IoT(Internet Of Things)

  

 

 

 

WPAN - ZigBee     

 

Technically, ZigBee is a kind of an application of IEEE 802.15.4. 802.15.4 defines various types of PHY implementation and a single (common) MAC layer that applies to all the different PHY implementation.  In terms of MAC layer, you may say ZigBee complies to 802.15.4 and in terms of PHY layer, you can say ZigBee implements a sub set of 802.15.4 PHY specification. In a very detailed implementation, there is some possibility that ZigBee PHY layer has a little bit of variations from 802.15.4 PHY specification since ZigBee has been in the market for pretty long time and it was commercialized even before every details of 802.15.4 PHY has clearly been defined. In addition, ZigBee has been driven by its own alliance group (ZigBee Alliance) as you might have seen how Bluetooth has been evolved/promoted by Bluetooth Alliance (or Bluetooth Special Interest Group). If you are seriously interested in the technical details of ZigBee, you would need to cover both IEEE 802.15.4 and various documents published by ZigBee Alliance.

 

Following is overall sturcture of ZigBee in comparison with 6LoWPAN. As you see here, ZigBee uses its own PHY (though it is very similar to 802.15.4 PHY) and the biggest difference is Network Layer. ZigBee uses its own proprietary Network Layer protocl. Since it is Proprietary, it was difficult to communicate to other network(non-ZigBee network), but now it is evolving into more common IP Network Layer concept.

 

 

 

PHY Property/Requirement

 

Property

Description

Data Rate

868 Mhz : 20 kbps

915 Mhz : 40 kbps

2.4 Ghz : 250 kbps

Range

10~20 m

Latency

Less than 15 ms

Channels

868 : 1 channel

915 Mhz : 10 channels, 2 Mhz Channel Spacing

2.4 Ghz : 16 channels, 5 Mhz Channel Spacing

Frequency Bands and PHY Type

868 : BPSK

915 Mhz : BPSK

2.4 Ghz : O-QPSK

Channel Access

CSMA-CA and slotted CSMA-CA

Addressing

Short 8 bit or 64 bit IEEE

 

 

Frame Structure

 

ZigBee uses two different PHY out of 6 different PHYs defined in IEEE 802.15.4. The two PHYs are BPSK and O-QPSK. Refer to BPSK Frame structure and O-QPSK Frame structure of IEEE 802.15.4 page