lect20 - 1.264 Lecture 20 Telecom: switched network and...

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1.264 Lecture 20 Telecom: switched network and data communications
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Intro to public network: major components [TE] Chapter 2 Local loop LATA connecting trunks T oll trunks Satellite (rare) T oll switch Local trunks Transmission Equipment Fiber optics Microwave (rare) Local switch Local switch Modem V ideo Telephone Fax machine Customer Premise Equipment Computer PBX Figure by MIT OCW.
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Telecom service areas Telephone Service Areas Central Office Exchange Boundary Wire Center Boundary Trunks Exchange A Exchange B Figure by MIT OCW.
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Frequency division multiplexing Traditional method of carrying multiple channels on a cable Similar to radio broadcasting with stations at different frequencies Being used again in ADSL, CATV, satellite, … Basic frequency is 4kHz band, based on voice needs Voice hears from 20 to 20,000 Hz, but mostly 300 to 3500 Hz Hybrid Transmitter Receiver Signaling Oscillator FDM Channels 2 to 12 To Distant Multiplex Terminal Signaling Leads Modulator FDM Channel 1 Modem Demodulator Transmit Receive Talking Path Signaling Leads Signaling Figure by MIT OCW.
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Time division multiplexing Time division samples each 4kHz channel Shannon/Nyquist: Sample at twice the bandwidth to code and decode allpossible information So, sample 8000 times/second at 8 bits per second (or 256 levels) Yields 64kb/sec channel, which is the basic rate of voice and data comm 24 64kb/sec channels is a T-1 (32 in Europe form an E-1) US channels are often only 56kb/sec because of bit robbing TDM used in fiber optic technologies; lasers can do both TDM and FDM (colors) Channel 24 Voice channel inputs Voice channel output Electronic sampler Pulse coder Pulse Decoder Filter Filter Filter Channel 24 Channel 1 Channel 1 Figure by MIT OCW.
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Transmission: PCM Framing in T-1 circuits 8,000 samples per second x 8 bits per sample 64,000 bits per channel x 24 channels 1,536,000 bits per second + 8,000 framing bits per second (not enough!) 1,544,000 bits per second total in T-1 circuit Framing used to keep sync between ends of circuit. If all 24 circuits are quiet, only framing bits keep the clocks at both ends of the circuit in sync (several framing systems used) T-1 is often the backbone for both voice and data, carries both T-1s operate over both copper and fiber. Higher bandwidth is fiber only.
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Mb/s / / / SONET Fiber bandwidth, beyond T-1 STS-1, or OC-1, is same as T-3, or 28 T-1s SONET can be divided into channels as small as 64kbps Voice carried at 64kbps Data carried at many rates Video carried at MPEG-1 (1.5Mb sec) or MPEG-2 (6Mb sec); these are highly compressed
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lect20 - 1.264 Lecture 20 Telecom: switched network and...

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