Review on WiFi-Nano: Reclaiming WiFi Efficiency Through 800 ns Slots

1. Review: “WiFi-Nano: Reclaiming WiFi Efficiency Through 800 ns Slots”
Bhavesh Singh
2010CS50281
1. Summary
1.1 Motivation
As the speed of WiFi physical layer increases from 1 Mbps to 1 Gbps, the efficiency
reduces from over 80% to 10%. It would be revolutionary if such a decrease in efficiency could
be ceased or kept as minimum as possible. In 802.11, the reasons for such efficiency drop is
known. So WiFi-Nano has tried to overcome these reasons and to solve the problem behind
each reason. There are three key overheads which are responsible for this – channel access,
data preamble and acknowledgement overheads. Some other overheads like collision are
also there. At 600 Mbps, the average channel access overhead is 500% of the packet (1500
bytes long) transmission time. While at 1 Mbps, it is 0.83%. Similarly the differences in other
overheads are also evident. For example the data preamble and ACK overheads together
increase from 15% at 54 Mbps to 44% at 600 Mbps. Such differences cannot be ignored and
solutions to these problems will make the better usability of WiFi with more and more
physical layer data rates. Since the issue of WiFi efficiency is bigger, it makes these problems
very significant which WiFi-Nano tried to solve.
600 Mbps
Overhead ~91%

2. 1.2 Contribution
The main contribution was to improve the efficiency of WiFi. Following are the contributions
made by WiFi-Nano
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Eliminated SIFS by speculative ack preamble transmission and thus reduced the ack
overhead.
Parallel implementation of preamble detection with preamble transmission. This
benefits in collision reduction and unfairness elimination.
Introduce shorter slot time of 800 ns and in turn improves the throughput of WiFi by
up to 100%.
Channel access and ack overhead was reduced.
Introduced the technique for sub-preamble detection and its realization by using a
lattice correlator.
Improved the air-time efficiency (fraction of time data was transmitted over the air)
of WiFi.
1.3 Methodology
Following methods were adopted to increase efficiency in WiFi-Nano
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800 ns slots : Instead of using 9 microseconds slots as in 802.11, 800 ns slots were
used in WiFi-Nano. It makes back-off efficient.
Speculative Preamble Transmission: Instead of waiting for multiple slots for detecting
preambles, nodes in Wifi-Nano speculatively transmit preambles as their back-off
counters expire, while continuing to detect preambles using self-interference
cancellation. Contention for channel access is carried out simultaneously with
preamble transmission. All the devices abort their transmissions midway except those
whose back-off counters expired the earliest. Thus average channel access time can
be reduced to 7.6 microseconds which was 101.5 microseconds with 9 microseconds
slot time and 100 bytes of packet size in 802.11 Wifi.
Speculative ACK: Instead of waiting for SIFS before transmitting the ACK preamble,
the receiver speculatively starts transmitting its ACK preamble as soon as it finishes
reception of the packet. The ACK transmission is then aborted midway upon detecting
errors in the received packet. Thus Speculative ACK Transmission allows WiFi-Nano to
eliminate SIFS and thus reduce the ACK overhead.
Working with Lattice Correlator: A novel Lattice Correlator was designed to enable
chained contention resolution in WiFi-Nano. In order to do this, their correlator must
provide two functions. First, devices are required to correlate sub-parts of a preamble.
Second, the need for roll-back requires that the exact position of the correlation be
known, since this will help accurately determine the beginning of the packet

