DIFS
The IEEE 802.11 family of standards describe the DCF
protocol, which controls access to the physical medium. A station must
sense the status of the wireless medium before transmitting. If it finds
that the medium is continuously idle for DCF Interframe Space (DIFS)
duration, it is then permitted to transmit a frame. If the channel is
found busy during the DIFS interval, the station should defer its
transmission.
DIFS duration can be calculated by the following method.
DIFS = SIFS + (2 * Slot time)
Standard |
Slot time (µs) |
DIFS (µs) |
SIFS (µs) |
IEEE 802.11b |
20 |
50 |
10 |
IEEE 802.11a |
9 |
34 |
16 |
IEEE 802.11g |
9 or 20 |
28 or 50 |
10 |
IEEE 802.11n (2.4 GHz) |
9 or 20 |
28 or 50 |
10 |
IEEE 802.11n (5 GHz) |
9 |
34 |
16 |
IEEE 802.11ac (5 GHz) |
9 |
34 |
16 |
IEEE 802.11g is backward compatible with IEEE 802.11b. When these devices are associated with same AP all the timing parameters are changed to 802.11b.
Short Interframe Space (
SIFS), is the amount of time in
micro seconds required for a wireless interface to process a received
frame and to respond with a response frame. It is the difference in time
between the first symbol of the response frame in the air and the last
symbol of the received frame in the air. A SIFS time consists of the
delay in receiver RF, PLCP delay and the MAC processing delay, which
depends on the physical layer used. In IEEE 802.11 networks, SIFS is the interframe spacing prior to transmission of an acknowledgment, a Clear To Send (CTS) frame, a block ack frame that is an immediate response to either a block ack request frame or an A-MPDU, the second or subsequent MPDU of a fragment burst, a station responding to any polling a by point coordination function and during contention free periods of point coordination function.
Channels
2.4 GHz (802.11b/g/n)
Graphical representation of 2.4 GHz band channels overlapping
Graphical representation of Wireless LAN channels in 2.4 GHz band
There are 14 channels designated in the 2.4 GHz range spaced 5 MHz
apart (with the exception of a 12 MHz spacing before channel 14).
Note that for 802.11g/n it is not possible to guarantee orthogonal frequency-division multiplexing (OFDM) operation thus affecting the number of possible non-overlapping channels depending on radio operation.
United States & Canada |
Europe |
Switzerland |
Japan |
Singapore |
China |
Israel |
Korea |
Turkey |
Australia |
South Africa |
Brazil |
Taiwan |
New Zealand |
183 |
4915 |
No |
No |
No |
No |
Yes |
No |
No |
No |
No |
No |
No |
No |
No |
No |
No |
184 |
4920 |
No |
No |
No |
Yes |
Yes |
No |
No |
No |
No |
No |
No |
No |
No |
No |
No |
185 |
4925 |
No |
No |
No |
No |
Yes |
No |
No |
No |
No |
No |
No |
No |
No |
No |
No |
187 |
4935 |
No |
No |
No |
No |
Yes |
No |
No |
No |
No |
No |
No |
No |
No |
No |
No |
188 |
4940 |
No |
No |
No |
Yes |
Yes |
No |
No |
No |
No |
No |
No |
No |
No |
No |
No |
189 |
4945 |
No |
No |
No |
No |
Yes |
No |
No |
No |
No |
No |
No |
No |
No |
No |
No |
192 |
4960 |
No |
No |
No |
Yes |
No |
No |
No |
No |
No |
No |
No |
No |
No |
No |
No |
196 |
4980 |
No |
No |
No |
Yes |
No |
No |
No |
No |
No |
No |
No |
No |
No |
No |
No |
7 |
5035 |
