Multiplexing theory

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Lecture Objectives

• Multiplexing– FDM– TDM (synchronous and asynchronous)– WDM

• Introduction to DWDM– How DWDM Works?– DWDM Features

• Introduction to SONET/SDH– Major Design Goals of SONET– SONET Frame Format– Frame Transmission– SONET Data Rates

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Multiplexing

• Multiplexing– The technique of transmitting multiple signals

over a single medium is called multiplexing– Multiplexing is a technique performed at physical

layer of the OSI model• Types of Multiplexing– FDM– DWDM– TDM

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Multiplexing• Multiplexing enables one physical medium to carry multiple

signals. • A transport network is a set of links between sites. To put

more than one call on each link is to give each call a time slot (a place in a schedule) and transmit several calls simultaneously.

• This process is known as time division multiplexing (TDM). With multiplexing, the end users have the illusion of being on their own private links.Transmission systems that are designed according to North American rules work with groups of 24 telephone calls (TI line).

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Frequency Division Multiplexing

• FDM• Useful bandwidth of medium exceeds required

bandwidth of channel• Each signal is modulated to a different carrier

frequency• Carrier frequencies separated so signals do not

overlap (guard bands)• e.g. broadcast radio• Channel allocated even if no data is there to transmit

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Frequency Division 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 the high-speed line and separates the multiplexed signals.

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Frequency Division Multiplexing

Analog signaling is used to transmits the signals.

Broadcast radio and television, cable television etc use frequency division multiplexing.

This technique is the oldest multiplexing technique.

Since it involves analog signaling, it is more susceptible to noise.

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Frequency Division Multiplexing(a) The original bandwidths.(b) The bandwidths raised in frequency.(c) The multiplexed channel.

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Frequency Division Multiplexing

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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:– Synchronous time division multiplexing

– Asynchronous or Statistical time division multiplexing.

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Synchronous TDM

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.

T1 and ISDN telephone lines are common examples of synchronous time division multiplexing.

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Synchronous TDM

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Synchronous TDM

• Data rate of medium exceeds data rate of digital signal to be transmitted

• Multiple digital signals interleaved in time• May be at bit level of blocks• Time slots pre-assigned to sources and fixed• Time slots allocated even if no data• Time slots do not have to be evenly

distributed amongst sources

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Synchronous TDM

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Statistical TDM

• In Synchronous TDM many slots are wasted• Statistical TDM allocates time slots

dynamically based on demand• Multiplexer scans input lines and collects data

until frame full• Data rate on line lower than aggregate rates of

input lines

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Asynchronous TDM

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FDM vs. TDM

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T1 Lines – 1.544 Mbps Data Rates

• T1 lines uses synchronous TDM• How we get 1.544 Mbps Data Rates ?

– A T1 line use DS-1 signaling, which provides for the multiplexing of 24 separate channels at a total speed of 1.544 Mbps

– Example:• Consider the example of average human voice which is approx.

4kHz (4000 Hz)• An analog to digital converter using PCM technique needs 127

quantization levels to represent human voice in digital form• Telephone systems uses 256 quantization levels (I.e. 28 bits per

sample to more precisely represent human voice )

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T1 Lines – 1.544 Mbps Data Rates• Recall that to create an accurate digital representation of an analog signal, you

need to sample the analog signal twice the highest signal frequency.• When digitizing voice, you need to sample the analog voice signal at 8000 times

per second, given that the highest frequency is 4000 Hz.• Since each T1 frame contains one byte (8-bits) of voice data for 24 channels, the

system needs 8000 frames per second to maintain 24 simultaneous voice channels.

• Each frame is 193 bits in length:(24 channels x 8 bits/channel) + 1 control bit = 193

• Thus, 8000 frames per second x 193 bits per frame = 1.544 Mbps

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Time Division Multiplexing

The T1 carrier (1.544 Mbps).

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Digital Hierarchy

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•Wavelength division multiplexing multiplexes multiple data streams onto a single fiber optic line.

•Different wavelength lasers (called lambdas) transmit the multiple signals.

•Each signal carried on the fiber can be transmitted at a different rate from the other signals.

•Dense wavelength division multiplexing combines many (30, 40, 50, 60, more?) onto one fiber.

Wavelength Division Multiplexing

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DWDM

• Short for Dense Wavelength Division Multiplexing, an optical technology used to increase bandwidth over existing fiber optic backbones

• Dense wavelength division multiplexing (DWDM) is a technology that puts data from different sources together on an optical fiber, with each signal carried at the same time on its own separate light wavelength.

• Using DWDM, up to 80 (and theoretically more) separate wavelengths or channels of data can be multiplexed into a light stream transmitted on a single optical fiber.

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DWDM

• DWDM works by combining and transmitting multiple signals simultaneously at different wavelengths on the same fiber.

• In effect, one fiber is transformed into multiple virtual fibers. So, if you were to multiplex eight OC -48 signals into one fiber, you would increase the carrying capacity of that fiber from 2.5 Gb/s to 20 Gb/s.

• Currently, because of DWDM, single fibers have been able to transmit data at speeds up to 400Gb/s.

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How DWDM Works • The technique takes each input source, assign a

uniquely colored laser (wavelength) to that source, and combines the multiple optical signals of the input sources so that they can be amplified as a group and transported over a single fiber.

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Wavelength Division Multiplexing

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DWDM Channels• A key advantage to DWDM is that it's protocol- and bit-rate-

independent.• Each WDM channel carries a time division multiplexed (TDM) signal.

• Since each channel is demultiplexed at the end of the transmission back into the original source, different data formats being transmitted at different data rates can be transmitted together.

• Specifically, Internet (IP) data, Synchronous Optical Network data (SONET), and asynchronous transfer mode (ATM) data can all be traveling at the same time within the optical fiber.

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DWDM Features

• Simultaneous Transmission Speeds– A single fiber optic line can support simultaneous

transmission speeds• Such as 51.84 Mbps, 155.52 Mbps, 622.08 Mbps and

2.488 Gbps.• These speeds are the speeds defined by OC-1, OC-3,

OC-12, and OC-48, the optical carrier (OC) specifications for high speed fiber optic lines.

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DWDM Features

• Support for Different Transmission Formats– A single fiber optic line can support different

transmission formats such as SONET (Synchronous Optical Network), SDH (Synchronous Digital Hierarchy) etc.

– DWDM-based networks can transmit data in IP, ATM, SONET /SDH, and Ethernet, and handle bit rates between 100 Mb/s and 2.5 Gb/s.

– Therefore, DWDM-based networks can carry different types of traffic at different speeds over an optical channel.

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DWDM Features

• DWDM is scalable– As the demands on a system and its applications

grow, it is possible to add additional wavelengths or lambdas onto the fiber, thus further multiplying the overall capacity of the original fiber optic system.

– Current technology allows for as many as 64 lambdas per fiber, with the possibility for 1000 lambdas within a few years

• DWDM is an expensive way to transmit signals from multiple devices

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DWDM Promises

• From a QoS standpoint, DWDM-based networks create a lower cost way to quickly respond to customers' bandwidth demands and protocol changes.

• In a system with each channel carrying 2.5 Gbps (billion bits per second), up to 200 billion bits can be delivered a second by the optical fiber.

• DWDM promises to solve the "fiber exhaust" problem and is expected to be the central technology in the all-optical networks of the future.

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WDM Supporting multiple speed transmissions

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