Understanding the Real-World Performance of Carrier Sense Kyle Jamieson, Bret Hull, Allen Miu, Hari Balakrishnan ACM SIGCOMM 2005 Workshop on Experimental Approaches to Wireless Network Design and Analysis (E-WIND), Presented by Chaegwon Lim 1/17/2019 *This material is borrowed from authors’ homepage and partly modified by the presenter.

Summary Let’s quantify how well carrier sense performs in real-world radio networks An experimental evaluation of the benefits and drawbacks of carrier sense Conclusion Current carrier sense schemes cannot take into account the capture effect So, we would like to propose the following direction for research. Let’s quantify… Let’s study diverse… Radio networks vary in the following key ways: - modulation type, e.g. AM narrowband vs. OFDM spread spectrum - network size, both network density and number of nodes - data rates, ranging from low (WSN-class) to high (metro-area networks) 1/17/2019

Outline Introduction Implementing carrier sense Benefits of carrier sense Drawbacks of carrier sense Conclusion Now I’ll talk about the various methods of carrier sense. After that I’ll discuss the benefits and drawbacks of carrier sense, showing some experimental results, and then conclude. 1/17/2019

Introduction Carrier sense is a crucial building block for many radio networks Wireless sensor networks Wireless local area networks Performance depends on carrier sense Application layer MAC layer Physical layer Carrier sense In the past 10 years there has been a lot of research aimed at wireless network protocols in the application and MAC layers. But carrier sense, which resides in the physical layer, is a crucial building block for such networks. Two examples are: - wireless sensor networks - wireless local area networks Carrier sense functionality is one of the high-level functions of the physical layer, and since every packet’s transmission is governed by carrier sense, good network performance depends on implementing carrier sense well 1/17/2019

Experimental setup Experimental testbed Sensor network 802.11b/g LAN Nodes 60 3 Radio Chipcon CC1000 Atheros 5212 Data rate 38.4 Kbps 1 to 54 Mbps Modulation FM narrowband OFDM/DSSS MAC B-MAC (software) 802.11 (hardware) For this work, we’ve used two very different network testbeds. The first is a large-scale sensor network with 60 sensor nodes. The second is a small-scale LAN testbed with three nodes. The thing to note in this chart is the diversity between the two platforms we chose. Because the LAN cards implement the MAC in hardware, we didn’t perform every experiment on both testbeds. 1/17/2019

Sensor network testbed 60-node Mica2 sensor network Six radio hops in diameter Ethernet backchannel to log packet receptions 16,076 sq. ft. [As written on slide.] http://mistlab.csail.mit.edu 100 ft. 1/17/2019

Outline Introduction Implementing carrier sense Benefits of carrier sense Drawbacks of carrier sense Conclusion Now I’ll talk about the various methods of carrier sense. After that I’ll discuss the benefits and drawbacks of carrier sense, showing some experimental results, and then conclude. 1/17/2019

How carrier sense works: energy detection Instantaneous signal strength Squelch (“noise floor”) Energy detect clear Energy detect busy Signal strength (dBm) There are four common methods of carrier sense. The first method is called energy detection. The radio measures the instantaneous signal strength received at the antenna as a function of time. The instantaneous signal strength is averaged to give a quantity called squelch, shown in blue here. - You can think of the squelch as the “noise floor” of the network When the instantaneous signal strength is less than the squelch, energy detect says that the medium is “clear;” when it is greater than the squelch, energy detect says that the medium is “busy.” Time 1/17/2019

How carrier sense works: other mechanisms Preamble detection Decorrelation amplitude Unique to spread-spectrum radios AGC (automatic gain control) unlock True when AGC adjusts rapidly Preamble Packet Spreading code × Received data Transmit data There are three other mechanisms radios use to perform carrier sense. Each packet begins with a unique preamble. If the radio hears a packet preamble on the channel, then it can infer with high probability that the channel is busy. A mechanism unique to spread-spectrum radios is decorrelation amplitude, which measures the amount of correlation between the incoming signal and the spreading code. Another mechanism is automatic gain control unlock, which becomes true when the AGC adjusts rapidly. 1/17/2019

Outline Introduction Implementing carrier sense Benefits of carrier sense Drawbacks of carrier sense Conclusion Now I’ll show you some experimental results that demonstrate how carrier sense improves network performance. 1/17/2019

Aggregate load lowers link delivery rate So let’s look at link quality in our large-scale sensor network. In this experiment, all nodes send at the same time, and log packet receptions. Here is the link delivery distribution, across the entire network, for only those links greater than 60% delivery rate, which are the links most likely to be of use to a routing protocol. The key shows the offered load of each sensor in packets per second. We see that at 4 pps, there are 360 links that achieve a delivery rate of greater than 70%. ~360 links > 70% at 4 pps WSN experiment with all nodes sending, carrier sense on 1/17/2019

