4 Central Gateway

The scope of this application report is not to provide a reference design for a central gateway, but to describe the technical aspects associated with enabling a multiple gateway environment when using the TI 15.4-Stack. Nonetheless, it does make sense to at least include a description of the parameters related to the TI 15.4-Stack that a central gateway implementation should consider.

Before describing the tasks associated with the central gateway, it is first useful to understand what a local gateway is, and what a local gateway does. As mentioned before, a local gateway is nothing more than a collector device with added capabilities to run protocol stack layers over the IEEE 802.15.4 e/g MAC and PHY. The collector device acts as a PAN coordinator for the TI 15.4 network, meaning that it is in charge of creating and maintaining the network, while also providing synchronization services to devices such as sensors (that have joined the network), and other network coordinators.

The reason behind providing synchronization services to other network coordinators is that before a collector forms a TI 15.4 network, it first needs to check that no other coordinator is operating on the same channel or with the same PAN ID.

Moreover, the local gateway must be able to identify which sensor devices can join the network and which sensor devices are currently active in the network. It does this by relying on an association procedure that verifies that the sensor device has the appropriate network key, and by adding approved sensor devices to an “accept list” if the current resources available on the PAN are sufficient. This “accept list” contains the following information:

• PAN ID of the device
• Short address
• Extended address
• Frame counter
• Device capability information

Consequently, a central gateway implementation must also provide synchronization services, and function as the coordinator for the local gateways that compose the network. Having said this, even though the central gateway must keep the information about the local TI 15.4 networks, it does not need to communicate with the PAN coordinators over the IEEE 802.15.4 e/g MAC and PHY.

The following information related to each local TI 15.4 network needs to be managed by the central gateway:

• Short address mapping: The central gateway should allocate short address spaces to each of the local PAN coordinators to avoid assigning duplicate short addresses to the devices in the network.
• MAC encryption key: Current SDK examples use pre-shared keys for the sensors and coordinators.
• 16-bit PAN identifier of the network
• 16-bit short address of the PAN coordinator
• 64-bit IEEE extended address of the PAN coordinator
• Security: Enabled or disabled
• Mode and operating channel: Network operating mode (beacon, non-beacon, frequency hopping)
• 16-bit short address of the sensor devices in the local network (assigned by the PAN coordinator)
• 64-bit IEEE extended address of the sensor devices in the local network
• Frame counter (replay protection)