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Comparing wireless networks

May 1, 2007 12:00 PM, Rodger Richey Senior Applications Manager Advanced Microcontroller Architecture Division Microchip T

There's no denying that wireless technologies work well keeping tabs on hospital equipment, monitoring patients' vital signs, and similar medical applications. But the large number of available technologies makes it confusing when selecting the right one for a job. A few considerations that focus on devices based on IEEE 802.11 and IEEE 802.15.4 standards should help clear the way to a best selection.

WiFi provides patient mobility

The terms IEEE 802.11 and WiFi are often used interchangeably. Technically, 802.11 is the standard and WiFi refers to the wireless LAN-(WLAN) devices based on the standard. The original release from 1997 is now called the legacy version. Four more revisions have been released since its implementation.

In general, WiFi networks require what are called access points, or routers. These have a built-in network adapter, antenna, and radio transmitter and hook up to the facility network with an Ethernet connection. WiFi systems broadcast every 100 ms at a 1-Mbit/s data rate. Thus, a WiFi device must operate minimally at this rate. The maximum rate is determined by the hardware.

WiFi networks have several advantages. For one, the systems are significantly less expensive than wired ones to install in existing buildings. And silicon pricing continues to decrease as the WiFi market expands. Also, WiFi is a global standard so it works the same in different countries.

In addition, many hospitals already have WiFi networks in place, so it is cost-effective to use equipment such as infusion pumps, pulse oximeters, and EKG and blood-pressure monitors with IEEE 802.11 transceivers. Patients appreciate the technology because it provides them more mobility. And physicians can access patient information from a central server. Lastly, WiFi-based tracking systems are useful for finding hospital equipment that has been moved or stored, especially when emergency situations arise.

A drawback is that most existing devices have a fairly high power consumption, making WiFi difficult to integrate into low-power, battery-operated equipment. Devices for embedded systems from companies such as Atheros Communication, Santa Clara, Calif, should soon solve this problem.

Think ZigBee for large networks

Another standard, IEEE 802.15.4, targets relatively smaller devices, such as weight scales, glucometers, spirometers, and thermometers. Transceiver costs are approaching less than $2, and small software stacks allow use of low-cost microcontrollers. Adding wireless capabilities to such products makes a lot of sense for price-conscious markets such as nursing homes.

There are several protocols — sets of standard procedures for transmitting or storing data — that build on IEEE 802.15.4. Although not yet widely deployed, ZigBee is probably the best known. One of its best features is interoperability. ZigBee defines communication methods so devices from any manufacturer in a star or mesh network can talk to each other. (A star network has one node that acts as a network coordinator. The nodes in a mesh network share routing responsibilities.) ZigBee-based networks can consist of over 65,000 devices.

Of course, these capabilities do not come free. Typical ZigBee protocol stacks (the software implementation of protocols) for network coordinators range in size from 64 to 96 Kbyte, and require a substantial amount of RAM and non-volatile memory to store routing information. End devices are typically 32 to 64-Kbyte.

Other protocols include the Simple Media Access Controller (SMAC) from Freescale Semiconductor Inc., Austin, Tex., and MiWi from Microchip Technology. These protocols target applications that don't require interoperability or large network sizes. SMAC provides a 2-Kbyte stack and limits network topology to point-to-point or star. MiWi provides more flexibility, supporting over 1,000 nodes and point-to-point, star, and limited mesh topologies. The MiWi protocol stack is 20 Kbyte for a coordinator and 4 Kbyte for an end device.

DATA RATES AND RANGES FOR IEEE 802.11
Protocol	Release Date	Frequency	Typical Data Rate	Max Data Rate	Indoor Range
Legacy	1997	2.4 GHz	1 Mbit/s	2 Mbit/s
802.11a	1999	5 GHz	25 Mbit/s	54 Mbit/s	∼30 m
802.11b	1999	2.4 GHz	6.5 Mbit/s	11 Mbit/s	∼30 m
802.11g	2003	2.4 GHz	25 Mbit/s	54 Mbit/s	∼30 m
802.11n	2006 (draft)	2.4 or 5 GHz	200 Mbit/s	540 Mbit/s	∼50 m
The data rates and ranges are for IEEE 802.11. The “a” and “g” revisions have the largest installed base.

RANGES FOR IEEE 802.15.4
Frequency band	Geography	Data Rate	Available Channels
868 MHz	Europe	20 Kbit/s	1
915 MHz	Americas	40 Kbit/s	10
2.4 GHz	Worldwide	250 Kbit/s	16
The geographies are the major regions that allow use of the frequency band. However, there are exceptions.

Other technologies

Other wireless technologies include

Proprietary RF such as Insteon from SmartLabs, Irvine, Calif., MicrelNet from Micrel Semiconductor Inc., San Jose, Calif., and Z-Wave from Zensys Inc., Fremont, Calif., each of which provides a base for wireless networking that is comparable to IEEE 802.15.4. The designer builds the modulation and the protocol.
Medical Implant Communication Services (MICS) targets implants for the human body that communicate with devices normally worn on the belt. MICS operates in the 402 to 405 MHz range and has a maximum 25 mW output power. This short range and low-output power keeps the technology from interfering with other wired or wireless networks.
Cellular technologies provide a way to monitor patients' heart rate and ECG/EKGs when they are outside of a hospital or doctor's office. However, the cost of hardware is high and requires service contracts. The Bluetooth communication standard provides one solution. Many cell phones use Bluetooth and can connect to headsets and other peripherals. A monitoring device, for example, can use a Bluetooth transceiver to connect to the cell phone to upload data to a physician.