Additions to cable spec enable QoS

With the development of products based on the Data Over Cable Interface Specification (DOCSIS) 1.1, cable networks are moving from best-effort service to the delivery of guaranteed service-level agreements (SLA) for critical business applications and voice traffic.

While best-effort service is adequate for basic Internet access, it is insufficient for the enhanced services required by enterprise networks. DOCSIS 1.0, which was developed by a consortium of cable operators and equipment vendors under the auspices of CableLabs, details the technical requirements for interaction between cable modems at customer premises and the Cable Modem Termination System (CMTS) at cable headends. The specification supports a single data stream from each cable modem with limited class-of-service differentiation available between modems. In this environment, all applications using a network contend for upstream bandwidth, so providing a defined quality of service (QoS) to a given application or user is problematic.

DOCSIS 1.1 defines enhancements to the media access control protocol of DOCSIS 1.0 to enable more sophisticated access methods over shared cable media.DOCSIS1.1 includes the following changes:

Packets are classified into service flows based on content. Therefore, each application can be mapped to a unique service flow.

Network access (upstream and downstream) is scheduled using one of a number of defined scheduling mechanisms, including constant bit rate, and real-time and nonreal-time polling.

Service flows may be configured through management or created and deleted dynamically in response to applications starting and stopping.

Fragmentation of large packets is required to allow low-latency services to operate on lower-bandwidth upstream channels.

These features allow support for multiple data streams from a cable modem and the ability to provide application-specific QoS treatment for each stream. As an interface standard, DOCSIS 1.1 does not regulate how vendors implement QoS as long as the external interfaces comply. Equipment manufacturers are therefore able to add value to the interface standard to offer increased intelligence to classify and treat traffic flows.

Also, cable operators can now offer SLAs to business customers for applications ranging from voice over IP to streaming video, and enterprise customers can now have the flexibility to support on-demand applications such as videoconferencing over cable networks and can receive guaranteed-bandwidth commitments for critical data applications. Network managers can cost-effectively deploy broadband access to remote branches while retaining flexibility for dynamic bandwidth allocation.

They can "turn up" or "turn down" bandwidth allocations to support specialized, high-bandwidth applications and increase bandwidth according to seasonal or even daily requirements. This flexibility extends from the customer premises to the backbone cable infrastructure. For example, a company can receive a guaranteed performance level for voice, another for VPN applications and yet another for an outsourced financial application. A high-performance router in the cable headend can inspect and classify each flow and ensure adequate bandwidth across the cable infrastructure.

DOCSIS 1.1 provides QoS for cable access networks. For applications to gain the maximum benefit from QoS, it must be provided end to end. Thus QoS-enabled traffic from an access network must be mapped to the QoS mechanisms - Differentiated Services, Multi-protocol Label Switching and ATM - used in the regional or backbone networks.

The edge router/CMTS providing this intelligent internetwork transition must implement the QoS mechanisms of both access and backbone networks. It must be capable of sophisticated traffic management and policing based on inspection of each traffic flow to guarantee flows from the cable modem to the core network of multiple providers. It must have sufficient performance to do these tasks at the full line rate of the interfaces supported.

White is chief technical officer at RiverDelta Networks. He can be reached at gwhite@riverdelta.com.