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CS335 Networking & Network Administration Wednesday, April 14, 2010 Long Distance WANS Digital Telephony Prior to networks being important Early telephone used analog signals which degrade over long distance needing amplifiers and introduce noise

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CS335 Networking &Network Administration

Wednesday, April 14, 2010

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Long Distance WANS

  • Digital Telephony

  • Prior to networks being important

  • Early telephone used analog signals which degrade over long distance needing amplifiers and introduce noise

  • Digital audio avoids noise by encoding original audio into digital form

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Audio digitization

  • Closest integer to signal = sampling = continuous stream of numbers

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Audio to digital converter

  • Sample the signal

  • Compute a number closest to level of signal

  • Continuous stream of numbers

  • Human voice has frequencies up to 4000 Hz

  • If sampled at twice the highest frequencies the original signal can be reconstructed from the samples

  • Digitized voice sampled 8000 times/sec (once every 125 microseconds)

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Pulse Code Modulation

  • World-wide standard for digital telephony

  • Need to choose a range of values

  • More values = more accurate but also more data

  • Uses integers between 0 and 255 for voice

  • PCM is the sampling scheme

  • Digitized data sent over long distance and converted back to audio at destination

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Synchronous communication

  • Telephone industry has complex digital communications systems – long distance

  • Digitized voice is synchronous or clocked

  • Data is moved at precise rates

  • Voice apps can’t tolerate delay or noise in the same way as data

  • Continuous transmission synchronized to have constant data rate

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Digital phone circuits

  • Point to point digital circuits - data uses different standards than voice

  • DSU/CSU’s (Data Service Unit /Channel Service Unit) at each end of phone circuit

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The demarc

  • Network Interface Unit (NIU) is the last piece of telephone company equipment

  • CSU portion handles line termination and diagnostics to test if line is connected

  • DSU translates data between the digital format of the phone circuit and the format needed by computer network

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Data rates

  • Fractional t1’s can also be leased

  • Phone company uses Time Division Multiplexing to subdivide T1 into 24/64 Kbps fractions =1536

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Intermediate capacity

  • Need more than a T1 but less than a T3

  • Inverse multiplexor allows multiple T1’s to be combined

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Highest Capacity Circuits

  • Synchronous Transport Signal (STS)

  • Optical Carrier (OC)

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SONET (Synchronous Optical Network)

  • In addition to STS and OC standards phone companies have defined SONET

  • Specifies details such as how data is framed

  • How lower capacity circuits are multiplexed into a high capacity circuit

  • How synchronous clock info is sent along with the data

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  • SONET Encoding

  • Size of a SONET frame depends on bit rate

  • STS-3 circuit each frame holds 2430 octets

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  • Digitized voice and data over conventional local loop wiring (twisted pair copper same as the analog phone system – POTS: plain old telephone service)

  • Two B channels operating at 64Kbps

  • B channels can be combined into a single channel to get 128 KbPS

  • One D channel operating at 16 Kbps as a control channel

  • When it was begun 64 Kbps was much faster than dialup modem, now it is an expensive alternative with little throughput

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  • Multiple flavors so we refer to it as xDSL


  • ADSL –Asymmetric digital subscriber line

  • Higher bit rate downstream than upstream

  • Most users request downstream data – browsing the internet

  • ADSL can operate at 6.144 Mbps downstream and 576 upstream

  • Doesn’t require changes in local loop wiring

  • Can run simultaneously over the same wires as phone service

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  • Adaptive to conditions on the local line in terms of choosing frequencies based on the interference that the modems find on the line when they power up

  • Probe many frequencies on the line between modems and select frequencies and modulation techniques that give optimal results

  • 286 separate frequencies or subchannels

    • 255 downstream and 31 upstream with 2 channels for control information

    • Choose frequencies above 4 Khz to not interfere with voice

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Cable Modems

  • Higher speed than telephone wiring

  • Less susceptible to EM interference

  • Already has infrastructure in place

  • Multiplexed onto the cable with other TV signal

  • Can deliver up to 36 Mbps, but is a shared capacity can be as little as 1/n where n=subscribers

  • CATV was never designed for upstream communication

  • Early attempts used a dual path approach – upstream data handled by a dialup modem

  • New approach is HFC )hybrid fiber coax) but it requires major upgrade of existing cable infrastructure

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Satellite and Wireless

  • Wireless is usually line-of-site

  • Receiving antenna needs to be in line with transmitter

  • Satellite also uses another path for upstream traffic, making it more complex