Tài liệu Lecture 28: Trunks and Multiplexing: pptx

This sharing by frequency band is known as Frequency division Multiplexing FDM.. Multiplexing: Multiplexing is a technique employed in many different situations where different signals

Trunks and Multiplexing:

What happens, when there is only one media available between two local offices, as shown below?

Local Office

Local Office

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In this case the media is the space and the link between the two local offices is the Microwave link

The users from the left want to communicate the users on the right They have to share the common

media This can be done in two ways:

First, the users can take turns; each one periodically uses the microwave link for a short period of time

This time-sharing is known as Time division Multiplexing (TDM)

Second, the bandwidth of the microwave link can be divided among logical channels, with each user

having exclusive possession of some frequency band Think the microwave link as a wide road The road is then divided into lanes and then each car is using their own lane This sharing by frequency

band is known as Frequency division Multiplexing (FDM)

• Sharing one media by multiple users is known as multiplexing

If the number of users are high then in the TDM each users have to wait for a long time to get their turn and is not a very practical way of sharing the media So, FDM is a better way of sharing the media

You could ask, why we are not establishing the same number of microwave links as the users But are limited by the one media The other microwave links have to use different frequency bands, again an example of FDM by the links

This problem of one media can be solved by using multiple copper cables or fiber optic strands This is again impractical in terms of costs

This problem of one media can be solved by transmitting copper cables or fiber optic strands All these media have their band limitation, copper have the lowest So we need lots of links and this is again impractical in terms of costs and some time impossible to lay the cables

Multiplexing:

Multiplexing is a technique employed in many different situations where different signals have to

share a medium, circuit or channel It enables more efficient use of circuits or facilities

A Multiplexer is a device that combines several simultaneous low speed circuits on one higher speed

circuit in such a way that each low speed circuit believes it has a separate connection to the host

• Multiplexing voice telephone calls means two or more conversations are sent simultaneously over

one circuit

• Multiplexing data means two or more messages are simultaneously sent over one circuit

• Multiplexers must be transparent; that is, have no effect on the end users connection compared to

if they had their own separate connection

• Multiplexed circuits generally have to have the same capacity (bandwidth) as the sum of the

circuits it combines

Combining the signals from four 9600 bps terminals into one signal of 38,400 bps

Combining 24 digital voice channels (64,000 bps) into one 1,544,000 bps (T-1) circuit

Combining 672 digital voice channels into one 44,373,000 bps (T-3) circuit

• A Demultiplexer does the opposite of a multiplexer; it separates a multiplexed signal back into its

separate signals, for distribution to multiple users

Types of Multiplexers:

1 FDM Frequency Division Multiplexing

2 TDM Time Division Multiplexing

Frequency Division Multiplexing: (Chapter-3)

• FDM is an analog technique where each signal or device is allocated a separate range of

frequencies to use on the channel

• This is similar to AM radio, for example, where each radio station transmits its signal on a different carrier frequency (the allocated spectrum is 1100KHz, from 550KHz to 1650KHz, and that can accommodate 1100KHz/10KHz = 110 channels)

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• The signals are all present on the medium at the same time but they don’t interfere with each other

because each one is modulated onto a different carrier frequency

• This FDM technique is less efficient, requires more maintenance and is less flexible than TDM

• The FDM schemes used around the world are to some degree standardized

• Group - 12 voice channels multiplexed together (12x4 kHz = 48 kHz Bandwidth)

• Supergroup - 5 groups multiplexed together (5x48 kHz = 240 kHz Bandwidth)

• Mastergroup - 10 supergroups multiplexed together (10x240 kHz = 2400 kHz Bandwidth)

• Other standards also exists - up to 230000 voice channels together (4x230000 = 920000 kHz Bandwidth)

Time Division Multiplexing: (Chapter-3) Time Division Multiplexing is used by the digital system where as FDM is used by the analogue

system

In TDM one character is taken from each terminal in turn and transmitted down the circuit

The analog signals are digitized in the end office by a device called a codec (coder-decoder) The codec makes 8000 samples per second because Nyquist theorem says that this is sufficient to

reproduce the original information back (4 kHz voice signal)

The sampling rate should be at least double the frequency of the signal being sampled

• TDM is a digital technique in which different digital signals take turns using the available channel

• Each signal uses the entire bandwidth for a short time and then waits for its turn again

