• shorter links
• the need for more wired or wireless backhaul
• unpredictable service due to the large interference domain

To provide longer range communications, a PtMP or PtP system can be used. This is normally used for applications such as fixed wireless access to homes or businesses, and smart grid backhaul, especially in medium density to rural areas. But problems with this model include:

• a lack of redundancy due to each client normally seeing only a single base station
• many base stations are required due to single hop
• the need to engineer each link
• incomplete coverage (each client must have a direct path to a base station, so some installations may be completely obstructed, and the network must be built very densely to minimize this)

To address some of these issues with each type of system, an architecture started to emerge a few years ago which combined PtMP backhaul along with omni-directional mesh. However, not only does this require two different solutions and sets of equipment, but the many of the issues are inherited from each type of system. For instance, while a subscriber connecting to the short-range mesh may benefit from the many mesh nodes available to choose from, there is still a need to engineer the backhaul links and there are still issues around interference with the short-range mesh. The PtMP system would still need to be built very densely to provide sufficient coverage and in order to compensate for the single hop links, and there still may be coverage holes due to obstructions. Also, the PtMP system lacks redundancy. And although it may be possible to use multiple meshes to heal around back-haul outages, this requires complex dynamic routing to be run between the backhaul network and the short-range mesh, and requires multiple adjacent short-range meshes which may not be present in situations where the meshes are islands within a larger sparse network.

One of the reasons that we chose to implement dynamic antenna pointing was to address both of these network architecture issues by providing a single system that can do both long-range backhaul and short-range meshing. In fact, while our first internal testbed ran over 7 hops that ranged from 10 yards to 300 yards, our first customer deployment connected mountain tops across 20 mile links. Over the shorter links the dynamically switched antennas have the isolation needed to avoid interference and to provide spectral reuse, while over longer links the antennas provide the gain needed to close the links at a decent modulation. These very different deployments use the same hardware, same protocol and exactly the same configuration – the only difference is the deployment locations.

Below are snapshots of two live deployments, one mostly PtMP (with one SkyExtender relay) and one dense mesh. The PtMP system has a mixture of links, from short to several miles, while the dense mesh has links of mostly under 100 yards. These systems are running equivalent hardware (although DualBands are used in the dense mesh), the same software, with the same basic configuration.

In some rural areas there is even a hybrid model that some customers use with the SkyPilot equipment where pockets of dense subscribers, connected to each other using short links, are interconnected using long distance links. For example, there are areas of rural Germany where a single SkyGateway connects over long links to SkyExtenders in different villages, which then mesh over shorter links with other SkyExtenders and SkyConnectors within the villages.

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Issues with layer 3 (IP):

• Only IP is transported: no AppleTalk, PPPoE, broadcast device discovery, or legacy Ethernet devices such as serial/Ethernet telemetry…
• No virtual LAN services: offering layer 2 pipes for virtual LAN services has become an extremely important offering for many service providers. IP networks do not inherently support this service, and additional equipment or protocols need to be layered on top in order to support it.
• Many layer 3 systems only support IPv4: IPv6 is very different from IPv4, so support needs to be explicitly added, and IPv6 is critical for large networks such as smart grids.
• IP demarcation issues: the interface from the wireless access equipment must support whatever dynamic routing protocol the operator has chosen (RIP, OSPF, BGP-4, …). And, the operator may need to run a dynamic IP routing protocol in order to support client mobility (while an Ethernet system would allow learning switches to interconnect gateways for fast, transparent roaming.)
• IP multicast support (for some types of video streaming) needs to be explicitly supported. IP Multicast forwarding is very different than regular IP forwarding, and involves different protocols.
• Slower re-routing: compared to Ethernet table learning, IP dynamic routing is slow.

Issues with layer 2 (Ethernet):

• Scalability limitations due to a large broadcast domain and Ethernet learning table size restrictions of external switches.
• Inter-subscriber security concerns due to layer 2 attacks directly between subscribers (ARP poisoning, rogue DHCP servers, …).
• Subscriber-to-network security concerns from Ethernet MAC address spoofing and ARP poisoning.

Since layer 3 is a higher layer protocol than layer 2, it seems to become a question of limitations versus problems. Is it better to live with the limitations of an IP transport or with the problems of an Ethernet transport?

