Posted 03/15/2000
Network Management: More than a roll of the Dice
Piecing Together Network Management for Data and Voice over
DSL
By Stefan Knight
DSL enables service providers to deliver multiple local telephone connections and high-speed Internet access over a single copper pair. While this revolutionary new technology creates tremendous business opportunities for local exchange carriers--incumbents as well as competitors--the challenge comes when the carrier needs to provision and manage these integrated services on what are two disparate networks.
The voice over DSL (VoDSL) market now is entering the second phase of development. The first phase saw the assessment of VoDSL technologies and the validation of the business case for carriers to deploy voice services over their DSL access networks. The second phase--which will be the most challenging for service providers and equipment suppliers alike--will see initial commercial deployments and the integration of VoDSL network services into data and voice operational support systems.
VoDSL delivers derived analog POTS lines over the DSL data network. This can be achieved by introducing two new devices, the IAD and the voice gateway (see "Voice and Data over DSL" diagram, page 38). An IAD sits at the customer premises of the upscale residence or the small business. Regular telephones, fax machines, PBXs, Ethernet LANs and PCs all connect to the IAD, which converts the analog voice traffic into digital packets and combines it with LAN data packets. The integrated traffic is carried simultaneously over a single DSL line to a DSLAM at the local exchange carrier's CO. The traffic then passes from the DSLAM to an ATM switch on the backbone, which switches the data traffic to a data router at an ISP, and the voice traffic to a voice gateway. The gateway converts the voice traffic back into time-division multiplexing (TDM) traffic and delivers it to a Class 5 switch.
Voice customer premises equipment typically is not managed in carrier voice networks. Phones, PBXs and key systems generally are not considered network elements that require management in the classic telecommunications management network (TMN) model. Even if a PBX or key system is supplied by the carrier as part of a managed service offering, the management of the voice service typically stops at the analog POTS port of the Class 5 switch. It is assumed copper continuity between the Class 5 switch and the premises constitutes a working voice service.
In the voice network, local telephone services are delivered to phones from Class 5 switches and DLCs. When DLCs were introduced, the issue of provisioning and managing another device in the voice network was addressed through standard interfaces: GR-303, TR-008 and, in Europe, V5. A mechanism called flow-through provisioning uses these interfaces to facilitate service creation between the Class 5 switch and the DLC. The timeslot management channel (TMC) and embedded operations channel (EOC) work in concert to provide complete automated provisioning and management of the DLC by the Class 5 switch. The TMC is used to assign and monitor DS-0 allocation between the two devices. The GR-303 EOC is used by the Class 5 switch to manage and monitor the state of the DLC and its interface groups. Each interface group is a virtual DLC with its own TMC and EOC channels for management. GR-303 allows for oversubscription of DS-0s between the DLC and Class 5 switch. The TMC is used to manage oversubscription assignments.
By contrast, automated provisioning is only starting to emerge in multivendor data access networks. The issue has been brought to a head by the need to scale DSL rollouts quickly. The elements of the data network typically are managed using simple network management protocol (SNMP) over an IP network. Each element has an IP address, which is used for management access. Each vendor develops a management system or integrates management applications in a common platform such as HP (Hewlett-Packard Co., www.hp.com) OpenView. Each data element is provisioned and managed individually with network alarms propagated to a central system. Global provisioning and management systems for the data network typically are restricted to single vendor environments.
VoDSL Requirements
The voice and data elements in the network are managed using separate systems and separate protocols. A common protocol in the voice network is transaction language one, better known as TL1, while the most commonly deployed protocol for data network management is SNMP. The gateway and integrated access device require knowledge of both networks to establish and maintain service. This is the deployment challenge for scaling integrated services using VoDSL.
The gateway utilizes a DLC interface to the Class 5 switch to preserve the management interface to the legacy network. The EOC and TMC are used to manage DS-0 allocation and assignment for a voice service. The interface to the data network is an ATM connection. Each DS-0 is mapped to an IAD POTS port across the access network. Making these connections and managing them in a large-scale voice deployment requires leveraging a flow-through provisioning and management model. In addition, deploying these services over the data network and deploying IADs brings about a variety of new management challenges. The major concerns are:
- How does the operator provision and manage voice and data services on subscriber IADs starting on day one and continuing down the line when there are moves, adds and changes?
- How does the network manager know when an IAD goes offline or has a service-affecting failure?
- With rate adaptive DSL, how will the network manager know when a subscriber no longer has enough bandwidth to provide all of the provisioned voice services?
- How does the network manager find out about dynamic issues in the data access network which may affect service such as jitter, congestion and excessive round trip delay?
- How does an operator troubleshoot a single-voice port outage on the IAD when there is no longer a copper maintenance loop test that can be performed?
- How does the gateway map a DS-0 to the appropriate ATM virtual path identifier/virtual channel identifier (VPI/VCI) terminating at the subscriber's IAD?
- To address these issues, two essential ingredients need to be added to a VoDSL solution:
- A new approach to element management system (EMS) capabilities--The EMS needs to be able to interact with both the voice and data OSSs and management systems to determine the correct mapping for voice services from the Class 5 switch across the various DSL access network elements.
