bacnet开发相关资料

produced a list of desirable attributes of a good protocol, and what the BACnet protocol

eventually became: Interoperability, Efficiency, Low Overhead, Highest Common

Multiplier, Compatibility with other applications and networks, Layered OSI model

Network, Flexibility, Extensibility, Cost Effective, Transmission Reliability, Apply to real-

time processes, Maximum Simplicity, Allow priority schemes, Medium access fairness,

and Stability under realistic loads.

application layer and a simple network layer are defined in the BACnet standard. BACnet

provides five options that correspond to the OSI data link and physical layers. Option 1 is

the logical link control (LLC) protocol defined by ISO 8802-2 Type 1, combined with the ISO

8802-3 medium access control (MAC) and physical layer protocol. ISO 8802-2 Type 1

provides unacknowledged connectionless service only. ISO 8802-3 is the international

standard version of the well-known "Ethernet" protocol. Option 2 is the ISO 8802-2 Type 1

physical layer. Option 4, the Point-To-Point protocol, provides mechanisms for hardwired or

dial-up serial, asynchronous communication. Option 5 is the LonTalk protocol. Collectively

these options provide a master/slave MAC, deterministic token-passing MAC, high-speed

contention MAC, dial-up access, star and bus topologies, and a choice of twisted-pair, coax,

or fiber optic media. The details of these options are described in Clauses 7 through 11.

features and requirements of BAC networks, including a constraint that protocol overhead

needed to be as small as possible. The reasoning behind the selection of the physical, data

link, network, and application layers for inclusion in the BACnet architecture is outlined in

technology in use on that network. Whereas the data link layer provides the capability to

address messages to a single device or broadcast them to all devices on the local network,

the network layer allows messages to be directed to a single remote device, broadcast on a

remote network, or broadcast globally to all devices on all networks. A BACnet Device is

uniquely located by a network number and a MAC address.

automatically using the network layer protocol messages defined in this clause. Network

layer protocol messages facilitate both the auto-configuration of routers and the flow of

messages to, and between, routers. BACnet routing capability may be implemented in

stand-alone devices or, alternatively, in devices that carry out other building automation and

control functions.

maintenance, and network layer congestion control.

Who-Is-Router-To-Network

This message is indicated by a Message Type of X'00' optionally followed by a 2-octet

network number. Who-Is-Router-To-Network is used by both routing and non-routing nodes

to ascertain the next router to a specific destination network or, in the case of routers, as an

through the router generating the message. It shall always be transmitted with a broadcast

MAC address.

I-Could-Be-Router-To-Network

This message is used to respond to a Who-Is-Router-To-Network message containing a

connection is not currently established.