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June 22, 2016

SETTING MINIMUM DATA RATES? – READ THIS FIRST.

 

Nowadays when you speak with a WLAN professional you will often hear the suggestion of setting or restricting minimum PHY rates to optimise your WLAN’s  performance.  Many professionals nowadays consider this to be one of the basic tasks that must be completed in the process of configuring and optimising a WLAN.

Configuring the minimum rates in a WLAN can have many benefits to your network’s performance including reduction of management overhead, removal of unnecessary RTS/CTS frames, better airtime utilisation, and enhanced throughput in the Extended Service Set.  It is an especially useful tool in High Density scenarios like big convention halls, sports stadiums, large lecture theatres and any other environment with many clients in a relatively small space.  Personally I set the minimum rates on all HD designs especially in the 2.4Ghz band!

(Yes, I have used 2.4Ghz in High Density deployments.  No I am not a magician.)

If done incorrectly however, or without a fundamental understanding of what you are actually changing, you may find that your optimisation does not always have the desired effects.  For instance, setting minimum data rates and then expecting this to somehow magically limit the coverage area of your AP… well that’s just a recipe for disappointment.

So let’s go through some of the basics of the various PHY Specifications, what minimum rates are, why we can and should set them to different levels and what EXACTLY we are changing with different settings.

PHY Specifications

WLANs work using the IEEE 802.11 standard and its amendments.  Some of these amendments are known as PHY specifications and define the modulation and coding of Wi-Fi signals that WLAN stations can use to communicate with each other.  The table below summarises the available data rates of each 802.11 PHY specification:

PHY SPECIFICATION

802.11 Amendment Frequency of Operation Supported Data Rates

DSSS

802.11 (original) 2.4GHz

1, 2 Mbps

HR-DSSS

802.11b 2.4GHz

1, 2, 5.5, 11 Mbps

ERP-OFDM

802.11g 2.4GHz

6, 9, 12, 18, 24, 36, 48, 54 Mbps

OFDM

802.11a 5GHz 6, 9, 12, 18, 24, 36, 48, 54 Mbps
HT-OFDM (Greenfields) 802.11n 2.4GHz / 5GHz

6.5 to 600Mbps

VHT-OFDM 802.11ac 5GHz

6 to 6933.3 Mbps

If you want a full breakdown of 802.11n/802.11ac MCS rates you can see them here

Backward Compatibility

802.11g (ERP-OFDM) has a minimum PHY of 6Mbps, but is also required to be backward compatible with 802.11b and 802.11 which both use the same 2.4GHz spectrum.  Even though the specified minimum data rate for 802.11g is 6Mbps, in practice a 802.11g radio will often use a minimum rate of 1 Mbps for the sake of backward compatibility with older clients that could be required to associate to the BSS.  Even a 2.4GHz 802.11n Access Point must be compatible with previous radio generations and client types and will often exhibit a minimum rate of 1 Mbps.

Thankfully a 5GHz 802.11n AP must only be backward compatible 802.11a and so the minimum PHY rate is 6 Mbps.  802.11ac APs only support 5GHz and so also have a minimum rate of 6 Mbps in an effort to maintain backward compatibility to 802.11a.

Preamble & PHY Header

Every single 802.11 frame (regardless of the PHY Specification) carries the same basic format.  The first thing to be transmitted is the preamble.  This is just a sequence of scrambled 1’s (DSSS / HR-DSSS) or simple waveforms (OFDM based PHY Specifications) that allows the listening station to synchronise with the incoming transmission.  It’s like having a code word or a sentence that makes someone aware that you want to talk to them.  AHEM! HEY YOU! LISTEN HERE! I’M TALKING!

The second thing that comes along is the PLCP Header.  Once the receiving stations have perked up and are now listening for the incoming message, the PLCP header gives the receiving stations some more information about the incoming transmission including:

  • The PHY Rate of the transmission of the 802.11 frame (MPDU)
  • How long the transmission will take (DSSS, HR-DSSS Only)
  • How much data is in the transmission (OFDM, ERP-OFDM, HT-OFDM, VHT-OFDM)

The PLCP Headers of 802.11n and 802.11ac carry a lot more information than the above, but it is out of scope for this discussion.

But wait, if the PLCP header defines the rate of transmission for the 802.11 frame, then…  Which rate does the PLCP Header use? Well, it depends on which PHY Specification you’re using.  The Preamble and PLCP Header are ALWAYS sent at the lowest rate defined for the relevant PHY Specification!

