This blog post provides guidelines
on best practices for configuring and deploying wireless backhaul on Wi-Fi
networks, and goes through the differences between and appropriate scenarios
for client bridges, repeaters, WDS Bridge links, and mesh networks.
The Options for Wi-Fi Backhaul
In a conventional wireless
network, each access point (AP) requires a wired Ethernet connection to provide
backhaul to the wired network infrastructure and ultimately the Internet. In some environments, however, it is either
impossible or prohibitively expensive to run an Ethernet cable to each AP. In such cases, Wi-Fi itself can be used to
provide wireless backhaul from the AP (or other network appliance, such as a
remote IP camera) to the wired network. Each
Wi-Fi backhaul link is referred to as a hop,
and it is possible to have a chain of multiple hops between the remote
wireless AP to the root wireless AP that has a wired connection to the network.
There are multiple options for
providing Wi-Fi backhaul to the remote APs.
Naturally, each option has both benefits and limitations. Most critically, each wireless hop introduces
latency, which adds in a linear fashion with the number of hops. Repeaters and mesh also inherently lower with
throughput and user capacity, often as a square of the number of hops.
It is
critical to understand your technical requirements and constraints, as well as
the benefits and limitations of each wireless backhaul option, when designing a
Wi-Fi network and selecting a particular Wi-Fi backhaul approach.
Option 1: Client Bridge
An access point operating in Client
Bridge mode provides Wi-Fi connectivity for a wired client device. A Client Bridge is intended to connect an
individual wired client device to a Wi-Fi network. This is depicted in Figure
1.
Figure 1:
Example of a network utilizing a client bridge.
When multiple wired client
devices are connected through a single Client Bridge, they share the same MAC
address on the network, namely the WLAN MAC address of the Client Bridge
itself. The multiple wired client
devices can still be configured with different Layer 3 static IP addresses, and
each wired device may or may not be able to obtain an independent Layer 3 DHCP
address, depending on the DHCP server.
Best
Practice: When using an AP in Client
Bridge mode, only connect one wired client device.
For typical applications, Client
Bridge mode is only utilized on single-band APs. For dual-band access points,
one radio (typically 5 GHz) will be configured to operate in client bridge
mode, while the other radio (typically 2.4 GHz) will be used for providing
Wi-Fi connectivity on an independent SSID to wireless client devices. Client Bridge mode is generally only
available on standalone APs, meaning that each AP must be configured
individually and cannot be managed or monitored from a centralized
controller. Client Bridge mode is
available on all EnGenius® single-band Electron™ and EnStation™ access points,
as well as dual-band APs in the Electron™ ECB series.
Option 2: Repeaters
An access point operating in Repeater
mode provides both Wi-Fi connectivity to client devices as well as providing a wireless
backhaul connection to one or more wired APs.
This is depicted in Figure 2. Repeaters
are intended for very small networks (e.g. home environments), where individual
repeater APs are used to fill in particular coverage gaps. Individual client MAC addresses are
preserved, though the VLAN (if any) is defined by the main access point’s SSID
that is being repeated.
Figure 2:
Example of a network utilizing a wireless repeater.
For dual-band access points, one radio
(typically 5 GHz) will be configured to operate in repeater mode, while the
other radio (typically 2.4 GHz) will be exclusively for providing Wi-Fi
connectivity to client devices. Note
that both Wi-Fi bands depend upon the repeater radio for backhaul. Since the repeater radio must spend half its
time providing Wi-Fi connectivity to client devices and half its time providing
wireless backhaul, the data capacity of a repeater 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% at each 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.
Repeater mode is generally
only available on standalone APs, meaning that each AP must be configured
individually and cannot be managed or monitored from a centralized
controller. Repeater mode is available
on all EnGenius® Electron™ ECB series access points.
Option 3: Point-to-(multi)point WDS
Bridge Links
A dedicated pair of APs,
usually with integrated directional antennas (such as the EnGenius®
EnStationAC), are configured to operate in WDS Bridge mode to create a
point-to-point link to provide wireless backhaul. The WDS Bridge link on the remote end is
connected to the remote AP via its wired Ethernet interface. From the
perspective of the rest of the network, this wireless connection looks like a
wired connection; in WDS Bridge mode, the wired Ethernet frame is encapsulated
and encrypted in a Wi-Fi packet on one end, transmitted across the wireless
link, and then de-encapsulated and decrypted on the other end. Thus, all wired Layer 2 information (i.e.
client MAC addresses, VLANs, etc.) are preserved across the WDS Bridge link.
Point-to-multipoint WDS Bridge links ae also readily possible, though be aware the
remote links collectively share the total available airtime bandwidth of the
link. This is depicted in Figure 3.
Figure 3:
Examples of point-to-point and point-to-multipoint networks utilizing
WDS Bridge links.
