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Re: TDMA - Reading the mail



>I understand what you are saying.  And in a one person to one person or
>point to point QSO this works great.  But in a point to multi-point
>environment where those "multi" points are not necessarily "well behaved",
>how can you do that?  If P51AA (North Korea for those who don't chase DX)

Good question, and I have several proposed solutions.

Approach #1. You allocate a timeslot to the round table. When somebody
in the group pushes the transmit button, his station first checks to
see that no one else is transmitting in the slot. If the slot is
clear, the station begins transmitting in the slot. If the station
doesn't see its own data come back to him after the appropriate
propagation delay, or if it sees another user in the slot, it stops
transmitting, waits a random amount of time and tries again.

Approach #2. Instead of allocating a timeslot to the group, you
allocate distinct timeslots to each user when they transmit, perhaps
by using the same algorithm above for a timeslot chosen at random by
each station. The members of the group demodulate all the slots
looking for packets directed to the group (every packet contains a
destination address and a source address).  All packets to the group
are decoded into audio and summed into the speaker at each receiver.

The nice thing about the second approach is that it lets more than one
person talk at the same time or to interrupt, just as in a physical
group meeting.  A full duplex two-party telephone-style conversation
would just be a special case of this same thing. It also lets
individual receivers mute individual transmitters (e.g., those making
disruptive remarks) without blocking them for everyone else as well.

The VAT tool used for voice conferencing on the Internet multicast
backbone works in much this way. I've always thought it was the right
approach.

Phil




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