Archives

April 2001 Issue
Intelligence to the Optical Edge: Smarter Routers Manage Traffic Waves
By

Hybrid fiber/coax (HFC) networks are now carrying more data traffic, which requires a variety of quality of service treatments. Optical networking combined with smarter edge routers can help solve this problem.

Cable operators are facing significant, potentially disruptive challenges as the volume of data traffic continues to increase at an exponential rate and the character of the traffic itself evolves from "best-effort" data transmission into the delivery of multiple services—each with its own quality of service (QoS) requirements. To transport this growing volume of traffic, cable operators are aggressively building out the optical infrastructure.

Increasing channel capacity along with rapidly falling costs-per-channel make optical transport networks both economically viable and attractive. The current generation of dense wavelength division multiplexing (DWDM) equipment is capable of supporting over 100 channels on each strand of cable, and each channel can support up to 10 Gigabits per second (Gbps) today, moving to 40 Gbps in the near future. To simplify and scale the delivery of multiple services and cope with constantly increasing traffic volume, the intelligence in the network must shift from the core to the edge. This requires high-performance optical edge routers that enable sophisticated and distributive traffic grooming and forwarding to take place at the periphery of the network.

Optical network evolution

Many current carrier networks consist of an underlying synchronous optical network (SONET) transport network with a hierarchical Internet protocol (IP) routing overlay. The transport network typically is bu; ; ieilt from tiers of SONET rings. This provides a time division multiplexing (TDM) infrastructure well suited for aggregating voice transmissions with predictable traffic patterns and well-defined data rates, but much less efficient for bursty data traffic.

The IP infrastructure is built with a hierarchical overlay infrastructure of routers. Various routing protocols are used to establish a mesh of logical, high-speed, point-to-point connections between routers over the underlying infrastructure. End-to-end IP connectivity is achieved through multihop packet forwarding along the paths between routers. Management and control is segmented at different layers of the hierarchy and also at different administrative domains within these layers. Such segmentation means that changes to the infrastructure must be coordinated between multiple entities and, as a result, are time-consuming and error-prone.

Next-generation optical networking

Operators can take advantage of the latest technical developments to overcome the limitations of current networks. A next-generation architecture is required. A promising candidate for this is based on deploying intelligence at the network edge with a less intelligent high-speed optical switched core. Each traffic flow can be classified at the edge of the optical network and assigned a different lightwave—or "lambda" l—for swift and efficient routing to its end destination.

The core is a l-based transport network constructed as a mesh of optical paths with IP routing used to select the best path between edge routers. The optical path comprises a combination of ls and physical fiber-optic cables. Operators can either use add/drop multiplexers (ADMs) to provide access to SONET rings, or deploy optical switches in headends for direct access to a flat optical network. This approach eliminates SONET ADMs and IP routing and switching overlays within the core, and results in an IP service delivery infrastructure with a single level, single hop logical mesh interconnection of IP routers.

The edge routers and optical mesh infrastructure work in concert to achieve end-to-end service flow definition and management. Engineers define QoS levels for given paths across the optical core and perform traffic engineering using lambda switched paths in the same manner as is done for multi-protocol label switched (MPLS) paths. The edge routers perform per-service flow policing and forwarding, and they map traffic to the appropriate lambda switched paths based on addressing information, QoS requirements and routing policies. Extensions to existing IP routing protocols are used to exchange the information required to establish the optical paths.

This approach allows the core optical network to be protocol-independent, with the edge devices providing translation and encapsulation as required for the transport of IP, asynchronous transfer mode (ATM), MPLS and native video streams. These next-generation optical networks will allow carriers to create a high-capacity core network that can be fed from cable infrastructure, digital subscriber line (DSL), wireless access or optical access networks. These local access networks can be connected to the core network through the optical edge routers using conventional IP routing, ATM or MPLS.

The high-performance optical core

Increased intelligence at the network edge allows carriers to build protocol-agnostic optical core networks that implement a relatively limited set of functions. Although the core network could be constructed as a SONET-like ring hierarchy, a mesh topology is expected to be dominant. A flat mesh offers cost advantages because redundant path ratios are 1:N rather than 1:1, and mesh topologies are easier to upgrade because these upgrades can be performed link-by-link without the need to upgrade an entire ring.

The primary function of the optical core is to provide high-speed optical transport of one or more wavelengths between edge routers. To achieve this, the core also must maintain a mechanism to manage paths between the edge routers. Mechanisms for fault detection and path restoration are required to support the high levels of availability needed. Alternate paths established to allow for fast-path switching following the failure of any given paths may be maintained as multiple routes to a destination by the IP routing protocols, with the lowest-cost path usually active.

