Deploying a Point-to-(Multi)Point Backhaul Network

Deploying a Point-to-(Multi)Point Backhaul
Network
Gabriel Reyes
Field Application Engineering, EnGenius Technologies, Inc.

This document summarizes the procedure for configuring and deploying a point-to-point (PTP) or pointto-multipoint (PMP) backhaul network utilizing EnGenius hardware.

In this document, a wireless link is defined as a wireless connection between a root node and one (or
more) remote nodes. The wireless link uses a technology called Wireless Distribution System (WDS)
bridging, which dedicates a radio to communicate with one or more other radios, specified by the MAC
address of the remote radio(s). When in WDS bridge mode, the radio cannot be simultaneously used for
client access. All Layer 2 traffic parameters, most especially MAC address of downstream clients and VLAN tags, are encapsulated and preserved across the link.

Point-to-(Multi)Point Overview
Typically, wireless links are used for outdoor / multi-building applications, so the typical deployment uses
outdoor access points with directional antennas. Sometimes, omni-directional antennas can be used at
the root node, if multiple remote nodes are deployed in different directions. An example of this is shown
in Figure 1.

Figure 1: Depiction of a point-to-multipoint wireless link.

Indoor deployments of WDS Bridge links are done in scenarios where installing Ethernet or fiber cabling
runs is either impossible or impractical, but power at each of the remote AP locations is available. It is best practice to utilize the 5 GHz band for wireless link applications, to take advantage of fewer sources
of interference from access points on the 2.4 GHz band as well as the larger channel selection and capacity of 40 MHz channels on the 5 GHz band with 802.11n technology (e.g. EnStation5), and 80 MHz channels on the 5 GHz band with 802.11ac technology (e.g. EnStationAC). The use of the 2.4 GHz band for wireless links should be reserved only for scenarios where the property is isolated from remote interference, where there is no co-located Wi-Fi coverage, and where there may be trees or other objects that partially obstruct the Fresnel zone of the path (e.g. a farm or nursery utilizing remote cameras for surveillance).

By convention, the root node is defined as the upstream side of the link (i.e. towards demarc on the
property) and the remote / slave node is defined as the downstream side of the link (i.e. towards the
remote AP and/or camera). Unlike other vendors, however, there is no distinction in the EnGenius
hardware – both sides of the link are configured in the same manner.

A typical WDS Bridge link is shown in Figure 2. Parallel (i.e. point-to-multipoint) WDS bridge links are
shown in Figure 3.

Figure 2: Logical network diagram of a point-to-point link.

Figure 3: Logical network diagram of a point-to-multipoint link.

In larger deployments, wireless links can be daisy-chained / relayed in series, as shown in Figure 4. In this scenario, a wireless link shall connect the demarc building to an intermediate building (“hub”), and then one or more additional wireless links shall connect the hub to another remote location. This technique is useful at properties where all of the remote locations do not have direct line of sight to the demarc, and/or where the number of backhaul links need to be physically spread out across multiple buildings.

Figure 4: Logical network diagram of a serial point-to-point link.

When only one or two devices are connected on the remote end to a slave node, a PoE switch is not required and the devices can be linked through the PoE (802.3af) pass-through port on the EnStationACs secondary LAN port, as shown in Figure 6. Note that the EnStationAC itself is powered via PoE+ (802.3at), which can be supplied either by the enclosed PoE+ injector or a PoE+ (802.3at) switch.

Figure 5: Connecting a PoE (802.3af) device such as a remote IP camera to the secondary LAN port of an EnStationAC.

For the older EnStation2 (2.4 GHz 802.11n) and EnStation5 (5 GHz 802.11n) products, remote powered
devices like IP cameras can be connected by cross-connecting the data ports on PoE injectors. Examples
of this are shown in Figure 6 and Figure 7. Note that the EnStation2 and EnStation5 require proprietary
PoE injectors (24V) that come with the access points. Additionally, the EnStation2 / EnStation5 have an
auxiliary Ethernet interface which can be used to connect up to two devices at the remote end without a
switch, as shown in Figure 8 and Figure 9, or to connect up to two devices without a switch at an intermediate relay location, as shown in Figure 10.

