P-TDMA-SYS: A TDMA System over Commodity 802.11 Hardware for Mobile Ad-Hoc Networks

Abstract

In some scenarios such as catastrophes and military operations, there is a need for communication without an infrastructure. In urgent cases, in-time and reliable channel access must be ensured. However, the channel access delay in widely used CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) network increases unpredictably when the channel is sensed busy. Therefore, TDMA (Time Division Multiple Access) gets more and more attention in mobile ad-hoc network research. This paper presents a TDMA network system (named as P-TDMA-SYS) that is designed to support real-time applications in mobile ad-hoc networks (MANETs) and successfully works on commodity 802.11 hardware. The key feature of P-TDMA-SYS is its microsecond synchronization and adaptive medium access policy. P-TDMA-SYS's implementation contains two major modules. One is the P-KER-MAC, a TDMA-based MAC protocol, running over commodity 802.11 hardware. The other is P-TCP, a cross-layer TCP congestion control algorithm specifically for P-KER-MAC. The presented TDMA-based MAC over commodity 802.11 hardware has been tested with a detailed accounting of the hardware and operation system overheads. Furthermore, P-TDMA-SYS has been compared with 802.11 CSMA/CA-based network in the single-hop/multi-hop scenarios. The test results show that our implementation achieves reliable throughput and low delay/jitter for supporting real-time applications.

A Sense-based Registration Process for TDMA in IEEE 802.11 Network

“…Node can transmit its data in active time, and can only listen in passive time. While [8] implement TDMA in application layer, and create application wide clock synchronization between nodes while [4] implements TDMA in mobile ad hoc network. [3], [8] doesn't declare specific mechanism in node registration, while [4] explain a brief mechanism in node registration by using idle sensing for node advertisement.…”

“…After broadcasting a beacon frame, access point enters idle state to listen registration request from nodes. In [4], nodes will send request frame when it sense idle. In reality, if more than one nodes sense idles and transmit the request message at the same time, collision will occur.…”

“…Some research has been done to maximize the quality of IEEE802.11 [2] use hybrid coding technique to maximizing bandwidth efficiency while some other researcher proposes the use of TDMA to replace CSMA/CA in IEEE 802.11 [3][4][5]. TDMA can eliminate fairness problem in CSMA/CA, because each node has exclusive transmission slot and get the same opportunity to send its data.…”