broadband strategies handbook.pdf - Khazar University

Figure 5.8 Number of Broadband Subscribers Worldwide, 2007–09, by Type ofWireline Technology350300250200150100500subscribers (in millions)Q1 07Q2 07Q3 07Q4 07Q1 08Q2 08Q3 08Q4 08Q1 09Q2 09Q3 09Q4 09DSL cable modem FTTxSource: Point-Topic.Digital Subscriber LineThe public switched telephone network (PSTN) line running to the subscriber’spremise has traditionally been copper wire, with a bandwidth of 3to 4 kilohertz (kHz). This narrowband channel offers an analog carrier originallyconfigured to provide a single telephone call. Two “twisted-pair” copperwires are used to support duplex communications (that is, the ability tosend and receive at the same time). The PSTN has also supported the capabilityfor narrowband Internet access, with subscribers using a modem todial up an ISP.DSL technologies use special conditioning techniques to enable broadbandInternet access over that same PSTN copper wire. Transmissionspeeds vary as a function of the subscriber’s distance from the telephonecompany switching facilities, the DSL version, the extent of fiber in the network,and other factors. DSL requires that the bandwidth over the copperline be separated between voice and data. A quartz crystal splitter is used tofilter the data channel when using the shared copper local loop for telephoneservice. Similarly the voice channel must be filtered when the line isused for broadband Internet access. Nonetheless, users can continue tomake and receive PSTN telephone calls when using DSL data services. As isthe case with dial-up access to the Internet, subscribers must have a modeminstalled between their computer and the copper wire. A DSL modem modulatesupstream signals to the Internet and demodulates downstream trafficto the subscriber.In addition to retrofitting their copper lines, telephone companies alsohave to upgrade their switching facilities in order to split traffic into voice222 Broadband Strategies Handbook