On detection of a path failure, traffic can be diverted to the alternate route immediately by the edge router without needing to wait for network-wide route convergence. Faster restoration can be provided by physical restoration techniques at the optical layer. Efforts are in progress to create standard mechanisms to enable edge devices to control end-to-end paths across the optical network.

The intelligent optical edge

In the next-generation optical network, edge routers will require the intelligence to classify packets and police traffic flows in real-time. This requires high-performance packet processing to support classification and forwarding of hundreds—or even thousands—of traffic flows. High-density, carrier-class edge routers are required so traffic flows from Data Over Cable Service Interface Specification (DOCSIS) access networks can be efficiently classified and routed either onto SONET rings or directly onto the optical core via MPLS, ATM virtual circuits or light paths. If traffic between the two endpoints does not justify a complete l, then the edge router may use an additional multiplexing layer such as MPLS to create an access hierarchy.

With carrier-class edge routing, operators may deploy increased intelligence at the edge of the hybrid HFC network and make the edge network core-agnostic. Operators can then evolve the core and edge networks independently as newer technologies are introduced.

Multiple system operators (MSOs) increasingly need to map traffic flows across both the cable network and the core optical networks of multiple service providers. This requires intelligent routers at the edge of the network to mark packets for these core networks according to differentiated services, ATM or MPLS standards and deliver them to the service provider point of presence via the optical network. This allows operators to provide end-to-end QoS treatments across core networks and backbone networks of their revenue sharing partners.

Traffic flow aggregation

The optical edge router must act as an aggregation point for IP traffic from the access network. In the access network, QoS will need to be applied on a per-flow basis. In the core network, aggregated QoS will be used. Therefore, the edge router must map individual flows into macro flows to which backbone QoS treatments can be applied. A macro flow consisting of multiple individual flows with common characteristics is mapped to a traffic-engineered label switched path or lambda switched path for the appropriate destination edge router. This allows multiple-access networks to use a common core mechanism. Traffic received from the core network must be mapped into individual flows for transmission to the access network with the required QoS. This requires the edge router to be capable of classifying traffic at the line rate of the core interfaces, which demands hardware-based, carrier-class performance levels.

Next-generation routers must deliver wire-speed flow classification, policing and QoS assignment. They also must support hierarchical, flow-based queuing, and have high-capacity wide area network and metropolitan area network (WAN/MAN) interfaces. Furthermore, they must offer carrier-class implementations of major routing protocols such as Border Gateway Protocol (BGP)-4, Open Shortest Path First (OSPF), Intermediate System to Intermediate System (IS-IS) and MPLS. Edge routers must offer per-flow accounting, configuration and management, and enable end-to-end service creation and per-flow statistics for billing, accounting and reconciliation.

Cable operators require next-generation edge routers that offer the performance and functionality necessary for aggregating cable flows onto the optical core network. Older router platforms lack the performance, scalability and flexibility needed to support demanding requirements at the edge of the optical network. Replacing a hierarchy of routers with edge routers and an optical mesh core can provide a low-latency, low-jitter path for IP traffic. When this is combined with a cable modem termination system (CMTS) and router platform that can perform the processing-intense filtering, forwarding, accounting and QoS functions at wire speed, applications such as voice and video can be confidently provided over the IP infrastructure.

Operators need intelligent optical edge routers to buildout network infrastructure and efficiently forward traffic across the core optical networks of one or more providers. They may then introduce data, voice and multimedia services for both corporate and residential subscribers and deliver QoS levels end-to-end across both the access and optical core networks. These carrier-class CMTS and routers require flexible optical interfaces to provide connectivity to the optical core.

Increasing edge intelligence

Operators need to add intelligence and performance to the edge of the network to link cable infrastructure more efficiently to the optical core networks of one or more providers. This will enable the carrier-class performance and reliability essential to delivery of next-generation services requiring end-to-end QoS from the cable modem to the core optical network of one or more providers.

These are exciting times for cable operators as they exploit market demand and emerging technologies to create multiple revenue streams and build closer bonds with residential and corporate subscribers. Optical networking enables the delivery of robust new services that traverse the cable network and allow subscribers to select from a broad range of services and providers.

Operators are evolving from hierarchical routed networks to flat networks in order to leverage the capacity, performance and efficiency of optical networking. With increased intelligence deployed at the network edge they can gain major business advantages. Creating tighter links with revenue-sharing partners and building closer bonds with subscribers, the race is on to build out and become the preferred broadband access medium. Next-generation, carrier-class optical edge routers will allow MSOs to take advantage of the new age of optical networking.

Gerry White is vice president and chief technical officer for RiverDelta Networks. He may be reached at .

 Back to April 2001 Issue

Access Intelligence's CABLE GROUP
Communications Technology | CableFAX Daily | CableFAX's CableWORLD | CT's Pipeline
CableFAX Magazine | CableFAX databriefs | Broadband Leaders Retreat | CableFAX Leaders Retreat