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9.7 Asynchronous Links An asynchronous link occurs when a node can receive packets from another node but it can’t send packets to that node. Whenever this happens, this link is not a good link to route packets. In ZigBee PRO, this problem is overcome by the use of the Network Link Status message. Every router in a ZigBee PRO network sends a periodic Link Status message. This message is a one hop broadcast message that contains the sending device’s neighbor list. The idea is this – if you receive your neighbor’s Link Status and you are either missing from the neighbor list or your receive cost is too low (in the list), you can assume that the link between you and this neighbor is an asynchronous link and you should not use it for routing. To change the time between Link Status messages you can change the compile flag NWK_LINK_STATUS_PERIOD, which is used to initialize _NIB.nwkLinkStatusPeriod. You can also change _NIB.nwkLinkStatusPeriod directly. Remember that only PRO routers send the link status message and that every router in the network must have the same Link Status time period. _NIB.nwkLinkStatusPeriod contains the number of seconds between Link Status messages. Another parameter that affects the Link Status message is _NIB.nwkRouterAgeLimit (defaulted to NWK_ROUTE_AGE_LIMIT). This represents the number of Link Status periods that a router can remain in a device’s neighbor list, without receiving a Link Status from that device, before it becomes aged out of the list. If we haven’t received a Link Status message from a neighbor within (_NIB.nwkRouterAgeLimit * _NIB.nwkLinkStatusPeriod), we will age the neighbor out and assume that this device is missing or that it’s an asynchronous link and not use it.
