The telecommunications industry is evolving rapidly. Wireline carriers are making significant investments in fiber infrastructures to deliver business, transport and residential services, and Carrier Ethernet is emerging as an important access and backhaul technology around the globe. Wireless carriers are scrambling to keep pace with a growing demand for mobile Internet services, and wireless equipment vendors are developing fourth generation (4G) technologies that can provide IP-based, high-speed broadband services for fixed, nomadic, and mobile users.

As wireless carriers move to 4G mobile technology, huge demands are being placed on carrier backhaul infrastructure. The multiple, high-bandwidth, quality-sensitive services that carriers have planned for 4G technology require an infrastructure that is packet-based, scalable and resilient, as well as cost-effective to install, operate, and manage.

An innovative, connection-oriented Ethernet technology, Provider Backbone Bridging-Traffic Engineering (PBB-TE) 802.1Qay, is emerging as a key solution for addressing the enormous 4G backhaul infrastructure challenge. Currently being standardized by the IEEE, PBB-TE promises to provide the resiliency, scalability and operational efficiency that wireless carriers require.

First generation (1G) mobile systems were analog and focused only on voice traffic. Second generation (2G) marked the transition from analog to digital systems. Third generation (3G) mobile systems evolved to support more bandwidth-hungry services, such as email, text messaging, and image sharing. (See Figure 1.)

Figure 1. Wireless standards evolution.

Typically, 3G mobile networks require two parallel backbone infrastructures: one consisting of circuit-switched nodes, and one consisting of packet-based nodes. This network infrastructure doubles the capital and operational expenses associated with deploying, maintaining, and operating 3G mobile networks.

4G mobile networks require a single, all-IP, packet-based backhaul infrastructure, providing carriers with a significant cost advantage. However, the number of mobile devices and multitude of services, such as traditional voice, voice conferencing, image sharing, video, and high-speed data, strains the infrastructure.

Four Requirements for the Ideal Carrier Ethernet Solution
Several 4G network characteristics have been established by international standards development organizations and forums. These requirements and performance targets are shown in Figure 2, along with their impact on air interface and/or infrastructure equipment. Generally, 4G standards are characterized by superior bandwidth, which sacrifices some of the mobility attributes.

Figure 2. 4G requirements and performance targets.

While most of the target 4G characteristics directly relate to the family of air interface standards, many directly influence the backhaul infrastructure requirements. These include Scalability, Resiliency, Topological Flexibility, and Improved Economics.

Requirement #1: Scalability
Improved Customer Scalability: Each successive wireless generation has experienced significant customer growth. Some early 4G network markets have seen end station counts (measured in Media Access Control [MAC] addresses) that are 2 to 5 times higher than initial estimates. Therefore, the 4G wireless backhaul infrastructure must be able to support tens to hundreds of thousands of MAC addresses per market.

IP Transport: IPv6 is an important network layer technology for 4G networks given the number of wireless and mobile devices moving to IP-based services. A Layer 2 transport backhaul infrastructure using IPv4 for management enables use of IPv6 network layer scalability without requiring Network Address Translation (NAT).

Base Stations: Markets require diverse numbers of base stations/towers. The 4G wireless backhaul infrastructure must be able to handle growing base station counts while retaining address and customer scalability.

Requirement #2: Resiliency
Stability: As 4G networks are deployed and expanded, the stability during backhaul infrastructure expansion and maintenance is a critical issue. Current stopgap implementations are prone to misconfiguration, causing traffic storms and costly network outages. There must be resilient, reliable backhaul infrastructure stability.

Predictable Low-Latency Data Transmission: Voice and other services reliant on fixed circuit-switched network delay require packet-based, low-latency, predictable data transmission.

Multi-Vendor Interoperability: Legacy Ethernet implementations often use vendor-specific proprietary control plane protocols to attempt to solve diverse backhaul architectures.

Optimized Bandwidth Plan: Traditional Ethernet backhaul technologies use loop prevention control plane protocols, such as IEEE 802.1w Rapid Spanning Tree (RST). Often, half of the backhaul capacity/paths are disabled when these protocols are used. In order to maximize backhaul utilization, enhanced techniques to manage redundant paths and overall bandwidth engineering are required.

Deterministic Bandwidth Guarantees: Some network redundancy schemes result in overloaded paths during fault conditions. To provide deterministic bandwidth, 4G wireless backhaul infrastructure must have predictable failover and resiliency schemes.

Pre-Defined Failover Actions: Legacy Ethernet’s connectionless nature weakens bandwidth and Quality of Service (QoS) configurability.

Requirement #3: Topological Flexibility
Base Station Site Interconnect Technology: Wireless and mobile operators face myriad challenges when interconnecting base stations. In some cases, copper or fiber access is available. In many instances, microwave links are more economical and readily deployable.

4G mobile backhaul infrastructure must have the flexibility to accommodate wireline copper, fiber, or wireless microwave and free space optical connectivity.

Requirement #4: Economics
Cost Effective: Given the competitive nature of wireline and wireless operators, the backhaul infrastructure solution must be cost effective to deploy, maintain, and operate.

Simplified Provisioning: Since mobile networks are constantly evolving through expanding markets and growing numbers of base stations and customers, network and service provisioning must be simple yet powerful.

