# MAC Layer Protocols for Internet of Things: A Survey
## 2 Short Range MAC Layer Protocols
### Radio Frequency Identification (RFID)  
Radio Frequency Identification (RFID) refers to a set of technologies that are aimed at identifying and recognizing elements (tags). An RFID system is basically composed of two types of devices: the identified devices (tags) and the device identifiers or readers. Tagged devices are triggered by RF (Radio Frequency) waves emitted by the reader devices and reply its identification (ID) tags. Readers handle data exchange between them. When necessary, readers send RF pulses interrogating the tags in the area. Tags reply to this question by submitting their tag IDs. Different classifications of RFID systems can be provided according to operating frequency, radio interface, communication range, tag autonomy (completely passive, semi-passive, active), and different standards have been ratified. Evolution of smart UHF (Ultra High Frequencies) RFID tags with embedded sensors and miniaturization of readers promotes this technology for high pervasive IoT ecosystems [9]. Figure 1 presents a brief summary of operation frequencies, transmission power level, and European regulation comments.  
Figure 1: RFID standards for short distance applications [10].  
The various devices identified by radio frequencies (RFIDs) such as wristbands, clothing, footwear, and others are a combination of a small microchip and an antenna integrated into a single casing uniquely identified electronically. When readers send their interrogation radio frequency pulse, tags transmit their identification information to the reader devices using radio frequencies. This transmission takes place depending on the proximity of the tag to the reader device, even though it does not have line of sight (LOS). The transmission range will depend on the class of device used. Transmissions occur from the low frequency (LF) bands at 124-135 KHz to ultra-high frequency band (UHF). There are three classes of RFID devices [11] as follows:  
* PRAT--Passive Reader Active Tag. The reader is passive and receives data from a battery-powered tag. The transmission range can reach up to 500 m depending on some characteristics of the system and transmission frequency used.
* ARPT--Active Reader Passive Tag. The reader is active and the identified tags are passive and powered by the energy harvested from the electromagnetic waves present in the air. In general, this power source can be a beacon transmitted by the reader to feed the tags and it receives back the transmission of the tag data. This is the most commonly used class.
* ARAT--Active Reader Active Tag. This class is where both the reader and the tags are powered by external power sources, but the tags only transmit their data when requested by the readers.  
There is a certain variety of standards for RFID systems. ISO (International Organization for Standardization)/IEC (International Electrotechnical Commission) 14443 [12] are the entities responsible for defining the behavior and properties of smart cards [13; 14]. The standard defines the nomenclature of the'reader device' as the Proximity Coupling Device (PCD) and the Tag Identified (TI) or, 'the object to be identified', is defined as the Proximity Integrated Circuit Card (PICC).  
One of the most commonly used identification standards in this case is the electronic product code (EPC) which contains a 96-bit structure in a string data format. This structure consists of eight initial bits that identify the protocol version followed by 28 bits representing the organization entity that produced such a label. The following 24 bits identify the type or class of the element and the remaining 36 bits are the unique serial identification of each particular element. These last two fields are used by vendors to assign identities to their devices [15]. Differentiated information such as Uniform Resource Locators (URLs) or some other more current pattern can also be used as identifiers as long as they meet the standardized format [16; 17].