Blueprint: Network Architecture

Backhaul Requirements of Emerging 4G and WiMax Deployments

by Greg Friesen, Director, Product Management

10/29/2007

Mobile WiMax and Ethernet based 4G networks are starting to be rolled out. These emerging deployments are being driven by the requirement for higher capacity mobile applications, and will be used to deliver mobile video, gaming, and data delivery, in addition to traditional voice. As these networks are being deployed, traditional T1/E1 backhaul solutions are no longer viable for the Ethernet based transport required for emerging services.

T1/E1 backhaul solutions not only have capacity limitations that are exceeded by 4G networks, they do not support the Ethernet transport requirements of 4G networks. This has caused many operators to consider wireless Ethernet backhaul for their 4G deployments. However, these new services are driving a completely new group of requirements on the wireless backhaul network. The key characteristics of the backhaul network which we will discuss and analyze how they can be delivered are:

Network Availability
Low Latency
Capacity Scalability
Network Reach
Total Cost of ownership
Enhanced Ethernet functionality

Path Diversity and Network Availability Levels

One of the primary requirements that all service providers have for existing and emerging services is high availability. Network availability requirements vary by operator from 99.9% to 99.999%. This wide range of availability levels must be delivered by the wireless network, with the 99.999% services being the hardest to deliver. In order to deliver this level of services, a new set of requirements is imposed on the wireless network. The wireless network must be in the licensed bands to avoid any potential interference which is unpredictable.

Once interference is eliminated by using the licensed bands, the two contributing factors to wireless service unavailability are the equipment and the air (rain fade) unavailability. The equipment unavailability can be hardened by using two redundant, parallel links, which protects against equipment failures. Lastly, high availability links need to be engineered. In some, cases when there are multiple links, it may not be possible to engineer the links to high enough availability to deliver 99.999% services. The best way of hardening these links is to introduce a ring configuration as shown in Figure 1.

This configuration will inherently also provide equipment protection. For wireless links, the major factor effecting path availability is the rain, however this can be reduced by providing diverse paths. The path diversity improvement factor (PDIF) is the measure of the air availability improvement through diversity. It provides a measure of the joint probability of two co-joined links failing simultaneously.

A PDIF of 5 or higher is quite common for a link of 10 Km. This results in the unavailability of the link being reduced by that factor. The service availability of a 99.99% path becomes 99.995%, and the unavailability of a 99.995% path becomes 99.999%. By introducing a mesh with 3 paths out of each node, the PDIF can be increased to10, further improving availability.

It is an important characteristic of the mesh network to be able to perform the ring and mesh switching in less than 100ms, to avoid service disruption. The effect of 1+1 and ring protection on service availability is shown in Figure 2.

Ultra-low latency Expectations for 4G Backhaul

The next key performance characteristic that is required of the backhaul network is ultra-low latency. The emerging set of voice and video services being delivered by 4G networks typically has a metro latency budget of about 10ms. 5 ms of this is usually allocated to the fiber network. This leaves 5 ms of delay for the wireless backhaul network. About half of this is allocated to the Ethernet switch layer, leaving about 2.5 ms for the wireless links. In an 8 node ring, this leaves a maximum latency of .3ms per link. This drives the requirement for a very low latency Ethernet system, and will be a key factor in backhaul technology selection.

The bandwidth intensive services that are being offered via 4G technologies will drive a new level of scalability. Today, 3G sites require ~10Mbps of capacity. A 3-4 sector WiMax site may consume as much as 100 Mbps. As multiple links are aggregated, this will quickly drive backhaul capacity requirements of 400 Mbps and beyond. To handle this capacity, important scaling features, including 256 QAM, and cross-polarization support will be required. With these, capacities of >800 Mbps can be delivered on a link. Another important part of this scalability, is the ability to increase the capacity remotely, without a site visit. This allows a cost effective, low capacity system to be deployed day one, when the network is first deployed. As customers are added and increased capacity is required, the backhaul network can rapidly scale, deferring costs until revenue is achieved. Software scaleable microwave systems, eliminate hardware swap outs and site visits, drastically reducing ownership costs.

