Technology Brief. Implementing DSL Service for ISPs with the SMS. The DSL Opportunity. DSL is Different from Dial

Transcription

1 Technology Brief Implementing DSL Service for ISPs with the SMS The DSL Opportunity Digital Subscriber Line (DSL) services represent a major opportunity for dialup-based Internet Service Providers (ISPs) to improve their competitive position by offering broadband connectivity to their subscribers while leveraging their existing provisioning, billing and marketing infrastructure. From the subscriber viewpoint, DSL represents a widely available solution that can deliver broadband multi-media services with download speeds of 384 kbps or more, a significant improvement over 56k modems. As the enduser market awareness of DSL continues to grow, many subscribers are switching to DSL from dialup Internet access. This is especially true of the power users segment, such as small businesses, telecommuters and home office users, who represent the higher end of many ISPs revenue base. Although there are many similarities between delivering Dial and DSL service, in many ways DSL departs from the dial-up network design, since it involves DSL-specific linetermination equipment and introduces network scaling issues, but at the same time offers richer revenue opportunities for the ISP that are made possible by utilizing the greater bandwidth. This white paper provides a background on the issues associated with implementing DSL service for ISPs, in a way that is both cost-effective and scalable from entry-level volumes up through thousands of subscribers, by using the Redback Subscriber Management System as the key network service component. DSL is Different from Dial In the dial world, individual subscribers initiate telephone calls from their modems, which are carried over the telephone network infrastructure (PSTN) and ultimately terminated on an ISP s remote access server (RAS), which then passes traffic to a router for access to the Internet. (Although there are variations on this model involving dial offload and wholesaling the basic model serves for our purpose in comparing with DSL.) Internet Service Provider Remote Access Server CO PSTN CO BGP Router Internet RADIUS AAA Server Single PPP Sessions, Individually Managed Figure 1: Current Dial Access Model 1 Copyright 2003 Redback Networks

2 In the DSL model subscribers connect from their premises via a DSL CPE over copper wire that is physically terminated on their DSL provider s DSL Access Multiplexer (DSLAM), which is located in a telecom central office (CO). The DSL provider, typically a LEC or CLEC acting as a wholesaler, aggregates traffic and sends it to the retail Internet Service provider via a high-speed connection such as a DS-3 or OC-3 circuit over their metro ATM network. As in the dial model the LEC owns the local loop and they provision the DSL service by connecting the copper pair to the DSLAM. In the original DSL model the equivalent to the RAS is missing and traffic is terminated directly on the ISP router. DSL Modem Copper Pair DSL Provider Central Office DSLAM IP Data Network Service Provider BGP Router Internet Voice RADIUS AAA Server Figure 2: Typical End-to-End DSL Architecture To Voice Network Benefits of DSL Service for ISPs From the perspective of an Internet Service Provider, DSL offers a number of key advantages and benefits that improve their competitiveness and enhance their revenue opportunities. These DSL benefits include: High-speed solution for subscribers that makes use of existing copper wiring Differentiation from other ISPs that haven t yet moved to offer DSL Competition against other broadband services, such as wireless and cable modems Opportunity to create a larger geographic service area throughout a Telco LATA A method for building subscriber loyalty, especially among small businesses and power users through value added services on the broadband connection Ingredients for Setting Up DSL Service The implementation of DSL services requires the ISP to address four key areas: Defining the business opportunity for the DSL service offering, including the projected numbers of subscribers, demographics, and growth prospects Contracting for a high-speed circuit from a DSL provider who provides optimal geographic coverage and a beneficial rate structure, to deliver subscriber traffic Implementing a method for terminating the delivery circuit and managing the required volumes of individual DSL subscriber sessions Integrating the DSL subscriber traffic into the existing ISP infrastructure in a manner that enables ongoing scalability of subscribers 2 Copyright 2003 Redback Networks

