Tuesday, January 2, 2018
Friday, November 24, 2017
What are Antennas
|Examples of increasing antenna gain. Top: Low gain dipole. Middle: High gain dipole. Bottom: High gain directional.|
- Just as antenna designers cannot build a perfect sphere, they cannot build a perfect cone. As a result, there is some amount of RF energy that is projected and received in the other directions. These are known as backlobes and sidelobes, as seen in the figure above. If two neighboring antennas are placed very close together, they can interfere with each other, thus a certain amount of separation distance (at least a few feet) is generally recommended when placing directional antennas next to each other.
- The beamwidth is defined by where the energy of the antenna drops by 3 dBi (i.e. half) of the peak. Thus, while the gain of the antenna is less beyond this beamwidth, it is also generally not zero, which needs to be accounted for in Wi-Fi design.
|Examples of different antenna types, with their typical potential for beamwidth and gain.|
APs with Internal Antennas
APs with External Antennas
Monday, October 16, 2017
What Has Happened
A Summary of How WPA2 Security Works
A Summary of the Vulnerability
How this Vulnerability Impacts Access Point Products and Networks
For More Information
Friday, September 8, 2017
Generally, three are two types of bandwidth throttling available on network equipment:
- Per-User Bandwidth Throttling: This limits the maximum amount of Internet bandwidth that each client device can consume
- Per-Subnet/VLAN Bandwidth Throttling: This limits the aggregate maximum amount of internet bandwidth that all client devices on the subnet / VLAN can consume at one time.
Unfortunately, setting the oversubscription ratio is an empirical exercise, and over time, the oversubscription ratio tends to decrease as more devices, each consuming more bandwidth, are connecting to your networks. In the pre-smartphone days, a 30:1 or even 40:1 was common for most wired / wireless networks.
Determining Appropriate Bandwidth Throttling Values
Of course, in reality complex networks are an Animal Farm (i.e. while all client devices are equal, some client devices are “more equal” than others). Thus, different classes of users will require different levels of service.
Tuesday, August 15, 2017
What is Tri-Band?
Why is Tri-Band Better than MU-MIMO?
|Multi-User Multi-In Multi-Out (MU-MIMO)|
- Increased overhead: The sounding frames and their responses consume airtime. While this is less than the presumptive gains of talking to multiple client devices simultaneously, it does indicate a loss. Most MU-MIMO access points only get a 1.7x - 2.2x increase in speed when talking downstream to three compatible client devices.
- Client device compatibility: The client devices need to be compatible with MU-MIMO in order to understand the sounding frames and to send the appropriate response. As of August 2017, there are still surprisingly few MU-MIMO compatible client devices on the market. There are some USB dongles available for PCs. The flagship mobile client device for MU-MIMO had been the Samsung Galaxy Note 7, which failed in the market for unrelated incendiary reasons. The Apple iPhone 7, while originally rumored to support it before its launch, quietly did not support MU-MIMO. Given Apple's notorious secrecy, we still don't know whether or not the upcoming Apple iPhone 8 will or will not support MU-MIMO.
- Client separation: MU-MIMO requires that the client devices it talks to simultaneously must be physically separated from each other. If the client devices are in too-close proximity, the beam forming won’t be able to successfully maximize the signal at one client and minimize the signal of the other (neighboring) clients.
- Downstream only: MU-MIMO only works for downstream traffic, from the AP to the client device(s). Upstream traffic from each client device to the AP must still happen one at a time, otherwise the AP will hear multiple client devices at once and won’t be able to distinguish between them.
The Takeaway Message
Monday, July 31, 2017
A Simplified Explanation of the Physics
A Simplified Explanation of the Math
- RSSI = Received signal strength indicator [dBm]
- d = distance of wireless link [m]
- f = operating frequency of wireless link [Hz]
- c = speed of light (i.e. 300 billion m/s) [m/s]
- PTx = transmit power of transmit radio [dBm]
- GTx = gain of transmit antenna, less cable losses [dBi]
- GRx = gain of receive antenna, less cable losses [dBi]
Monday, April 10, 2017
The Options for Wi-Fi Backhaul
Option 1: Client Bridge
Option 2: Repeaters
Option 3: Point-to-(multi)point WDS Bridge Links
Point-to-Multipoint WDS Bridge Network Example
Option 4: Mesh Networks
Mesh Network Terminology and Best Practices
- Root Node (a.k.a. Gateway Node): This is an access point with a wired connection to the wired switch infrastructure. The remote nodes establish wireless backhaul connections to the root node. Note that the wired connection utilized by a root node can either be (1) a direct Ethernet or fiber-optic connection to the wired switch infrastructure or (2) a wired connection to a separate WDS Bridge wireless point-to-(multi)point link on an independent channel.
- Remote Node: This is an access point without a wired Ethernet connection. Backhaul to the network is established via a wireless connection to a root node or to other remote nodes. Note that the remote AP still requires electrical power, so an Ethernet connection to a PoE injector is common, though the “network” end of the PoE injector may not be connected at all or may only be connected to a wired client device, such as an IP camera.
- Minimize the number of hops, so as to minimize the total wireless latency and throughput penalty of the network
- Maximize the signal strength of each hop, so as to maximize the achievable Wi-Fi data rates between the mesh radios on each hop. For maximum data rates in 802.11ac, the received signal strength indicator (RSSI) would ideally be in the -40 dBm to -50 dBm range, though this is usually unachievable in practice since omni-directional antennas are typically used to create the widest field of view to neighboring APs. Data rates should be above -65 dBm for decent data rate performance between hops.
- Balance the load on each AP, so as to account for the number of associated client devices and the total throughput consumption on each AP. The throughput load stacks as the number of hops increase, so intermediate remote nodes that are heavily utilized with client traffic will not give as many resources to downstream remote nodes.
- Mesh AP Mode: In this mode, the wireless radio acts like a repeater, providing both Wi-Fi connectivity to client devices as well as providing a backhaul connection to one or more remote APs. For single-band mesh access points, this is the only operational mode available. For dual-band access points, one of the bands (typically 5 GHz) will be configured to operate in this mode. The other band (typically 2.4 GHz) will be exclusively for providing Wi-Fi connectivity to client devices. Note that both Wi-Fi bands depend upon the mesh radio for backhaul. Since the mesh radio must spend half its time providing connectivity to client devices and half its time providing backhaul, the data capacity of the mesh radio for both backhaul and for Wi-Fi client connectivity is reduced by 50%. When there are multiple hops, the data capacity is reduced by 50% per hop. Thus, for two hops, the total data capacity is only 1/4, for three hops it is 1/8, for four hops it is 1/16, and so forth.
- Mesh Point Mode: In this mode, available only in dual-band APs, the wireless mesh radio (typically 5 GHz) only provides wireless backhaul, and the other radio (typically 2.4 GHz) only provides Wi-Fi connectivity to client devices. Operationally, the mesh radio operates like a dynamic WDS bridge link, so while each hop still introduces latency which adds linearly, there is no 50% throughput penalty per hop, since the mesh radio is not also servicing client devices on the same radio and can be devoted exclusively to backhaul. Since Wi-Fi access to client devices is restricted to only one radio (typically 2.4 GHz), the overall client capacity of the AP is that of a single-band AP. Furthermore, even dual-band 802.11ac client devices will only be able to connect at 802.11n data rates on the 2.4 GHz radio.