Figure 6: Cross-connecting PoE injectors for a single remote device (e.g. remote IP camera).

Figure 7: Wiring configuration for one AP per slave backhaul link.

Figure 8: Wiring configuration for two APs per slave backhaul link.

Figure 9: Wiring configuration for two cameras per slave backhaul link.

Figure 10: Wiring configuration for two cameras per intermediate relay point backhaul link without a PoE switch.

In general, a dedicated radio shall be used in a design of a wireless link, connecting to remote access points or cameras. In instances where only a single remote AP and/or camera is required, a dual-radio access point can be used with the 5 GHz radio is dedicated to the WDS bridge link. Note that Wi-Fi clients will only be served on the 2.4 GHz radio in this case, as shown in Figure 11.

Figure 11: Logical design diagram showing a single remote AP connected via WDS on its 5 GHz radio. Clients are served on the
2.4 GHz band only for such APs.

Figure 12: Logical design diagram showing an omni-directional AP at the root location, for connecting multiple remote locations
that do not fall within the 18o directional envelope of a standard EnStation. Clients may optionally be served with Wi-Fi on the
2.4 GHz band from such omni-directional APs.

Configuration Best Practices
For point-to-multipoint networks, a maximum of eight remote nodes (EnStationAC) or four remote notes
(EnStation2 / EnStation5) can be configured to connect to a root node. It is generally best practice to keep
the number of remote / slave nodes to one or two per wireless link. This is done for several reasons:

               - It limits the total amount of traffic going over any particular wireless link
               - It minimizes the effect of an outage due to equipment failure
               - It leaves some design margin in case additional locations for remote access are required

In the absence of external interference for short distance links (under ½ mile), the following sustained
data throughputs are achievable:

               - EnStationAC (802.11ac, 80 MHz channel): 400 Mbps
               - EnStation5 (802.11n, 40 MHz channel): 80 Mbps
               - EnStation2 (802.11n, 20 MHz channel): 40 Mbps

At larger distances, the signal gets weaker and the wireless data rates get slower, leading to a lower overall data throughput. Signal is measured by the receive signal strength indicator (RSSI), which indicates how well a particular radio can hear the remote connected client radios. For point-to-(multi)point applications, the optimal RSSI on each end of the wireless link is between -40 dBm and -50 dBm to achieve the highest possible data rates. 

Setting Appropriate Transmit Power
The best practice is to pre-configure the radios with a transmit power of 17 dBm and validate that a link
is properly established (which serves to validate security and MAC address settings as well). Once the
access points are physically mounted in place, look at the RSSI readings on each radio and adjust the
transmit power settings on each side of the link up or down to get the RSSI to within the -40 dBm to -50
dBm range.

If the signal strength is greater than -35 dBm (typical for wireless links under 50 feet), then the electronic
amplifiers get saturated because the signal is too strong, which degrades throughput performance. In
such scenarios, turning down the power to minimum (11 dBm) may be insufficient, and if so we
recommend purposely misaligning the antennas.

If the signal strength is less than -75 dBm (typical for very long distance shots over 4 miles), it may be
difficult to sustain a link reliably or to achieve high throughputs, especially in the presence of external
interference. For long distance shots, EnGenius recommends using laser tooling to optimize the antenna
alignment so as to maximize the signal.

