======= Review 1 ======= *** Originality/Novelty:: Rate the novelty of the paper. Somewhat novel (2) *** Quality of presentation:: Layout, English, ... Good (3) *** Summary:: Please describe the main contributions and the key weakness of the paper. This paper deals with the weakness of 802.11p MAC protocol which performs poorly on delivery ratio for dense vehicular network environments. Due to the constrain of the Contention Windows (CW) specified in 802.11p, the one-hop broadcast (sharing beacon messages among neighbors) at the level of the MAC layer performs poorly. The authors made a good point that the performance could be improved if the CW were adjusted based on the current network density (i.e., number of neighboring nodes). In the week side of this paper, the authors' findings seem to be somewhat in the initial stage of the research, and more matured results can be obtained through further research. *** Your recommendation:: accept/reject Accept if room (3) *** Comments:: Please provide detailed descriptions that support your scores and your suggestion to the authors if any. It would be nice if you can provide precise differences between unicasting and broadcasting for adjusting CW: you indicated briefly that retransmission could be one of the factors in III, but then you stated on the end of IV that the linear dependence found in the case of unicast messages also holds for broadcast traffic. Why didn't you show some result graphs for the unicasting case. It would be nice if you showed a pictorial diagram (i.e., intersection road) for simulation environments for readers. You indicated that the beaconing rate was 10Hz. This seems unrealistic for a high dense network in network perspectives. I believe it will be much easier and more feasible in design and implementation perfectives if an adaptive method is applied to the network layer rather than MAC layer because the beaconing (the major vehicular application traffics as you cited) is a part of network layer operations in WAVE. ======= Review 2 ======= *** Originality/Novelty:: Rate the novelty of the paper. Somewhat novel (2) *** Quality of presentation:: Layout, English, ... Good (3) *** Summary:: Please describe the main contributions and the key weakness of the paper. The main contribution of the paper is to analyze the impact of the min CW on the MAC layer performance in a broadcast VANET environment. The paper motivates the need to adaptively adjust the min CW based on node density and sheds light on balancing min CW with beacon reception probability. Thus, an optimal min CW is desirable to meet these two requirements of low contention and high reception probability. *** Your recommendation:: accept/reject Accept if room (3) *** Comments:: Please provide detailed descriptions that support your scores and your suggestion to the authors if any. The weakness of the paper is that the proposed adaptive contention window scheme is not fully developed. How CW can be adjusted adaptively is still left at the crust of the issue... ======= Review 3 ======= *** Originality/Novelty:: Rate the novelty of the paper. Somewhat novel (2) *** Quality of presentation:: Layout, English, ... Good (3) *** Summary:: Please describe the main contributions and the key weakness of the paper. This paper looks at using an adaptive contention window to improve MAC (802.11) performance for broadcast messages. This adaptive window has been considered before as noted by the authors but becomes more relevant for safety application VANET which require a more reliable broadcast. The paper considers beacon broadcast for analysis but does not include performance evaluation of safety messages *** Your recommendation:: accept/reject Accept if room (3) *** Comments:: Please provide detailed descriptions that support your scores and your suggestion to the authors if any. This paper takes another look at using dynamic contention windows in MAC to improve performance of broadcast packets which would be useful for safety applications. However, there are no performance analysis specifically for safety packet broadcasting. *********************************************************************************************************************************************************************************************** Comments for previous version submitted at GlobeCom 2010 *********************************************************************************************************************************************************************************************** ======= TPC review 1 ======= *** 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. Marginal work and simple contribution. Some Flaws (2) *** Novelty and originality: Rate the novelty and originality of the ideas or results presented in the paper. Minor variations on a well investigated subject. (2) *** Quality of presentation: Rate the paper organization, the clearness of text and figures, the completeness and accuracy of references. Well written. (4) *** Strong Aspects: Comments to the author: what are the strong aspects of the paper This paper deal with a very important functionality on VANET environment which is the support of broadcast communications using IEEE 802.11 for safety applications. More precisely, the authors started by the analysis of impact of the minimum contention window on the MAX layer performance. There after, they propose a solution for adapting the contention window to network density in order to improve the reception probability of the broadcast messages. *** Weak Aspects: Comments to the author: what are the weak aspects of the paper? The paper contribution contribution is limited. The authors focused mainly on the evaluation of the contention window impact on the packet reception probability. However, the authors proposed a small contribution highly inspired from the related works but unfortunately the evaluation of that proposal is very limited. I suggest to the authors to deeply investigate the evaluation section of the proposal. Finally, I suggest to the authors to compare their proposal to other related works. *** Recommended Changes: Recommended changes. Please indicate any changes that should be made to the paper if accepted. I