PDF of this tutorialThe dynamics of the analog-modem market can be traced back to July 1968 when, in its landmark Carterfone decision, the FCC ruled that "the provisions prohibiting the use of customer-provided interconnecting devices were unreasonable."
On January 1, 1969, AT&T revised its tariffs to permit the attachment of customer-provided devices (such as modems) to the public switched network—subject to the following three important conditions:
- The customer-provided equipment was restricted to certain output power and energy levels, so as not to interfere with or harm the telephone network in any way.
- The interconnection to the public switched network had to be made through a telephone company-provided protective device, sometimes referred to as a data access arrangement (DAA).
- All network-control signaling such as dialing, busy signals, and so on had to be performed with telephone-company equipment at the interconnection point.
By 1976, the FCC had recommended a plan whereby current protective devices would be phased out in favor of a so-called registration plan. Registration would permit direct switched-network electrical connection of equipment that had been inspected and registered by an independent agency such as the FCC as technically safe for use on the switched network.
In the post-war era, heavy emphasis on information theory led to the profound and now famous 1948 paper by Claude Shannon providing us with a concise understanding of channel capacity for power and bandlimited gaussian noise channels—our analog telephone channel.
C = Bw * Log2(1+S/N)
This simply states that the channel capacity, C, is equal to the available channel bandwidth, Bw, times the log base 2 of 1 plus the signal-to-noise ratio in that bandwidth. It does not explain how to accomplish this, it simply states that this channel capacity can be approached with suitable techniques.
As customers started buying and using modems, speed and reliability became important issues. Each vendor tried to get as close to the limit expressed by Shannon's Law as they could. Until Recommendation V.32, all modem standards seemed to fall short of this capacity by 9 to 10 db S/N. Estimates of the channel capacity used assumed bandwidths of 2400 Hz to 2800 Hz, and S/N ratios from 24 db to 30 db and generally arrived at a capacity of about 24,000 bits per second (bps). It was clear that error-correction techniques would have to become practical before this gap would be diminished.
Modems of the 1950's were all proprietary—primarily FSK (300 bps to 600 bps) and vestigial sideband (1200 bps to 2400 bps). These devices used or were built upon technology from RF radio techniques developed during the wartime era and applied to wireline communications.
International standardization of modems started in the 1960s. In the 1964 Plenary, the first CCITT Modem Recommendation, V.21 (1964), a 200 bps FSK modem (and now 300 bps) was ratified and is (still) used in the V.34/V.8 handshake. The preferred modulation progressed to 4 Phase (or 2X2 QAM) in 1968, and to 4X4 QAM with V.22bis in 1984. Additionally, in 1984, the next major technological advancement in modem recommendations came with V.32 and the addition of echo cancellation and trellis coding. Trellis codes, first identified by Dr. Gottfred Ungerboeck, were a major breakthrough in that they made it practical to provide a level of forward error correction to modems, realizing a coding gain of 3.5 db, and closing over a third of the gap in realizing the Shannon channel capacity. Recommendation V.32bis built on this and realized improvement in typical-connection S/N ratios and increased the data rates to 14,400 bps.
As work on V.34 started in earnest (1989/90), a recognition of further improvement in the telephone networks in many areas of the world was evident. With this recognition, the initial goal of 19,200 bps moved to 24,000 bps and then to 28,800 bps. The newer V.34 (1996) modem supports 33,600 bps. Such modems achieve 10 bits per Hertz of bandwidth, a figure which approaches the theoretical limits. Recently, a number of companies have introduced a 56.6-kbps analog modem designed to operate over standard phone lines. However, the modem is asymmetrical (it operates at normal modem speeds on the upstream end), it requires a dedicated T1/E1 connection to the ISP site to consistently reach its theoretical limits. For users without such a line the modem offers, inconsistently at best according to reports, a modest gain in performance.
However, the bandwidth limitations of voice band lines are not a function of the subscriber line but the core network. Filters at the edge of the core network limit voice-grade bandwidth to approximately 3.3 kHz. Without such filters, the copper access wires can pass frequencies into the MHz regions. Attenuation determines the data rate over twisted-pair wire, and it, in turn, is a function of line length and frequency. Table 1 indicated the practical limits on data rates in one direction compared to line length.
