Information Technology Reference
In-Depth Information
connected with S1. In the jargon of wireless communication, we say
L1, L2, and S2 have formed an
infrastructure basic service set
(IBSS).
In principle, L1, L2, and S1 might all communicate directly with
each other over the wireless network. However, as shown in the fig
ure, L1 and L2 can be out of range of each other even if they are
connected to the same stationary computer. Further, if L1 and L2
were just barely out of range and L1 were to move around, its sig
nals might be received by L2 at some times and not at other times.
To resolve such potential troubles, the access point in any IBSS re
lays any data sent by any portable machines in its IBSS. This guar
antees that all computers with wireless connections to an access
point hear the same information.
The transmission of data follows much the same approach used
with an Ethernet connection—with one addition. Because several
computers will be sending data over the same media (e.g., radio
waves), messages are broken into pieces, and computers wait until
they hear no messages from other machines. However, for wireless
communications, listening for the lack of another transmission may
not be sufficient. For example, in our scenario, L1 and L2 might
each have data to transmit, but they may be out of range of each
other. If they both started transmitting at the same time, then trans
mission to S1 could be muddled, but neither would know about the
transmission of the other. (Even if they could hear each other in
principle, their own transmission could overpower whatever they
might try to hear, and they still might not realize that concurrent
transmission had occurred.)
To resolve these concurrenttransmission problems, before send
ing a message, a computer first sends a request to transmit to an
other machine. The data are not sent until the initiating machine re
ceives a message back to proceed. If no permission is forthcoming, a
computer must wait for awhile and then ask again for permission.
In this setting, if two requests are sent at the same time but the re
quests interfere with each other, then each party must wait until
new requests are reissued and are transmitted clearly. If one request
is received clearly, then it can be acknowledged and that machine
can proceed. If L1 and L2 make simultaneous requests that are re
ceived, then S1 can respond to just one at a time.
Let's assume now that L1, L2, and S1 have connections to each
other. They can all continue operating smoothly in their own IBSS,
