Wireless Local Loop

A wireless local loop (WLL) is a generic term for an access system that uses a wireless link to connect subscribers to their local exchange in place of conventional copper cabling. Wireless local loop—also known as fixed wireless access (FWA), or simply fixed radio—entails the use of analog or digital radio technology to provide telephone, facsimile and data services to business and residential subscribers. Depending on the existing telecommunications infrastructure, demand for services, and local market conditions, this technology can be both a substitute and a complement to copper wire in the local loop. WLL systems can help eliminate the backlog of orders for telephone service, which is estimated at over 50 million lines worldwide.

There are many WLL technologies operating in several radio frequencies and which adhere to different wireless standards. Most of them operate in a similar manner as cellular telephony, but WLL is fixed, not mobile. WLL systems provide rapid deployment of basic phone service in areas where the terrain or telecommunications development makes installation of traditional wireline service less attractive and less cost-effective. WLL systems can be easily integrated into the wireline public switched telephone network (PSTN) and can usually be deployed within a month of equipment delivery, far more quickly than traditional wireline installations which can take several months for initial deployment and years to grow capacity to meet the continually growing demand for communication services.

WLL systems also offer increased implementation and design flexibility. They can be used to provide first-line communication services in areas where there is no wireline infrastructure, or they can be implemented selectively as alternatives for wired feeder, distribution, or drop, as well as in competitive situations where there is convergence of the fixed and mobile markets.

WLL systems require minimal planning and can be deployed quickly, offering first-line telephone service to thousands of subscribers in a matter of months, instead of years. This is because operators can avoid having to deal with frequent wired local loop build-out issues which can be capital intensive. With WLL systems, construction costs are minimal and there is no need to arrange for rights-of-way for buried cable, both of which can dramatically slow down first-line service growth.

With WLL systems, operators can deliver service where it is needed, when it is needed—helping to reduce financial risk by ensuring faster payback on capital investments, especially if the system adheres to industry-accepted standards and protocols. Open standards and protocols allow operators to create efficient multiple-vendor systems, basing their technology planning and purchasing decisions on quality, effectiveness, and value without being locked into a single equipment vendor.

WLL technology is also generally compatible with existing operations support systems (OSS), as well as existing transmission and distribution systems. WLL systems are scalable, enabling operators to leverage their previous infrastructure investments as the system grows.

WLL solutions include analog systems for medium-to-low-density and rural applications. For high-density, high-growth urban and suburban locations, there are WLL solutions based on the digital standard optimal for wireless local loop use, Code Division Multiple Access, or CDMA. TDMA (Time Division Multiple Access) and GSM (Global System for Mobile telecommunications) systems are also offered. In addition to being able to provide higher voice quality than analog systems, digital WLL systems are able to support higher-speed fax and data services.

Although WLL systems are often based on mobile wireless technology, it is principally a fixed service. With the location of the subscribers known, a WLL system deployment can be tailored to provide user coverage at less cost than a comparable mobile system. However, WLL vendors such as Ericsson, Lucent Technologies, Motorola, Nokia, and Nortel offer complete network solutions that can serve both WLL and mobile cellular subscribers. The difference between fully mobile and WLL subscribers is the tariffing and numbering arrangements. WLL customers are typically charged using wireline tariffs and the numbering plan is similar to the wireline numbering plan. Mobile subscribers are charged according to mobile tariffs and they may have a different numbering space.

WLL subscribers receive phone service through a radio unit linked to the PSTN via a local base station. The radio unit consists of a transceiver, power supply, and antenna. It operates off AC- or DC-power and may be mounted indoors or outdoors, and it usually includes battery back-up for use during line power outages. On the customer side, the radio unit connects to the premises wiring, enabling the customer to use existing phones, modems, fax machines, and answering devices (Figure 1). The use of a cordless phone can provide mobility within the home or office.

Figure 9.1: The fixed wireless terminal is installed at the customer location. It connects several standard terminal devices (telephone, answering machine, fax, computer) to the nearest cell site Base Transceiver Station (BTS).