Carrier sense improves link delivery rates Now let’s look at the same link qualities, with carrier sense turned off. At 4 pps, only 80 links in the network achieve a link quality greater than 70%. So carrier sense is avoiding collisions under high load. Only 80 links in the network are > 70% without CS Carrier sense avoids collisions under high load 1/17/2019

Carrier sense improves throughput Now let’s look at the rate of packets received by all the nodes in our network, at an offered load of 1 pps per node. We look at links greater than 60%, which are most likely to be useful to routing layers. We see that carrier sense improves the net link-level throughput. Large-scale experiment with an offered load of 1 pps/node 1/17/2019

Outline Introduction Implementing carrier sense Benefits of carrier sense Drawbacks of carrier sense Conclusion Now let’s discuss some drawbacks of carrier sense. These flaws speak to why we think there is room for improving carrier sense. 1/17/2019

Sender-side decision; receiver-side collision Will any transmissions collide with mine? R S The fundamental problem with CS is that although carrier sense is sender-side decision, collisions happen at the receiver. So carrier sense is at best a sender-side heuristic at the sender for predicting what will happen to the packet when it gets to the receiver. Carrier sense is at best a heuristic for predicting transmissions’ success 1/17/2019

Exposed terminals fool carrier sense Missed transmission opportunity R S S΄ R΄ One way the fundamental problem with carrier sense can manifest is the exposed terminal problem. Suppose S has a packet to send to R, but S’ is already transmitting to R’. S will carrier sense busy, and defer its transmission, but R could have received the packet. This represents a missed transmission opportunity for S. Carrier sense indicates busy, yet the transmission would have succeeded (S, S’ are exposed terminals) 1/17/2019

Carrier sense misses transmit opportunities Now let’s quantify missed transmission opportunities. This figure plots link delivery rate as a function of two variables. The first is the instantaneous signal strength, measured at the sender just before each transmission. The second is the squelch, again measured just before each transmission. The sender uses the energy detect method of carrier sense. Below the dotted line, the instantaneous signal strength is less than the squelch. Carrier sense waits until the instantaneous signal strength is less than the squelch before transmitting. This figure shows that when carrier sense transmits, link delivery rates are quite high at ¼ pps per node. Large-scale experiment with CS energy detect, 0.25 pps per node 1/17/2019

Carrier sense misses transmit opportunities When we turn carrier sense off, nodes transmit even when the instantaneous signal strength is greater than the squelch, above the dotted line. A substantial fraction of the link delivery rates above the dotted line are high. These points represent opportunities carrier sense missed to transmit. If each node built up this graph based on probes, we speculate that they could take advantage of these missed transmission opportunities. Large-scale experiment with carrier sense off, 0.25 pps per node 1/17/2019

Capture fools carrier sense Missed transmission opportunity A R B Another way the fundamental problem with carrier sense can manifest is capture. A, B, and R are each within transmission range to each other. A starts to transmit to R over a low-quality link. B defers. However, B is closer to R than A, so if B were to transmit, R could receive B’s transmission. This could represent a missed transmission opportunity for B, because carrier sense would prevent B’s transmission. R captures B’s transmission despite A’s concurrent transmission 1/17/2019

Capture prevalent at low bit rates Collision In this experiment, 802.11 nodes A and B transmit simultaneously and without carrier sensing the channel. At high bit-rates, A and B’s packets collide, resulting in low delivery rates for both nodes. At lower bit-rates, however, B achieves a high delivery rate to R, so if B can determine that A’s transmissions are fruitless, B should disable carrier sense in these situations. Disclaimer: not always, in the interest of avoiding starvation. At some low 802.11 bit rates, node B should disable carrier sense 1/17/2019

Hidden terminals fool carrier sense Carrier sense is free! S R S’ A final way the fundamental problem can manifest itself is in hidden terminals. Carrier sense indicates free, yet both transmissions fail (S, S’ are hidden terminals) 1/17/2019

Conclusion and future research An experimental evaluation of the benefits and drawbacks of carrier sense Current carrier sense schemes are oblivious to the capture effect Future work To study effects of global interference in a spread spectrum system. To design and Implement a system to increase network capacity by modifying the carrier sense decision process. We have presented an experimental evaluation of the benefits and drawbacks of carrier sense. More experimental results are in the paper. In the future we plan to look at algorithms individual nodes can use to track correlations between signal strengths and packet reception rates. Another idea we plan to pursue is to use a congestion control algorithm and turn off or reduce the amount of carrier sense at the MAC layer. 1/17/2019