3 8Mbps down- 3.5 miles Asymmetrical stream, 1Mbps (5.5 km) upstream RADSL RADSL 600Kbps-7Mbps 3.5 miles Symmetrical downstream, (5.5 km) 64Kbps-1Mbps asymmetrical upstream G.993.1 55Mbps down- 1 mile Symmetrical stream, 15Mbps (1.5 km) upstream asymmetrical VDSL2 G.993.2 55Mbps down- 2.5-3 Symmetrical VDSL2- stream, 30Mbps miles 12MHz upstream (4-5km) symmetrical G.993.2 100Mbps in each 0.3 miles Symmetrical VDSL2- direction (0.5 km) 30MHz asymmetrical DSL Technology HDSL is the oldest of the DSL technologies. It has been in full use for over a decade, and it is most commonly used by telcos to provision T-1 or E-1 services. HDSL allows carriers to do SO at a reduced cost because it does not require special repeaters, loop conditioning, or pair selection in order to deliver those services. HDSL reduces the cost of provisioning T-1/E-1 because of the way it delivers the bandwidth (see Figure 12.1). A traditional T-1/E-1 environment makes use of two twisted-pairs. Each pair carries the full data rate, which is 1.5Mbps with T-1 and 2.048Mbps with E-1. Because each pair is carrying such a high data rate, higher frequencies need to be used; as a result, repeaters need to be spaced roughly every 0.5 to 1 mile (900 to 1,800 m). Furthermore, no bridge taps are allowed in the tra- ditional T-1/E-1 environment. HDSL is a symmetrical service, meaning that it provides equal bandwidth in both directions. In addition, it is full-duplex, so it allows communication in both directions simultaneously. HDSL modems contain some added intelligence, in the form of inverse multiplexers. Because of these multiplexers, each pair carries only half of the data rate (and the receiver combines the two to deliver the full data rate), and as a result, those bits can ride in the lower range of frequencies, thus extending the distance over which they can flow without the need for a repeater. The allocation of bandwidth on HDSL depends on whether it is operating on T-1 or Local Exchange Remote Terminal Customer 1.544-2.048Mbps Traditional T-1/E-1