The WLL subscriber has access to all the usual voice and data features, such as caller ID, call forwarding, call waiting, three-way calling, and distinctive ringing. Some radio units provide multiple channels, which are equivalent to having multiple lines. The radio unit offers service operators the advantage of over-the-air programming and activation to minimize service calls and network management costs.

The radio unit contains a coding and decoding unit that converts conventional speech into a digital format during voice transmission and back into a nondigital format for reception. Many TDMA-based WLL systems use the 8-Kbps Enhanced Variable Rate Coder (EVRC), which became a published Telecommunications Industry Association (TIA) standard (IS-127) in January 1997. EVRC provides benefits to both network operators and subscribers.

For operators, the high-quality voice reproduction of the EVRC does not sacrifice the capacity of a network nor the coverage area of a cell site. An 8-Kbps EVRC system, using the same number of cell sites, provides network operators with greater than 100 percent additional capacity than the 13-Kbps voice coders that are deployed in CDMA-based WLL systems. In fact, an 8-Kbps EVRC system requires at least 50 percent fewer cell sites than a comparable 13-Kbps system to provide similar coverage and in-building penetration.

For subscribers, the 8-Kbps EVRC uses a state-of-the-art background noise suppression algorithm to improve the quality of speech in noisy environments typical of urban streets where there is heavy pedestrian and vehicular traffic. This also is an advantage compared with traditional landline phone systems which do not have equivalent noise suppression capabilities.

Depending on vendor, the radio unit may also include special processors to enhance call privacy on analog WLL systems. Voice privacy is enhanced through the use of a Digital Signal Processor (DSP)-based speech coder, an echo canceler, a data encryption algorithm, and an error detection/correction mechanism. To prevent eavesdropping, the low bit rate encoded speech data is encrypted using a private key algorithm, which is randomly generated during a call. The key is used by the DSPs at both ends of the communications link to decrypt the received signal.

The use of a DSPs in the radio units of analog WLL systems also provides subscribers with other benefits, such as improved fax and data transmission.

Regulation | Wireless LAN

In addition to assigning licensed and unlicensed spectrum for various wireless applications, often in concert with the international standards community, the FCC can set aside spectrum for various applications that are in the public interest. In mid-1997, the FCC set aside 350 MHz of free, unlicensed radio spectrum for a new category of terminal equipment suitable for schools and others seeking to install wireless LANs under the National Information Infrastructure (NII) initiative.

The new category of terminals, so-called NII/SuperNet terminals, include PCs and laptops equipped with radio receivers to transmit data over short distances at up to 25 Mbps. This is an alternative to poking through thick classroom walls to install a wired LAN.

The FCC also made provisions to allow part of the band to carry traffic throughout a campus or even a community. The highest 100 MHz of the band has a power limit 20 times higher than the lowest 100 MHz of the band. This will enable remote LAN access, though probably not in dense urban areas. The move satisfies the FCC's policy goal of promoting WAN applications, such as rural telemedicine and community access to classroom networks.

Wireless LAN Standard

A wireless LAN standard has been in the making since 1990. In mid-1997, the IEEE 802.11 committee finally made its most significant progress in issuing a standard. The standard defines a transmission rate of up to 2 Mbps over infrared or radio frequency bands. The standard also includes the media access control protocol Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA), which means devices implementing the standard can interoperate with wired Ethernet LANs.

The new standard specifies a choice of three different physical layers. Members of the 802.11 working group decided that a choice of physical-layer implementations was necessary so systems designers and integrators could choose a technology that matches the price, performance, and operations profile of a specific application.

The standard provides for an optical-based physical-layer implementation that uses infrared light to transmit data. It also provides two radio frequency (RF) based physical-layer choices: Direct Sequence Spread Spectrum (DSSS) and Frequency Hopping Spread Spectrum (FHSS). Both operate in the 2.4-GHz ISM band.

