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We always talk about packets, data packets, but physically, if they are transmitted through the fiber and wireless technology solutions, what kind of signals are, how they are made? Square waves?

Peter Long
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1 Answers1

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They are different and vastly varied depending on the medium and also on the signalling standard.

In the Synchronous Digital Hierarchy, the STM64 and STM 256 signals used on backbone optical fiber networks at 10GBps and 40GBps, they are baseband waves, theoretically square edged but practically are almost Gaussian owing to dispersion in the optical fiber. This dispersion results from material dispersion and, more importantly, from the wavelength dependence of the eigenvalue equation that defines the fiber modes.

STM64/256 signals are organized through wavelength division multiplexing into many channel groups on single fibers.

In the very latest trunk networks, the data are encoded into the phase of the optical wave itself, with various groups experimenting with 500GBps polarization / quadrature phase shift keying. This is not common.

Optical fibers mostly, as in SDH, work like very fast on/off telegraph wires.

Further out in the network, a whole host of different modulation schemes are used. Wifi networks use phase / frequency modulated carriers, with 64 QAM being common. Specialist modulation schemes are used in rural / high noise environments, with CDMA (orthogonal codeword signalling) sometimes used.

Selene Routley
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  • Thanks Wet, I was not aware about SONET, so packet is a Gaussian wave or a gauss function? This gauss function is integrated in chip of modems or in multiplexing transport station ? Very interesting but for Twisted Pair what is packet, same gauss wave ? You have some technical title or reference about this processing, I would like to explore these transport systems. – Peter Long Dec 27 '16 at 23:37
  • Data packets are a higher level of description than the physical layer, which denotes the modulation and transmission coding. Those are per the ISO or IP layers. The data packets are parts of a frame, which is described in the next layer (or two, depends on the standard used) up. A frame will typically include data from different sources or types, multiplexed. A user gets one slot, say, and that's his/her data packet. Look at a basic book on data communications,the modulation is the lowest level layer,then the link layer.At the physical layer can also be multiple access, diferent transmissions – Bob Bee Dec 27 '16 at 23:42
  • thanks bob, lowest level is so delta-sigma modulation like this ? But this modulation system is integrated in this chips ? – Peter Long Dec 28 '16 at 19:55