No |
No |
No |
No |
Yes |
No |
No |
No |
No |
No |
No |
No |
No |
No |
No |
8 |
5040 |
No |
No |
No |
No |
Yes |
No |
No |
No |
No |
No |
No |
No |
No |
No |
No |
9 |
5045 |
No |
No |
No |
No |
Yes |
No |
No |
No |
No |
No |
No |
No |
No |
No |
No |
11 |
5055 |
No |
No |
No |
No |
Yes |
No |
No |
No |
No |
No |
No |
No |
No |
No |
No |
12 |
5060 |
No |
No |
No |
No |
No |
No |
No |
No |
No |
No |
No |
No |
No |
No |
No |
16 |
5080 |
No |
No |
No |
No |
No |
No |
No |
No |
No |
No |
No |
No |
No |
No |
No |
34 |
5170 |
No |
No |
Indoors |
Client only
[clarification needed] |
No |
Yes |
No |
Yes |
Yes |
Indoors |
No |
Indoors |
Indoors |
No |
Indoors |
36 |
5180 |
Indoors |
Yes |
Indoors |
Indoors |
No |
Yes |
Yes |
Yes |
Yes |
Indoors |
Yes |
Indoors |
Indoors |
No |
Indoors |
38 |
5190 |
No |
No |
Indoors |
Client only |
No |
Yes |
Yes |
Yes |
Yes |
Indoors |
No |
Indoors |
Indoors |
No |
Indoors |
40 |
5200 |
Indoors |
Yes |
Indoors |
Indoors |
No |
Yes |
Yes |
Yes |
Yes |
Indoors |
Yes |
Indoors |
Indoors |
No |
Indoors |
42 |
5210 |
No |
No |
Indoors |
Client only |
No |
Yes |
Yes |
Yes |
Yes |
Indoors |
No |
Indoors |
Indoors |
No |
Indoors |
44 |
5220 |
Indoors |
Yes |
Indoors |
Indoors |
No |
Yes |
Yes |
Yes |
Yes |
Indoors |
Yes |
Indoors |
Indoors |
No |
Indoors |
46 |
5230 |
No |
No |
Indoors |
Client only |
No |
Yes |
Yes |
Yes |
Yes |
Indoors |
No |
Indoors |
Indoors |
No |
Indoors |
48 |
5240 |
Indoors |
Yes |
Indoors |
Indoors |
No |
Yes |
Yes |
Yes |
Yes |
Indoors |
Yes |
Indoors |
Indoors |
No |
Indoors |
52 |
5260 |
DFS |
Indoors/DFS/TPC |
Indoors/DFS/TPC (otherwise limited to 100mW instead of 200mW) |
Indoors/DFS/TPC |
No |
Yes |
DFS/TPC |
Yes |
Yes |
Indoors |
DFS/TPC |
Indoors |
Indoors |
No |
DFS/TPC |
56 |
5280 |
DFS |
Indoors/DFS/TPC |
Indoors/DFS/TPC (otherwise limited to 100mW instead of 200mW) |
Indoors/DFS/TPC |
No |
Yes |
DFS/TPC |
Yes |
Yes |
Indoors |
DFS/TPC |
Indoors |
Indoors |
Yes |
DFS/TPC |
60 |
5300 |
DFS |
Indoors/DFS/TPC |
Indoors/DFS/TPC (otherwise limited to 100mW instead of 200mW) |
Indoors/DFS/TPC |
No |
Yes |
DFS/TPC |
Yes |
Yes |
Indoors |
DFS/TPC |
Indoors |
Indoors |
Yes |
DFS/TPC |
64 |
5320 |
DFS |
Indoors/DFS/TPC |
Indoors/DFS/TPC (otherwise limited to 100mW instead of 200mW) |
Indoors/DFS/TPC |
No |
Yes |
DFS/TPC |
Yes |
Yes |
Indoors |
DFS/TPC |
Indoors |
Indoors |
Yes |
DFS/TPC |
100 |
5500 |
DFS |
DFS/TPC |
DFS/TPC (otherwise limited to 500mW instead of 1W) |
DFS/TPC |
No |
No |
No |
No |
Yes |
DFS/TPC |
DFS/TPC |
Yes |
DFS |
Yes |
DFS/TPC |
104 |
5520 |
DFS |
DFS/TPC |
DFS/TPC (otherwise limited to 500mW instead of 1W) |
DFS/TPC |
No |
No |
No |
No |
Yes |
DFS/TPC |
DFS/TPC |
Yes |
DFS |
Yes |
DFS/TPC |
108 |
5540 |
DFS |
DFS/TPC |
DFS/TPC (otherwise limited to 500mW instead of 1W) |
DFS/TPC |
No |
No |
No |
No |
Yes |
DFS/TPC |
DFS/TPC |
Yes |
DFS |
Yes |
DFS/TPC |
112 |
5560 |
DFS |
DFS/TPC |
DFS/TPC (otherwise limited to 500mW instead of 1W) |
DFS/TPC |
No |
No |
No |
No |
Yes |
DFS/TPC |
DFS/TPC |
Yes |
DFS |
Yes |
DFS/TPC |
116 |
5580 |
DFS |
DFS/TPC |
DFS/TPC (otherwise limited to 500mW instead of 1W) |
DFS/TPC |
No |
No |
No |
No |
Yes |
DFS/TPC |
DFS/TPC |
Yes |
DFS |
