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Many businesses have become increasingly aware of the importance of implementing a thorough network security strategy to safeguard valuable network data from intruders. Modems that offer security features usually provide two levels of protection: password and dial-back. Password protection requires the user to enter a code, which is verified against an internal security table. Many modems can store multiple passwords.
The dial-back feature offers a higher level of protection. Incoming calls are prompted for a password, and the modem either calls back the originating modem using a number stored in the security table or prompts the user for a telephone number and then calls back.
Security procedures can be implemented before the modem handshaking sequence, rather than after it. This effectively eliminates the access opportunity for potential intruders. In addition to saving connection establishment time, this method uses a precision high-speed analog security sequence that is not even detectable by advanced line monitoring equipment.
For the highest level of security, some modems even support the Data Encryption Standard (DES). Although DES has been around since 1977, it is still one of the most effective means of protecting data. DES-based encryption software uses an algorithm that encodes 64-bit blocks of data and uses a 56-bit key. The length of the key imposes a difficult decoding barrier to would-be intruders because 72 quadrillion (72,000,000,000,000,000) keys are possible.
Modems use two types of transmission techniques: asynchronous or synchronous. The users operating environment determines whether an asynchronous or synchronous modem is required.
During asynchronous transmission, start- and stop-bits frame each segment of data during transfer to distinguish each bit from the one preceding it. Synchronous transmission transfers data in one continuous stream; therefore, the transmitting and receiving data terminal equipment (DTE) must be synchronized precisely to distinguish each character in the data stream.
Most mainframes and minicomputers use synchronous protocols, whereas PC-to-PC communications are typically asynchronous. Users who require both PC-to-PC and PC-to-mainframe communications can purchase modems that support both types of transmissions. The software that comes with the modem usually supports several emulation techniques for file transfers between hosts and PCs.
Those who require more out of the PC-to-host link than simple file transfer can look to such software as Attachmate Corp.s Extra Personal Client 6.1and Wall Data Inc.s Rumba Office 95/NT 5.0. Both products offer advanced data-query capabilities. These and other host-access suites offer a variety of connection types and methods within one product, usually installed off a single CD-rom. At a minimum, such products can connect to an IBM 3270 or AS/400 host without having to depend on IBMs DOS-based drivers.
Wireless modems are required to transfer data over public wireless services and private wireless networks. These modems come in a variety of hardware configurations: standalone, built-in, and removable PCMCIA card.
Newer modems are programmable and therefore capable of being used with a variety of wireless services using different frequencies and protocols. There are even modems that mimic wireline protocols, allowing existing applications to be run over the wireless network without modification.
Private wireless networks operate in a range of unique frequency bands to ensure privacy. Using radio modems operating over dedicated frequencies within these frequency bands also permits the transmission of business-critical information without interference problems. Furthermore, the strategic deployment of radio modems can provide metropolitan area coverage without the use of expensive antenna arrays.
Such modems are designed to provide a wireless, protocol-independent interface between host computers and remote terminals located as far away as 30 miles. Most provide a transmission rate of at least 19.2K-bps point-to-point in either half- or full-duplex mode. Some radio modems even support point-to-multipoint radio network configurations, serving as a virtual multidrop radio link that replaces the need for expensive dedicated lines (see Exhibit 1). In this configuration, one modem is designated as the master, passing polling information and responses between the host and terminals over two different frequencies.
In multidrop configurations, a radio network is capable of supporting one type of asynchronous or synchronous polling protocol. Because such modems perform no processing or interpreting of the protocol, the host (or front-end processor) must generate all required protocol framing, line discipline, node addressing, and data encapsulation. Depending on the vendor, these modems may be equipped with an integral repeater to maintain signal integrity over longer distances.
Exhibit 1. Radio Modem Configuration.
Regardless of the transmission technology or the hardware configuration used, the modem must be tuned to the frequency of the service providers wireless network to operate properly. Until recently, modems were offered in different versions, according to the wireless network the modem would connect to. This delayed product development and inflated the cost of manufacturing, which was passed on to users in the form of higher prices for equipment.
To overcome these problems, chip manufacturers have developed programmable chipsets that are not limited to a specific networks radio frequency. Newer wireless modems are computer configurable. Within specified frequency ranges, the transmit and receive frequencies are independently selectable via software.
Not only can modems be programmed for multifrequency use, they can provide seamless integration of multiple media wireline and wireless through a common programmable interface. This is accomplished with a chipset that supports both wireline and wireless communications. Special software used with the chipset provides a method for connecting cellular phones to modems, which is important because cellular phones lack dial tones and other features used by modems on the wireline phone network. The software makes it appear that those features exist.
Although todays computers powered by Pentium and PowerPC chips are better equipped than ever to handle multimedia and video, they face a bottleneck that, in most cases, offers top speeds of no more than 28.8K bps or 33.6K bps over dialup lines. A new type of modem, the cable modem, has emerged for delivering entertainment and information services, including Internet access, to television sets and PCs over the installed base of ordinary twisted-pair wiring. Using traditional coaxial cable installed by CATV operators, these modems can deliver speeds of up to 1,000 times that of todays analog modems.
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