A light-weight wireless handshake

Kate Lin, Yung-Jen Chuang, Dina Katabi
Appears in: 
CCR April 2012

In many wireless systems, it is desirable to precede a data transmission with a handshake between the sender and the receiver. For example, RTS-CTS is a handshake that prevents collisions due to hidden terminals. Past work, however, has shown that the overhead of such handshake is too high for practical deployments. We present a new approach to wireless handshake that is almost overhead free. The key idea underlying the design is to separate a packet's PLCP header and MAC header from its body and have the sender and receiver first exchange the data and ACK headers, then exchange the bodies of the data and ACK packets without additional headers. The header exchange provides a natural handshake at almost no extra cost. We empirically evaluate the feasibility of such lightweight handshake and some of its applications. Our testbed evaluation shows that header-payload separation does not hamper packet decodabilty. It also shows that a light handshake enables hidden terminals, i.e., nodes that interfere with each other without RTS/CTS, to experience less than 4% of collisions. Furthermore, it improves the accuracy of bit rate selection in bursty and mobile environments producing a throughput gain of about 2x.

Public Review By: 
Bhaskaran Raman

Wireless systems often face this trade-off: information exchange is necessary for performance enhancement, but such information exchange itself has a high cost. A classical example is that of RTS-CTS packet exchange in WiFi systems, which can help in various aspects such as hidden node detection, effective rate adaptation, channel estimation in MIMO systems, etc. However, prior work has shown the cost of RTS-CTS to be quite high. This paper proposes an interesting idea: separate the data header from the data, and ACK header from the ACK, and use the header exchange as an inexpensive proxy for RTS-CTS exchange. While an attractive idea on the surface, separating the header from the actual body raises questions about whether the packet's decodability will be affected. Through a software-radio based prototype, this paper demonstrates that the header separation has insignificant effect on packet reception ratio. Further, the paper experimentally shows that severe hidden terminal scenarios can be effectively avoided, with the header-exchange. Experimental results are also presented to indicate that rate adaptation can function about twice as effectively with such header-exchange, than without. The reviews agreed that the idea of separating the header from the body, is novel for wireless systems. Worthy of further exploration are issues such as: (1) Would the SIFS interval need to be lengthened in a real implementation, due to the extra computations necessary in the dataACK header exchange? (2) What benefits, if any, would accrue in practice for other systems requiring feedback, such as MIMO beamforming systems, if the data-ACK header exchange were to be used for this purpose?