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Recently, I had a conversation with some peers discussing the differences between slot times and preambles. I thought it would be good to post that information here since my research helped solidify the differences for me. Special thanks to Devin for "checking my six" and verifying my thoughts.

First let's cover some definitions of slot times and preambles:

Short Radio Preambles - The radio preamble (sometimes called a header) is a section of data at the head of a packet that contains information that the wireless device and client devices need when sending and receiving packets. You can set the radio preamble to long or short.

Short?¡é?€?¡±A short preamble improves throughput performance. Cisco Aironet Wireless LAN Client Adapters support short preambles but early models of Cisco Aironet's Wireless LAN Adapter (PC4800 and PC4800A, prior to 2000) require long preambles.

Long?¡é?€?¡±A long preamble ensures compatibility between the wireless device and all early models of wireless LAN adapters. If these client devices do not associate to newer APs, you should use long preambles.

Short Slot Times - The amount of time a device waits after a collision before retransmitting a packet. You can increase throughput on 802.11g, 2.4-GHz radios by enabling short slot time (most .11g radios enable this by default). Reducing the slot time from the standard 20 microseconds to the 9-microsecond short slot time decreases the overall backoff, which increases throughput. Backoff, which is a multiple of the slot time, is the random length of time a station waits before sending a packet on the LAN.

Many 802.11g radios support short slot time, but some do not. When short slot time is enabled, the wireless device uses the short slot time only when all clients associated to the 802.11g, 2.4-GHz radio support short slot time. Short slot time is an 802.11g-only feature and does not apply to 802.11a radios.

SUMMARY: Slot times should transition from 20us to 9us when a "pure" .11g environment exists for that AP. -- JB

Here's what the IEEE 802.11 Handbook: A Designer's Companion has to say about them:

Page 60:

A Short Preamble subfield is transmitted by an AP or a mobile STA in an IBSS in Beacon, Probe Response, Association Response, and Reassociation Response frames to indicate the availability of the short preamble option when using an IEEE 802.11b PHY. When set to one, this subfield indicates that the use of short preambles is allowed in the BSS. When set to zero, this subfield indicates that the use of short preamble is not allowed in the BSS. In a mobile STA that is not part of an IBSS, the Short Preamble subfield in Association Request and Reassociation Request frames indicates the capability of the STA to send and receive the short preambles of IEEE 802.11b.

NOTE: In Long preambles, the PLCP Synch subfield is 128-bits. In Short preambles, the PLCP Sync subfield is only 56-bits. -- JB

Page 61:

The Short Slot Time subfield in Beacon, Probe Response, Association Response, and Reassociation Response frames sent by the AP indicates the length of the slot time that is currently in use in the BSS. STAs indicate their capability to support short slot times in the Association Request and Reassociation Request frames. When an association from a STA that does not support short slot times is accepted by an AP, the AP indicates that short slot times may not be used in the BSS by transmitting the value of the short slot time subfield as a zero at the next Beacon frame after accepting the association. All STAs associated with the AP must abide by the current setting of the Short Slot Time subfield as it is transmitted by the AP.
---------------------------

SUMMARY:
Most 802.11g (ERP) radios enable short slot times by default. If you have a .11b/g radio and there are .11b (HR-DSSS) clients associated to that AP, then, per the standard (see above), the AP MUST switch to "Short Slot Time = 0" in the Beacon frames. HR-DSSS and DSSS radios only understand long (20us) slot times. Short slot times (9us) only exist when .11g-only (ERP) devices are associated to a .11g (ERP) AP. Typically, you don't "disable" short slot times; instead, the AP enables/disables them depending on the type of associated client radios (PURE ERP-OFDM = 1, Mixed = 0). Some vendors allow you to turn it off if you want, but there's no sense in doing so.

Here's how you can verify the short slot time status in your WLAN environment:
- Create a pure .11g environment (verify there are no .11b clients associated to the AP)
- Capturing Beacon frames using a wireless protocol analyzer
- Look at the Slot Time subfield in the Beacon frame

If the Slot Time subfield shows a binary 0, it means the slot times are 20us (short is disabled). If it shows a binary 1, then short slot times (9us) are enabled. See attached to see what it will look like if you used Network Instruments Observer to do the capture.



In this Beacon frame, both short slot times and short preambles are disabled.

Hope this helps,
Joel

Hi Joel:

Good stuff. I hope we see more posts like this.

Additional "short" ideas:

IEEE 802.11 clause 17 OFDM and clause 19 ERP-OFDM frame preambles are shorter than "short" frame preambles -- 16 microseconds versus 72 microseconds for clause 18 HR/DSSS and clause 19 ERP-CCK.

Only clause 18 HR/DSSS stations and clause 19 ERP stations can recognize "short" frame preambles.

Clause 17 OFDM normal slot time is the same amount of time as clause 19 ERP-OFDM "short" slot time.

Stations in an all ERP BSS may use short slot time when transmitting ERP-OFDM frames, but not when transmitting ERP-DSSS or ERP-CCK frames such as are frequently used for beacons.

Unlike ERP protection mechanisms that solve contention problems across BSSs sharing the same BSA, short slot time may be used by one BSS while an overlapping BSS uses long slot time. This combined with a smaller minimum contention window allows ERP stations an access advantage over their more plodding 2.4 GHz brethren.

