June 1998

BY PAMELA MOFFIT DODGE

As telecommunications evolves from a technology to a service industry, the main issues driving ATM management are providing competitive services with Quality of Service (QoS) guarantees, and giving subscribers control over their portion of the public network. When managed properly, an ATM backbone network can bundle a host of protocols (IP, frame, voice) into a common infrastructure. Such infrastructures address competitive issues by providing the ideal basis for offering competitive services with QoS guarantees. QoS guarantees open up opportunities for service providers to offer such services as voice, SNA (Systems Network Architecture), and LAN, among others.

UNDERSTANDING BASICS
ATM management consists of taking a common infrastructure and dividing it into services. The underlying technologies of ATM (such as Variable Bit Rate and non-real-time service classes) allow service providers to define service-oriented, rather than technology-oriented, solutions. For example, instead of simply offering customers Constant Bit Rate (CBR) services, providers can use ATM management to offer voice services. Although managing the physical elements of the ATM network (such as routers, switches, etc.) remains important, the driving issue becomes managing the services.

Managing services involves several factors. These include defining the service, defining its QoS and traffic profile file parameters (such as bandwidth allocations, discard rate, congestion control, etc.), assigning the service to the subscriber, and providing the subscriber with tangible proof of the quality of that service over time. Many service providers now are offering Service Level Agreements (SLAs) that define QoS, as well as reports based on actual services delivered over time.

ATM networks differ from existing technology most dramatically in their ability to define different levels of QoS for individual connections. Individual Virtual Connections are characterized by numerous connection attributes including cell rate or bandwidth, delay experienced, variation in mean delay, cell burst sizes and spacing, and cell-loss maximum guarantees provided.

CLASSES OF SERVICE
The ATM Forum has defined four distinct classes of service to categorize the different combinations of connection attributes. These classes are Constant Bit Rate (CBR), real-time Variable Bit Rate (rt-VBR), non-real-time Variable Bit Rate (nrt-VBR), and Available Bit Rate (ABR)/Unspecified Bit Rate (UBR).

In some ATM switches, the buffers supporting Variable Bit Rate traffic can be further subdivided to create as many as ten distinct QoS classes. These ten classes would include one CBR class, four rt-VBR classes, four nrt-VBR classes, and one ABR/UBR class. Segregating the different traffic classes ensures that conditions within individual subdivisions will not affect other subdivisions, even under conditions of extreme load or traffic bursts. For instance, CBR traffic such as voice and video would not be disrupted by bursty data traffic.

Available Bit Rate
Buffer capacity is most important for supporting the ABR/UBR class of service for bursty computer traffic. ABR means that a given virtual circuit connection can increase its cell transfer rate to utilize all of the available bandwidth (a dynamic attribute that changes as new connections are initiated) on a particular link. To be effective, ABR requires a complex rate-throttling protocol to operate hop-by-hop across the switches in a connection path. ABR also requires ATM endstations (workstations in ATM LANs, or customer-premises routers in WAN Internetworks) to actively participate in end-to-end, rate-setting algorithms. Although the ATM Forum has largely defined the conceptual framework for end-to-end rate-based flow control for ABR services, it is clear that it may be several years before most ATM customer premises equipment is able to effectively support it.

Unspecified Bit Rate
UBR is a more recent development from the ATM Forum. It suggests that bursty data connections should have an unspecified bit rate and burst at maximum line speed to fully utilize available link bandwidth and switch buffers. When coupled with Early Packet Discard (EPD) and Fair Queuing, UBR provides a very effective data transport service known as UBR+. EPD allows switches to discard an entire packet (composed of multiple cells) in the event of congestion. Fair Queuing ensures that UBR connections get throttled proportionately to their transfer rate, so that high cell rate users do not monopolize the available bandwidth. UBR+ is considered a more likely near-term solution to ATM congestion control that will be used until the installed base of workstations and customer premises routers is upgraded to participate in the ABR flow control algorithms.

Service Class Challenges
Although the ATM technology supports these varying classes of service, the management challenge now is to provision, monitor, and control these services on a per QoS level. For example, when setting up a CBR Switched Virtual Circuit (SVC), let's call it IP voice. The proper network elements throughout the network must support and be configured to enable a guaranteed priority delivery rate.

Once the elements are provisioned, the IP voice service needs to be monitored as a service, not as a series of discrete elements. Look for new network management tools that are SVC based rather than element based to provide this insight. In addition, reporting and sharing of information with end users will expand to add QoS levels.

CUSTOMER NETWORK MANAGEMENT
Although end users are increasingly considering public network services created from these different traffic classes, many are reluctant to give up the control they now have with private networks. ATM management is addressing this issue by giving service providers the ability to offer subscribers a level of control over their portion of the public network. This includes information on network throughput, delay, availability and utilization, and network views proving delivery of service classes. Service providers can give subscribers printed reports containing such information or can provide them with Customer Network Management (CNM) capability in which they can directly monitor their portion of the network.

Increasing Control
CNM enables service providers to share control with customers by sharing the information over which they formerly had complete control. Services based on CNM technology provide a number of functions, including allowing end users to monitor their subscribed portion of the service provider network for activity and performance information such as circuit utilization, error performance, and mean downtime. Other CNM services allow users to view configuration information about Committed Information Rate (CIR) levels, Quality of Service settings, and outstanding faults. Such services enable 24-hour access to network data so end users can monitor link, congestion, and service quality at any time.

By enabling access to Quality of Service (QoS) and Service Level Agreement (SLA) information, CNM allows enterprise network managers to maintain a high degree of control while their traffic is being carried over the public network. CNM services also provide customers with the information necessary to proactively plan for network growth before there is a major bandwidth crunch. And, CNM gives service providers a competitive opportunity to reformat information to make it useful to the customer's business.

BILLING MANAGEMENT
Billing is another important issue driving ATM management today. To stay competitive, service providers must have the ability to bill customers based on service types and actual service consumed rather than at flat rates. This way, customers pay only for the service they actually use and are able to see the various types of service they have been consuming. QoS billing lets users see how much they are paying for voice, SNA, and LAN, among other services rather than how much they paid overall. And service providers can create services with different QoS classes and charge different dollar amounts for those services. For example, a CBR SVC that delivers a higher degree of availability can be charged at a higher rate than UBR Best Effort services.

ATM management enables service providers to further differentiate themselves by creating billing plans that align with enterprise business practices. This capability is especially significant for accounting and billing operations that can become the basis for a number of differentiated, revenue-generating services. Some of these include usage- and time-based Switched Virtual Circuit (SVC) and Permanent Virtual Circuit (PVC) call-billing plans and new tools for customer bandwidth-usage or call duration billing, among others. Benefits to end users as a result of new ATM management technologies include services targeted to actual business consumption of public network services.

CONTINUED ATM EVOLUTION
As the telecommunications industry evolves and managing services becomes a critical success factor, the benefits of a properly managed ATM backbone network are evident. By bundling all types of traffic into a flexible, common infrastructure, ATM backbone networks provide customers with a single solution to handle all of their traffic. For service providers, this infrastructure not only provides the basis for differentiated services, but also enables the QoS capabilities and customer control that will be essential to keep pace in a service industry.

Pamela Moffitt Dodge is director of network management product marketing, Core Systems Division, for Ascend Communications Inc. Ascend is a leading provider of technology and equipment solutions for telecommunications carriers, Internet service providers (ISPs), and corporate customers worldwide. Ascend delivers a comprehensive set of best-of-breed solutions in the key areas required to build a high-performance, cost-effective public and private network infrastructure from end to end. For more information, contact the company at 510-769-6001 or visit their Web site at www.ascend.com.