The total bandwidth of a standard television coaxial cable is close to 1 GHz it can deliver hundreds of 6 MHz analog video channels to residential customers. It could easily provide a 6 MHz channel for digital data using well-understood modulation techniques, more than enough to support just about any digital data service. In reality, however, the top throughput rate is typically limited to 10 Mbps or less by the Ethernet interfaces used.
Many industry analysts predict cable technology will continue to lead over DSL, with an estimated 19,000 business cable connections by 2005. At least one service provider plans to begin offering a non-shared/dedicated cable service sometime in 2001. It runs over hybrid fiber/coaxial networks (HFC) and uses switched Ethernet over cable to create virtual channels of up to 40 Mbps for each user.
The signal is received by any cable modems active on the local LAN segment. The downstream throughput can be 27 Mbps or 40 Mbps, depending on the quality of the channel. The cable modem at the client end translates the radio frequencies into packets, determines if the packets are destined for its address, and, if so, sends the packets to a computer or to a LAN, often at only 1.5 Mbps. Ethernet is the only current choice, but Universal Serial Bus (USB) and PCI are being considered as possible interfaces.
Over Cable Service Interface Specifications (DOCSIS)
A key to cable equipment interoperability is the radio frequency interface (RFI) specification. The architecture of the DOCSIS RFI consists of three major components:
The DOCSIS architecture defines one downstream channel to send signals to all cable modems, and multiple upstream channels on non-overlapping frequencies.
The cable infrastructure has two major problems, both of which will be costly for the cable providers to fix which means they will delay deployment:
Some cable operators avoid both of these issues alltogether by using a standard analog modem over a dial-up telephone line for upstream data. However, this solution increases cost, adds the usual call-setup delay, and may limit the top downstream speed (related to TCP acknowledgements).
The primary fix to the ingress noise problem is deploying fiber optic cable from the head end out into the network. The resulting infrastructure is know as "hybrid fiber coaxial" (HFC).
Another option is to connect fewer households to a given segment of cable. Both solutions have economic impact.
Fiber optic cable also increases the aggregate capacity of the cable network, decreases the cost of ownership, and reduces the number of amplifiers needed to boost signals on the coaxial segment. As of Spring 1999, probably less than 20 percent of homes are connected via an upgraded cable system.
Even more disturbing are the security implications on a shared-medium network, each node has the ability to see all the traffic to all nodes. A NIC in promiscuous mode can intercept all the traffic on a cable segment. DOCSIS calls for DES encryption to be installed on each modem. This, of course, will add cost to the modems.
Some cable modems have a MAC bridge built in, which prevents customers from seeing unicast frames addressed to another node. Because this does not block broadcast traffic, however, it is still possible for users to perform denial-of-service attacks on their neighbors.