Setting Appropriate Channels
In non-rural environments, the entire 2.4 GHz band (channels 1 – 11) and the UNII-1 and UNII-3 portions
of the 5 GHz band (channels 36 – 48 and 149 – 165) are commonly used by dual-band access points for
Wi-Fi. These Wi-Fi access points can create significant co-channel interference (CCI) with point-to(multi)point links and degrade their performance. It is therefore recommended that wireless point-to-point links be established on the 5 GHz band and that channels are restricted to the UNII-2 and UNII-2e
portions of the band (channels 52-64 and 100-140). When utilizing multiple point-to-(multipoint) links,
please keep the neighboring channels independent and use an alternating sequence to maximize their separation. The following alternating scheme for adjacent / co-located wireless links is recommended. Note that halving the channel size will halve the maximum data capacity of the link, but will double the number of independent channels:

               - 80 MHz (EnStationAC): 100, 52, 116
               - 40 MHz (EnStation5/EnStationAC): 52, 100, 116, 132, 60, 108, 124
               - 20 MHz (EnStation5/EnStationAC): 52, 100, 116, 132, 60, 108, 124, 56, 104, 120, 136, 64,                                                                     112, 128

Considerations for Video Surveillance Applications
Point-to-(multi)point links are commonly used for surveillance applications, where one or multiple IP
cameras need to be positioned at a remote location. When designing such applications, it is important
that the bandwidth requirements per camera are understood, so as not to exceed the capacity of the
backhaul link. It is therefore important to consider the quantity of cameras, number of pixels per camera,
and frame rate. Furthermore, H.264 (or better) encoding should always be used to maximize video data
compression when utilizing wireless links.

When deploying IP cameras for remotely located video surveillance across a property, it is best practice to evaluate and explicitly quantify the average data rate per camera, which is based on number of pixels,
frame rate, and compression scheme (H.264 or better encoding is always recommended when using
wireless links). The total number of cameras per wireless link should be limited to 50% - 60% of the total
capacity of the link. This ensures that there will be sufficient bandwidth on the wireless link in case of link
degradation due to external interference effects, as well as leaving some design margin in case additional
cameras are added to the video surveillance design at the last minute or at a future point in time.

When calculating the throughput link budget of a wireless point-to-(multi)point system, it is important to
remember that all downstream hops have an additive effect on the total upstream throughput. An
example of this is shown in Figure 13.

Figure 13: Example link budget calculation for multiple point-to-point shots in series.

EnGenius Access Point Models Supporting Point-to-(Multi)Point
All EnGenius Electron APs can be configured in WDS bridge mode to establish point-to-(multi)point links.
For most applications, using models with internal directional antennas will be appropriate, though in some
cases, the use of omni-directional antennas is appropriate at the root to reach remote locations in opposite directions. The full list of EnGenius APs that can be configured in WDS Bridge mode are shown
in Table 1. The models highlighted in green are specifically designed for WDS Bridge applications and thus
recommended for most applications, though can also be operated as single-band access points.

Table 1: List of EnGenius Access Points capable of WDS bridge mode.

* Denotes AP models with external antennas
† Denotes AP models with internal direcOonal antennas

Design Example #1: RV Park
Figure 14 shows an RV park using EnGenius equipment to supply managed Wi-Fi. EnGenius Neutron
EWS860 outdoor access points are located at all of the poles with red pins (other poles in the park are
shown with yellow pins). There are approximately 40 client devices per access point during peak usage
times. All access points require point-to-multipoint backhaul to the main distribution frame containing
the AP controller, router, and Internet feed, which is in the Registration building at the lower right corner
of the property. There is fiber running to one distribution pole, with all other links being wireless utilizing
EnStationAC.

Figure 14: Design example of a point-to-multipoint network spanning an RV park to provide Wi-Fi and wireless backhaul.

For channelization on the 5 GHz band, the access points are all set with static 40 MHz channels on UNII1
(36 – 48) and UNII-3 (149 – 161). The point-to-multipoint links are all set with static 80 MHz channels
on UNII-2 (52 – 64) and UNII-2e (100 – 140) bands to maximize capacity. This is shown in Table 2.

Table 2: AP locations with individual static channel and power settings per band for RV Park application.