To deal with the issue of MAC address scalability, fortunately switch learning tables have greatly increased in size. And even if an Ethernet learning table overflows, the standard behavior is to replace the oldest entry, which is often from an inactive device. And data is still forwarded in any case, so the total number of devices supported on a network is much larger than the size of the switches’ Ethernet learning tables.

And to deal with both the large broadcast domain issue and lack of security between subscribers due to potential layer 2 attacks, many switches have a feature called “protected ports” (which SkyPilot has implemented as “Peer to Peer Control”). This feature can selectively block layer 2 forwarding between ports and VLANs of an Ethernet switch, or between subscribers within a virtual LAN within the SkyPilot system, in cases where the users of those ports or VLANs are not from the same administrative domain (for example, not employees of the same company). And since this control can be done on a VLAN basis, an operator can use this control to provide some groups of subscribers direct layer 2 access while limiting the layer 2 access of other users, such as home Internet subscribers, to only the router that leads to the Internet.

And even if users of different protected ports or VLANs need to communicate at layer 3 (for some cases of VoIP, gaming, file sharing, …) then several simple methods are available to allow that communication at layer 3 or above, such as the “local proxy ARP” feature of most routers or /30 IP subnetting at the subscriber level.

So with the control of protected ports and VLANs in both the wireless system and any external network switches, the potential of attacks between subscribers (such as ARP poisoning and rogue DHCP servers) can be completely avoided, and the only attacks left are attacks directly from subscribers to the network (such as to the first hop router). These layer 2 attacks fall into two specific cases: MAC spoofing and ARP poisoning. In both of these attacks one user intentionally mimics the Ethernet MAC address of another user, which causes a temporary Denial of Service (DoS). These can not effectively be used as data intercept attacks, so data is not compromised. And the denials of service are extremely short, especially in the case of MAC address spoofing where the attack only lasts until the real user sends a single Ethernet frame. And since the attacker is easily identified and their access can simply disabled, these attacks are not actually very common, and are ineffective.

And an alternative or supplementary tool that an operator can use to address many of these issues is filtering. SkyPilot devices support filters that range from the Ethernet MAC layer up to the IP port level. For example, instead of (or in addition to) disabling peer to peer communication using protected ports, an operator can simply configure UDP port filters to prevent rogue DHCP servers.

So by having addressed these Ethernet scalability and security concerns, the edge network can take advantage of the benefits of an Ethernet transport, including:

• Simple IP address management: IP addresses can be handed out in a number of ways (DHCP, static, PPPoE, …) and they can be assigned independently of the point of attachment.
• Support for any Ethernet device, such as IPv4, IPv6, IP multicast, NetBIOS, AppleTalk,  and legacy Ethernet devices.
• Seamless, fast intra-network mobility.
• Virtual LAN services (private LANs can be configured across the network).
• Simple layer 2 demarcation at the base-station (no IP routing protocol requirements).

An important aspect of Ethernet is that using it as a transport method does not mean a lack of IP services. IPv4, IPv6 and virtually every layer 3 protocol has an Ethernet convergence function, so if a device talks Ethernet then it can run over an Ethernet transport system without any special support from the network devices. And, even if a device such as a wireless mesh node provides Ethernet transport, it can also include an IP stack for its own communication, such as remote management. And IP-aware filters can be added to devices that are providing only an Ethernet transport service. So, Ethernet transport does not mean “no IP”.

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For omni-directional PtMP systems, the frequency re-use factor is basically how often a frequency gets reused within the overall network, and re-use factors of 3, 4, 7, 9 and 12 are common. A frequency re-use factor of 4 means that 4 different frequencies are used, with base-stations that have adjacent coverage each operating on a different single frequency, and each frequency is reused on each 4th base-station.

A problem with this type of network is that obviously many frequencies are required, which may not be possible in many limited frequency bands such as 3.65 GHz or 4.9 GHz. And since only a fraction of the total available bandwidth is used at each base-station, the capacity of each base-station is reduced. Additionally, since each base-station is only providing a single frequency, there is no frequency or base-station redundancy at the subscriber level, so interference or a failure of a base-station will cause a complete outage for any affected subscribers.

