Collision

When the reader sends the request to the tag, not only one tag responds simultaneously to a reader’s signal, their communication signals might interfere with one another. This interference is referred to as a collision and typically results in a failed transmission. In order for a reader to communicate with multiple tags, a method for collision free tag communication must be employed. These methods are referred to as anti-collision methods.

An anti-collision method must be employed if an application will typically have more than one tag communicating with a reader at the same time. Unlike standard networking however, RFID tags pose a number of problems that arise from the very limited resources that they are provided with. First, they can afford only limited computation power. Second, state information, such as what portion of the tags identifier has already been read, may be unreliable. Third, collisions may be difficult to detect due to widely varying signal strengths from the tags. Finally, as in most wireless networks, transponders cannot be assumed to be able to hear one another.

A common classification of anti-collision algorithms, either probabilistic or deterministic, is based upon how the tags respond during the anti-collision algorithm.

There are two types of tag anti-collision protocols for RFID systems: probabilistic methods and deterministic methods.[3] The probabilistic methods are based on ALOHA protocol. The ALOHA based protocols reduce the probability of occurring tag collisions how tags respond at the different time. The deterministic methods are tree based protocols. The tree based protocols keep splitting the group of colliding tags into two subgroups until all tags are identified.

In probabilistic algorithms, the tags respond at randomly generated times. There are several variations of probabilistic protocols depending on the amount of control the reader has over the tags. Many probabilistic algorithms are based on the Aloha scheme in networking.

Deterministic schemes are those in which the reader sorts through tags based on their unique identification number. The simplest deterministic scheme is the binary tree algorithm.

Anti-collision algorithms which are used in RFID systems will affect its overall cost and speed. [4] In general, how to judge an RFID anti-collision algorithm is mainly dependent on how many timeslots are consumed for identifying all tags, the complexity of the algorithm, and how much data is transmitted. Therefore a good anti-collision algorithm should consist of the following factors: lower timeslots, simple in calculation, low cost, lower power, and fast reading speed.