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Notice the transmission rate of 160 KHz duplex. Duplex mode is necessary since the B channels are utilized for voice traffic. Imagine having a one-sided conversation (simplex mode) with the.... (fill in the blank). Standard ANSI T1.601 (ITU I.431) implementations employ echo cancellation to separate the transmit and receive signals at each end of the transmission path. Without echo cancellation, the users think they are in the bottom of the same well when talking over DSL. Users should make sure DSL modems are ANSI compliant, else there will be problems of the unwanted kind.
Since DSL bandwidth is 0 to 80 KHz (U.S.), analog POTS cannot be simultaneously provisioned on the same circuit. But, a DSL modem can be connected to each end of a single POTS line to effectively give two digital voice lines. The gain in voice channels is possible by using each B channel as a separate voice line. The installation and use of two DSL modems to double the number of voice channels is called an (au) pair gain application.
Figure 9-4. Maximum ADSL transmission distance
Figure 9-5. ADSL streams
Various consumer services drive the demand for ADSL technology. We would love to have one box in our homes, and businesses, that can connect to the Internet and video service providers while receiving and sending faxes, providing videotelephony and video conferencing, and still give us the same POTS we are accustomed to enjoying. Will the day arrive when all home communications will be provided for out of a single box? Probably not. The ADSL modem will be a termination box, similar to a CATV set-top box, with the various services transported from the box through the home over appropriate cabling. However, some enterprising companies will try to sell us a one-box-fits-all product with integrated TV, computer, and telephone functionality. I can only envision an ugly fit.
ADSL is characterized by speeds of 1.5 MHz to 10 MHz downstream, 16 to 640 Kbps upstream, and 1.5 kilometer maximum transmission distance. See Figure 9-4.
Since most of the data traffic is one way, from service provider to user, such as an Internet connection, and there is much less traffic volume from the user to the service provider, the term asymmetric was attached to DSL to distinguish the primarily one-way ADSL medium from the full duplex, two-way DSL traffic medium. See Figure 9-5.
ADSL is used for Internet access, VOD, remote LAN access, and interactive multimedia. In the future it will be used with circuit switched networks, packet switched networks, such as IP routers, and ATM switched networks.
ADSL modems incorporate error correction algorithms, called CRCs, intended to reduce the effect of impulse (white) noise on digitally compressed video signals. Impulse noise is noise generated in a random fashion such as the noise heard on AM radio caused by lightning. The error correction algorithm introduces approximately 20 microseconds (msec) of signal delay to the bit stream. The artificially introduced 20 msec delay for digitally compressed video signals is anathema to LAN- and Internet-based data communications applications. ADSL modems must know whether to apply error correction or not to any particular signal.
ADSL modems incorporate adaptive line algorithms that enable the modem to deal with noisy lines in an "intelligent" way. Let us meet the DSL family. One wifey, one hubby, no pets, and no kids. Suppose the hubby is watching television, say the Super Bowl, one night when the wifey decides to wash her hair. All is well until she plugs the 10,000 watt hair dryer with a 1.8 MHz AC motor into the bathroom outlet. Now, on cable or broadcast TV the picture will get considerable interference in it. The picture will be buggered up enough that the hubby will probably shout some amount of unmentionable remarks. But our DSL family remains one happy bunch of lovey-dovey birds. The DSL modem actually detects the frequency of interference, which is 1.8 MHz, and blocks out that frequency and so many above and below for use. The DSL hubby never notices any picture degradation. Smart machine, eh? The preceding discussion is for educational purposes only. I have not seen any 1.8 MHz, 10,000 watt hair dryers... yet. So, adaptive line algorithm is a cryptic way of saying the machine does not use the portion of the bandwidth that has a high noise level for the moment.
Connected to the ADSL modem on the customer premises side will be stereos, televisions, computers, fax machines, telephones, and virtually any other audio, musical, video multimedia gadget, and data contraption you can think of. Simultaneous reception of various services and communications upstream and downstream are features of ADSL modem technology that analog modems can only dream of. Want to overdose on technology? Hurry! Hurry! Hurry! Get your ADSL modem connected to all your electronic goodies now!
Future use of ADSL technology is limited only by the demands of a communications-hungry world. Circuit switched, packet switched, cell switched, operating in duplex mode, able to receive and transmit simultaneously, monitoring and evaluating user input such as service specific selections, ADSL modems must be much more intelligent than just plain vanilla flavor voice frequency analog modems. ADSL modems must be smart modems, able to distinguish between and among all the bewildering choices of services and service options available.
High Data Rate Digital Subscriber Line (HDSL) is characterized by two transmission speeds, 1.544 Mbps and 2.048 Mbps, with a bandwidth dependent upon the particular information encoding scheme used. A transmission speed of 1.544 Mbps (T1) duplex requires two twisted pair lines from the source to the destination. A transmission speed of 2.048 Mbps (E1) duplex requires three twisted pair lines from source to destination.
HDSL is used for interconnecting T1/E1 service, interoffice trunks, LAN/WAN access, server access, cellular antenna stations, digital loop carrier systems, and interexchange POPS. HDSL transmits 1.544/2.048 Mbps in bandwidths ranging from 80 KHz to 240 KHz, depending upon the data encoding technique. The consumer will not really see HDSL as it will be used in local exchange carrier (LEC) trunking.
Very High Rate DSL (VDSL) is sometimes called BDSL, VADSL, or even ADSL. In June 1995, the ADSL Forum T1E1.4 group chose VDSL as the official moniker. Confusing, eh? The key to recognizing any particular flavor of DSL is the transmission distance and the rate of transmission. In this case, VDSL is good for a maximum of 1,500 meters.
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