3 Defining the Business Opportunity The first step for an ISP in offering any new service is taking the time to define the business opportunity. This is a direct outgrowth of any ISP s business, which is to know their subscriber base. In addition to determining the geographic distribution and numbers of subscribers, with DSL it is also necessary to know which of the types of DSL service will appeal to them. Since many DSL providers do not necessarily offer all the different variants of DSL, the type of service a subscriber base needs will form part of the wholesale provider selection criteria. The most popular types of DSL service: IDSL ISDN Digital Subscriber Line: Generic term for a basic symmetrical service that uses digital modems to achieve speeds of up to 144 kbps in both directions over ISDN-loop equivalent distances (about one mile). Easier to provision than other forms of DSL and extends farther but limited bandwidth. ADSL Asymmetric Digital Subscriber Line: Transmits from 1.5 Mbps to 9 Mbps downstream (to the subscriber) and from 16 kbps to 640 kbps upstream, depending on line distance. This is the most common form of DSL service for residential subscribers who mainly download from the Internet. SDSL Symmetrical Digital Subscriber Line: Delivers service from 144kb up to T1 speeds in both directions depending on the service contract. Generally for business where symmetrical speed between branch offices or to customers is required. HDSL High data rate Digital Subscriber Line: DSL CPE on either end of one or more twisted pairs deliver up to T1 or E1 speeds. At present T1 requires two lines and E1 requires three. This is generally a limited availability business service. VDSL Very high data rate Digital Subscriber Line: DSL CPE operating at data rates from 12.9 to 52.8 Mbps with maximum reach ranging from 4500 feet to 1000 feet of 24 gauge twisted pair. Generally used in multi-tenant or multi-dwelling applications. Due to the presence of many competing DSL providers in the market, there are variations on these basic DSL categories, which attempt to differentiate gradations of service capabilities and there are always developments in technology that can lead to new service types. In practice most residential DSL service is ADSL based. Basics of Service Delivery The critical components of successful service establishment: 1) Matching the DSL provider s service offerings to ISPs requirements 2) Determining the size of aggregate pipe that is needed 3) Understanding the relationship between the ISP and the DSL provider Because the various DSL providers serve different local geographic areas and offer different types of DSL service, it is important for the ISP to match their available offerings with the services that a subscriber base really needs. Generally, one of the best methods for assessing the geographic match is to simply get the DSL provider to supply an overview of CO office code prefixes (e.g. the next 3 digits after the area code) for which they offer DSL services. Then it is a relatively straightforward matter to match 3 Copyright 2003 Redback Networks

4 existing and/or projected subscriber populations against the providers serviceavailability data. An ISP is essentially contracting for the DSL provider to aggregate the individual subscribers and to deliver those subscriber circuits via an aggregated circuit, such as a full or fractional DS-3 or maybe multiple T1 lines for smaller volumes of subscribers. Basically, this bandwidth estimation process involves the combination of the following four key factors: The projected total number of subscribers Number of subscribers using each type of service & their individual peak bandwidth requirements (e.g. 256 kbps consumer service vs. 768 kbps business service vs. 1.5 Mbps T1 replacement service) The aggregate of their simultaneous peak bi-directional bandwidth requirements if everyone was on line at the same time (e.g. number of consumers ADSL users times 256 kbps + number of business users times 768 kbps.) The upper-bound over-subscription factor that will still allow acceptable quality of service levels based upon estimates subscribers expectations. The over-subscription estimate will be a critical factor in determining both the size and the average utilization of an aggregate circuit from the DSL provider. For instance, with no over-subscription factor, a single 45 Mbps DS-3 circuit would only support about 175 ADSL users at 256 kbps each. Of course, most of the time such an aggregate circuit would be substantially under utilized. On the other hand, using a 5-to-1 over-subscription ratio can boost the number of subscribers to more than 1000 sharing the same DS-3, thereby greatly lowering the cost per user and greatly increasing the circuit s overall utilization. The balancing issue is the point at which overall bandwidth constraints don t satisfy the users level of service expectation. Such service expectations are likely to vary greatly between power users and casual users and an ISP should consider implementing a tiered service offering. Once a DSL provider that best fits overall requirements is identified and the aggregate size of the pipe has been estimated, the next step is to define and negotiate the parameters of the contractual arrangement. While DSL providers don t publicly indicate what specific items they are willing to negotiate, it is important to keep in mind that many aspects of the service can be tailored to meet the ISP s requirements. For instance, many DSL providers will consider waiving the engineering and installation charges for a DS-3 circuit in exchange for a commitment as short as one year. In essence, they are counting on achieving a pass-through benefit from the same increased level of subscriber loyalty and stability that the ISP is expecting to achieve through offering DSL to their customers. In addition, many DSL providers are willing to start out by provisioning only a portion of the DS-3 circuit s total bandwidth capacity. Obviously, this could give a very attractive entry point and incremental growth path to match the circuit costs with the actual numbers of subscribers at any point. Cost-Effective and Scalable Circuit Termination Once the DSL circuit is defined and ordered, the next major issue is how to terminate all the subscribers sessions in a cost effective manner that is consistent with the existing 4 Copyright 2003 Redback Networks