Design Example #2: Urban Surveillance
The following is a surveillance application at an apartment complex in Brooklyn, NY. There are 59
apartment buildings, each equipped with 16 cameras and an NVR. All buildings have line-of-sight to the
main distribution frame (MDF). A central security operations center is located in the MDF building, with
the requirement to be able to view multiple cameras across the complex. A series of point-to-multipoint
links are established from the main building to connect to the 58 other buildings, as shown in Figure 15.

Figure 15: Design example of a point-to-multipoint network spanning rooftops in an urban environment for surveillance.

Because of the data requirements, the use of 80 MHz channels was required. A shielded metal tower
was constructed to keep the neighboring EnStationAC units on the roof of the MDF isolated from each
other so that channels could be re-used, as shown in Figure 16.

 

Figure 16: Custom tower used to isolate neighboring EnStationACs on the roof of the MDF.

As the buildings were brick and the links were all above the roof line, the interference from Wi-Fi in the
apartment units was minimal so channels 36 and 149 were also used in this case. The complete
channelization plan is shown in Table 3. 

Table 3: AP locations with individual static channel and power settings per band for urban surveillance application.

Configuring the Access Points for Point-to-(Multi)Point Applications
To create a wireless link, the radio on the access must be configured for “WDS Bridge” mode. Once that
is done, the MAC address of the access point other side of the wireless link. For point-to-multipoint
applications, the MAC addresses of the multiple slave nodes shall all be specified on the root node. On
the slave nodes, only the MAC address of the root node is listed.

The configuration procedure is as follows:

     (1) Log into the Access Point
                 a. http://192.168.1.1
                 b. Default Username: admin
                 c. Default Password: admin
     (2) Go to the Operation Mode screen.
                 a. Set the access point into WDS mode (for dual band APs, do this for the 5 GHz radio)
                 b. Set the Country / Region
                 c. Provide a unique name for the AP

Figure 17: Operation Mode screen [EnStation2].

     (3) Go to the IP Settings screen. Provide a unique static IP address for the AP on your LAN.

Figure 18: IP Settings screen [EnStation2].

     (4) Go to the WDS Link Settings screen.
                 a. Security Type: AES (WEP and TKIP are deprecated. Do not use.)
                 b. AES Passphrase: <8-64 characters> (Choose a passphrase. Must match on both sides of
                     link.)
                 c. MAC Addresses: Enable and enter the MAC address(es) of the remote access points.
                    (Note, only the root node should have multiple MAC address listings for up to 4 slave nodes
                     when creating a point-to-multipoint link. When doing links in serial, use a separate slave
                     node for the first link and co-located root node on a different channel for the second link.)

Figure 19: WDS Link Settings screen [EnStation2]

Figure 20: WDS Link Settings of a 5 GHz root node wireless point-to-multipoint link in WDS Bridge mode [EAP1750H].

      (5) Go to the Wireless Network screen.
                  a. Set wireless mode: 802.11n or 802.11n/ac only (Disable 802.11a)
                  b. Channel HT mode:
                            i. 2.4 GHz radios: 20 MHz                                                                                                            ii. 5 GHz 802.11n radios: 40 MHz
                          iii. 5 GHz 802.11ac radios: 80 MHz                                                                                         c. Extension Channel: <ignore> (This is not relevant for 20 MHz channels, and will be
                     selected automatically for 40 MHz / 80 MHz channels
                  d. Channel / Frequency: <unique> (Choose a unique, non-overlapping channel for your band.
                      In large deployments, channels can be re-used but make sure they are not on neighboring
                      / co-located links. Also be sure to not conflict with your AP channels if deploying                                   802.11n/ac access points. If EnGenius has supplied a design, a recommended channel                             allocation is generally included. These should be considered “starting points”, as external                         Wi-Fi sources present at the site may necessitate selecting alternate channels with less                           interference.
                           i. 2.4 GHz Channels (20 MHz): 1, 6, 11
                          ii. 5 GHz 802.11n Channels (40 MHz): 36, 44, 52, 60, 100, 108, 116, 124, 132, 1401,
                                                                              149, 157
                         iii. 5 GHz 802.11ac Channels (80 MHz): 36, 52, 100, 116, 1321, 149

Figure 21: Wireless Network Settings screen [EnStation2].

