** Celebrating 20 Years of Wi-Fi ** pioneer Dr. Alex Hills discusses Wi-Fi performance issues and the importance of measurements
20 years ago today (September 15, 1999) is the day Wi-Fi was officially introduced. Today everyone -- from our grandparents to our toddlers – uses Wi-Fi! Good or bad, it exists practically everywhere indoors, some places outdoors, and it is complemented by cellular LTE signals almost everywhere. Dr. Alex Hills, the author of this post, is our distinguished advisor at Omny IQ. He is a pioneer in Wi-Fi deployments and a renowned inventor and expert on wireless technologies.
Dr. Alex Hills and his team built the world’s first large Wi-Fi network. As described in his book, Wi-Fi and the Bad Boys of Radio, the team overcame major obstacles to create the first wireless campus at Carnegie Mellon University. It was an unheard-of idea when Dr. Hills started the project in 1993. The new network, which came to be called “Wireless Andrew,” was the prototype used by many others to build Wi-Fi networks now in use around the world.
Our Omny IQ team includes early contributors to the Wi-Fi industry. Vivek, our founder / CEO, built with his team Netgear’s market-leading Wi-Fi business from its dawn back in 2000 to the global consumer adoption of Wi-Fi home networks. Our team of embedded Wi-Fi firmware developers and Wi-Fi analytics experts are technical experts in Wi-Fi, networking, analytics and data science.
Delivering optimal Wi-Fi service to all users, whether in a home, campus or office, continues to be very challenging. Fortunately, the industry continues to address these challenges, and advanced methods and standards are being introduced to the market.
Wi-Fi Performance Issues, Measurements and Problem Solving
by Dr. Alex Hills
The performance of a Wi-Fi network can be affected in a variety of ways by:
A wireless transmission is attenuated – that is, weakened -- by distance. Intervening obstructions cause additional reductions in signal strength.
The layout and construction of buildings influence the coverage areas of wireless nodes. Wi-Fi signals can be successfully used at ranges up to 300 meters in an open environment, but ranges are typically reduced to 20 to 60 meters indoors, where walls and partitions are part of the environment. Wood, plaster and glass are not serious barriers to Wi-Fi radio transmissions, but brick and concrete walls can have a big effect. The greatest obstacles commonly found in residential and office environments are metal objects like desks, filing cabinets, reinforced concrete, and elevator shafts.
Radio signals can also suffer “multipath” distortion when they are reflected by walls, furniture, equipment and other objects in the environment. When this happens, Wi-Fi signals follow multiple paths from transmitter to receiver, and numerous copies of the same transmission arrive at the receiver, each showing up at a slightly different time. The delayed duplicates can corrupt the direct (line-of-sight) signal, causing transmission errors.
Any or all of these problems can reduce a node’s useful coverage area and degrade the quality of the service received by clients attempting to communicate through the node.
Radio interference can be caused by transmissions from outside a Wi-Fi network. The source might be a nearby radar station or an unlicensed -- but legal -- radio transmitter. Microwave ovens can cause radio interference if they are not properly shielded. Interfering transmissions like these can degrade or completely obliterate a Wi-Fi signal.
Radio noise occurs naturally but also comes from man-made sources like electrical machinery, automobile engines and fluorescent lighting. Like radio interference, radio noise can degrade or obliterate a Wi-Fi signal.
Overlapping coverage areas and Wi-Fi signals received from nearby networks
When the coverage areas of a network’s nodes overlap, as in Wi-Fi mesh networks, Wi-Fi signals can be received from more than one node (access point) and/or client, and the result can be poor performance. Similar difficulties can be caused by Wi-Fi signals received from a nearby Wi-Fi network. These problems are related to the “multiple access” method used in Wi-Fi.
Congestion caused by more traffic than a node can handle
Congestion can occur in a Wi-Fi network when many Wi-Fi clients are attempting to send traffic through a single node. The problem is exacerbated by Wi-Fi’s multiple access method.
Multiple access in Wi-Fi networks
Because multiple Wi-Fi clients share a single channel to communicate with a node, a multiple access scheme is needed to coordinate transmissions. The scheme is called “carrier sense multiple access with collision avoidance” (CSMA/CA). This method is what is commonly used in connected devices like smartphones, internet speakers, streaming TVs, routers and gateways that are based on the IEEE 802.11ac and 802.11n Wi-Fi standards.
On a single channel, only one station -- a node or client -- can successfully transmit at a time. When transmissions overlap, a “collision” occurs, but CSMA/CA has a way to resolve some of the problems that might be caused by these collisions.
With the CSMA/CA protocol, each station listens before transmitting. If a station hears another one transmitting, it defers and waits until the channel is free. If two stations attempt to send at the same time, neither will hear the other and their transmissions will collide. When this happens, neither transmission is received correctly, and repeat transmissions are needed.
