Motorola 557925-001-00 ユーザーズマニュアル
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Media Access Control
802.11 wireless devices are half-duplex; meaning they cannot transmit and receive
simultaneously. Access to the media is controlled by carrier detect media access - listening on
the specified frequency, and transmitting in a time when no other signal is present. Using carrier
detect many devices can share the same RF medium. Adding CTS/RTS further increases the
number of co-channel devices; although CTS/RTS itself reduces the total bandwidth available to
the client.
simultaneously. Access to the media is controlled by carrier detect media access - listening on
the specified frequency, and transmitting in a time when no other signal is present. Using carrier
detect many devices can share the same RF medium. Adding CTS/RTS further increases the
number of co-channel devices; although CTS/RTS itself reduces the total bandwidth available to
the client.
Carrier Detect Range
Carrier detect media access operates within the maximum distance that a signal will be received
above the noise floor of a neighboring device. This is the carrier detect range. When laying out
the network topology, it is important to factor the carrier detect range. For the Wireless WallPlate,
the carrier detect range is -94dBm, the sensitivity level of the receiver. Note that the carrier
detect range is not a physical distance; however, some attempt is made in this document to
correlate physical distances to the important RF domain since we all live in the real world.
above the noise floor of a neighboring device. This is the carrier detect range. When laying out
the network topology, it is important to factor the carrier detect range. For the Wireless WallPlate,
the carrier detect range is -94dBm, the sensitivity level of the receiver. Note that the carrier
detect range is not a physical distance; however, some attempt is made in this document to
correlate physical distances to the important RF domain since we all live in the real world.
Pathloss
Pathloss is defined as the ratio of transmit to received power, normally expressed in dB. It can be
measured by subtracting the receive signal strength in dBm from the transmit signal power
in dBm. Pathloss calculations are always one-way; from a transmitter to a receiver. In theory,
the return path should be the same, but does vary. It is not uncommon to see asymmetrical
pathloss.
It is desirable for a client device to have a low pathloss to more than one WallPlate. The client
driver normally associates with the AP having the strongest signal (lowest pathloss).
Because of the material used in hotel construction, pathloss is difficult to mathematically
calculate. It must be measured. While this implies a comprehensive site survey; in fact, some
basic assumptions can be made from a relatively simple site survey using only the received
signal strength (RSSI).
measured by subtracting the receive signal strength in dBm from the transmit signal power
in dBm. Pathloss calculations are always one-way; from a transmitter to a receiver. In theory,
the return path should be the same, but does vary. It is not uncommon to see asymmetrical
pathloss.
It is desirable for a client device to have a low pathloss to more than one WallPlate. The client
driver normally associates with the AP having the strongest signal (lowest pathloss).
Because of the material used in hotel construction, pathloss is difficult to mathematically
calculate. It must be measured. While this implies a comprehensive site survey; in fact, some
basic assumptions can be made from a relatively simple site survey using only the received
signal strength (RSSI).
Signal Loss in Material
Most RF pathloss discussions focus on the losses in free space, without consideration for the
attenuation through different wall material. In fact, the attenuation that occurs in a 27cm concrete
block wall is nearly equal to the signal loss over 6 meters in free space! (using the formula
Pathloss in dB = (Loss at 1 Meter) +
attenuation through different wall material. In fact, the attenuation that occurs in a 27cm concrete
block wall is nearly equal to the signal loss over 6 meters in free space! (using the formula
Pathloss in dB = (Loss at 1 Meter) +
α * 10 * LOG(Distance in meters) + Xσ). To simplify this
process; we can combine the loss through free space in one room with the loss through one wall
material and arrive at a value that can be applied cumulatively through multiple rooms.
material and arrive at a value that can be applied cumulatively through multiple rooms.
bath
bed
bed
TV/
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k
RoomLoss in dB =
(Loss through wall material) + (Loss in free space in room)
(Loss through wall material) + (Loss in free space in room)
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