3. 
transmission. Each packet of WiFi-Nano is preceded by a PN sequence comprising
several short but distinct 800ns PN sequences PN1, PN2, ···, PNn. The lattice correlator
takes as input the received signal, and generates
, (N is the number of 800ns PN
sequences) outputs corresponding to the correlations obtained from each continuous
sub-part of the preamble e.g., [PN1, PN2], [PN3, PN4, PN5] etc. Detection of a spike
in any of these inputs provides two pieces of information. First, the presence of an
ongoing transmission, and second, the start time of the beginning of the reception.
The start time of beginning of the packet reception is determined by the position of
the last 800ns PN sequence.
Probabilistic Collision Resolution: Since potential collisions can be detected in each
800ns slot, WiFi-Nano uses a novel contention resolution scheme to resolve collisions
on the fly. Finally, note that when more than two packets collide in a given slot, the
number of collisions can be approximately estimated by the number of correlation
spikes that occur within a single 800ns slot (this is because the slot boundaries of
different nodes are not perfectly aligned due to differences in propagation delays).
Upon detecting k − 1 distinct spikes in a single slot, rather than using 50%, each device
continues transmitting with a probability of . Thus, the probabilistic collision
resolution mechanism in WiFi-Nano avoids payload collisions with a high probability,
thereby significantly reducing the collision overhead
1.4 Conclusion
The main objective of this research was to examine the efficiency by reducing the 9
microseconds slot used in Wifi to 800 ns. The 800 ns slot size needs some alteration in the
conventional 802.11 Wifi like the preamble transmission and detection was done in parallel,
which is achieved by using speculative transmission of preambles and analogue interference
cancellation. Also SIFS is eliminated by speculative ACK transmission. Furthermore, a novel
lattice correlator is designed that correlates to parts of the preamble and is able to accurately
determine the start time of detected preambles, which is a key requirement for accurate
rollback of speculative preamble transmissions. Also, Nodes in Wifi-Nano abort transmissions
probabilistically since nodes are able to detect collisions during the preamble transmission
phase. This way, the packet collisions are avoided with high probability.
2. Critique
2.1 Lack of information about DIFS
The paper talked about the elimination of SIFS in WiFi-Nano by speculative ACK preamble
transmission but had not talked about DIFS time. Whether, DIFS is there in WiFi-Nano or it is

4. also eliminated, is also not clear in the paper. Since the slot time is reduced to 800 ns in WiFiNano, there must be an impact on DIFS as well if it is there. Generally DIFS is equal to (SIFS +
2 * slot time). Since SIFS in WiFi-Nano has been eliminated, so DIFS should be equal to 2*800
i.e. 1600 ns if it is present. Also they haven’t talk much about the size of contention window.
What should be the size of contention window so that both fairness and efficiency of WiFi
can be maximized?
2.2 Challenges in speculative preamble transmission
Nodes may unduly count down their back-offs while other nodes’ preamble are being
transmitted. This would result in some wastage of resources. The preamble may be detected
as late as 5 slots after the preamble transmission, the channel is perceived as free. Due to
this, there is some overhead in efficiency also. This problem is not discussed in the paper.
2.3 Why 800 ns slot?
Why the author has chosen this figure of 800 ns as the slot time? The author surely want
to decrease the overheads by reducing the slot time. Does the overhead linearly dependent
on slot time or not? There must be some relation between the slot time and the overheads.
The author neither mention why he picked 800 ns slot nor he discussed the relation between
slot time and overheads.
3. Synthesis
Wifi-Nano is designed to increase the efficiency of WiFi with high data rates. The
technology can be tested on smaller data rate WiFi (11 Mbps or 54 Mbps) and study the
results. Since these WiFis already have greater efficiency, so it might not greater impact on
these but even a 10% improvement in efficiency would be great if it will be able to do that.
There are various techniques discussed in the paper for solving some problems like
collision overhead, fairness etc. These solutions can be explicitly used in other scenarios, for
example, we can use Probabilistic collision resolution in 802.11 or other variants of WiFi. We
can use the novel lattice correlator designed for Wifi-Nano in other purposes as well.
The paper has presented four techniques to improve the efficiency of WiFi. They are
speculative preamble transmission, probabilistic collision resolution, counter roll back and
speculative ACK transmission. All the four techniques are very specific and can be used
explicitly for other purposes.
Likewise the SIFS is eliminated from the WiFi, one more extension could be to eliminate
the DIFS time as well. There may be some way possible to eliminate it as well.