# LBTM: Listen-before-Talk Protocol for Multiclass UHF RFID Networks
## 2 Related Works  
Contention protocols arise in many practical communication networks, and in most cases, they consider homogeneous nodes and traffics between them. For example, the well-known nonpersistent carrier-sense multiple access (CSMA/CD) that uses a back-off algorithm, such as that employed in local area networks [9], is an example of this kind. However, legacy CSMA/CD does not support service differentiation or priority access to the channel. The work [10] presented an analytical approach to this problem by assigning different durations for the contention windows to each traffic class. This idea is similar to LBTM, although in LBTM, only the first contention window adds additional back-off time to avoid starvation of the lowest priority readers.  
Other important back-off approaches incorporating multiclass differentiation are _Predictive_[11], _Sliding Contention Window_[12], _Gentle DCF/Probabilistic_[13], _Polynomial Backoff_[14], and _Fibonacci Backoff_[15] schemes. With the exception of [10; 14], these proposals show significant degradation when the number of contenders rise up. In the case of _Fibonacci Back-off_[15], the authors focused only on tag collisions and did not address reader-to-reader collisions.  
None of the previous approaches have been used or proposed yet for an RFID network. Most of the earlier algorithms can be adapted to some extent to work in this context. However, it would be necessary to do performance studies since the traffic characteristics of an RFID environment are quite different from those considered in the related works. This is indeed the primary goal of our work.  
On the other hand, several works have dealt with different traffic-load-balancing issues in RFID networks, but are mainly focused on readers positioning or other aspects unrelated to the reader-anti-collision protocol. Works of this type are [16; 17; 18; 19; 20; 21]. None of these proposals provide mechanisms to share the channel during operation. One recent mechanism with this aim was introduced in [22], which presented a distributed reader-to-reader anti-collision protocol based on Petri nets to assign interrogation time slots to the different readers--although, as with most works on RFID, the authors did not consider multiclass traffics.  
Random-access protocols aimed at multiclass RFID or general-purpose Internet of Things (IoT) networks are scarce. To some extent, works exist for machine-to-machine (M2M) cellular networks, like in [23], where authors introduce a Time Division Multiple Access (TDMA) protocol for heterogeneous traffic in 3 GPP Long Term Evolution networks. More similar to our environment, in [24], authors study the application of Aloha protocol for multiclass networks. They approximate the solution by maximizing a global utility function designed as a weighted sum of all classes' utility functions. The solution found assures maximum utility when the higher priority classes have a higher probability of accessing the medium. In our approach, we also design a global utility function to permit the selection of the optimal network operation point. In [25], authors propose an irregular repetition slotted Aloha (IRSA) [26] scheme consisting of transmitting packet replicas and exploiting interference cancellation at the receiver for achieving higher throughput. However, this kind of multitransmission scheme is not suitable for RFID networks, where communication between readers and tags has to be free of interfering sources.