Previous | Table of Contents | Next |
Although the TSS V.42 Recommendation supports MNP error control, the V.42bis Recommendation which is a data compression scheme that requires the use of the V.42 protocol does not support a secondary method of compression. This theoretically means that for a portable computer users V.42bis modem to operate in a compressed mode, it must communicate with another V.42bis-compatible modem. Fortunately, most V.42bis modems also include MNP support through class 5, enabling users to communicate in both a data compressed and error control mode with another MNP modem.
The key difference between V.42bis and MNP class 5 or class 7 is in their method of data compression. V.42bis uses a technique known as Lempel-Ziv, which can operate on single characters as well as on strings. In comparison, MNP compression primarily operates on single characters and is slightly less efficient. When using a V.42bis modem that incorporates V.32 modulation, the 9,600-b/s operating rate of the modem may achieve a throughput of as much as 28,400-b/s when data is highly susceptible to compression.
Interface vs. Operating Rate. To effectively use data compression, users are required to set their modems interface data rate to exceed its operating rate. To understand why this is required, Exhibit 2 illustrates the relationship between the interface speed and the operating rate of a modem.
Exhibit 2. Interface Rate Vs. Operating Rate
If the interface speed equals the operating rate, data compression will not increase the throughput of the modem. To increase the modems throughput, users must set the interface rate to at least twice the modems operating rate. For example, if the modem operates at 2,400 b/s, the interface rate should be set to at least 4,800 b/s. This allows more bits in the form of characters to enter the modem per unit of time than can be transmitted. Then, data compression can attempt to reduce the bits entering the modem so they can be transmitted at the modems operating rate, increasing the throughput through the modem.
Flow Control. A second consideration users must take into account when using either error control by itself or with data compression is flow control. To see why flow control is necessary, Exhibit 2 shows the interface speed set to 4,800 b/s, while the operating rate is 2,400 b/s. If the modem cannot effectively compress data, perhaps because of the composition of the data, the rate data enters the modem will exceed the rate it is placed onto the line. Although the modem has a buffer, eventually it will overflow and data will be lost. Therefore, a method is required to regulate the flow of data into the modem. This method is called flow control.
Most modems support at least two methods of flow control, referred to as XON/XOFF and CTS/RTS. The XON/XOFF method of flow control results in the modem transmitting an XOFF character to the portable computer to disable transmission and an XON character to enable transmission to resume. Because the computer may require an ability to regulate data from the modem if users are printing received data or performing another mechanical related operation, they can also use XON and XOFF from the portable computer to the modem to enable and disable transmission from the modem to the portable computer.
The CTS/RTS method of flow control refers to the use of clear to send CTS and request to send RTS control signs on the interface between the computer and the modem. When the modem regulates flow control, it will drop the CTS signal to inform the computer to stop transmission and raise the CTS signal lead to inform the computer to resume transmission. When the computer regulates the flow of data from the modem, it will toggle the RTS signal lead.
In setting flow control, users must ensure that their computer and modem are set to use the same method. Concerning which method to select, if users expect to transfer files that may contain control characters, including an XOFF, they should use the CTS/RTS method of flow control. Otherwise, the reception of an XOFF would inadvertently turn off the computer.
To enable most of the features discussed in this section requires the use of the users modems command set. Although most modems claim Hayes Microcomputer Products command set compatibility, such compatibility only governs basic modem operations. To enable or disable MNP operations, V.42 error control, V.42bis data compression, or flow control requires the use of the modems extended command codes. Because a modems extended command set codes are not standardized, users should refer to their modems reference manual to determine the codes to use to enable or disable the previously discussed features.
The remainder of this chapter focuses on the step-by-step procedures portable computer users must follow to access several popular packet-switching networks, electronic mail systems, and an information utility. Included for each are the customer service telephone number users can call to obtain an appropriate local telephone access number, a WATS access number if applicable, log-in procedures, including communications configuration parameter settings necessary to access each system, and a summary of commands when appropriate for accessing a special feature of the system.
Customer service telephone numbers:
The log-in procedures for British Telecom (BT) TMNET are:
Primary setting: | 8 data bits, 1 stop bit, no parity |
Secondary settings: | 7 data bits, 1 stop bit, mark parity |
7 data bits, 1 stop bit, space parity |
where: | [Ctrl] | Optional control characters | |
Ctrl-X | Transmit data flow control | ||
Ctrl-R | Receive data flow control | ||
Ctrl-I | Interactive data session request | ||
Ctrl-V | Volume bandwidth request | ||
Ctrl-H | Half-duplex request | ||
Ctrl-P | Even parity request | ||
Ctrl-W | Wipe log-in string | ||
username | Users network identification | ||
destination | Address of computer connected to BT TYMNET accessed | ||
password | security mechanism that verifies use of user name |
Previous | Table of Contents | Next |