Types of Modems

Standard Modems

Most standard modems today are either internal or external.

An internal modem is used inside of the computer and connects directly to the I/O BUS. The internal modem does not require a separate power supply as it gets it's power from the computer's internal BUS nor does an internal modem require a serial port or connecting cables to that port. An internal modem will contain a 16550A UART or equivalent circuitry, which will aid in fast data throughput to the computer. Internal modems are usually cheaper than external modems as well. Internal modems are a little more difficult to install than external modems and an available ISA slot must be present to install it into the computer.

On an external modem it is helpful that the modem be connected to a 16550A UART serial port to be assured of the maximum data throughput and of course, an available serial port is necessary. An external modem connects to a serial port on the PC or TI and requires the use of a connecting cable between the PC (or TI) and modem, a power connection and of course a place to put the modem itself. The advantage of an external modem is that external modems can be easily moved from one computer to another and the lights on the modem itself can aid in the diagnosis of any problems. Furthermore, an external modem can easily be reset by turning it off whereas an internal modem can not just be turned off without turning off the system.

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Fax Modems

These allow you to send and receive faxes. The fax part of the modem sends/receives data that is interpreted as a picture. One can create documents on a computer (resumes, papers, thesis proposals) and send them direclty from the application to a fax machine or a computer with a fax modem. Instead of sending the print job to the printer you're sending it to the fax modem. The information is sent over telephone lines to a remote fax machine or fax modem. That is, any file in your computer can be sent to a fax machine or another computer with a fax modem. One can fax résumés, thesis proposals, etc. to a fax machine in a matter of minutes.

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Intelligent Modems

Intelligent modems are more expensive modems that contain internal read only memory (ROM) coding and microprocessor chips to provide sophisticated communications protocols and diagnostic checking within the modem itself. Some of these intelligent modems not only perform digital-to-analog conversation but also operate as multiplexers, security restrictor devices, encryption devices, error detection and retransmission devices.

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Short Haul Modems

Short Haul modems provide transmission of data at medium to high rate in either direction (full duplex). They are also used in the control of remote devices in their network. In a short haul modem, you use your own wire pair cable to transmit direct electrical signals. This type of modem is also called line driver. This device consists of a box approximately the size of a cigarette pack. Because short haul modems do not require an external power source, getting their power through the serial port, they sometimes are referred as modem eliminators. These modems are generally used in business and not the private user. The farther the signal has to travel; the amount of data is reduced. Generally short hauls are synchronous, full duplex.

For Example:

Telebyte M 431: V.35 Interface, full duplex on single twisted pair, up to 128 Kbps, built-in diagnostics, bi-directional handshake, 9600 bps at 3.6 miles, and cost: $575.

CIM M 74: V.35 Interface, full duplex on two twisted pair, up to 19.2 Kbps, bi-directional handshake, .6 Kbps at 10 miles, and cost: $155.

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Wireless Modems

Wireless modems transmit the data signals through the air instead of by using a cable. They sometimes are called radio frequency modem. This type of modem is designed to work with cellular technology, and wireless local area networks. Wireless modems use two types of transmission to transfer their data; radio transceivers and infrared (IR). Radio transceiver modems have three ways of transmitting data; transceiver-transceiver, transciever-sattellite-transceiver, and cellular phone. Radio transceiver-transceiver can be used as point-point or point-multipoint operation and generally transmit at the frequency of 900 MHz. Radio transceiver modems have advantages and disadvantages when compared with a wired modem.

Advantages are: no costly wiring to maintain, no costly wiring to install, no down time waiting on a connection, can connect through a hub to communicate with a wired modem, and no fees to pay for a leased line.

Disadvantages are: higher initial cost of equipment ($500-$899), with satellite and cellular modems higher operating cost, security harder to maintain, short transmission range; generally 20 miles unless you use repeaters, data transmission speed goes down as range increases, and loss of data.

As you can see there are good and bad reasons for transceiver modems. IR modems can be used as point-point or with the use of a conversion box, can be connected through a telephone line. IR is very limited in transmission range, 35ft or less. The biggest advantage that IR has is with its use by the person who travels and needs to communicate with their home base.

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Cable Modems

When cable was first introduced, it was designed for rural areas with poor reception. It is now an enhanced network offering over 500 site of video, music entertainment, and interactive services (web TV). Due to its huge bandwidth, cable is able to deliver a greater amount of data at much faster speeds than analog or Integrated Services Digital Network (ISDN) device. With these kind of speeds and the data size, we have new high-end applications available, such as real-time video, this technology is not quite fully born yet and is still in the development stage.

Lacks of standards are hindering development of an acceptable system. Most cable systems are a one-way system and were not setup for two-way communication needed for modems. According to the research done by the Yankee Group, more than 90% of the cable systems are one-way, making the system impossible for these systems to use Radio Frequency (RF) return path.

In order for one-way systems to take part in this data capability, the smart solution would be to create a hybrid system that used the cable bandwidth for downstream delivery and an analog or ISDN connection for the upstream. This is just what US Robotics and Zenith has done. By working together and using Zenith's "HomeWorks Universal" cable modem and US Robotics "Total Control Enterprise Network Hub" at the cable headend. This system will use US Robotics Total Control Enterprise Network hub to send upstream request through the use of ISDN or standard analog connections. The high-speed downstream data will be supplied by Zenith's Metro Access cable modem system.

