Data Communications Theory

Lecture-7

Dr. Anwar Mousa

University of PalestineFaculty of Information Technology

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The electromagnetic spectrum; propagation in free-space and the atmosphere

In this section we shall consider the physical properties of free-space electromagnetic waves, and how the atmosphere

influences the propagation of electromagnetic waves. In the following sections, we shall describe how these

properties have determined the selection of frequencies for communication .

The electromagnetic spectrum is divided up into a number of

bands

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The higher frequencies of the electro-magnetic spectrum

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The electromagnetic spectrum ;Microwave : from 1GHz – 300 GHz S band: 2-4 GHzC band: 4-8 GHzX band: 8-12 GHzKu band: 12-18 GHzK band: 18-27 GHzKa band: 27-40 GHz..................................................................................................VLF/LF band: 3 KHz - 300 KHzMF band: 300 KHz - 3 MHzHF band: 3 MHz - 30 MHzVHF band: 30 MHz - 300 MHzUHF band: 300 MHz - 3 GHzSHF band: 3 GHz - 30 GHzEHF band: 30 GHz - 300 GHz

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The electromagnetic spectrum ;AM broadcast band(MF band). The band of frequencies

extending from 535 to 1705 kHz.The 117 carrier frequencies assigned to AM broadcast stations begin at 540 kHz and progress in 10 kHz

steps to 1700 kHz.The FM broadcast band (VHF band) consists of that portion

of the radio frequency spectrum between 88 MHz and 108 MHz. It is divided into 100 channels of 200 kHz each.

TV chennels (VHF,UHF bands) VHF(54 MHz-88 MHz, 174 MHz-216 MHz), UHF(470 MHz-806 MHz) . It is divided

into 69 channels of 6 MHz each .

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The electromagnetic spectrum; propagation in free-space and the atmosphere

Propagation of waves in free-space is different from that in cable or waveguides.

The power of free-space waves obey an inverse square law. For each doubling of the distance

between the source and receiver, a 6dB loss is experienced .

For all frequencies up to millimetre-wave frequencies, this free-space loss is the most

important source of loss. Because of it, free-space systems usually require

much more power than cable or fibre systems .

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Propagation in free-space

When waves traveling in free-space are obstructed, new waves result from the interaction. There are four types of interaction :

.1Reflection. This occurs when a wave meets a plane object. The wave is reflected back without distortion .

.2Refraction. This occurs when a wave encounters a medium with a different wave speed. The direction and speed of the

wave is altered ..3Diffraction. This occurs when the wave encounters an edge.

The wave has the ability to turn the corner of the edge. This ability of waves to turn corners is called diffraction. It is

markedly dependent on frequency -- the higher the frequency, the less diffraction. Very high frequencies (light)

hardly diffract at all; "light travels in straight lines ".

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Reflection, refraction and diffraction

.4Scattering. Catch-all description of wave interactions that are too complex to be described as reflection, refraction or

diffraction. Typically the result of scattering is to remove radiation of the wave, and re-radiate it over a wide range

of directions. Scattering too is strongly frequency dependent. Usually it will increase with frequency .

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Propagation through the atmosphere

From the point of view of wave propagation, there are two layers .

The troposphere is the lowest layer of the atmosphere. It extends (typically) from the surface to a height of 50 Km. It contains all the Earth's weather, all the liquid water, most of the water vapour, most of the gaseous

atmosphere, and most of the pollution .The ionosphere extends from the top of the troposphere

into outer space. The ionosphere plays a crucial historical role in radio communication. It consists of oxygen molecules that are ionised by the action of the sun.

During the day, the quantity of ions rises. At night, the ions recombine to form uncharged oxygen molecules .

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Propagation through the atmosphere

The ionisation of the atmosphere converts the ionosphere into a plasma: an electrically neutral gas of positive and

negative charges .

A plasma has a wave-speed that is a strong function of frequency .

The consequence of this is that, to a low frequency wave, the ionosphere behaves as a mirror.

Waves are simply reflected. This permits a mode of propagation in which the wave bounces forwards and

backwards between the ionosphere and the Earth .

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Propagation through the atmosphere

It was this mode of propagation that permitted Marconi, to achieve cross-Atlantic radio communication, and, in the

process, discover the ionosphere.

The Earth itself also acts as a mirror for electromagnetic waves .

As the frequency increases to around 50MHz, the ionospheric effect reduces, and at higher frequencies

becomes invisible .

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Propagation through the atmosphere Using the ionosphere for radiowave propagation

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High Frequency (HF, 3MHz -30MHz) propagation

The propagation of HF consists of two waves .The sky-wave, that bounces from the ionosphere, and the ground-wave, that propagates along the ground.

