The primary factor when designing a wireless system is appreciating that the radio frequency characteristics cannot be altered regardless of equipment manufacturer, says Ian Poulett.
Implementation of Wireless Ethernet solutions is migrating into the industrial arena. Wireless solutions are not limited to Ethernet but cover GSM, GPRS, Bluetooth and Serial Data. The primary factor when designing a wireless solution is appreciating that the radio frequency characteristics cannot be altered regardless of equipment manufacturer.
This article will highlight the above issues and ensure that they do not turn into major problems when implementing such a network.
* Industrial Ethernet - the migration of Ethernet from the idyllic office environment onto the factory floor to ensure critical applications have a resilient and fault tolerant network have been significantly aided by many IEEE standards that ensure that all equipment will interoperate and work correctly.
In the Industrial Ethernet market, leading vendors have striven to ensure that networks can pass critical data deterministically and with a method of redundancy built into the physical communications medium to ensure that any cable loss will have a minimal impact to data throughput.
They are also producing networking products that are simple to connect, easy to identify and diagnose faults and most importantly will not be affected by environmental conditions.
The fact that IEEE standards are followed and exceeded ensures that the above points aid any network design and implementation.
Furthermore as wired solutions have been around for a while the majority of known problems when installing and commissioning copper or fibre optic cable networks can be eradicated by following rules that have been developed over a long period of wide and varied installations.
The key factor when designing wired networks is remembering the physical medium that is being used to transport data can be used to pass a single protocol or multiple data types.
This makes a physical copper or fibre solution far more secure - a primary factor when designing a secure and resilient network.
* Wireless - in the majority of applications the term 'wireless' often brings a single and simple conclusion that all 'wireless' devices and protocols all have the same characteristics - this cannot be further from the truth.
The Radio Frequency (RF) spectrum ranges in frequency ranging from DC all the way through to natural daylight.
As we are all aware but often forget the RF spectrum is totally open and does not enable anyone to have a guaranteed frequency that is only available to a unique party.
Critically, RF is easily affected by a wide range of obstacles that have a direct and adverse effect on the transmission path at a particular radio frequency.
The RF frequencies that are being used in Ethernet and other applications highlighted within this document are invisible to the human eye.
Critically, all frequencies are open to anyone; do these characteristics provide peace of mind for critical applications?
Unlike RF and as highlighted previously; copper and fibre systems offer dedicated transmission paths that can pass control data, video, voice and a multitude of other protocols.
However, if required, a single protocol can be passed down a dedicated fibre or copper path to provide additional network resilience against protocol conflicts or to provide enhanced security.
Therefore, copper and fibre solutions offer a physical visible solution that allows people peace of mind that the data is being passed down a secure medium.
Therefore, RF has two major characteristics that require clarification before any network can be designed and implemented - failure to comply with these will result in a network deemed not fit for purpose.
The characteristics can be summarized as follows.
* RF solutions can be affected by physical location or environmental characteristics.
* RF solutions can be monitored, sniffed, modified and cracked by off the shelf equipment.
A quick examination of RF characteristics highlights the susceptibility to interference by external sources.
Long Wave frequencies are used by the vast majority of the world's naval forces to allow communications.
As long distances are possible we can therefore ascertain that the RF is not seriously affected by physical or environmental affects that will cause major interference.
However, although the data can be passed significant distances the data rate is very, very low - not suitable for high speed applications as detailed within this document.
High Frequency communications (infrared) are used in industrial applications.
Although high data rates are possible and achievable the physical characteristics make Infrared easily susceptible to interference from dust, moisture and the vast majority of airborne contaminations.
Visible light has similar properties to Infrared and is affected by physical boundaries such as solid walls and doors and can also be attenuated by mist, fog and rain etc Therefore in summary - low frequency RF.
* Greater powers of penetration.
* Not susceptible to interference.
* Travels further in free space.
* Low data rates.
High frequency RF.
* Absorption in most materials (air/walls).
* Easy susceptible to interference.
* Travels less distance in free space.
* High data rates.
Typical frequencies.
* AM radio - 535kHz 1.7MHz.
* Short wave radio - 5.9MHz 26.1MHz.
* Citizens Band (CB) Radio - 26.96MHz 27.41MHz.
* Television stations - 54 88MHz for channels 2 through 6.
* FM radio - 88MHz 108MHz.
Television stations - 174 220MHz for channels 7 through 13.
* Wireless Ethernet - with Wireless Ethernet solutions the primary radio frequency are 2.4GHz or 869MHz.
