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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:
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Data Modulation: a set of waveforms that is understood as
symbols by readers and tags.
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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.
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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).
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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
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Description
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NA Value
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Europe Value
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T0
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Foward Link Symbol Period
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14.25 μS
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66.7 μS
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1/T0
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Forward Link Data Rate
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70.18 kbps
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15 kbps
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T0/2
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Recerse Link Symbol Period
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7.125 μS
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33.3 μS
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2/T0
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Reverse Link Data Rate
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140.35 kbps
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30 kbps
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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)
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Tdata1 (μS)
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Re (kbps)
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6.25
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9.375
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128
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12.5
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107
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12.5
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18.75
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64
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25
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53.3
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25
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37.5
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32
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50
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27
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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.
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