It is imperative that WiMAX service providers build and maintain their own backhaul networks. It is unlikely that any other service provider of any type (fiber, telco, CATV) can adequately support the demands for a carrier grade WiMAX network given the needs for high aggregate bandwidth, independence from potential competitors and demanding Quality of Service and security needs.

Considerations

It is important to note that the backhaul network supporting an access network must carry the aggregate traffic of all subscribers at any one time. In an enterprise market, an example would be 1,000 data T1s (1.54 Mbps) equaling 1.54 Gigabits per second of aggregate bandwidth over the network. A residential application servicing 1,000 HDTV sets (one small US suburb) at 19 Mbps of aggregated bandwidth demand would equal 19 Gbps.

While some planners might plan for oversubscription that would be some fraction (say, one-tenth) of that 1.54 or 19 Gbps, a millimeter wave solution (60/70/80 Gbps) is still the best technology for that network and allows a good deal of flexibility in future proofing that backhaul network. The evolution of Wi-Fi presents a good historical example. The first Wi-Fi access points offered 2 Mbps of throughput followed by 11 Mbps (802.11b), 54 Mbps (802.11g) and soon almost 200 Mbps (802.11n?).

The challenge then becomes: how will that backhaul network be architected to deliver as close to 100% availability as possible? Failure of even one node could wreak financial disaster on the service provider.

Backhaul Architectures

Since the dawn of the telegraph, network planners have had to plan their networks to maximize reliability. This is largely a function of how the network is laid out or architected. The following paragraphs describe network architectures and their advantages and disadvantages.

The PSTN is best described as a star network where central offices are nodes on the network. For the technology of the time (copper wire connecting central offices), it was the most efficient.

Figure 1 A star network is not the best architecture for a backhaul network supporting WiMAX

A star network is vulnerable in that it offers single points of failure on those lines connecting the central offices. If any of those links should break (see Figure 1 above), the central office it serves and the tens of thousands of subscribers it serves will be out of service.

Mesh

Mesh architecture captures the high reliability envisioned by the early Internet pioneers. Mesh potentially offers advantages to the service provider including negating the need for a separate backhaul network and improved reliability. The reliability function of a mesh network is notable in that if one node fails, backhaul traffic could be routed around that failed node minimizing the service disruption to only those subscribers directly served by that failed base station. There is a lot of industry buzz around mesh networks and some success in the Wi-Fi market, however, mesh technology has yet to materialize in either the WiMAX access or the millimeter wave markets.

As illustrated in Figure 2 below (current Wi-Fi mesh products, for example), a pure mesh network architecture links every base station. That is, every base station radio that services the access network pulls double duty as a backhaul radio. For the WiMAX service provider planning a high capacity, high reliability network, a mesh architecture may not be the best choice of architectures. The chief detraction to this strategy is that valuable spectrum that should be used for subscriber access is being used for backhaul. Some industry analysts speculate that backhaul needs might consume 50% of available access spectrum.

A pure WiMAX mesh network remains an engineering challenge. At the time of this writing, no WiMAX mesh product has been commercially deployed. A complex routing technology must be built into the WiMAX radio. Many vendors claim that the marriage of a WiMAX radio and router at a WiMAX base station might adequately function as a WiMAX mesh. Few service providers have the in-house resources to do the necessary research and development to produce a WiMAX mesh solution.

Figure 2 Wi-Fi mesh networks require no separate backhaul network

This brings the discussion the "meshing" backhaul radios such as the millimeter wave products that compliment WiMAX network in their gigabit plus throughput and multiple kilometer ranges. The advantage of this would be that a) the backhaul would use spectrum separate from that being used for WiMAX access and b) the same advantages of meshing that apply to existing Wi-Fi mesh enhancing reliability by routing traffic around ailed links would apply. Figure 3 below illustrates a meshed backhaul network.

Figure 3 Backhaul network in mesh architecture

At the time of this writing, no mesh millimeter wave backhaul product is available on the open market. Millimeter wave vendors state their products are intended as bridges and not routers. That is, in Internet-speak, they link two points point-to-point (bridging) and do not perform a routing capability necessary to support meshing. Figure 4 below illustrates the technical difference between a bridge and a router.

