Wi-Fi is a Passion


Site Survey specific scenarios

Every site survey is unique, so there are a lot of special surveying scenarios. As already mentioned there are surveys that you need a badge in order to access some areas. Surveying means that you need to walk everywhere, where access points are or need to be deployed. In an airport, this means also behind TSA or in baggage belt areas. As a non-traveller, you cannot just walk in and start walking around. This means arranging receiving a special badge or maybe an escort, or it may even require some extra training. There are warehouses or plants that require you have safety training or wear some PPE (Personal Protection Equipment) before you can do the survey.

Designing for multiple floors is not per se harder than single floors, but instead of the access points on the floor that can cause interference, access points from the below and/or above floor can interfere. This makes creating a channel plan harder. We know now that building materials can decrease the signal strength, but also in open rooms is some loss in dB. Free space path loss can be calculated by 20log(d) – 27.55 where d is the distance and the outcome will be a loss in dB.

For applications, you need some special requirements. The signal strength for Voice over IP needs a minimal signal strength of -67 dBm Dräger technology asks in some papers even -55 dBm, and other telemetry technology requires -65 dBm. Both voice and video need to use codecs to work and video can be real time (live) or buffered (streaming). In both scenarios QoS needs to be implemented. Multimedia packages come in varying sizes, long or short packages. Both have their benefits and disadvantages. For example, consider the total amount of management overhead. With smaller packages, there will be more management overhead than with bigger packages. When you have a big chunk of video data package and it gets lost, you miss that frame and that has impact on the quality of the video. With longer packages, latency will increase. Fragmentation is also a disadvantage of long package and a benefit for short packages, as smaller packages do not need to be fragmented. As you see, real-time networks are harder to design than ‘simple’ data networks.

Location services become also more important in some industries. In hospitals, you see that they tag mobility devices like infusing pumps, so they do not lose them in the hospital. For this can done with RFID together with triangulation techniques.

When you design for multiple channels, you do not just look at the -67 dBm value, but also where ends the signal. You want to look where you see the access point still at -85 dBm. Why this number? That is because of the Clear Channel Assessment (CCA) threshold.

With outdoor surveys and planning you need to take into consideration that the earth is not flat. After 7 miles, the earth bulge starts impeding on the Fresnel Zone. The first Fresnel Zone needs to be clear, and something as simple as a tree with leaves (in the spring) or without leaves (in the fall) has influence on the Fresnel Zone. Weather conditions can influence too, for example, does it snow a lot, or is there a lot of smog or fog around this area? Beside the point-to-point links or point-to-multipoint links, there is also mesh networks or hotspot-type networks. With hotspot-type of networks, a wire is at the location where the access point is placed. With mesh networks, there is a root mesh access point and couple mesh access points. Those access points do not have a wire cable, only a power cable. They are self-healing and will find the best path among the mesh access points. Mesh networks are more tolerant of failures as it is based on a standard 802.11s, but most of the time it is a proprietary implementation. The advantage is there are no cables because it can be hard to have cables in certain locations. It is possible to have a dedicated bridge link for outside hotspot, but that is not like a mesh network with multiple hops as seen below.