3

I am trying to understand the technology behind NFC. I have a basic understanding of the protocol, the physics behind the technology and the immense power it brings to the wide range of applications that could be there.

Now I am trying to understand the the different modes in which an NFC enabled device (may be a smart phone) can operate.

I understand that the device may take up the role of a reader/writer or a card (card emulator). However, what I do not understand is what is the need for having the initiator and the target modes of operation ?

My understanding is :-

In a passive communication mode between 2 NFC enabled devices, one device takes up the active role (thus acting as the reader/writer) and the other device takes up the passive role (and thus acting as the card emulator).

And we know that the active device (in a passive mode of communication between 2 NFC devices) is the one that creates an RF field and constantly polls to find out nearby NFC enabled devices that are ready to act as a card emulator.

So is it not just obvious that the the device acting as an active device (taking up the reader/writer role) is the one that would be the initiator of the communication ? Why is a separate initiator and target ** mode required when it can be inferred easily from the **reader/writer or card role ?

qre0ct
  • 5,680
  • 10
  • 50
  • 86

1 Answers1

3

You got it right:

The distinction between target and initiator is only of significance in the peer-to-peer communication mode (aka Android Beam). In this mode one device powers the field and acts like a card reader/writer. This is the initiator. The other device takes over the target role and technically acts like a card emulation.

The difference between target and initiator is significant in the LLCP protocol which implements the peer-to-peer functionality. Reason for that is, that the target cannot send data on it's own. It first has to be asked from the initiator if it wants to send something.

The LLCP protocol (among other things) abstracts this difference and lets the initiator poll the target at regular intervals. If you write communication code on top of this protocol (or anywhere higher in the protocol stack) you don't have to worry about the target/initiator mode at all anymore.

For completes sake here is also a operation mode within peer-to-peer where the two devices are powering the field (called active mode), but this is rarely used because support for it is not that great.

To answer your question:

Why is a separate initiator and target ** mode required when it can be inferred easily from the **reader/writer or card role

The naming is there to avoid confusion. Take peer-to-peer initiator mode for example: Also it is technically 95% identical to reader/writer mode there are still some minor differences. You don't want your peer-to-peer initiator to connect to a real card but only to peer-to-peer targets for example.

Naming this mode initiator instead of reader/writer avoids confusion.

Nils Pipenbrinck
  • 83,631
  • 31
  • 151
  • 221
  • Thanks. But there is still a bit of confusion and I would appreciate any help here as well. When a device takes up an active role, it becomes reader/writer and hence by default the initiator, right ? So a device that has the "reader/writer" mode set will simply start sending commands to the device that has the "card" mode (and hence by default the target) set, right? So again, the question, why will this second device, that has the "card" mode set, explicitly need to set "target" mode again, when it is implicitly already understood?(And the same with the 1st device as well) – qre0ct Nov 28 '14 at 14:24