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Solving Cable Telephony Bandwidth Blues

BY LINDEN DeCARMO

Contrary to public perception, cable networks do not have infinite bandwidth. In reality, they contain serious bottlenecks that can cripple telephony applications. In this article, we'll explore the architecture of cable networks, reveal their limitations for cable telephony, and explore how the PacketCable standard proposes to solve these bandwidth issues.

Evolving Architecture
Cable companies, also known as Multi-Service Operators (MSOs), have invested billions of dollars in deploying coaxial (or coax) networks. Unfortunately, coax is overwhelmed by the transportation demands of packet-based multimedia content. Therefore, cable companies have been migrating to Hybrid Fiber Coaxial (HFC). HFC is called a hybrid since portions of the network remain coax, while performance-critical sections use higher-capacity fiber.

MSOs chose HFC because it has greater bandwidth, is more reliable than coax, and most importantly, does not require that consumers' homes be rewired. While HFC has significant advantages for multimedia transport, the way it is deployed in cable environments can be problematic for telephony applications.

To reduce cost and complexity, MSOs use shared bandwidth for their HFC networks. Shared bandwidth networks transport a fixed quantity of data that cannot be adjusted as more devices are added to the network. Therefore, the performance of these networks is inversely related to the number of network users (a greater number of users must battle over fixed bandwidth).

Cable networks also operate in broadcast mode: packets transmitted on HFC are broadcast (or simultaneously made visible) to all computers on the network. Consequently, broadcast networks require an arbitration scheme since only one device should broadcast at a time. If there is no arbitration, two or more devices may try to simultaneously transmit a packet (this is known as a collision). Network administrators dread collisions because they degrade performance by requiring retransmission of the affected packets.

 

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