HP Vm200 User Manual

							Chapter 5: Wireless configuration
5
Wireless configuration
Contents
Wireless coverage......................................................................................................... 5-2
Factors limiting wireless coverage...................................................................... 5-2
Configuring overlapping wireless cells............................................................... 5-3
802.11n best practices.................................................................................................. 5-7
Supporting legacy wireless clients ...................................................................... 5-7
Channel width ........................................................................................................ 5-9
Radio configuration ................................................................................................... 5-10
Radio ..................................................................................................................... 5-10
Regulatory domain .............................................................................................. 5-10
Operating mode ................................................................................................... 5-11
Wireless mode...................................................................................................... 5-11
Channel width ...................................................................................................... 5-12
Channel ................................................................................................................. 5-12
Detecting rogue APs .................................................................................................. 5-13
Scanning modes ................................................................................................... 5-13
Viewing scan results ............................................................................................ 5-14
Scanning for rogue APs ...................................................................................... 5-15
Creating a list of authorized access points....................................................... 5-15
Viewing wireless information ................................................................................... 5-16
Viewing all connected wireless clients ............................................................. 5-16
Viewing wireless statistics for the radio ........................................................... 5-18
Viewing throughput for wireless clients ........................................................... 5-22 
						

							Wireless configuration
Wireless coverage
5-2
Wireless coverage
As a starting point for planning your network, you can assume that when operating at high 
power, the V-M200 radio provides a wireless networking area (also called a wireless cell) of 
up to 300 feet (100 meters) in diameter. Before creating a permanent installation, you should 
always perform a site survey to determine the optimal settings and location for the V-M200.
The following sections provide information on wireless coverage. A tool that can help 
simplify planning a secure wireless network is the HP RF Planner. For more information, see 
the RF Planner Admin Guide.
Factors limiting wireless coverage
Wireless coverage is affected by the factors discussed in this section.
Interference
Interference is caused by other APs or devices that operate in the same frequency band as the 
V-M200 and can substantially affect throughput. Several tools are available to diagnose 
interference problems as they occur.
Select Wireless > Rogue AP detection to view detailed information about all wireless 
APs operating in the immediate area so that you can effectively set the operating 
frequencies. This feature also makes it easy for you to find rogue APs. See Detecting 
rogue APs on page 5-13. 
Select Status > Wireless to view detailed information about packets sent and received, 
transmission errors, and other low-level events. 
Select Status > Client data rate matrix to view information about data rates for all 
connected client stations. This makes it easy to determine if low-speed clients are 
affecting network performance. 
CautionAPs that operate in the 2.4 GHz band may experience interference from 2.4 GHz cordless 
phones and microwave ovens.
Physical characteristics of the location
To maximize coverage of a wireless cell, the V-M200s are best installed in an open area with 
as few obstructions as possible. Try to choose a location that is central to the area being 
served. 
Radio waves cannot penetrate metal; they are reflected instead. The V-M200 can transmit 
through wood or plaster walls and closed windows. However, the steel reinforcing found in 
concrete walls and floors may block transmissions or reduce signal quality by creating 
reflections. This can make it difficult or impossible for a single V-M200 to serve users on 
different floors in a concrete building. Such installations require a separate V-M200 on each 
floor. 
						

							Wireless configuration
Wireless coverage
5-3
Configuring overlapping wireless cells
When the radio is operating in the 2.4 GHz band, overlapping wireless cells occur when two 
or more APs are within transmission range of each other. This may be under your control, 
(for example, when you use several cells to cover a large location), or out of your control (for 
example, when your neighbors set up their own wireless networks). In either case, the 
problems you face are similar.
NoteOverlapping channels do not occur when the radio is operating in the 5 GHz band. All 5 GHz 
channels are non-overlapping.
Performance degradation and channel separation
When two wireless cells operating on the same frequency overlap, throughput can be reduced 
in both cells. Reduced throughput occurs because a wireless user that is attempting to 
transmit data defers (delays) transmission if another station is transmitting. In a network 
with many users and much traffic, these delayed transmissions can severely affect 
performance, because wireless users may defer several times before the channel becomes 
available. If a wireless user is forced to delay transmission too many times, data can be lost.
Delays and lost transmissions can severely reduce throughput on a network. To view this 
information about your network, select Status > Wireless. For recommendations on using 
this information to diagnose wireless problems, see the online help for this page.
The following example shows two overlapping wireless cells operating on the same 
frequency. Since both APs are within range of each other, the number of deferred 
transmissions can be large.
Cell 1
Channel = 1Cell 2
Channel = 1
AP AP 
						

