We’ve been talking in previous articles about features and characteristics of 802.15.4, the level 2 ISO radio interface defined by the IEEE. This article will focus on more of the low level radio characteristics of the standard:
But before that let’s review what 802.15.4 is and isn’t:
· It is a short range communication standard for ISM (Instrument, Scientific and Medical) applications. Depending on the specific frequencies used it can go as far as several kilometers but usually 15.4 nodes are found within 10 to 100 meters of each other.
· It isn’t a Bluetooth replacement – 15.4 has no capability to transfer voice communications.
· It is a low cost standard designed to add radio communications to small, stand alone, often battery powered sensors and actuators.
· It isn’t a LAN replacement – At the low data rates it offers (115Kb) you’d be very unhappy connecting your laptop to a server over 15.4. But if you wanted a temperature update from a thermometer every 5 minutes, that’s a perfect low data rate 15.4 applications.
· It is a low power, low duty cycle technology. Nodes using 15.4 sleep most of the day and when they transmit, it’s in short bursts with tiny amounts of energy.
· It isn’t a high speed network. Best case data transfer is only 250Kb/sec.
· It is an intermittent connection network where nodes are expected to come and go and constant connections are unusual.
· MOST OF ALL IT ISN’T a NETWORK OR APPLICATIONS STANDARD. There is nothing in the 802.15.4 standard concerning routing data or anything that defines the contents of messages. Higher level protocols like Wireless Hart or Zigbee do that. 802.15.4 is just a mechanism to move some amorphous data from one node to another through the air.
There are certain key attributes of 802.15.4 radios:
PHYSICAL CHARACTERISTICS – The 802.15.4 standard defines three frequency spectrums, 868MHz (Europe), 915Mhz (North America, Australia) and 2.4GHz (Most of the world). Within these frequencies, one or more channels are defined. A wireless communication channel is a transmission path from one node to another. The more communication paths or channels that are available, the more simultaneous conversations that can occur. 802.15.4 Defines a single channel for 868MHz, 10 channels for 915MHz and 16 channels for 2.4Ghz operation.
BANDWIDTH – 15.4 is a low bandwidth network solution. At 2.4Ghz you can get 250Kb/s. It’s even less for 868Mhz (20Kb/s) and 915MHz (40Kb/s).
POWER – 15.4 devices transmit a very low power signal compared to 802.11 (WiFi) and 802.3 (Bluetooth). Typical power for a 15.4 transmission is 1mw. That’s about 30 times less than your cell phone.
LOW DUTY CYCLE OPERATION – 802.15.4 is a network designed for device applications that require infrequent data transmission. A lot of 15.4 applications require so little power that batteries can be used in devices with an expected lifetime of ten years or more.
SPREAD SPECTRUM – All 15.4 radios incorporate spread spectrum modulation techniques spread the data signal across the frequency spectrum. This improves the signal to noise ratio and improves it’s immunity to interference.
ENERGY DETECTION & LINK QUALITY – All 15.4 radios are required to incorporate Energy detection & Link Quality assessment.
OPEN CHANNEL ASSESSMENT – When multiple channels are available a 15.4 radio will use the Energy Detection & Link Quality circuitry to assess the relative chance of communicating on a particular channel and select the optimum channel.
There is, of course, a lot more that could be described about 15.4 radios. There are things like how it uses orthogonal signaling to trade off bandwidth to recover sensitivity and the settling times on power up but that gets really geeky and it beyond the needs of anyone who’s not designing CMOS radio circuits.
What’s pretty interesting though is that data receiving is a lot more power hungry operation than transmitting. It’s much more power efficient to blindly transmit than to blindly receive for the same amount of time. It turns out that there are a lot more filters active on receive than on transmit so receiving consumes vastly more power. This becomes very important if you’re planning on using a 15.4 radio in a battery powered application.
In the next segment of this series we’ll discuss the how 15.4 defines access to the channels provided by the radio.