OSDP Protocol - Quick Highlights
BUS: RS485 : 9600 Baud (Typical - Supports up to 115200 Baud)
BUS Devices: 32 Devices (Typical - Supports up to 128 Slave BUS devices, using a single master -> multiple slaves topology)
Error Detection: Yes
Encryption Support: Optional AES-128 encryption and CMAC chaining.
Device Capability Reporting: Yes, OSDP-compliant devices support a Capability Report response packet
Primary Use: Access COntrol Reader Support, including Wiegand, ABA, 200 bit binary, raw data and LED / Beeper control.
Here are highlights of SIA's current OSDP specification, and how they apply to everyday access control and security system installation challenges.
Ease of implementation
One of the biggest advantages of using OSDP to control and monitor devices in an access control application is the ease of implementation for programmers and system developers. Card read data is formatted in Wiegand, ABA, 200 bit binary, or raw format. Driving the LED and audible beeper in alternating patterns can be accomplished in one command. Polls and Responses ensure reliable supervision.
Scalable
Up to 126 addressable devices are allowed on one RS-485 line. However, it is more practical to split these up among several channels to reduce electrical loading and poll/response cycle times. A typical configuration is 32 devices per line.
Secure Channel
Secure Channel mode defines a method of implementing encryption, key management, and authentication on an OSDP connection. Using AES-128 encryption and CMAC chaining (see FIPS 197), this method allows relatively easy implementation in control panels and peripheral devices, even if writing in Assembler.
Enhanced audio and visual output
OSDP enhances the user experience through improved sound and visuals at the remote access controlled site. A former typical problem when trying to blink LEDs faster than once every 2 seconds, was that network interface latencies could cause the LED to appear to go out or stay on randomly, due to asynchronous effects (arrythmia). OSDP protocol improves LED, audio or configurable digital output, by setting timed functions which are carried out locally, either with an OSDP-compliant reader or a legacy reader and a conversion device. OSDP also provides a common format for textual displays at the card reader.
Features
A variety of devices can be accommodated using OSDP. For this reason, it is vital to gain a checklist of supported features for each device. For instance, if a single red LED is the only visual indicator, then sending the "turn LED blue" command would be meaningless. Each OSDP-compliant device must support the Capability Report response.
Secure Channel communications
Since many installations require secure channel communications, an authentication/encryption scheme is defined in the OSDP™ specification. It has been determined and accepted that AES-128 is more than enough encryption for the small amount of data being transferred (usually 8 bytes). Keys are managed via inherently secure (install mode) connection or programming cards. AES-128 is relatively easy to implement in low-cost embedded peripheral devices.
Flexibility
Although many card reader/door interface functions are well defined, a manufacturer-specific command allows any unique function to be easily implemented, such as uploading new firmware, configuring the device, etc.
The OSDP specification is meant to define a low-cost means of implementing a standard 2-wire communications protocol. However, there is no limitation to using alternative media such as TCP/IP, Wireless, Fiber Optics, or any other half or full-duplex connection. Gateway products and protocol translators are readily available.
OSDP is not limited to RS-485 twisted pair communication. Any half or full duplex data communication medium is usable with the proper gateway or protocol translator.