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Using Pseudowire to Keep Up in Cell Backhaul

Mon, 12/12/2011 - 5:06pm
Chris York, product marketing manager at DragonWave

Wireless carriers evolve their networks, forcing cable to compensate.

Due to their extensive all-packet residential networks, cable operators have been in a strong position to address the burgeoning demand for mobile backhaul and business services. The biggest impediment to satisfying that demand is that cable networks built to support primarily residential video and data services do not meet the stringent service-level agreement (SLA) requirements of large enterprise customers.

The packet-based architectures adopted by MSOs mean they cannot directly interface with 2G/3G mobile networks or legacy business services; non-packet and packet-based networks are so fundamentally different that connecting them will always be a challenge.

To meet the challenge of supporting legacy TDM services for 2G or 3G backhaul, legacy business PBX, point-to-point leased lines, or ATM/Frame Relay services over a packet-only (Ethernet, IP or MPLS) MSO transport network, operators have two options: running parallel access networks – at significant cost and operational complexity – or deploying pseudowire technology, which allows legacy traffic to be transported transparently over a packet network.

Another important challenge faced by cable operators is in expanding the location and reach of their infrastructure. MSO networks have excellent coverage in residential areas near homes and small businesses, but their existing networks do not extend to most large enterprises or base station towers.

To expand their service coverage, MSOs tend to build out their fiber plant where it is economical to do so, as well as adopt high-capacity packet microwave solutions as a simple and cost-effective extension to their existing wired infrastructure.

MULTI-GENERATION MOBILE BACKHAUL
New mobile devices and advanced applications have driven a 500 percent increase in wireless data traffic over the last three years alone, triggering a major shift toward IP-optimized packet mobile networks. Yet, while the future of mobile networks is unquestionably packet-based, the majority of existing mobile base stations still have TDM interfaces.

Because wireless carriers will continue to support legacy 2G and 3G networks even as they roll out 4G technology, cable operators will continue to have to support both TDM and packet interfaces, which again recommends the two primary approaches mentioned above: running parallel networks or using pseudowire technology.

The benefit of running parallel or hybrid networks to support both packet-based traffic and TDM traffic with dedicated transport networks is that all traffic is handled in its native form without any protocol conversion. The downside is the operational complexity of managing two separate networks and maintaining the additional cross-connects, cabling and other infrastructure. This approach also results in significant idle capacity and reduced efficiency when compared to a single packet network.

The downside of parallel networks is making the use of pseudowires more attractive. Pseudowire runs over all common packet transport, including Ethernet, IP and MPLS. Pseudowire access gateways (as shown in Figure 1) offer MSOs a simple solution to supporting both TDM and packet interfaces to converge voice and data over a single packet-based network.

Pseudowire is a packet-based encapsulation technology that transparently handles TDM/ATM/Frame Relay services end-to-end over a packet transport network, without the need for complex conversion or bearer traffic protocol processing, and as a result is compatible with most existing Ethernet/MPLS switches and IP routers. In the case of cable operators, only pseudowire “end points” need to be added to an existing packet network to start deploying TDM services – the “cloud” between these end-points is essentially the same with or without pseudowire.

Pseudowire is also a well-established technology, extensively supported by standards organizations such as the MEF and BBF, and it’s quickly becoming part of the standard “packet networking” knowledge base of network design and support engineers.

Using pseudowire technology, cable operators can offer a flexible backhaul solution, with any combination of TDM or packet traffic, for a smooth migration to 3G and 4G networks. Operators maintain a single converged packet network with lower complexity, simplified installation and management, and reduced capital and operational costs when compared to running parallel networks with the added complexity and cost of TDM switching. While some operators are partial to maintaining separate networks due to their familiarity with legacy networks and protocols, cable operators have much greater expertise in operating packet-based networks, making a converged packet architecture a compelling solution.

Figure 1
Positioned for the future all-IP network, pseudowire systems deliver a packet-based architecture that provides complete support for TDM circuits, while providing the following key benefits:

  • Lower capital expenditures, maintenance and management costs
  • Simple migration to the next-generation network without compromising legacy traffic
  • Higher network utilization, lowering costs of transmission
  • Implementation of strategic investments in IP, MPLS and Ethernet infrastructures – freeing resources that are currently invested in maintaining legacy infrastructure
  • Advanced synchronization support and clock recovery mechanisms
  • Flexible future-proof architecture.

For nodal applications, cable operators can leverage multi-service packet node devices, which combine multi-generation voice and data and cell site switching within a single unit – reducing the number of devices needed at the cell site to reduce cost and increase overall efficiency.

