1. COF: Exploiting Concurrency for Low Power Opportunistic Forwarding
ICNP, 2015, San Francisco
COF: Exploiting Concurrency for Low Power Opportunistic Forwarding
Daibo Liu1, Mengshu Hou1, Zhichao Cao2,
Yuan He2, Xiaoyu Ji3 and Xiaolong Zheng2
1 University of Electronic Science and Technology of China
2 Tsinghua University
3 Hong Kong University of Science and Technology

2. Motivation t Low power wireless sensor networks Duty-cycled mode
Asynchronous wake-sleep schedule
Short preamble transmission
Built upon CSMA
Exclusive mode for channel access t
Alice …… ……
I prepare send a packet. I will not stop sending until the ACK comes!
Bob ……
Channel is busy. I will keep waiting until the channel is free!
In low power wireless sensor networks, energy efficiency is a fundamental issue in the design of forwarding protocols.
A primary low power mechanism is duty cycling. In the duty-cycled mode, a node periodically switches its radio state between on (awake) and off (sleep).
Usually, the sleep schedules of different nodes are unsynchronized, the sender has to wait until the receiver turns on the radio. During the waiting period, the sender has to continuously transmit the same data packet (which is called data preamble) until the receiver’s acknowledgement is received.
Furthermore, the medium access control (MAC) protocol is built upon CSMA in low power WSNs. Neighboring nodes utilize wireless channel in exclusive mode. Hence, the long preamble transmission time could incur long idle waiting time.
* Blind waiting in duty cycles is generally energy-inefficient and limits network throughput.
With CSMA mechanism, senders transmit data
Packets in exclusive mode to avoid collision!
Exclusive transmission mode usually results in long
waiting time, extra energy consumption.

3. No Motivation Opportunistic forwarding meeting duty cycle and CSMA
Multiple forwarding choices
Temporal diversity: different awake time
Spatial diversity: different locations
Alice
Bob No
To shorten the waiting time, a practical approach is opportunistic forwarding, which takes the earliest forwarding opportunity instead of a deterministic forwarder.
In low power opportunistic forwarding, the forwarding opportunities include all the neighbors that are awake and offer sufficient routing progress.
Despite that the waiting period is shortened, however, as we have observed, the current collision avoidance based MAC is still conservative for duty-cycled opportunistic routing.
Specifically, in low power opportunistic forwarding, the multiple forwarding choices with temporal and spatial diversity increase the chance to tolerate collision in concurrent transmissions. The interference from a specific neighbor is likely to have different influence on different candidate forwarders. If any one of the potential forwarders can successfully decode the sender’s packet under interference, opportunistic forwarding should promote rather than arbitrarily suppress such a transmission opportunity.
Should Alice and Bob transmit data packets
always in exclusive mode?

4. Motivation Opportunistic exposed terminal (OET)
Two senders are within each other’s carrier sense range
Benefit for performance by exploiting concurrency
Network throughput
Energy efficiency
Transmission latency
Alice
Bob
We use the term opportunistic exposed terminal (OET) to denote such a phenomenon.
As shown in the figure, two senders, Alice and Bob, are within each other’s carrier sense range. However, concurrent transmission can bring more benefits than transmission in exclusive mode. Benefit can be defined as total throughput, energy efficiency, or end-to-end delay. So, in OET, the concurrency should be exploited to further improve the performance of low power opportunistic forwarding.
In opportunistic exposed terminal, concurrent opportunity
should be exploited to improve performance!

5. Motivation Empirical study Low power opportunistic forwarding
OET in different scenarios (100+ samples)
Performance: delay, total throughput
Gap = Performance of with CSMA - Performance of without CSMA
Potential decrement
of transmission delay
Potential improvement
of total throughput
To show the space for concurrency in low power opportunistic forwarding, we conduct experiments in indoor testbed. In each experiment, we randomly select two nodes as senders. They are within the carrier sense range of each other and continuously generate data packets to forward. We repeat the experiments by setting the two senders’ CSMA mechanism to be enabled and disabled in the same scenario. We compute the average single hop transmission delay gap and the average throughput gap between the two settings of each experimental scenario.
The experiments are conducted for more than 100 times.
As shown in the figures, when CSMA is disabled in low power opportunistic forwarding, a great portion of transmissions can achieve shorter single hop delay (the plus zone) and finally achieve higher throughput (the minus) than the CSMA-enabled case. This observation indicates that there are many data transmissions (about 63.7% in our experiments) suffering from the opportunistic exposed terminal problem. Exploiting concurrency of transmissions can alleviate this problem.
Concurrent transmission in opportunistic exposed terminal
is benefit for network performance.

