======= Review 1 ======= *** Strong aspects: Comments to the author: what are the strong aspects of the paper The paper is written well. It considers an interesting problem and summarizes the existing solutions to the problem and clearly explains possible implementations issues. Overall, the paper looks like a nice short survey with some interesting simulation results. *** Weak aspects: Comments to the author: what are the weak aspects of the paper? The paper does not add much to the existing solutions. For example, the authors slightly modify some of the existing solutions in Sections IV, by introducing parameters \lambda and \alpha, whose values are analyses through simulation. It seems that the optimum value of these parameters defer from one scenario to another so a fixed value cannot be used everywhere. *** Recommended changes: Recommended changes. Please indicate any changes that should be made to the paper if accepted. In Section IV.C, it says "CW should be decremented when the channel is idle for an important amount of time". The statement "an important amount of time" is vague. It is good to know whether, based on the simulation results, the optimum values of \lambda and \alpha significantly changes over different scenarios It will add to the contribution of the paper if the authors come up with their own contention window adaptation and compare it with the existing solutions. *** Relevance and timeliness: Rate the importance and timeliness of the topic addressed in the paper within its area of research. Good (4) *** Technical content and scientific rigour: Rate the technical content of the paper (e.g.: completeness of the analysis or simulation study, thoroughness of the treatise, accuracy of the models, etc.), its soundness and scientific rigour. Solid work of notable importance. (4) *** Novelty and originality: Rate the novelty and originality of the ideas or results presented in the paper. Some interesting ideas and results on a subject well investigated. (3) *** Quality of presentation: Rate the paper organization, the clearness of text and figures, the completeness and accuracy of references. Well written. (4) ======= Review 2 ======= *** Strong aspects: Comments to the author: what are the strong aspects of the paper Good focus, organization, and presentation. Solid technical work. The topic of improving MAC performance for vehicular communication is a timely topic and a difficult problem. *** Weak aspects: Comments to the author: what are the weak aspects of the paper? The conclusion made in this paper, ie that adaptive CW would improve performance is kind of expected, especially given that this has already been shown previously for other 802.11 contexts. The paper did not articulate why we could not readily expect the same result for vehicular context. The outcome is therefore more like an evidence of something that is intuitively expected. *** Recommended changes: Recommended changes. Please indicate any changes that should be made to the paper if accepted. The major comment is to lift the novelty of the paper. There seems to be some new ideas, e.g. proposal of methods to use vehicle data (e.g. number of stops etc.) to estimate density, but the authors did not explain clearly which of the 5 methods were taken from existing literature and which are totally new only proposed in this paper for the first time. Assuming that the ideas in Section IV D and E are new, more detailed analysis of these techniques would enhance the novelty aspect of the paper. For algorithm D (stop time based), the authors mentioned only positive aspects, e.g. no additional hardware is needed and stop time can be calculated from speedometer data, but it would be good to also highlight if there are any issues that need to be addressed. For example, use of odometer data in the MAC layer would be kind of cross-layer design and may introduce delay in making the odometer readings available at the MAC layer. *** Relevance and timeliness: Rate the importance and timeliness of the topic addressed in the paper within its area of research. Excellent (5) *** Technical content and scientific rigour: Rate the technical content of the paper (e.g.: completeness of the analysis or simulation study, thoroughness of the treatise, accuracy of the models, etc.), its soundness and scientific rigour. Solid work of notable importance. (4) *** Novelty and originality: Rate the novelty and originality of the ideas or results presented in the paper. Some interesting ideas and results on a subject well investigated. (3) *** Quality of presentation: Rate the paper organization, the clearness of text and figures, the completeness and accuracy of references. Excellent. (5) *********************************************************************************************************************************************************************************************** Comments for previous version submitted at ICCCN 2011 *********************************************************************************************************************************************************************************************** ======= Review 1 ======= *** Contributions: What are the major issues addressed in the paper? Do you consider them important? Comment on the degree of novelty, creativity, impact, and technical depth in the paper. Estimation of local density for contention window adaption in Vehicular Network. As part of this paper, the authors have identified five different methods, and compared them via simulations. *** Strengths: What are the major reasons to accept the paper? [Be brief.] Five different methods, which are based on a) beacons b) collided packets c) idle time measurements d) stop times e) vehicle speed, have been studied. *** Weaknesses: What are the most important reasons NOT to accept the paper? [Be brief.] 1. These methods have been taken from the prior works (except c). There is no clear motivation given reason behind adopting these algorithms 2. Two metrics have been used for comparison, however no motivation is provided for selecting these metrics. 