Therefore as there are two RF options there are two characteristics that have to be taken into account.
* 2.4GHz - operating on the 2.4GHz frequency are two wireless Ethernet standards in addition to the other protocols operating such as Bluetooth.
Following on from previous notes the higher the frequency the higher the data rate but the transmission distance is significantly reduced.
In addition, within the UK the power of the transmitter is limited.
Therefore in summary although 2.4GHz does provide good data rates for Ethernet connections it is limited with transmission distances and transmission power.
* 802.11b.
Maximum data rate is of 11MBits/s.
Maximum distance in factory is 100m.
Can be stopped by walls and major obstructions.
* 802.11g.
Maximum data rate is of 54MBits/s.
Maximum distance in factory 30m.
Will be stopped by walls and major obstructions.
The 802.11 standard covers a wide range of Wireless Local Area Networks (WLANS) standards of which 802.11b and 802.11g are two of the current standards.
Currently 802.11b is the standard that is being used in factory automation as the characteristics of the frequency and the employed DSSS to aid transmission is suited to factory automation.
However, although 802.11g has a significantly higher data rate by implementing OFDM rather than DSSS to increase the speed of data throughput they still operate at 2.4GHz.
Common interference on the 2.4GHz frequency is as follows.
* Microwave Ovens (hence why 2.4GHz is affected by water as the RF tries to heat the water rather than passing through it).
* Bluetooth Devices.
* Open RF spectrum - un-limited devices can operates at 2.4GHz without any laws governing bandwidth utilization, etc * 869MHz.
The 869MHz frequency has been used within the UK (and the most of Europe) for passing serial and telemetry data for a number of years.
The frequency is used and implemented in a wide range of applications primarily as it operates within a 'license-free' bandwidth.
However, this frequency is governed by OFCOM (within the UK) that set a number of standards that ensure that the frequency is not hi-jacked by a single user.
Therefore, in summary 869MHz, although has a reduced data throughput (ideal for serial communications and telemetry applications) it is governed by OFCOM so that in principle all users of such a frequency are guaranteed that their network will function correctly.
* 869MHz.
Maximum data rate of 76KBits/s.
Maximum distance in factory 5km.
Not easily stopped by walls and major obstructions.
* Building a network - as Wireless networks are to be employed in factory automation and process control environments in a similar vein to wired networks a secure and resilient system may need to be implemented.
There are a wide range of applications where point to point, point to multipoint, network repeating, network bridge and network routing are all available and possible regardless of the radio frequency employed.
To ensure that radio communications across a network are handled correctly and follow the correct protocol a network Access Point is required as the focal point.
As the network focal point or 'Access Point' (AP) this unit handles all network traffic and therefore all RF network traffic passes through this unit.
As all data is passed through this unit the network loading must be a consideration - an 11MBits/s bandwidth will be shared by all devices on the local network.
Each radio therefore must receive authorization from the network AP prior to logging onto the network.
Whether this is a simple point to point radio link where two radios are required or a complex multi-site system - the AP handles all of the network traffic.
Therefore, unlike a wired system where data can be passed from unit to unit locally without the need of passing through a central Switch, a wireless network cannot form any part of a deterministic network! Wireless networks operate in a pseudo half duplex format.
Therefore, as the AP forms the pivotal portion of any network and that a RF solution may form part of a linear or star topology in an environmentally challenged location a radio site survey must be paramount prior to implementation of a RF network.
* Radio site surveys - wired solutions are implemented using either a copper or fibre medium.
If any faults do occur equipment is readily available that enables faults to be quickly located and fixed.
However, as highlighted previously RF can be easily affected.
RF does not travel the same distance in all directions with the same level of power.
Walls, doors, lift shafts and people can attenuate the signal making RF irregular and unpredictable.
Therefore, assuming that a simple omni-directional antenna will overcome all installation requirements will lead to failure.
The aim of a correctly implemented site survey is to supply adequate information to ensure the correct number of access points and general placement of radio units and in particular antenna type and position will make a network as resilient as physically possible.
A basic site survey is implemented with a number of steps, as follows.
* Facility diagram - obtaining a facility diagram enables the Engineer to gain an understanding of the obstructions and overall layout of the facility.
If possible GPS co-ordinates should be taken to ensure correct placement of antenna etc * Equipment location - the distance between the actual location of wired network equipment and network RF equipment or AP are affected by the frequency.
These distances should be kept to a bare minimum.
