Definition of "Baseband," "Broadband," and "Carrier"
In the thread entitled "When is broadband broad enough?", our esteemed colleague in Australia, David Clayton, calls attention to the use of the term "broadband," a word that seems to appear in every news report concerning the internet and connectivity. The use of the terms "baseband," "broadband," and "carrier" have always had a contextual meaning, but more importantly, the definition seems to vary based on who is using the term and why. I am fascinated by language, and always observe word usages carefully whenever I read documents published in different eras. In my mind, the first definition of these three words is always the electrical one - but as David pointed out, not everyone shares that view.
I have here an older (1930's era) copy of the venerable text "Principles of Electricity as Applied to Telephone and Telegraph Work," published in many editions during the last century by the Long Lines Department of AT&T. The word "broadband" appears countless times throughout this edition, but even in this unassailable text it is not always used in the electrical engineering sense. In the electrical sense, "broadband" would refer to a signal the spectral energy of which occupies a broad frequency band, as the name implies. How wide this spectrum must be to qualify as broad is open to some debate, but in the case of any channelized FDM system where carriers of different frequencies are transmitted simultaneously over the same physical medium there can be no such debate - that's broadband. This remains true even if the content of the multiple carriers was originally just one channel of message content, split across multiple modulated carriers. Thus, a typical xDSL stream carrying one channel of user content, even flying solo on a "dry" copper pair, would qualify as broadband by this definition.
But here is where semantics rears it ugly head. In that same text (often referred to simply as "Principles"), a discussion of time-division multiplexing of separate message channels of low speed (telegraph) data into a single higher bitrate stream refers to this as a "broadband" service. Electrically, this stream is anything but broadband - in the case of a conventional alternate-mark encoded bitstream (as this was), the actual electrical signal would appear on an oscilliscope as square wave, albeit a somewhat phase-distorted one. If you were to send all marks and no spaces, the square wave would be quite perfect indeed - and have virtually all of its spectral energy concentrated on a very specific frequency. This, electrically, is the very definition of the narrowest of narrow bands, which is often conflated with "baseband." The use of the word "broadband" to refer to this TDM telegraph stream conforms to the definition David Clayton was discussing, i.e. carrying more than one service, or in this case multiple channels of message content.
The term "baseband" is equally subject to being defined based on context, but even allowing for that, its very meaning seems to have been construed in recent decades. Baseband, in the electrical sense, originally referred to a message signal that had not been "frequency-shifted." A good example of this would be a 1,000 hertz audio tone carried by a standard voice band telephone line. The 1,000 hertz tone playing in your ear from the telephone receiver is of course transduced directly from an electrical waveform of the very same frequency carried on the copper pair. If you were to drop the electrical signal on this copper pair into an old-style analog FDM carrier system, it would be used to modulate a higher frequency carrier wave, or, in VERY archaic language, "frequency-shifted out of the base band." (This is not to be confused with frequency-shift keying, a modulation scheme for radiotelegraph signals that is very far removed from this discussion.)
These days, however, the term baseband has come to mean something very different, particularly in relation to transport of digital message content. Its early use in digital systems applied to any encoded message stream in which the symbol rate (the true meaning of "baud") was directly correlated to the actual bitrate of the message content. Simplex telegraphy is an example of this. A more recent example would be an RS-232 connection, in which the electrical signal state in each symbol slot is directly correlated to a 1 or a 0 in the actual message bit stream. In recent decades, however, ethernet over twisted pair has become king for local area networks, and here is where markets wield their power. The various popular flavors of twisted pair ethernet all have names that contain the word "base," e.g., 100baseT. Sure enough, the "base" is short for baseband, and this misnomer, applied to twisted-pair ethernet at the beginning, has stuck. Putting aside gigabit ethernet (which splits the message data stream across more than one physical circuit), even lowly 10baseT is not actually baseband in the electrical sense. Complex voodoo encoding schemes in all versions of ethernet result in a symbol rate that is indeed lower than the actual content bit rate. Thus, the use of the word "baseband" here is strictly applicable only in the sense that David referred to, i.e., it is used to connect a single node on a network, or at least talk to only one node at a time over a given physical connection. And of course, "baseT" is what everyone in the marketplace calls it, which is all that really matters if you want to do business with everyone else.
This brings us to the word "carrier," a word which often appears in the same sentences as the words "baseband" and "broadband." This word has the hardest life of all - in addition to the electrical meaning and the number-of-channels of message content meaning, this word has yet a third job, that of defining what business a company is in. In the electrical meaning, "carrier" of course refers to a signal which is modulated by the message signal to produce a complex transmitted signal from which the message content can be demodulated at the receiving end. There are too many ways of doing this to even mention, and if you want a good lesson in carrier modulation you can ask your favorite Ham radio enthusiast to explain it. This would be a good first step, because you can't even hope to comprehend the various schemes for doing this with digital message content unless you understand the analogue signal techniques first. Too many textbooks that attempt to explain digital system modulation schemes tend to confuse encoding techniques with modulation. The two are often hoplessly intertwined, such as with QPAM, but they are indeed separate issues (and both are way beyond the scope of this article). An exploration of the encoding and modulation schemes used for modern digital content transport systems will lead the astute reader to one conclusion: These systems are such a complex amalgam of techniques developed over more than a century that it is pure folly to attempt to describe any of them using only one word. This is probably the biggest reason that these semantic debates will always be with us.
The second definition of "carrier" as it relates to the number-of-channels of message content meaning is perhaps the most common usage. Everybody is familiar with "T-carrier," the undisputed king of digital transport of voice for decades. Electrically, a true T1 is in fact a baseband signalling system, virtually identical to the multiplexed telegraph channels described above. But everyone calls it T-carrier because the single bitstream can contain as many as twenty-four separate channels of message content, in effect, "carrying" these channels. The name was applied from the very beginning, and this has always made sense, especially when one considers that it was developed as a replacement for N- and O- carrier systems, which were "carrier" systems in BOTH the electrical sense and the number-of-channels of message content sense.
The last definition of "carrier" is the business meaning - and this may be the most important of all, because without Carriers (with a capital "C"), we wouldn't be able to have this discussion. Carriers, of course, are the service providers that carry all the content that spews from our keyboards, cameras and mouths. Your phone service provider is a Carrier, as is your cable TV compnay, ISP, etc. It is from this meaning that we get the term "Carrier-grade equipment."
There is also a second, elevated class of Carriers - those who operate long-haul networks that cross continents and oceans and whose customers are in fact themselves Carriers. These Carriers never have to deal with us lowly end-user customers, instead providing transport for companies that do, making them Carrier's Carriers. This inspires the following inevitable statement: "Carrier's Carriers carry Carrier's content carefully and continuously on optical carrier." And I suspect that somewhere, on a sunny beach where one of their undersea cables meets land, there is a girl selling sea shells.
All comments to this article are welcome, even if they are to tell me I got it all wrong.
Copyright 2010 A.J. Bennett. All rights reserved. Copyright 2010 Telecom Digest. All rights reserved.
Jim Bennett
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