2.2 IP LAN for high speed CN backbones

This entry is part 7 of 11 in the series Lon over IP (709.1/852)

Since the IP Ethernet LAN channels such 100/1000 Base-T, can support much higher traffic speeds than CN channels, there are well suited as a backbone for large CN installations. This also makes it easy to attach monitoring and management tools. They just need an 852 interface.A n example is show in the figure below.

IP LAN for CN/IP Backbone

Fig. 2.2:  IP LAN for CN/IP Backbone

When building 852 channels with a large number of devices there is a potential problem with using IP. The typical IP packet is sent unicast. This does not match very well how CN channels work. Most CN channels are multi-drop shared channels where any packet on the channel can  be seen by any of the devices. This is not true with IP Unicast. The IP stack will drop any  unicast packets not addressed to the receiving host. This means to replicate the multi-drop nature of the CN channel multiple unicast messages must be sent, potentially one for every device in the 852 channel. This causes an exponential explosion in traffic. Say you have 64 devices in the IP-852 channel. Every CN packet on the CN side could become 63 IP packets on the 852 side. This is an exponential increase in the traffic that can quickly overwhelm the IP stack on the CN/IP routers. The 852 protocol ameliorates this problem in several ways.

The first is through selective forwarding. When selective forwarding is supported, each of the CN/IP routers in an 852 channel gets a copy of the routing tables for all the other routers on the 852 channel. Each router when forwarding a packet then checks the destination address of a packet received on is CN side against the routing table of each of the other 852 routers to see if a particular router would forward the packet across to its CN side. If not then the originating router does not send the IP packet. This way for channels with configured routers the IP side traffic is minimized and only broadcast CN packets get sent to all the 852 routers.

The second method that 852 can use to manage IP traffic is it supports IP multicast. With IP multicast addresses, all the devices can share a common IP address that the IP stack will propagate up to the application. Thus if an 852 channel needs to support CN networks with a lot of peer-to-peer traffic between CN channels or a lot of broadcast messages, the 852 channel could  use IP multicast instead of or in addition to unicast. The following figures illustrate the differences between multicast and unicast.


Fig. 2.3:  Unicast


Fig. 2.4:  Multicast

The third method 852 can use to manage IP traffic is the aforementioned packet aggregation. The maximum un-fragmented UDP packet size is 548 for a typical IP header. This is much larger than the typical CN packet. Since 852 can aggregate multiple CN Data packets into one IP packet,  the exponential increase in unicast traffic can be significantly reduced.

For CN Data packets 852 uses UDP which is a low latency protocol much better suited to the low latency typically expected on CN channels. For channel management packets, 852 can use either TCP or UDP packets. Because TCP is more complicated, UDP is the usual. But UDP has one drawback for large numbers of 852 devices on a single channel, that is, the size of a management packet can get bigger than 548 bytes. For this reason, 852 supports segmenting of 852 management packets when using UDP. This provides a lightweight facility similar to TCP.

Series Navigation2.1 Multi-site building automation networks over an IP WAN2.3 852 to 852 Bridging Router
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