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ADSL, VDSL, VADSL, HDSL, DSL, SDSL, BDSL -- enough for several dizzy spells. Most of these acronyms have relatively clear definitions, but they often suffer confusion, with one another and with other acronyms. (We must say that none will enjoy the fun of ATM, universally believed to mean Automatic Teller Machine, when we insiders know it really refers to that famous occult, Another Telecommunications Medium.)
This monograph hopes to define these terms. Rather than put them in alphabetical order, we have arranged them in chronological sequence of the basic terms, with synonyms, accidents, and a few related terms described briefly thereafter. We also eschew brevity. Glossaries often leave one panting for more. We hope to restore breathing with as little excess verbiage as possible, but with enough words to convey an impression of what a term really means, spiced up with a bit of history and a few application comments. A small table at the beginning summarizes the picture, and we relent at the end, with a terse, alphabetical Glossary.
Copper Access Transmission Technologies
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Name Meaning Data Rate Mode Applications
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V.221 Voice Band Modems 1200 bps to Duplex3 Data communications
V.32 28,800 bps
V.34
DSL Digital Subscriber 160 kbps2 Duplex ISDN service
Line Voice and data comm
HDSL6 High data rate Digital 1.544 Mbps4 Duplex T1/E1 service
Subscriber Line 2.048 Mbps5 Duplex Feeder plant, WAN, LAN
access, server access
SDSL Single line Digital 1.544 Mpbs Duplex Same as HDSL plus
Subscriber Line 2.048 Mbps Duplex premises access for
symmetric services
ADSL Asymmetric Digital 1.5 to 9 Mbps Down8 Internet access, video on
Subscriber Line 16 to 640 kbps Up demand, simplex video, remote
LAN access, interactive
multimedia
VDSL7 Very high data rate 13 to 52 Mbps Down Same as ADSL plus HDTV
Digital Subscriber 1.5 to 2.3 Mbps Up9
Line
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1. Designations are not acronyms, but CCITT recommendation numbers
2. 192 Kbps divides into two B channels (64 kbps), one D channel
(16 kbps) and link administration.
3. "Duplex" means data of the same rate both upstream and downstream
at the same time.
4. Requires two twisted-pair lines
5. Requires three twisted-pair lines
6. A new system called SDSL, for Single Line DSL, operates at 1.5
or 2.0 Mbps duplex over one line
7. Also called BDSL, VADSL, or, at times, ADSL. VDSL is ANSI
and ETSI designation.
8. "Down" means downstream, from the network to the subscriber.
"Up" means upstream.
9. Future VDSL systems may have upstream rates equal to downstream,
but on much shorter lines.
Copper Access Technologies
We are quite used to voice-grade data modems, and their limitations. Voice grade modems presently transmit up to 28.8 kbps over a common telephone line, but the practical limit only twenty years ago was 1.2 kbps. No one believes we can go much faster than 33.6 kbps in the future, however. Voice grade bandwidth does not exceed 3.3 kHz. Modems like V.34 achieve 10 bits per Hertz of bandwidth, a startling figure that approaches theoretical limits. Not only that, V.34 modems transmit and receive simultaneously, in the same band. And you can buy one for under $200. We have these modems because of almost sublime advances in algorithms, digital signal processing, and semiconductor technology.
Voice grade modems operate at the subscriber premises end of voice grade lines and transmit signals through the core switching network without alteration; the network treats them exactly like voice signals. This has been their singular power, that, despite rather slow speeds compared to terminals today, they can be connected immediately anywhere a telephone line exists, and there are nearly 600 million such locations.
Bandwidth limitations of voice band lines do not come from the subscriber line, however. They come from the core network. Filters at the edge of the core network limit voice grade bandwidth to 3.3 kHz. Without filters, copper access lines can pass frequencies into MHz regions, albeit with substantial attenuation. Indeed, attenuation, which increases with line length and frequency, dominates the constraints on data rate over twisted pair wire. Practical limits on data rate in one direction compared to line length (of 24 gauge twisted pair) are:
DS1 (T1) 1.544 Mbps 18,000 feet
E1 2.048 Mbps 16,000 feet
DS2 6.312 Mbps 12,000 feet
E2 8.448 Mbps 9,000 feet
1/4 STS-1 12.960 Mbps 4,500 feet
1/2 STS-1 25.920 Mbps 3,000 feet
STS-1 51.840 Mbps 1,000 feet
Subscriber loop plant configurations vary tremendously around the world. In some countries 18,000 feet covers virtually every subscriber; in others, such as the United States, 18,000 feet covers less than 80% of subscribers. However, the 20% or so remaining have lines with loading coils which cannot be used for any DSL service (including ISDN) without removing the coils. Most telephone companies have had programs to shrink average loop length underway for a number of years, largely to stretch the capacity of existing central offices. The typical technique involves installation of access nodes remote from central offices, creating so-called Distribution Areas with maximum subscriber loops of 6000 feet from the access node. Remote access nodes are fed by T1/E1 lines (now using HDSL) or fiber. In suburban communities a Distribution Area connects an average of 1500 premises; in urban areas, the figure is double, about 3000 premises. Of course the number of premises served dwindles as service data rates increase. A Fiber to the Curb system (FTTC) offering STS-1 rates may only be within reach of twenty homes in some suburban areas.
