An Anti-Collision Algorithm for Active RFID

Posted on March 27, 2008. Filed under: Uncategorized |

Authors –  Nan Li, Xiaohui Duan, Yakun Wu, Shi Hua, Bingli Jiao

Year September, 2006

Published in Wireless Communications, Networking and Mobile Computing, 2006. WiCOM 2006.International Conference.

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In this paper the authors have discussed various existing anti-collision algorithms like ALOHA, CSMA/CA with RTS/CTS and finally they proposed their own algorithm DCMA (dual channel multiple access) which over comes short comings of the existing anti-collision algorithms.  According to power and modulation modes, there are two types of the RFID system: the active RFID system and the passive RFID system.  The Active  RFID systems are suitable for longer distance applications ranging up to 30 meters. 

For an active RFID system using CSMA/CA as anti-collision algorithm it must uses some physical monitoring functions inorder to avoid collision.  The use of CSMA/CA with RTS/CTS allow the virtual monitoring functions and suitable for long packet size transmission where as CSMA/CA for short packet size only.

In ALOHA there are no monitoring functions at all and is suitable for short packet size transmission only.  As the packet size increases the probability of collision and hence the channel rate decreases which inturn result in high power consumption.  Also in CSMA/CA that uses RTS/CTS the clients are in receiving modes even during their back-off periods and thus increases power utilization and also when some client added to the system of which RTS collide with the data already in possession then this would result in collision and thus increases power consumption.  Inorder to overcome these shortcomings they designed DCMA which not only avoids physical monitoring functions but also fit for longer data transmission with high channel rate and minimum power consumption. 

In this design both the client and host uses two channel modes namely control mode (1) and data mode (2).  When some client wants to send data to host he would first switch its mode from sleep mode to control mode and sends RTS, once this control information is received by the host he sends back CTS to client and switches its channel mode to 2 i.e. data mode.  Once the CTS signal received by the client he starts sending packet of information and acknowledge ACK signal to host by switching its channel mode to 2.  Once completed the ACK signal is sent back to the client by the host which upon receipt switch back its mode to sleep mode.  There is no chance of collision even if some client wants to send some control information while being the host is engaged in receiving some data, since because of the use of separate channels.


Channel Rate Utilization:

The rate is decreasing with the increase in number of clients. Because more clients result in more collisions so more invalid occupancy of the channel, results in lower channel rate. Among the three protocols DCMA have higher channel rate utilization than CSMA/CA and ALOHA which has power channel rate utilization since it does not uses any sort of monitoring functions so during long data transmissions there is a maximum probability of collision. 

 Power efficiency:

Also the consumption increases as clients increases (due to more collisions).  The power consumption of DCMA is the lowest of all since because of power down mode during back-off period, and its collisions are fewer. CSMA/CA is lower, although CSMA/CA is always in receiving condition even in back-off period. So, even with fewer collisions, the power consumption of CSMA/CA is relatively higher and power consumption of ALOHA is too high again due to high collision probability during long packet size transmission.

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