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Physical Layer

Physical Layer. Dr. Sanjay P. Ahuja, Ph.D. Fidelity National Financial Distinguished Professor of CIS School of Computing, UNF. Multiplexing.

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Physical Layer

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  1. Physical Layer Dr. Sanjay P. Ahuja, Ph.D. Fidelity National Financial Distinguished Professor of CIS School of Computing, UNF

  2. Multiplexing • Transmission channels are expensive. It is often that two communicating entities do not fully utilize the full capacity of a channel. For efficiency, the capacity is shared. This is called multiplexing. • There are n inputs to a multiplexer. The multiplexer is connected by a single data link to a de-multiplexer. The link is able to carry n separate channels of data. • The mux combines data from n input lines and transmits over a higher capacity data link. The demux accepts the multiplexed data stream and separates the data according to channel and delivers them to the appropriate output lines.

  3. Multiplexing • Transmission channels are expensive. It is often that two communicating entities do not fully utilize the full capacity of a channel. For efficiency, the capacity is shared. This is called multiplexing. • There are three types of multiplexing: • Frequency Division Multiplexing (FDM) • Time Division Multiplexing (TDM) • Wavelength Division Multiplexing (WDM)

  4. Frequency Division Multiplexing (FDM) • The frequency spectrum (bandwidth) is divided among the logical channels, single frequency bands are allocated to different users. E.g., in radio broadcasting different frequencies are allocated to different radio stations. • FDM is used with analog signals. • When 3000 Hz wide voice-grade telephone channels are multiplexed using FDM, 4000 Hz is allocated to each channel to keep them well separated (known as a guard band).

  5. Time Division Multiplexing (TDM) • In time division multiplexing, several connections share the high bandwidth of a channel. Here multiple signals (digital) are carried on a single channel by interleaving portions of each signal in time. • TDM is a digital multiplexing technique and is used for digital data only.

  6. The T1 Carrier (Example of TDM) • The T1 carrier consists of 24 voice channels muxed together. • On the transmitting end of a T1, a codec (coder-decoder) samples the analog amplitude of 24 4000 Hz voice lines each at 8000 samples per second, or 125 µsecond/sample for each of the 24 channels (see Nyquist theorem below). • Each of the 24 channels gets to insert 8-bits into the output stream, 7-bits for data and 1 bit for signaling. • Nyquist Theorem: A signal of bandwidth W can be completely captured by taking 2Wsamples per second. • For a 4 KHz voice signal, all information can be recovered by sampling at 8000 samples/second. Sampling faster isn't useful but sampling slower than the Nyquist limit means that some changes in the signal will be missed and data lost.

  7. The T1 Carrier • Each T1 frame consists of 24 * 8 = 192 bits + 1 bit for framing = 193 bits/frame. • There are 8000 frames generated per second (or 125 µsecond/frame). • The gross data rate of T1 = 8000 frames/sec * 193 bits/frame = 1.544 Mbps • For transmitted digital data, the 24th channel is used for synchronization and is considered as overhead.

  8. TDM • TDM allows multiple T1 carriers to be muxed into higher order carriers. • 4 T1 channels muxed onto 1 T2 channel (6.312 Mbps). • 7 T2 channels muxed onto 1 T3 channel (44.736 Mbps). • 6 T3 channels muxed onto 1 T4 channel (274.176 Mbps). Multiplexing T1 streams into higher carriers.

  9. Wavelength Division Multiplexing (WDM) • WDM is used to carry many signals on one fiber.

  10. Wavelength Division Multiplexing (WDM) • 4 fibers come together at an optical combiner, each with its energy present at a different wavelength (λ). • The 4 beams are combined onto a shared fiber. At the far end, the beam is split up over as many fibers as there were on the input side. Each output fiber contains a special filter that filters out all but one wavelength (λ). • The resulting signal can be routed to their destination or recombined for additional muxed transport. • Today we have WDM products that support 96 channels of 10 Gbps each for a total of 960 Gbps. • When the number of channels is very large and the wavelengths are spaced closed together (0.1 nm – 0.4 nm), the system is referred to as Dense WDM (DWDM).

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