The increasing capacity requirements of the 4G networks will drive the deployment of the higher modulation systems discussed above. With these higher modulation systems, there is a link budget reduction. In order to meet the link length requirements, new options must be considered. One of these options, is to deploy a higher output power radio head, which will provide increased link length through increased link budget. Additional link range can also be achieved through a technique called adaptive modulation, combined with priority queuing.

Adaptive modulation will switch from the current modulation to a lower modulation during a rain fade or multipath event. This will ensure the link is still active, but at a reduced throughput. By doing this, a service provider can engineer a link to 99.999% availability for QPSK (delivers about 70 Mbps in a 50MHz channel), while having a lower availability for the higher modulations. The adaptive modulation will work in co-ordination with priority queuing. During a modulation switch, the highest priority traffic will be serviced, while the low priority traffic will be dropped. This allows the high value services, such as voice to maintain very high availability, while the lower priority services will have a slightly lower but acceptable availability.

Backhaul Link Range Comparisons

The effect of these two techniques on link range, which can more than quadruple the effective distance is shown in figure 3, which is based on 99.995% availability in Denver, CO, for a 256QAM link. The links engineered to 99.995% with adaptive modulation will support 99.995% at QPSK, and 99.96% at 256 QAM.

Link range requirements are also reduced by adopting a ring architecture, will allow intermediate hops, rather than long, direct hub and spoke links. The ring architecture will also harden the availability of links, allowing them to be engineered to a lower individual availability, and therefore increasing their effective reach.

4G Backhaul Cost Factors

The 4G business case is very sensitive to the backhaul network cost. The wireless backhaul network cost in turn is dominated by the ongoing lease costs, rather than the upfront install and equipment cost. This makes it very important to minimize recurring antenna lease and indoor cabinet leasing costs. Figure 3 shows that through adaptive modulation, the antenna diameter can be reduced more than 2 sizes. This can reduce the monthly recurring antenna lease costs by more than 50%. Ongoing lease costs can also be drastically reduced by using an all outdoor system. The first step towards this is to use an all outdoor modem and radio, eliminating the need for indoor rack space for the modem. Of course the outdoor equipment must be fully weather and temperature hardened. The next step to truly eliminating any indoor requirements is to place the Ethernet switching, batteries, rectifiers, surge protectors, and fans all outdoors. By integrating all of these components into a weather hardened outdoor unit, all indoor space can be eliminated, providing tremendous recurring cost savings.

Enhanced Ethernet Capabilities for 4G Backhaul Network

The last important requirement of the 4G backhaul network is enhanced Ethernet functionality. This encompasses a few key items. The first item, is that it is important that the transport layer supports Jumbo frames, for protocols such as MPLS. The backhaul layer also needs to support flow control, with pause frames, to be able to provide back pressure to the Ethernet switching layer, when the link is being overloaded in capacity. The Ethernet backhaul network must also be fully layer 2 transparent in order to fully support 4G services and roaming capabilities Lastly, 802.1p and DSCP prioritization need to be supported in order to handle multiple traffic types.

In conclusion, it is apparent that a new network backhaul architecture must be considered in order to support the emerging 4G services. The key characteristics and resulting architecture requirements of this new architecture are:

Network Availability -- 50 Ms ring and mesh protection switching
Low Latency - Ultra-Low latency link
Capacity Scalability - 256 QAM, Dual-Polarization support and adaptive modulations
Network Reach -- Ring architecture, adaptive modulation
Total Cost of ownership -- Adaptive modulation, high power, All outdoor radio, all outdoor auxiliary equipment
Enhanced Ethernet functionality --Layer 2 transparency, prioritization, flow control, jumbo frames

Through this combination of features, a cost-effective, carrier-grade backhaul network can be built. This network will provide scaleable highly networkable Ethernet bandwidth that will enable the next-generation of 4G services.

About the Author

Greg Friesen is Director, Product Management for DragonWave.

About DragonWave

DragonWave designs, markets and supports broadband, wireless networking products for service providers and enterprises requiring reliable, predictable, interference-free, high-bandwidth transmission of real-time, IP applications. DragonWave products meet the demands of a wide range of applications as well as delivering a value proposition that enables operators and service providers an "invest as you grow" capability that leverages profitable growth. DragonWave is headquartered in Ottawa, Canada's high-technology capital.

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