Wireless Ethernet - Online Article

Introduction

The advantages of Ethernet are many: Ethernet components are commodity items and thus are low cost; Ethernet can be used as a transport medium regardless of the PLC communications protocol by encapsulating the Modbus or Profibus protocol within the TCP/IP packet; the multi-drop nature of Ethernet provides a straightforward expansion path; and the destination devices that the information is sent to are already on Ethernet networks.

But even with these advantages Ethernet shares one disadvantage: cable still needs to be run around the plant. In a typical installation, the cost of running the cable will exceed the cost of the rest of the equipment. And when equipment must be moved, additional cabling expenses are incurred.

Now there is a solution: Wireless Ethernet, which provides the benefits of Ethernet without the need for cables. Wireless technologies in office and factory environments have a speckled past. Some vendors have been guilty of over-promising and under-delivering. However, when properly understood and deployed, wireless devices can provide reliable, robust communications.

The first category to distinguish between wireless technologies and products is licensed versus unlicensed. As the overwhelming majority of wireless Ethernet devices operate in unlicensed frequency bands, this discussion will be limited to unlicensed wireless technologies, in particular to spread spectrum technology in the 2.4GHz band employed by almost all wireless Ethernet products. The 2.4GHz band is also attractive as it is license-free around the world.

Spread spectrum radio technology spreads the information signal over several frequencies. By so doing, interference on a single frequency does not block the signal. There are two types of spread spectrum technology: direct sequence (DS) and frequency hopping (FH).

DS spreads the signal by multiplying the data stream by a pseudo-random noise signal of higher frequency than the data stream. This causes the resulting signal to be spread over a bandwidth equal to the frequency of the pseudo-random noise signal. The ratio of the spread signal to the unspread signal is called the processing gain. It is expressed in dB and reflects the amount of signal impairment that can occur without a loss of information. For the latest 802.11b systems, the information signal is spread over 22MHz of bandwidth. Using more complex modulation techniques, they are able to maintain 10dB of processing gain.

FH on the other hand, continuously varies the carrier frequency of the information signal. It transmits a small burst on data on one carrier frequency then changes to another carrier frequency and transmits another burst. By 'hopping' to frequencies in a pseudo-random pattern, systems have a high degree of immunity to jamminmg and multi-path fading. Last summer, the FCC issued new rules that require hopping over just 25 frequencies and with 5MHz of bandwidth. These new rules are consistent with the ETSI rules that govern European radio operation.

To decide which technology is best for an application, it is helpful to understand the factors that affect radio transmission. In indoor applications, including factories, two primary factors are multi-path fading and interference. Multi-path fading occurs when multiple copies of the signal arrive at a radio at the same time but with varying phase. This causes signals to cancel each other to some degree, resulting in a `faded' or reduced strength signal.

Interference occurs when another RF source generates a signal at a frequency `of interest that is of higher field strength than the intended signal. The interfering device does not need to be another radio. In the 2.4GHz frequency band, microwave ovens and welding equipment can be sources of interference. While interference acts to reduce throughput by requiring retransmissions, multi-path acts to reduce range.

There is much debate over whether DS or FH provides the better solution but in our experience, FH radios have outperformed DS radios in factory and industrial applications.

A third factor affecting performance is the receive sensitivity of the radios. Receive sensitivity is defined as the minimum signal strength needed to receive signals at a given bit error rate. As such, radios with a better receive sensitivity will achieve better range. Receive sensitivities are given in negative dBm numbers indicating the reduction in a 1mW signal that is required. For example, the SEM2410 wireless Ethernet bridge has a receive sensitivity of -93dBm at a bit error rate of 10-5.

Another important factor is that, others things being equal, the higher the over-the-air data rate, the poorer the receive sensitivity. Multirate radios, such as 802.11b, that operate at several over-the-air rates, will specify receive sensitivities for each rate. Thus, while the receive sensitivity at 11Mbps may be only -76dBm, at 1Mbps, the receive sensitivity might be -88dBm.

