When a new device wants to join a WirelessHART network, it begins by listening. It listens for advertisements to join the network broadcast by devices already in the network. These broadcasts contain the channel map identifying channels used by the network, the 4 bit join priority of the device, the superframes and their join links. This allows the joining device to know who to contact as well as when and how.
The channel map designates which frequencies the network actually uses. The join priority helps the joining device determine to which device already in the network it should link by specifying number of hops to the gateway, signal strength, number of descendants, and remaining battery life of the advertising device. The join links in the superframes tell the joining device when devices already in the network will be listening for new devices.
All WirelessHart transmissions are coded. Even the advertisement to join is coded with a standard WirelessHART key. This serves like an I.D. badge to differentiate WirelessHART traffic from other wireless transmissions. Before a device can start listening for opportunities to join the network, it must be given a network join key. This is usually programmed during installation.
The network manager should recognize the join key for the device. When it sends a join reply, it includes the network key. It will also send address information for the new device.
The device must regularly transmit keep-alive messages. If the network ever looses contact with the device, it will not be allowed to rejoin the network until a new join key has been programmed.
“The name’s Hart. W. L. Hart.”
The security guard examined the ID. Everything seemed to be in order, and the man was here at the right time. “You got your security pass?” he asked the tall, sophisticated stranger standing before him.
Hart pulled the card his contact had given him the day before from his suite pocket. The code embedded on the magnetic stripe should give him access to the inner sanctum of this high-tech den of thieves. As he ran it through the reader the light on the door turned green and he heard the distinctive click of a thick bolt moving to unlock the large steel door.
As he stepped into the room he saw a large screen to the left and another steel door in front of him. Once he was all the way in, the first door closed behind him.
“Welcome, Mr. Hart,” said a large face which had appeared on the screen. “I am The Manager. I am issuing you a new card which will give you access to every room in the complex. You will need this card to open any door, including the exits. You see, we keep close tabs on our associates.”
A card appeared in a slot under the door. Hart picked it up and started the swipe to unlock the inner door.
“Oh, one more thing, Mr. Hart,” said the boss on the screen. “If you do exit the complex, you will not be able to get back in. Do you understand?”