The Home Works Universal modem uses an RS-232 cable connecting the cable modem to the US Robotics or legacy modem of ISDN terminal adapter in the subscriber's home. The cable modem sends standard AT commands to the analog device or the ISDN adapter. This system will be software upgradable as the needs arise and will be optimized by the use of US Robotics Quick Connect.

Zenith's Home Works Universal cable modem will accomplish the following tasks:

Network management is available, using standard SNMP commands to the modem. Security is provided in several levels in the cable system. The cable modem is assigned a password. This password is required for the cable modem to pass data up the dial-up link. Invalid users will be denied access to the network. By filtering in the cable modem passes only data packets that are addressed to that computer. This prevents users from snooping the network and viewing others network packets.

This is a promising and exciting system available for data transfer. This hybrid system will allow one-way lines to be used until more two-way lines are available. This new telco return path system is combining the strengths of the Zenith cable modem and US Robotics dial-up technology to become an industry standard and exposing many subscribers to a faster data transfer and internet access.

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Total Control Enterprise Network Hub - T1 Interface & Modems

A dedicated T1 is often considered the entry-level connection for business Internet access. With a data rate of 1.54 Mbps, a T1 connection can support one to hundreds of users.

The WebCC T1 connection has two parts; the Local Loop circuit and the Internet Access. The Local Loop is a point-to-point circuit provided by a telecommunications company to connect the customer premises to a WebCC Point of Presence (POP). WebCC handles these connections at every step, ensuring a single point of contact for the customer. Once your data packets traverse the local loop, they enter the Internet via our ATM switching fabric.

The Total Control Enterprise Network Hub from USRobotics is a network hub designed to integrate data communications components into a single chassis. The advantages from this product are many: cost savings, a single manufacture point of contact, fewer installations, less maintenance, centralized management, and easy upgrades.

This product's first iteration included T1 interface and modems, the design engineers have provided capacity for much more. The Total Control Enterprise Network Hub uses a modular midplane architecture, that virtually supports every type of dial-up application, with X.25; entry to Ethernet, Token Ring and Frame Relay-based networks. The device has integrated NETServer remote access servers, and recently PRI (primary rate interface) ISDN capabilities; and still has capacity in reserve for high-bandwidth application, new high-speed transport, and increased density flow.

This hub is capable of integrating local and wide area networks and managing their data. It houses SNMP-based network management card, 0-16 application/interface cards in a rack-mountablechassis. Because of the "midplane" design, cards are installed from both the front and the back of the chassis; this allows each chassis to designed for their environment.

The application cards are installed from the front and they include: analog/digital quad modems, PRI cards, remote access servers, X.25 PADs, single and dual T1 cards, management card, Windows NT server, etc. And in the back network interface cards are installed supporting Token Ring, Ethernet, Frame Relay or RS-232 connections. This chassis also has two power supplies in the front, one is used for the necessary operations and the other is used as a back up. This device has four separate buses, uses to distribute incoming and outgoing signals and they are Network Management Bus, Time-Division Multiplexing (TDM) Bus, a Packet Bus, and a General-Purpose Bus. Each one of these buses was designed for the type of data it carries.

The Network Management Bus is a 16 Mbps, synchronous serial bus that communicates with all of the application and interface cards, sixteen channels go from the management card to the other cards an sixteen channels return from the application and interface cards to the management card. All 32 channels are 512-Kbps, with a bandwidth of 8 Mbps in each direction, giving a total transmission capacity of 16 Mbps. This hub is designed so that any card that fails will not cause a total system failure. If a card does fail it can be "hot swapped", removed and replaced while the hub is continuing to run.

The TDM Bus is a 16-Mbps serial bus communicating with the T1 or ISDN cards and the modem cards. Using time-division multiplexing, the bus provides 256 full duplex 64-Kbps segments of bandwidth called DS-Os. The TDM bus is a collection of 64Kbps-time slot and there is no networking protocol. The bus is organized to the AT&T CHI highway standard.

Packet Busies a 32-bit parallel bus, capable of operating at clock speeds up to 33 Bus, for a maximum capacity of 1 Gbps. This bus carries data from the modems to any of several gateway cards, such as NETServer remote access server or an X.25 PAD. The bus can transmit data to or from a LAN or a WAN or connect a dial-in caller to LAN-based mail server. The protocol used on the packet bus is standard IEEE 802.2; the hardware employs the NUbus standard developed by Apple. General Purpose Bus was designed for future expansion; it does not currently carry data traffic. It can be used as a backup bus for one of the others or it could be used to run the Ethernet and Token Ring LAN segments. The Total Control Enterprise Network Hub will be primarily installed at telephone company sites, value added networks, corporate sites, Internet service providers, and Online services. This bus will enable its customers to add or exchange the different cards as their communications needs evolve.

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Another type of modem is PCMCIA modems that are build specifically for the PCMCIA slot in a laptop. For laptops an internal modem (PCMCIA) is almost a necessity to prevent having to carry an external modem around with your laptop.

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If you have comments or suggestions, email me at enriquej@mama.indstate.edu

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