For sectorized base-stations, a frequency re-use factor of 3 is commonly used, and adjacent sectors do not use the same frequency. For instance, if a base-station has 3 sectors, each sector would be 120 degrees wide for 360 degree coverage and each sector would use a different frequency from a total of 3 frequencies. Problems with this architecture include:

• Lack of frequency diversity at the subscriber: a subscriber physically resides in one primary sector (and frequency), so if that frequency is being interfered with by a different base-station (in a licensed band) or a different network (in an unlicensed or “lightly” licensed band) then the subscriber could lose service. And if directional antennas are used at the subscriber, which is almost always the case in order to increase the link gain, then redundancy is not even availabe from other base-station locations.
• Lower antenna gain: the frequency re-use factor dictates the sector beam-width (antenna gain is directly related to beam-width), and, in order to get 360 degree coverage, wide antenna beam-widths are needed. And even if a single frequency were used multiple times on a single base-station (which usually requires some sort of coordination), such as in a F1,F2,F3,F1,F2,F3 pattern, each sector would still only be at most 60 degrees. In a dynamically switched antenna system, like SkyPilot’s, this constraint does not exist, and the antenna beam-width can be much smaller which results in higher antenna gain.
• Multiple frequencies are needed: just like in the omni-directional case, in some bands there are a limited number of available frequencies (or frequencies are expensive in licensed bands).  And in unlicensed bands there may not be multiple clean channels. And if a single channel is sub-divided, which many systems do not even support, each sector would only have a fraction of the total bandwidth.

With SkyPilot’s dynamic antenna switching, 8 high-gain 45 degree sectors are shared using a single radio, so a single frequency can be provided with 360 degree coverage while still providing the benefits of a high-gain antenna. Even though SkyPilot provides the resiliency of a mesh networking architecture, this spectral reuse flexibility has allowed many service providers to deploy large PtMP deployments in which each base-station provides synchronous connectivity to  low-cost subscriber equipment.

In situations where multiple channels are available, an omni-directional PtMP system loses any extra capacity that could have been gained, due to the required frequency re-use. By simply using multiple base-stations with the SkyPilot system, all of the additional channel capacity can be provided at each base-station location. And, each channel is provided over 360 degrees, compared to sectorized PtMP architectures which only provide each frequency on particular sectors, so with the SkyPilot equipment, there is frequency and base-station equipment redundancy to each subscriber (even if the subscriber uses a high-gain directional antenna).

And, of course, there is the additional benefit of meshing for additional range, routing around obstructions, and increasing system capacity by relaying through shorter high-modulation links (instead of wasting base-station bandwidth by communicating to a long range subscriber at low modulation, a high-modulation relay can be used).  But, these benefits are all extra, since even in a pure PtMP environment there is significant benefit from dynamic antenna switching.

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Focusing on that last claim, simultaneous transmissions and receptions over long-distance links simply do not work in the real world. This is based on physics. If a high-power radio is transmitting at +30dBm while another co-located radio is trying to receive a signal at -90dBm, then the +30dBm transmission will completely swamp the -90dBm reception. That’s a 120dB difference in signal levels, and to put that in perspective, the transmitted signal level is 1 trillion times stronger than the received signal level.

To combat this problem, multi-radio systems have traditionally tried using combinations of:

• filtering (but filtering is expensive and can not provide anywhere near the needed 100dB+ of isolation)
• physical separation of antennas (but using extremely long cables to externally mounted antennas is lossy, expensive, and doesn’t fit onto a pole – so it’s impractical to get the level of isolation required)
• increasing received signal strength by only allowing very short links (but this is still insufficient, and would only allow for very short links)
• lab demos (it’s common to see cabled multi-radio setups showing simultaneous active radios in the lab, but this is just smoke and mirrors where the cables are providing artificial path isolation and are allowing unrealistically high received signal levels)
• lowering modulation (where the radios are allowed to interfere and simply drop modulation and rely on CSMA – but then there is no capacity benefit to using multiple radios)
• requiring channel separation (but even skipping an entire channel is not enough, which you can see from radio vendors’ published “alternative channel rejection” values)
• and (in theory), trying to schedule around tx/rx situations (but, this requires symmetric upstream and downstream traffic, and is impractical)

Even if different channels are used, a typical wireless specification like 802.11a only requires adjacent channel rejection of up to 16 dB and alternate channel rejection up to 32 dB. So even different channels don’t help, and different frequency bands are sometimes recommended (for instance, one channel at 5.2Ghz and the other at 5.8Ghz) which is impractical due to power restrictions and availability.