A table summarising the Modulation and Coding of the Preamble and PHY Header for different PHY Specs is shown below:

Modulation & Coding

PHY Data Rate

PHY Specification

Preamble

PLCP Header

PLCP Header

DSSS

DBPSK

DBPSK

1 Mbps

HR-DSSS  (Long PPDU format)

 

(Short PPDU format)

DBPSK

DBPSK

1 Mbps

DBPSK

DQPSK

2 Mbps

ERP-OFDM

NA

BPSK R=1/2

6 Mbps

OFDM

NA

BPSK R=1/2

6 Mbps

HT-OFDM (HT-Greenfield)

NA

BPSK R=1/2

6.5 Mbps

VHT-OFDM

NA

BPSK R=1/2

6 Mbps

The DSSS / HR-DSSS Preamble is actually made up of a Sync Field and a Start of Frame Delimiter (SFD).  The Sync Field and SFD are both constructed of randomised 1’s as modulated bits and so have Modulation / Coding information associated with them.

In comparison, the training sequences sent with the ERP-OFDM / OFDM / HT-OFDM / VHT-OFDM preambles are not actually modulated bits.  They are simply a sequence of specific waveforms or symbols that must be correctly interpreted by the receiver to synchronise with the transmitter.  This is why they don’t have modulation / coding associated with them.

Observation #1:

The PHY Rate of the Preamble and PLCP Header is ALWAYS sent at the rate defined for the relevant PHY Specification!  It doesn’t matter if you set your minimum rate to 48Mbps.  The first part of every transmission, the Preamble and the PLCP header will be sent at the MOST ROBUST modulation scheme defined by the PHY Specification you are using.  That means if you are using a 2.4GHz 802.11n AP with full backward compatibility, the MOST ROBUST modulation and coding rate will be DBPSK with a PHY data rate of 1 Mbps.

The MAC Header

Immediately after the PLCP Header, comes the 802.11 frame or Mac Protocol Data Unit (MPDU).  The 802.11 frame or MPDU is sent at the Data Rate specified in the PLCP header.  Every MPDU starts with a MAC Header that contains the MAC Layer Addressing  information (where the frame is from and where it is headed) and a Duration/ID field.  The Duration/ID Field warns any stations that can decode the MAC Header to update their NAV and remain quiet for any future frame transactions that are required after the current frame.

What does Setting the Minimum Rate Change?

The MAC Protocol Data Unit and the MAC Header are sent at the rate specified by the transmitting station in the PLCP Header.  When you set the Minimum Rate for a BSS, you are effectively only setting the minimum rate that may be used for transmitting the MPDU, NOT for the Preamble or PLCP Header.

  1. Setting minimum rates does NOTHING to the size of the coverage area of the BSS.
  2. The PLCP Header will still be sent out at the lowest possible rate defined by the PHY Specification.
  3. You will still cover just as great an area as if you had not touched the minimum rates for the BSS.
  4. Every time a station transmits, every other station that can decode the PLCP Header will be silenced by the transmission.
  5. The number of stations affected by channel contention by other stations HAS NOT CHANGED.
  6. The only thing that has changed is the speed at which we send the MPDU.  Put another way, the only thing we have changed is the maximum amount of Airtime we use sending a specific amount of data.  We’ve improved our efficiency and thereby increased the capacity of the channel, but we have not reduced the CONTENTION AREA.

Ok so I gave minimum rates a bit of a bad rap there.  They might sound quite useless after that list, but hold on a second.

Setting the minimum rate can have a great effect on your WLAN’s performance when trying to limit the amount of air time used up by Management Frames sent on the Wireless LAN and by stations sending data.  Remember that in a BSS, Beacons, Probe Requests and Probe Responses are all sent at the lowest common rate supported by the AP and Client.  This means that if we set our minimum rate to 6Mbps instead of 1 Mbps we will reduce the amount of airtime used for management overhead.  This leaves us with more free airtime to send actual user data!  The biggest effect of changing this setting will be seen in High Density environments with multiple SSIDs that are all beaconing on the same channel.

Setting the Minimum Rate Too High

I have heard some WLAN engineers talk about pushing the minimum rate of a BSS to 24Mbps or 48Mbps.  In my view this can be a bad idea for the following reasons:

  1. The useable coverage area of the AP can become much smaller prompting APs to be placed closer together.  However the area of contention covered by the AP’s PLCP Header remains the same.  This actually increases contention and interference between APs!
  2. Management overhead is typically sufficiently minimised in Very HD environments using a 12Mbps minimum rate, even on 2.4GHz Radios. (I’m talking about a Stadium here)
  3. Management Frames can become hard to decode in some locations, causing clients to drop their connections or miss notifications for queued/buffered traffic etc.
  4. There is also little evidence in my experience that setting high minimum rates prompts sticky clients to roam between APs,  it simply helps them drop more packets.
  5. Wireless is a dynamic medium with many stochastic events and effects on the channel.  Limiting the data rate selection algorithm to only higher rates can cause an AP to suffer higher re-transmit rates and suffer more dropped frames to clients which is exactly what we DON’T want.  Remember, a frame sent at 12 Mbps once is WAY, WAY better than a frame sent at 24Mbps twice or worse one sent at 48 Mbps 4 times or more!

