtually the same components of the PSTN Direct Connect network, except that the MTSO/MSC replaces the PSTN-to-Radio Interconnect system.
The key components of an MTSO/MSC-based network are:
§ Mobile Telephone Switching Office/Mobile Switching Center (MTSO/MSC), which performs the billing and database functions and provides a E1 or T1 interface to the PSTN
§ Cell Site equipment including the Base Transceiver Station (BTS)
§ Fixed Wireless Terminal (FWT)
§ Operations and Maintenance Center (OMC)
For digital systems such as GSM and CDMA, the radio control function is performed at the Base Station Controller (BSC) for GSM or the Centralized Base Site Controller (CBSC) for CDMA.
In GSM systems, there is a Base Station System Controller (BSSC), which includes the Base Station Controller (BSC) and the transcoder. The BSC manages a group of BTSs, acts as the digital processing interface between the BTSs and the MTSO/MSC, and performs GSM-defined call processing.
In CDMA systems, there is a Centralized Base Site Controller (CBSC), which consists of the Mobility Manager (MM) and the transcoder subsystems. The MM provides both mobile and fixed call processing control and performance monitoring for all cell sites as well as subscriber data to the switch.
As in PSTN-based networks, the FWT in MTSO/MSC-based networks is a fixed radio telephone unit that interfaces to a standard telephone set acting as the transmitter and receiver between the telephone and the base station.
Operations and maintenance functions are performed at the OMC. As in PSTN-based networks, the OMC in MTSO/MSC-based networks is responsible for the day-to-day management of the radio network and provides the database and statistics for network management and planning.
The PSTN Direct Connect network is appropriate when there is capacity on the existing local or central office switch. In this case, the switch continues to provide the billing and database functions, the numbering plan, and progress tones. The MTSO/MSC architecture is appropriate for adding a fixed subscriber capability to an already existing cellular mobile network or for offering both fixed and mobile services over the same network.
While MTSO/MSC-based and PSTN Direct Connect networks are implemented using existing cellular technologies, proprietary WLL solutions are designed specifically as replacements for wireline-based local loops. One of these proprietary solutions is Nortel's Proximity I, which is used in the UK to provide wireline-equivalent services in the 3.5-GHz band. The TDMA-based system was designed in conjunction with U.K. public operator Ionica, which is the source of the "I" designation. The I Series provides telecommunications service from any host network switch, providing toll quality voice, data, and fax services. The system is switch independent and is transparent to DTMF tones and switch features.
The Proximity I system architecture consists of the following main elements:
§ Residential service system (RSS), which is installed at the customer premises and provides a wireless link to the base station
§ Base station, which provides the connection between the customer's RSS and the PSTN
§ Operations, Administration, and Maintenance system, which provides such functions as radio link performance management and billing
Residential Service System (RSS). The RSS offers two lines, which can be assigned for both residential and home office use, or for two customers in the same 2-km area. Once an RSS is installed, the performance of the wireless link is virtually indistinguishable from a traditional wired link. The wireless link is able to handle high-speed fax and data via standard modems, as well as voice. The system supports subscriber features such as call transfer, intercom, conference call, and call pick-up.
The RSS has several components: a transceiver unit, residential junction unit (RSU), network interface unit, and power supply. The transceiver unit consists of an integral 30 cm octagonal array antenna with a radio transceiver encased within a weatherproof enclosure. The enclosure is mounted on the customer premises and points toward the local base station.
The RJU goes inside the house where it interfaces with existing wiring and telephone equipment. The Proximity I system supports two 32-Kbps links for every house, enabling subscribers to have a voice conversation and data connection for fax or Internet access at the same time. At this writing, work is under way to develop systems that can handle ISDN speeds of 64 Kbps and beyond. Further developments will result in RSSs that can handle more lines per unit for medium-sized businesses or apartment blocks.
The network interface unit, mounted internally or externally, is a cable junction box that accepts connections from customer premises wiring. The unit also provides access for service provider diagnostics and contains lightening protection circuitry.
The power unit is usually mounted internally and connects to the local power supply (110/220V AC). The power unit provides the DC supply to the transceiver unit. A rechargeable battery takes over in the event of a power failure and is capable of providing 12 hours of standby and 30 minutes of talk time.
Base station. The base station contains the radio frequency equipment for the microwave link between the customer's RSS and the PSTN, along with subsystems for call-signal processing, frequency reference, and network management. This connection is via radio to the RSS and by microwave radio, optical fiber, or wireline to the local exchange. The base station is modular and can be configured to meet a range of subscriber densities and traffic requirements. The base station has several components: transceiver microwave unit, cabinet, power supply, and network management module.
The base station's dual antenna transceiver microwave unit provides frequency conversion and amplification functions. Each unit provides three RF channels, the frequency of which can be set remotely. The unit can be configured for a maximum of 18 RF channels. The antennas are available in omnidirectional or sectored configurations, depending on population densities and geographical coverage. An omnidirectional system can support 600 or more customers, while a trisectored antenna can serve more than 2000 customers. Base stations in rural areas can be sited up to 20 km from a subscriber's premises.
The base station can be configured with either an internal or external cabinet. The internal cabinet is for location in an equipment room, while the external cabinet is weather-sealed and vandal-proofed for outside locations. Both types of cabinets house the integrated transceiver system, transmission equipment, optional power system, and batteries. A separate power cabinet provides DC power to the base station from the local 110/220-V AC source. This cabinet may include battery backup with battery management capability and power distribution panel that provides power for technicians' test equipment. The network management module is the base station polls individual RSS units to flag potential service degradation. Reports include link bit error rate (BER), signal-to-noise ratio, power supply failure, and the status of the customer standby battery.
The connection from the base stations to the local exchange on the PSTN is via the V5.2 open standard interface. In addition to facilitating interconnections between multivendor systems, this interface enables operators to take full advantage of Proximity I's ability to maximize spectrum utilization through allocation of finite spectrum on a dynamic per-call basis, rather than on a per-customer basis. Concentration allows the same finite spectrum to be shared across a much larger number of customers, producing large savings in infrastructure, installation, and operations costs for the network operator.
Operations, administration, and maintenance. OA&M functions are implemented through an element manager accessed through a field engineering terminal. In Nortel's Proximity I, the element manager is built around Hewlett Packard's OpenView. Communications with the network of base stations and customer equipment is done through the Airside Management Protocol, which is based on the OSI Common Management Information Protocol (CMIP). The field engineering terminal can operate in a remote operations center, but is primarily intended for use by on-site maintenance engineers who are responsible for the proper operation of the base stations.
All the applications software in the customer premises equipment is downloadable from the element manager. This software provides the algorithms which convert analog voice signals into 32-Kbps digital ADPCM, which provides toll quality voice transmission. Other applications software includes algorithms for controlling the draw of battery-delivered power, in the event of a 110/220-V AC power failure.
Via the Air Interface Protocol, the customer equipment is able to provide the element manager with information about its current status and performance, the most useful of which are measurements taken during the transmission of speech. This allows the management system to flag performance degradation for corrective action.
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