Building an efficient headend for data
This is the second part of a series addressing the major headend architectural and operational issues in deploying data services over a broadband cable network. These articles examine ways to optimize the headend architecture for broadband data services, the major ingredients in designing a headend to support such services, key considerations when deploying data services and ways of integrating data services into your existing cable TV systems.
This second part of the article series discusses:
- Multi-tiered services: considerations in setting up a headend to provide multiple classes of service, for maximum revenue generation.
- Scalability: how to architect the headend to allow the operator to scale services efficiently from initial small systems to large-scale services.
- Wide-area network interface: how to interface the headend most efficiently to a wide-area network that supports Internet, Intranet and other data services.
There is considerable interest in providing superior Internet access services to the residential customer through the high speeds available from cable modems. However, little attention has been focused on the significant opportunity in addressing the needs of the small office, home office (SOHO) and corporate business customers. In order to support such services, robust cable modem systems, such as those based on S-CDMA (synchronous-code division multiple access) technology, have a distinct advantage. S-CDMA provides high upstream bandwidth, with the resilience to operate in high noise environments, while providing support for operator-defined multiple service tiers — from best-effort residential Internet access to guaranteed premium business services.
Such services may be tailored to the specific requirements of the customer, designed to reflect usage, value and demand. High-end premium services for the business customer are typically less cost-sensitive and command higher revenues than residential best-effort IP-based services .
Within the cable network, multiple tiers of service allow the network provider to offer guarantees of bandwidth minimums and maximums, as well as priorities. At the time of service provisioning, the service class is established to define how the headend controller allocates bandwidth to the user and to provide appropriate parameters for billing. The use of ATM transport technology between the router and cable modem enables flexible management of guaranteed minimum and maximum bandwidth, traffic latency and priority to accommodate constant bit rate (CBR), unspecified bit rate (UBR), and unique combinations of both services.
Cable modem systems based on advanced transmission technology provide the transport robustness, reliability and high bandwidth to support high data rates and multiple tiers of premium services simultaneously.
It's important that the quality of service required by the end user extends beyond the bandwidth control within the cable network and into the IP network. This requires that the provisioning server provides information to allow the mapping of the user's IP address to pre-defined address ranges, such that the IP routed network can apply class of service forwarding (such as filtering/firewall protection, prioritization, policy routing) in the backbone or WAN.Service configurations
Figure 1 illustrates some of the configurations that may provide an expanded set of differentiating data services to different customers.
- Home user/single PC and telecommuting. IP addresses are allocated to each user's PC using centralized DHCP (dynamic host configuration protocol) servers which dynamically allocate addresses from multiple IP address "pools." The network operator may provide different IP address pools through the provisioning server, reflecting operator-defined tiers of service for particular groups of customers. By mapping like groups of customers into different IP address ranges, conventional IP routers can provide filtering, prioritization and forwarding control. Telecommuters can use specialized client software, such as the point-to-point tunneling protocol (PPTP)  to establish a private and secure IP tunnel to their corporate backbone.
- Small office, home office (SOHO) and remote office. Smaller organizations require Internet access from a large portion of their LAN, such that the modem provides an interface between the headend equipment and the corporate backbone across the cable network. Security issues may be mitigated through application-level security client-server software available through third-party providers using standards-based cryptographic specifications. Once again, premium service tiers provide the quality of service required by such users.
Isolated school LAN/small LAN. A group of PCs that requires no firewall are connected to the cable modem to provide access to the Internet. Two examples of this type of customer are a small, isolated school classroom LAN and a business that operates a LAN with Internet content servers only. Each PC is configured with its own unique IP address through the same mechanism as for the home user with a single PC.
In the special case of the business that provides Internet content, the bandwidth asymmetry normally associated with typical Internet applications (e.g., Web-browsing) is reversed. Only the unique combination of S-CDMA and ATM-based managed grade of service control allows the network operator to offer the high levels of upstream bandwidth required.
- Large business/corporate — private virtual network. Any large organization may use the cable network and Internet to form a virtual private network (VPN) between its national or regional sites. With this size business, the modem connects to the customers' own router, which may provide a termination point to an IP tunnel established between it and the corporate backbone network. IP-based applications such as virtual private networks and point-to-point connections can be offered by the network operator to provide specialized, connection-oriented and secure IP services. A network address translator, as part of the firewall, may be used to translate between the public Internet and the privately managed corporate LAN/WAN IP address space.
Network operators need to ensure that the cable modem system follows a scalable path that tracks the level of subscriber penetration and the nature of the services being offered. It is thus essential to consider both network architecture and physical placement of the data service equipment.
Figure 2 shows a hierarchical cable TV network with a high-speed data service overlay. As discussed in the first part of the article, the ability to grow in a cost-effective, incremental and graceful manner requires that the architecture supports enhancements in capacity and capability, such that all deployed equipment is upgradable as system needs evolve.
RF concentration promotes the efficient use of the limited return spectrum, allowing the network operator to move under-utilized and heavily capital intensive fixed costs to service-driven variable costs. Data-carrying RF channels can be added as additional capacity is required.
Robust and highly reliable transmission technology allows the operator to spread the cost and bandwidth capacity of the headend controllers over a large user population (on both large tree-and-branch coaxial networks or through HFC node aggregation). Robust S-CDMA-based systems offer a uniquely high level of node aggregation, allowing operators to add headend equipment gradually as it is required. Because equipment is kept to the minimum necessary to satisfy the service requirement, operations and capital costs, and headend space are also reduced.
As service penetration rises, the capacity and complexity of the data network also increases. It is essential that the data network accommodate the growth in an incremental fashion that does not force continual rearrangement and disturbance of the network. A fully routed, hierarchical IP network allows a well-understood and standard approach to manage the complexity of packet routing across the cable and backbone networks. In a similar manner, management of the data network and service provisioning must also scale with the increasing number of users. Access systems based on S-CDMA allow operators to scale systems efficiently, because the same headend controller and management equipment that supports early subscribers can grow as subscriber penetration increases.WAN interfaces
The connection to the wide area network provides connectivity between the network operator's private domain and the Internet and other service providers. Several standards and specification development bodies have defined the data link and physical layer combinations used to carry IP-based data traffic to and from the backbone and wide area networks: ATM over STS-3c and DS-3; FDDI; 802.3 over 10BASE-T and 100BASE-T; Ethernet over 10BASE-T and 100BASE-T.
Operators should use a standards-based approach to accommodate future system growth. Cable modem systems based on advanced modulation technology provide the robustness, high bandwidth and service management capabilities to address the broadest market with the greatest efficiency.
Editor's note: This article is part 2 of a series. Part 1 appears in the August issue, page 76.
| "Data Over Cable: A segmented strategy holds the key to success," by Doug Saqui and Fred Uno, Montgomery & Associates.|
| "Point-to-Point Tunneling Protocol — PPTP," Internet Draft, draft-ietf-pppext-pptp-00.txt, June 1996.|