An important consideration for many operators implementing a converged packet pseudowire solution is the need for highly robust synchronization. Existing 2G (and some 3G) cellular networks require precise synchronization information, which is traditionally delivered by their TDM connections. A packet-based solution must support advanced network synchronization via Synchronous Ethernet (SyncE), which locks the timing of the Ethernet physical layer, and 1588v2 or Precision Time Protocal (PTP), a Layer 2 standard for precision clock synchronization.

While these synchronization standards are of key importance to converged mobile networks, they do not, in and of themselves, define the clock recovery mechanisms – an essential element in converged networks. It is therefore essential that, in addition to supporting these standards, the converged packet solution must also provide a highly robust clock recovery algorithm to filter out packet delay variation (PDV), network impairment and other scenarios, including: packet loss and re-ordering, jitter, wander, heavy load, load step changes, load slow changes/bursts, latency changes and re-routing, network condition changes, and temperature changes. Any pseudowire solution should therefore be carefully evaluated to ensure it has a robust clock recovery implementation.

BUSINESS SERVICES OPPORTUNITIES
Cable operators have had great success selling Ethernet access services to business customers. The competitive cost points and high capacity of their offerings have made them a highly attractive option when compared to more traditional leased-line services. However, many businesses remain unaddressed due to their existing ATM, Frame Relay or voice systems, which do not interface directly with the Ethernet-based service offerings of cable operators.

To address this largely untapped market, MSOs can build out dual-access networks, which provide dedicated transport for their different service offerings, but – like in the case of a hybrid backhaul network – the high capital and operational costs, and long deployment horizons, make this a less attractive option for most cable operators.

Many cable operators are addressing this opportunity and their existing service offerings by deploying multi-service gateways. Multi-service access devices leverage pseudowire technology to enable operators to meet stringent mobile backhaul and business service quality of service (QoS) and SLA requirements, while offering an evolution path as they transition to next-generation packet networks.

Using these devices, legacy ATM, Frame Relay and TDM traffic are supported seamlessly by pseudowire alongside high-capacity packet connections. In addition, operators have the ability to introduce new E-Line and ELAN connectivity to business customers. Figure 2 illustrates the implementation of a multi-service gateway.

Figure 2
The benefit to businesses is a simple, highly flexible connection that can carry any mix of legacy or packet-based traffic in a completely transparent fashion. Cable operators gain new revenue streams while maintaining an efficient converged packet network that is optimized for all future high-capacity services and applications.

EXTENDING BEYOND THE WIRED NETWORK
Cable operators have significant fiber coverage within metropolitan areas, enabling them to deliver backhaul and other connectivity to a large number of sites. There are, however, significant new opportunities just beyond their wired network infrastructure. One such example is microcellular backhaul.

The surge in mobile demand is driving a need for many new mobile base stations and the use of microcellular underlay networks. The higher-density small cells used in these deployments operate below the clutter level to improve coverage, increase spectral reuse and offload traffic from the over-burdened macrocellular network.

MSOs are also in a strong position to offer backhaul to both existing base stations, as well as new microcell locations, but they must reach beyond their existing network to do so. The most obvious option is to build out their fiber plant – with nearly unlimited capacity and high performance, fiber is an excellent choice. The downside to fiber is the cost to deploy, which can easily reach $500,000 per mile. In the case of microcells, which are located in non-traditional locations such as lamp poles and traffic lights, fiber is certainly not likely to be a viable backhaul solution. This is where packet microwave comes in.

Cable operators are recognizing the complementary value of packet microwave as a cost-effective solution to extend their wired infrastructure to support mobile backhaul. Next-generation packet microwave systems offer many significant advantages for operators, including:

  • All-outdoor units for rapid installation and operational simplicity
  • Pure-packet or TDM + packet interfaces
  • Multi-gigabit capacity
  • Interference-free, licensed operation
  • Low latency for voice, video and future time-sensitive applications
  • Advanced traffic queuing and QoS support
  • Lowest total cost of ownership.

Packet microwave solutions are equally adaptable to other fiber or cable extension applications. MSOs can extend backhaul services into lower-density areas not currently covered by fiber, and business connectivity can be offered to new sites not currently being served. All of these opportunities can be addressed quickly and at much lower cost points than wired alternatives.

Figure 3
Cable operators are reaching beyond their existing service offerings in both mobile backhaul and business services. Emerging technologies such as multi-service access devices and pseudowires enable cost-effective multi-generation base station backhaul and a smooth evolution path for operators as they struggle with an ever-changing traffic mix. Multi-service gateways enable a host of new high-value and high-revenue business services.

Finally, packet microwave solutions extend coverage well beyond the cable operator’s existing fiber build to address new and existing backhaul opportunities and reach new business sites. With all of these high-value opportunities, MSOs can ensure the healthy growth of the business for many years to come.

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