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Overview of EPCglobal Class 1 Generation 2 and Comparison with 1st Generation EPCglobal Standards

Related Topics:
EPC Compatible Readers - RFID Solutions - Wireless RFID Readers


Author
Daniel Dobkin
Enigmatics 

Introduction

EPCglobal was formed in 2003 to take the activities of the Auto-ID Center forward into a non-profit standards framework supporting use of RFID in the supply chain and in other applications. As part of this mission, EPCglobal sought to create a single worldwide standard for

Daniel Kurtz
WJ Communications

the UHF RFID reader-tag air interface. This second-generation standard (“Gen 2”) was developed in 2004 and became publicly available in 2005 [1]; it has also been submitted to the International Standards Organization (ISO) with the intention that it should become part of the ISO-18000 series of RFID standards, as ISO18000-6C. Earlier EPCglobal protocols, the Class 0 Generation 1 [2] (“Class 0”) and Class 1 Generation 1 [3] (“Class 1”) standards, have seen wide commercial deployment and may be familiar to the reader.
 

 
Numerous vendors have announced upgrades for existing RFID readers, and new reader models, to support Gen 2 tags, and by the end of 2005 several tag manufacturers were able to supply Gen 2 tags in quantity. In this series of short articles, we will try to provide an introduction to those parts of the standard that directly affect users, to help them understand what the terminology means and make optimal use of the new capabilities provided by Gen 2 tags and readers. We will also compare each aspect of the standard to the Class 0 and Class 1 standards to clarify distinctions and their significance.

 

Overview: Protocol Challenges and Gen 2 Solutions

Any passive RFID protocol must provide certain basic functions:

  • Data Modulation: a set of waveforms that is understood as symbols by readers and tags.

  • Packet structure: preambles, training symbols, and timing conventions that enable tags and readers to synchronize to each other’s clocks, and to recognize commands, parameters and data.

  • Command set: commands and responses that make it possible to read (and optionally manipulate) information stored on tags, in particular their identifying numbers (which will generally be EPCglobal-compliant electronic product codes,EPCs).

  • Collision arbitration: provisions for allocating the wireless medium when more than one tag is in the field of a reader, in order to resolve potential collisions between tags contending to transmit information back to the reader. This is also sometimes known as singulation.

Let’s look at how the EPCglobal Gen 2 standard deals with these challenges. Many of

the more technical aspects of the standard are transparent to most users, and will only be

briefly reviewed here.

 

Data Modulation

Modulation is the change made in a signal in order to send information. For example, conventional analog broadcast radios use amplitude modulation (AM) or frequency modulation (FM) to send voice and music over a wireless link to the listener. Each RFID standard employs one modulation scheme for the Forward Link (reader-to-tag) and another for the Reverse Link (tag-to-reader). The modulation schemes reflect the different roles of reader and tag. The reader must send enough RF power to keep the tag powered. A passive tag does not transmit its own signals but modulates by changing the phase or amplitude of the reader’s transmitted signal that is being backscattered from its antenna.

 

Forward Link

In all three EPCglobal protocols, a reader sends signals to tags by changing its output power level between two states. This is known as amplitude-shift keying (ASK). These states are known as the on state and off state, although there may still be some small amount of transmitted power even in the “off” state.

 

Both Class 0 and Class 1 data symbols are encoded as an off state of (Toff) followed by an on state (Ton). The total time (T0=Ton+Toff) for both parts is constant. In other words, both have a constant Forward Link data rate. Class 0 has three data symbols (‘0’, ‘1’ and ‘Null’) with varying duty cycles (ratio of Ton to T0). There is some flexibility in choosing the data rate and the exact duty cycles for the different symbols; a typical example is shown in Figure 1.

 

Class 1 uses just two data symbols, Data ‘0’ and Data ‘1’. Data ‘0’ is off for T0/8; data ‘1’ is off for 3*T0/8. Also shown in Figure 1 is an alternative Class 1 encoding scheme. Class 1 specifies two modes of operation, a fast mode for North America and a slower one for Europe, as shown in Table 1.

 

Table 1: Class 1 Forward and Reverse Link parameters.

Parameter

Description

NA Value

Europe Value

T0

Foward Link Symbol Period

14.25 μS

66.7 μS

1/T0

Forward Link Data Rate

70.18 kbps

15 kbps

T0/2

Recerse Link Symbol Period

7.125 μS

33.3 μS

2/T0

Reverse Link Data Rate

140.35 kbps

30 kbps

The Gen 2 specification allows much flexibility in specifying the Forward Link. Gen 2 symbols begin with an on state followed by an off state. The off pulse is of a fixed duration, or pulse width, denoted PW. Symbols are distinguished by varying the interval between off state pulses. Thus, this technique is sometimes referred to as pulse-interval encoding (PIE). Like Class 1, Gen 2 uses just two data symbols, Data ‘0’ and Data ‘1’.

The total time of a Data ‘0’ is called Tari. The Data ‘1’ symbol is allowed to be between 1.5*Tari and 2*Tari. Since the Data ‘0’ and Data ‘1’ are not of the same length, there is no fixed Gen 2 Forward Link data rate. Instead, there is an “effective data rate assuming equiprobable data,” Re = 2/(TData0 + TData1). Tari must be between 6.25us and 25us, resulting in effective data rates between 27 and 128 kbps, as shown in Table 2.

Table 2: Gen 2 Forward Link data rates assuming equiprobable data.

Tari (μS)

Tdata1 (μS)

Re (kbps)

6.25

9.375

128

12.5

107

12.5

18.75

64

25

53.3

25

37.5

32

50

27

 

Figure 1: Comparison of reader data encoding for the three EPCglobal standards.

 

Gen 2, as we have seen, offers a wide array of allowable Forward Link waveforms. This permits tradeoffs between read rate, read range and occupied transmit bandwidth. The occupied bandwidth of the reader’s transmitted signal varies with the pulse width of the shortest feature in the signal, and differing amounts of spectrum are available in different jurisdictions. For example, European (ETSI) standards restrict operation to a much narrower bandwidth than the US, requiring slower operating rates.

Next

 

More Topics

EPC Compliant Readers EPC Compliant Tags EPC Tagging Systems EPC Tags
Passive RFID Tags RFID Labels RFID Stickers Active RFID Systems
 

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