Even the combination of many of these techniques is nowhere near sufficient to allow for simultaneous transmissions and receptions in a single device over reasonable link distances. So, while a multi-radio story might sound extremely compelling, and has somehow even found its way into some RFP requirements, there are many hidden technical challenges that make it not feasible in the real world.

And there is another important factor in the single versus multiple radio debate – in an “access network”, where the majority of traffic is flowing to and from a gateway node, the bottleneck is almost always the gateway. So, in order to increase capacity of the overall mesh, multiple radios can simply be used at the gateway (which I’ll talk about how to most effectively do in regard to SkyPilot devices in another post). So, if you happen to have access to a live mesh “access network”, try monitoring the utilization of all of your non-gateway radios versus your gateway radios, and this will show you how much money you’ve stranded on your poles.

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This diagram shows how a traditional single radio mesh system has its bandwidth reduced due to a large interference domain allowing only a single device to transmit at a time (note: the circles show the communication range, while the interference range will usually have a radius many times larger).

A multi-radio system could use several frequencies to allow multiple transmissions to take place at the same, reducing some of these interference conditions (however, not only does this require multiple clean channels, but there are some pitfalls that will be analyzed in a future post).

So an obvious question is, “How does SkyPilot’s dynamic antenna switching affect system capacity?” The answer is that even though the SkyPilot system uses a single backhaul radio, it can still provide 1/n the channel capacity simultaneously to each device due to the dynamic antenna switching.

In addition to all of the previously discussed benefits of dynamic antenna switching, such as higher link budget, interference avoidance and point-to-point power levels, the largest benefit is probably from something called “spectral re-use”. Basically, spectral re-use is a benefit of using dynamically switched high-gain antennas where multiple transmissions can take place simultaneously, on the same frequency, in very close proximity.

For example, the dynamic point-to-point link formed by the high-gain antennas allows a first-hop transmission to not interfere with a third-hop reception, even on the same channel. And while one first-hop device is relaying, spectral re-use allows many other devices to simultaneously communicate, such as allowing the gateway to transmit to another first-hop device. That is why we always recommend at least 2 first-hop devices. This allows the gateway, and most other devices within the mesh, to be continuously active, so the capacity of the overall system is equal to the capacity of the gateway radio.  This allows at least 1/n to be delivered to each device simultaneously, equivalent to the multi-radio mesh system and much higher than traditional single radio systems.

And by only consuming a single channel, additional channels can be employed in order to multiply overall system capacity (plus, it is often difficult to find the multiple clean channels that multi-radio architectures require). But, the use of multiple radios in context of traditional mesh networks and the SkyPilot system will be explored in a future post.

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• The radio’s transmit power
• The transmit antenna gain
• The receive antenna gain

For this discussion, we’ll ignore other common link budget parameters, such as cable and path losses, since we are looking at power and antenna gain on a particular link.

Most regions restrict a device’s EIRP (Effective Isotropic Radiated Power), which is essentially the radio’s transmit power plus the transmit antenna gain.  To comply with EIRP limits, each device must either reduce its radio’s output power, reduce its transmit antenna gain, or both.

Since SkyPilot’s products use high gain antennas, the radio’s output power must be reduced to comply with EIRP limitations.  Due to this, there is a common misconception that SkyPilot’s high-gain antennas are not beneficial in EIRP-restricted regions.