In the standard, a 2-Mbps peak data rate is specified for DSSS with optional fallback to 1 Mbps in very noisy environments. The standard defines the FHSS implementation to operate at 1-Mbps and allows for optional 2-Mbps operation in very clean environments.

The 802.11 media access control (MAC) can work seamlessly with standard Ethernet via a bridge to ensure that wireless and wired nodes on an enterprise LAN are logistically indistinguishable and can interoperate. The 802.11 MAC is necessarily different from the wired Ethernet MAC, but any such differences are masked by an access point that connects a wireless LAN channel to a wired LAN backbone.

The wireless LAN standard uses a carrier sense multiple access with collision-avoidance MAC scheme, whereas standard Ethernet uses a collision-detection scheme.

The roaming provisions built into 802.11 also provide several advantages. The 802.11 standard includes mechanisms to allow a client to roam among multiple access points that can operate on the same or separate channels. For example, an access point transmits a beacon signal at regular intervals that includes a time stamp for client synchronization, a traffic indication map, and indication of supported data rates. Roaming clients use the beacon to gauge the strength of their existing connection to an access point. If the connection is weak, the roaming station can attempt to associate itself with a new access point.

Although the 802.11 standard addresses roaming, it is with the understanding that all the access points in an installation are manufactured by the same vendor. The standard does not ensure that clients can roam among access points from different vendors. With the advent of 802.11 products, however, users may want to mix and match access points. For example, some customers might need standard commercial-grade bridges in the office but want ruggedized bridges for the factory floor.

To address multivendor roaming, Aironet Corp., Digital Ocean, Inc., and Lucent have collaborated to develop the Inter Access Point Protocol (IAPP) specification. That will extend the 802.11 multivendor interoperability benefits with comprehensive roaming protocols. Several others, including IBM, have voiced support for IAPP as a necessary step toward true multivendor interoperability.

The 802.11 specification adds features to the MAC that can maximize battery life in portable clients via power-management schemes. Power management causes problems with wireless LAN systems because typical power-management schemes place a system in sleep mode (low or no power) when no activity occurs for a user-definable time period. This can cause a sleeping system to miss critical data transmissions. To support clients that periodically enter sleep mode, the 802.11 standard specifies that access points include buffers to queue messages.

The 802.11 standard also addresses data security. The standard defines a mechanism through which the wireless LANs can achieve Wired Equivalent Privacy (WEP). The optional WEP mechanism is especially important because RF transmissions—even spread-spectrum transmissions—can be intercepted more easily than wired transmissions.

The next step in the evolution of the 802.11 standard will likely be the incorporation of higher data rates, expected in the 10 Mbps and above range in the 5.2-GHz band. The higher speed could encourage development of such applications as streaming video, telephony, and multimedia.

Network management | Microwave

Microwave systems can be managed using proprietary tools or the de facto standard Simple Network Management Protocol (SNMP) using a proxy agent. The proxy agent unit is a standalone device capable of managing single microwave terminals or, in the case of a network hub point, the unit can manage multiple microwave terminals. The proxy agent continually polls the managed devices for changes in alarm and status information and updates its locally stored Management Information Base (MIB). Events such as major and minor alarms cause the device to generate enterprise specific traps directed to the network management system (NMS). General alarm and status information stored in the MIB is made available to the NMS in response to SNMP's Get and GetNext requests.

The setup and management of microwave-based LANs is becoming much easier. Lucent Technologies, for example, offers a Windows-based site survey tool to facilitate remote management, configuration and diagnosis of the installed base of WaveLAN wireless LANs, which include access points and adapters that are available in 900-MHz and 2.4-GHz versions.

WaveMANAGER/CLIENT makes it easy for system administrators to monitor the quality of communications at multiple WaveLAN stations in a wireless network. It can also be used to verify building coverage, identify coverage patterns, select alternate frequencies, locate and tune around RF interference, and customize network access security.