3. Other possible metrics are delay, and number of collisions. Too large windows result in larger delays, plots related to the delays will be important. 4. No justification provided for the values provided in Table 1, which are the values chosen for the tuning parameters used in five estimation methods. 5. Only one trend, which is however well proven already, is proved. That, using adaptive methods for updating contention window shows superior performance over the default algo 802.11p 6. In the context of wireless mesh/ad hoc networks, these type of algos are well studied already. *** Detailed Comments: Please provide detailed comments that will be helpful to the TPC for assessing the paper. Also provide feedback to the authors. 1. These methods have been taken from the prior works (except c). There is no clear motivation given reason behind adopting these algorithms 2. Two metrics have been used for comparison, however no motivation is provided for selecting these metrics. 3. Other possible metrics are delay, and number of collisions. Too large windows result in larger delays, plots related to the delays will be important. 4. No justification provided for the values provided in Table 1, which are the values chosen for the tuning parameters used in five estimation methods. 5. Only one trend, which is however well proven already, is proved. That, using adaptive methods for updating contention window shows superior performance over the default algo 802.11p 6. In the context of wireless mesh/ad hoc networks, these type of algos are well studied already. *** Recommendation: Your overall rating. Likely Reject (top 50% but not in top 35%, needs more work) (2) ======= Review 2 ======= *** Contributions: What are the major issues addressed in the paper? Do you consider them important? Comment on the degree of novelty, creativity, impact, and technical depth in the paper. This paper studies contention window adaptation in vehicular networks by estimating the local density of the vehicles. The problem they studies is important as the minimum contention window size profoundly affects the MAC protocol performance in VANET. The authors propose five different methods for adapting contention window size in a vehicular environment and compare them through simulations. The technical depth of this paper is appropriate for ICCCN. The impact of the paper could not be fully appreciated unless the authors address some concerns (see weakness section). *** Strengths: What are the major reasons to accept the paper? [Be brief.] The problem they study is important to the development of Vehicular Ad-Hoc Networks. Their result could create an interesting discussion on this topic. Besides, simulation work is solid in their paper and could give some insight to the readers. *** Weaknesses: What are the most important reasons NOT to accept the paper? [Be brief.] The significance of the work is unclear. The technial approaches can also be improved. *** Detailed Comments: Please provide detailed comments that will be helpful to the TPC for assessing the paper. Also provide feedback to the authors. First, the model and the performance of 802.11 LAN have been extensively studied in the past decades. Even though the authors propose five different methods, the significance of each method has not been fully addressed. For example, the collided packets and the idle time counting method are very similar. Besides simulations, the reviewer encourages the authors to study their methods in a theoretical perspective, which could increase the impact of the paper. Secondly, the performance metrics used in their paper is very limited. For example, they do not study the impact of each method they propose on the average waiting time, which is critical in VANET. Thirdly, the proposed methods are heuristic, and it is unclear why only these methods should be tried, and how they compare to the optimal solution. Minor comments: the proposed methods are not for density estimation, but for contention window size adaptation. So the authors may revise their title, abstract etc to reflect this. *** Recommendation: Your overall rating. Likely Reject (top 50% but not in top 35%, needs more work) (2) ======= Review 3 ======= *** Contributions: What are the major issues addressed in the paper? Do you consider them important? Comment on the degree of novelty, creativity, impact, and technical depth in the paper. This is a very interesting paper that presents and compares five different mechanisms for estimating traffic density in a V2V communication environment. *** Strengths: What are the major reasons to accept the paper? [Be brief.] The paper addresses an important problem in an area of growing interest (V2V communication), and provides a good comparison of several novel heuristic solutions. *** Weaknesses: What are the most important reasons NOT to accept the paper? [Be brief.] The comparison is done in simulation only, which reduces the accuracy of the results compared a real deployment. However, since V2V systems are not yet widely deployed, it is hard to avoid this shortcoming in evaluating V2V protocols. *** Detailed Comments: Please provide detailed comments that will be helpful to the TPC for assessing the paper. Also provide feedback to the authors. The paper presents mechanisms for estimating traffic density in a V2V environment. These mechanisms are used to control the contention window size used for broadcast messages. Since broadcast messages are unacknowledged, no adaptive mechanism is used to change the contention window size in response to transmission failures. Instead, the authors propose five different heuristics that attempt to estimate traffic density and hence compute the right contention window for each broadcast in a model-based "open-loop" fashion. A simulation-based evaluation is used to compare these heuristics to each other as well as to regular 802.11 performance. It is shown that some heuristics achieve a remarkable improvement over the current