Data Communication ch08 multiplexing

Chapter 4 Reuse

Frequency Division Multiplexing

Time division multiplexing

Synchronous time division multiplexing

Statistics time division multiplexing

Asymmetric Digital user line

 

Chapter 4 Reuse

Efficient use of high-speed data communication links-Reuse

The capacity of a single data link can be shared by multiple information sources.

Reasons why reuse is widely used

The higher the data rate, the higher the cost-effectiveness of transmission facilities. Most data communication devices require a relatively low data rate.

8.1-Frequency Multiplexing

OFDM (Frequency Division Multiplexing)

Conditions for using OFDM:

The valid bandwidth of the transmission media exceeds the required channel bandwidth.

Method:

Each signal is modulated to different carrier frequencies.

Each modulated signal requires a certain amount of bandwidth, which is called a channel.

To prevent mutual interference, channels are separated by protection frequency bands.

Used to simulate signals. Digital data must be first modulated as analog signals.

 

8.1-Frequency Multiplexing

Total Bandwidth B of the S (t) of the OFDM signal

 

8.1-Frequency Multiplexing

Two problems to be addressed by the OFDM system:

Crosstalk

Traffic adjustment Noise

8.1-Frequency Multiplexing

Analog Carrier System

To adapt to different capacities of various Transmission Systems

At&t (USA) designed a hierarchical OFDM System

Group)

12-channel voice (4 KHz per channel) = 48 khz

Range: 60 kHz to 108 kHz

Supergroup)

60 channels

OFDM of five base group signals on the carrier between 612 kHz and kHz

Master Group)

10 super

....

 

Wavelength Division Multiplexing

It is a multiplexing method used in optical fiber communication.

Optical Signals of different wavelengths are transmitted through the same optical fiber.

Similar to frequency division multiplexing

Uses a narrowband optical array with different sources to form a broadband optical array.

Prism used for wavelength division multiplexing and multi-channel Decomposition

Differences from multimode Signals

 

8.2 synchronous time-division multiplexing

TDM (time division multiplexing)

Conditions for synchronizing TDM:

The data rate achieved by the transmission media exceeds the data rate of the transmitted signal.

Method:

Each signal is temporarily cached.

Sequential Scan cache to retrieve data

Form a composite digital data stream

Finally, the signal transmitted on the transmission media can be digital or analog. If it is the latter, the digital signal must be modulated.

 

Features

Time-based staggered transmission of multiple digital signals

It can be bit-level, character-level, or data block-level interleaved

The time slot is pre-allocated to the data source and is fixed.

Allocate time slots even if there is no data

 

TDM Link Control

No header or tail

No Data Link Control Protocol required

Traffic Control

The data rate of the multiplexing line is fixed.

When the receiver of one channel cannot accept data, other channels still want to continue

The corresponding data source is forced to be aborted.

This causes some idle time slots

Error Control

The system of each channel detects and handles errors by itself

 

8.2 synchronous time-division multiplexing

Frame demarcation of TDM with no sign or sync characters

Therefore, the synchronization mechanism must be provided.

Additional Digital Frame Group Technology

Each TDM frame is appended with a control bit.

It looks like another channel-"control channel"

Identifiable bit mode for Channel Control

For example, the alternate 01010101... It cannot appear on the data channel

Use this control bit mode to compare the incoming control bit on each channel

8.2 synchronous time division multiplexing-pulse Filling

Problem-Synchronize different data sources

Independent clock downtime for different data sources

There is no simple proportional relationship between data rates of different data sources

Solution-pulse stuffing

The output data rate (excluding the frame location bit) is higher than the sum of the input data rate

Fill in additional null bits or pulses for the input signal to match the local clock

Insert a fill pulse at a fixed position of the frame and remove it when resolving the replay.

8.2 synchronous time-division multiplexing

Digital Carrier System

TDM Grading System

SONET/SDH

STS-1: 51.84 Mbps

Multiple STS-1 to form STS-N Signal

8.3 statistical time division multiplexing (STDM)

Symptom: No connected devices are transmitted at any time

As a result, many time slots are wasted in TDM synchronization.

Principle of STDM: dynamically allocates time slots as needed

Repeat scanning input lines and collecting data until the frame is full

The data rate on the line is lower than the total rate of all input lines

 

Comparison between TDM and STDM

In the frame structure of STDMAddress Field

The output data rate of the STDM multier can be lower than the sum of input data rates, although the average total input volume is smaller than the capacity of the multiplexing link, however, there is still a way to increase the efficiency of STDM during peak hours when the instantaneous input exceeds the capacity. It allows one frame to compress the data of multiple data sources. Each time slot has a larger overhead, you can use a relatively small bit of relative address.

8.3 statistical time division multiplexing (STDM)

Feature: the output data rate is lower than the sum of the input data rate

Problems may occur during peak hours

The solution is to temporarily cache the excess input.

An example of such system behavior (Table 8.6)

Make the cache size as small as possible to reduce latency

 

8.3 statistical time division multiplexing (STDM)

I-number of input sources

R-data rate of each input source, effective capacity of the Unit bps m-multiplexing line, unit bps a-Average time ratio of each input source transmission, 0 <A <1 k = m/IR-ratio of reusable link capacity to the maximum input sum, a <k <1 Air <m <IR for example, for a given data rate m, if K = 0.25, the number of devices processed is four times that of the synchronization TDM with the same multiplexing link capacity.

 

Think of STDM as a single server Queue with a fixed service time and Poisson (random) arrival, you can get:

Utilization P = A/K

Note the following conclusions:

As utilization increases, cache requirements and latency also increase

Utilization above 80% is obviously not expected

The utilization remains unchanged, and the average latency decreases as the link capacity increases.

The average cache size used depends only on, and does not depend on M directly.

 

8.4 Asymmetric Digital user line (ADSL)

ADSL-asyuncrical Digital Subscriber Line

Link between User device and network

Local Loop

Use the already installed twisted pair Cable

The bandwidth ranges from 0 ~ 4 kHz audio-Level Signal

But it can carry a wider spectrum.

1 MHz or above

8.4 Asymmetric Digital user line (ADSL)

Features:

Asymmetric

The downstream stream capacity is far greater than the upstream stream capacity

Frequency Division Multiplexing

Minimum 25 kHz for voice

Traditional telephone service (Pots-plain old telephone service)

Echo Cancellation (echo cancellation) or OFDM is used to provide two frequencies for upstream and downstream streams.

Use OFDM in each band

Advantages of ECHO offset Technology

Enable more downstream bandwidth in the lower Spectrum

Upstream stream channels are easy to expand upwards

Distance: 5.5

 

8.4 Asymmetric Digital user line (ADSL)

8.4.2 discrete multi-tone

DMT-discrete Multitone

Features: Multiple Carrier signals at different frequencies

First, each channel sends some bits to determine their signal-to-noise ratio.

Transmit more bits using subchannels with better signal-to-noise ratio

ADSL/DMT uses 256 4 kHz downstream subchannels

Each sub-channel carries 0 ~ 60 kbps Data Rate

Total channel capacity: 15.36 Mbps

Damage to Mbps to 9 Mbps

 

Summary of this Chapter

Frequency Division Multiplexing

Time division multiplexing

Synchronous time division multiplexing

Statistics time division multiplexing

Asymmetric Digital user line