Once of the complications when tunneling CN protocols over an IP WAN is that there is no guarantee that packets won’t take more than one router through multiple IP switches and routers endemic to IP WANS. When this happens packets can get out of order. This can be problematic for an automation network depending on the type of messages being sent. 852 provides a solution for this problem by sequentially numbering CNData packets between 852 devices. This way a n 852 router can detect and reorder CNData packets before forwarding them onto the CN channel. The escrow time used to hold out of order packets is tunable for the WAN delays. Stale packet detection also helps in the rare case that a packet is delayed but not out of order. In addition, because 852 provides channel management of virtual channels, one can mix and match devices from different IP subnets all on the same channel. This makes it convenient to integrate devices from remote locations in multi-building or multi-site connection of automation networks. IP having remote WAN support also means that one can do remote protocol analysis of a CN channel. This has become a common feature for CN/IP routers.
Although the 852 standard does not have a standard approach to handing NAT. All the commercially available CN/IP routers have a NAT solution. This has become standardized in the next generation 852.1 protocol.
In addition some WAN channels have limited bandwidth, so the packet aggregation feature of 852 enables packing many CN packets into on larger IP packet to reduce traffic. The Adept GR4 router even has an optional low bandwidth serial transaction mode to limit the bandwidth consumed by management packets. Moreover, the 852 md5 authentication provides security over open WAN networks to prevent spoofing or reception of fake sensor data or actuator commands. These are all features that other CN tunneling approaches have to add on a vendor by vendor basis. Because IP WiFi is an inexpensive and ubiquitous technology, 709.1 networks can leverage this through 852 in applications where no extant wiring is available. There is a also a compact WiFi enabled version of the Adept router, but any of the LON/IP routers can be used over a WiFi channel with the addition of an inexpensive WiFi to Ethernet router access point. One advantage of using 852 on an IP WiFi network is that one can now have roaming connections which can be advantageous when doing network setup and debugging task.
Fig. 2.1: Multi-site building automation network with internet connectivity