							Wireless configuration
Wireless coverage
5-4
The solution to this problem is to set the two networks to different channels with as great a 
separation as possible in their operating frequencies. This reduces crosstalk and enables 
client stations connected to each V-M200 to transmit at the same time. 
Selecting channels
For optimal performance when operating in the 2.4 GHz band, select an operating frequency 
that is different by at least 25 MHz from the frequency used by other wireless APs that 
operate in neighboring cells. 
Two channels with the minimum 25 MHz frequency separation always perform worse than 
two channels that use maximum separation. It is always best to use the greatest separation 
possible between overlapping networks.
With the proliferation of wireless networks, it is very possible that the wireless cells of APs 
outside your control overlap your intended area of coverage. To choose the best operating 
frequency, select Wireless > Rogue AP detection to generate a list of all APs that operate 
near you and their operating frequencies.
The set of available channels is automatically determined based on the Country setting you 
define by selecting Management > Country. This means that the number of non-
overlapping channels available to you varies by geographical location, which affects how you 
set up your multi-cell network.
Cell 1
Channel = 1Cell 2
Channel = 6
AP AP 
						

							Wireless configuration
Wireless coverage
5-5
Sample channel selections
For example, when operating in 802.11b mode, the V-M200 supports the following 
14 channels in the 2.4 GHz band. 
However, the number of channels available for use in a particular country are determined by 
the regulations defined by the local governing body. The following table shows the number of 
channels that are available in North America and Europe. 
Since the minimum recommended separation between overlapping channels is 25 MHz (five 
cells) the recommended maximum number of overlapping cells you can have in most regions 
is three. The following table gives examples relevant to North America and Europe for 
channels in the 2.4 GHz band. 
ChannelFrequencyChannelFrequency
1
2
3
4
5
6
72412
2417
2422
2427
2432
2437
24428
9
10
11
12
13
142447
2452
2457
2462
2467
2472
2477
RegionAvailable channels
North America  1 to 11
Europe 1 to 13
North AmericaEurope
cell 1 on channel 1
cell 2 on channel 6
cell 3 on channel 11
cell 1 on channel 1
cell 2 on channel 7
cell 3 on channel 13 
						

							Wireless configuration
Wireless coverage
5-6
In North America, you can reduce transmission delays by using different operating 
frequencies as shown in the following figure.
Alternatively, you can stagger cells to reduce overlap and increase channel separation as 
shown in the following figure. 
Cell 1
Channel = 1Cell 2
Channel = 6Cell 3
Channel = 11
AP AP AP
Cell 1
Channel = 1Cell 2
Channel = 6Cell 3
Channel = 11Cell 4
Channel 1
AP AP AP AP 
						

							Wireless configuration
802.11n best practices
5-7
This strategy can be expanded to cover an even larger area using three channels as shown in 
the following figure.
802.11n best practices
This section provides recommendations on how to best use 802.11n wireless technology, 
especially when legacy (a/b/g) clients must also be supported. 
Supporting legacy wireless clients
The 802.11n standard is very similar to the 802.11g standard, in that both provide mechanisms 
to support older wireless standards. In the case of 802.11g, protection mechanisms were 
created to allow 802.11b and 802.11g wireless devices to co-exist on the same frequencies. 
The data rates of 802.11g (6, 9, 12, 18, 24, 36, 48 and 54 Mbps) are transmitted using 
Orthogonal Frequency Division Multiplexing (OFDM) modulation, while the data rates of 
802.11b are transmitted using Direct Sequence Spread Spectrum (DSSS) modulation. Since 
older 802.11b-only clients cannot detect OFDM transmissions, 802.11g clients must “protect” 
their transmissions by first sending a frame using DSSS modulation. This frame (usually a 
CTS-to-self or RTS/CTS exchange) alerts 802.11b clients to not attempt to transmit for a 
specified period of time.
Cell 1
Channel = 1
Cell 5
Channel = 11Cell 2
Channel = 6
Cell 6
Channel = 1Cell 3
Channel = 11
Cell 7
Channel = 6Cell 4
Channel 1
Cell 8
Channel 11
AP AP AP AP
AP AP AP AP 
						

							Wireless configuration
802.11n best practices
5-8
If protection is not used, 802.11b clients may transmit a frame while an 802.11g frame is 
already being sent. This leads to a collision and both devices need to re-transmit. If there are 
enough devices in the network, the collision rate will grow exponentially and prevent any 
useful throughput from the wireless network.
802.11n clients face the same problem as described for 802.11g clients. Legacy a/b/g clients 
cannot detect the High Throughput (HT) rates that 802.11n uses. So to avoid causing 
excessive collisions, 802.11n clients must use the same protection mechanisms when a legacy 
client is present. Even the most efficient protection mechanism (CTS-to-self) causes a 
substantial decline in throughput; performance can decline by as much as 50 percent. The 
802.11n clients can achieve maximum data rates only when the legacy clients are not present.
Available 802.11n modes
Supported wireless modes are determined by the regulatory domain. Available options may 
include one or more of the following:
802.11n (5 GHz): (Pure 802.11n) Up to 300 Mbps in the 802.11n 5 GHz frequency band.
802.11n/a: (Compatibility mode.) Up to 270 Mbps for 802.11n and 54 Mbps for 802.11a in 
the 5 GHz frequency band.
802.11n (2.4 GHz): (Pure 802.11n) Up to 144.4 Mbps in the 802.11n 2.4 GHz frequency 
band.
802.11n/g: (Compatibility mode.) Up to 130 Mbps for 802.11n and 54 Mbps for 802.11g in 
the 2.4 GHz frequency band. Only use this setting when support for 802.11g is necessary.
802.11n/b/g: (Compatibility mode.) Up to 130 Mbps for 802.11n, 54 Mbps for 802.11g, and 
11 Mbps for 802.11b in the 2.4 GHz frequency band. Only use this setting when support for 
802.11b is necessary.
NoteThe V-M200 Radio can also be set to legacy a/b/g values with no 802.11n support.
802.11n (5 GHz) and 802.11n (2.4 GHz)
HP refers to these two modes as Pure-N. When the V-M200 radio is in either of these modes, it 
will not allow non-802.11n clients to associate. Legacy clients can see the V-M200, and may 
attempt to associate, but they will be rejected. The V-M200 makes this determination based 
on the supported rate set that the client presents during its association request. If the client’s 
rate does not match the 802.11n rate, it is not allowed to associate.
In these modes, the V-M200 will not use protection when sending HT frames to associated 
clients. If legacy V-M200s or clients are using the same channel, this may lead to collisions. In 
the 5 GHz band, this will probably not be a common problem since the band isn’t heavily 
used. However in the 2.4 GHz band, this mode may cause serious performance deterioration 
for everyone on the channel (both the 802.11b/g and 802.11n clients).
The V-M200 will still signal associated clients to use protection when they send data. The 
V-M200 does this via a field in the beacons that it sends. So clients sending data to the V-M200 
will use protection, but data sent from the V-M200 will not be protected. 
						

