## 4 Tree-Based Protocols
### Window Based Protocols  
In the majority of tree-based protocols, tags respond with their full ID or with the bits from the last query, when the query sent by the reader matches the tag ID prefix. Figure 14 shows an example of a communication slot between the reader and the tag. To reduce the number of bits transmitted by the tag, a window method has been proposed [48; 49; 50]. In the identification process, many slots ultimately collide, resulting in a huge waste of bits. Protocols using the window method reduce the number of bits transmitted by the tags. The window is defined as a bit-string of length _ws_ bits transmitted by a tag in a slot. This bit-string is computed on the reader side, respecting the condition \(0\ <\ ws\ <\ k\). It is shown in Figure 15. Most tree-based protocols use a fixed tag response during the identification process, but some use different operational process methods with a dynamic response that is based on window synchronization. Figure 14: Example of a communication slot between the reader and one tag. Figure 13: An example of STT protocol. The Query window Tree protocol (QwT) is a memoryless tree-based protocol that applies a dynamic bit window to QT [48; 50]. Tags respond directly depending on the current query. QwT tags compare their ID value with the query received and transmit a certain number of bits, managed by the reader. This reduces the complexity of passive tags, their energy consumed, and the identification time. A reader and tag flow chart for QwT are shown in Figure 16a,b. When tags appear in the interrogation area, the reader will broadcast to them by transmitting a query length of \(L\) bits. Tags will respond if their ID prefix matches the query sent by the reader, but with the previously specified number of bits. One of the main features is that the total number of collisions is decreased by transforming potential collisions into partial successful slots. This is a new type of slot, called go-on slots. The previously explained window methodology is implemented in the QwT protocol. The window allows tags to transmit only the bit-string instead of their full ID. If tags match a reader query, they will synchronously transmit the next adjacent _ws_ bits of the ID. This protocol uses cyclic redundancy check (CRC) in order to differentiate between the types of tag responses. Accordingly, the slot types that can occur in the QwT protocol can be classified into 4 groups:  
Figure 16: Flow chart of QwT protocol: (**a**) for reader; (**b**) for tags. Figure 15: Window synchronized answer. * Collision slot. When the reader cannot differentiate the answer, they will create two new queries by appending '0' and '1' to the former query [\(q_{1}\),\(q_{2}\)...\(q_{L}\)]. The window size _ws_, will remain unchanged, with the value used in the previous query. * Idle slot. When there is no response, the reader will discard the query and retain the same _ws_ as that of the last command. * Go-on slot. This occurs when at least one tag responds with a window and the reader is able to understand it. If \(L+ws<k\) is not true, the reader will transmit a new query created from the former query and received window. During this query, the reader will append an updated _ws_ value. * Success slot. This is a type of go-on slot where the reader successfully receives the last part of the tag ID and \(L+ws=k\). Then, the reader can save the tag, calculate the new _ws_, and continue with the identification process. Using the QwT protocol, the reader computes _ws_ using the expression (1), where \(\beta\) is an adjustable parameter. This heuristic function is used to provide dynamism to the value of _ws_. It can only be applied to the go-on and Success slots, since in a Collision or Idle slot, _ws_ will be held unchanged. The proposed protocol maintains the memoryless feature of QT in that it is an applied bit window procedure. It provides a decrease in the number of tag-transmitted bits, but increases the number of slots and reader-transmitted bits.