VACETS Regular Technical Column

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September 10, 1996

Internet Over Cable

Cable Networks

To better understand the nature of Internet access over cable, it is helpful to review how the cable network infrastructure works. The overall structure or topology of existing residential cable TV networks follows the tree and branch architecture. In each community, a head end (the originating point for cable TV signals) is installed to receive satellite and traditional over-the-air broadcast television signals. These signals are then carried to subscribe rs' homes over coaxial cable that runs from the head end throughout the commu nity.

Each 6 MHz TV channel is transmitted in analog form over adifferent 6 MHz of enclosed spectrum on the cable. Multiple channels are sent over the same cable using Frequency Division Multiplexing (FDM). Because different channels are sent at different frequency offsets (e.g. 54-60 MHz, 60-66 MHz, etc.), this form of transmission is referred to as broadband. (In contrast, baseband transmissions all take place in the base band starting at 0 Hz.) The speed of this access depends on the equipment used to modulate digital computer information on to cable's analog TV channels. Such equipment provides bandwidths ranging from 500 Kbps to over 10 Mbps.

To achieve geographical coverage of the community, the cables emanating from the head end are branched into multiple cables. When the cable is physically split, a portion of the signal power is split off to send down the branch. The signal content, however, is not split: the same set of TV channels reach every subscriber in the community. The network thus follows a logical bus architecture. With this architecture, all channels reach every subscriber all the time, whether or not the subscriber's TV is on. Just as an ordinary television includes a tuner to select the over-the-air channel the viewer wishes to watch, the subscriber's cable equipment includes a tuner to select among all the channels received over the cable.

Because the signals decrease in strength as they travel several miles through the cable to subscribers' homes, amplifiers have to be deployed throughout the plant to restore the signal power. The more times the cable is split and the longer the cable, the more amplifiers are needed in the plant.

Two Way Transmission Television, as its critics have long noted, is a one-way street: programs are broadcast to viewers, but no mechanism is built in for viewers to talk back to producers, or to send their own video (or Internet) signals into the network. This fact is reflected in typical residential cable TV network implementations, which transmit signals only in the direction from the head end to the subscriber (referred to as the downstream direction).

Many of new interactive services require transmission from the subscriber as well, with bandwidth requirements for such upstream transmission varying tremendously depending on the service. Pay-per-view movies could be ordered over the network with only a few low-bandwidth clicks of the remote control (or mouse), while telephony would require a larger (but still relatively small) 4 KHz of bandwidth. Internet applications span a range of upstream

requirements, from low-bandwidth email to high-bandwidth video sent from a home-based World Wide Web server.

  1. Enabling upstream transmission requires three types of technical changes to the network:
  2. Spectrum must be allocated for the signals traveling in the upstream direction. The range of spectrum from 5 to 42 MHz is typically dedicated to upstream transmission. This range generally provides a maximum of 4 usable upstream channels.
  3. Amplifiers must include circuitry (diplex filters) to separate the upstream and downstream signals and amplify each direction separately, in the right frequency range. Rather than upgrading existing amplifiers in their tree and branch networks, most network providers have elected to incorporate upstream capability into the new amplifiers they need to purchase to pass the higher-bandwidth signals of hybrid fiber and coax networks.

Downstream transmission from the head end is broadcast: the same signal is sent on all the wires. In contrast, upstream transmission is inherently personalcast: each subscriber is trying to place a different signal onto the network. When going up the tree these different signals must eventually share the same piece of transmission spectrum. Some form of access method is needed to arbitrate which signal is actually carried. Access methods come in different varieties, such as Time or Frequency Division Multiple Access (TDMA or FDMA), Carrier Sense Multiple Access (CSMA), etc. Which method is appropriate depends on the expected application.

Upstream transmission also presents a technical challenge because multiple noise sources invariably accompany the multiple signal sources. Careful plant engineering is often required for the upstream signal to arrive in recognizable form.

Cable Modems

The cable modem is a key component of Internet access which allows the user's PC to connect with the cable network and the Internet beyond. Cable modems in production or under development can be grouped into three broad categories:

  1. Devices which route IP (Internet Protocol) messages exclusively, the Motorola product being a good example.
  2. Devices capable of supporting IP and other network protocols such as Ethernet. Current vendors include LANCity and Zenith.
  3. Devices which implement the ATM (Asynchronous Transfer Mode) cell and signaling formats. These products are further down the road.

For consumers, the most immediate requirement is the Internet, but businesses are certainly interested in the point-to-point Ethernet capabilities of cable modems.

Cable Modem Issues

The salient issues surrounding a cable company's selection of a cable modem are as follows:

Network management - Cable Operators must be able to remotely monitor and manage the functionality of customers' modems on the network to ensure network reliability. Data layer encryption - this will provide a level of network security beyond what the end user installs on his/her own PC. Cable modem operations - the cable modem should be conveniently co-located with the user's PC and should have various operational features like power level control for transmission, the ability to support more than one PC on a Local Area Network, and the ability to detect its location in the network. Telephony return option - for smaller cable operators who haven't upgraded their cable network to full two way capability the option exists to use the telephone as the return path for Internet traffic. Here, the customer still gets the full benefit of high speed Internet reception over cable, but uses the telephone line for upstream data. Price - cable modem cost must be comparable to that of a high end telephone modem: i.e. $300 - $400. Cable modem rental as a portion of the monthly Internet over cable service cost will likely be an option for the customers.

Cable Modem Vendors

Several cable operators have announced large orders for Motorola CyberSurfer modems, which are emerging as an early favorite among large cable companies. Other major vendors include Zenith, LANCity, Hybrid Intel/AT&T, Hewlet Packard, Nortel, Scientific Atlanta and General Instrument.

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