Communications Protocol

Standard Gestalt Packet Frame

All communications between the pyGestalt virtual nodes and Gestalt-based physical nodes rely on the standard Gestalt packet format, shown below:

Byte Position	Description
Byte 0	Start Byte (72 = unicast, 138 = multicast)
Byte 1	Destination Address (Byte 0)
Byte 2	Destination Address (Byte 1)
Byte 3	Destination Port
Byte 4	Packet Length
Byte 5 -> N-1	Payload
Byte N	Checksum

• Start Byte: Besides beginning the packet, this byte also defines whether the packet is unicast or multicast. A unicast packet will only be received by the node whose address matches that specified in the packet. A multicast packet, on the other hand, will be received by all nodes on the Gestalt network.
• Destination Address: Each node on the Gestalt network is assigned a unique two-byte address, which is mirrored in the corresponding virtual node. In this way, messages can be routed back and forth between a virtual node and its physical counterpart.
• Destination Port: Just as addresses are used to pair virtual and physical nodes, port numbers are used to pair individual service routines. For example, a service routine in the virtual node on Port 10 would communicate to a corresponding firmware service routine on Port 10. The port number is a single byte. Ports 1-9 and 255 are reserved by the Gestalt library.
• Packet Length: The total length of the packet minus 1, in bytes. For example, a packet consisting of 15 bytes total (including the payload), would have a length byte equal to 14.
• Payload: A variable-length payload. This is used to transmit data between service routines on the virtual and physical nodes. It is possible (and common) for the payload to have zero length.
• Checksum: A checksum generated from the sequence of bytes in the packet, and used to guarantee that the packet has not become corrupted in transmission. Gestalt uses the CRC-8 checksum algorithm.

Serial Communication Protocol

Gestalt nodes typically communicate over a asynchronous serial connection, often via a USB-to-serial converter. The format is 8 data bits, no parity bits, and 1 stop bit.

Standard Baud Rates

Node Type	Baud Rate
Standard Gestalt Node	115.2 kbps
Arduino-Based Gestalt Node	38.4 kbps

The standard baud rates above were based on the clock frequency of the microcontroller. Custom Gestalt nodes use a 18.432Mhz crystal, which was selected to have zero rate error at 115.2kbps (and all standard baud rates). Because Arduinos rely on a 16Mhz crystal, communication speeds are limited by the rate error. A non-standard speed of 76.8kbps would have been ideal, but is unsupported in Linux. Therefore, we use a standard baud rate of 38.4 kbps. This has a rate error of 0.2%.

Typical Packet Exchange Rates

Packet throughput is limited not only by the communication baud-rate, but also internal delays in the USB-> serial device driver, the operating system, and within the pyGestalt framework. We measured the actual round-trip packet exchange rate, measured in packets per second, for several types of nodes. You can see the full results here (including how to run the tests yourself), or the summaries below. The pyGestalt framework itself is currently able to process 385 round-trip packets per second as an upper limit to overall system throughput.

Arduino Uno Node, pyGestalt running on Mac OSX:

Payload (Bytes)	0	5	15	20	25	45	55	115	200
Packets per sec.	93	63	55	35	29	28	16	10	8

Networked Gestalt Node, pyGestalt running on Mac OSX:

Payload (Bytes)	0-15	20	25	30	40	85	100	150	200
Packets per sec.	63	52	44	41	37	28	19	14	14