Automated Network Monitoring: While many legacy technologies like TDM contain extensive monitoring capabilities, traditional Ethernet lacks troubleshooting and fault detection. 4G wireless backhaul infrastructure requires network and service monitoring, as well as fault detection, isolation, repair, and verification capabilities.

Using PBB-TE in 4G Wireless Backhaul Networks
In early 2007, IEEE 802.1 commissioned a project to standardize Provider Backbone Transport (PBT) as PBB-TE. Known as IEEE 802.1Qay, the effort will produce a standard that defines enhanced Ethernet-based techniques for transporting services across diverse network topologies using MAC header encapsulation. PBB-TE, shown in Figure 3, has emerged to address current Layer 2 bridging limitations that relate to resiliency and scalability.

Figure 3. Mobile Backhaul using IEEE 802.1Qay PBB-TE.

PBB-TE eliminates the need for non-edge switches to perform MAC address learning and unknown address flooding. Instead, point-to-point tunnels are provisioned using a comprehensive management platform. Rather than using conventional Ethernet control plane protocols such as IEEE 802.1w RSTP and IEEE 802.1s MSTP to prevent loops and provide resiliency, the management platform traffic engineers the operator's network, which utilizes more capacity, pre-defines failover scenarios, and optimizes service performance and assurance.

Figure 4 depicts PBB-TE equipment located at the Point of Presence (POP) and at each base station location. Redundant PBB-TE tunnels take divergent paths back to the POP to provide deterministic, reliable failover.

Figure 4. Redundant PBB-TE tunnels.

The topological flexibility associated with PBB-TE enables 4G cells to grow and expand as market penetration and customer acquisition dictates. A logical view of the same 4G market is shown in Figure 5. In this example, each base station has a primary and backup tunnel configured back to the POP.

Figure 5. Redundant 4G PBB-TE tunnels.

Base station traffic is forwarded along the primary tunnel. Each primary tunnel is protected by one or more backup tunnels. Multiple techniques are used to provide efficient tunnel failover and service restoration in the event the backhaul infrastructure links become unreliable or inoperable.

Tunnel Resiliency Techniques
PBB-TE provides a variety of tunnel resiliency techniques. One technique involves IEEE 802.1ag Connectivity Fault Management (CFM) frames, which are known as Continuity Check Messages (CCMs). CFM provides network, path and service-level in-band management capabilities. Primary and backup tunnels are monitored using CFM CCM frames. Each tunnel endpoint sends CCMs at preconfigured intervals to monitor the status of the tunnel. A disruption in the reception of CCMs causes tunnel failover to occur. Base station traffic is then automatically switched to the backup tunnel.

Another technique involves ITU-T Recommendation G.8031/Y.1342, which defines Ethernet Protection Switching (EPS). This recommendation defines point-to-point Virtual Local Area Network (VLAN)-based protection schemes including 1+1 and 1:1 protection switching architectures.

The 1+1 protection scheme implies the base station traffic is permanently sent across the primary and backup tunnels. The tunnel endpoint discards the backup tunnel traffic until detection of a primary tunnel failure. The Automatic Protection Switching (APS) protocol synchronizes the 2 tunnel endpoints.

The 1:1 protection scheme signifies that the base station traffic is only sent across the backup tunnel upon detection of a failure. Again, the APS protocol synchronizes the tunnel endpoints.

While this recommendation is useful for basic point-to-point topologies, it is not intended for more complex topologies like multiple rings or mesh architectures, and will have limited applicability in 4G mobile backhaul infrastructures.

While the CFM resiliency technique has advantages, such as the ability to work across multiple rings and mesh architectures, its inherent scalability is often challenged. In order to achieve rapid failover in the 50-100 ms range, the CCM interval must be ~10 ms. Depending on the number of tunnels and services, a small CCM interval may overwhelm some networking equipment. Some implementations, in order to satisfy a given CCM interval demanded by the failover requirement, may sacrifice management plane responsiveness, such as provisioning, traffic statistics collection and other important tasks. Derivations of the CFM CCM approach include path-based failure detection and propagation. Such schemes may improve failover determinism without causing undue stress on the networking equipment.

Solution Found!
Wireless carriers around the globe are faced with increasing demands for new mobile Internet services. These growing service demands are driving a move to IP-based, high-speed broadband services that only new 4G technologies can provide.

However, wireless carriers implementing 4G mobile technologies are realizing these new technologies place huge demands on their backhaul infrastructure. Carrier Ethernet's innovative new connection-oriented technology, PBB-TE, is emerging as the ideal solution for meeting the demands of 4G technologies. With PBB-TE, 4G mobile operators can create a robust, packet-based backhaul infrastructure that is scalable, resilient, and more cost-effective to install, operate, and manage.

 About the Author
Taylor Salman is Director of Global Marketing for Ciena Corporation, with responsibility for Wireless and MSO markets. He has more than 20 years experience in telecommunications engineering, and more, covering a broad array of data networking technologies. He joined Ciena from RCN Business Solutions. Prior to RCN, he was Director, Business Development and Marketing for OPNET Technologies. Other prior experiences include engineering roles for wireless defense communications networks, network consulting at Booz Allen and Hamilton, and architectural analysis and development of a Big LEO satellite telephony system. For more information, email tsalman@ciena.com or visit www.ciena.com.

What’s your take on this subject? Leave a comment and get the conversation going.