In step 4 of the CSMA/CA protocol, a station that successfully transmits a frame
ID: 3539170 • Letter: I
Question
In step 4 of the CSMA/CA protocol, a station that successfully transmits a frame begins the CSMA/CA protocol for a second frame at step 2, rather than at step 1. What rationale might the designers of CSMA/CA have had in mind by having such a station not transmit the second frame immediately (if the channel is sensed idle)?In step 4 of the CSMA/CA protocol, a station that successfully transmits a frame begins the CSMA/CA protocol for a second frame at step 2, rather than at step 1. What rationale might the designers of CSMA/CA have had in mind by having such a station not transmit the second frame immediately (if the channel is sensed idle)?
Explanation / Answer
Suppose that wireless station H1 has 1000 long frames to transmit. (H1 may be an AP
that is forwarding an MP3 to some other wireless station.) Suppose initially H1 is the
only station that wants to transmit, but that while half-way through transmitting its first
frame, H2 wants to transmit a frame. For simplicity, also suppose every station can hear
every other station%u2019s signal (that is, no hidden terminals). Before transmitting, H2 will
sense that the channel is busy, and therefore choose a random backoff value.
Now suppose that after sending its first frame, H1 returns to step 1; that is, it waits a short
period of times (DIFS) and then starts to transmit the second frame. H1%u2019s second frame
will then be transmitted while H2 is stuck in backoff, waiting for an idle channel. Thus,
H1 should get to transmit all of its 1000 frames before H2 has a chance to access the
channel. On the other hand, if H1 goes to step 2 after transmitting a frame, then it too
chooses a random backoff value, thereby giving a fair chance to H2. Thus, fairness was
the rationale behind this design choice.
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