6. Motivation Empirical study – On the other hand
Low power opportunistic forwarding
OET in different scenarios (100+ samples)
Performance: delay, total throughput
Gap = Performance of with CSMA - Performance of without CSMA
If concurrent transmission is out of control, it will adversely
shrink network throughput and increase transmission delay!
Degraded throughput
caused by collisions
Adversely increased
transmission delay
On the other hand, if concurrent transmission is out of control, it may induce serious data collisions, and results in longer transmission delay (minus zone) and degraded network throughput. If we can avoid the misled concurrency, the dash areas are the potential improvement space for low power opportunistic forwarding.

7. Key Insights
CSMA-based mechanism is too conservative to exploit potential forwarding opportunities;
It is profitable to achieve concurrent transmission in OET (opportunistic exposed terminal);
Improper concurrent transmission is adverse to network performance.
The empirical studies shed light on the potential of exploiting concurrency for low power opportunistic forwarding.
We should allow senders to concurrently transmit in the presence of opportunistic exposed terminal, however, the improper concurrent transmission is adverse to network performance.

8. Our Goal
Detect potential concurrency opportunity under OET, exploit the concurrency to improve performance of low power opportunistic forwarding
Motivated by the above-mentioned results, we propose the design of COF.
COF will detect potential concurrency opportunity under OET, and further exploit the concurrency to improve performance of low power opportunistic forwarding.

9. concurrent transmission
Overview of COF
A and B achieve
concurrent transmission
A Concurrency with B
Overhear preamble
Okay, let’s see the overview design of COF.
Node A and B are under the opportunistic exposed terminal. When A intends to transmit a data packet, B is transmitting at this time.
So, A keeps overhearing the data preamble transmitted by B. Once successfully decoding a preamble, it queries the expected benefit of concurrent transmission with B and confirms that the concurrency can gain more benefit than the individual transmission of B. Then, A disables carrier sensing temporarily and begins to transmit data preambles for a period of one duty cycle. By overhearing the data preamble of A, B will do the same decision and then disable its carrier sensing and concurrently transmit.
Overhear the ongoing transmission
Check the expected benefit of concurrency
Make concurrent decision

10. Outline of COF Design How to quantify the benefit of concurrency?
Link quality under the condition of concurrent transmission
Online measurement approach
Then we introduce how to quantify the benefit of concurrency, which refers to the link quality under the condition of concurrent transmission, and an online measurement approach.

11. Conditional Link Quality
Conditional packet delivery rate (CPDR)
Concurrent transmission: Alice and Bob
CPDR(Alice|Bob), involving
Data delivery rate (DDR)
Transmitted from Alice to each forwarder
ACK receipt rate (ARR)
Replied from each forwarder to Alice
Alice
Bob
CPDR(Alice|Bob) denotes the probability that Alice can successfully
forward a packet to its forwarder set when Bob is transmitting.
To quantify the benefit of concurrency, we first introduce the computation of link quality under the condition of concurrent transmission.
We mark it as CPDR, conditional packet delivery rate.
In this case, Alice and Bob are concurrently transmitting, CPDR(Alice | Bob) denotes the probability of Alice’s transmission success rate when Bob is concurrently transmitting. CPDR refers to the Data delivery rate (transmitted from Alice to its forwarders) and ACK receipt rate (replied from a forwarder to Alice).

12. Data delivery rate (DDR)
DDR(Alicei|Bob)
The probability that i can successfully receive Alice’s transmission when Bob is transmitting
Data
Data
Data
Alice
Bob
Data
Data
The data delivery rate (DDR) is defined as the probability that forwarder i can receiver Alice’s message when Bob is also transmitting.

13. ACK receipt rate (ARR) ARR(iAlice|Bob)
The probability that Alice can receive the replied ACK from forwarder i when Bob is transmitting
ACK
Data
Data
Alice
Bob
ACK
Data
And the ACK receipt rate （ARR) is defined as the probability that Alice can receive the replied ACK from the forwarder i when Bob is transmitting at this time.

