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发帖数: 2469 | 1 How To Fix Wi-Fi Roaming
Published: Monday, 22 January 2018 09:21
Written by Tim Higgins
Tags:BasicsHow ToWiFi
Introduction
In the beginning, the Wi-Fi gods created the router. And it was good, at
least for small spaces. But then the people discovered Wi-Fi dead spots and
became displeased with the gods, gnashing their teeth and shaking their
fists.
So the Wi-Fi gods created Wi-Fi extenders and all was good again. Until the
people discovered that their devices would not always use those extenders,
preferring to stay connected to their old, familar Wi-Fi router. And the
people were unhappy once again, feeling their gods had tricked them.
So much for allegory. So why won't your devices behave and connect to the
extenders or access points you've added to your Wi-Fi network? Or the shiny
and expensive new mesh Wi-Fi system you ripped and replaced your old setup
with? And why, when you seek help for fixing your sticky devices, do you
hear maddening stories from other people that say their devices roam
perfectly well using the same router/extender/Wi-Fi system you use?
What is Roaming?
Note: This article will use access point or AP to refer to the devices
creating your Wi-Fi network. This includes wireless routers, mesh nodes, Wi-
Fi systems, Wi-Fi extenders, etc. In the end, they all meet the definition
of an access point, i.e. a network device that enables Wi-Fi devices to
connect to a wired network.
STA (station) will be used to refer to wireless client devices.
Let's start by defining roaming. It is simply the process of moving an
established Wi-Fi network association from one access point to another
without losing connection. The process is illustrated in the diagram below,
borrowed from Revolution Wi-Fi's excellent article.
If the process is fast—often referred to as "seamless roaming"—the
transition can be quick enough to not drop a VoIP call (typically 50-150 ms)
or interrupt a video stream.
Wi-Fi Roaming
Wi-Fi Roaming
[Image credit: Revolution Wi-Fi
But not everyone needs "seamless" roaming. Many of us just want our devices
to eventually (multiple seconds are fine) switch to the closest AP when we
move from one room to another. Either way, the mechanics of roaming are the
same.
The roaming process consists of four steps. Each step is initiated by the
STA:
Looking for roam targets - This can occur when a STA reaches a scanning
threshold level (preemptive roaming --usually based on RSSI) or when the STA
decides to roam. Preemptive scanning is preferred to roam-time, since it
minimizes roam time.
Deciding when to roam - The STA typically monitors RSSI (signal level) to
determine when to roam. When the signal level of the current connection
reaches a set value, the STA starts the roaming process. Roaming can also be
triggered by packet retries or missed beacons. But RSSI is most commonly
used.
Deciding where to roam - This is the "secret sauce" of the roaming process,
so you usually won't find it documented anywhere. The target AP chosen is
typically the one offering the strongest signal in the current band. But
other roaming criteria are possible, including signal-to-noise ratio (SNR),
link rate, band and combinations of these.
Making the move - A STA must authenticate and associate with the new AP. If
the new AP has the same SSID as the old, this process can be sped up
significantly, minimizing the chance of connection drop. If the target AP
has a different SSID, there are more steps in the authentication and
association process, increasing the chance of a connection drop or glitch.
Devices Rule
The STA is the boss when it comes to roaming, making almost all decisions
about when and where to roam. But the AP can influence the roaming process
through the methods below. In all cases, however, the STA makes the final
decision on where to roam.
Delaying responses to STA authentication requests. This is typically used to
band-steer STAs by delaying the authentication probe response on the band
the AP doesn't want the STA to connect to. Since the STA sees the AP on the
preferred band respond first, it usually connects there.
Refusing to authenticate a STA. This is used to limit AP load, i.e. number
of associated STAs.
De-authenticating, i.e. forcing a STA to disconnect. This is usually done as
a last resort, for obvious reasons.
Providing an optimized list of APs to roam to, aka Neighbor Reports (802.11k)
Providing information about the traffic load on other APs (802.11v)
Speeding up the roaming process by providing fast authentication (802.11r)
Influence is the important word here. To paraphrase an old saw, you can lead
a STA to an AP, but you can't make it connect. You can also disconnect a
STA, but you can't (or shouldn't) make it stay disconnected. APs play
hardball with witholding authentication or enforcing deauthentication at the
risk of being blamed for being flaky. After all, the network was working
just fine before you changed to that fancy new router, wasn't it?
The other key concept is that 802.11k,v and r must be supported in both the
AP and STA in order to be used in the roaming process. 11r support is
particularly problematic, because some older STAs can't process 11r
information from APs and won't be able to connect at all if 802.11r support
is enabled.
802.11k and/or v also help minimize roaming times by doing a lot of the work
the STA would normally have to do by making its own scans. But their value
is more in influencing a STA's where to roam by putting more preferable APs
at the top of the lists provided to the STAs. But in the end, the STA still
decides whether and how to use the information provided by the APs.
Mesh Systems vs. APs
Contrary to what consumer Wi-Fi marketeers would like you to believe, Mesh
Wi-Fi Systems contain no magic to smoothly guide your devices to the best
connection. All the points in Devices Rule section above apply whether you
are dealing with a router + extender, router + AP, mesh Wi-Fi system or
multiple APs. The difference boils down to which roaming enhancement
technologies are supported, where they are implented and how well they are
implemented.
The only advantage a Wi-Fi system has over other ways to build a multi-AP
network is that it is designed as a system. So if mesh system supports 802.
