Data communications and computer networks: a business user’s approach

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Data Communications and Computer Networks: A Business User’s Approach

Chapter 5
Multiplexing : Sharing a Medium

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Last time

Data Communications and Computer Networks Chapter 5

Making connections
• Synchronous vs asynchronous (temporal) • Duplex vs simplex (directional)

Continue making connections – multiplexing
• Many into one; one into many (spatial)
•Will use time andfrequency to do it.

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Introduction
Under the simplest conditions, a medium can carry only one signal at any moment in time. For multiple signals to share one medium, the medium must somehow be divided, giving each signal a portion of the total bandwidth. The current techniques that can accomplish this include •frequency division multiplexing (FDM) •time division multiplexing (TDM)•Synchronous vs statistical

•wavelength division multiplexing (WDM) •code division multiplexing (CDM)
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Multiplexing
Multiplexor (MUX) Demultiplexor (DEMUX)
Sometimes just called a MUX

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Multiplexing
• Two or more simultaneous transmissions on a single circuit.
– Transparent to end user.

• Multiplexing costs less.

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Data Communications and Computer Networks Chapter 5

FrequencyDivision Multiplexing
Assignment of non-overlapping frequency ranges to each “user” or signal on a medium. Thus, all signals are transmitted at the same time, each using different frequencies. A multiplexor accepts inputs and assigns frequencies to each device. The multiplexor is attached to a high-speed communications line. A corresponding multiplexor, or demultiplexor, is on the end of thehigh-speed line and separates the multiplexed signals. 6

Data Communications and Computer Networks Chapter 5

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Data Communications and Computer Networks Chapter 5

Frequency Division Multiplexing
Analog signaling is used to transmits the signals. Broadcast radio and television, cable television, and the AMPS cellular phone systems use frequency division multiplexing. This technique isthe oldest multiplexing technique. Since it involves analog signaling, it is more susceptible to noise.
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Data Communications and Computer Networks Chapter 5

Time Division Multiplexing
Sharing of the signal is accomplished by dividing available transmission time on a medium among users. Digital signaling is used exclusively. Time division multiplexing comes in two basic forms:
1.Synchronous time division multiplexing, and 2. Statistical, or asynchronous time division multiplexing.

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Data Communications and Computer Networks Chapter 5

Synchronous Time Division Multiplexing
The original time division multiplexing. The multiplexor accepts input from attached devices in a round-robin fashion and transmit the data in a never ending pattern. T-1 and ISDN telephonelines are common examples of synchronous time division multiplexing.
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Data Communications and Computer Networks Chapter 5

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Data Communications and Computer Networks Chapter 5

Synchronous Time Division Multiplexing
If one device generates data at a faster rate than other devices, then the multiplexor must either sample the incoming data stream from that device more often than itsamples the other devices, or buffer the faster incoming stream. If a device has nothing to transmit, the multiplexor must still insert a piece of data from that device into the multiplexed stream.
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Data Communications and Computer Networks Chapter 5

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Data Communications and Computer Networks Chapter 5

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Synchronous time division multiplexing So that the receiver may staysynchronized with the incoming data stream, the transmitting multiplexor can insert alternating 1s and 0s into the data stream.

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Data Communications and Computer Networks Chapter 5

Synchronous Time Division Multiplexing
Three types popular today: •T-1 multiplexing (the classic) •ISDN multiplexing •SONET (Synchronous Optical NETwork)

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The T1 (1.54 Mbps) multiplexor stream is a...
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