Wireless: RSSI & SNR

Unanswered Question
Mar 31st, 2010

Looking for a troubleshooting guide for wireless clients, comparing RSSI and SNR values when connected to LAP(s).

Currently using Cisco WCS 6.0 for troubleshooting wireless clients.

Please advise.

Thanks,

Colm

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Overall Rating: 4 (2 ratings)
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Leo Laohoo Wed, 03/31/2010 - 17:22

Very broad topic to ask.  Can you be more specific or elaborate further?

colmgrier Fri, 04/02/2010 - 04:24

example:

WCS displays that a wireless client has a RSSI -80 and SNR 20.

What I'm looking for is a graph/table that will compare the RSSI and SNR values and display what connection will be achieved (strong/weak/low).

What is the recommended RSSI and SNR vaules for DATA and VOICE wireless clients.

Thanks,

Colm

George Stefanick Sat, 04/03/2010 - 08:58

VOICE - RSSI -67 (no higher) SNR 25 (no lower)

DATA - RSSI - 73ish (no higher) SNT 20ish (no lower)

Of course you have some wiggly room with DATA...

colmgrier Sat, 04/03/2010 - 12:02

Thanks George for your feedback. Customer are using Cisco 1141 Access Points.

Is there a specification guide showing all RSSI values max and min values.

Thanks,

Colm

David_Mitchell Tue, 11/16/2010 - 08:20

Cheers George very useful.  I have been looking for a simple answer to this for some time and here it is!

George Stefanick Tue, 11/16/2010 - 16:50

David,

No worries. Remember to rate helpful post as it helps others navigate to quicker answers. Cheers and happy holidays !

BRYN JONES Mon, 04/19/2010 - 03:32

I found this from a trawl of the web that I did a while back...does this help?

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Determining signal strength

The wireless standard 802.11b operates in the 2.4-2.485GHz (gigahertz) radio frequency (RF) band; RF is measured in decibels (dB). Wireless cards often come with client software that displays signal strength in dB or dBm (a variant of dB that provides an exact correlation to the power of the radio signal in watts).

Note: The minimum power sensitivity on most 802.11b clients is -96dBm (very low). If your software displays "Signal/Noise" or "SNR" in dBm, you can convert this to dB by subtracting the minimum power sensitivity, -96dBm, from the number displayed as "SNR" or "Signal/Noise". For example, if the Signal/Noise is -47dBm, you would convert this to dB as follows:

-47dBm - (-96dBm) = 49dB

If your wireless card software is indicating that the signal-to-noise ratio (SNR) is greater than 10dB, you are getting the maximum available bandwidth, or 11Mbps (megabits per second). An SNR higher than 10dB won't increase the amount of bandwidth beyond this maximum (in the example above, with an SNR of 49dB, the bandwidth is still 11Mbps, the maximum available rate). When the SNR drops below 10dB, however, the maximum data rate drops:

SNR

Maximum data rate

8dB

5.5Mbps

6dB

2Mbps

4dB

1Mbps

Even though the maximum data rate goes down, the connection will still be maintained as long as you have an SNR of 4dB or greater.

Possible problems

Other factors can affect the quality of your wireless connection. The list below is incomplete, but it may offer some explanation for poor performance that occurs even when the signal strength is good:

  • Multipath: In      general, an RF signal grows wider as it is transmitted farther. As it      spreads, the RF signal will meet objects in its path that will interfere      with the signal in various ways (e.g., by reflecting it). When the signal      is reflected by an object (e.g., a metal object) while moving toward a      receiver, multiple wave fronts are created, one for each reflection point.      This can result in a large number of waves, depending on how many      reflecting surfaces the original signal encounters. Many of these      reflected waves are still moving toward the receiver, creating a condition      known as multipath. As the number of reflective surfaces increases, the      signal deteriorates.
  • Near/far: Near/far is a problem that can happen when multiple wireless users have      devices that are very near an access point, much closer than a user who's      on the radio signal boundary. The farthest device cannot be heard over the      traffic from the devices closer to the access point. The only solution is      to move the more distant device closer to the access point.
  • Hidden node: When      you turn on an 802.11b-capable laptop or handheld device, it immediately      scans the airwaves for access points. It quickly evaluates the signal      strength of the available access points, and the number of users per      access point. Based on this, the device will choose the access point with      the strongest RF signal and the fewest users. In hidden node situations,      at least one client (node) is unable to "hear" one or more of      the other clients connected to the same access point. Usually this is      because of some physical obstruction between it and other users. As a      result, there can be problems in the way the clients share the available      bandwidth, causing data "collisions", or bit errors. When a bit      error occurs, the clients need to re-transmit the data. These collisions      can result in significantly degraded data transmission rates in the      wireless network.
  • Dynamic Rate Shifting (DRS): The terms Adaptive (or Automatic) Rate Selection (ARS) and Dynamic Rate      Shifting (DRS) denote how bandwidth is adjusted dynamically by wireless      clients. This adjustment in speed occurs as distance increases between the      client and the access point (or possibly if interference increases). As      the distance grows greater, the signal strength will decrease to a point      where the current data rate cannot be maintained. As the signal strength      drops, the client will drop its data rate to the next lower specified data      rate, for example, from 11Mbps to 5.5Mbps, or from 2Mbps to 1Mbps.

Throughput

Throughput is a measure of the speed of your wireless connection. Defined as the amount of data transmitted in a given time period, throughput is based on many factors. Three important factors are described below:

  • Interference from another radio      frequency source: IU uses the 802.11b wireless standard,      which operates in a frequency range that is unlicensed, meaning the      Federal Communications Commission allows anyone to use it. Unfortunately      for 802.11b users, microwave ovens and 2.4GHz cordless phones operate in      the same frequency band. The radio signal emanating from such devices can      severely degrade or completely destroy an 802.11b signal.
  • Security: 802.11b      networks, if not secure, are susceptible to hacking and data theft by      unauthorized users. Any sensible network owner will use some form of      security. This adds more overhead to the wireless data packets; this      overhead (bits of data) uses valuable bandwidth, but is absolutely      necessary. Although 802.11b allows for 11Mbps maximum throughput, a user      will typically get only about 5.5-6Mbps of data. IU uses a virtual private network (VPN)      for encryption and authentication. Another method is wired equivalent      privacy (WEP), but it is much less secure than VPN.
  • Distance: Greater      distances between the transmitter (access point) and receiver (client)      will cause the throughput to decrease because of an increase in the number      of errors (bit error rate). 802.11b recognizes these bit errors and      requires that the bits be retransmitted. 802.11b is configured to make      discrete jumps to specified data rates (11, 5.5, 2, and 1Mbps). If 11Mbps      cannot be maintained because of bit errors and degrading signal strength,      then the device will drop to 5.5Mbps, then to 2Mbps, and then to 1Mbps,      with eventual loss of the connection. Remember, too, that because of      overhead, you'll get only about half of the available bandwidth.

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