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 point- to-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.

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.

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:

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

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.

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.

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

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.

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

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 UNII- 1 (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.

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.

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.

Figure 17: Operation Mode screen [EnStation2].

Figure 18: IP Settings screen [EnStation2].

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].

Figure 21: Wireless Network Settings screen [EnStation2].

1 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].

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).

Figure 24: Connections / WDS Link List screen to view list of connected access points [EnStationAC].

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.