							Wireless configuration
802.11n best practices
5-9
NoteNote that some people may refer to this mode as Greenfield, which is not correct. Greenfield 
is an 802.11n-specific preamble. The V-M200 does not support this preamble and therefore 
does not support Greenfield mode.
The Pure-N modes can be used when there is no legacy wireless traffic present in or around 
the premises on the channels that will be used. All client devices must support 802.11n. 
802.11n/a, 802.11n/b/g
These modes are referred to as compatibility modes. 802.11n/a, which supports 802.11n and 
802.11a clients in the 5 GHz spectrum, is the default mode of the V-M200 radio. 802.11n/b/g 
supports 802.11n and 802.11b/g clients in the 2.4 GHz spectrum. In either of these modes, the 
V-M200 allows both 802.11n and legacy clients to associate. The V-M200 advertises protection 
in the beacon when legacy clients are associated or operating on the same channel. This 
alerts the associated 802.11n clients to use protection when transmitting. The V-M200 also 
uses protection when necessary while sending HT data. 
When to use these modes: these compatibility modes should be used when legacy clients are 
present in the network. HP recommends 802.11n/a or 802.11 n/b/g as the typical operating 
mode. Both modes allow for all wireless clients to connect and they use protection to avoid 
causing interference.
802.11n/g
This mode is the same as 802.11n/b/g except that 802.11b clients are prevented from 
associating. The V-M200 does not advertise 1, 2, 5.5 and 11 Mbps as supported rates in its 
beacons or Probe-Responses. The V-M200 does not tell 802.11g clients to use protection, and 
this can cause collisions with any 802.11b clients present on the same channel. 
When to use this mode: this mode should only be used in special cases where 802.11b clients 
are causing problems in the network.
Channel width
When operating in the 5 GHz band, the V-M200 enables you to use the standard channel width 
of 20 MHz or a double width of 40 MHz. 40 MHz widths are achieved by using two adjacent 
channels to send data simultaneously. The advantage of using a 40 MHz width channel is that 
the available bandwidth is doubled leading to much higher throughput.
When operating in the 2.4 Ghz band, a channel width of 20 MHz is automatically selected and 
cannot be changed.
When operating in the 5 GHz band, the Auto 20/40 MHz option should be used as the 
channel width.
When a channel width of 20 MHz is used, channel usage is the same as in legacy mode. In the 
2.4 GHz band, channels 1, 6, and 11 can be used without overlapping. In the 5 GHz band, each 
channel is separate, with no overlapping. 
						

							Wireless configuration
Radio configuration
5-10
When Auto 20/40 MHz is selected, the V-M200 radio uses a 40 MHz channel width. However, 
both 20 and 40 MHz clients can associate. The channel selected on the radio page is the 
primary channel and the secondary (or extension) channel is located adjacent to it. The 
secondary channel is either above or below depending on which channel was selected as the 
primary. In the 5 GHz band, the channels are paired: 36 and 40 are always used together, 44 
and 48 are always used together, etc. 
Radio configuration
To define configuration settings for the V-M200 radio, select Wireless > Radio to open the 
Radio configuration page.
Radio
Select this checkbox to activate the radio. 
Regulatory domain
NoteThis option is not available on V-M200s delivered with a fixed country setting.
Indicates the geographical region in which the V-M200 operates. To change the domain, click 
the domain name or select Management > Country. 
CautionWireless radios are governed by different regulatory standards depending on the region in 
which they are installed. By setting the regulatory domain, the V-M200 will only allow you to 
configure wireless settings in accordance with the regulations in the selected domain. 
Therefore, the settings that are available on this page may not include all options that are 
described in this section. Please ensure that the V-M200 is operating in accordance with 
channel, power, indoor/outdoor restrictions and license requirements for the intended 
country.