The unit ships with three of iRobot's Virtual Wall Lighthouse modules. These are small IR beam transmitters that have two modes.

Thankfully, iRobot provides you with a few options to give you more control over where your Roomba goes. The 880 comes with two Virtual Wall/Lighthouse devices for you to place around your home to ...

Aside from simply closing a door or placing a physical object in its way, iRobot includes a single Virtual Wall with the Roomba 960 which allows users to place the virtual barrier in one of two ways.

As before, some Roomba models come with “Virtual Walls” that confine Roomba to a single room. The 560 ships with a pair of vastly improved “Virtual Wall Lighthouse” barriers.

Basically, each Lighthouse/virtual wall unit has a RF transmitter/receiver (along with the Home Base and 560) for communicating with each other.

The Roomba 790 is also equipped with the Virtual Wall Lighthouse, which helps to contain the Roomba in one room until it has completed its cleaning mission.

The Virtual Wall Lighthouse can be set to three different ranges of virtual wall, up to eight feet in length.

The iRobot Roomba 880 outshines its Roomba 790 predecessor and carries the brand to a new level of robot vacuum awesomeness.

Roomba describes the feature thusly, [A]djust to Lighthouse, set the distance of the door opening and place the Virtual Wall Lighthouse outside of the doorway to the room you want Roomba to clean.

The winner will receive: iRobot® Roomba® 880 with HEPA Filter, Side Brush, 2 AeroForce™ Extractors, Home Base™ with EU Line Cord, 2 Virtual Wall® Lighthouse™ Devices (4 C Batteries ...

Basically, each Lighthouse/virtual wall unit has a RF transmitter/receiver (along with the Home Base and 560) for communicating with each other.