Cable Modems

Introduction
Cable modem Internet access is the leading broadband Last Mile technology in both the consumer and the business markets. It is called an "always-on" service, because the user does not have to dial-up the provider and create a connection, in order to use the service — it is always available.

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.

How it works
Figure 1 shows how a cable data network operates. At the head end, a cable modem connects to an Ethernet router port. The router also has a WAN connection to the Internet. The cable modem translates Ethernet packets into radio frequency signals that are mapped into an unused 6-MHz channel and broadcast to all homes, typically via an HFC node (see infrastructure, below).

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.

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Data Over Cable Service Interface Specifications (DOCSIS)
In 1996, several cable operators formed a group to develop a specification for sending data over the television cable system infrastructure, including the equipment, service provisioning, security, and interfaces. The original specification is known as DOCSIS 1.0; an upgraded version, DOCSIS was last updated in December 2000.

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 Cable Modem Termination System (CMTS), installed in the head end, or main facility, of the cable operator
  • The HFC cable network wiring infrastructure
  • The cable modem, installed at the customer premises

The DOCSIS architecture defines one downstream channel to send signals to all cable modems, and multiple upstream channels on non-overlapping frequencies.

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Media Access Control
There are two choices for media access control for a cable modem:

  • Contention mode — works well under light load
  • Time-Division Multiplexing (TDM) mode — works well under a heavier load or when guaranteed throughput is needed

Deployment Issues
There are a couple of architectural issues that are slowing the deployment of cable services:

  • Cable Infrastructure: Most cable systems were designed for one-way downstream signal flow.
  • Shared media: Cable installations "share" the cable medium among perhaps hundreds of users in the same neighborhood.

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The Cable Infrastructure

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:

  • Prior to 1990, cable infrastructures included one-way amplifiers every 2,000 feet or closer. This, of course, blocks all upstream traffic. (After that, cable operators began to install amplifiers that split the bandwidth into an upstream and a downstream band. Frequencies between 5 MHz and 42 MHz were designated as upstream and 91 MHz to 857 MHz was designated as downstream.)
  • The upstream frequency band is highly susceptible to ingress noise, caused by interference from such things as household appliances and dimmer switches. Since amplifiers amplify noise as well as the signal, the noise tends to concentrate as signals get closer to the head end. This typically limits the upstream throughput to somewhere between 200 Kbps and 2 Mbps.

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).

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Hybrid fiber coaxial (HFC)
Hybrid Fiber/Coax (HFC) System - According to the Operations Support System Interface Specification (OSSI) glossary, HFC is:

A broadband bidirectional shared-media transmission system using fiber trunks between the headend and the fiber nodes, and coaxial distribution from the fiber nodes to the customer locations.

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.

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Shared Media
There are both performance and security issues with shared media. As with other shared media environments, throughput is reduced by the presence of simultaneous users. This means that the high performance early adopters of the technology will enjoy is only temporary: as more users are signed up, the performance will be degraded. Reducing the number of users reduces profitability.

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.

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