14. Bi-directional link quality
Cpdr between Alice and i, considering
DDR(Alicei|Bob)
ARR(iAlice|Bob)
Data
Data
Data
Data
Data
ACK
Alice
Bob
Alice
Bob
ACK
Data
Data
Data
The CPDR between Alice and forwarder i is only a part of
the CPDR between Alice and its forwarders set.
Then, COF can compute the bi-directional link quality between Alice and the forwarder i, by considering both the data delivery rate (DDR) and ACK receipt rate (ARR).
Note that the CPDR between Alice and forwarder i is only a part of the CPDR between Alice and its all forwarders.

15. Conditional Packet Delivery Rate
Computation of CPDR
Multiple forwarders, e.g., FAlice={C, D, E}
CPDR between Alice and its forwarders set
Bob’s CPDR(Bob|Alice) is computed in the same way
By considering all potential forwarders, COF could compute the expected conditional packet delivery rate between Alice and its forwarder set under the influence of Bob’s transmission.
In the same way, Bob can compute the expected CPDR under the influence of Alice’s transmission.

16. Benefit of Concurrency
Expected throughput gain (EGain)
Comparison
Concurrent throughput, T(Alice, Bob)
Original throughput without concurrency, T(Bob, null)
Overall benefit
Benefit threshold ω If
Concurrency is permitted, otherwise, denied.
T(Alice, Bob) = CPDR(Alice | Bob) + CPDR(Bob | Alice)
T(Bob, null) = CPDR(Bob | null)
We define the expected benefit as the expected gain of throughput of a period of one transmission.
After computing the conditional packet delivery rate between each node and its forwarder set.
COF first compute the concurrent throughput of both Alice and Bob.
In order to achieve concurrent transmission, as a new entrant, Alice should query the original throughput of Bob without concurrent transmission. And then, Alice can compute the overall benefit of concurrency.
If the benefit is larger than omega, COF permits the concurrent transmission, otherwise, the concurrency is denied.
EGain(Alice, Bob) = T(Alice, Bob) – T(Bob, null)
EGain(Alice, Bob) > ω
denotes the expected throughput of one transmission of Bob without suffering the influence of Alice

17. Online Measurement In multi-hop WSNs, each node acts as both
Sender
With a forwarder set
Forwarder
With a children set
Data sequence number (DSN) indicates each transmission
COF using bitmap to record
Ongoing transmitter (as sender)
Transmission state of each delivery (as sender)
ACK reply count for a child’s data delivery (as forwarder)
OK, les’s show the online measurement of conditional link quality in distributed manner.
COF uses DSN to denote each transmission, and uses bitmaps to record the following information: who the ongoing sender is, what the transmission result is, and how many ACKs are replied by each forwarder.

18. Online Measurement Small bitmaps
Alice maintains a bitmap for each neighbor
Each of R1 and R2 maintains one bitmap for Alice R1 R2
Alice
Bob
By collecting forwarders’ bitmap information, Alice can
completely know: which transmission is fail,and further
know which replied ACK is lost.
Collected for Alice
For example, as shown in the figure, Alice’s concurrent neighbor could be Bob, R1, R2, or null (three is no ongoing transmission). Each transmission can be denoted as a unique DNS.
Alice maintains a bitmap for each neighbor to record the concurrent information. For each transmission, there are four different states, state 0 denotes there is no concurrent transmission, state 1 denotes the transmission is replied by one of its forwarder, state 2 denotes an unacknowledged transmission, and 3 denote the starting of a new data transmission.
At the same time, each forwarder maintains a bitmap for Alice to record the number of replied ACK corresponding to each decoded transmission (denoted as a unique DSN).
By collecting forwarders’ bitmap information, Alice can completely know: which transmission is fail, and further know which replied ACK is lost.
0: With no concurrency
DSNs
1: ACKed transmission
Number of replied ACKs
2: Retransmission
3: Starting of a new data
R1, R2 records the number of replied ACK for Alice’s each data transmission
Alice records the number of transmissions for each data corresponding to a concurrent neighbor

19. Online Measurement Collected for Alice Corresponding to each
transmission (DSN)
The number of replied ACKs
Acknowledged or not (state 1)
Corresponding to each data
packet (multiple DSNs)
Transmission count
Received (nonzero) or not of each transmission
Corresponding to each transmission and each forwarder, using the number of replied ACKs and the number successfully replied ACK, Alice could compute the ACK receipt rate.
In the same way, corresponding to each packet and each forwarder, using the number of transmission count, and the number of received data copies (with different DSNs), Alice can compute the data delivery rate.
According to the the ACK receipt rate and data delivery rate corresponding to each forwarder, COF further computes the conditional link quality under the influence of Bob’s transmission.
Computing ARR(iAlice|Bob),
i is R1 or R2.
Computing DDR(Alicei|Bob),
i is R1 or R2.