* Access point locations - where Access points are required then a good indication of locations can be ascertained from the facility diagram - is there adequate overlap?
* Dynamic site survey - using an actual AP and network equipment in conjunction with a wide range of antenna will enable signal strength, data rate and signal quality to be gauged and noted.
Using a GPS can aid the final installation as small movements in antenna location and positioning can have an effect on the above readings.
For fault finding and diagnostic purposes modification to Antenna orientation and location due to new equipment or new buildings will have an effect (positive or negative) on the network.
The primary resource of providing accurate site surveys is experience.
After carrying out a large number of site surveys Engineers will gain valuable experience of the common interference paths and methods of increasing signal strength and quality.
* Network security - wired network solutions are predominantly secure.
It is inherently difficult to gain access to a fibre optic network and gather information on the passing data.
However, as highlighted previously a wireless network is open to all - depending on the actual frequency implemented will have a major effect on security considerations.
* 2.4GHz systems - the numbers of devices that implement communications using 2.4GHz are wide, varied and increasing.
A simple demonstration of this is to search for Bluetooth enabled devices or search for Wireless Access Points in a busy city centre or conference facility using a laptop or PDA that are Bluetooth or Wireless Enabled.
Gaining access to these networks is often prohibited by the implementation of a number of security protocols.
* Basic network security - password protection and MAC address filtering are the simplest levels of security available that provide limited protection.
Hiding a network SSID (Service Set Identifier) also reduces the likelihood of gaining access to such a network.
However, the vast majority of industrial based wireless network equipment automatically hide the AP's SSID and disable DHCP to try and reduce any network leakage.
* Wireless encryption - WEP - the WEP, or Wired Equivalency Protocol, offers 64 or 128Bit encryption and was originally designed to work in conjunction with 802.11b.
Today, the flaws within the encryption are well known and various tools are available on the marketplace that enables either the 64 Bit or 128 Bit encryption to be cracked in around 15 min.
* WPA - Wi-Fi Protected Access (WPA) was generated to fill the holes that had been found in WEP.
In the majority of applications a pass phrase is generated that must be keyed into all devices that wish to gain access to a network.
In principle encryption keys are automatically changed and devices automatically re-authenticate after a set period.
* AES - the Advanced Encryption Standard (AES) is used in the full implementation of WPA 2.
This security protocol is to date un-breakable and therefore used in military and government applications.
* Overhead - as with all communication systems the more complex the security the greater the amount of bandwidth will be taken by the security protocols.
However, regardless of the overhead the level of encryption is directionally proportional to the perceived level of threat.
* 869MHz - unlike the 2.4GHz frequency the 869MHz frequency is by nature difficult to capture or eavesdrop.
As highlighted previously 2.4GHz units can easily be monitored by standard laptops and PDA's.
However, 869MHz is a frequency that cannot be readily be monitored by a standard laptop and PDA.
However, as each installation of Wireless Ethernet is proportional to the perceived level of threat the same levels of encryption should readily be made available within an 869MHz radio.
* Wireless network redundancy - if Wireless Networks are to flourish in areas where wired networks currently dominate the marketplace the final part of the key is the ability of any system to quickly recover from a network failure.
Spanning Tree Protocol can be implemented in many Wireless networks.
As per wired networks multiple paths or loops between network devices will cause a network to fail.
When STP is enabled each device on the network is given a number - the unit with the lowest number is usually the device that will manage the complete network.
The tree spans the complete network and ensures that only a single path between devices is enabled - the secondary path is blocked or enabled as a redundant path.
The network loops are therefore eradicated and the redundant paths are only enabled when a network failure occurs.
* Conclusion - it is clear that Wireless Ethernet is a valid option when considering implementation of a new network.
The costs associated with installing new cabling systems can be prohibitive and therefore wireless solutions can be a feasible alternative.
It has been highlighted that as long as the user fully understands the limitations of a Wireless network are based on the RF characteristics that cannot be altered, then a resilient system can be designed.
Either a 2.4Ghz or 869MHz solutions have pros and cons.
As previously highlighted understanding the basic RF design limitations will enable a user to understand and select the correct frequency for the application.
Once the correct frequency has been selected a Radio site survey will ensure that a system should work during and after system commissioning.
The correct placement of antennas will ensure a reliable system will function during possible environmental changes.
Finally, if network resilience is required the Spanning Tree Protocol should enable multiple paths between wireless devices to be managed correctly, therefore eradicating any network loops, etc * About the author - Ian Poulett is with Westermo Data Communications.