- VoDSL embedded operations channel (EOC)--An EOC between the gateway and the IAD is needed to facilitate service creation and service assurance.
Diagram: Voice and Data Over DSL
A successful VoDSL solution extends the role of the gateway EMS to incorporate protocol interfaces to both the voice and data management networks, enabling the gateway to communicate with both networks. The gateway's EMS and VoDSL EOC play active roles in facilitating service creation and ongoing service assurance for voice services delivered to subscribers over DSL data networks (see "Integrated Network Management" diagram, below).
Diagram: Integrated Network Management
Service Creation
Service creation is the first consideration for scalable VoDSL deployments. Carriers need to be able to turn up thousands of ports per day with complete automation from the customer service representative's (CSR) screen all the way to the phone of the subscriber. Flow-through provisioning from the Class 5 switch to the gateway ends at the egress ATM port of the gateway. To that point, the Class 5 switch and gateway GR-303 or TR-008 interface can be provisioned like a standard DLC. The missing information element is the subscriber's broadband access address, the ATM VPI/VCI down which all voice services should travel. This information is readily available from the data network if the gateway EMS has implemented the appropriate data communication protocols and APIs to either "pull" it from these systems or allow it to be "pushed" to the gateway at the time of DSL service provisioning. Each carrier management network is different. The gateway requires an open and flexible protocol interface to bridge the gap between legacy and new OSSs.
Once the mapping between the gateway and the IAD occur, all information about the voice services that have been provisioned can, in principle, flow through to the IAD. The IAD can be entirely plug and play for voice. In practice, the EOC concept needs to be extended to the IAD. The gateway and its management system have full knowledge of the voice service parameters keyed in by the CSR and provisioned into the voice network. This knowledge needs to be communicated to the IAD in band from the gateway. The solution for this is the VoDSL EOC, which borrows the appropriate, standard management protocol from the data network, SNMP.
ATM networks use SNMP over ATM adaptation Layer 5 (AAL5) for management. This standard is called integrated local management interface (ILMI) and effectively provides an EOC capability between the ATM switch and ATM CPE. The VoDSL EOC utilizes a similar approach, SNMP over ATM adaptation Layer 2 (AAL2). Voice transport uses AAL2 for an ATM DSL network or AAL2 payload encapsulated in Frame Relay Forum 8 compatible frames for a frame relay DSL access network. SNMP over AAL2 is a standards-based management mechanism. This transport can be used to query a voice service management information base in the IAD and to allow complete flow-through provisioning of the voice service. This solves the service creation dilemma. The VoDSL EOC is currently undergoing standardization work within the ATM Forum's (www.atmforum.com) Voice Telephony Over ATM (VTOA) working group.
So, should the VoDSL EOC be used for data provisioning as well?
The data and voice services are carried over separate paths to separate networks for termination. Data is terminated at a router or broadband access server, and voice is terminated at the VoDSL gateway. Many times the voice service will be directed to one carrier network and the data service to a separate ISP network.
For initial deployments, it is most appropriate for the data service to be provisioned and monitored by the data management system using SNMP over IP over the data channel to the IAD. The voice service can be managed by the gateway using SNMP over AAL2. The voice connection and management is independent of the data setup. In fact, with VoDSL the subscriber receives dial tone whether or not the IP data service is active or properly configured. The data and voice OSS as well as personnel can continue to be leveraged with the linkage between the two occurring as a protocol interface between the back-office management systems and the gateway management system.
Service Assurance
Service creation is just the beginning of a successful VoDSL deployment. After creation comes assurance. Subscribers have an unyielding expectation that going off hook on a phone provides dial tone. The voice network is incredibly reliable in this sense. The data network is getting more reliable every day, or carriers would not contemplate putting in VoDSL. But the data network is dynamic, which requires a higher level of policing to ensure services are delivered appropriately with their intended quality. ATM networks, which were designed from the beginning to deal with such issues, are ideally suited for mixed voice and data traffic.
The VoDSL EOC is the essential means of assuring the integrity of the voice path from the gateway across the data network all the way to the IAD. This in-band management channel uses the same AAL2 virtual channel as all of the derived voice calls and provides the essential communication to the network manager of important service- affecting events. Such events include the loss of communication with an IAD due to a failure as well as network performance issues like jitter, latency and bandwidth availability to deliver the SLA promised the subscriber. In effect, the IAD becomes another network element with northbound alarm generation capabilities.
The gateway filters IAD alarms to the gateway EMS for back-end processing. Here, the gateway EMS demonstrates application-level intelligence to ignore storm conditions as when all of the IADs come online simultaneously after a power outage or network outage.
There are unique challenges for the service provider deploying toll-quality voice services using VoDSL to paying subscribers. Each of these challenges can be met and overcome by implementing the appropriate management mechanisms in the VoDSL network. These operational considerations are perhaps the most important attribute of a VoDSL network solution. A service provider's success with VoDSL will depend upon the ready availability of management solutions that address these network concerns.
Stefan Knight is director of product marketing at CopperCom Inc. (www.coppercom.com). He can be reached at sknight@coppercom.com.