You now have enough information to be a network planner, presuming the marketing department has handed you a stable list of applications. If that list does not include digital live television or HDTV (but does include video on demand and Internet access), then a data rate of 1.5 Mbps per subscriber terminal downstream may suffice, and you can offer it to virtually everyone within 18,000 feet, the nominal range of ISDN. For subscribers with shorter lines, either to a central office or remote access node, you can offer more than one channel to more than one premises terminal. If digital live television is on the list, then you have to offer at least 6 Mbps, and you may be limited to 4500 foot distances to supply more than one channel at a time. (This fact is the heart of telephone company interest in wireless broadcast digital TV, and the consequent Balkanization of its future network.) Clearly HDTV, demanding as much as 20 Mbps, only goes over the shortest loop length.
Of course, this offering of digital services over existing twisted-pair lines requires transceivers, special modems capable of dazzling data rates when one considers the age and original intentions of twisted-pair wiring technology. It turns out that this effort to use twisted pair for high speed information began many years ago.
DSL -- Digital Subscriber Line
The basic acronyms for all DSL arrangements came from Bellcore, so we must blame them for the basic confusion between a line and its modems. In general we say that DSL signifies a modem, or a modem pair, and not a line at all. Yes, a modem pair applied to a line creates a digital subscriber line, but when a telephone company buys DSL, or ADSL, or HDSL, it buys modems, quite apart from the lines, which they already own. So, DSL is a modem, not a line. This confusion becomes quite important to avoid when we talk about prices. A "DSL" is one modem; a line requires two.
DSL itself, apart from its later siblings, is the modem used for Basic Rate ISDN. A DSL transmits duplex data, i.e., data in both directions simultaneously, at 160 kbps over copper lines up to 18,000 feet of 24 ga wire. The multiplexing and demultiplexing of this data stream into two B channels (64 kbps each), a D channel (16 kbps), and some overhead takes place in attached terminal equipment. By modern standards DSL does not press any transmission thresholds, but its standard implementation (ANSI T1.601 or ITU I.431) employs echo cancellation to separate the transmit signal from the received signal at both ends, a novelty at the time DSL first found its way into the network.
DSL modems use twisted-pair bandwidth from 0 to about 80 kHz. (Some European systems use 120 kHz of bandwidth.) They therefore preclude the simultaneous provisioning of analog POTS. However, DSL modems are being used today for so-called pair gain applications, in which DSL modems convert a single POTS line to two POTS lines, obviating the physical installation of the second line wiring. The telephone company just installs the analog/digital voice functions at the customer premises for both lines, and presto, two from one.
T1 or E1
In the early sixties engineers at Bell Labs created a voice multiplexing system that first digitized a voice signal into a 64 kbps data stream (representing 8000 voltage samples a second with each sample expressed in 8 bits) and then organized twenty four of them into a framed data stream, with some conventions for figuring out which 8 bit slot went where at the receiving end. The resulting frame was 193 bits long, and created an equivalent data rate of 1.544 Mbps. The structured signal was called DS1, but it has acquired an almost colloquial synonym -- T1 -- which also describes the raw data rate, regardless of framing or intended use. AT&T deployed DS1 in the interoffice plant starting in the late sixties (almost all of which has since been replaced by fiber), and by the mid-seventies was using DS1 in the feeder segment of the outside loop plant.
In Europe, and at CCITT (now ITU), the collection of world PTTs other than ATT modified Bell Labs original approach, as they were wont to do, and defined E1, a multiplexing system for 30 voice channels running at 2.048 Mbps. In Europe E1 is the only designation, and stands for both the formatted version and the raw data rate.
