RFID Basics - EPC Gen2 Memory

As per How to Select a Correct RFID Tag – Standards & Mandates, this standard is officially called EPC Radio-Frequency Identity Protocols Class 1 Generation 2 UHF RFID protocol for communications at 860–960 MHz. It was developed by EPCglobal, Inc. in 2004, which is now GS1, and was approved as ISO 18000-6C in July 2006. It defines air interface parameters for tags operating within the frequency range of 860–960 MHz and allows for use of different frequencies in different regions from within this range.

For more information, read How to Select a Correct RFID Tag – Standards & Mandates.

The following graphic depicts the Generation 2 memory layout. Gen2 tags can four banks of non-volatile memory – Reserved Memory, EPC Memory, Tag Identification (TID) Memory and User Memory. Each memory region is accessed in parts, that are 16-bit long, called words, starting at word 0.

The reserved area of memory holds the tag’s Access and Kill passwords, with a 32-bit “Kill” password that allows a tag to be permanently silenced.
• The Kill command only executes if the password has been set (that is, is non-zero).
• The default Kill password value is zero.
A 32-bit Access password allows the Tag to transition to the Secured state. A tag in the Secured state can execute all Access commands, including writing to locked blocks.
Kill and access passwords must be accepted prior to being used. The kill password is a 32-bit value stored in reserved memory 00h to 1Fh, MSB first. Remember that:
• A tag that does not implement a kill password acts as though it had a zero-valued kill password that is permanently read/write locked.
• A tag does not execute a kill operation if its kill password is zero.
• An interrogator uses a tag’s kill password once, to kill the tag and render it silent thereafter.
• The default (that is, un-programmed) value is zero.
The access password is a 32-bit value stored in reserved memory 20h to 3Fh, MSB first. Remember that:
• Tags with a non-zero access password is required an interrogator to issue this password before transitioning to the secured state.
• The default (that is, un-programmed) value is be zero.
A tag that does not implement an access password acts as though it had a zero-valued access password that is permanently read/write locked.

EPC Memory consists of the 16-bit Cyclic Redundancy Check (CRC-16), which is an error detection code, a 16-bit Protocol Control (PC) and starting with Word 2, the EPC number (96 to 496 bits).

The PC is further subdivided:
• First 5 bits indicate the EPC memory length, next bit indicates whether the tag has or does not have user memory.
• The bit 6 identifies the existence of the Extended Protocol Control Word 1 (XPC_W1) in the word 21 of EPC Memory.
• The bit 7 identifies whether the data in the EPC data area conforms to a GS1 EPCGlobal™ (set to 0b) or an ISO/IEC 15961 coding scheme (set to 1b). The actual coding scheme is identified in the Application Family Identifier (AFI) bit field.

User memory is optional and can have various sizes, usually 512 bits and up to 4 or 8K of memory in some tags. This memory is used for additional data that is not suitable or does not fit into the EPC memory, such as for instance temperature or maintenance logs, expiration dates and other data.
The first few bits are a Data Storage Format Identifier (DSFID) as specified in ISO15961 and ISO15962. The DSFID specifies the format for the remainder of the user memory bank. The DSFID is usually eight bits in length, but may be extended as per ISO15961. When the DSFID specifies Access Method 2, the format of the remainder of user memory is “packed objects”. This format is recommended for use in EPC applications.

TID Memory carries the unique tag identifier that cannot be changed or erased. This ID identifies the tag itself, rather than item it is applied to.

This standard defines EPC tag data formats for Generation 2 tags. It defines how the EPC is encoded on the tag and how it is encoded for use in the information systems layers of the EPC Systems Network. The standard includes specific encoding schemes for EPC General Identifier (GID).
extended as per ISO15961. When the DSFID specifies Access Method 2, the format of the remainder of user memory is “packed objects”. This format is recommended for use in EPC applications.

This standard defines EPC tag data formats for Generation 2 tags. It defines how the EPC is encoded on the tag and how it is encoded for use in the information systems layers of the EPC Systems Network. The standard includes specific encoding schemes for EPC General Identifier (GID).
extended as per ISO15961. When the DSFID specifies Access Method 2, the format of the remainder of user memory is “packed objects”. This format is recommended for use in EPC applications.

*Explanation: Note that the letter “S” in the term “SGLN” does not stand for “serialized” as it does in SGTIN. This is because a GLN without an extension also identifies a unique location, as opposed to a class of locations, and so both GLN and GLN with extension may be considered as “serialized” identifiers. The term SGLN merely distinguishes the EPC form, which can be used either for a GLN by itself or GLN with extension, from the term GLN which always refers to the unextended GLN identifier. The letter “S” does not stand for anything. (Source GS1.org)