5 ISP network infrastructure. Because the DSL model does not have a direct equivalent to the Remote Access Server, ISPs often have simply tried to use their backbone routers to terminate a DSL provider s DS-3 circuit. This creates problems in two key areas: inadequacy of the router to handle the traffic, and complexity in moving the traffic onto the ISP infrastructure. Routers are designed to dynamically communicate with thousands of networks via a limited number of individual connections. Router designs were never intended to terminate thousands of simultaneous individual connections or subscribers. Over the years, IP router designs have been optimized for use in dynamic clustered topologies that scale overall system performance by adding interconnected routers, rather than by scaling the number of connections to a specific router. In essence, routers are optimized to communicate with thousands of networks, but by actually connecting to only a relatively few neighboring routers at a time. Introducing the Redback Subscriber Management System The Subscriber Management System (SMS) was designed in response to ISP s requirements for terminating 1000s and even tens of thousands of individual DSL subscriber sessions. It fits in the network operational model in a similar role to the dial Network Access Server however the SMS is designed specifically to address the issue of broadband subscriber management. The Subscriber Management System provides an intelligent solution for terminating high-speed circuit connections, such as DSL, from the subscriber side, and creating groomed IP streams optimized for backbone routers. Essentially, the SMS aggregates T1, DS-3 and/or OC-3 circuits full of subscriber sessions on the frontside and outputs groomed IP streams on the backside. DSL Modem Copper Pair Central Office DSLAM IP Data Internet Service Provider RedBack SMS Mbps Ethernet BGP Router Internet Voice RADIUS AAA Server Figure 3: DSL Architecture with SMS Used at ISP POP Using an SMS improves the flexibility of DSL provider choices as well as it allows an ISP to provision services from two or more DSL providers, in order to compensate for gaps in geographic coverage and/or service offerings. Without an SMS, the termination and management of traffic from different DSLAMs can be problematic as it would require multiple termination devices and the associated purchase and management costs. On the other hand, an SMS can simultaneously accept high concentrations of traffic from multiple DSLAMs, while independently performing all of the aggregation, management and conversion functions necessary to output router-ready IP streams to the backbone. Another advantage of using an SMS is the ability to terminate either ATM or Frame Relay streams, depending upon which mode the DSL provider supports. Rather than having to limit their DSL provider choices or to invest in redundant internal switches to handle ATM and Frame Relay, the ISP can invest in a single SMS with confidence that 5 Copyright 2003 Redback Networks

