# LBTM: Listen-before-Talk Protocol for Multiclass UHF RFID Networks
## 5 Grocery Shop Example  
In our first example, we have considered a setup with only two types of traffic, closely resembling a grocery store scenario like the one described in [33]. These are described as follows:  
* **Inventory traffic.** A set of readers realizes a continuous inventory of the items distributed in the store area. The goal of these readers is to maximize the ratio of items correctly inventoried. This metric is called MSL (Minimum Stock Level) [34], and it measures inventory accuracy. MSL is defined as the time-average ratio of time where the percentage of known tags divided by the total number of tags is over a given minimum threshold. For example, an MSL with a threshold of 90% and accuracy of 99% indicates that in at least 99% of the queries (done at randomly chosen time moments) to the inventory database, 90% of the reported data will be exact. Note that items constantly appear, due to replacement tasks; or disappear, if they are picked up by customers. Thus, yielding to frequent changes in the inventory.
* **Checking-out traffic.** This represents the customers leaving the store. In this case, the customers wait in a shared exit queue until a checking-out gate is available. The number of tags per customer is random, and the batch for each particular buyer is read independently. This traffic has a high priority, and the goal is to minimize the total response (waiting plus interrogation) time of the clients.  
The simulation layout is shown in Figure 4. It consists of 4 easy-pass checking-out gates and 16 ceiling readers configured with the following parameters:  
Figure 3: Example of the power received by the tags using two different link budget setups in the simulator, one considering the Friis model and the second assuming a cardboard substrate for the tags.  
* The inventory cycle time for the ceiling readers is 15 s. That is, the time between two consecutive interrogation processes of a ceiling reader.
* The checking-out readers have the minimal cycle time allowed by the ETSI EN 302-208-1 standard [27] of 100 ms.
* Each shelf in the store may contain up to 250 items (tags), and each ceiling reader manages four shelves. Therefore, a total number of up to 1000 tags have to be read at each interrogation process.
* The number of items (tags) picked by each customer is a random variable with a mixed Poisson-lognormal (PLN) distribution [35], which has been found to model this kind of process correctly.
* The clients arrive at the exit queue at a rate of \(\lambda=1\) customer (batch) per second.
* Simulation stops after 4000 clients have left the store.
* The replacement task is performed every 15 min in a random number of shelves (between 1 and 4).
* When a customer leaves the store, the same number of items (tags) as those in its batch are selected at random and removed from the shelves.
* All readers cause mutual interferences. In the best case, the minimum interference corresponds to two readers installed in the ceiling at 2.5 m height and which are separated by almost 29 m. The interference signal is reflecting in the ground and is received with a power of slightly below \(-23\) dBm, much higher than the tags' backscattered signals.