Channel 140 (40 MHz) or 132 (80 MHz) is only available if channel 144 is available on the access point.         

Figure 22: 5 GHz portion of wireless Network Settings screen [ENH1750EXT].

     (6) Go to the Wireless Advanced Setting screen. Most settings here should be left on default unless
          you are specifically advised to change them by an EnGenius Field Application Engineer.
                 a. Data Rate: Auto [default]. This allows the AP to dynamically adjust the data rate based
                                    on the strength and condition of the wireless link.)
                 b. Transmit Power: 17 dBm [initial]. Must be set on both sides of link based on distance and
                                            any obstructions. When the link is established, the RSSI of the other side                                             should be between -65 dBm to -40 dBm. If the signal is weaker than -65                                               dBm, the data rate will not be optimal and thus you will not get the full                                                 capacity of the link. If the signal is stronger than -35 dBm, then the signal                                             is too strong, the amplifiers in the AP will be saturated, and the link                                                     quality will degrade. For very short links (<< 50 feet), minimal power                                                 level of 11 dBm should be set and potentially the links may need to be                                                 purposely misaligned to get the power level below -35 dBm.
                c. RTS/CTS Threshold: 2346 [default]. This is a protection mechanism to allow newer APs to
                                                talk to older 802.11 clients. Not used in WDS.
                d. Distance: 1 km [default]. For very long links, additional delays for                                                                     message acknowledgements are sometimes required. For most applications, this                                   can be left at default.
                e. Aggregation: Enable, 32 Frames, 50000 bytes [default]. Frame aggregation is a mechanism                                       used by 802.11n/ac to reduce overhead. Leave enabled at default settings.
                f. Wireless Traffic Shaping: Disabled [default]. This provides bandwidth limits per                                                                         SSID. Generally we want to maximize the capacity of the link, so                                                         leave this disabled.

Figure 23: Wireless Advanced Settings screen [EnStation2].

To see if the link is established and to verify the RSSI values, click on the “WDS Link List” tab (or
“Connections” tab on 802.11ac models).

Powering the Wireless Links
The EnStationAC is powered via 802.3at, and can be powered from any standard PoE+ (802.3at) switch or
injector. A PoE+ injector is included with the hardware.

All of the outdoor 802.11n access points with internal directional antennas (e.g. EnStation2, EnStation5, ENS500, ENS202) are powered via 24V passive PoE injectors, instead of standard 802.3af (i.e. PoE at 48V, up to 15.4W) or 802.3at (i.e. PoE+ at 48V, up to 30W) used to power network devices, such as access
points and cameras. A 24V PoE injector is included with the hardware.

EnGenius offers the EPD4824, which is a 48V PoE active to 24V passive PoE converter. This converter is powered from of a conventional PoE / PoE+ network switch, and thus does not require a separate power cord for the PoE injector. For applications where remote PoE switches are deployed for APs and cameras, the use of this converter for power is highly recommended.

Figure 25: EPD4824 converter to go from PoE 48V (802.3af) to PoE 24V (proprietary).

There is also a third party appliance, the POE24 from Digital Loggers, which provides an 8 port 24V PoE
injector with a web power switch for powering 24V devices in a 1U chassis. This appliance can be deployed in demarc and hub locations where several 24V PTP/PMP access points are located. This device can be configured to ping the IP address of the remote end of each link and power cycle the particular PoE port if the link is detected as offline:

Spec sheet: http://www.digital-loggers.com/poe24spec.pdf 
Web site: http://www.digital-loggers.com/poe24.html 

Figure 26: Digital Loggers POE24 8 port 24V PoE injector.

 

Check out EnGenius Forum about similar topic: 

http://community.engeniustech.com/topic/1022-deploying-a-point-to-multipoint-backhaul-network/