Analog Dialup Services

The analog modem dialup service gained popularity because of constant increases in speed and support from modem manufacturers. The initial offering started with speeds from 300 to 2400 bps, then to 9600 bps, 14.4 kbps, and so on. The term modem is an abbreviation for modulator-demodulator. Because the modem operates in the 4-kHz bandwidth, it requires analog to digital and digital to analog conversion on both ends. Part of this technology uses extended code correction and compression, which allows the speed to increase up to a factor of 4.

A significant event of the modem offering was in 1968, when the Federal Communications Commission (FCC) regulations permitted CPE to be connected to the Public Switched Telephone Network (PSTN). Another major event in the history of network communications was a paper written in 1948 by Claude Shannon, which established the theoretical limits of the speed that the modem can operate. The equation (see Chapter 2, "Telecommunication Basics") is called Shannon's limit, or Shannon's Law, and defines the upper theoretical limit of the data rate if proper technology is used.

The first attempt for standardization occurred in 1968, where the Comité Consultatif International Téléphonique et Télégraphique (CCITT), which is today called the International Telecommunication Union Telecommunication Standardization Sector (ITU-T), established a recommendation known as V.21, which defines the 200 (300 bps) modem speed. V.21 is still in use today as part of the V.34 and V.8 handshake that establishes basic modem compatibility. In 1984, the V.32 standard defined echo-cancellation and trellis coding. These standards provide some degree of forward error correction (FEC) techniques, and V.32b defines data rates of up to 14.4 kbps. The V.34 standard supports data rates of 24,

28.8, and 33.6 kbps, and coding 10 bits in one sinusoid (10 bits/Hz), which approaches the theoretical limit of Shannon's Law.

The announcement of 56 k modems by US Robotics, based on using T-carriers for dial-in service, was a major breakthrough. Analog modems operate over non-conditional circuits; not all 256 levels of pulse code modulation (PCM) coding are recognizable. The modems use 7 instead of 8 bits, which yields 128 levels of PCM and consequently 7 bits x 8000 = 56 kbps. Without any special requirements, this service requires a T1 or ISDN digital trunk on one end that supports 56/64 kbps, and a regular analog line on the other end. Both ends of the connection are usually asymmetric. From the modem side, the downstream connection has a fairly good chance to achieve 56 kbps most of the time because quantizing digital to analog is not susceptible to noise and interference on the line (see the section, "PCM" in Chapter 2).

Nevertheless, such things as local loop issues, bridge taps, crosstalk, and so on usually decrease the data rate to a more common 48 to 50+ kbps. The upstream direction is different. Analog to digital transformation is susceptible to different types of interference. Regardless of the connection speed reported by the dial management applications, the upstream speed is usually 28.8 or 33.6 kbps. This fact is not visible to the end user because the Internet is more of a downstream than upstream service.

A remote access implementation is usually based on access servers, or remote access servers, from which the remote user connects to the corporate network access server (NAS) by using a designated phone number(s). (See more about access servers at www.cisco.com/ warp/public/44/jump/access_servers.shtml.)

Some telecommunication companies that provide global dialup service from all over the world use global dialer software, where the user chooses the country and the available phone numbers within the country to connect to the corporate network. The dial numbers are predefined and populated in a dial manager application that is standard on a user's laptop or PC.

In the U.S., an option for dial services is to use 800 toll-free numbers, where the enterprise owns the toll-free number, and pays a fixed per-minute usage fee. The 800 numbers eliminate the need for a more expensive and cumbersome calling card option to connect from long distance locations. Be aware that many hotels now charge a usage fee for 800 calls made from the room, which can significantly increase the dialup cost.

Another analog dial option is to use any ISP to connect to the Internet and use VPN-based solutions to connect to the corporate network. This solution is the most cost-effective and all signs in the dial market show that VPN will be the preferable dial solution in the near future. One significant feature of using a dedicated concentrator for VPN over dial is the option to use compression (LZS, STAC), so that the overhead of VPN packets can be overcome and yield better performance. Also, today's VPN client software is built right into many popular operating systems.

You learn more about dialup technologies in Part II, "Dial."

14 Chapter 1: Remote Access Overview

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