CHANNEL CONTENTION SOLUTION FOR TETRA DIRECT MODE DM GATEWAYS

MOTOROLA

Technical Developments

CHANNEL CONTENTION SOLUTION FOR TETRA DIRECT MODE DM GATEWAYS

by Steve Valentine

ABSTRACT

Two TETRA direct mode gateways present on the same direct mode radio communication channel are assigned specific talk-group procedures for use in the case where both gateways are affiliated to the same talk group. Each gateway only attempts to set up a talk-group call after a random period of time, specific to that gateway, has elapsed. If a gateway detects traffic signaling activity during this time then the call set-up message will not be sent.

INTRODUCTION

This paper describes an improved channel access mechanism intended to minimize signaling message collisions between DM Gateways operating on the same radio frequency channel within the same geographic location.

PROBLEM TO BE SOLVED

The current DM Gateway standard does not provide any channel access specific procedures to handle the case where a DM Gateway receives a presence signal from another DM Gateway present on the same free DM channel. Currently the standard wonld allow a DM Gateway to commence signaling on the DM channel immediately, because the channel is assumed free.

Thns in the case where the two DM Gateways are operating on the same DM channel, and are both affiliated members of the same voice and data (V +D) talkgroup, there is a high probability that both DM Gateways will attempt to set-up the talkgroup call on the DM channel simnltaneously.

If this happens the communication shall fail on the DM channel due to collision. There is currently no mechanism defined within Tetra DMO Gateway

Motorola, Inc. 2000

standard (ETS 300 396-5) to avoid collisions between DM Gateways operating on the same talk group.

DM CHANNEL ACCESS PROCEDURES

The TETRA standard states that both DM subscribers and DM Gateways must conduct channel monitoring to determine the current state of the DM channel prior to transmission. Where fast call setup is desired, this channel monitoring shall be conducted when the radios are idle on the DM channel.

The TETRA DM Gateway standard also pennits a DM Gateway to transmit a presence signal periodically on the selected DM channel. Detection of the DM Gateway signaling informs all radios monitoring the DM channel including other DM Gateways that the DM Gateway is present and available. The presence of this DM Gateway broadcast on the channel does not mean that the DM channel is busy, as presence signals can be transmitted on free channels or during traffic transmissions.

The current TETRA standard allows a DM Gateway to begin call set-up procedures on the selected DM channel for a circuit mode call originated on the V+D side, if its channel surveillance procedures indicate that the DM channel is free.

In the special case of a DM Gateway which observes a channel change, for example from busy to free when a previous call on the DM channel in which the DM Gateway did not participate has now stopped, the DM Gateway shall choose an integer R randomly...