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Subscriber loop configurations vary all around the globe. In some places, 18,000 feet from the local exchange to the subscriber premises may cover every subscriber in the loop. But in other places, 18,000 feet may not. In the United States, it is estimated that 18,000 feet will cover about 80 percent of all subscribers. The 20 percent of subscribers outside the 18,000 feet zone use loading coils that prevent the use of DSL technology on those lines. The coils will have to go. Installing remote access nodes, thereby creating an ADSL distribution area, allows the removal of the loading coils, virtually shrinking the distance from subscriber to local exchange to an average of 6,000 feet. The practice of installing distribution areas is an ongoing event that telephone companies implemented several years ago to increase the capacity of the existing central/local office. Remote access nodes are connected to T1/E1 lines/fiber passing HDSL data. Distribution areas cover an average of 1,500 suburban subscribers and an average of 3,000 urban subscribers. The number of subscribers in a distribution area is greatly dependent upon the subscribed data rates, as each distribution area has a maximum data rate ceiling in its service area that is, of course, directly related to transport capacity.
Service | Required Bandwidth |
VOD (MPEG) | 1.5 Mbps |
Internet/multimedia | 1.5 Mbps |
live digital TV | 6.0 mbps |
HDTV | 20 Mbps |
Table 9-2. Consumer services required bandwidth
Table 9-2 details the bandwidth requirements of several popular consumer services. VOD requires a minimum 1.5 Mbps bandwidth, making 56.6 Kbps analog modems too lightweight to supply VOD services. But ADSL modems, if you are within 18,000 feet of a distribution area, have sufficient bandwidth muscle to provide VOD services.
An ADSL modem combines the total multiplexed data stream that includes the service data, such as a request to view a particular VOD channel, and maintenance channels from the service provider to the user into blocks of data; appends error correction codes, called cyclic redundancy check (CRC), to each block of data; and ships the data block out to the user. The users modem receives the data block, decodes the error correction codes, then repairs the data block, if necessary, within limitations. The modems do have an option to allow the construction of superblocks of data by interleaving data within portions of the blocks. The use of superblocks allows the modem to correct specific combinations of bit errors within certain spans of bits which helps the quality of data and video signals.
A review of some basic terminology is appropriate at this point. Downstream means the transmission path from the network, or local access, to the subscriber. Upstream means the transmission path from the subscriber to the network. Duplex refers to the ability to carry on a simultaneous two-way conversation over a single transmission path. Simplex refers to the ability to carry on a one-way conversation over a transmission path. The DSL designations in the following discussions are in accordance with ANSI and ETSI naming conventions.
Transmission Technology | Maximum Data Rate | Applications |
analog modem | 56.6 Kbps | low speed data communications |
DSL | 160 Kbps | ISDN voice and data communications |
HDSL | 1.544-2.048 Mbps | T1/E1, feeder plant, LAN/WAN access |
SDSL | 1.544-2.048 Mbps | T1/E1, feeder plant, LAN/WAN/premises access |
ADSL | 1.5-10 Mbps | Internet, VOD, Remote LAN, multimedia |
VDSL | 13-52 Mbps | Internet, VOD, Remote LAN, multimedia, HDTV |
Table 9-3. Copper twisted pair transmission technologies
Compare any of the DSL modem technologies with the voice frequency analog modem. From 200 bps in the early days to 56.6 Kbps now, voice frequency analog modems have evolved over the years in an almost herculean effort in the race to keep pace with the demand for faster data communications. Voice frequency analog modems, while undoubtedly occupying a place in our communications networks for some years to come, and a hallowed place in the Communications Hall of Fame, located somewhere near Dallas, Texas (in my closet), have lost the race to DSL modems.
The maximum bit rate is selectable by the user at the time service is ordered. This is a "you pay for what you get" conceptthe more bandwidth you order, the more it will cost. Bit rates are selectable in increments with 32 Kbps the most common increment. In actual practice, residential users will probably not get an opportunity to select bit rates. Each type of service such as video or Internet will come with some particular bit rate that is tailored to the needs of the service. Yet those users who are interested in data transmission/reception service, typically business customers such as remote LAN access to ATM, will probably be able to select the bit rate they wish to transmit/receive.
DSL is, of course, the acronym for Digital Subscriber Line. This is the DSL that is a specific transmission protocol and speed. This DSL really should be called SLMDSL for Slow Like (cold) Molasses DSL. DSL transmission rate is 160 Kbps duplex. DSL is used for Basic Rate ISDN and consists of two B channels of 64 Kbps each and one D channel of 16 Kbps. The two B channels are used for voice and data traffic. The D channel is used for management (signaling and routing) purposes.
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