The WDS Bridge links are
statically established, so that each WDS Bridge AP only accepts connections
from pre-defined radios. WDS Bridge
usually requires dedicated hardware at each remote location operating on
independent channels, though some APs allow for one radio (typically the 5 GHz)
to be in WDS bridge mode and the other radio (typically the 2.4 GHz) to be in
AP mode to provide Wi-Fi service client devices.
Best
Practice: WDS Bridge with dedicated 5
GHz only access points is generally recommended for most networks requiring
both wireless backhaul and high bandwidth and/or high user capacity Wi-Fi. While each hop
adds latency, there is no throughput or user capacity degradation, since the
point-to-(multi)point backhaul link is solely dedicated to wireless backhaul,
with Wi-Fi access for client devices being handled by separate access points.
For large networks consisting
of multiple remote nodes, a WDS Bridge backhaul network requires its own design
effort to ensure appropriate bandwidth capacity and channel utilization. WDS Bridge mode is generally only available
on standalone APs, meaning that each AP must be configured individually and
cannot be managed or monitored from a centralized controller. WDS Bridge mode is available on all EnGenius®
Electron™ and EnStation™ access points.
Point-to-Multipoint WDS Bridge Network Example
Figure 4
shows an example of an outdoor Wi-Fi network at an RV park utilizing
point-to-(multi)point links to provide wireless backhaul to APs mounted on
light poles. The colored lines indicate the
point-to-(multi)point WDS Bridge links implemented with EnGenius® EnStationAC
access points.
Figure 4:
Example of a wireless network utilizing point-to-(multi)point links for
backhaul to outdoor wireless APs.
Red markers indicate the
location of outdoor dual-band APs, and yellow markers indicate the location of additional
light poles that were available at the property. To maximize wireless backhaul capacity, all
of the WDS Bridge links utilized 80 MHz channels in the UNII-2 and UNII-2e
bands (i.e. DFS channels 52-64, 100-112, and 116-128). The 5 GHz radios on the dual-band APs were
set to use 40 MHz channels on the UNII-1 and UNII-3 bands (i.e. channels 36-40,
44-48, 149-153, and 157-161), so as to avoid co-channel interference with the
point-to-multipoint backhaul network.
Option 4: Mesh Networks
In a mesh network, the AP uses
its own radio to provide a wireless backhaul to other APs on the network,
eventually reaching an AP with a wired Ethernet connection to the wired
backhaul infrastructure and the network.
In this sense, a mesh network is a network of repeaters, though mesh is
designed to operate automatically and more intelligently on a large scale. A mesh network creates a set of “dynamic WDS
Bridge” links, using routing algorithms to automatically calculate the most
optimal wireless path through the network back to a wired root node. This makes mesh networks relatively robust to
the failure of an individual AP; in a process referred to as “self-healing”, the
routing algorithms will automatically calculate the “next best” path through
the network if an AP in the path goes offline.
Since the routing functions are done automatically within the mesh
software, mesh networks are actually fairly straightforward to set up and are
thus scalable to cover large geographic areas.
All wired Layer 2 information (i.e. client MAC addresses, VLANs, etc.)
are preserved across the mesh link. Examples
of mesh networks are shown in Figure 5 (for home / SOHO
environments) and Figure
6
(for larger campus-wide environments).
Figure 5: An example of a home / SOHO mesh network,
utilizing EnGenius® EMR3000 mesh routers.
Figure 6:
An example of a large campus mesh network, utilizing EnGenius®
EWS1025CAM mesh cameras.
The mesh network control architecture
can either be centralized or distributed. With a centralized control architecture, an AP
controller is required to calculate and coordinate the mesh parameters for each
AP. This architecture, however, limits
the scalability of the mesh network to the capacity of the AP controller. In a distributed control architecture, such
as the EnGenius® Neutron™ series and EMR3000 product, each AP operationally acts
like a router, continuously sharing information about its connection status to its
neighbors, and each AP uses this information to compute its own optimal mesh
path. In a distributed architecture, an AP controller can be optional, though
is generally extremely useful in providing centralized real-time monitoring of
the mesh network, as well as establishing the core initial mesh network
parameters, such as mesh ID, encryption, etc.
Unfortunately, mesh networks
have significant limitations, most notably in the loss of throughput and user
capacity, which scales geometrically as the number of wireless hops increase,
as well as the increase in latency, which scales linearly as the number of
wireless hops increase.
Accordingly, mesh networks are not suitable for high
bandwidth or latency-sensitive applications.
Because of these performance limitations, it is generally recommended
that mesh networks be avoided unless no other viable backhaul options
are available. Mesh networks should only be used in environments where
providing Ethernet data wiring to access points or cameras is impossible or
cost-prohibitive.
Mesh networks were originally
trendy in the mid-2000s, as a way of both providing metropolitan Wi-Fi coverage
as well as coverage for large outdoor properties where wiring was prohibitively
expensive, such as RV parks, garden-style apartment complexes, marinas,
etc. While many mesh networks were
successfully deployed, most of these efforts ultimately failed, especially in
metropolitan Wi-Fi. Early mesh networks
relied upon single-radio APs on 2.4 GHz using 802.11g. When dual-band APs were introduced, only
802.11a was available on the 5 GHz band, which still led to very low throughputs
as the number of hops increased.
