PDF of this tutorialUltimately the appropriate protocol technology will depend on an array of applications considerations, such as size and population density of the geographic area (rural versus urban) and the service needs of the subscriber base (residential versus business; POTS versus data access). In fact, there are many good reasons why different wireless technologies will serve some applications better than others. The challenge for WLL vendors is to identify the optimal wireless protocol for their unique application needs, then reduce cost per subscriber through silicon and deliver integrated solutions to the marketplace.
WLL will be implemented across five categories of wireless technology. They are digital cellular, analog cellular, personal communications network (PCN)/personal communications service (PCS), cordless telephones 2nd generation (CT–2)/digital European cordless telecommunication (DECT), and proprietary implementations. Each of these technologies has a mix of strengths and weaknesses for WLL applications.
Analog Cellular
Given its wide availability resulting from serving high-mobility markets, there is significant momentum to use analog cellular for WLL. There are currently three main analog cellular system types operating in the world: advanced mobile phone system (AMPS), Nordic mobile telephone (NMT), and total access communications systems (TACS). AMPS and its cousin narrowband advanced mobile phone system (NAMPS) dominate the analog cellular market with 69 percent of subscribers, while TACS has 23 percent and NMT has only 8 percent.
As a WLL platform, analog cellular has some limitations in regards to capacity and functionality. Due to widespread deployment, analog cellular systems are expected to be a major wireless platform for WLL, at least in the short term. Given its characteristics, analog cellular is best suited to serve low-density to medium-density markets that don't require landline-type features. Analog cellular is forecasted to account for 19 percent of the WLL subscribers in the year 2000.
Digital Cellular
These systems have seen rapid growth and are expected to outpace analog cellular over the next few years. Major worldwide digital cellular standards include global system for mobile communications (GSM), time-division multiple access (TDMA), Hughes enhanced TDMA (E–TDMA), and code-division multiple access (CDMA). GSM dominates the digital cellular market with 71 percent of subscribers.
Digital cellular is expected to play an important role in providing WLL. Like analog cellular, digital cellular has the benefit of wide availability. Digital cellular can support higher capacity subscribers than analog cellular, and it offers functionality that is better suited to emulate capabilities of advanced wireline networks. Its disadvantage is that it is not as scalable as analog cellular. It is forecasted that approximately one-third of the installed WLLs will use digital cellular technology in the year 2000.
Although GSM currently dominates mobile digital cellular, there has been little activity in using GSM as a WLL platform. Since GSM's architecture was designed to handle international roaming, it carries a large amount of overhead that makes it unwieldy and costly for WLL applications. In spite of these limitations, it is likely that GSM WLL products will be developed over the next few years. CDMA appears to be the standard best suited for WLL applications. CDMA employs a spread-spectrum modulation technique in which a wide range of frequency is used for transmission and the system's low-power signal is spread across wide-frequency bands. It offers higher capacity than the other digital standards (10 to 15 times greater than analog cellular), relatively high-quality voice, and a high level of privacy. The main disadvantage of CDMA is that it is only now beginning to be deployed on a wide scale.
PCS/PCN
PCS/PCN incorporates elements of digital cellular and cordless standards as well as newly developed radio-frequency (RF) protocols. Its purpose is to offer low-mobility wireless service using low-power antennas and lightweight, inexpensive handsets. PCN is primarily seen as a city communications system with far less range than cellular. PCS is a broad range of individualized telecommunications services that let people or devices communicate regardless of where they are. Some of the services include personal numbers assigned to individuals rather than telephones, call completion regardless of locations (find me), calls to the PCS customer that can be paid by either the caller or the receiver, and call-management services that give the called party greater control over incoming calls.
It is not clear which standards, if any, will dominate the WLL portion of PCS/PCN. The candidate standards are CMDA, TDMA, GSM, personal access communication systems (PACS), omnipoint CDMA, TDMA, upbanded CDMA, personal handyphone system (PHS), and digital cordless telephone United States (DCT–U). These standards will probably be used in combination to provide both WLL and high-mobility wireless services.
PCS/PCN has the advantage of being designed specifically to provide WLL by public wireless operators. The main weakness of PCS/PCN is that it is not yet commercially available.
CT–2/DECT
Cordless telephony was originally developed to provide wireless access within a residence or business between a base station and a handset. Since the base station is still hard-wired to the PSTN, this is not considered WLL. For the purposes of this study, DECT is considered WLL when a public network operator provides wireless service directly to the user via this technology.
Although DECT does not appear to be ideally suited for rural or low-density applications, it has some significant advantages in medium-density to high-density areas. Cordless telephony has advantages in terms of scalability and functionality. As compared to cellular technology, DECT is capable of carrying higher levels of traffic, provides better voice quality, and can transmit data at higher rates. The microcell architecture of DECT allows it to be deployed in smaller increments that more closely match the subscriber demand, with reduced initial capital requirements.
Proprietary Implementations
Proprietary WLL systems encompass a variety of technologies and configurations. These systems are considered proprietary because they are not available on public wireless networks and are typically customized for a specific application. They generally do not provide mobility. This makes proprietary technology most effective for applications that cannot cost-effectively or time-effectively be reached by landline alternatives. Proprietary systems are, therefore, positioned to provide basic fixed wireless telephony in low-demand and medium-demand density applications.