suggest to the authors to deeply investigate the evaluation section of the proposal. Finally, I suggest to the authors to compare their proposal to other related works. ======= TPC review 2 ======= *** Relevance and timeliness: Rate the importance and timeliness of the topic addressed in the paper within its area of research. Acceptable (3) *** 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. Marginal work and simple contribution. Some Flaws (2) *** Novelty and originality: Rate the novelty and originality of the ideas or results presented in the paper. Minor variations on a well investigated subject. (2) *** Quality of presentation: Rate the paper organization, the clearness of text and figures, the completeness and accuracy of references. Well written. (4) *** Strong Aspects: Comments to the author: what are the strong aspects of the paper This paper discusses the impact of the size of the contention window, which is used in IEEE 802.11p backoff algorithm, on the delivery of broadcast frames in a VANET environment. The performance evaluations were conducted using simulations and several scenarios were assessed. The obtained results are discussed and some general conclusions are derived. Finally, the authors propose to adjust dynamically the size of the contention window depending on he load of the system (node density and traffic load). The paper is well organized and quite easy to follow. *** Weak Aspects: Comments to the author: what are the weak aspects of the paper? Despite the interest of hellping to understand the impact of the contention window on the delivery ratio of broadcast messages in VANET, there are no novel results, which were not expected. Broadcasting in MANET has been extensively investigated in the literature. See for example the following references: Y.-C. Tseng, S.-Y. Ni, Y.-S. Chen, and J.-P. Sheu.The broadcast storm problem in a mobile ad hoc network. Wirel. Netw., 8(2/3):153–167, 2002. B. Williams and T. Camp. Comparison of broadcasting techniques for mobile ad hoc networks. In MobiHoc ’02: Proceedings of the 3rd ACM international symposium on Mobile ad hoc networking & computing, pages 194–205, New York, NY, USA, 2002. ACM Press. Several proposals were made to address the issue of broadcasting in MANET. For example, take a look to the following reference: W. Peng and X. Lu. Ahbp: An efficient broadcast protocol for mobile ad hoc networks. J. Comput. Sci. Technol., 16(2):153–167, 2001. We understand that the ealuated environment is not the classical MANET, but a special case of MANET (i.e. VANET), with it own specificities. However, the obtained results are somewhat expected. The general behavior is not surprising and is well known. The main interest of the paper would be the proposal of a dynamic contention window adjustment for broadcasting (although the idea is a very classical one). Unfortunately, the idea is not well investigated. Encourage the authors to pursue their efforts in that direction. *** Recommended Changes: Recommended changes. Please indicate any changes that should be made to the paper if accepted. The authors should take a look to the references that we provided in the previous section. They should also investigate more deeply different approaches which aims at dynamically adjust the contention windows. Several proposals were made for unicast. We believe that some approaches, like probabilistic estimations of the system load, could be adapted with little modifications) to broadcast. ======= TPC review 3 ======= *** 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) *** Strong Aspects: Comments to the author: what are the strong aspects of the paper Well written paper. I like the proposal idea. The proposal seems to be applicable. *** Weak Aspects: Comments to the author: what are the weak aspects of the paper? real experimentation would give a real valkue on the results *** Recommended Changes: Recommended changes. Please indicate any changes that should be made to the paper if accepted. It would be good if the authors can say some words on the simulation environments: methods, tools etc ... ======= TPC review 4 ======= *** 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. Valid work but limited contribution (3) *** 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. Readable, but revision is needed in some parts. (3) *** Strong Aspects: Comments to the author: what are the strong aspects of the paper The paper deals with very important aspect of VANET applications: namely the safety messages broadcast which are characterized as being short but more frequent. The article presents excellent understanding for the problem involved and the complex relation between the contention window and number of contending vehicles. Valuable results are given in Sections II – IV *** Weak Aspects: Comments to the author: what are the weak aspects of the paper? Considering Section V, although the use of RTS/CTS packets improves performance it does not completely solve the hidden terminal problem and it introduces other problems for high density nodes. It is not clear in this section how to determine the number of hidden nodes and no information was given about the radio channel model and the causes of the shadowing in an intersection of roads. The curves shown in Fig.4 are similar to those shown when hidden nodes are not considered which suggests the number of hidden notes is too small to have any major effects on the system performance. I was looking for thorough investigation of this problem and hope the authors will get back to this subject and do more research. The effects of hidden nodes on the contention window and system performance need further work. The results presented in Section VI are not up to expectation considering the title of the section. Here one expects to find methods for optimizing the contention window according to node density. However, the results offer a simple solution to such a difficult subject and I hope the authors continue researching the subject. *** Recommended Changes: Recommended changes. Please indicate any changes that should be made to the paper if accepted. I recommend re-writing sections V and VI according to my comments in ‘Weak Aspects’.