Until recently, T1 and E1 circuits were implemented over copper wire by using crude transceivers with a self-clocking Alternate Mark Inversion (AMI) protocol. AMI requires repeaters 3000 feet from the central office and every 6000 feet thereafter, and takes 1.5 MHz of bandwidth, with a signal peak at 750 kHz (U.S. systems). To a transmission purist, this is profligate and ugly, but it has worked for many years and hundreds of thousands of lines (T1 and E1) exist in the world today.
Telephone companies originally used T1/E1 circuits for transmission between offices in the core switching network. Over time they tariffed T1/E1 services and offered them for private networks, connecting PBXs and T1 multiplexors together over the Wide Area Network (WAN). Today T1/E1 circuits can be used for many other applications, such as connecting Internet routers together, bringing traffic from a cellular antenna site to a central office, or connecting multimedia servers into a central office. An increasingly important application is in the so-called feeder plant, the section of a telephone network radiating from a central office to remote access nodes that in turn service premises over individual copper lines. T1/E1 circuits feed Digital Loop Carrier (DLC) systems that concentrate 24 or 30 voice lines over two twisted pair lines from a central office, thereby saving copper lines and reducing the distance between an access point and the final subscriber.
Note, however, that T1/E1 is not a very suitable service for connecting to individual residences. First of all, AMI is so demanding of bandwidth, and corrupts cable spectrum so much, that telephone companies cannot put more than one circuit in a single 50 pair cable, and must put none in any adjacent cables. Offering such a system to residences would be equivalent to pulling new wire to most of them. Secondly, until recently no application going to the home demanded such a data rate. Thirdly, even now, as data rate requirements accelerate with the hope of movies and high speed data for everyone, the demands are highly asymmetric -- bundles downstream to the subscriber, and very little upstream in return -- and many situations will require rates above T1 or E1. In general, high speed data rate services to the home will be carried by ADSL or VDSL (or similar types of modems over CATV lines).
HDSL -- High data rate Digital Subscriber Line
HDSL is simply a better way of transmitting T1 or E1 over twisted pair copper lines. It uses less bandwidth and requires no repeaters. Using more advanced modulation techniques, HDSL transmits 1.544 Mbps or 2.048 Mbps in bandwidths ranging from 80 kHz to 240 kHz, depending upon the specific technique, rather than the greedy 1.5 MHz absorbed by AMI. HDSL provides such rates over lines up to 12,000 feet in length (24 ga), the so-called Carrier Serving Area (CSA), but does so by using two lines for T1 and three lines for E1, each operating at half or third speed.
Most HDSL will go into the feeder plant, which connect subscribers after a fashion, but hardly in the sense of an individual using a phone service.
Typical applications include PBX network connections, cellular antenna stations, digital loop carrier systems, interexchange POPs, Internet servers, and private data networks. As HDSL is the most mature of DSL technologies with rates above a megabit, it will be used for early-adopter premises applications for Internet and remote LAN access, but will likely give way to ADSL and SDSL in the near future.
SDSL -- Single line Digital Subscriber Line
On its face SDSL is simply a single line version of HDSL, transmitting T1 or E1 signals over a single twisted pair, and (in most cases) operating over POTS, so a single line can support POTS and T1/E1 simultaneously. However, SDSL has the important advantage compared to HDSL that it suits the market for individual subscriber premises which are often equipped with only a single telephone line. SDSL will be desired for any application needing symmetric access (such as servers and power remote LAN users), and it therefore complements ADSL (see below). It should be noted, however, that SDSL will not reach much beyond 10,000 feet, a distance overwhich ADSL achieves rates above 6 Mbps.
ADSL -- Asymmetric Digital Subscriber Line
ADSL followed on the heels of HDSL, but is really intended for the last leg into a customer's premises. As its name implies, ADSL transmits an asymmetric data stream, with much more going downstream to the subscriber and much less coming back. The reason for this has less to do with transmission technology than with the cable plant itself. Twisted pair telephone wires are bundled together in large cables. Fifty pair to a cable is a typical configuration towards the subscriber, but cables coming out of a central office may have hundreds or even thousands of pairs bundled together. An individual line from a CO to a subscriber is spliced together from many cable sections as they fan out from the central office (Bellcore claims that the average U.S. subscriber line has twenty-two splices). Alexander Bell invented twisted pair wiring to minimize the interference of signals from one cable to another caused by radiation or capacitive coupling, but the process is not perfect. Signals do couple, and couple more so as frequencies and the length of line increase. It turns out that if you try to send symmetric signals in many pairs within a cable, you significantly limit the data rate and length of line you can attain.
