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<br /> 20 | P a g e <br />City of San Leandro, CA <br />Fiber Master Plan <br />Digital Subscriber Line (DSL) <br /> <br />DSL is a wireline technology that uses high frequencies, which are not used by analog <br />voice calls, to transmit digital data over traditional copper telephone lines faster than <br />modems. DSL-based broadband provides transmission speeds ranging from several <br />thousand bits per second (Kbps) to millions of bits per second (Mbps), generally ranging <br />from 1.5 Kbps to 10 Mbps. DSL operates over the phone line—in parallel with voice traffic <br />so calls are not affected—which plugs directly into a computer or router at the customer’s <br />site. The other end of the phone line connects to a DSL line card in the telephone <br />company’s central office or remote cabinet. Each user’s data is multiplexed with their <br />neighbors’ over high-capacity fiber, transported to internet interconnection points, then <br />routed over internet backbones to their online destinations. <br /> <br />There are different types of DSL: <br /> <br />• Asymmetrical Digital Subscriber Line (ADSL/ADSL2/ADSL2+) provides faster <br />speed in the downstream direction than the upstream direction. This is fine for most <br />customers who receive a lot of data but do not send much. <br />• Symmetrical Digital Subscriber Line (SDSL) – SDSL has the same speeds as <br />ADSL and is used typically by businesses that generate online content or for <br />services such as video conferencing, which need significant bandwidth both to and <br />from the internet. <br />• Very-high-bit-rate Digital Subscriber Line (VDSL) – is a new generation of <br />technology that provides up to 52/16 Mbps. It is more sensitive to line quality and <br />requires a more expensive line card. <br /> <br />The availability and speed of DSL service depends on the distance from the customer to <br />the closest telephone facility known as a central office. Telephone lines were optimized <br />for voice communications and conditioned to eliminate high frequency noise. <br />Consequently, some telephone lines cannot handle DSL, and others must be modified to <br />support the service. Multiple DSL lines can be bonded to provide higher speeds, but the <br />cost multiplies, too. <br />Digital Carrier Systems <br /> <br />Most commonly known as T-1s, this is the digital telephone standard in the US and has <br />been the mainstay of corporate telecom for years. This service uses a four-wire interface <br />to deliver 1.5 Mbps, which can be subdivided into 24 channels when bonded together. <br />While not falling within today's federal definition of broadband, this is the way many <br />companies get internet access and connect their various facilities. T-1s are almost <br />universally available from local service providers, although they may charge for mileage <br />and other things that make the service rather expensive. The digital services hierarchy <br />extends to multi-megabit services and fits with the even higher bandwidth optical carrier <br />services.