OpenBridge is an open protocol to link DMR servers. Protocol designed in cooperation between BrandMeister team and Kurt OE1KBC.
OpenBridge is very simple protocol that allows to send call streams between servers.
Protocol is based on MMDVM protocol and supports DMRD packets only.

At current moment protocol doesn't solve question of routing information exchange.

Basic principals

  • No connection establishment, no keep-alives
  • DMR call streams only, multiple calls at the same time
  • Both sides should have configured end-points
  • Secured by using HMAC-SHA1 signs
  • IPv6 support

Frame format

  • DMR payload
    • 53 bytes of MMDVM's DMRD message
    • Slot bit should be clear
    • Repeater ID field should be filled with peer ID of server
    • Stream ID should identify each unique transmission
    • Sequence number should start from 0 on each transmission
    • Any kind of call is accepted (group, private, data, CSBK, etc)
  • HMAC-SHA1 signature
    • 20 bytes in length
    • Plain text key should be used
    • Key should be unique for each pair of endpoints