Monday, April 10, 2017

Wireless Backhaul Best Practices

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:
  1. Minimize the number of hops, so as to minimize the total wireless latency and throughput penalty of the network
  2. 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.
  3. 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). 

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