利用无线网络并发传输优化广播确认策略

需积分: 0 96 浏览量更新于2024-10-27 收藏 5.41MB PDF 举报

"SMACK - A SMart ACKnowledgment Scheme for Broadcast Messages in Wireless Networks" 本文是发表在SIGCOMM '09会议上的一篇研究论文，主要探讨了一种名为SMACK（SMart ACKnowledgment）的新颖广播确认策略，该策略应用于无线网络中的多播消息传输。SMACK的目标是利用无线网络中的并发传输特性来显著降低可靠多播的开销，并提高诸如任意播、广播、领导者选举等常见组通信原语的效率。传统的无线网络协议设计通常将干扰和同时传输视为需要避免的问题，这往往导致无线网络在功能上受到有线网络协议的限制。然而，作者们提出，无线协议可以利用并发传输的优势，以大幅度降低成本，尤其是在直接可达节点之间的多点通信场景中。 SMACK方案的核心思想是通过智能的确认机制来优化广播过程。在无线环境中，当一个节点广播消息时，其他接收节点可以同时发送确认信息，而不是依次单独响应。这种并行的确认过程可以显著减少网络中的通信延迟和重传次数，从而提高整个系统的效率和可靠性。论文中，作者们深入分析了并发传输如何与现有的无线网络协议协同工作，以及如何通过适当的接口设计来支持这种新的通信模式。他们还可能讨论了SMACK在不同网络环境和条件下的性能，包括网络负载、信道条件和节点密度等因素的影响。此外，为了证明SMACK的有效性，文章可能包含了一系列的仿真和实地实验结果，这些结果展示了SMACK相比于传统方法在减少传输成本和提高通信速度方面的显著优势。通过这种方式，SMACK不仅为无线网络的可靠多播提供了一种创新解决方案，而且为未来无线网络协议的设计提供了新的思考方向。 SMACK方案揭示了无线网络中并发传输的潜力，挑战了传统观念，为无线网络的高效组播通信开辟了新的可能性。这一研究对于理解无线网络的内在特性，以及设计更适应无线环境的协议具有重要意义。

(a) Subcarrier assignment in a network

(b) Non-contiguous OFDM transmission

Figure 1: Schematic illustration of ACKs using OFDM

the time to gather reliable acknowledgments for broadcasts. We

use the physical layer to combine the responses from the different

nodes. Upon receiving a successful broadcast message from the AP

the clients use their pre-deﬁned subcarriers to transmit a



as an

ACK.

To summarize, the protocol has the following steps:

1. When nodes join the network, the AP assigns each node a

unique “membership id”, which is a small integer.

2. An AP sends the broadcast message using conventional PHY

speciﬁcations for 802.11a/g.

3. On receiving the broadcast message all clients decode the

message (if possible).

4. If a client successfully decodes the message, the client then

uses the single orthogonal subcarrier speciﬁed by the mem-

bership identiﬁer to indicate it has recei ved and decoded the

message

5. The AP receives the composite time domain signal of all

OFDM subcarriers and performs an FFT to obtain the fre-

quency domain representation of the signal. After perform-

ing demodulation the individual acknowledgments can be re-

cov ered. A one in the n

bit position can be mapped as an

ACK from one of the N (number of subcarriers) clients.

Due to the conversion between the time domain and frequency

domain, relatively tight timing synchronization is needed for the

composite additive signal to be decoded at the AP – in other words,

all the responding stations must transmit at about the same time;

ho wever, that time synchronization is provided by the broadcast

message itself as explained in §5.2.

To understand how much more efﬁcient it is to use physical layer

signaling, consider the costs of transmitting a message using the

802.11g PHY that is the basis for our extension. A normal mes-

sage requires a 20μs preamble to be transmitted and then, at best

assuming the 54Mbps modulation rate, each 48 × 6 bits takes one

OFDM symbol time (4μs) to transmit. Thus, a 64 byte message,

which can’t actually even contain the Ethernet addresses in a stan-

dard 802.11g packet would take at least 20 + 4 × 3 or 32μseconds.

After a 16μs “SIFS” period for a 20MHz channel [13], clients

would normally respond using a similar message format. Thus, a

single response to a standard 802.11g packet would take another

≈ (32 + 16) = 48μs.

By comparison, using physical layer signaling 53 clients can pro-

vide a single bit of information within two OFDM symbol periods,

or a total of 8μs (as detailed in §5.2), or one-sixth the time for a

single station to respond using standard messages. This means that

using the proposed protocol, the time needed for a single station

will be reduced by about an order of magnitude; when the num-

ber of potential respondents increases, that time is reduced by two

orders of magnitude.

3.2 Extending Link Layer Broadcasts

As Figure 1(a) makes clear, we have mainly worried about pro-

viding a reliable broadcast for a “single hop” wireless network.

We’d still like to have reliable broadcasts in multi-hop wireless net-

works. Such protocols usually use link layer ﬂooding which often

requires re-broadcast and leads to a common phenomenon called a

broadcast storm [16]. This problem is especially elevated by the

lack of ACKs – without an acknowledgment, it’s unclear which

nodes have received messages. ACK-based broadcast schemes that

degenerate the broadcast mechanism into multiple unicast commu-

nication increases network overhead and latency. Given a reliable

ACK as a basic operation, we can obviously improve on scalable

broadcast algorithms [25, 22]. More importantly, we can use the

time of arrival information available at the physical layer to further

improve the performance of reliable network broadcasts. We show

how to do this in §10.

4. ROBUSTNESS OF SMACK

4.1 Against Varying Signal Power

The reliable broadcast acknowledgment scheme described in §3

typically caters to a network of directly reachable nodes. The sig-

nal power from these clients may vary widely. Setting a single

threshold for all these clients would be difﬁcult if the received sig-

nal power of each of the subcarriers at the AP vary in a broad range.

Hence, we propose to adjust the transmission power of tone trans-

mitters/clients such that the received power of the subcarriers from

different clients at the AP are comparable and within tolerable lim-

its, ensuring that the weaker signal does not get lost due to the high

po wer of the stronger signal. The dynamic transmit power adjust-

ment of the clients can be decided based on existing channel as-

sessment techniques as done in CDMA [1]. The calibration of the

transmit power control mechanism based on the channel condition

is kept as future work. In this way, we can set a single threshold

to detect all the clients in the network, as the received power of the

individual subcarriers become similar after adaptive power control.

To detect the farthest client, we need to detect its signal. We ar-

gue that the weakest client’ s signal at the AP is not only detectable,

but also decodable if a packet is transmitted. Otherwise normal

802.11 communication with that client will not have been possible.

In case our proposed protocol fails to detect acknowledgment from

the weakest client, the fallback mechanism to retransmit to that par-

ticular client will ensure reliable delivery of the broadcast message

to the client.

剩余11页未读，继续阅读

mybest28

粉丝: 1
资源: 1

利用无线网络并发传输优化广播确认策略

smack-im-4.3.4-sources_www.im.43_im_

smack_3_3_0 源码

javasmack源码-udesk_mchat_android:udesk_mchat_android

openfire-4.1.4-1.x86_64.rpm

ji-shi-tong-xin-xi-tong.zip_xinshijitv com_xinshijitv网站

asmack-android-7_asmack的android7.0的jar包

javasmack源码-ops_skill_summary:您必须熟练掌握的devops工具或脚本

smack_src_3_1_0

smack_src_3_2_2

smack_4_1_4

最新资源