Happily, the preponderance of target applications for digital subscriber services are asymmetric. Video on demand, home shopping, Internet access, remote LAN access, multimedia access, specialized PC services all feature high data rate demands downstream, to the subscriber, but relatively low data rates demands upstream. MPEG movies with simulated VCR controls, for example, require 1.5 or 3.0 Mbps downstream, but can work just fine with no more than 64 kbps (or 16 kbps) upstream. The IP protocols for Internet or LAN access push upstream rates higher, but a ten to one ratio of down to upstream does not compromise performance in most cases.
So ADSL has a range of downstream speeds depending on distance:
Up to 18,000 feet 1.544 Mbps (T1)
16,000 feet 2.048 Mbps (E1)
12,000 feet 6.312 Mbps (DS2)
9,000 feet 8.448 Mbps
Upstream speeds range from 16 kbps to 640 kbps. Individual products today incorporate a variety of speed arrangements, from a minimum set of 1.544/2.048 Mbps down and 16 kbps up to a maximum set of 9 Mbps down and 640 kbps up. All of these arrangement operate in a frequency band above POTS, leaving POTS service independent and undisturbed, even if a premises ADSL modem fails.
As ADSL transmits digitally compressed video, among other things, it includes error correction capabilities intended to reduce the effect of impulse noise on video signals. Error correction introduces about 20 msec of delay, which is much too much for LAN and IP-based data communications applications. Therefore ADSL must know what kind of signals it is passing, to know whether to apply error control or not (this problem obtains for any wire-line transmission technology, over twisted pair or coaxial cable). Furthermore, ADSL will be used for circuit switched (what we have today), packet switched (such as an IP router) and, eventually, ATM switched data. ADSL must connect to personal computers and television set top boxes at the same time. Taken together, these application conditions create a complicated protocol and installation environment for ADSL modems, moving these modems well-beyond the functions of simple data transmission and reception.
VDSL -- Very high data rate Digital Subscriber Line
VDSL began life being called VADSL, because at least in its first manifestations, VDSL will be asymmetric transceivers at data rates higher than ADSL but over shorter lines. While no general standards exist yet for VDSL, discussion has centered around the following downstream speeds:
12.96 Mbps (1/4 STS-1) 4,500 feet of wire
25.82 Mbps (1/2 STS-1) 3,000 feet of wire
51.84 Mbps (STS-1) 1,000 feet of wire
Upstream rates fall within a suggested range from 1.6 Mbps to 2.3 Mbps. The principal reason T1E1.4 decided against "VADSL" was the implication that VDSL would never be symmetric, when some providers and suppliers hope for fully symmetric VDSL someday, recognizing that line length will be compromised.
In many ways VDSL is simpler than ADSL. Shorter lines impose far fewer transmission constraints, so the basic transceiver technology is much less complex, even though it is ten times faster. VDSL only targets ATM network architectures, obviating channelization and packet handling requirements imposed on ADSL. And VDSL admits passive network terminations, enabling more than one VDSL modem to be connected to the same line at a customer premises, in much the same way as extension phones connect to home wiring for POTS.
However, the picture clouds under closer inspection. VDSL must still provide error correction, the most demanding of the non-transceiver functions asked of ADSL. As public switched network ATM has not begun deployment yet, and will take decades to become ubiquitous, VDSL will likely be asked to transmit conventional circuit and packet switched traffic. (Indeed, a recent telephone company RFQ describes a VDSL-type transceiver with three circuit-switched video channels and a single ATM channel.) And passive network terminations have a host of problems, some technical, some regulatory, that will surely lead to a version of VDSL that looks identical to ADSL (with inherent active termination) except its capability for higher data rates.
VDSL will operate over POTS and ISDN, with both separated from VDSL signals by passive filtering.
Other Terms
VDSL had been called "VASDL" or "BDSL" or even "ADSL" prior to June, 1995, when T1E1.4 chose "VDSL" as the official title. The other terms still linger in technical documents created before that time and media presentations unaware of the convergence. ETSI TM3, the European counterpart to T1E1.4, has also adopted "VDSL," but temporarily appends a lower case "e" to indicate that, until the dust settles, the European version of VDSL may be slightly different than the U.S. version. This is the case with both HDSL and ADSL, although there is no convention for reflecting the differences in the name. The differences are sufficiently small (mostly surrounding data rates) that silicon technology accommodates both.
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