Keep it simple

As DSL moves closer toward mainstream deployment and more users come online, the ability to keep per-user provisioning costs down becomes increasingly important. At the core of this challenge is the need to simplify the provisioning of permanent virtual circuits, which play a central role in carrying end users' sessions over a DSL network.

Yet while today's standard DSL systems have made strides toward reducing PVC provisioning, incumbent local exchange carriers (ILECs), competitive LECs and ISPs still face a significant - and costly - provisioning burden. Two key technologies can be used to tackle this challenge: employing point-to-point protocol for data services and integrating Layer 2 Tunneling Protocol (L2TP) functionality into the DSL access multiplexers (DSLAMs). With such an integrated DSL solution, PVC management can be significantly simplified, while still maintaining quality of service (QOS) for advanced applications such as voice over IP and streaming video.

Here and now

Most of today's DSL networks are heavily leveraged on ATM to primarily provide data services (Figure 1). Typically, a separate PVC is provisioned for each user to a broadband access server (BAS), which then tunnels the traffic into ISP routers (or to routers leading to other service providers such as corporate networks or online banking services) of the end user's choice. Placed at the ingress to the ATM network, the BAS allows multiple end-user sessions to be transported to any given ISP router using L2TP.

This allows end users to use service selection - to access multiple destinations while minimizing the number of PVCs running to ISP routers. Because of the limited amount of processing power on typical BAS devices, and depending on the number of customers being offered DSL-based services, numerous BAS devices may be required within the same DSL network.

While the use of BAS devices has facilitated service selection and minimized the number of PVCs that must be provisioned by the ISP, PVC provisioning with this model still can be a challenge. As DSL providers look to scale their services upward to meet the needs of a growing customer base, they still must manually configure a separate PVC for each additional end user. A number of factors can make handling these PVCs complicated and costly. Tackling any of these factors will reduce PVC provisioning complexity and lead to improvements over today's standard DSL model.

- One factor is the need to provision PVCs each step of the way - at each network element. For the deployment depicted in Figure 1, each new user signing on for DSL services will require a separate PVC between the customer premises equipment and the DSLAM, the DSLAM and the BAS, and the BAS and the router. Furthermore, given the limited processing ability of many BAS devices, ISPs may be forced to deploy more than one BAS to accommodate their growing base of customers. Any model that reduces the number of network elements will yield provisioning and management efficiencies.

- The need to coordinate provisioning activities between the ISP and the access provider also can increase the burden of PVC provisioning, especially on a daily basis. This requirement arises because PVC provisioning often must go through the DSLAM. It is a particular concern when the BAS is co-located at the ISP. In such cases, the access provider, which manages the DSLAM and sells the DSL service, and the ISP, which sells the service application, must closely coordinate their efforts to provision PVCs. Add to this the fact that there are possibly dozens of ISPs with which the access provider must interact. Just as it is often difficult to establish communication processes within a single large company, it is even more difficult to establish effective communications on a daily basis between companies, many of which offer competitive services. Allowing these companies to work autonomously will enable greater efficiencies in the provisioning process.

Nailed-up bridging and routing services are often not scalable from an address administration perspective, leaving ISPs with no easy way to address security, billing, authentication and administration. Adopting a model that gives ISPs an easy handle on such provisioning tasks will further reduce the burden of PVC provisioning.

These are just some of the factors that lead to increased provisioning burdens. As DSL moves closer toward mass deployment, with so many more end users to manage, meeting these challenges is becoming increasingly important.

An integrated DSL infrastructure

One promising approach that lets providers tackle these challenges is to use an integrated DSL model (Figure 2). There are two key differences between the integrated model and the model employing BAS devices. First, point-to-point protocol (PPP) is used to transport data traffic within the DSL network. Second, L2TP functionality is integrated within the central office DSLAM (as opposed to the BAS).

The integrated model works by setting up a PPP session - running over ATM - that extends from the end user to the selected ISP router. PPP over Ethernet or PPP over ATM is used to transport DSL traffic between the end user's modem and the DSLAM. The PPP session is temporarily terminated at the DSLAM, and the DSLAM then automatically maps the PPP session onto an L2TP tunnel running to the appropriate ISP. ATM PVCs carry the L2TP tunnels between the DSLAM and the ISPs. End users then can enjoy user-initiated service selection, easily breaking off a PPP session to one ISP and establishing a new session with another dynamically, without the need for intervention from the access provider. They also can set up multiple accounts with multiple ISPs for applications, such as children's ISPs and telecommuting. This also can be accomplished without intervention from the access provider.

As with many of today's typical deployments, the integrated model minimizes the number of PVCs that must be connected to ISP routers. Since many PPP sessions can be multiplexed over the same PVC using L2TP, only one PVC must be set up between the DSLAM and each ISP router. This ATM PVC only needs to be provisioned once, when the DSLAM is first installed or when an ISP wants to add its services. No changes to the PVCs are necessary as end users are added or as they change services, thus minimizing PVC churn.

