Remote access requires two basic components: a remote computer and a remote access system on the network (typically a computer or device running special software called the Remote Access Server). The remote computer connects to the remote access server and then operates just as it would if it were a workstation on the network.
The device most commonly used to connect computers over a public analog phone line is a modem (a contraction of MOdulator/DEModulator ). A sending modem converts digital signals from the computer into analog signals that can be transmitted over telephone lines and other analog media. On the receiving end, the modem changes the analog signals back to digital signals.Because telephone lines can be found almost everywhere, this method of remote communication is readily available to everyone with access to a phone line.
Modems change the digital ones and zeros into analog signals that can be transmitted over telephone lines. The pattern of these analog signals encodes the data for transmission to the receiving computer. The receiving modem then takes the analog signals and turns them back into ones and zeros. This method is slower than a completely digital transmission, but data can travel over longer distances with fewer errors.
A modem can be either internal or external. The key difference between the two is the amount of configuration required. You must configure internal modems with an IRQ and an I/O address, as well as a virtual COM port address, to ensure that they function properly. External modems simply hook to a serial port and don’t require nearly as much configuration.
Configuring an Internal Modem
Internal modems are on expansion cards that fit into a computer’s expansion bus. To that end, themodem you install must be designed to work in the type of expansion bus slot in the computer.Configuring an internal modem is much like configuring any other hardware device. You must setthe IRQ, I/O address, and virtual COM port so that they don’t conflict with other devices.
The only one of these configuration parameters that you haven’t seen before is the virtual COM port. A virtual COM port is a logical designation given by the operating system for a serial port. A computer can have only two physical serial ports, but it can have as many as four logical ports. Each physical port must be associated with a logical port so that the operating system can use it. The same is true for modems (since they are, in fact, serial devices). You must set a modem to use a COM port that is not being used by any other device.
Each COM port shares an IRQ address with another port. COM1 and COM3 share IRQ 4, and COM2 and COM4 share IRQ 3. If two devices are set to different COM ports (COM1 andCOM3, for example) but have the same IRQ, the modem may not function properly. It is usually best to set an internal modem to COM4 because COM1 is most likely to be in use, while COM2 (which shares the IRQ with COM4) is less likely to be used.
Configuring an External Modem
Although almost all modems used today are internal, there are some situations in which external modems may be more appropriate (such as modem pools). Also, some people prefer external modems because they can see the modem’s status lights. Most internal modems use software status lights, which don’t work if the hardware or software is failing. When using external modems, you have two considerations: available serial ports and the UART type.
Available Serial Ports
With the large number of external serial expansion devices available—including modems, cameras, and printers—a spare serial port is often not available, making the use of an internal modem one of your only solutions. Universal Serial Bus (USB) allows up to 127 devices (including modems) to be chained off of a single USB port. This technology eliminates the need for multiple serial ports on a computer, as well as the need to use an internal modem.
UART stands for Universal Asynchronous Receiver-Transmitter and is the chip that manages serial communications. Each set of serial ports shares a UART. The type of UART chip determines the maximum port speed that a particular serial port can handle. There are two main types: the 8250 series and the 16550 series. The primary difference between them is the capacity of the port buffers. Internal modems have built-in, high-speed UART chips, so this isn’t an issue with internal modems.
The 8250 chips have 8-bit buffers that are limited to a maximum speed of 9600bps and are typically found in PCs manufactured before 1986 (before the IBM AT). Starting with the IBM AT, computers have the faster 16450 and 16550 UARTs. These chips use 16-bit buffers and transmit data at a maximum speed of 115,200bps. Any modem faster than 9600bps that will be connected to a PC for remote access requires the use of 16550 UARTs in the PC to get the maximum possible speed. Otherwise, connection speed will be limited to the fastest output speed of the 8250 UART, 9600bps.
When configuring a workstation for remote access, you must configure the software to recognize the modem in addition to configuring the hardware. In Windows 2000, for example, you do so using Device Manager. To open the Device Manager, follow these steps:
Additionally, you must configure the software to initiate and maintain the connection. This means configuring the dialer software (the client software that uses a local modem to dial the remote access server) and the network protocols that the communications will use, including the following:
Even if the hardware is configured properly, the software may not initiate a connection.
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