2.6 KiB
SSS7 Modems
SSS7 is the Seidenstrasse Signaling System 7.
This repository contains the schematics its the hardware, along with a description of the physical layer and the transport layer. The actual application layer will be developed independent of this specification.
Physical Layer
The physical layer is based on a RS248 three wire bus (a differential pair for data and ground), using the MCP2551 is used as transceiver chip. Since it is actually a CAN-Transceiver, it has a well defined behaviour in case of collisions (no short circuits possible) and it allows to read back the bits which are actually on the bus while sending. This enables any device with a decent UART (microcontroller, raspi ...) to communicate over long distances (2000m+). The data is send at 9600 Baud (aka. 9,6kbit/s) with a maximum baudrate error of +-5%, to ensure that even the slowest micro controller can participate in the communication. A slow bitrate also necessary to keep communication and collision detection reliable over longer distances.
Transport Layer
A device is allowed to send data after the bus has been idle for at least 24 bit times (aka. 2.5ms). After this period the sender is allowed to send a single frame. The frame format is simply the first byte is the length of the entire frame in bytes (= N + 3), followed by N bytes of payload. The reaming 2 bytes of the are a 16bit CRC checksum over the frames payload. If a received frame is shorter than it length field or if its CRC sum does not match its payload, any receiving devices should assume a collision and drop the frame. Additionally each frame is associated to a priority between 0 (highest) and 5 (lowest), which is not send over the bus. Instead the priority is used to calculate the necessary back off in case of a collision.
The sender has to detect collisions by reading back each byte written to the bus immediately after writing it.
If the read byte does not match the written byte, a collision occurred.
In this case the sender has to wait for random back off interval until attempting a retransmit.
The back off interval is calculated as (8 * bit_time) * (priority + rand(1,10)),
where bit_time = 1 / 9600 s and priority is the frames priority.
Device receiving on the bus are not necessarily able to detect collisions,
unless they show up as timing error at their UART.
Therefore receiving devices should rely only on the length and the CRC sum of frame to detect collisions.
Licenses
The schematics and pcb layout files in hardware are released under the CERN Open Hardware Licence v1.2.