How to Limit the Range of the Preamble and PLCP Header.

Up until now I have only addressed the effects of setting Minimum Rates.  In this section, we will see how we can control the modulation of the Preamble and PLCP Header.

So here is the first thing, you can’t really limit it all that much.  The only way to limit the preamble and PLCP Header is to force the AP not to respect any kind of backward compatibility with older standards.  This may offer you quite considerable shrinkage in the area covered by a preamble and PLCP Header provided you are in a Free Space environment with no obstacles.   if you are indoors however, it may offer less benefit.  Even with any shrinkage, you will still be using BPSK R=1/2 modulation and coding and the range of the PLCP Header will STILL be much greater in comparison to the useful range of the cell.

Let’s look at an example.  Assume that we designed our network to cover to -65dBm to clients throughout the coverage area.  We want to see where our furthest preambles will be heard by our own APs.  Assume we use a high end 4×4:4 802.11ac Wave 2 AP in our design.  This AP has a receive sensitivity of 1Mbps at -101dBm and 6Mbps at -95dBm.  Sure, I have gained about 6dB, which in an outdoor environment equates to halving the coverage range of the PLCP Header.  But indoors, that difference in coverage area may not be as great due to the absorptive effect of physical obstacles like walls, cabinets, furniture etc.

Enabling OFDM-Only Mode

This is by far THE MOST powerful tool in your arsenal.  By simply setting a 2.4GHz 802.11g/n radio to “OFDM-Only” or “802.11g-Only” mode (the naming of this setting differs between vendors) you will immediately accomplish the following:

  1. Force the preambles / PLCP Headers of ALL Management and Data frames to use the OFDM format only.
  2. Banish all 802.11 / 802.11b clients from connecting to your WLAN.
  3. Reduce the use and need of RTS/CTS frames for protecting legacy clients.
  4. Set the minimum rate of the WLAN to 6 Mbps, preventing associated STAs from negotiating DSSS/CCK rates.
  5. All management frames will also be sent at 6Mbps by default, reducing management overhead.

This setting will give you both the ability to set the Preamble and PLCP Header modulation type (OFDM, BPSK, R=1/2) and will also ensure that MPDUs are sent at 6Mbps saving you airtime.


EDIT: Primoz Marinsek, Jim VajdaAndrew von Nagy, and Keith Parsons got me to think about the above very carefully.  This is a revised list after their input.  I would like to thank them for their contributions.  Secondly, if you’re wondering when RTS/CTS modes can be activated by older clients, check out my article about 802.11 PHY Compatibility.


If you are a Ruckus Wireless customer, you can set this for each WLAN from the GUI of the SmartZone Controller.  Just tick the box that says “Enable OFDM-only”.  You should do this in EVERY 2.4GHz network you deploy except when a caveman in a forklift waves an old PSION scanner at you.

Note: I do not intend to insult  men driving forklifts, their technique with a wooden club is generally unmatched.

Enabling N-Only / HT-Greenfields Mode

This used to be a setting I liked using on the 5GHz radios of my WLANs unless I had to support a specific client device.  I would force the 5GHz radios to use only the HT-GreenField PPDU format, preventing 802.11a only stations from associating to the 5GHz WLAN.  My logic here was that very few 802.11a stations are in circulation and those that do support 802.11a are usually dual band.  So I’d force old clients to 2.4GHz.  The value of operating in HT-greenfields mode is marginal in comparison to HT-Mixed mode and doesn’t offer as great a leap as OFDM-Only mode above.

802.11ac:

All the new APs I deploy today are 802.11ac, which defines a single PPDU format that is backward compatible with 802.11n and 802.11a.  I don’t use N-Only mode on my 5GHz WLANs anymore, simply because the option does not exist for an 802.11ac AP.

Management Tx rate

Before I sign off this blog, I feel I should mention my least favourite option for configuring minimum rates simply for the sake of it.  Setting the Management Tx Rate is similar to setting the minimum rate for the BSS, except it does not place any limitation on the minimum BSS rate for client devices.  The management Tx rate simply sets the minimum rate used for the MPDU in management frames.  It can be useful for reducing management overhead without limiting the available rates for clients.  For example you could set the management rate to 2Mbps which would effectively halve your management overhead from beacons.  Personally, I dislike this feature as it can introduce a disparity between the useful range of management frames and clients if you make the difference large enough.

I generally just stick to using OFDM-Only and then setting the minimum BSS rate to a value of 6 or 12 Mbps depending on remaining management overhead.

Anyway, thats all for now folks, hope this helped!

Rob