Information Technology Reference
In-Depth Information
Only one computer at a time can transmit its message along
the cable.
All machines on the network receive every message.
Each machine reads a packet header to determine the in
tended recipient, and the message is saved by only the desig
nated computer(s).
In order to enforce the sending of only one message at a time,
machines check that the cable is not already in use before
starting a transmission. Also, if two computers start transmit
ting at the same time, they both listen while transmitting to
detect this condition, and then they cease their work for a
random amount of time before trying again.
These ideas carry over to wireless communications, as well as
with infrared signals or radio waves replacing the Ethernet cable. In
a wireless setting, communication involves breaking messages into
packets and utilizing headers to identify which machine is to receive
the data. Further, because communication involves infrared signals
or radio waves, all wireless computers in an area share the airwaves,
receive the same messages, and determine if they are the intended re
cipient of each message. Also, two computers could start transmit
ting messages at the same time, thereby interfering with each other.
However, for wireless communications, the lack of a designated
cable adds several complications:
Frequencies used for computer radio transmission must be
controlled so that computers can communicate with each
other without interfering with other electrical devices, such as
television sets, radios, or garagedoor openers.
Several wireless systems may be active in one location, just as
one room might contain cables for several Ethernet cables for
different networks.
Electrical interference in an area may limit the speed at which
data can be transmitted reliably.
The transmission of radio waves can overwhelm a local re
ceiver, so it may be difficult for a machine to determine
whether another computer starts broadcasting a message con
currently with the first.
