# LBTM: Listen-before-Talk Protocol for Multiclass UHF RFID Networks
## 1 Introduction  
Multireader RFID deployments (also known as RFID networks) can be subject to constraints imposed by the different nature of the traffics accessing each reader. For example, in the same facility, some readers may work in the background doing a low-priority, continuous inventory process, while others may be serving the customers' checking-out processes and have a higher priority. These systems are said to have multiclass traffics, and specific solutions may be used to balance the performance of each class. For example, in the previous case, the customers' class traffic waiting times can be reduced at the expense of a more imprecise continuous inventory process. These types of trade-offs will be required in actual environments, where readers access heterogeneous sources of tags. RFID networks experience reader-to-reader interferences since neighbor reader signals easily jam the weak tags' responses. Therefore, it is necessary to arbitrate access to the shared interrogation channel using different mechanisms. The standard EPCglobal Class 1 Gen2 [1] introduces Multiple-Interrogation-Mode (MIM) or Dense-Interrogation-Mode (DIM), which alleviate the reader-to-reader interferences by making the tags respond in the sidebands of the readers' transmission channels. Another approach is using listen-before-talk (LBT), which senses the channel for an interval to determine if it is free before proceeding with the transmission. Otherwise, it defers its transmission to a later time. Depending on the country's regulations, LBT can be regarded as an optional mechanism for RFID networks. The EPC Global report [2] shows that it is in use in Japan, South Korea, Latvia, and the Russian Federation, while other countries use frequency-hopping (FH) [3] or the ETSI normative (ETSI 300-220 [4]), whose polite-spectrum-access (PSA) mode is a type of LBT. In this paper, which is an updated and enhanced version of our previous work [5], we propose a simple variation of the LBT protocol, named LBTM (LBT Multiclass), to handle multiclass traffics in RFID environments. When a reader tries to occupy the channel, it has to wait a fixed listen time (LT)--plus a random contention time (CT) if the channel is busy--until the channel is free. Then, the reader can start the interrogation of the tags nearby. The core idea of LBTM is to modify the access loop if the channel is busy by waiting \(\Delta\) LT periods until probing the channel again, \(\Delta\) being a parameter that depends on the traffic class and that is higher for the lower priority classes. In short, a higher \(\Delta\) allows expanding the back-off period for low-priority classes, and a relatively lower \(\Delta\) to prioritize traffic. This operation still allows for transmission opportunities for the lowest class readers, since after the back-off period, all readers behave similarly. Therefore, LBTM only provides soft QoS guarantees (i.e., statistical bounds), rather than hard QoS limits achieved with other disciplines like the preemptive head-of-the-line (HOL) one [6]. On the other hand, LBTM is limiting the starvation of the lowest class traffics, thus being suitable for heterogeneous environments. In this work, we aim at describing the operation of LBTM and studying how to set up and characterize a realistic network layout. Conceptually, the operation of the LBTM mechanism in a network can be considered as a single-server system (if all readers are mutually interfering, the channel is a resource that can only be accessed by one reader at a time) with separate traffic queues per traffic class (_polling model_) and finite-users, since the number of readers of each class is finite. The queue discipline is Random Order of Service (ROS) since users are served at no particular order but randomly (see, e.g., Chapter 2 in [7] and references therein). Besides, interarrival and service times are non-exponential. These types of polling models are complex to analyze and would require multiple simplifications, yielding a lack of results' reliability.