IEEE 802.15.4 PHY Layer

 

PropertyRange

 

Raw Data Rate868 MHz:20 Kb/s;

915 MHz:40 Kb/s;

2.4 GHz: 250 Kb/s

Range10-20 m
LatencyDown to 15ms
Channels

 

868/915 MHz: 11 Channels, 2.4 GHz: 16 Channels
Frequency BandTwo PHYs: 868 MHz/915 MHz and 2.4 GHz
AddressingShort 8-bit or 64-bit IEEE
Channel AccessCSMA-CA and slotted CSMA-CA
TemperatureIndustrial temperature range -40 to +85 C

 

The Physical (PHY) layer is concerned with the interface to the physical transmission medium (radio, in this case), exchanging data bits with this medium as well as with the layer above (the MAC sub layer). More specifically, its responsibilities towards the physical radio medium include:

1. Channel assessment

2. Bit level communications (bit modulation, bit demodulation, packet synchronization)

The PHY layer also offers the following services to the MAC sub layer:

PHY Data Service: Provides a mechanism for passing data to and from the MAC sub layer.PHY Management Services: Provides mechanisms to control radio communication settings and functionality from the MAC sub layer.

PHY functionalities:

1) Activation and deactivation of the radio transceiver

2) Energy detection within the current channel

3) Link quality indication for received packets

4) Clear channel assessment for CSMA/CA

5) Channel frequency selection

6) Data transmission and reception

The operating frequency bands are depicted in the figure below.

channel-structure
The standard specifies two PHYs:
1) 868 MHz/915 MHz direct sequence spread spectrum (DSSS) PHY (11 channels)
    1 channel (20Kb/s) in European 868MHz band
    10 channels (40Kb/s) in 915 (902-928) MHz ISM band
2) 2450 MHz direct sequence spread spectrum (DSSS) PHY (16 channels)
    16 channels (250Kb/s) in 2.4GHz band
    2.4 GHz PHY can be used to attain higher throughput and lower latency / lower duty cycle.
phy-layer

PHY packet fields:

1) Preamble (32 bits) – synchronization

2) Start of packet delimiter (8 bits) – shall be formatted as “11100101”

3) PHY header (8 bits) – PSDU length

4) PSDU (0 to 127 bytes) – data field

The packet structure of the IEEE 802.15.4 PHY layer is depicted in Figure above. The first field of this structure contains a 32-bit preamble. This field is used for symbol synchronization. The next field represents the start of a packet delimiter. This field of 8 bits is used for frame synchronization. The 8-bit PHY header field specifies the length of the PHY service data unit (PSDU). The PSDU field can carry up to 127 bytes of data.

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IEEE 802.15.4: CASE STUDY

The main design objective of the IEEE 802.15.4 open standard is to support the wireless connectivity of a vast number of industrial, home, and medical applications, including automotive monitoring and control, home automation, ubiquitous and pervasive health care, gaming, connecting devices to a PC, and sensor rich environments.

Such applications require a small, low cost, highly reliable technology that offers long battery life, measured in months or even years, and automatic or semiautomatic installation. The IEEE 802.15.4 standard supports these requirements by trading off higher speed and performance for architectures that benefit from low power consumption and low cost.

Most of these applications require only low to medium bit rates (up to some few hundreds of kbps), moderate average delays without too stringent delay guarantees, and for certain nodes it is highly desirable to reduce the energy consumption to a minimum. The physical layer offers bit rates of 20 kbps (a single channel in the frequency range 868 –868.6 MHz), 40 kbps (ten channels in the range between 905 and 928 MHz) and 250 kbps (16 channels in the 2.4 GHz ISM band between 2.4 and 2.485 GHz with 5-MHz spacing between the center frequencies). There are a total of 27 channels available, but the MAC protocol uses only one of these channels at a time; it is not a multi channel protocol.

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