However, since the SkyPilot system uses high-gain directional antennas on both transmit and receive, the link budget is still increased due to the antenna gain.  Let’s look at an example by comparing two identical links, with different antennas, in a region restricted to 30 dBm EIRP:

• Link 1: radio output power of 24 dBm + transmit antenna gain of 6 dBi + receive antenna gain of 6 dBi = 30 dBm EIRP and comparable link budget of 36 dBm
• Link 2: radio output power of 12 dBm + transmit antenna gain of 18 dBi + receive antenna gain of 18 dBi = 30 dBm EIRP and comparable link budget of 48 dBm

So, even though the use of a high-gain antenna on link 2 resulted in the radio’s output power needing to be turned down by 12 dB in order to meet the 30 dBm EIRP limit, the actual link budget is 12 dB higher than link 1 due to the receive antenna gain.  In free-space, this would result in 4 times the range (or an increase in modulation, depending on how you want to spend the link budget).

To put it more simply, EIRP restrictions limit how big your mouth is, but not how big your ear is.  And of course there are still the other benefits of using directional antennas, such as causing less interference and being less susceptible to interference.

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Regarding interference, transmitting with a directional antenna reduces the interference caused to other devices. Receiving with a directional antenna reduces the amount of interference received from other devices. Both of these compound and allow for many simultaneous devices in an extremely dense area such as municipal and utility smart grid networks. It is common to have two pairs of nodes communicating simultaneously just a few blocks apart.

Capacity is also increased since multiple communications can take place simultaneously within the same network. For instance, while a gateway is communicating to one first hop node, other first hop nodes in the same network can communicate to second hop nodes. In this way, the gateway is always active and overall system capacity is increased, which is important for very dense networks with many users in a small area.

And since both long range communications and dense node clustering are enabled by the same underlying technology, these characteristics can coexist in a single network. Many of our customers’ networks contain a wide range of node densities and it is common to see long range links connecting dense pockets of users in downtown areas or apartment complexes, along with individual paths to office buildings or power substations – all in a single network. Since our protocol dynamically schedules communications and controls the pointing of directional antennas, it applies equally well to many levels of network density.

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• Clocks need to be synchronized: Devices participating in a synchronous protocol obviously needed synchronized clocks. This can be provided in several ways, including external clock sources such as GPS or over-the-air clock synchronization. SyncMesh uses a combination of the two, which leverages the accuracy of GPS clocks with the low cost of over-the-air synchronization.
• Clocks need to be very accurate: This usually requires expensive clock crystals that are accurate over a wide temperature range. SyncMesh provides an extremely accurate clock source by utilizing an over-the-air calibration protocol along with an internal calibration algorithm that maintains accuracy even with inexpensive crystals.
• Inefficiencies: Many synchronous, slotted protocols are inefficient due to their simple Time Division Multiple Access (TDMA) MAC layers, which assigns fixed slots to each user. To overcome this, SyncMesh uses a dynamic slot allocation scheme which assigns all slots in real time.
• Lack of interoperability with other systems: Since many outdoor wireless systems leverage unlicensed frequencies, multiple systems may need to share the spectrum. Carrier sensing systems may be able to (in theory) share the spectrum by avoiding simultaneous use, while more complex synchronous systems will probably not understand each other. However, we’ve already seen that carrier sensing has issues, and many systems ‘tweak’ their carrier sensing and back-off protocols to get an unfair advantage over other users of the spectrum. SyncMesh handles multiple users of the spectrum by pointing antennas – the high link budget point-to-point link can avoid interference from other systems, while its directional nature minimizes interfering with other systems. And with a dynamical directional system, if one path is not idle, others likely will be.
• Complexity: WiMAX-like synchronous systems are much more complex than asynchronous 802.11 systems. That is a large reason why WiMAX CPEs are more expensive than 802.11 clients, and why WiMAX base stations are significantly more expensive than 802.11 access points. SyncMesh has been developed over a period of 6 years and runs on top of off-the-shelf 802.11 silicon, which lowers cost.