Yes |
DFS/TPC |
120 |
5600 |
No |
DFS/TPC |
DFS/TPC (otherwise limited to 500mW instead of 1W) |
DFS/TPC |
No |
No |
No |
No |
Yes |
DFS/TPC |
No |
Yes |
DFS |
Yes |
DFS/TPC |
124 |
5620 |
No |
DFS/TPC |
DFS/TPC (otherwise limited to 500mW instead of 1W) |
DFS/TPC |
No |
No |
No |
No |
Yes |
DFS/TPC |
No |
Yes |
DFS |
Yes |
DFS/TPC |
128 |
5640 |
No |
DFS/TPC |
DFS/TPC (otherwise limited to 500mW instead of 1W) |
DFS/TPC |
No |
No |
No |
No |
Yes |
DFS/TPC |
No |
Yes |
DFS |
Yes |
DFS/TPC |
132 |
5660 |
DFS |
DFS/TPC |
DFS/TPC (otherwise limited to 500mW instead of 1W) |
DFS/TPC |
No |
No |
No |
No |
No |
DFS/TPC |
DFS/TPC |
Yes |
DFS |
Yes |
DFS/TPC |
136 |
5680 |
DFS |
DFS/TPC |
DFS/TPC (otherwise limited to 500mW instead of 1W) |
DFS/TPC |
No |
No |
No |
No |
No |
DFS/TPC |
DFS/TPC |
Yes |
DFS |
Yes |
DFS/TPC |
140 |
5700 |
DFS |
DFS/TPC |
DFS/TPC (otherwise limited to 500mW instead of 1W) |
DFS/TPC |
No |
No |
No |
No |
No |
DFS/TPC |
DFS/TPC |
Yes |
DFS |
Yes |
DFS/TPC |
149 |
5745 |
Yes |
in study, SRD (25 mW) |
No |
No |
No |
Yes |
Yes |
No |
Yes |
No |
Yes |
No |
Yes |
Yes |
Yes |
153 |
5765 |
Yes |
in study, SRD (25 mW) |
No |
No |
No |
Yes |
Yes |
No |
Yes |
No |
Yes |
No |
Yes |
Yes |
Yes |
157 |
5785 |
Yes |
in study, SRD (25 mW) |
No |
No |
No |
Yes |
Yes |
No |
Yes |
No |
Yes |
No |
Yes |
Yes |
Yes |
161 |
5805 |
Yes |
in study, SRD (25 mW) |
No |
No |
No |
Yes |
Yes |
No |
Yes |
No |
Yes |
No |
Yes |
Yes |
Yes |
165 |
5825 |
Yes |
in study, SRD (25 mW) |
No |
No |
No |
Yes |
Yes |
No |
Yes |
No |
Yes |
No |
Yes |
Yes |
Yes |
Devices supporting the latest 802.11ac (draft) standard are now being certified by
the Wi-Fi Alliance. 802.11ac promises various improvements over
802.11n. Starting from the first IEEE 802.11 standard in 1997 to the
latest 11ac standard, there have been improvements in various aspects of
802.11 networks - speed of the network being one of the major
improvements. This article attempts to explain the PHY data rates of
11b, 11a/g, 11n and 11ac.
Modulation
The above diagram shows the modulation techniques used in different 802.11 PHYsical layers. 802.11 systems support two (Ignoring the optional and obsolete types) types of modulation - DSSS/CCK and OFDM. This article is not going to cover the details of DSSS/CCK (a good intro can be found here) and OFDM. The below figure gives an overview of both the techniques.
Data Rate
DSSS/CCK (11b Date Rates)
DSSS/CCK data symbol is formed by spreading and modulating the data bits. The data rate depends on :
- Chips per second (CSec) = 11,000,000
- Chips per symbol (CSym) = 11 (DSSS) or 8 (DSSS with CCK)
- Bits per symbol (NBits) = 1 or 2 (DSSS), 4 or 8 (DSSS with CCK)
Data Rate = (CSec/CSym)*NBits
Chips per second |
Chips per symbol |
Bits per symbol |
Data Rate (Mbps) |
11,000,000 |
11 |
1 |
1 |
11,000,000 |
11 |
2 |
2 |
11,000,000 |
8 |
4 |
5.5 |
11,000,000 |
8 |
8 |
11 |
OFDM
DSSS/CCK uses the
entire available bandwidth as one single channel. OFDM divides the
channel into multiple(overlapping) sub-channels. The data rate depends
on:
- Symbol duration (in other words symbols per second) (SDur)
- Symbol duration depends on "Guard Interval" between symbols.