Many clients using a single node can cause frequent collisions. Multiple repeat transmissions are needed, and all users experience delays. This is why congestion caused by many clients trying to use a single node can be a serious problem in Wi-Fi networks.
The CSMA/CA protocol also explains why problems arise when a node and its associated clients receive co-channel signals from other nodes or clients in the network. When this happens, the node and its clients, following the CSMA/CA protocol, all defer. And, if a collision occurs between a transmission from a distant station and a transmission from one of the node’s associated clients, retransmission may be needed even though the colliding transmissions were intended for different nodes.
The same thing happens when co-channel signals are received from a station that is part of a completely different Wi-Fi network -- for example, a neighbor’s network. This is why it’s important to avoid situations where two co-channel nodes have overlapping coverage areas – a condition called “co-channel coverage overlap.”
The new IEEE 802.11ax standard, marketed as “Wi-Fi 6,” uses "multiple user – multiple input multiple output" (MU-MIMO) and "orthogonal frequency division multiple access" (OFDMA) techniques to improve Wi-Fi performance to levels not possible with earlier Wi-Fi standards. The new standard also uses better power control methods to avoid interference with neighboring networks.
Wi-Fi Measurements and Monitoring
Many performance problems can be eliminated or mitigated by measuring certain variables in a Wi-Fi network and making adjustments when needed.
As in any IP network, speed and latency are important. In a Wi-Fi network, it’s helpful to measure the speed (in Mbps) and latency (in msec) of the connection from an external location (e.g., an ISP router) to each node (AP router and extender) in the network. Thus, to the extent possible, we would like to be able to perform a speed test to each of these and a ping test to each to measure latency.
Further, because nodes in the network can become overloaded, it is helpful to measure utilization (in percent) for each of them. Utilization monitoring is important because high utilization can cause severe congestion due to the properties of Wi-Fi’s multiple access method.
And, as we have described, Wi-Fi performance problems often arise because of the radio environment in which the network operates.
To ensure good signal quality, it’s important to measure the strength (in dBm) of signals being received from clients by nodes and the strength of signals being received from nodes by clients. We would also like to measure the strength of signals being received by mesh AP routers and extenders from each other.
We would like to monitor the signal strengths (in dBm) and affected channels of interfering Wi-Fi signals being received from other Wi-Fi networks, which can cause poor performance if they result in co-channel overlap. And we would like to identify the source (e.g., SSID and MAC address) of the interfering Wi-Fi signals.
Since non-Wi-Fi signals can cause interference, it would also be helpful to measure the strengths and affected channels of non-Wi-Fi interfering signals being received.
And it would be helpful to know the noise level (in dBm) and affected channels of both natural and man-made noise being received within a Wi-Fi network.
Problem Discoveries and Remedies
With these measurements we are more effective at discovering network problems that can degrade performance and correct them. Here are some examples, which are being addressed by Omny IQ’s Preventive Care system:
A - Poor quality on a link between a network’s primary node and the ISP can cause performance problems (as indicated by speed and latency measurements). If the link speed is low and/or its latency is high, one should work with the ISP to diagnose and correct the problems on this link.
B - A Wi-Fi node can become overloaded if it has a high utilization. One way to remedy this problem is to move or steer some of the node’s associated clients’ connections to a different node.
C - The quality of the radio links that interconnect a router and its extenders in a Wi-Fi mesh network may be poor. This would be indicated by low RSSI and/or low SNR measurements on these links. Possible remedies are to increase the transmit power and/or receiver sensitivity of the router and extenders. Another remedy is relocation of the router and/or extenders so that they are closer together.
D - The quality of the radio links connecting clients to nodes are poor if low RSSI and/or low SNR are measured on one or more of these links. This situation may be corrected by steering clients experiencing poor signal quality to another node. Another remedy may be to increase the coverage area (by increasing transmit power and receiver sensitivity) of the node, but this step requires caution because it could cause another problem, excessive co-channel coverage overlap.
E - Channels being used for links between nodes (routers and extenders) and between nodes and clients are subject to interference from nodes in other Wi-Fi networks (due to co-channel coverage overlap), interference from non-Wi-Fi sources, and/or noise from man-made or natural sources. It may be possible to correct these problems by carefully changing the nodes’ channel assignments and then moving clients to the newly assigned channels.
Like seven earlier launches, the new satellites were lifted into the sky by a SpaceX Falcon 9 rocket’s 1.7 million pounds of thrust, and all those watching were cheering. See the video. Now Iridium has all 75 new satellites – 66 operational and 9 in-orbit spares – in their final positions. They’re providing worldwide coverage to aircraft, ships, vehicle and pedestrian users around the world – literally everywhere.