11k Neighbor Reports, you can be sure all mesh points support it and
reasonably sure the feature has been tested. But since 802.11k depends on
STA support, there is no guarantee that your device, or mix of devices, will
smoothly roam.
This doesn't mean all Wi-Fi systems support all roaming enhancement
technologies or that all implementations of the technologies operate equally
well. Some products don't support 11k,v or r while others might support
only one or two standards and may throw in their own proprietary roaming "
encouragement" techniques that clash with those implemented in STAs.
In the end, "your mileage may vary"... a lot, when it comes to fast, trouble
-free Wi-Fi roaming. Assume nothing; check the specs! SmallNetBuilder is
here to help by tracking down roaming assistance information on all tested
Wi-Fi products and adding it to our Wi-Fi System and Access Point Finders.
We also hope to add it to the Router Finder, at least for newer products.
Don 't assume all new routers support 802.11k,v, or r, either, or any
roaming enhancements. After all, they're meant to be the only thing
providing Wi-Fi in your network. For example, ASUS is in the process of
adding its AiMesh technology to many of its newer AC routers, allowing any
combination of them to create mesh Wi-Fi networks. But when asked which of
its routers support 802.11k,v or r, ASUS replied that its routers support
only proprietary roaming methods.
Standardization
Forunately, help is on the way. The Wi-Fi Alliance announced its Wi-Fi
CERTIFIED Agile Multiband certification program in December 2017. Agile
Multiband certification is available for both APs and STAs and is built on
802.11k,v,r plus 802.11u and "Wi-Fi Alliance-defined technologies".
802.11u isn't relevant to home networks; it's intended to aid network-to-
network roaming. Wi-Fi CERTIFIED Agile Multiband Highlights describes the "
Wi-Fi Alliance-defined technologies" as "supplements information exchanged,
identifies preferred channels, bands or APs to increase intelligent Wi-Fi
network management".
You need to dig into the Wi-Fi CERTIFIED Agile Multiband Technology Overview
and Wi-Fi Agile Multiband Technical Specification v1.1 to get into the
details. But the key takeaway is the Wi-Fi Alliance is now providing a way
to ensure a common set of Wi-Fi roaming enhancements are implemented in a
CERTIFIED product.
It remains to be seen, however, whether manufacturers will incur the
additional time and expense required for this new certification. As is
typical, the first certified products are primarily reference designs. The
only Wi-Fi CERTIFIED Agile Multiband products you can actually buy right now
are Intel's Wireless-AC 8265 adapter and Ruckus' R710 Access Point and
ZoneDirector 1200 Controller. Not exactly consumer stuff. And remember, you'
d need certified APs and STAs to get any help from Agile Multiband.
What You Can Do
So it should be clear there are no sure solutions when it comes to trouble-
free Wi-Fi roaming. But you can increase your chances of success by
following these guidelines.
Don't Roam In The First Place - Don't assume that a multi-AP Wi-Fi system is
the right solution for you. For smaller areas, upgrading your router might
be a better solution than adding an extender to your existing router.
Upgrading most any N class router to any AC router will increase effective
Wi-Fi coverage, even for single and dual-stream Wi-Fi devices. And if you
already have an AC router, upgrading to a four-stream AC router might do the
trick. This is backed up by data; many of the top-ranked routers in the
Router Ranker have four-stream designs. (All testing is done with a two-
stream STA.)
Use A Wi-Fi System - If you can't get your desired Wi-Fi performance from
one Wi-Fi router, switching to a Wi-Fi system is more likely to produce
smooth roaming. Since they are designed with newer Wi-Fi chipsets, they are
more likely to support 802.11k,v or r roaming enhancement technologies. And
since they are designed as systems, all APs will support whatever roaming
assistance is designed into the product.
Roaming assistance is most effective when supported on all APs in a system.
This won't be the case if you add an extender or AP to an existing router.
Your old router is unlikely to support any of the newer standards and may
not support band-steering either.
Building a system from all the same make/model APs can also increase your
chances of smoother roaming. But, again, check the specs. Half of the APs in
our recent 2x2 AP round-up have no roaming assistance features.
Don't Overcrowd APs/mesh points - Too many APs in too small an area will
almost guarantee terrible roaming behavior. Some devices will never reach a
level low enough to trigger roaming. Others may bounce back and forth
between APs with similar signal levels, causing connection instability. So
you might not need all three mesh nodes that come in a three-pack "mesh"
system. And skip the temptation to buy an Orbi three-pack unless you have a
really big area to cover.
Don't Create AP Gaps - The inverse of the previous point is also true. If
there is too little or no overlap between access points, you'll get a
dropped connection. Fortunately, Wi-Fi systems are good at guiding proper
mesh node placement, as are today's Wi-Fi extenders. | m*******0
发帖数: 19 | 2 长文章给个链接就行了。
一语不发，上来就复制粘贴。
好歹给个中心思想，你的点评，你到底是支持还是贴上来做反例。 | f******o
发帖数: 2469 | 3 文化要学习 不懂你在说什么
还有贴链接会被杀
【在 m*******0 的大作中提到】
: 长文章给个链接就行了。
: 一语不发，上来就复制粘贴。
: 好歹给个中心思想，你的点评，你到底是支持还是贴上来做反例。
| S**C
发帖数: 2964 | 4 给个TR;DR吧
【在 f******o 的大作中提到】
: 文化要学习 不懂你在说什么
: 还有贴链接会被杀
| f******o
发帖数: 2469 | 5 不愿意读不学习 不勉强你
【在 S**C 的大作中提到】
: 给个TR;DR吧
|
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