WaveMANAGER/CLIENT offers five basic functions:

§  Communications Indicator is located on the Windows 95 taskbar and provides mobile users graphical, real-time information on the level of communication quality between a WaveLAN station and the nearest WavePOINT access point.

§  Link Test Diagnostics verifies the communications path between neighboring WaveLAN stations, as well as between a WaveLAN station and WavePOINT access points within one wireless cell. Link Test Diagnostics measure signal quality, signal-to-noise ratio and the number of successfully received packets.

§  Site Monitor is used to ensure optimal placement of WavePOINT access points. While carrying a WaveLAN-equipped computer through the facility, Site Monitor graphically displays changing communication quality levels with the various access points installed in the building. This tool makes it easy to locate radio dead spots or sources of interference.

§  Frequency Select manages RF Channel selection. It enables the user to choose from up to eight different channels (in the 2.4-GHz frequency band). WaveMANAGER/AP is used to select access point frequencies.

§  Access Control Table Manager enables the system administrator to provide extra levels of security by restricting access to individual computers in a facility.

LAN implementation | Microwave

LAN implementation

There are some short haul microwave systems used for corporate LANs that do require an FCC license because they operate in a more crowded portion of the spectrum. This is the case with the 18-GHz frequency band, which is used by Motorola's Altair product line, for example. The FCC allocates licenses for the 18-GHz region of the spectrum for five channels, each consisting of two 10-MHz bands in the 18- to 19-GHz band. The two-channels-per-license arrangement permits full-duplex communications using one channel in each direction.

While a license may be construed as a liability, it offers the guarantee of interference-free communication. Unlike unlicensed spectrum, such as that often used by spread-spectrum technologies, licensed spectrum gives the license-holder a legal right to an interference-free data communications channel. Owners of wireless LANs operating on unlicensed spectrum are continuously at risk of unauthorized interference destroying their data communications capabilities, and do not always have legal recourse.

Since Motorola has obtained licenses for 18-GHz operation in all U.S. metropolitan areas with populations above 30,000, Altair microcells in these areas are fairly well protected from unauthorized interference. Motorola customers do not have to deal directly with the government or wait for approval to operate Altair equipment. Upon purchasing the equipment, they fill out a simple registration form, and immediately place their equipment into operation. Motorola's Frequency Management Center dedicates and coordinates frequencies to specific customer locations. In the event a customer moves the Altair system to another location, Motorola provides an 800 number to report that fact in compliance with FCC requirements. Motorola handles frequency coordination for the customer at the new location.

LAN extension

Corporations are making greater use of wireless technologies for extending the reach of LANs where a wired infrastructure is absent or costly. Wireless bridges and routers can extend data communication between buildings in a campus environment or between buildings in a metropolitan area.

Wireless bridges. Short-haul microwave bridges equipped with directional antennas provide an economical alternative to leased lines or underground cabling. Because they operate over very short distances—less than a mile—and are less crowded, they are less stringently regulated and have the additional advantage of not requiring an FCC license.

The range of the bridge is determined by the type of directional antenna. A 4-element antenna, for example, provides a wireless connection of up to 1 mile. A 10-element antenna provides a wireless connection of up to 3 miles.

Directional antennas require a clear line of sight. To ensure accurate alignment of the directional antennas at each end, menu-driven diagnostic software is used. Once the antennas are aligned, and the system ID and channel are selected with the aid of configuration software, the system is operational. Front-panel LEDs provide a visual indication of link status and traffic activity. The bridge unit has a diagnostic port, allowing performance monitoring and troubleshooting through a locally attached terminal or remote computer connected via modem.

Because it is fully compatible with the IEEE 802.3 Ethernet standard, microwave bridges support all Ethernet functionality and applications without the need for any special software or network configuration changes. For Ethernet connections, the interface between the microwave equipment and the network is virtually identical to that between the LAN and any cable medium, where retiming devices and transceivers at each end of the cable combine to extend the Ethernet cable segments. Typically, microwave bridges support all Ethernet media types via AUI connectors for thick Ethernet (10Base5), 10Base2 connectors for thin Ethernet, and twisted-pair connectors for 10BaseT Ethernet. These connections allow microwave bridges to function as an access point to wired LANs.