20. Evaluation Implementation Evaluation Performance TinyOS-2.1.1
Combining COF with ORW
Evaluation
Indoor testbed, 40 Telosb nodes
One-hop network, multiple-hop network
Performance
Overhead
Reliability
Energy efficiency
Transmission efficiency
COF is implemented in TinyOS, by combining with ORW and CTP.
The experiments are conducted in an indoor testbed with 40 Telosb nodes to test the performance of COF, including the effect of overhead on performance, the exploited concurrency in low power opportunistic forwarding, reliability, energy efficiency and transmission efficiency.

21. Overhead of COF Effect of overhead on performance
COF exploits the free space of both network probe and data packet to share the bitmap information. The overhead is very limited.
We conduct experiments to compare the performance of ORW and ORW combining with COF to quantify the effect of overhead on performance, in the same system setting, network scenario, and with extremely low traffic load.
We compute the average single hop delay and average radio duty cycle of all nodes in the Figure. The extra overhead of COF brings 0.9% extra delay and 0.75% extra energy consumption.
Exploiting free space of probe/data packets to exchange bitmap information
Indoor testbed experiments: ORW vs. ORW+COF
Identical network setting and scenario
Extremely low data traffic load
The overhead of COF induces 0.9% extra delay and
0.75% extra energy consumption.

22. Multi-hop Network Performance
Almost the same packet
delivery ratio
By using COF, ORW can
reduce 18.9% energy
consumption
Indoor testbed with 40 Telosb nodes
ORW vs. ORW+COF
Identical network setting and scenario
Reliability, energy efficiency, transmission efficiency, delay
We also conduct experiments in multi-hop network to compare the performance of ORW and that of ORW combining with COF. By computing the mean packet delivery ratio (PDR), radio duty cycle, mean one hop transmission count, and each node’s end- to-end delay, we plot the cumulative distributions in figures.
Overall, the cumulative distribution of nodes’ PDR of ORW- COF is almost the same as that of ORW.
But the exploiting of concurrency significantly reduces the expected waiting time of each transmission, so as to reduce the radio duty cycle by 18.9% compared with that of ORW.

23. Multi-hop Network Performance
Exploiting concurrency, the average transmission count
is slightly increased (6.3%) due to collision.
Indoor testbed with 40 Telosb nodes
ORW vs. ORW+COF
Identical network setting and scenario
4 minutes Inter-packet interval
Reliability, energy efficiency, transmission efficiency, delay
By exploiting concurrency, data collision is increased compared with that ORW. The average transmission count is increased by 6.3%. However, the concurrent transmission could significantly reduce the waiting time. By using COF, ORW reduces almost 41% end-to-end delay.
Overall, COF can significantly improve the performance of low power opportunistic forwarding with little cost.
By using COF, ORW reduces almost 41% end-to-end delay.

24. Summary Exploiting concurrency for low power opportunistic forwarding
Quantify the benefit of concurrency
Online measurement
Concurrent decision
Real implementation of COF in TinyOS2.1.1
Evaluation in indoor testbed
Okay, let me summarize this work!
In this work, we protocol COF to exploit potential concurrency in low power opportunistic forwarding.
To achieve this goal, we propose a method to measure conditional link quality, then quantify the benefit of potential concurrency.
According to the benefit restriction, COF makes a concurrency decision to permit or deny the concurrent opportunity.
We implement COF in TinyOS and combine it with the traditional opportunistic forwarding.
We conduct evaluation in indoor testbed to test the performance of COF.

