RFID Tag-Reader Interaction Session Layer

fits and mechanisms of each mode are described in the following subsections.

2.4.3.1 Global Scroll. The first, more primitive, method is called global scroll. In this method the reader sends a single request out and all tags that receive it respond. This mode allows for more rapid reads of a single tag, but if multiple tags are in the area, only the strongest response signal is read. The trade space created by the rapid read times of a single tag make this ideal for high speed applications, where only a single tag will be within view of the reader at a time, like assembly lines. An illustration of the reader to single tag interaction is shown in Figure 2.14. In the single tag example the reader sends the query and the tag responds. All future interactions from the reader to the tag will include the tag’s SUID. This prevents a new tag from entering the area and interfering in the ongoing

Figure 2.14: Global Scroll Single Tag Interaction

Figure 2.15: Global Scroll Multiple Tag Interaction

exchange. In the case of multiple tags, as shown in Figure 2.15, we can see all three tags respond. Tags two and three’s responses are represented as a weaker signal by the thinner and dashed lines. In this case, the reader responds only to tag one’s stronger signal, once again including tag one’s SUID to let the tags know who it is talking to.

2.4.3.2 Inventory. The second read method is called inventory and is based on the Time Slotted ALOHA protocol. The original ALOHA protocol was developed at the University of Hawaii Manoa campus as a computer network topology [13]. It had no collision prevention technique. If a computer had data to send, it tried to send it. If a collision occurred, it would wait and try again at a later

point in time. Time slotted ALOHA was an improvement on this as it introduced time slots. A node could only begin transmission at the beginning of a slot, and the message had to fit inside the time slot. This improved throughput and did not require central coordination. It did require a common time reference between the nodes. This common time reference is not available in RFID tags, particularly passive tags. The solution for this was to modify the slotted ALOHA protocol to be centrally coordinated, or polled in this case. The reader serves as the central coordinator and will start an inventory round by advertising the number of slots it is using. This is an integer and can range from 1 to 1024, determination of the optimal number of slots depends on the number of tags expected to be in range. This advertisement of the number of slots occurs in the Query command message, 2.16 (a). The tags each then pick a random number between 0 and the number of slots minus 1. This slot choice is shown in Figure 2.16 in parasynthesis after each tag. Each time the reader sends out a QueryRep short command, 2.16 (b-d), the tag increments its internal counter. When the counter and the selected slot number match the tag sends an ACK response including the tag’s SUID. In Figure 2.16 (e,f) two tags had chosen slot 2 and responded to the reader in this same slot. Upon seeing the multiple backscatter responses the reader can either try to filter the stronger one out and respond to it or ignore both. Our example ignores both and sends the next QueryRep (g). Tags one and two, having not been acknowledged now await the beginning of a new inventory round where they will pick new random slots. In step (g), Tag 3 sees the new QueryRep and increments its counter to match the random slot it choose earlier and responds, (h).

Figure 2.16: Inventory Query Round

Since it is the only tag to respond in this slot the reader sends an ACK including the tag’s SUID,(i), similar to the way a teacher would call on a student in a classroom. Further commands are then executed depending on the reader’s configuration, (j). Once the interaction is complete and since the reader has checked all slots it will start a new inventory round, allowing tag one and two to try and report again.

2.5 Software Defined Radios

Software defined radios have been utilized as experimental platforms for RFID research at the University of Washington. Michael Buettner is the author of the Gen2 RFID reader and listener [14]. This software package is widely used in SDR circles as the building block for any RFID related system. Chris Paget used it when he set his RFID tag read distance record of 217 feet [15]. Mr. Buettner also used the software as a basis for several papers on RFID technology [16–18]. Of particular interest

to this research is his paper titled“A Software Radio-based UHF RFID Reader for PHY/MAC Experimentation.” Buettner provides a solid background on the actual interaction of the reader and tag, as well as a software platform that can be easily modified to capture data not readily available on commercial RFID readers. The rest of this section provides an overview of the USRP1 main board and Flex 900 daughtercard used in this research.

2.5.1 USRP 1 Mainboard. The USRP 1 main board is manufactured by