Figure 4 A bridge (backhaul radio) is a point-to-point only function as opposed to a router that router that routes over a wider network

Just as no true mesh product has arrived in the WiMAX market, no mesh product is shipping in the millimeter wave backhaul market.

Ring

Of the backhaul architectures discussed above, the ring is the most popular among millimeter wave applications. The ring architecture is best known from SONET ring architectures popular in fiber optic networks. Figure 5 below illustrates a simple ring architecture.

Figure 5 A simple ring network architecture

The success of the SONET ring architecture lay in its consecutive point network technology. The logic in consecutive point technology is that the data flow in a network is either clockwise or counter clockwise in a circular pattern. If one link in the consecutive point network were to fail, the intelligence in the network immediately senses this and reroutes traffic in the opposite direction thus minimizing the impact of the failure on that link. Figure 6 below illustrates a consecutive point network.

Figure 6 A consecutive point network reverses direction of flow in the event of a network outage thus minimizing the impact of the outage

As a ring network grows, there is the potential for ever more subscribers (regardless of the percentage of the total number of subscribers) to be without service in the event of the failure of one link on the network. A ring or consecutive point network's availability (reliability) is enhanced by architecting it as a Figure 8. This way, in the event of failure on one link, the percentage of subscribers without service is further minimized.

Figure 7 Extending a consecutive point network into a Figure 8 architecture can improve reliability

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The "gotchas" of Backhaul: QoS and Availability


The two chief detractors from QoS are latency and jitter. Latency and jitter can multiply when traffic must route over multiple hops. That is, there is a "cost" for traffic for every additional hop it goes through. VoIP and video (potentially the bread and butter for the WiMAX service provider) are particularly vulnerable to latency and jitter induced when the packet stream must pass through the WiMAX base station and a number of backhaul radios until reaching the fiber point of presence (POP) connecting to a fiber backbone that is optimized to reduce latency and jitter using such technologies as multi protocol labeling system (MPLS). Latency occurs in both the wireless portion (over the air) and in radios and routers in a network. Ergo, the fewer hops a subscriber's packets have to make from the home or office to their destination (and vice versa).

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Figure 8 Latency calculation across a wireless network

In engineering a wireless backhaul network, the network architect should minimize the number of hops from subscriber to fiber POP. As Figure 8 above illustrates, latency is cumulative across any network. There is latency both in over the air links as well as in the wireless radio and across the IP backbone. Too many hops over too many links could add unacceptable levels of latency and jitter detracting from the QoS for VoIP and video.

High Availability

Perhaps the simplest means of ensuring the highest availability and reliability in a network is to build in redundancy where ever possible so that there is no single point of failure. In a wireless backhaul network, this is accomplished by doubling the radios in the network such that if one radio (link) goes down, its backup radio can immediately take up the load. While this may double the total price tag for backhaul radios, it is much less expensive than alternative
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Figure 9 Redundant connections to multiple IP backbones are critical to providing superior levels of availability in a backhaul network

It should be noted that having redundant radios in the backhaul network will do little good if the internet backbone provider servicing the backhaul network suffers a network outage. For that reason, the service provider should plan for multiple IP backbone service provider connections on their network. Not only does this provide a disaster recovery solution, it also presents the WiMAX service provider the opportunity to shop IP backbone providers for pricing and advantageous service level agreements (SLA). The WiMAX service provider can load balance between multiple IP backbone service providers for the optimum mix in pricing, SLA (reliability/availability), and geography (distance between fiber points of presence - POP).


Conclusion

This article explored the challenges of architecting a wireless backhaul network to support a WiMAX access network. That network must be able to support the bandwidth demands both current and future for both residential (think IPTV at 19 Mbps per TV) and businesses (multiple megabits per employee per business).

Any WiMAX access network must be carrier grade and completely and favorably with the PSTN's alleged "five 9s" of reliability. Network architecture must first be planned with an eye to providing the most "fail safe" backhaul network possible. This is accomplished by planning to minimize the percentage of subscribers affected by a network outage as much as possible.

As the network must support time sensitive application such as VoIP and HDTV video, the backhaul network must offer the optimum in QoS in the form of minimizing latency and jitter which is best accomplished by minimizing the number of hops between the fiber POP and subscriber.