With the wide adoption of
dual-band access points with 802.11ac, there has been renewed interest in mesh
for both Wi-Fi access and surveillance applications. Accordingly, several startup companies, as
well as established vendors like EnGenius®, have introduced mesh Wi-Fi products
utilizing 802.11ac. While the data rates
of 802.11ac are approximately 25 times larger than the 802.11a data rates of a
decade ago, the number of client devices and their bandwidth demands have also
grown exponentially during that time.
The fundamental limitations of mesh networks are therefore still the
same, and thus mesh may ultimately again prove to be a passing fad.
Nonetheless, mesh networks are
the only viable option in many cases.
The sections below highlight how to best design and deploy mesh
networks, so as to maximize their performance and mitigate their inherent
limitations.
Mesh Network Terminology and Best Practices
The access points in a mesh
network are categorized as either root nodes or remote nodes:
- 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.
The path from a particular remote
node back to a particular root node can require connections via multiple
intermediate remote nodes, and this wireless link in this chain is referred to
as a hop. The mesh routing algorithm selects the most
optimal route through the network. The
optimization function used by the mesh APs is generally proprietary to each AP
vendor, but typically attempts to balance several, often conflicting,
parameters, such as the following:
- 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.
Because of the competing
tradeoffs in this optimization process, mesh networks can often result in
counter-intuitive and/or sub-optimal topologies.
Best
Practice: The network design should cluster the APs
into groups consisting of up to four remote nodes that are only one hop away
from a root node. Thus, at least 20% of your APs, distributed roughly
evenly throughout the property, should be root nodes. Each remote node is therefore nominally only
one hop away from a root node. In the
event of a failure of a root node, the nearby remote nodes will then only be 2-3
hops away from another root node. This approach
generally requires creating additional root nodes, which can be done either by running
Ethernet or fiber-optic cable to the particular remote locations, or by
establishing dedicated point-to-(multi)point WDS Bridge links to create
“wireless wires” from the root AP back to the wired network.
Best
Practice: Each root node should be set on a static
independent channel, and each remote node should be set to “auto channel”. This
is done to maximize the airtime capacity of the overall network, so that
multiple neighboring root nodes do not create self-interference. The remote nodes are set to auto-channel so
that they can fail over to a different root nodes in the event of the failure
of their primary root node. When utilizing
point-to-(multi)point WDS Bridge links to establish root nodes, these must also
be on static independent channels, and thus must be accounted for in the
overall channelization plan.
Both root nodes and remote nodes
can generally operate in one of two modes:
- 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.
Best
Practice: Mesh APs should generally be
configured to operate in Mesh Point mode. The loss of
bandwidth capacity from lacking wireless 5 GHz wireless connectivity is minor
compared to the loss of bandwidth capacity from losing 50% of bandwidth per
hop. This also allows for the transmit
power of the mesh radios to be set at their maximum value, so
as to provide the maximum signal strength between nodes without being
imbalanced with the low transmit power capability of most 5 GHz client devices.
In both operational modes, the
overall data capacity of a mesh AP is reduced as compared to the same AP
operating in a conventional configuration with a wired Ethernet connection to a
wired switch infrastructure. Accordingly, a mesh Wi-Fi network will never have the same
level of throughput and client capacity of a conventional Wi-Fi network.
Mesh Network Example
Figure 7 shows an example mesh network
deployed using the Best Practices highlighted above. This is an RV park with 437 spaces spread
across a roughly 2000’ x 1000’ area. The
main distribution frame (MDF) is in the southwest corner of the property, and
trees in parts of the property preclude direct line-of-sight to many locations.
Figure 7:
Example of a mesh network, utilizing point-to-multipoint links to create
additional root nodes.
The red links and bubbles
indicate WDS Bridge links from the MDF to each of the root APs. In some cases, multiple WDS Bridge links in
series need to be established. The point
to point links are designated by Master or Slave with a letter and number
index. (For example, the WDS Bridge link
going between the MDF and G8-R is designated link D, with [Master D] connected
to [Slave D1]).
The other colors and bubbles
represent the root and remote APs in Mesh Point mode, and the nominal mesh
links between the remote APs and the root APs.
In the figure, each group is designated with a group number and an index
to indicate that it is a root node or remote node. (For example, in the right, the root node is
designated [G8-R] and the nominal remote nodes are designated [G8-1] to
[G8-4].)
The point-to-(multi)point WDS Bridge utilizing
80 MHz channels on the UNII-2 and UNII-2e bands (i.e. channels 52-64, 100-112,
116-128). Each root AP is set to a
static 40 MHz channel on the 5 GHz band in the UNII-1 and UNII-3 bands (i.e.
channels 36-40, 44-48, 149-153, and 157-161).