Because of the long wavelengths, HF communications are not effected by the troposhpere, and can usually bend around

even large objects such as hills .The ionosphere permits communication over great distances.

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Very High Frequency (VHF) and Ultra High Frequency (UHF) propagation

These frequencies are not effected by the troposphere, but are too high to exploit the ionosphere .

In addition their decreased wavelength makes them increasingly less able to diffract around obstacles .

At UHF in particular, areas in deep radio shadow can experience very poor reception .

In urban areas, most reception is due to scattered arrivals .Experiments have shown that the free-space loss is increased

in urban areas, to the extent of following a fourth power law .VHF is used for FM radio stations and UHF for TV

transmissions .

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Very High Frequency (VHF) and Ultra High Frequency (UHF) propagation

When the received signal consists of a number of scattered arrivals, an effect called fading occurs .

According to the location, the many arrivals may interfere constructively or destructively .

The phase relationship between the arrivals changes as the location of receiver changes.

This causes a moving receiver to experience fluctuations between strong and weak signals .

This effect is known as fading .

SONET / SDH

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What is SONET & SDH?SONET and SDH are a set of related standards for

synchronous data transmission over fiber optic networks. SONET is short for Synchronous Optical NETwork SDH is an acronym for Synchronous Digital Hierarchy. SONET is the United States version of the standard

published by the American National Standards Institutue (ANSI).

SDH is the international version of the standard published by

the International Telecommunications Union (ITU).

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Optical Level Electrical Level Line Rate (Mbps) SDH Equivalent

OC-1 STS-1 51.840 –

OC-3 STS-3 155.520 STM-1

OC-12 STS-12 622.080 STM-4

OC-48 STS-48 2488.320 STM-16

OC-192 STS-192 9953.280 STM-64

OC-768 STS-768 39813.120 STM-256

OC-1536 STS-1536 79,626,120 STM-512

OC-3072 STS-3072 159,252,240 STM-1024

STS (electrical signaling levels called Synchronous Transport

Signals)

OC: Optical Carrier

STM: Synchronous Transport Modul

The following table lists the hierarchy of the most common SONET/SDH data rates:

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The SONET/SDHThe "line rate" refers to the raw bit rate carried over the

optical fiber. A portion of the bits transferred over the line are designated

as "overhead ."The overhead carries information that provides OAM&P

(Operations, Administration, Maintenance, and Provisioning) capabilities such as framing, multiplexing, status, and

performance monitoring .The "line rate" minus the "overhead rate" yields the

"payload rate" which is the bandwidth available for transferring user data such as packets or ATM cells .

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The SONET/SDHThe SONET/SDH level designations sometimes include a

"c" suffix (such as "OC-48c") .The "c" suffix indicates a "concatenated" or "clear" channel .

This implies that the entire payload rate is available as a single channel of communications (i.e. the entire payload

rate may be used by a single flow of cells or packets) .The opposite of concatenated or clear channel is

"channelized." In a channelized link the payload rate is subdivided into

multiple fixed rate channels. For example, the payload of an OC-48 link may be

subdivided into four OC-12 channels .

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The SONET/SDHIn practice, the terms STS-1 and OC-1 are sometimes used

interchangeably, though the OC-N format refers to the signal in its optical form .

It is therefore incorrect to say that an OC-3 contains 3 OC-1s: an OC-3 can be said to contain 3 STS-1s.

Note that the typical data rate progression starts at OC-3 and increases by multiples of 4 .

As such, while OC-24 and OC-1536, along with other rates such as OC-9, OC-18, OC-36, and OC-96 may be defined in some standards documents, they are not available on a wide-

range of equipment.As of 2007, OC-3072 is still a work in progress.

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The SONET/SDHSynchronous optical networking, is a method for

communicating digital information using lasers or light-emitting diodes (LEDs) over optical fiber .

The method was developed to replace the Plesiochronous Digital Hierarchy (PDH) system for transporting large

amounts of telephone and data traffic

and to allow for interoperability between equipment from different vendors.

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The SONET/SDHSynchronous networking differs from PDH in that the exact

rates that are used to transport the data are tightly synchronized across the entire network ,

made possible by atomic clocks.SONET/SDH is a synchronous network using synchronous

TDM multiplexing .All clocks in the system are locked to a master clock .

This synchronization system allows entire inter-country networks to operate synchronously ,

greatly reducing the amount of buffering required between elements in the network.

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The SONET/SDHBoth SONET and SDH can be used to encapsulate

earlier digital transmission standards, such as the PDH standard, or used directly to support

either ATM or so-called Packet over SONET/SDH (POS) networking .

As such, it is inaccurate to think of SDH or SONET as communications protocols ,

but rather as generic and all-purpose transport containers for moving both voice and data.

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Structure of SONET/SDH signals SONET and SDH often use different terms to describe

identical features or functions .

