art@ACC.ARPA (01/13/86)
This is not really about tcp/ip, but the excerpt came off this newgroup and someone may find it interesting since most protocols usually get down to a real comm channel. > The broadband communications use one 6MHz channel, > and can handle 10Mbits/sec. (Yes, it is possible to get more bits in > a channel than its bandwidth. This has always seemed to me to violate > some basic principle, but sophisticated communications technology can > get more bits/sec than Hz.) Remember that these types of communication channels are really analog circuits. Whereas digital channels have basically only two variables (binary state and bit duration), analog channels have such variables as frequency, amplitude, phase change. Analog channels use these variables to carry more information in a given unit of time. 9600 4-wire modems still have to communicate over phone circuits which have a useful bandwidth somewhat over 3kHz. This is accomplished by sending 4 bits at a time, 2400 groups/second. The four bits are encoded as one of eight phase shifts (3 bits) and one of two amplitudes (4th bit). The time unit of a particular modulation state is named after Baudot and is called a "Baud", and the rate at which the modulation state changes is the "Baud Rate". Thus, most 9600 bit/second modems are not really "9600 Baud" modems. But, most 300/1200 bit/second modems use Frequency-Shift-Keying (FSK) where the binary state of the data directly controls which of two frequencies is being transmitted. In this case the bit/second rate is the same as the baud rate. Most broadband systems are based on CATV technology developed for video distribution. These systems use a range of frequencies broken into 6mHz bands. By modulating the carrier frequency within a 6 mHz band, each band can carry separate information. The 802.4 token bus protocol used in GM's MAP specifications uses two pairs of adjacent 6mHz channels, one pair for transmitting and the other for receiving. The channel is modulated with a technique called Duobinary AM-PSK which uses both amplitude and phase to convey information at a 10 mHz rate. Unless laboratory developments change things, fiber optics will be used digitally. A laser will generate monochromatic light which is either on or off. But Bell Labs has demonstrated that the light can be switched (and detected) many billions of times per second over useful distances. (24 telephone voice channels only use 1.544 Mbit/sec) <Art@ACC.ARPA> ------
CERF@USC-ISI.ARPA (01/15/86)
Art, about light pipes - recent reports indicate that it is feasible to modulate light by heterodyning as in radio carriers. This is an analog method (as opposed to baseband on/off signalling) which permits much better signal to noise ratios, longer distances between repeaters, and increased total bandwidth usable in a single mode fiber which might otherwise have to use wavelength multiplexing to achieve increased capacity with multiple channels each running in digital on/off mode. Vint P.S. as for 24 telephone channels on the 1.544 Mbps T1, there are commercial compression systems out (one made by Paul Baran!) which will put up to 96 voice channels on a single 1.544 mbps T1 link.
leong%ANDREW@PT.CS.CMU.EDU (John Leong) (01/15/86)
General Interest ..... T1 is the lowest TDM digital transmission rate used by the phone companies for long line transmission. The standard bit rates are : DS1 : 1.544M (24 voice)@* DS2 : 6.312M (4 * DS1, 96 voice)@* DS3 : 44.736M (7 * DS2, 672 voice)@* DS4 : 274.176M (6 * DS3, 4032 voice) Most DS1 or T1 data are done with twisted pairs. Fibre is normally used only for DS4 or T4 data rate which is heavily used for (real long distance) inter-citiy traffics. Leong