Like conventional Ethernet bridges, microwave bridges perform packet forwarding and filtering to reduce the amount of traffic over the wireless segment. The microwave bridges contain Ethernet address filter tables, which help to reduce the level of traffic through the system by passing only the Ethernet packets bound for an interbuilding or intrabuilding destination over the wireless link. Since the bridges are "self-learning," the filter tables are automatically filled with Ethernet addresses as the bridge learns which devices reside on its side of the link. That way, Ethernet packets that are not destined for a remote address remain local. The table is dynamically updated to account for equipment that is either added or deleted from the network. The size of the filter table can be 1000 entries or more, depending on vendor.

With additional hardware, microwave bridges have the added advantage of pulling double-duty as a backup to local T1/E1 facilities. When a facility degrades to a pre-established error-rate threshold or is knocked out of service entirely, the traffic can be switched over to a wireless link to avoid loss of data. When line quality improves or the facility is restored to service, the traffic is switched from the wireless link to the wireline link.

Wireless routers. Wireless remote access routers scale wireless connect geographically disbursed LANs by creating a wireless WAN over which network traffic is routed at distances of 30 miles or more using spread-spectrum technology.

Unlike wireless bridges which simply connect LAN segments into a single logical network, wireless routers function at the network layer with IP/IPX routing, permitting the network designer to build large, high performance, manageable networks. Wireless routers are capable of supporting star, mesh, and point-to-point topologies that are implemented with efficient MAC protocols. These topologies can even be combined in an internetwork.

A polled protocol (star topology) provides efficient shared access to the channel even under heavy loading (Figure 1). For small-scale networks, a CSMA/CA protocol supports a mesh topology (Figure 2). Clusters of nodes can be connected using a point-to-point protocol when building large-scale internetworks (Figure 3). The single-hop node-to-node range is up to 30 miles, depending on such factors as terrain and antennas, with a multiple hop range extending on the order of a hundred miles.

Figure 1: In the star topology, remote stations interconnect with the central base station, and with other remote stations, through the base station. Only one location needs line-of-sight to the remotes. Networks and workstations at each location tie into a common internetwork. The maximum range between the central base station and remote stations is approximately 15 miles.

Figure 2: In the mesh topology, each site must be line-of-sight to every other. A CSMA/CA protocol ensures efficient sharing of the radio channel. The range with omni-antennas is up to 3.5 miles.

Figure 3: The point-to-point topology is useful where there are only two sites. It can also function as a repeater link between clusters of sites. The range using directional antennas is up to 30 miles.

End-to-end SNMP supports management of the radios and the wireless WAN along with the remainder of the enterprise network using industry standard SNMP tools. The result is a manageable network with reach extending to metropolitan, suburban, rural, remote, and isolated areas.

Applications of wireless routers include remote site LAN connectivity and network service dissemination. Organizations with remote offices such as banks, health care networks, government agencies, schools, and other service organizations can connect their computing resources. Industrial and manufacturing companies can reliably and cost-effectively connect factories, warehouses, and research facilities. Network service providers can distribute Internet, VSAT, and other network services to their customers.

A WAN built with wireless routers exploits the tariff-free wireless "infrastructure." A wireless WAN offers a substitute for a wired infrastructure with its associated costly service fees. In areas where a wired infrastructure is absent or underdeveloped, the use of wireless routers newly enables internetworking. Performance is comparable to commonly used wired WAN connections, approaching T1 speeds with a 1.3 Mbps data rate.

A user-supplied PC hosts the wireless router device through a connection to the PC's parallel printer port. Software includes an installation utility, network drivers, management, and IP/IPX routing. No radio license is required in the United States and many other countries, simplifying the network development process compared to licensed microwave.