WLAN: Smoothening Wireless Expansion

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

Scenario 1
Supports	Up to 32 users
Equipment	Two 802.11b or 802.11b/g access points
Throughput	Up to 11Mbps for 802.11b Up to 54Mbps for 802.11g
Security	WPA2-PSK VPN gateway for remote access
Wired network access	Broadband ISP or leased T1
TCO/benefit ratio	$20,000 TCO to deliver a benefit of $300,000 over a three-year period
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.

Source: Intel

Scenario 2
Supports	Up to 150 users
Equipment	Twelve 802.11b or 802.11b/g access points
Throughput	Up to 20 Mbps to company network; Up to 11 Mbps for 802.11b Up to 54 Mbps for 802.11g
Security	WPA2 with RADIUS server for authorization and authentication and VPN gateway for remote access
Wired network access	Leased line
TCO/benefit ratio	$60,000 TCO to deliver a benefit of $1,000,000 over a three-year period
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.

Source: Intel

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 security, 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 technologies.

Non-unified WLAN Architecture

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.