In article . com,
mazerom wrote:

1. what will happen when you have two transmitters(digital spectrum) @
the same carrier freqeuncy and is received by a superhet receiver
exactly at the same time, although unlikely (2.4ghz ISM band)

The same thing that happens in any similar situation at lower
frequencies. The two carriers will interfere. Unless they are
accurately locked to a common frequency reference (unlikely) there
will be a significant beating between them. The information-carrying
sidebands of the two transmissions are likely to interfere, and the
transmissions are likely to be garbled at their intended reception
points. The degree of garbling/interference will depend on the signal
strengths and also on the modulation scheme used.

2. what are ways and means to prevent this collision?csma/ca?rts/cts
setup?how?

There are several approaches which have been used.

The simplest is probably CSMA/CA, where each stations listens, doesn't
transmit when it detects a carrier, and waits for some variable amount
of time (often randomly-chosen) after carrier goes away before it
starts transmitting. You'll tend to find this used in ad-hoc networks
of various sorts, where there's no central coordinating authority for
the right-to-transmit. For example, most amateur-radio AX.25 packet
networks here in the US work this way. Collision rate tends to go up,
and bandwidth utilization maxes out and decreases when there are many
stations trying to operate on a single frequency.

The classic problem with CSMA/CA on radio is the "hidden node"
problem. If two nodes can't hear one another, they can't avoid one
another... and their transmissions can often both be heard at other
stations and will interfere.

A second approach is an RTS/CTS system, coordinated by a central node
which can be heard by all other nodes. This is a common mode on
802.11b networks, where coordination is performed by the 802.11b
access point.

A third approach is a polling approach, where the outlying nodes never
transmit (not even an RTS) until specifically polled by the central
authority-node. I believe that many of the AX.25 packet networks in
Europe use this approach, in order to minimize collisions and improve
bandwidth utilization over what's possible with CSMA/CA.

If there's no central authority to handle polling or RTS/CTS, then
some form of ad-hoc "token passing" can be used, where each node
receives an explicit handoff of the right-to-transmit from another
node, and either sends what it has to send, or forwards the right-to-
transmit packet to the next node in the sequence.

Time-division multiplexing (e.g. slotted Aloha) is another method...
nodes coordinate to agree on which time slots each is allowed to
transmit in. This can minimize or eliminate collisions (except
perhaps during setup) but it can waste bandwidth, because timeslots
can go unused if the nodes which "own" them don't have anything to say
at the moment.

So - there are lots of approaches - which one is best depends a great
deal on the environment and application.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
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