• Data from different signals is interleaved; each signal having its own timeslot, much like boxcars on

a train where each car carries freight for one user The difference is that the sequence of boxcars

repeats itself endlessly

• TDM generally more efficient than FDM, because it does not need guard bands Guard bands could

be used to transmit data

• TDM systems may waste bandwidth if all time slots are not unused

North America’s T Carrier Digital Circuits form an interconnected synchronous digital

communications system of leased circuits using Time Division Multiplexing

T Carrier Circuit

Voice Channels

Speed(bits/second) Equivalent

T-1 Circuit

Time division multiplexing allows multiple T1 carriers to be multiplexed into higher-order carriers Fig.2-28, Tanenbaum

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Statistical Time Division Multiplexing (STDM): (Fig 4-24, p 131)

• STDM is the exception to the rule that the capacity of the multiplexed circuit must equal the sum of

the circuits it combines

• Statistical Time Division Multiplexers can handle more terminals simultaneously than a Time

Division Multiplexes can

• They are based on the probability that not all terminals have to send data at the same time; therefore

you can actually have more terminals sharing the line than you might expect

• They require internal buffering of the data being sent so data can be temporarily stored at times of

heavy traffic

• They also require addresses be attached to all data so the destination terminal is known since this is

not like TDM where each terminal would be assigned a timeslot which never changes

• Timeslots are allocated dynamically based on demand

• STDM provides more efficient use of the circuit because more terminals can share it

simultaneously

• Example:

12 terminals connected at 9600 bps each

Using TDM they would need 12 x 9600 or 112,00 bps capacity

With STDM these 12 terminals could probably share a 56,000 bps circuit without a significant loss in performance, thereby saving the cost of leasing the more expensive circuit

Inverse Multiplexers combine several low speed circuits to transport a higher speed signal The PPP

(Point to Point Protocol) used for Internet connections through a serial port, is actually an inverse multiplexer

Wavelength Division Multiplexing (WDM) - is the nothing but the Frequency division Multiplexing

But this is called WDM because the combination or separation of light beams of different wavelengths are accomplished by using Prisms also called diffraction gratings Prisms refract light and the

refraction angle depends on the wavelength of the light This is used for fiber optic channels

Remember different wavelength means different frequency signals

The wavelength is related to frequency as follows:

Wavelength = Velocity of the wave / frequency

λ = v/f , the velocity of light is constant (3 x 108

m/sec) in one particular media when f = 100 MHz, λ = 3 m

when f = 1 GHz, λ = 0.3 m

Ref: Fig.2-25 Tanenbaum, p.120

In WDM the necessary condition is that the incoming channels has to use different frequencies, i.e., the light sources have to have different wavelength (in the range of 0.8 to 1.6 micrometers)

The bandwidth of a fiber optic cable is about 25,000 GHz But we use only a few GHz because it is the electronic circuit limitation that makes currently impossible to produce high bandwidth signal and convert that for optical media

Wavelength Division Multiplexing is preferred because it uses passive devices (prisms, not electronic circuits) to multiplex different optical signal and made it possible to utilize the bandwidth of a single optical fiber

One optical fiber can carry 25,000 channels of 1 GHz bandwidth, and each 1 GHz channel can carry 250,000 voice channels In total 25,000 x 250,000 = 6,250,000,000 voice channels

Permits up to 10 simultaneous circuits to be transmitted on a fiber optic cable by sending each signal

on a separate wavelength of light This multiplexing will greatly increase the carrying capacity of the already high bandwidth optical fiber

SONET (Synchronous Optical NETwork):

It is an Optical TDM system The equivalent CCITT recommendations are called SDH (Synchronous

Digital Hierarchy) Virtually all the long distance telephone traffic in North America uses trunks running SONET in the physical layer

A SONET system consists of switches, multiplexers, and repeaters, all connected by fiber

The basic SONET channel (STS-1, Synchronous Transport Signal-1) has a data rate of 51.84 Mbps

Electrical Optical Optical STS-1 OC-1

(Optical Carrier)

51.84

STS-12 OC-12 STM-4 622.08 STS-18 OC-18 STM-6 933.12 STS-24 OC-24 STM-8 1244.16

The ATM runs at 155 Mbps, the intention is to carry ATM cells over SONET OC-3c (OC-3

concatenated, from a single source) trunks