Руководство По Проектированию для Cisco Cisco Aironet 350 Mini-PCI Wireless LAN Client Adapter
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Enterprise Mobility 4.1 Design Guide
OL-14435-01
Chapter 3 WLAN Radio Frequency Design Considerations
Manually Fine-Tuning WLAN Coverage
Mandatory, Supported, and Disabled Rate Modes
You can use the data rate settings to choose which data rates the wireless device can use for data
transmission. There is a direct correlation between data rates, range, and reliability. The lower the data
rate, the greater the reliability and range for a given power setting. Sites vary for specifics, but a
reasonable rule of thumb for carpeted space is an order of magnitude of increased reliability for every
time you halve the data rate. Range is generally affected by a factor of a 30 percent increase
(approximately) for every halving of data rate. Managing the square footage of the area covered within
a -67 dBm edge can be effectively managed using this technique. Setting the data rates to match client,
application, or user needs is an effective RF design element that should be considered before deploying
APs.
transmission. There is a direct correlation between data rates, range, and reliability. The lower the data
rate, the greater the reliability and range for a given power setting. Sites vary for specifics, but a
reasonable rule of thumb for carpeted space is an order of magnitude of increased reliability for every
time you halve the data rate. Range is generally affected by a factor of a 30 percent increase
(approximately) for every halving of data rate. Managing the square footage of the area covered within
a -67 dBm edge can be effectively managed using this technique. Setting the data rates to match client,
application, or user needs is an effective RF design element that should be considered before deploying
APs.
Data rates are expressed in megabits per second. You can set each data rate to one of three modes:
•
Mandatory—Allows transmission at this rate for all packets, both unicast and multicast. The data
rate on at least one of the APs must be set to Mandatory, and all clients that associate to the AP must
be able to physically support this data rate on their radio to use the network. Additionally, for the
wireless clients to associate to the AP, they must be able to currently receive packets at the lowest
mandatory rate and their radios must physically support the highest mandatory data rate. If more
than one data rate is set to mandatory, multicast and broadcast frames are sent at the highest common
mandatory transmission rate of all associated clients (the lowest mandatory receive rate of all of the
clients). This allows all clients to receive broadcast packets. The lowest mandatory rate is normally
set at 1 Mb/s.
rate on at least one of the APs must be set to Mandatory, and all clients that associate to the AP must
be able to physically support this data rate on their radio to use the network. Additionally, for the
wireless clients to associate to the AP, they must be able to currently receive packets at the lowest
mandatory rate and their radios must physically support the highest mandatory data rate. If more
than one data rate is set to mandatory, multicast and broadcast frames are sent at the highest common
mandatory transmission rate of all associated clients (the lowest mandatory receive rate of all of the
clients). This allows all clients to receive broadcast packets. The lowest mandatory rate is normally
set at 1 Mb/s.
•
Supported—Allows transmission at this rate for unicast packets only. The AP transmits only unicast
packets at this rate; multicast and broadcast packets are transmitted at one of the data rates set to
mandatory. The wireless clients always attempt to transmit and receive at the highest possible data
rate. They negotiate with the AP for the highest data rate set to supported or mandatory to transmit
and receive unicast packets. The wireless client devices are able to receive broadcast or multicast
packets at any mandatory rate at or below the negotiated rate.
packets at this rate; multicast and broadcast packets are transmitted at one of the data rates set to
mandatory. The wireless clients always attempt to transmit and receive at the highest possible data
rate. They negotiate with the AP for the highest data rate set to supported or mandatory to transmit
and receive unicast packets. The wireless client devices are able to receive broadcast or multicast
packets at any mandatory rate at or below the negotiated rate.
•
Disabled—The AP does not transmit data at this rate.
Lowest and Highest Mandatory Rate Settings
Multiple clients associated to the AP can have different transmission rates, depending on interference,
obstacles, or their distance from the AP. For example, if an 802.11b client is far from the AP and can
only transmit and receive at a speed of 1 Mb/s because of the distance, it would be able to associate to
the AP because the lowest mandatory rate (see
obstacles, or their distance from the AP. For example, if an 802.11b client is far from the AP and can
only transmit and receive at a speed of 1 Mb/s because of the distance, it would be able to associate to
the AP because the lowest mandatory rate (see
) is set to 1 Mb/s. If a second 802.11g client
associates to the AP at 54 Mb/s, the AP would transmit broadcasts and multicasts at 1 Mb/s because this
is the highest mandatory rate that all clients can receive. If the lowest mandatory rate was set to 5.5 Mb/s,
the 802.11b client would not be able to associate to the AP because it could not receive broadcast packets
at the lowest mandatory rate.
is the highest mandatory rate that all clients can receive. If the lowest mandatory rate was set to 5.5 Mb/s,
the 802.11b client would not be able to associate to the AP because it could not receive broadcast packets
at the lowest mandatory rate.
In
, note that the highest mandatory setting is 11 Mb/s. The highest mandatory rate tells the
AP what rate the client radios must be able to physically transmit at. This does not mean that they are
actually transmitting and receiving packets at that rate, it just means that the radio physically supports
that rate; the wireless client needs only to be able to receive packets at the lowest mandatory rate.
802.11b devices would be able to associate to the AP shown in
actually transmitting and receiving packets at that rate, it just means that the radio physically supports
that rate; the wireless client needs only to be able to receive packets at the lowest mandatory rate.
802.11b devices would be able to associate to the AP shown in
because their radios can
physically transmit at 11 Mb/s. If a higher data rate (such as 18Mb/s) was set to mandatory, only 802.11g
clients would be able to associate to the APs.
clients would be able to associate to the APs.
Setting any of the OFDM rates (rates above 11mb/s) to mandatory disables 802.11b connectivity. This
can, for example, allow the administrator to exclude 802.11b clients from the AP by requiring an
802.11g data rate or setting a minimum transmission rate of all clients by disabling 802.11 rates. The
reason this might be done is that the same 1500 byte packet at a lower data rate takes a longer time to
transmit, and thus, lowers the effective data rate for all wireless clients associated to the AP.
can, for example, allow the administrator to exclude 802.11b clients from the AP by requiring an
802.11g data rate or setting a minimum transmission rate of all clients by disabling 802.11 rates. The
reason this might be done is that the same 1500 byte packet at a lower data rate takes a longer time to
transmit, and thus, lowers the effective data rate for all wireless clients associated to the AP.