[출처]Telephonyonline
The cellular telephone revolution of the past twenty years has redefined the world ideas about when and where people can be reached, creating nytime, anywhere·availability. Business enterprises know they can increase productivity and improve customer service with mobile voice communications within their facilities, and most companies have long used cellular phones to deliver this capability. As wireless LANs have begun taking hold across the corporate landscape, however, enterprises have started exploring voice over WLAN (VoWLAN) as an alternative to cellular telephone coverage. The thinking is that an in-house mobile wireless infrastructure makes enterprises masters of their own destiny, integrates wired and wireless telephony in the same IP infrastructure, reduces calling charges, and also addresses the problem of highly variable cell phone coverage inside some buildings.
But before jumping on the VoWLAN bandwagon, enterprise IT managers should ask themselves what they really want from voice mobility, and what they will get from VoWLAN in return for the cost and effort required. When the alternatives are compared, most companies find that they can get far better service and scalability with lower costs by extending cellular coverage throughout their facilities.
Mobile voice challenges
In a perfect world, every employee should have clear, reliable voice connectivity anywhere within a facility, whether it 바보 on the factory floor, in the warehouse, in stairwells, or in administrative offices. Voice users are accustomed to clear and reliable connections, and business-class wireless voice demands the same quality of service. But buildings, furniture, equipment, and adjacent wireless LANs create a relatively hostile environment for easy propagation of wireless signals, so achieving parity in the quality department can be a problem.
For cellular phone users, the building바보 outside walls seriously degrade the signal from the nearest cellular base station. Employees often migrate toward windows to improve the clarity of their connections, since interior walls, furniture, and office equipment all impede the signal.
In addition, outdoor cellular base stations were designed to serve mobile users traveling through the cell hen enterprise cellular users from within a building use an outdoor base station for ongoing connectivity, that station overall effectiveness at handling outdoor traffic can be compromised by the increased load.
In-building cellular systems overcome these issues. Essentially, an in-building cellular system is comprised of a micro cellular base station or repeater, which provides the cellular signal/channels, and a distributed antenna system (DAS) that distributes the signal and extends coverage throughout the building. In-building wireless eliminates interference issues by extending the signal, and it eliminates the capacity problem for outdoor base stations by offloading an enterprise traffic to a dedicated base station inside the building.
Figure 1. In-building wireless extension architecture
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VoWLAN users face the same basic indoor interference issues from walls and furniture, but Wi-Fi frequency coverage can also be limited by other equipment on the same frequencies (cordless phones, microwave ovens, or wireless LANs in neighboring offices). Since Wi-Fi frequencies are unlicensed, there바보 no real way to avoid such interference or police it.
Figure 2. Typical VoWLAN architecture
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The next hurdle is deployment. Even if a solution is technically workable, the hassle and disruption of deploying it may make it more trouble than it 바보 worth. As corporate migration to IP PBXs has shown, changing something as fundamental as a telephone system requires large expenditures and a lot of retraining, and it generally involves more than a few startup glitches.
Another challenge is management and maintenance. Enterprise IT organizations are busy enough managing their existing infrastructure and applications without having to take on a whole new set of problems. And wireless brings its own unique set of challenges. In the short term, higher management costs can easily overwhelm any theoretical savings to be gained from an all-IP voice system.
Technology choices
With these challenges in mind, let now compare VoWLAN and in-building cellular technologies.
With VoWLAN, enterprises envision an all-IP phone system where every user has one handset for both desktop and mobile use. Ideally, this handset should also integrate with the cellular network, but even the most ardent VoWLAN proponents recognize that full Wi-Fi/cellular integration with reasonably priced handsets is a few years away. Setting aside Wi-Fi/cellular integration, however, Wi-Fi still has its issues.
Wi-Fi (802.11) was designed to support IP data (not voice) connections within a limited area. To cover large areas, it 바보 necessary to deploy multiple Wi-Fi access points (APs). Wi-Fi vendors would have us believe that setting up IP voice is a simple matter of adding a few APs to the network, but the reality is a lot more complicated. For example:
* Standard 802.11 APs are relatively passive devices that are unaware of one another. As a result, the network is unable to load-balance user traffic among multiple APs. When an AP becomes overloaded, it simply stops accepting requests for connections.
