How do we connect large, campus-wide WLANs?

Q: Is it possible to connect separate, disparate wireless LANs, such as those found on a university campus? What issues can we expect if we try this? – Harshini, no city given.

For any large-scale WLAN deployment, such as those on a university campus, several requirements must be considered when designing the architecture and integrating it with an existing LAN. One important requirement – minimize the increase in IT staff support needed.

Historically, there have been few successful ways to do this. One is to run a parallel infrastructure for your WLAN with separate routers, Ethernet switches, and trunks. Your WLAN connects to your wired LAN at a single point through a firewall. The access points, which are connected to the Ethernet switches using Ethernet cables, are invisible to this infrastructure that communicates with mobile devices.  The access points simply relay packets between mobile devices and the wired infrastructure, ultimately passing these packets through the firewall onto the LAN.  Another method is to deploy a large virtual LAN (VLAN) that carries only WLAN traffic, and directs the VLAN to a firewall that connects to the LAN.

For mobile devices to be able to roam about the campus wirelessly in a traditional WLAN environment, either all the mobile devices have to support mobile IP, or the WLAN has to use a single large subnet. Otherwise, connections get dropped when a device roams on the WLAN from one subnet to another, finding itself with an invalid network address.

Both of these solutions suffer from the problem that access points must be managed individually, yet coordinated collectively to provide the best coverage with minimal interference.

Another idea has emerged where management of access points is centralized within a hierarchical system of WLAN controllers (this model is currently being explored by the IETF via the Lightweight Access Point Protocol). Controllers, in the form of an appliance or switch, allow users to deploy access points on existing LANs, segregate the WLAN traffic from the LAN traffic, and provide separate subnet support for mobile devices. Roaming is allowed without the device concerns for incorrect addresses or needs for any special protocol support. Also, mobile IP is not required.