General protocol rules

The SCOM protocol connects two SCOM-enabled devices. Both sides of a SCOM Communication link have equal rights, at the SCOM level none of them is privileged (no master/slave architecture). The basic rules of data exchange are:

The device can send a packet at any time, as long as it is not waiting for the acknowledgment of a previously sent packet.
The device that received a correct packed immediately sends an acknowledgment. If the receiving device it's busy sending it's own packet, the acknowledgment is sent immediately after the packet was transmitted.
After sending the packed, the device waits for an acknowledgment for a specified amount of time. When the acknowledgment comes, the device is ready to send another packet. If the acknowledgment doesn't come, the packet is retransmitted with highest priority. After a specified number of failed retransmissions, the link is reported to be broken.

SCOM device identification

Each SCOM device is identified by a 32-bit class identifier (type of device) and 128-bit unique device ID. Both of these properties are user-defined, and set up during SCOM initialization.

SCOM packets

Each SCOM packet (a.k.a. frame) has the following properties:

type - an 8-bit number defining type of a packet
priority - (0..7) defines packet priority, that controls packet transmission order
acknowledgement - defines whether the packet requires acknowledgment (default) or not
payload - user-defined data payload

Single-frames and multi-frames

A single SCOM frame can carry SCOM_MAX_DATA_LENGTH bytes at most. Such frame is called a single-frame. In case the user payload is larger than SCOM_MAX_DATA_LENGTH, it is possible to send the whole payload using built-in SCOM fragmentation. In such case the payload is divided and carried by multiple so called multi-frames. Using SCOM API it is possible to divide and glue back a larger payload in a convenient way.

SCOM services

The SCOM protocol supports the concept of standardized services. the services group SCOM packets and allow to extend the range of frame types. Each service is identified by an 8-bit identifier, that is also a common frame type for service-related packets. In case of such service frame, the payload field is shortened by 1 byte, and the resulting space is used as an additional frame type specifier in the given service.