Multiplexing

What? Deterministic

Transmission of multiple signals (users) across a single Data Link.

Why? Channel Bandwidth sharing between multi users, spatial (FDM/WDM) or temporal (TDM/STDM), with minimal control overhead

, yet possible BW waste!

How? Dividing the link’s BW into “n” sub-channels. Spatial :

Frequency Division Muxing or (FDM). Temporal:

Wavelength division Muxing over fiber links. (WDM). Time Division Muxing or (TDM).

(Also, modern UDWDM) Or both TDM(Temporal)&FDM(Spatial)

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Figure 6.2 Categories of multiplexing

– FHSS

A)

FDM: (Analog Transmission over different carrier frequencies)

It is an “analog” muxing where the total link BW is divided into n channels (sub BWs), each carries a user modulated signal over a carrier frequency.

Channels are separated by “guarding bands” to prevent overlapping (crosstalk).

Since, each channel suppose to carry an analog signal, the carrier frequency should not interfere with the data signal.

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Figure 6.4 FDM process

Analog to Analog

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Figure 6.5 FDM demultiplexing example

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Figure 6.7 Example 6.2

The Guarding band is to alleviate the signals crosstalk problem. FDM hierarchy is used by telephone companies such as AT&T to combine signals from lower BW to higher BW lines (carriers).

FDM applications:

1) Radio broadcasting. AM: 530 KHz 1700 KHz

FM: 88KHz 108 MHz Each station needs: AM 10 KHz only but FM 200KHz

2) TV: Each station needs 6MHz of BW. 3) 1st generation cellular phones, a user is assigned two 30 KHz channels one

to Tx and other to Rx.

Wave Division Muxing:

Analog Muxing over optical fibers

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Figure 6.10 Wavelength-division multiplexing

(Ultra Dense WDM: # of channels from 1024 up to 32,000 –theoretical [research at UCSD]- each channel is 2.5 Gb/s!)

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Figure 6.11 Prisms in wavelength-division multiplexing and demultiplexing

B) Time_Division_Muxing (TDM)

I) Synchronous Time is slotted and ownership of the link total BW is rotated among users temporally in a round-robin fashion over time slots “ 1 n”.

.

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Figure 6.12 TDM

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Figure 6.13 Synchronous time-division multiplexing

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Figure 6.15 Interleaving

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Figure 6.17 Example 6.9

Notice that: The data rate of the muxing link is n times faster than the input data rate per every line (why?). Fig 6.17 shows every input line with 100 kb/s, whereas the Mux output data rate is 4X100 kb/s (400 kb/s).

When some of the input lines have half the rate of the others. Multilevel Muxing:

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Figure 6.19 Multilevel multiplexing

Multiple Slot Allocation: When an input line has multiple rates of the others.

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Figure 6.20 Multiple-slot multiplexing

Pulse Stuffing: (bit padding) When the bit rates of the input lines are not multiple integers of each other.

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Figure 6.21 Pulse stuffing

Pulse Stuffing: insertion of dummy bit (in the line with lower data rate).

Frame Synchronization Mux-Demex synchronization is very important maintain the integrity of the frames exchange between source and destination.

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Figure 6.22 Framing bits

A “framing bit” is added at the beginning of a frame, alternating 0,1,0,1… between frames.

Digital Signal Service Hierarchy (DS-N)

Telephone companies use TDM via DS_n digital hierarchy.

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Figure 6.23 Digital hierarchy

• T lines implements (carry) the above DS-n services: T-1 carries DS-1 up to T-5 carries DS-5. T-1 line data rate: 24 PCM channels 24 X 8 bits + 1 (framing) = 193 bits to be transmitted every sample time 125 micro sec/sample (why?) yielding a data rate of (193/125) X 106 = 1.544 M b/s

Notice that the sampling rate is 8000 sample/sec, since the TP line BW is 4 KHz and by Nyquist we sample twice the channel BW (8000 sample/sec), hence 1/8000 = 125 micro sec/sample

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Figure 6.24 T-1 line for multiplexing telephone lines

• Tn are digital lines to carry digital data from digital or analog sources (in this case after using Delta Code Modulation, DCM).

II) Statistical TDM (STDM)

:

• TDM is inefficient in case of a user not using its assigned time slot (wasting BW).

• In STDM, slots are allocated based on data availability (dynamic), instead of static assignment of time slots, one per user, i.e., instead of having one local queue per user's station, as done in TDM, we will have one global system queue to feed the Mux every Δt (original time slot of TDM) in the STDM. In STDM, the global queue will be served via the n time slots (n-servers), instead of the single local queue single time slot

server at each station in the TDM.

Problem? Yes, at the destination “Demux”, directing the frames to the correct destination?

Solution add address to each Muxed frame with its corresponding destination user.

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Figure 6.26 TDM slot comparison

Spreading Spectrum (SS):

• As in the Muxing, SS is used mainly over wireless domain to combine different users’ signals over a large BW channel, but not mainly for sharing such BW, instead it might be for a critical security and/or alleviating signal multipath fading problem goals.

• Wireless domain links

adopts the SS since security is inherently vulnerable (broadcast open media for eavesdrop)

• SS achieves security via adding redundancy to the input signal, spreading it over much larger BW’s that envelopes the signal.

BWspr.spect. >> BWoriginal_signal

There are two major types of SS:

A)

Frequency Hopping Spread Spectrum (FHSS)

• It uses M different frequencies, carriers, to modulate the original signal, each for a duration of “hop” time, cycling every M hops.

• A pseudorandom code generator is used to cyclic frequency pattern for modulating the input signal, repeated every cycle.

• An intruder, if successful, can access only one hop worth of information, since the hoping sequence is not known.

• For better BWss utilization, we can encode M different users (channel) per every hop, by carefully laying out users hopping sequences.

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Figure 6.30 FHSS cycles

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Figure 6.31 Bandwidth sharing

B)

Direct Sequence Spread Spectrum (DSSS)

• Instead of using M different frequencies in FHSS, we encode (spread) every bit of the input data (NRZ) into a “chip” of n bits (NRZ). Then, it divides the generated code into symbols using O-QPSK (Orthogonal QPSK) to modulate them (broadband Tx over wireless scarce

BW).

• Spreading n-bit chip sequence codes might be one of the following types:

(Ref: http://www.wirelesscommunication.nl/reference/chaptr05/cdma/codes/codes.htm)

1) Pseudorandom-Noise (PN), 2) Walsh Hadamard sequences (mostly by CDMA), 3) Barker code (n=11 bit) , IEEE802.11 DSSS standard, 4)Maximum Length sequences, 5) Gold codes, and 6) Kasami codes.

• The required chip rate is n times the original bit rate.

• Since the intruder does not know the “chip” encoding, the system is secure.

• If different stations use different “orthogonal” chip encoding, interference is alleviated, otherwise it is difficult to recover data at the receivers.

• Bandwidth sharing is important over wireless; it can be achieved only by making chips of many users “orthogonal” (independent). Hence, such users can use the same link simultaneously without interfering with each other.

(for more details see Chapter 12, MAC sublayer)

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Figure 6.33 DSSS example