Question: When one ERP station in an all ERP ESS allowing short preambles and short slot time selects a 2 Mbps rate and has to use both long preambles and long slot time, does the AP change either or both of the corresponding capability information subfields, and which features are used with frames exchanged between the one station and the AP, and which features are used for multicast frames from the AP?

I hope this helps. Thanks. /criss

Question: When one ERP station in an all ERP ESS allowing short preambles and short slot time selects a 2 Mbps rate and has to use both long preambles and long slot time, does the AP change either or both of the corresponding capability information subfields, and which features are used with frames exchanged between the one station and the AP, and which features are used for multicast frames from the AP?

What is the right answer to this?

The CWAP guide states this...

When only ERP stations are present in the BSS, the access point uses an OFDM PHY (and thus OFDM preambles) for the beacon frames. When NonERP station associates to the BSS, the access point uses the DSSS PHY (and thus DSSS preambles) for the beacon frames.
When the NonERP stations are all short-preamble capable, the access point sends the beacon with a short preamble. When any of the NonERP stations are long-preamble-only capable, the access point sends the beacon using a long preamble. When a NonERP station sends a probe request frame to the access point using a long preamble, the access point must reply with probe response frame using a long preamble. When a NonERP station sends a probe request frame to the access point using a short preamble, the access point must reply with a probe response frame using a short preamble. This is sometimes considered the ?¡é?€??preamble echo?¡é?€?? rule, though it is not called by this name in the 802.11 series of standards.

I would think:

Multicast frames from the AP will not get Acknowledgements from the station, especially if they stations enter the power-saving mode and buffer the multicast traffic and must wait on the DTIM interval. They are generally sent at the lowest basic rate this case 2 mbps?

Hi compughter:

Granted that frames transmitted to a multicast receiver address are never acknowledged.

Now how about preamble length and slot time length? Will an AP use a mix of these features to suit each client or only those features advertised in the last capability field?

I hope this helps. Thanks. /criss

Hi Criss:

Did a bit of research again:

Here is what I found so far:

IEEE Std 802.11g?¡é???¡é-2003
(Amendment to IEEE Std 802.11?¡é???¡é, 1999 Edition (Reaff 2003)
as amended by

IEEE Stds 802.11a?¡é???¡é-1999, 802.11b?¡é???¡é-1999,
802.11b?¡é???¡é-1999/Cor 1-2001, and 802.11d?¡é???¡é-2001)

It states:

The Capability Information field contains a number of subfields that are used to indicate requested or advertised optional capabilities.

The length of the Capability Information field is 2 octets. The Capability Information field consists of the following subfields: ESS, IBSS, CF-Pollable, CF-Poll Request, Privacy, Short Preamble, Packet Binary Convolutional Code (PBCC), and Channel Agility, Short Slot Time, and DSSS-OFDM.

The format of the Capability Information field is illustrated in Figure 27. No subfield is supplied for ERP as a STA supports ERP operation if it includes all of the Clause 19 mandatory rates in its supported rate set.


APs (as well as STAs in IBSSs) shall set the Short Preamble subfield to 1 in transmitted Beacon, Probe Response, Association Response, and Reassociation Response management MMPDUs to indicate that the use of the Short Preamble option, as described in 18.2.2.2, is allowed within this BSS.

To indicate that the use of the Short Preamble option is not allowed, the Short Preamble subfield shall be set to 0 in Beacon, Probe Response, Association Response, and Reassociation Response management MMPDUs transmitted
within the BSS.

Slot Times

STAs shall set the Short Slot Time subfield to 1 in transmitted Association Request and Reassociation Request MMPDUs when the MIB attribute dot11ShortSlotTimeOptionImplemented anddot11ShortSlotTimeOptionEnabled are true.

Otherwise, the STA shall set the Short Slot Time subfield to 0 in transmitted Association Request and Reassociation Request MMPDUs.

If a STA that does not support Short Slot Time associates, the AP shall use long slot time beginning at the first Beacon subsequent to the association of the long slot time STA.

APs shall set the Short Slot Time subfieldin transmitted Beacon, Probe Response, Association Response, and Reassociation Response MMPDUs to indicate the currently used slot time value within this BSS.

STAs shall set the MAC variable aSlotTime to the short slot value upon transmission or reception of Beacon, Probe Response, Association Response, and Reassociation Response MMPDUs from the BSS that the STA has joined or started and that have the short slot subfield set to 1 when the MIB attribute dot11ShortSlotTimeOptionImplemented is true.

STAs shall set the MAC variable aSlotTime to the long slot value upon transmission or reception of Beacon, Probe Response, Association Response, and ReassociationResponse MMPDUs from the BSS that the STA has joined or started and that have the short slot subfield set to 0 when the MIB attribute dot11ShortSlotTimeOptionImplemented is true.

STAs shall set the MAC variable aSlotTime to the long slot value at all times when the MIB attribute dot11ShortSlotTimeOptionImplemented is false.

When the dot11ShortSlotTimeOptionImplemented MIB attribute is not present, or when the PHY supports only a single slot time value,

Exactly what i was looking for. After all these years, the post and the comments are really helpful in understanding some of the most confusing details from the 802.11 stds. 

As noted above, shorter preambles will increase throughput.  However, a longer preamble is more likely to be heard and can help in some environments.

This is especially true for clients with lower signal output or poorer EVM.