The main difference between the two:SONET can use either of two different basic framing

units while SDH has one

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The basic unit of transmissionThe basic unit of framing in SDH is an STM-1 (Synchronous

Transport Module level - 1), which operates at 155.52 Mbit/s.

SONET refers to this basic unit as an STS-3c (Synchronous Transport Signal - 3, concatenated) ,

but it is otherwise identical in size, bit-rate, and high-level functionality.

SONET offers an additional basic unit of transmission, the STS-1 (Synchronous Transport Signal - 1), operating at 51.84

Mbit/s - exactly one third of an STM-1/STS-3c.

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Concatenated signalsIn normal operation, an STS-n signal is made of n

multiplexed STS-1 signal.Sometimes we have a signal with a data rate higher than

what an STS-1 can carry.In this case, SONET allows us to create an STS-n signal

that is not considered as n STS-1 signal; it is one STS-n signal that can not be demultiplexed into n STS-1 signal

(STS-nc).STS-3c can not be demultiplexed into three STS-1 signals.An STS-3c can accommodate 44 ATM cells, each of 53

bytes.

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SONET LAYERS The SONET standards includes four functional layer:

the photonic, the section, the line and the path layer.They corresponds to both the physical and data link

layer in OSI..1The path layer

The path layer is responsible for the movement of a signal from its optical source to its optical destination.

Path layer overhead is added at this layer.STS multiplexers provide path layer functions

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SONET LAYERS .2The line layer

The line layer is responsible for the movement of a signal across physical line.

Line layer overhead is added to the frame at this layer.

STS multiplexers and ADM provide line layer functions

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SONET LAYERS .3The Section layer

The line layer is responsible for the movement of a signal across physical section.

It handles framing, scrambling and error control.

Section layer overhead is added to the frame at this layer.

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SONET LAYERS .4The Photonic layer

Corresponds to the physical layer of OSI Model.Includes physical specification of the optical fiber

channel,SONET uses NRZ encoding with the presence of

light representing one and the absence of light representing Zero .

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Framing

In packet or frame oriented data transmission (such as Ethernet), a frame usually consists of a header and a payload,

with the header of the frame being transmitted first, followed by the payload (and possibly a trailer).

In synchronous optical networking, this is modified slightly. The header is termed the overhead and the payload still exists,

but instead of the overhead being transmitted before the payload, it is interleaved,

with part of the overhead being transmitted, then part of the payload, then the next part of the overhead, then the next part

of the payload, until the entire frame has been transmitted .

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MultiplexingThree STS-1 signals may be multiplexed by time-division

multiplexing to form the next level of the SONET hierarchy, the OC-3 (STS-3), running at 155.52 Mbit/s .

The multiplexing is performed by interleaving the bytes of the three STS-1 frames to form the STS-3 frame, containing

2,430 bytes and transmitted in 125 microseconds.Higher speed circuits are formed by successively aggregating

multiples of slower circuits, their speed always being immediately apparent from their designation.

For example, four STS-3 signals can be aggregated to form a 622.08 Mbit/s signal designated as OC-12 or STM-4.

OC-12 STS-12 622.080 STM-4OC-3 STS-3 155.520 STM-1

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MultiplexingThe highest rate that is commonly deployed is the OC-192 or

STM-64 circuit, which operates at rate of just under 10 Gbit/s .Speeds beyond 10 Gbit/s are technically viable and are under

evaluation.[ Few vendors are offering STM-256 rates now, with speeds of nearly

40Gbit/s .]Where fiber exhaust is a concern, multiple SONET signals can

be transported over multiple wavelengths over a single fiber pair by means of Wavelength division multiplexing ,

including Dense Wave Division Multiplexing (DWDM) and Coarse Wave Division Multiplexing (CWDM). DWDM

circuits are the basis for all modern transatlantic cable systems and other long-haul circuits.

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SONET/SDH and relationship to 10 Gigabit/// Ethernet

Another fast growing circuit type amongst data networking equipment is 10 Gigabit Ethernet

(10GbE) .This is similar in rate to OC-192/STM-64, and, in its

wide area variant, encapsulates its data using a light-weight SDH/SONET frame.

OC-192 STS-192 9953.280 STM-64However, 10 Gigabit Ethernet does not explicitly

provide any interoperability at the bitstream level with other SDH/SONET systems .

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SONET equipment

SONET regeneratorSONET Regens extend long haul routes in a way similar to

most regenerators,A regenerator is a repeater that takes a received optical signal

(OC-n) (that has already traveled a long distance ), demodulates it into the corresponding electrical signal (STS-

n), regenerates the electrical signal, and finally modulates the electrical signal into its corresponding OC-n signal .

