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AP Placement and Cell Sizing: The two key objectives of WLAN is to provide connectivity (or coverage) at all desired locations and to provide reasonable capacity to cater to the bandwidth needs of client applications. Depending on the type of WLAN equipment deployed and the intended applications, companies may be forced to overlay two sets of access points-upgrade client devices, add extra access points because of weak technology that limits capacity-and to spend additional money on pre-deployment studies as well as ongoing configuration.
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Wireless Standards
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Feature
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802.11b
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802.11g
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802.11a
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Available RF channels
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3 non-overlapping
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3 non-overlapping
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8 or more non-overlapping (varies by country)
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Maximum data rate/channel
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11 Mbps
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54 Mbps
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54 Mbp
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Frequency band
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2.4 GHz
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2.4 GHz
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5 GHz
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Typical range
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100 ft at 11 Mbps; 300 ft at 1 Mbps
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50 ft at 54 Mbps; 150 ft at 11 Mbps
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40 ft at 54 Mbps; 300 ft at 6 Mbps
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| Three industry-wide WLAN standards have been ratified by the Institute of Electrical and Electronics Engineers (IEEE) |
Locations with high user density, or with users running communication-intensive or interactive applications require better capacity than other locations. Bandwidth aggregation through multiple overlapping cells, or use of densely packed small-sized cells are two ways to provide higher capacity at desired locations.
Accounting the Cost: To determine the cost of the deployment and how technology can help, it's important to understand the elements that comprise the total cost of deployment. Every access point (AP) requires an associated Power over Ethernet (PoE) port, a cable run to the place where the AP will reside, and the time associated with deploying all of these. An access point, for instance, that costs $150 can climb up in the $400-550 range when the planning, labor, and PoE are taken into account.
| To determine the cost of the deployment, it's important to understand the elements that comprise the total cost of deployment |
Knowing the limitations of 802.11 and how advanced systems are designed to help can save on equipment as well as labor costs. Costs can be lowered by implementing thin wireless access points (APs), particularly in larger networks. Traditional wireless deployments utilise thick APs, each configured individually.
Limited Channels: Because of limited number of available channels in 802.11 wireless LAN technologies, channels need to be re-used beyond certain re-use distance. Without proper assignment of channels and power calibration of access points, co-channel interference can degrade the performance of wireless LAN. The 802.11b and 802.11g wireless LAN standards operate in the 2.4 GHz band. This band only offers three non-overlapping channels. Each channel is a separate 'pipe' of bandwidth. All clients attached to the access point share that same bandwidth.
The task of the channel assignment process is two-fold-reduce the interference among neighboring cells, and provision enough capacity.
Supporting Mobility and Automation: The obvious aspect where a wireless network excels above wired is mobility. Having the ability to connect anywhere, anytime is a powerful motivator for wireless. However, the biggest downside to having the wrong pervasive WLAN design is the ongoing tuning and reconfiguration of the network.
If the WLAN system requires RF planning tools to deploy, this will be an ongoing task. Adding new desktops, moving an employee, or even renovating a part of the office building in a way that was not originally predicted, will change the RF environment and, therefore, require changes to the network. If those changes are not handled by the system automatically, the IT manager must manually intervene with software tools. This process is commonly referred to as RF spectrum management and is the bane of IT staffers' lives.
| Typical scenarios of WLAN deployment in a small-to-medium sized business |
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Scenario 1
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Supports
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Up to 32 users
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Equipment
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Two 802.11b or 802.11b/g access points
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Throughput
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Up to 11Mbps for 802.11b
Up to 54Mbps for 802.11g
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Security
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WPA2-PSK VPN gateway for remote access
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Wired network access
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Broadband ISP or leased T1
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TCO/benefit ratio
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$20,000 TCO to deliver a benefit of $300,000 over a three-year period
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As per application needs, two 802.11b or 802.11b/g access points can be sufficient to support up to 32 WLAN users, with traffic routed through a four-port hub. If the office has fewer than 15 WLAN users, the second access point can allow for future growth while serving as a failover device in case the first access point malfunctions.
A small gateway capable of 2 Mbps throughput serves as an optional VPN gateway for remote users to log in to the company network. A broadband ISP or a leased T1 line would typically provide up to 1.5 Mbps of bandwidth-coupling well with the bandwidth provided by the gateway and wireless access points. Since the WLAN can easily keep up with the Internet connection, you get the benefits of wireless without worrying about introducing bottlenecks into the network.
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Source: Intel
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Scenario 2
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Supports
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Up to 150 users
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Equipment
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Twelve 802.11b or 802.11b/g access points
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Throughput
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Up to 20 Mbps to company network;
Up to 11 Mbps for 802.11b
Up to 54 Mbps for 802.11g
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Security
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WPA2 with RADIUS server for authorization and authentication and VPN gateway for remote access
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Wired network access
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Leased line
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TCO/benefit ratio
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$60,000 TCO to deliver a benefit of $1,000,000 over a three-year period
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By deploying twelve 802.11b or 802.11b/g access points, a medium-sized business can support up to 150 wireless users at Ethernet-like speeds. The VPN gateway is capable of 20 Mbps throughput and enables remote users to log into the company network via VPN.
In this scenario, a series of switches aggregate and manage traffic from the wireless access points, which connect to the Internet over a leased line. The access points can be powered by Power-over-Ethernet (PoE) using the Category 5 cable connecting access points to the wired network-resulting in significant cost savings in construction and wiring for each access point.
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Source: Intel
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Mobility needs to be supported at two different levels-link-layer and IP-layer. 802.11 specifications support link-layer mobility by providing a mechanism for a client to detect new access points and switch across access points based on signal strength measurements.
Handoff Between APs: When a client roams from one access point to another, the time between disconnecting from the first access point and reconnecting to the second access point is non-zero. For some clients, this process can take up to several seconds. If a micro-cell configuration is deployed to increase throughput, then this handoff problem is exponentially worse.
One should bear in mind that there are inherent limitations to the number of connections to an access point. A good rule of thumb for designing is that each access point can support 20-30 simultaneous users. Applications that don't demand high bandwidth and/or low latency will suffer performance when the number of simultaneous access point users is too great. How the access points connect to the backbone is important as well.
The Right Tools
Wireless LAN applications continue to mature in features and usability. Higher speeds, increased , quality of service (QoS) and centralized management are just a few of the wireless developments in the past few years, and more advances are coming. On the operational side also the configurations and the security settings are quite simpler compared to other managed .
| Non-unified WLAN Architecture |
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| The wired and wireless networks remain separate, with the interface between the two being a standard Ethernet connection |
What enterprises need are WLAN systems that are designed specifically for pervasive Wi-Fi access in enterprises. These systems are cost effective as they handle the issues of pervasive WLAN deployment automatically, require no RF planning, do away with quality-killing handoffs, can eliminate interference problems and enable greater scalability.
Centralized AP management has become a popular method of wireless installation that moves all intelligence from the APs to an appliance. A centralized wireless deployment allows for the Virtual LAN (VLAN) to be extended over the existing wired network. Configuration changes are applied at the management switch instead of at each AP. Since the APs are communicating with a central device, advanced capabilities, such as automatic channel and power configuration and rogue detection, are possible. In addition, each thin AP generally costs significantly less than its more feature-rich cousin.
The downside to a centralized model is the up-front costs. The central management switch is usually expensive. However, if the deployment involves many APs, or wireless expansion is anticipated in the future, the up-front costs of a centrally managed application are often eclipsed by the benefits.
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