1 Chapter 12 Access and Interconnection Technologies2 Internet Access Technology: Upstream and Downstream • Internet access technology refers to a data communications system that connects an Internet subscriber to an ISP – such as a telephone company or cable company • How is access technology designed? • Most Internet users follow an asymmetric pattern – a subscriber receives more data from the Internet than sending • a browser sends a URL that comprises a few bytes • in response, a web server sends content3 Narrowband and Broadband Access Technologies • A variety of technologies are used for Internet access • They can be divided into two broad categories based on the data rate they provide – Narrowband – Broadband4 Narrowband Access Technologies – Refers to technologies that deliver data at up to 128 Kbps – For example, the maximum data rate for dialup noisy phone lines is 56 Kbps and classified as a narrowband technology5 Broadband Access Technologies – generally refers to technologies that offer high data rates, but the exact boundary between broadband and narrowband is blurry • many suggest that broadband technologies deliver more than 1 Mbps • but this is not always the case, and may mean any speed higher than dialup6 The Local Loop and ISDN • Local loop describes the physical connection between a telephone company Central Office (CO) and a subscriber – consists of twisted pair and dialup call with 4 KHz of bandwidth • It often has much higher bandwidth; a subscriber close to a CO may be able to handle frequencies above 1 MHz • Integrated Services Digital Network (ISDN) – ISDN offers three separate digital channels – designated B, B, and D (usually written 2B + D) • The 2 B channels (each 64 Kbps) are intended to carry digitized voice, data, or compressed video – Both of the B channels can be combined or bonded to produce a single channel with an effective data rate of 128 Kbps • The D channel (16 Kbps) is used as a control channel7 Digital Subscriber Line (DSL) Technologies • DSL is one of the main technologies used to provide high-speed data communication services over a local loopAsymmetrical Digital Subscriber Line (ADSL) • link between subscriber and network • uses currently installed twisted pair cable • is Asymmetric - bigger downstream than up • uses Frequency division multiplexing • has a range of up to 5.5km http://www.cs.tut.fi/tlt/stuff/adsl/node6.html#kuspektri9 Digital Subscriber Line (DSL) Technologies • ADSL is the most widely deployed variant – and the one that most residential customers use • ADSL uses FDM to divide the bandwidth of the local loop into three regions – one of the regions corresponds to traditional analog phone service, which is known as Plain Old Telephone Service (POTS) – and two regions provide data communication 286 separate frequencies called sub-channels 254+2 (sub-channels allocated for downstream data transmission 31 allocated for upstream data transmission10 Local Loop Characteristics and Adaptation • ADSL technology is complex – because no two local loops have identical electrical characteristics • ADSL is adaptive – That is, when a pair of ADSL modems are powered on, they probe the line between them to find its characteristics – agree to communicate using techniques that are optimal for the line • ADSL uses Discrete Multi Tone modulation (DMT) – that combines frequency division multiplexing and inverse multiplexing techniques – Let’s see how….Discrete Multitone (DMT) • A multiplexing technique commonly found in digital subscriber line (DSL) systems • The basic idea of DMT is to split the available bandwidth into a large number of subchannels – DMT then combines hundreds of different signals, or subchannels, into one stream • Each subchannel is quadrature amplitude modulated (QAM) – if SNR is good – Recall: eight phase angles, four with double amplitudes • The DMT line code, as defined in ANSI T1.413-1998, divides the useful bandwidth of the standard two wire copper medium used in the PSTN, – which is 0 to 1104kHz, into 256 separate 4.3125kHz wide bins – Each is called sub-carriers – Each sub-carrier is associated with a discrete frequency or tone, – Each tone has a frequency of 4.3125kHz * n, where n = 1 to 256, and is essentially a single distinct data channel (total of 1056 KHz) © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved. 11Discrete Multitone (DMT) – Better channel Utilization • Another demultiplexing technique • Idea: If some subchannel can not carry any data, it can be turned off and the use of available bandwidth is optimized http://www.cs.tut.fi/tlt/stuff/adsl/node22.html#kudmtex High attenuation at higher frequencies13 Local Loop Characteristics and Adaptation • Two of the upstream channels are reserved for control information • Upstream: – A maximum of 255 sub-carriers can be used to modulate data in the downstream direction – Sub-carrier 256, the downstream Nyquist frequency, and sub-carrier 64, the downstream pilot frequency, are not available for user data • Downstream – maximum of 31 low frequency sub-carriers can be used to modulate data in the upstream direction – Sub-carrier 32, the upstream Nyquist frequency, and sub-carrier 16, the upstream pilot frequency, are again not available for user data, limiting the total number of available upstream sub-carriers to 3014 Local Loop Characteristics and Adaptation • There is a separate modem running on each sub-channel, which has its own modulated carrier – Carriers are spaced at 4.1325 KHz intervals to keep the signals from interfering with one another • Two ends assess the signal quality at each frequency – Use the quality to select a modulation scheme – If a particular frequency has a high signal-to-noise ratio • ADSL selects a modulation scheme that encodes many bits per baud – If the quality on a given frequency is low • ADSL selects a modulation scheme that encodes fewer bits per baud15 The Data Rate of ADSL • How fast can ADSL operate? ADSL can achieve – a downstream rate of 8.448 Mbps on
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