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Simply put, internetworking is required whenever we need to couple dissimilar LAN technologies and control the traffic exchanged between them.

There are various situations where this occurs. Let's look at each one...

One of the jobs of the Network Layer is to provide a means for routing BACnet messages between logically different networks.

In the figure shown, the upper network segment has several workstations and a field panel which regularly exchange information. The lower network has several field panels which communicate with each other for control and monitoring purposes. So normally, there is no traffic between these two segments. By isolating them onto separate network segments, the upper and lower devices need not bother each other with overlapping traffic.

In those instances when, for example, one of the workstations needs some information from a device on the lower network, it can send a message which contains extra routing information which is used to convey the remote destination of the message. In this case, a special device is required which is called a router. A router is a BACnet device which couples two or more network segments together and passes messages back and forth, only when necessary. The router may bridge together two of the same type of LAN segments, such as Ethernet to Ethernet, but more typically routers also serve the function of coupling different LAN types together, for example Ethernet to MS/TP.

One very important role for routers is in dial-up communications. Dial-up is a special case for several reasons.

First, in the BACnet view the two networks on either side of a dial-up connection are separate logical networks. The router which initiates the dial-up through a modem, establishes a logical connection with the answering router. Once the connection is established, the two routers together behave like a single router in a hardwired connection. For this reason, in BACnet such dial-up routers are called half-routers.

It is possible for a router to be both a hardwired full router, and a dial-up half-router at the same time.

The dial-up router has a fairly complex job. Besides establishing and maintaining a connection, and the normal tasks of a full router once connected, the router must also watch for circular connections. This can occur when a network has more than one dial-up router connected to it. It is theoretically possible for network A to have two simultaneous connections with network B through two dial-up routers on each network. The routers can detect this situation and force one connection to be closed. This prevents "broadcast storms" which are streams of messages being forwarded endlessly across both networks.

Even if traffic is not an issue for your BACnet system, there are often compelling economic reasons to consider sharing LAN infrastructure. In most larger buildings, universities and campus office parks, there is often a substantial investment in LAN technology to link the computers in various buildings together for MIS and administrative functions. Typically Ethernet is used for this type of network. The building automation system with BACnet can take advantage of this infrastructure and use the same LAN for some amount of BAS traffic, without having to incur the substantial costs of installing and maintaining the LAN wiring, repeaters, bridges, routers etc. In these cases, other types of BACnet LANs can be coupled into the "corporate or university backbone" LAN with a router.

Other than new buildings, or total retrofits, most facilities which embrace BACnet will have the problem of older systems which are already in place. How can these older systems migrate or integrate with BACnet?

The answer is to use a special device called a gateway. Unlike a router, which sends and receives BACnet messages over different LANs, a gateway must potentially not only deal with different LAN technology, but also different concepts and approaches in the application side of the message. With BACnet routers, only the LAN technology changes, but the messages themselves are the same: they always use the BACnet Application Layer and its concepts.

So the gateway must potentially do a very difficult job. Some of the ideas and concepts in the proprietary system, may have no straightforward equivalent in BACnet, and of course the reverse is also true. The gateway is not just a LAN translator, but it is also a concept translator.

One common approach to gateways is that each proprietary system is viewed as an "island" with the gateway being the only bridge to that island.

Using Existing LAN Infrastructures

One of the advantages to BACnet's transport flexibility is the option to utilize existing LAN infrastructures. While traditionally many building automation systems have used dedicated communications wiring, BACnet enables another option which in many cases has compelling financial advantages.

Because BACnet can use the international standard ISO8802-3 ("ethernet") as a transport medium, in many facilities BACnet devices can be added to an existing ethernet LAN. When this is done, the costs associated with creating a large multi-building LAN, just for the building automation system, can be eliminated. The cost of wiring, routers, repeaters, bridges and maintenance which would normally be a burden can be eliminated by using a portion of the bandwidth on the existing LAN for automation communications.

Experience has shown that properly implemented automation systems, based on BACnet, do not place a substantive additional burden on existing LANs, since the nature of BACnet communications is generally intermittent.

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