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The only advantage I can think of using 16-bit instead of 64-bit addressing on a IEEE 802.15.4 network is that 6 bytes are saved in each frame. There might be a small win for memory constrained devices as well (microcontrollers), especially if they need to keep a list of many addresses.

But there are a couple of drawbacks:

  • A coordinator must be present to deal out short addresses
  • Big risk of conflicting addresses
  • A device might be assigned a new address without other nodes knowing

Are there any other advantages of short addressing that I'm missing?

Jonatan
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  • Saving 6 or 12 bytes saves time both for the transmitter and all the receivers. So it's a considerable network-wide power savings for low-energy networks. – dwhall Mar 27 '15 at 03:26

2 Answers2

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You are correct in your reasoning, it saves 6 bytes which is a non-trivial amount given the packet size limit. This is also done with PanId vs ExtendedPanId addressing.

You are inaccurate about some of your other points though:

  • The coordinator does not assign short addresses. A device randomly picks one when it joins the network.
  • Yes, there is a 1/65000 or so chance for a collision. When this happens, BOTH devices pick a new short address and notify the network that there was an address conflict. (In practice I've seen this happen all of twice in 6 years)
  • This is why the binding mechanism exists. You create a binding using the 64-bit address. When transmission fails to a short address, the 64-bit address can be used to relocate the target node and correct the routing.
kyork
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The short (16-bit) and simple (8-bit) addressing modes and the PAN ID Compression option allow a considerable saving of bytes in any 802.15.4 frame. You are correct that these savings are a small win for the memory-constrained devices that 802.15.4 is design to work on, however the main goal of these savings are for the effect on the radio usage.

The original design goals for 802.15.4 were along the lines of 10 metre links, 250kbit/s, low-cost, battery operated devices.

The maximum frame length in 802.15.4 is 128 bytes. The "full" addressing modes in 802.15.4 consist of a 16-but PAN ID and a 64-bit Extended Address for both the transmitter and receiver. This amounts to 20 bytes or about 15% of the available bytes in the frame. If these long addresses had to be used all of the time there would be a significant impact on the amount of application data that could be sent in any frame AND on the energy used to operate the radio transceivers in both Tx and Rx.

The 802.15.4 MAC layer defines an association process that can be used to negotiate and use shorter addressing mechanisms. The addressing that is typically used is a single 16-bit PAN ID and two 16-bit Short Ids, which amounts to 6 bytes or about 5% of the available bytes.

On your list of drawbacks:

  • Yes, a coordinator must hand out short addresses. How the addresses are created and allocated is not specified but the MAC layer does have mechanisms for notifying the layers above it that there are conflicts.
  • The risk of conflicts is not large as there are 65533 possible address to be handed out and 802.15.4 is only worried about "Layer 2" links (NB: 0xFFFF and 0xFFFE are special values). These addresses are not routable/routing/internetworking addresses (well, not from 802.15.4's perspective).
  • Yes, I guess a device might get a new address without the other nodes knowing but I have a hunch this question has more to do with ZigBee's addressing than with the 802.15.4 MAC addressing. Unfortunately I do not know much about ZigBee's addressing so I can't comment too much here.

I think it is important for me to point out that 802.15.4 is a layer 1 and layer 2 specification and the ZigBee is layer 3 up, i.e. ZigBee sits on top of 802.15.4.

This table is not 100% accurate, but I find it useful to think of 802.15.4 in this context:

+---------------+------------------+------------+
|  Application  | HTTP / FTP /Etc  | COAP / Etc |
+---------------+------------------+------------+
|  Transport    |  TCP / UDP       |            |
+---------------+------------------+   ZigBee   |
|  Network      |   IP             |            |
+---------------+------------------+------------+
|  Link / MAC   | WiFi / Ethernet  | 802.15.4   |
|               |    Radio         |   Radio    |
+---------------+------------------+------------+
Doddie
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