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...