* While IP data packets can be easily delayed a short period without affecting performance, IP voice is a very time-sensitive application, and packet delivery delays will rapidly reduce quality of service to the point where calls are indecipherable by the hearer or are disconnected.
* Voice and data traffic contend for bandwidth on a Wi-Fi network, so Wi-Fi vendors recommend using a lot of APs. Under current guidelines, a Wi-Fi AP supports no more than seven voice users with reasonable quality, and only if no other data traffic is present. Vendors often recommend quadrupling the number of APs in the WLAN to support reasonable coverage and QoS, or segregating voice traffic onto a second set of APs on a different frequency (such as 802.11a).
* Most Wi-Fi APs can distinguish between IP voice packets and regular IP data packets today, and cannot independently control the quality of service (QoS) for each type of traffic. The 802.11e standard should help improve Wi-Fi QoS capabilities, but it not in deployment yet.
In contrast, today mobile cellular networks were designed to support voice traffic for millions of users over very large geographical regions. Because of these requirements, cellular networks handle voice and data traffic much differently. For example:
* The system base stations and base station controller are always aware of every active user, and automatically coordinate client access among base stations to balance the overall client load. If a base station is unable to handle a client request, it hands that client off to the next nearby base station.
* Base stations control QoS in both directions to every client, so transmit power is optimized for individual user circumstances. A user behind a filing cabinet would send and receive a stronger signal to make up for the furniture interference.
* One base station radio can handle anywhere from 30 to 85 simultaneous users without QoS degradation, and base stations are engineered to support multiple radios, allowing hundreds of simultaneous voice calls and data sessions. Voice and data traffic are handled separately in a cellular network, rather than contending for the same bandwidth.
* Base station controllers automatically coordinate channels among their connected base stations to avoid interference. Cellular frequencies are licensed to each wireless carrier by the FCC, so there is no interference between multiple carrier networks that may be available in any area.
The right wireless voice solution
The only thing remaining in our comparison of Wi-Fi and cellular voice is to consider the technical attributes of each alternative against the business requirements.
* Enable pervasive voice coverage with high QoS Cellular technology inherently offers far higher QoS for voice users than VoWLAN, and its use of licensed frequencies and higher-capacity radios helps ensure high call quality in potentially difficult wireless environments.
* Minimize IT administration and equipment costs Wi-Fi data networking equipment is relatively simple, but Wi-Fi voice will require new site surveys, new APs, and more management effort to ensure QoS and to minimize interference from other sources. Alternatively, by extending cellular coverage indoors, companies can continue using existing infrastructure and wireless phones without user disruption, new training, or additional network management costs.
* Minimize network service and per-minute charges In-building wireless equipment costs can usually be shared with the carrier or building owner, and most cellular carriers now have rates as low as $39.95 per month for corporate users with unlimited mobile-to-mobile calls within the same network. While VoWLAN carries no per-minute charges, the savings is outweighed by the additional management, equipment (APs and handsets), and user training costs.
Enterprises clearly want one network that can handle voice and data traffic. Fundamentally, the choice is between using a network designed for data (WLAN) to serve much more demanding voice applications, and using a network designed for voice (cellular) to support data traffic. Given that cellular carriers have offered data service for years and are rapidly rolling out higher-speed data options like EV-DO and HSDPA, it appears that a true, high-bandwidth, dual-purpose network will be available from cellular providers long before WLAN technology catches up.
Enterprises and other large organizations have been using in-building cellular systems for years to enable reliable and pervasive mobile voice services, and there are no good reasons to walk away from this technology today. VoWLAN systems may make sense years from now, when all carrier networks run on IP voice, Wi-Fi networking issues are resolved, and dual-mode handsets are reasonably priced. But for the present and near future, there is no compelling reason for enterprise IT planners to abandon something that works well and struggle with a nascent technology that faces multiple birthing pains.
