Previous | Table of Contents | Next |
The key difference between an analog modem designed for use on the public switched telephone network and a cable modem is in the bandwidth of the channels they are designed to use. Cable TV uses RG-11 cable for the main CATV trunk and RG-59 cable from trunk distribution points into and through residences and offices. Both types of coaxial cable have 75 ohms impedance and support broadband transmission, which means that two or more channels separated by frequency can be simultaneously transported on the cable.
A cable-TV broadcasting infrastructure uses 6M Hz channels within the bandwidth of RG-11 and RG-59 cable to transmit a TV channel. Most CATV systems are currently unidirectional, which means that TV signals are broadcast from the CATV system operator without any provision for receiving a return signal. This transmission limitation is gradually being overcome as CATV operators begin to add bidirectional amplifiers to their networks that, when they are installed, will support transmission from subscribers in the reverse direction to conventional TV signal broadcasts. This will enable CATV systems to support the standardized transmit frequency range of 5M Hz to 42M Hz, and receive a frequency range of 54M Hz to 550M Hz, with 6M Hz cable TV channels.
By using one or more 6M Hz cable TV channels, a cable modem obtains the use of a bandwidth that is 1,500 times greater (6M Hz/4K Hz) than that provided by a voice channel on the switched telephone network. This means that the modem can support a signaling rate of twice the bandwidth, or 12M baud, on one TV channel, based upon the Nyquist theorem, before the occurrence of inter-symbol interference.
The primary difference between cable modems currently being used in field trials are in their use of one or more 6M Hz TV channels within the band of channels carried by a coaxial cable, and their methods of attachment to the CATV network. One cable modem manufactured by Zenith Network Systems, a subsidiary of Zenith Electronics, of Glenview IL, operates on 6M Hz channels at 4 M bps to the subscriber, using a special filtering technique to prevent data channels from interfering with adjacent information, which can be in the form of either data or video, that would co-exist with the data transmission provided by the cable modem. The uplink or return data rate occurs at 500K bps. Modem modulation is biphase shift key (BPSK), which means that two bits (bi) are encoded in each phase change, and the modems phase changes are shifted in phase from one to another. This modem is also frequency-agile, which means it can be set to operate on any standardized channel on a broadband CATV system.
The Zenith cable modem is actually one portion of a series of components required for a PC to use the modem. A complete transmission system requires the use of a Zenith cable modem, Ethernet 10BASE-T adapter card with a 15-conductor pin connector, and a 15-conductor shielded cable to connect the cable modem to the adapter. Exhibit 1 illustrates the cabling required to connect a PC to a CATV network via the use of a Zenith Network Systems cable modem.
Exhibit 1. Cabling for a Zenith Network Cable Modem System.
When the adapter card is installed in the PC, it in effect turns the computer into a client workstation. Because the adapter is an Ethernet 10BASE-T card, this means that the channel being used by the cable modem operates as one long CSMA/CD LAN, with each PC user competing with other PC users for access to the channel. This means that the CATV operator should segment its cable distribution system to limit the number of cable modems attached to any segment, similar to the manner in which conventional LANs are limited with respect to the maximum number of workstations that can be connected to the LAN.
The connector labeled R on the rear of the cable modem is a reverse cable connector designed for networks that use a single coaxial cable. The second connector, labeled F, represents a forward cable connector that would be used if the modem were connected to a cable system that uses two cables. In such a system, one cable is dedicated to conventional CATV broadcasting through one-way amplifiers, which precludes reverse transmission on the same cable. This type of system also requires the use of a second cable to obtain a transmission capability in the reverse direction.
In addition to the previously described cable modem based upon the exclusive use of RF technology and biphase shift key modulation, Zenith Electronics Corporation announced a high-speed cable modem architecture. This architecture is based on the use of 16-VSB (vestigial sideband), a technique developed by Zenith as part of the organizations high-definition research, as well as the 256 quadrature amplitude modulation technology. Through the use of more complex modulation techniques for which more data bits can be represented by one signal change, the Zenith modem architecture can support data rates up to 40M bps on a 6M Hz cable channel.
Recognizing the fact that many cable TV systems will be limited to one-way transmission for the foreseeable future because of the time and cost associated with upgrading the CATV infrastructure, Zenith plans to support a range of options and speeds for upstream data transmission. According to Zenith, both telephone (analog modulation) and RF return path transmission capabilities will be incorporated into different versions of this new family of cable modems. For many cable modem applications, such as Internet operations, the use of the switched network for a return path should provide an acceptable level of performance. The rationale for this is best noted by examining the communications interaction between a potential cable modem user and the cable network as a user searches out and accesses various points on the World Wide Web.
Previous | Table of Contents | Next |