protocol. *** Recommendation: Your overall rating. Definite accept (top 15%, excellent paper) (5) *********************************************************************************************************************************************************************************************** Comments for previous version submitted at Networking 2011 *********************************************************************************************************************************************************************************************** ======= Review 1 ======= *** Contributions: What are the major issues addressed in the paper? Do you consider them important? Comment on the degree of novelty, creativity, impact, and technical depth in the paper. The paper presents a methodology to evaluate node density in a vehicular network. The proposed solution is reasonable, paper is generally well written. *** Strengths: What are the major reasons to accept the paper? [Be brief.] The topic is important, the proposed solution is reasonable, the paper is generally well written. *** Weaknesses: What are the most important reasons NOT to accept the paper? [Be brief.] How does the proposed solution perform when generate traffic is bursty? Performance assessment should be improved. *** Detailed Comments: Please provide detailed comments that will be helpful to the TPC for assessing the paper. Also provide feedback to the authors. The paper presents a methodology to evaluate node density in a vehicular network. The proposed solution is reasonable, paper is generally well written. The topic is important, the proposed solution is reasonable, the paper is generally well written. In the paper it is not discussed with enough details how the proposed solution perform when generate traffic is bursty, that is, traffic sources spend portions of the time transmitting and the remaining in a idle state. Furthermore, in general the performance evaluation should be improved by running more simulations in several scenarios, with heterogeneous traffic types. *** Familiarity: Rate your familiarity with the topic of the paper. Familiar (3) *** Recommendation: Your overall rating. Likely Reject (top 50% but not in top 30%, needs more work) (2) ======= Review 2 ======= *** Contributions: What are the major issues addressed in the paper? Do you consider them important? Comment on the degree of novelty, creativity, impact, and technical depth in the paper. The major issue addressed in this paper is that some system parameters (in particular contention window) need to be dynamically adapted to suit the local node density. This is a known issue and has be tackled to some extent(e.g. see reference "Dynamic Adaptation of Joint Transmission Power and Contention Window in VANET" VTC 2009, by Danda B. Rawat et al.) There are reasonable amount of prior work in local density estimation itself. The major contribution is the simulation study of several ways of local density estimation and its impact to reception reliability. *** Strengths: What are the major reasons to accept the paper? [Be brief.] This is a well written an well structured paper. The simulation results show some improvements. The local density estimation methods are simple and implementable. *** Weaknesses: What are the most important reasons NOT to accept the paper? [Be brief.] Most of the solutions presented in the paper seem to be common sense to the reviewer. *** Detailed Comments: Please provide detailed comments that will be helpful to the TPC for assessing the paper. Also provide feedback to the authors. The statement Section 3, "If a station contending for transmission senses the channel busy, it has to wait for a number of idle slot times ...", is an incorect *** Familiarity: Rate your familiarity with the topic of the paper. Expert (4) *** Recommendation: Your overall rating. Likely Reject (top 50% but not in top 30%, needs more work) (2) ======= Review 3 ======= *** Contributions: What are the major issues addressed in the paper? Do you consider them important? Comment on the degree of novelty, creativity, impact, and technical depth in the paper. •Improve the performance of the IEEE.802.11 protocol by adjusting the minimum contention window depending on the local node density in vehicular to vehicular communication •Present and simulates five possibilities for adapting contention window •Yes it is important, this parameter is neglected by the ETSI Technical Committee and the minimum contention window has an influence on the MAC Layer in vehicular environment •Degree of novelty and creativity are middle, because the proposed solutions are based on some known studies. •Authors touch in depth technical detail to explain the problem *** Strengths: What are the major reasons to accept the paper? [Be brief.] Paper is well written and easy to unterstand. the paper looks at IEEE 802.11p based vehicular networks, which, is a very relevant and current topic. Simulation topic is presented to support the presented idea. *** Weaknesses: What are the most important reasons NOT to accept the paper? [Be brief.] The authors use simulation with JIST/SWANS which does not have valid Phy/Mac models. No sufficient details are provided about the 802.11p implementation used by the authors for the simulation study. Hence the accuracy of the results (esp the methods used to estimate node density locally) is questionable. The idea central to the paper builds up directly on related work exple [7] and is a minor extension of previous work. *** Detailed Comments: Please provide detailed comments that will be helpful to the TPC for assessing the paper. Also provide feedback to the authors. See section on Weaknesses: JIST/SWANS does not have valid Phy/Mac models. The idea central to the paper builds up directly on related work exple [7] and is a minor extension of previous work. *** Familiarity: Rate your familiarity with the topic of the paper. Some knowledge (2) *** Recommendation: Your overall rating. Likely Reject (top 50% but not in top 30%, needs more work) (2)