The integrated model also provides other ways to simplify the provisioning process. In fact, the integrated model addresses each of the challenges discussed above, while still maintaining the QOS offered by ATM for native ATM applications. For example, applications that may require constant bit-rate such as voice or video can be nailed up in PVCs from the end user to the ISP without compromising their QOS needs.

Fewer networks elements

One key advantage of integrating L2TP functionality within the DSLAM is that it removes one step in the provisioning process. Specifically, since the DSLAM communicates directly with the ISP router, the integrated model eliminates the need to channel traffic from the DSLAM to the BAS and from the BAS to the ISP router.

This results in fewer network elements to manage, while simultaneously optimizing traffic flow through the ATM network. Integrating L2TP functionality and service selection into the DSLAM also eliminates the potential need for more BAS devices to support a growing customer base because the integrated DSLAM server will handle all the traffic of the DSLAM as necessary.

The reduction of network elements benefits ISPs and access providers. With less equipment to buy, upfront costs are lower. Details are simplified, including installation, network management and space requirements. There are also fewer points of failure. And integrated network management is concisely managed through the DSLAM only, and not an additional box. All these factors make management simpler, more streamlined and significantly more cost-effective.

Of equal importance is the fact that the integrated model allows for a cleaner line of separation between the two different businesses: ISPs on the one side and the access provider on the other. Recall that if PVCs must be provisioned through the DSLAM, and particularly if the BAS is co-located with the ISP, it can be difficult for these various parties to coordinate provisioning. Add to this the fact that there are possibly dozens or even hundreds of ISPs with which the access provider must interact. By contrast, provisioning a PVC within the integrated model simply involves two steps, which are more clearly delineated along business lines. First, when an ISP partners with the access provider, a PVC is established from the ISP's routers to all of the DSLAM equipment. This is done once and lays in place the PVCs needed for tunneling. After this, the ISP can negotiate with the end user to establish DSL service. Then, when customers are introduced, the access provider provisions! the PVC from the customer's mod em to the DSLAM to provide the basic DSL service.

As the ISP begins to sign customers for specific services, the ISP tells the customer how to provision its PPP service, in exactly the same way it does for 56 kb/s modem dial-up service today. A small postcard or an e-mail may be sent with PPP username and password information. This information is entered into the end user's PC or the modem, and service is enabled.

Thus the access provider and ISP do not need to coordinate every provisioning task. ISPs can turn up end users' service more quickly and efficiently, and access providers don't need to worry about ongoing per-user PVC provisioning.

An end-to-end PPP also facilitates this separation of duties, allowing the ISP and the end user to negotiate the service themselves without the access provider's involvement. Again, with the PPP session established from the end user to the ISP router, no intervention is required on the part of the access provider, which only provides the "pipe" as the PPP sessions come and go. In fact, once the DSL service is established, end users can add, drop and change ISPs on their own, and the access provider does not have to engage in the PVC provisioning associated with service selection and end user churn.

Seamless integration

Using PPP for data services provides another advantage by giving end users and ISPs a model with which they already are familiar. Specifically, an end-to-end PPP session set-up follows the same method used today for dial-up access to ISPs, allowing them to perform such functions as user identification and authentication, domain name system and IP address information, and session delineation based on existing methods, standards and operational systems. Using PPP resolves dynamic user sessions and leverages remote access server functionality in the routers, such as Radius, IP administration, security and billing.

End users, for their part, are only required to provision their own PPP user data in the form of a username and password - a process they already employ today for traditional 56 kb/s services. As stated, these users can dynamically add, drop and change ISPs using a simple user menu, without intervention from an access provider. This can be a simple extension of dial-up networking based on the PPP over Ethernet standard, which likely will become integrated into the desktop operating system, thus mitigating the need for any user setup procedure. Already, the feature eliminates the need for complex routing modems to be used at the customer premises, which typically results in extensive help desk costs to service providers. The user is not required to know anything about IP addresses, subnet administration, gateway devices or domain name servers. To establish service, the user can simply connect a modem and enter a username as password.

While the integrated DSLAM uses PPP for data, an end-to-end ATM system still provides QOS for value-added applications such as voice over IP or streaming video. Specifically, since there is a complete ATM path from the user to the ATM network, native constant bit-rate services such as voice and video can easily bypass the integrated BAS in the DSLAM, passing through the DSLAM for non-PPP services.

Using an integrated BAS DSL model based on PPP for data services and integrated L2TP functionality within the DSLAM can provide a range of benefits to ISPs and access providers alike, reducing the cost and complexity of PVC provisioning.

This is becoming increasingly important in the broadband access arena as DSL providers look to achieve mass deployment and fend off challenges from competing broadband solution providers such as cable modem vendors. Hiccups and delays in the provisioning process lead directly to customer malcontent and could potentially fuel the desire for alternative services to DSL. It is critical, therefore, that access providers and ISPs adopt the easiest method of deploying service.