How can we provide for WLAN mobility?

Are there ways to provide wireless LAN mobility over the enterprise without creating a flat radio frequency domain?

Q: Are there ways to provide wireless LAN mobility over the enterprise without creating a flat radio frequency domain? – Brad, Seattle

A: Yes, Brad, there is a Santa Claus. The WLAN mobility problem boils down to basic IP routing. When a mobile device acquires an IP address on a subnet (subnet 1), and it roams to another subnet (subnet 2), the packets destined for the device continue to be routed to subnet 1.

Traditional 802.11 access points were not designed for large-scale deployments, and as a consequence, typical deployments would require that all of the access points in a building (or whatever mobility domain is desired) reside on a single flat network. These types of network gymnastics are not always natural and most often require large virtual LAN (VLAN ) networks.

Some folks would argue that this problem is not significant since their user community does not typically move while using their mobile devices. Unfortunately, radio frequency works in such a way that a mobile device may need to associate with another access point for various reasons, such as interference caused by someone walking between the user and the access point. So mobility in the 802.11 world does not necessarily equate to physical mobility.

The Internet Engineering Task Force (IETF) has been working on a specification called Mobile IP, which was designed to solve this problem. However, this solution requires software on the client, and has therefore not been widely deployed. A couple of years ago a new class of appliance came to the market to address the mobility problem without the need for software on the client. These devices typically require that they be located in the wiring closet, and have direct connection to the access points. But it is important to note that there is no standard protocol for providing this function – so you may notice drastic differences in deployment requirements, features, and performance.

More recently, some vendors announced new features on access points that provide roaming across subnets, using a derivative of Mobile IP (commonly called Proxy Mobile IP). The issue that has been raised regarding this approach is the need to modify the operation of the existing wired infrastructure, something that many IT managers are very reluctant to do.

Some of the new WLAN switches also provide inter-subnet mobility features, and include direct access to the 802.11 protocol. As such, they are capable of protocol hand-off latencies designed for real-time applications (such as voice over IP).