802.3 versus 802.11

802.11 (and more recently 802.11b) is a wireless Ethernet standard designed to promote interoperability between vendors' office LAN products, with the goal to foster competition - resulting in lower costs. 802.11 defines not only the MAC layer but also the PHY layer for 802.11 radios. This refers to the over-the-air protocol used by the radios. While the goals of this standard are worthy, progress in achieving interoperability has been limited.

802.3 is the standard that defines 10BaseT Ethernet (802.3u defines 100BaseT ). These refer to wired Ethernet. Wireless devices that are 802.11 compliant, are actually 802.3 compliant on the connection to the network and 802.11 compliant in the over-the-air protocol. The important point is that while a wireless Ethernet device must be 802.3 compliant to connect to a wired Ethernet network, it does not need to be 802.11.

Deploying Wireless Ethernet

The first step is to determine how much data is to be transmitted and how quickly it must be transmitted. Also, the amount of latency that can be tolerated must be understood. The first item will determine the amount of throughput needed. The latency will determine how much data can be sent at one time. Throughput and latency are inversely proportional. Regardless of how robust a radio link is, there will be times where a transmission is unsuccessful the first time and must be retransmitted. This retransmission is automatic and is not seen by the network, but it does introduce additional latency. Thus while a longer data transmission increases throughput, when a retransmission is required, a longer data transmission increases the latency. In some systems, the latency is fixed. In other systems, the latency is adjustable by adjusting how quickly the radio hops from one frequency to another.

Once the required throughput is determined, the narrowing down of potential devices can begin. Keep in mind that more throughput needs higher over-the-air data rates which results in poorer receive sensitivity which results in shorter range which increases the cost of the system. Thus care should be taken not to overestimate throughput needs. Special care should be taken with the 802.11b multi-rate devices. While they boast an 11Mbps over-the-air rate providing 7Mbps throughput, if there is insufficient signal strength or quality, they will automatically throttle back to a lower rate. Also note that they will have different ranges for the different data rates.

The architecture of the wireless Ethernet network should be determined next. How many remote devices will be connected wirelessly will figure in the decision of whether to operate one point-to-multipoint network, multiple point-to-point links or some combination of the two.

This will impact the required throughput per device. If multiple links are employed, each link will need less throughput. If multiple links are to be employed in a single location, the ability of the wireless devices to operate in the presence of other devices must be considered. For example, typical 802.11b devices have just 3 non-overlapping channels. This means that only 3 separate links can be operational in the same location. By contrast, frequency hopping systems have 64 hopping patterns which can support up to 16 co-located networks.

Many people become confused when considering wireless Ethernet in regards to 802.11. As discussed briefly above, the wireless devices need to be 802.3 compliant to connect to the wired network. As long as this is requirement is met, the over-the-air protocol can be whatever is best suited for the particular application. This is especially important as the 802.11 specification was developed for office LAN applications. In those applications, vendors are trying to match up as close as possible to 10MB Ethernet network speeds. These applications envision many access points tying into the wired network in an office environment. Thus the emphasis is on speed not range since wired Ethernet is prevalent throughout most offices. In a factory environment, however, range is much more important as the aforementioned cost of wiring factories is high.

The location of the wireless Ethernet devices deserves some attention as well. The location of the factory device will determine the general location of the remote wireless Ethernet device and the nearest point of the wired network will control the base wireless device, there are still some considerations that can have an impact. As a rule of thumb, the antennas for all the devices should be placed as high as possible without placing them behind an obstruction. While 2.4GHz is considered a line-of-sight frequency band, indoors, particularly in factories, there are sufficient surfaces to reflect the signals to provide communications between two devices without line of sight.

Conclusion

Ethernet is making its way onto the factory floor. Wireless Ethernet devices offer the benefits of Ethernet without the wiring. Given an understanding of how wireless devices operate and with some careful planning, wireless Ethernet can provide reliable, robust communications in even the noisiest factory environment.

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