Physical Layer Dr. Sanjay P. Ahuja, Ph.D. discusses multiplexing techniques used at the physical layer including frequency division multiplexing (FDM), time division multiplexing (TDM), and wavelength division multiplexing (WDM). TDM is used to carry multiple digital signals over a single channel by interleaving portions of each signal in time. For example, a T1 carrier uses TDM to multiplex 24 voice channels together by sampling each channel at 8000 samples/second and inserting 8-bit samples into a serial data stream. WDM carries many signals on a single optical fiber by using different wavelengths of light to represent different channels.

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 2W samples 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 (Example of TDM)
 Conversion from Analog to Digital signal with sampling

8. The T1 Carrier (Example of TDM)
 Conversion from Analog to Digital signal with sampling (pulse code
modulation)

9. 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.

10. 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.

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

12. 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).

13. Erbium Doped Fiber Amplifier (EDFA)