Other responsibilities of the PHY layer include detecting transmissions from new nodes, and assessing the quality of links with other nodes.
The IEEE 802.15.4 MAC
The medium access layer (MAC) specification defines how multiple 802.15.4 radios operating in the same area will share the airwaves. It employs CSMA-CA (Carrier Sensing Multiple Access with Collision Avoidance) to control when a node is allowed to transmit. (A collision occurs when two nodes transmit at the same time.)
The MAC layer also governs any security protocols applied to the link.
The maximum length of an IEEE 802.15.4 packet is 127 bytes. This includes a two byte CRC (cyclic redundancy check) value, used for error-checking.
There is an optional mechanism to ensure that data is transmitted successfully, using an Acknowledgement bit. When this bit is set, the receiver must acknowledge that the data has been received, by sending back an acknowledgement transmission. If this acknowledgement is not received within a certain time, the transmitter will retransmit the data, and do this for a fixed number of times before declaring that there is a transmission error. However, receiving an acknowledgement simply indicates that a frame was properly received by the receiver's MAC layer. It does not, however, indicate that the frame was processed correctly. It is possible that the MAC layer of the receiving node received and acknowledged a frame correctly, but due to the lack of processing resources, a frame might be discarded by upper layers.
Page 10 of 73 For Evaluation Purposes Only Matrix Multimedia Zigbee Solution Course Notes
In all radio communication, the signal (the information) is modulated onto a carrier wave, by using it to change slightly (modulate) a property of the carrier wave. The three main types of modulation are:
- amplitude modulation (AM) - the information signal changes the amplitude (height) of the carrier wave;
- frequency modulation (FM) - the signal changes the frequency of the carrier wave slightly;
- phase modulation (PM) - the signal causes slight changes in the phase of the carrier wave. This is the basis of the modulation used in ZigBee and it will be further examined shortly.
In the case of ZigBee, the signal is digital, either logic 0 or logic 1. The term 'keying' is used to describe digital modulation, referring back to early telegraphy when a Morse key was tapped to send a signal.
With PSK (phase-shift keying), the phase is changed by 1800 when the signal changes from logic 0 to logic 1, or back again. The next diagram illustrates this:
Illustration of PSK10010SignaltransmittedEffect oncarrier
QPSK (quadrature phase-shift keying) uses phase-shifts of 900 instead of 1800. This allows four changes in phase instead of only two. As a result, the phase of the carrier can represent two binary digits, 00, 01, 10 and 11, effectively doubling the bandwidth of the carrier.
O-QPSK (Offset QPSK) is a variation which uses different values of phase-shift to minimise the changes in amplitude that take place in the resulting signal.
Page 11 of 73 For Evaluation Purposes Only Matrix Multimedia Zigbee Solution Course Notes
The principle of DSSS is shown in the following diagram:
Although the signal is binary, either logic 0 or logic 1, it is not transmitted as a single change in amplitude, frequency or phase. Instead, these two possible states are transmitted as different series of bits, called 'chips'. Each chip is transmitted on its own sub-channel, simultaneously with the other chips.
For example, eleven chips could be used to distinguish between logic 0 and 1:
Logic 1 = 00110011011 Logic 0 = 11001100100
These sequences are so different that even if substantial corruption occurs during transmission, logic 0 and 1 can still be distinguished.
1.5.4 IEEE 802.15.4 Device Types
The standard defines two types of device:
- a full-function-device (FFD) - These typically perform network management functions such as routing, coordination, networking formation, and other management functions;
- a reduced-function-device (RFD) - These typically interact directly with the application processes and sensors. These nodes (often referred to as leaf nodes) contain the firmware and hardware that perform data capture, control functions, and other application specific functions. They can also be mobile, depending on application, and therefore have stringent requirements for low power and memory space.
FFD devices usually have higher power requirements to permit "always on" operation to facilitate network routing, data analysis/aggregation, and other more demanding operations. An FFD can talk to RFDs and other FFDs, whereas a RFD can only talk to a FFD.
Page 12 of 73 For Evaluation Purposes Only Matrix Multimedia Zigbee Solution Course Notes
The ZigBee network
As we have seen, the MAC layer of the IEEE 802.15.4 introduced two types of physical device, the full-function-device, FFD and the reduced-function-device, RFD. The main differences are summarised in the following table, which introduces new concepts which will be developed in this section: Full Function Device