Transcript commandable objects

INFO 203
IT for Engineers
BACnet
Dr. Jennifer Booker
INFO 203 week 10
1
www.ischool.drexel.edu
Networking Review
• Detailed overview of computer networking
introduced the layers of networking, their
services, and their protocols
– Application layer, home to the applications we
use daily – HTTP, FTP, SMTP, POP3, IMAP,
MIME; and DNS behind the scenes
– Transport layer, home to TCP and UDP
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Networking Review
– Network layer, home to IPv4 and IPv6, plus
routing protocols within an autonomous
system (RIP, OSPF) and among them (BGP)
– Link layer, home to Ethernet and PPP, plus
the support protocol ARP
– Physical layer, where protocols describe how
to implement a link layer protocol over a
particular medium (e.g. PPP over a copper
wire)
INFO 203 week 10
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BACnet
• Now use this understanding of networking
to investigate BACnet (Building
Automation Control Network) to enable
interoperability among building systems
and devices
• BACnet was developed in 1987 by
ASHRAE and first standardized in 1995 by
ANSI and in 2003 (ISO 16484)
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BACnet
• BACnet can be based on Ethernet or IP
networks, using a twisted pair link protocol
MS/TP (see EIA-485)
• Object-oriented systems capture data and
the functions they can use in objects
– BACnet is object-oriented, and defines 54
standard objects that can be used or not
independently of each other
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BACnet services
• BACnet provides a range of services
– Object access
– Alarm and event management
– Scheduling
– Trending
– Files
– Device and network management
– Web services
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BACnet testing
• BACnet devices and applications are
tested by the BACnet Testing Laboratory
(BTL), in accordance with ASHRAE
Standard 135.1
• Applications include HVAC, fire, security,
lighting, elevators, etc.
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Devices
• BACnet devices act as a controller,
gateway, or user interface
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Objects
• Objects manage physical or logical
information, e.g. analog or binary inputs,
control algorithms, or other applications
• The 54 standard objects means many
kinds of devices can be managed without
developing new specific code for them
– But non-standard objects are allowed
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Objects
• Objects define and object identifier and a
collection of properties
• Properties may have read only or
read/write ability, and can be required or
optional
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Services
• BACnet services are requests from one
device to another
– object access (read, write, create, delete);
– device management (discover, time
synchronization, initialize, backup and restore
database);
– alarm and event (alarms and changes of state);
– file transfer (trend data, program transfer);
– virtual terminal (human machine interface via
prompts and menus).
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Transport and Network
• BACnet allows several transport protocols
• BACnet can run over seven network types
– IP (easy and fast but expensive)
– MS/TP (Master-Slave/Token Passing)
– Ethernet (if no IP routers are involved)
– ARCNET (rarely used)
– PPP (used only for dial-up connections)
– LonTalk (passes messages only)
– Zigbee (connect to local wireless devices)
INFO 203 week 10
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Routing
• A major feature of BACnet is its ability to
join many subnetworks together
• A BACnet router might join different
BACnet networks from different network
types
– For example Broadcast Management Devices
(BBMDs) are used to join BACnet/IP subnets
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Interoperability
• The five areas of interoperability provided
by BACnet are defined by the BACnet
Interoperability Building Blocks (BIBBs):
– Data sharing
– Trending
– Scheduling
– Alarm & Event Management
– Device & Network Management
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Data Sharing
• Data can be sent uni- or bi-directionally
between devices for archives, reports,
sensor readings, control messages, or
other purposes
• It uses a client-server model
• Read and Write properties are allowed
– There are 16 levels of priority available for
controlling writing to commandable objects
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Data Sharing
• Data can also be shared via a Change of
Value (COV) mode
• COV reduces network traffic by only
reporting larger changes in data values on
a subscribed device
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Trending
• A Trend Log object collects data samples
from devices
• Then a supervisory workstation reads the
trend log periodically
• Helps cut down on network traffic
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Scheduling
• Schedules can be maintained in devices to
time controlling them
• Dates, times, day of week, week of month,
and month can be used to manage
schedules
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Alarm & Event Management
• Data can be sent based on alarm limits or
event triggers
• Logs and summaries can be generated
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Device & Network Management
• This service allows detection of new
devices (“device binding” based on
network number and MAC address),
objects within devices, establish
communication, synchronize time, and
reboot devices
• A process like ARP helps identify devices
and objects automatically
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Interoperability Conformity
• The Protocol Implementation and
Conformance Statement (PICS) tells what
BACnet features have been implemented
in a particular device
• Based on BIBBs, which address the five
interoperability areas already discussed
• Testing approved by BTL
conforms to ASHRAE 135.1
INFO 203 week 10
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BACnet over IP (B/IP)
• ANSI/ASHRAE 135-1995 defines how
BACnet works over IP-based networks
• We want B/IP devices to
– Communicate with each other
– Send and receive local broadcast messages
– Receive global broadcast messages
• Devices have unique IP addresses but
routers need to know a network number
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B/IP
• Uses UDP because it’s already well known
and doesn’t mean creating a new IP
protocol code
• In a simple intranet, devices are identified
by IP address and port number
• The link layer protocol is not specified
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VLL
• BACnet defines a virtual link layer (BVLL)
to help higher layers view BACnet
consistently no matter what link protocol is
actually used
• Devices communicate via unicast
messages and the B/IP addresses
– Broadcast messages are limited to the subnet
not the regular IP broadcast address
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Broadcast
• Broadcast messages are not routed to
allow large B/IP networks without flooding
traffic
• Consider IGMP (group management
protocol) but instead create the BACnet
Broadcast Management Device (BBMD)
– Passes broadcast messages from one subnet
to another; does little else
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BBMD
• After rejecting star and ring architectures,
BBMD was based on a fully connected
star
– All BBMDs are connected to all other BBMDs
logically
– Each BBMD must know the whole network
architecture, and uses the same Broadcast
Distribution Table (BDT)
– There are BDT Read and Write messages
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Foreign devices
• A foreign device has an IP address
outside the B/IP subnet it wants to join,
such as a dial-up connection via SLIP or
PPP
• BBMDs use messages to register foreign
devices and maintains a table of them
– This foreign device table can be read and is
then acknowledged
– Can delete entries too
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Foreign devices and networks
• BBMDs can forward broadcast messages
to foreign devices and receive those
messages from them too
• B/IP devices can IP devices outside of the
BACnet, using the B/IP address to extract
the IP and port number data needed
• Broadcast messages can also be sent to
non B/IP networks or vice versa
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Connecting B/IP networks
• Two or more B/IP networks can be joined
• Need to make sure device identifiers are
unique across all networks (internetworkwide)
– Can often share a BACnet network number
• Must connect the networks with a BACnet
router
– The router is registered as a foreign agent on
both/all networks
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Connecting B/IP networks
• The same device can be a BBMD and
BACnet router
• Within a B/IP router the ports are assigned
B/IP network addresses
• Directly connected networks can receive
local broadcast or multicast messages
• Other networks are considered ‘remote’
and are reached via a router
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References
• Introductory material through slide 21 from
http://bacnet.membershipsoftware.org/files/HomepageIntroduction%20to%20BACnet/BACnet%20Introduction
%20-%20V3-1.pdf
• Protocol details (slides 22-30) from
http://www.bacnet.org/Tutorial/BACnetIP/index.html
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