Information Technology Reference
In-Depth Information
channels back to CCCH. Otherwise, retransmis-
sion will be made after a timeout expires until the
maximum number of retransmissions is reached.
The channel will be tuned back to CCCH upon
the maximum number of retransmissions or the
end of the slot.
Remark:
1) Although the quality of egress
channels can be obtained in LQI
TX
, this informa-
tion may not be up to date. Hence, relying on the
receiver to use CTS to reserve the channel is a
better way. 2) ACK is used to reserve the channel
at the transmitter while the new RTS is for chan-
nel renegotiation.
The
hidden terminal problem
is handled as de-
picted in Figure 2(b). Suppose at first
i
is sending
to
j
on channel 11 and
k
needs to send a packet to
w
. In one-channel MAC protocols, this may result
in one of the three cases: neither
i
nor
k
can send;
collision happens at
j
; or
k
will back off.
In our MQ-MAC, according to the above
design, RTS is broadcast with un-preferred chan-
nel 11, asking
w
to select a channel via CTS;
w
chooses channel 14 according to (1). With the
help of CTS and ACK,
k
can use channel 14 to
forward data to
w
. With different channels,
i
and
j
, and simultaneously
k
and
w
, can communicate
without blocking each other.
Even another delicate situation can be handled.
Suppose that, right after
i
has sent a RTS to
j
,
k
also sends a RTS to
w
, and then
k
receives
j
's CTS
before
w
's CTS, both trying to reserve channel
11. In such a case, as
j
reserved channel 11 first,
k
will give up channel 11 and send out another
RTS to renegotiate with
w
. Finally,
i
and
j
settle
on channel 11 while
k
and
w
settle on channel
14. Simultaneous communications via different
channels are still possible. Hence, hidden terminal
problem is handled.
Furthermore, the
exposed terminal problem
is also handled as shown in Figure 2(c). In one-
channel MAC protocols, when this happens,
normally either
k
will not transmit or
k
still uses
the same channel. Yet, interference between both
communication pairs still exists due to the usage of
the same channel. In our proposed MQ-MAC, by
overhearing
i
's RTS and ACK,
k
is able to select
Figure 2. Proposed MQ-MAC protocol and how it handles hidden/exposed terminal problem
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