Overview of Modems

In general, modulator -demodulators (modems) are devices that modulate digital signals into audible sounds and demodulate audible sounds into digital signals. Included in this section are some basic facts about telephone companies, how telephone companies deal with switched services, and the limitations that this places on data connections. Besides the information provided in Chapter 1, "Remote Access Overview," and Chapter 3, "The Cloud," this section also includes information on ways that telcos are improving connection parameters.

A local telco takes the signal from the analog line that terminates at a remote user 's residence and digitizes it to 64 kilobits per second (kbps). This is done so that each call can take the same amount of bandwidth and can be easily placed into larger circuits, such as a Primary Rate Interface (PRI) or channelized digital service 3 (DS3). A 64-kb bandwidth per call is limiting in the amount of data that a modem can push. However, this bandwidth assists the telephone company because they can multiplex hundreds of calls together for ease of transport from one central office (CO) to another.

The quality of digital signals sent over long distances is better than that of analog signals sent over long distances. This is an advantage for modems when calling outside your local CO. Digital signals can be amplified by a device called a repeater. When the signal-to-noise ratio (SNR) drops below acceptable margins, repeaters are often inserted into the line. While amplifying analog signals, the noise is part of the analog signal and is also amplified, which results in lost data because of the noise at a point further down the line.

There are two different types of basic modems. The first is an analog modem, which is the type of modem installed on a user's computer. These can be internal (hardware card installed on a system, motherboard) or external (independent, free-standing hardware). Examples of these modems are 3Com/US Robotics Sportster, Lucent WinModems, and Best Data V.90 modems. These modems demodulate the signals coming from an analog line and pass that data off to your computer. They also modulate the data into analog signals and send it down the analog line to your provider.

The second type of modem is a digital modem. Digital modems are found in most dial access servers that interface directly with the 64-kbps trunks from the telco in the form of a PRI or a channelized DS3. They decode the digital signals directly off the 64-kbps trunk and pass them to the dial-in server. They also place the digital signals directly on the 64-kbps trunk and pass them to the end user.

Digital modems were key to breaking the 33.6-kbps speed barrier that analog modem to analog modem calls could not surpass. With digital modem to analog modem calls, the digital modem can send up to 53,333 bps to the analog modem by placing the digital signals directly on the line, but the analog modem can only send at speeds up to 33,600 bps back to the digital modem because it has to perform a digital to analog conversion prior to sending the data.

Table 5-1 describes the different standards that have evolved modems to today's speeds. The different modulations and the speeds achieved by each of them throughout the years are listed in Table 5-1.

Table 5-1. Modulations and Their Connection Speeds

Standard	Speeds Achieved
CCITT V.21	300 bps ( falls back to 150 and 110 bps)
CCITT V.22	1200 bps (falls back to 600 bps)
CCITT V.22 bis	2400 bps
CCITT V.32	9600 bps (falls back to 7200, 4800, and 2400 bps)
CCITT V.32 bis	14,400 bps (falls back to 12000 bps and V.32)
V.32 ter (terbo) [*]	20,000 bps
V.Fast [*]	28,800 bps Preliminary implementations of V.34
V.FC [*]	28,800 bps An attempt to make V.Fast more compatible between different implementations
ITU-T V.34	28,800 bps (falls back to 26,400, 24,000, 21,600, 19,200, 16,800, and 14,400 bps; V.32 bis; and V.32)
X2 [*]	U.S. Robotics 56,000 bps
K56Flex [*]	56,000 bps
ITU-T V.90	56,000 bps FCC limited to 53,333 bps (falls back to 52,000, 50,667, 49,333, 48,000, 46,667, 45,333, 44,000, 42,667, 41,333, 40,000, 38,667, 38,400, 37,333, 34,667, 34,000, 33,600, 33,333, 32,000, 31,200, 30,667, and 29,333 bps and V.34)
ITU-T V.92	56,000 bps Same as V.90 with added features including Modem on Hold and Quick Connect

^[*] Not a standard, but worth mentioning because they helped pave the way for the standards to evolve .

NOTE

CCITT (Consultative Committee for International Telegraph and Telephone) changed its name to ITU-T (International Telecommunications Union Telecommunication Standardization Sector) on March 1, 1993.

The analog signal loss mentioned previously is easy to notice with modems because they are noise-sensitive devices. After you reach a range of about 15,000 cable feet from your CO, the quality drops to a point where 56k/V.90 is no longer available. The further out you are from the CO, the slower the connection speed becomes.

NOTE

The worst case I have recorded is 42,800 cable feet from a CO, where most modems did not train-up at all, and the ones that did only connected at 4800 to 9600 baud at best. Surprisingly, this was in the city of San Jose, Californiaone of the most populous and up-to-date areas in terms of carrier services.

As speeds increase towards the 56 k limit, the factors that affect a modem's ability to connect and remain at those speeds become more apparent. The number one limiting factor is distance from the CO. The second limiting factor is the SNR (S/N ratio). If there is a lot of crosstalk from other lines, your line will have more noise, which limits your connections. Another factor is the telco's cloud that your call travels across on its way to its final destination.