1. Error analysis and low-cost error correction for the crowded wireless 2.4 GHz band Mark Grossman ’11, Jens Mache Lewis & Clark College
1. Introduction
Sensor networks
are an “Emerging Technology That Will Change the World“ [1]
are distributed systems of small networked sensors
allow for an instrumentation of the physical world at an unprecedented scale and density
enable new generations of applications and the "Internet of Things"
Wireless communication
most sensor networks use the standard (ZigBee, etc.)
operate in the 2.4 GHz ISM (industrial, scientific, and medical) band
are subject to interference from other devices such as Wi-Fi ( a/b/g/n), Bluetooth devices, video devices, cordless phones, car alarms, child monitors and microwave ovens
2. Our experiment
We developed code to monitor radio traffic between compliant devices. We constructed an experiment in which a standardized packet was broadcast from one device at a constant rate. A second device (operating in promiscuous mode) received these packets and determined if the packet contained errors. If the packet was erroneous, further analysis was performed to classify the error pattern into one of several categories.
c8 d ff ff f #A..S O..
f a e af
ce 71 7a 8a qz.
ERR_LENGTH
*****
c8 6b ff ff f A.k O..
f a e b
ERR_DSN_OR_FCS
c8 de ff ff f A O..
f a e f
ERR_SINGLE (24)
bb ff ff f A O..
f a e cc
ERR_HALFBYTE (4)
c8 2a ff ff f A.* O..
f a e
ERR_EXTENDED (10)
c8 e ff ff f A O..
f a
ERR_MULTI (28)
c A.swwwwwwwwwwww
wwwwwwwwwwww723Y D
ERR_MULTI (96)
c ff ff f A O..
f a e ab a
: Length
c : Frame Control Field
: Sequence Number
: Destination PAN
ff ff : Destination Address
f : Source Address
7f a
e : MAC Payload
9a ab : Frame Control Sequence
4. Analysis & Conclusions
Our results showed that the proportion of correctable errors was significant (~35%) and, furthermore, perhaps fewer than 5% of the remaining errors could be corrected with an existing, expensive error correction scheme (FEC). This data enabled our colleague [2], who is implementing the error correction algorithm, to optimize his methods for the greatest amount of error correction with the least amount of overhead.
This experiment could be repeated under different conditions to determine the error profiles of various environments.
Error control
The standard method to deal with transmission errors is retransmission, also known as automatic repeat request (ARQ).
Sensor networks have to minimize communication in order to improve energy consumption and thus lifetime (batteries cannot be replaced).
Our long-term goal is to investigate (and develop) an alternative to ARQ: correcting bit errors using Cyclic Redundancy Checks (CRCs) which are already present and used for error detection.
e.g. for polynomial x3 + x + 1, CRC( ) = 5.
Thus, error correction table T = { correct, 7, 6, 4, 5, 1, 3, 2 }.
If transmits as , CRC = 3.
T[3] = 4 correctly indicates that bit 4 was corrupted.
Acknowledgements & References
This work is supported by an NSF REU and grant CNS
[1] MIT Technology Review, “10 Emerging Technologies That Will Change the World”
[2] Travis Mandel & Jens Mache, Investigating CRC Polynomials that Correct Burst Errors, Proceedings of the International Conference on Wireless Networks, 2009