25. Thank you!
Q&A
That is all! Thank you! Any question?

26. Backups

27. Concurrent Decision COF process If there is no detected transmitter,
Beneath Routing layer;
Above Physical layer;
Interacting with MAC protocol.
Free channel: COF is inactive;
Busy channel: MAC calls COF
to forward.
COF process
If there is no detected transmitter,
Continue overhearing
Make decision according to benefit restriction
If EGain>ω, permitted concurrency
Otherwise, denied concurrency
COF sits beneath routing layer and above physical layer. It interacts with MAC layer.
To transmit a message, the default MAC protocol should first check the wireless channel.
If channel is free: MAC deals with data forwarding without activating COF
If channel is busy: Mac delivers data forwarding event to COF, and activates it.
Once COF is activated, if there is no detected transmitter, it continues overhearing and does nothing.
Otherwise, it makes a concurrency decision according to the benefit restriction.

28. Exploiting Concurrency Opportunity
Topological demo
‘’ORW,CS off,no ack’’ makes full use of concurrency without
suffering from ACK loss. It obtains the best performance in
the presence of opportunistic exposed terminal.
Presence of opportunistic exposed terminal
Indoor testbed:
Extremely high traffic load
Collecting data packets by deleting duplication
ORW, CS, ack (enabled CSMA, requiring ACK)
ORW+COF
ORW, CS off, no ack (disabled CSMA, without requiring ACK)
Here, we conduct experiment to quantify the throughput gain by adopting COF in presence of opportunistic exposed terminal.
Before the experiment, we first let all nodes take turns to broadcast beacons (100 beacons per node) to identify the signal strength and link quality between each pair of nodes.
Then, we carefully select nodes to construct the target topology as shown in the left part of the figure. For each target topology, we adopt three opportunistic forwarding protocol stack to test the overall throughput.
We select a large number of target topologies in our indoor testbed, and plot the evaluation results in the figure.
As shown in the figure,‘ORW,CS off,no ack’makes full use of concurrency without suffering from ACK loss. It obtains the best performance in the presence of opportunistic exposed terminal.
By using COF, ORW can further improve its throughput by 64%, which is very close to‘ORW,CS off,no ack’.
By using COF, ORW can further improve its throughput by 64%,
which is very close to ‘’ORW,CS off,no ack’’.

29. Setting CN to 80 can achieve good performance
Update of CPDR
Moving average
Cpdr(Alicei|Bob)=(1-θ)∙ Cpdr(Alicei|Bob)
+ θ ∙ Cpdrnew(Alicei|Bob)
Cpdr(iAlice|Bob)=(1-α) ∙ Cpdr(iAlice|Bob)
+ α ∙ Cpdrnew(iAlice|Bob)
Experimental test:
α and θ are variable
θ =Ni/CN, α= Nj/CN
Ni and Nj denote the used DSNs
Setting CN to 80 obtaining good performance
Transmission efficiency and delay
As we have observed, an ACK can be accurately delayed to send;
an ACK in time domain can be used to report a pending data and denote a specific sender;
children nodes can synchronize according to the same probe transmitted from receiver.
Then, time is divided into multiple slots, each sender will reply an ACK in a unique slot to report the need of data transmission.
Setting CN to 80 can achieve good performance

30. Setting ω to 0.55 can achieve good performance
Benefit Threshold
The value of ω directly affects the number of concurrencies and efficiency
Smaller ω: more concurrencies and low efficiency
Larger ω: less concurrencies and high efficiency
Experimental test:
Performance
Transmission efficiency
Transmission delay
ω is set to 0.55
This slide gives a general view of how receiver infer the slot sequence number of each received ACK.
For example, S2 replies an ACK in the first slot, and S1 replies in the third slot. R receiving the first ACK at T4, and receives the second ACK at T5.
T base is the fixed duration between the transmitting time of reservation probe and the starting time of dividing time into slots.
So, receiver could infer the slot sequence number of each received ACK according to the equation.
Now, we want to ask how can senders reply ACKs in different slots to avoid collision?
Setting ω to 0.55 can achieve good performance

31. High probability to decode
Motivation
Empirical study
During the waiting period:
Overheard preambles
Waiting time
High probability to decode
at least one preamble
Long waiting time

32. Benefit of Concurrency
The consistent decision
First, Alice checks
Egain(Alice|Bob)> ω (1)
If (1) is valid, then Alice checks
Egain(Bob|Alice)> ω (2)
If both (1) and (2) are valid, then
Alice and Bob will concurrently transmit
As we have observed, an ACK can be accurately delayed to send;
an ACK in time domain can be used to report a pending data and denote a specific sender;
children nodes can synchronize according to the same probe transmitted from receiver.
Then, time is divided into multiple slots, each sender will reply an ACK in a unique slot to report the need of data transmission.