Slide 12 of 27
Cable modem services have been specifically vilified -- mostly by telcos! -- because their shared bandwidth results in additional security and performance issues that do not affect ADSL.
Let's start by reviewing the problem. In the cable modem network topology, customer premises that share a cable segment also share bandwidth; the service is a shared medium service for those customers, more or less analogous to the shared medium in a coax-based Ethernet LAN. ADSL, on the other hand, runs over the customer's point-to-point local loop back to the C.O. and, therefore, does not share bandwidth.
Well, sort of. In fact, all users on the Internet eventually share facilities and resources. It is absolutely correct that all premises in a neighborhood share a cable segment in the cable modem model. But while ADSL users own the wire, they share the capacity of the DSLAM in the C.O. Why does this matter? Because a well-engineered cable modem network will provide adequate service while a poorly-engineered DSLAM will suffer poor performance. The point -- a well-engineered service will do fine; bandwidth contention on a cable modem network as a root of bad performance compared to ADSL is a red herring.
What is much more of a real problem is security. Indeed, the shared nature of cable modem services makes the potential security problems very real; browsing the network neighborhood can take on whole new meaning if every house on the block shares a cable! We'll address that issue in upcoming slides.