- Bits per symbol (NBits)
- Coding rate (CRate)
- Some bits are used for error correction and do not carry data
- Number of sub-channels (NChan)
- Depends on channel width(20/40/80/160)
- 11n/11ac have more sub-channels in the same bandwidth compared to 11a/11g
Data Rate = (1/SDur)*(NBits*CRate)*NChan
11a/11g Rates
Modulation |
NBits |
CRate |
NChan |
SDur (micro sec) |
Data Rate (Mbps) |
BPSK |
1 |
1/2 |
48 |
4 |
6 |
BPSK |
1 |
3/4 |
48 |
4 |
9 |
QPSK |
2 |
1/2 |
48 |
4 |
12 |
QPSK |
2 |
3/4 |
48 |
4 |
18 |
16-QAM |
4 |
1/2 |
48 |
4 |
24 |
16-QAM |
4 |
3/4 |
48 |
4 |
36 |
64-QAM |
6 |
2/3 |
48 |
4 |
48 |
64-QAM |
6 |
3/4 |
48 |
4 |
54 |
11a/11g Data Rates
Note: The above table shows date rates for 20MHz channel width. 5,10 MHz channel widths are not shown.
11n/11ac Data Rates
11n and 11ac data rate improvements are due to:
- The number of sub-channels in 11n and 11ac is more than 11a and 11g.
- Higher coding rate (5/6)
- 11ac also uses 256-QAM which further increases the data rate.
- Wider Channels (40,80,160)
- Multiple Spatial Streams (MIMO) (Nss)
Modulation |
NBits |
CRate |
NChan |
Data Rate (Mbps)
SDur=4us |
Data Rate (Mbps)
SDur=3.6us |
BPSK |
1 |
1/2 |
52 |
6.5 |
7.2 |
QPSK |
2 |
1/2 |
52 |
13 |
14.4 |
QPSK |
2 |
3/4 |
52 |
19.5 |
21.7 |
16-QAM |
4 |
1/2 |
52 |
26 |
28.9 |
16-QAM |
4 |
3/4 |
52 |
39 |
43.3 |
64-QAM |
6 |
2/3 |
52 |
52 |
57.8 |
64-QAM |
6 |
3/4 |
52 |
58.5 |
65 |
64-QAM |
6 |
5/6 |
52 |
65 |
72.2 |
256-QAM (11ac) |
8 |
3/4 |
52 |
78 |
86.7 |
256-QAM (11ac) |
8 |
5/6 |
52 |
86.7 |
96.3 |
11n and 11ac Data Rates (20 MHz, 1 SS)
The entries highlighted are 11ac only rates and the rest are common to 11n and 11ac.
Updated on 20/08/2014:
Note: The
last entry,i.e, MCS9 (256 QAM, 5/6 rate) is
not allowed in 11ac 20 MHz channel.
The maximum data rate is achieved with MIMO and maximum channel width.
Channel Width |
NBits |
CRate |
NChan |
Data Rate (Mbps)
SDur=3.6us (Nss=1) |
11n Max Rate
(Nss=4) |
20 Mhz |
6 |
5/6 |
52 |
72.2 |
288.8 |
40 Mhz |
6 |
5/6 |
108 |
150 |
600 |
11n Max Data Rates
Channel Width |
NBits |
CRate |
NChan |
Data Rate (Mbps)
SDur=3.6us (Nss=1) |
11ac Max Rate
(Nss=8) |
20 Mhz |
8 |
5/6 |
52 |
96.3 |
577.8
(Nss=6) |
40 Mhz |
8 |
5/6 |
108 |
200 |
1600 |
80 Mhz |
8 |
5/6 |
234 |
433.3 |
3466.4 |
160 Mhz |
8 |
5/6 |
468 |
866.7 |
6933.6 |
11ac Max Data Rates
Updated on 20/08/2014: Max valid spatial streams for 11ac 20 MHz channels corrected to 6.
Summary
The improvement
in data rate from 11b to 11ac is due to various factors: OFDM, Wider
Channels, QAM, Higher coding rate and MIMO. The figures below depicts
the improvement in data rate due to each factor(excluding MIMO).