Using PBB-TE in 4G Wireless Backhaul Networks

In early 2007, IEEE 802.1 commissioned a project to standardize Provider Backbone Transport (PBT) as Provider Backbone Bridging – Traffic Engineering. Known as IEEE 802.1Qay, the effort will produce a standard defining enhanced Ethernet-based techniques for transporting services across diverse network topologies using MAC header encapsulation. PBB-TE has emerged to address current Layer 2 bridging limitations that relate to resiliency and scalability.

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 utilizing more capacity, pre-defining failover scenarios and optimizing service performance and assurance.

Benefits	Description
Improved resiliency	No customer MAC address learning in backhaul infrastructure No flooding of unknown MAC addresses Reduced likelihood of traffic storm Standards-based control plane = IEEE 802.1ag CFM, ITV-T Y.1731 Explicit primary and backup paths Enables fast and predictable failover
Improved scalability	Switches at each base station only learn attached customer MAC addresses and backhaul addresses (not transiting customer MACs) Only the POP-located backbone edge bridge, which terminates PBB-TE tunnels, learns all customer MACs
Improved service predictability	True traffic engineering Since 4G networks have configurable channel bandwidth (e.g., from 5-20 MHz), PBB-TE tunnels can accommodate a wide range of service types and bandwidth plans Configurable bandwidth for services and tunnels Committed Information Rate, Excess Information Rate Improves network utilization Optimized paths minimize frame delay (e.g., latency)
Interoperable, standards-based	Data plane = IEEE 802.1Qay PBB-TE Control plane = IEEE 802.1ag CFM, ITV-T Y.1731 Eliminates use of proprietary, vendor-specific protocols

The following diagram depicts PBB-TE equipment located in the POP and at each base station location. Redundant PBB-TE tunnels take divergent paths back to the POP to provide deterministic, reliable failover.

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 depicted in the following diagram. In this example, each base station has a primary and backup tunnel configured back to the POP.

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. This recommendation defines point-to-point 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 two 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.

Relevant 4G Mobile Standards

The following 4G mobile standards will benefit from utilizing IEEE 802.1Qay PBB-TE as a component of the wireless backhaul infrastructure:

• IEEE 802.16 Worldwide Interoperability for Microwave Access (WiMAX)
  • Fixed, nomadic, portable, and mobile wireless broadband connectivity without the need for direct line-of-sight to a base station.
• HiperMAN
  • WiMAX variation created by the European Telecommunications Standards Institute (ETSI) Broadband Radio Access Networks (BRAN) group.
  • Operates in the 2-11GHz range and is seamlessly interoperable with subset of IEEE 802.16a-2003.
• iBurst
  • Uses technology known as High Capacity Spatial Division Multiple Access (HC-SDMA), recently standardized by Alliance of Telecommunications Industry Solutions (ATIS).
• Long Term Evolution (LTE) – also known as UMTS release 8
  • UMTS-based wireless broadband Internet system with voice and other services added.
• Ultra Mobile Broadband
  • Improved CDMA2000 mobile phone standard for next generation applications and requirements.
• WiBro
  • Service name for mobile WiMAX in Korea market.

• Part 1:
  Introduction
• Part 2:
  Wireless Evolution to 4G
• Part 3:
  4G Network Characteristics and Requirements
• Part 4:
  Using PBB-TE in 4G Wireless Backhaul Networks
• Part 5:
  Summary