6 they ve not sacrificed any of their future flexibility. From an investment flexibility standpoint, the SMS concept enables an ISP to initially deploy a minimal system configuration with a single I/O card to handle the incoming DS-3 and outgoing Fast Ethernet connections, and then to smoothly scale up the system as required to match actual growth. The SMS s level of capacity should provide ample growth for even the most ambitious ISP. At the same time, the ISP s initial investment in an SMS remains comparable with that of a standard mid-sized router and offers increasingly better cost/benefit ratios as the system requirements scale upward. Fast and Easy Circuit Set-up When it comes to setting up new circuits, the SMS is designed for optimal ease of administration, with a short straightforward recipe, regardless of whether the DS-3 feed is carrying ATM or Frame Relay traffic. An example of the required steps, using an industry familiar command line interface, are as follows: ATM Circuit Set-up: 1. Plug in the circuit 2. Set Clock Source to: Line 3. Set Cell Delineation to match the telco switches requirements (e.g. PLCT ) 4. Turn Scrambling On or Off as required 5. Configure the PVC 6. Configure the Subscriber Frame Relay Circuit Set-up: 1. Set Interface Type (typically to DTE) 2. Set LMI Type (typically to ANSI ) 3. Set Clock Source to: Line 4. Configure the PVC 5. Configure the Subscriber Integrating DSL within the ISP Infrastructure Beyond circuit termination, there is also the issue of how to manage hundreds or thousands of new subscribers from an accounting, authentication and access standpoint. This essentially comes down to how the ISP s existing RADIUS functionality, currently utilized by the dial RAS, is deployed in a broadband environment. By acting as a standard RADIUS client, the SMS provides the network administrator with seamless manageability over all DSL subscribers, bringing them directly under the umbrella of established user-management methods. The SMS integrates seamlessly with an ISP s existing RADIUS AAA systems, greatly simplifying the task of managing subscriber access and security and accounting and integrating with existing billing systems. In addition, an SMS can simultaneously create and manage multiple RADIUS-compliant authentication domains. This allows an ISP to automatically direct any user s session to any one of multiple IP networks or services. This capability, called multiple contexts, is a major breakthrough because it overcomes the previous requirement to have each DSL circuit logically nailed up to a fixed connection in the router environment. Essentially, 6 Copyright 2003 Redback Networks

7 the SMS now allows any user session to enjoy the kind of dynamic connections previously achievable only in the low-speed dial-up model. The Subscriber Management System effectively provides three critical benefits: 1. The scalability of a specialized ATM termination device with the capacity to simultaneously handle many thousands of subscribers 2. The routing functionality of multiple routers to optimally direct subscriber traffic into the backbone environment (each SMS context represents a virtual IP router) 3. The ability to dynamically direct user-level protocols (such as PPP) across the entire Subscriber-to-Router virtual connection. In addition, the integration between the SMS and RADIUS server allows for setting up highly efficient traffic-shaping profiles that smoothly match the IP traffic to the specific service provisioned by each ATM virtual circuit. Because the relatively unlimited bandwidth of the backbone and LAN must be trimmed to fit within the constraints of each subscriber s DSL bandwidth, traffic shaping can be a critical factor to avoid over-running the parameters established for the WAN link. By setting up Service Profiles within the SMS for each of the connection services offered, the ISP can quickly create a key component of the overall traffic management structure. With an SMS, the ISP has a wide degree of flexibility to shape traffic by a variety of criteria, such as per-port, per-pvc, per-subscriber, etc. Both receive and transmit traffic can be shaped via policing and rate-limiting functions. If appropriate, the SMS traffic shaping capabilities could even be used to migrate away from standard flat-rate DSL services by overlaying shaping parameters for usage rate structures or even tiered services that reflect different gradations of QoS commitments. The Bottom Line The bottom line is that the ability to offer a range of DSL connectivity options is quickly becoming a requirement for remaining competitive as an Internet Service Provider. The good news is that even a small to mid-sized ISPs can now cost effectively institute a competitively rich set of DSL services without an insurmountable investment in multiple new routers or a complex re-engineering of their existing network infrastructures. By following the logical steps of understanding their customer base and mapping those needs to the available DSL offerings, ISPs can readily define and negotiate appropriate provider relationships to support both their initial requirements and growth projections. Then, by deploying scalable Subscriber Management Systems to terminate and manage the hundreds to thousands of incoming DSL subscriber circuits, ISPs can immediately begin taking advantage of the new revenues from offering DSL services. 7 Copyright 2003 Redback Networks