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Some of the problems associated with carrier sensing (CSMA) and collision avoidance (CA) protocols include:

• Acknowledgment overhead: This is compounded over long distance links due to propagation time.
• Exponential back-off: This is compounded in outdoor networks, where re-transmissions are common due to interference, which causes latency to increase exponentially.
• “Hidden Nodes”: This is a classic problem with 802.11 CSMA, where carrier sensing at the transmitter does not sense interference at the receiver. This is greatly compounded in outdoor networks, where obstructions and long distances between the transmitters normally results in them not being able to hear each other.
• “Exposed Nodes”: This is a classic problem with 802.11 CA, where the RTS message between a transmitter and receiver causes other potential transmitters to become idle when they could have transmitted successfully to a different receiver. This is greatly compounded in a mesh network, where there are normally many active receivers.
• CA overhead: The collision avoidance overhead due to the RTS-CTS-Data-ACK exchange requires 4 propagation times, which results in large overhead on long-distance links.
• CSMA failures: In a small office or cafe, all stations can normally hear each other, which allows them to properly carrier sense and avoid collisions. In an outdoor wireless network, many stations can not normally hear each other, resulting in collisions which cause nodes to experience exponential back-off.
• Ad-hoc architecture: When connecting to an access point in a small office or cafe, all communications occur between the stations and the access point (which is called infrastructure mode) and not directly between stations. This means that most of the transmissions will never collide since all downlink transmissions are from a single device, the access point. In a mesh network using either ad-hoc mode or infrastructure mode there are many simultaneous transmitters and receivers, and all transmissions may collide.
• Unfairness: Another classic problem with 802.11 is MAC layer unfairness, and the problem greatly increases in outdoor networks. Due to the increasing back-off during retransmissions, nodes with fewer retransmissions are more likely to gain access to the channel than nodes that are retransmitting. Additionally, nodes that sense the channel becoming idle earlier are more likely to get access to the channel, and over long distances this results in unfairness to some nodes due to their location.

These problems are basic issues with asynchronous protocols such as 802.11, and all of these problems are drastically increased in outdoor wireless networks. Most people have experienced performance problems related to these issues in offices or cafes, but in outdoor mesh networks the impact of these problems is greatly increased, sometimes resulting in a complete collapse of the MAC layer.

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Regarding the latter, to avoid the challenges of dynamically pointing antennas, some multi-antenna systems use a separate radio for each antenna (or subset of antennas). This has several problems, with the most obvious problem being cost. Even though there is now the availability of inexpensive 802.11 radios, these radios have many hidden costs due to:

• amplifiers
• increased processing power and processor interconnect
• increased node size
• increased power consumption

However, there is a bigger problem with using multiple radios – self-interference. Even if the radios each use separate frequencies and guard bands (which is impractical due to the limited number of channels in many frequency bands), all radios interfere on some level. This can be seen by looking at an 802.11 radio’s published adjacent channel rejection values, which is basically the amount of interference from communications on an adjacent non-overlapping channel. The problems due to this self-interference are magnified by the characteristics of outdoor wireless, such as high levels of external interference and weak signal reception due to long links and high amounts of obstruction.

To address the issues of cost and limited channel availability, a reduced number of radios is sometimes used. For instance, some systems use 2 or 3 radios per node. However, a reduced number of radios means a reduced number of antennas, which means either very low gain antennas are used, or 360 degree coverage is not provided. Both of these restrictions are a large problem for an outdoor mesh system.

To mitigate the interference issues, the most obvious solution is to provide high levels of isolation between the radios and between the antennas. Traditionally, this would mean expensive filters and large amounts of physical shielding which is expensive and increases node size. However, it is impractical to cost effectively provide a sufficient amount of isolation in a mesh node, given typical outdoor wireless scenarios where the received signal may be under -90dBm while the transmissions might be at +30dBm. Adjacent, or even alternate, channel rejection along with filters and physical isolation are not enough to provide anywhere near the level of isolation required. So, interference between the radios is not addressed, and results in decreased link modulation and reduction in link range, which are the two main reasons one would use a directional antenna in the first place.

Another general technological issue with using a radio per directional antenna is that such a system can’t take advantage of steerable (adaptive beam-forming) antennas. Steerable antenna technology allows an antenna’s pattern to be electronically adjusted, so a radio per beam can not be used since there are no fixed beams.

All of these issues can be addressed by using a synchronous protocol to coordinate all transmissions so that a single radio can be switched among many antennas (or between beam-steering weights). And even though a single radio architecture may not seem to have the capacity of a multiple radio architecture, a multiple radio system can not take advantage of additional radio capacity due to self-interference. And, the real bottle-neck of a mesh network is almost always at the bandwidth injection point (gateway), which means the use of multiple radios in the majority of nodes in a mesh network is wasted money.

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