Since the late 1990s, SONET regenerators have been largely

replaced by Optical Amplifiers .

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SONET equipment STS (Synchronous Transport Signals) Multiplexer/

Demultiplexer

Mark the beginning points and endpoints of a SONET linkProvide interface between an electrical tributary network and

the optical networkAn STS multiplexer multiplexes signals from multiple

electrical sources and creates the corresponding OC signals. An STS demultiplexer demultiplexes an optical OC signal

into corresponding electricalsignals.

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SONET equipment

SONET Add-Drop Multiplexer (ADM)SONET ADMs are the most common type of SONET

Equipment .SONET ADMs allows insertion and extraction of signals.SONET ADMs can add STSs coming from different sources

into a given path or can remove a desired signal from a path and redirect it without demultiplexing the entire signal.

Instead of relying on timing and bit positions, add/drop multiplexing use header info such as addresses and pointers

to identify individual streams.

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SONET equipment

TerminalsA terminal is a device that uses the services of a

SONET network.For example, in the Internet, a terminal can be a

router that sends packets to another router at the other side of a SONET network.

SONET is used as a transport network to carry loads from other WANs.

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SONET Network ArchitecturesCurrently, SONET (and SDH) have a limited number of

architectures defined .These architectures allow for efficient bandwidth usage

as well as protection (i.e. the ability to transmit traffic even when part of

the network has failed ,)and are key in understanding the almost worldwide usage

of SONET and SDH for moving digital traffic .The three main architectures are:

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SONET Network Architectures.1Linear networks

Linear networks can be point-to-point or multipoint

Point-to-point NetworksA point-to-point network is normally made of an STS

multiplexer, an STS demultiplexer and zero or more regenerators with no add/drop multiplexers.

The signal flow can be uidirectional or bidirectional.

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SONET Network ArchitecturesMultipoint networks

A multipoint network uses ADMs to allow the communications between several terminals .

An ADM removes (drops) the signal belonging to the termial connected to it and adds the signal

transmitted from another terminal.Each terminal can send data to one or more

terminals.The signal flow can be uidirectional or bidirectional.

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SONET Network ArchitecturesLinear APS (Automatic Protection

Switching), also known as 1+1 :To create protection against failre in linear networks,

SONET defines Automatic Protection Switching (APS).

APS means protection between two ADMs or a pair of STSMux/Demux_

Three schemes are common:

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SONET Network Architectures1 .ONE-PLUS-ONE APS

There are normally two lines: one working line and one protection line .

Both lines are active all the time.The sending multiplexer sends the same data on both lines;

the receiver multiplexer monitors the line and chooses the one with the better quality .

If one of the lines fails, the receiver selects the other line.The sheme is inefficient because two times the bandwidth is

required .

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SONET Network Architectures2 .ONE-to-ONE APS

There are normally two lines: one working line and one protection line .

The data are sent on the working line until it fails.At this time the receiver , using the reverse channel, informs

the sender to use the protection line instead.The failer recovery is slower than that of the one-plus

scheme but this scheme is more efficient because the protection line is used only when the working line if failed .

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SONET Network Architectures3 .ONE-to-Many APS

Similat to the one-to-one scheme except that there is only one protection line for many working lines.

When a filer occurs in one of the working lines, the protection line takes control until the failed

line is repaired. It is not as secure as the one-to-one scheme!

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SONET Network Architectures.2UPSR (Unidirectional Path Switched

Ring) :A type of ring network

A UPSR is a unidirectional network with two rings: one ring used as the working ring and the opther as the

ptoterction ring.The idea is similar to the one—plus-one scheme in a

linear network.The same signal flows through both rings, one clockwise

and the other counterclockwise.

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SONET Network ArchitecturesI t is called UPSR because monitoring is done at the path

layer.A node receives two copies of the electrical signal at the

path layer, compares them and chooses the one with the best quality.

If a part of a ring fails, the other ring still can guarantee the continuation of data flow.

UPSR, like the one-plus-one scheme, has fast failure recovery, but it is not efficient because we need to have

two rings that do the job of one !

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SONET Network Architectures.3BLSR (Bidirectional Line Switched Ring) :A type of ring network

Communications is bidirectional, which means that we need two rings for working lines.

We also need two rings for protection lines.The operation is similar to on-to-one APS scheme.

If a working ring in one direction between nodes fails, the receiving node can use the reverse ring to inform the upstream

node in the failed direction to use the protection ring.

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SONET Network ArchitecturesCombination of rings

SONET uses a combination of interconnected rings to create services in a wide area.

A SONET network may have a regional ring, several local rings and many site rings to give service to a wide area .

These rings can be UPSR. BLSR, or a combination of both .

Mesh Networks

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Next-generation SONET/SDH SONET/SDH development was originally driven by the need to transport

multiple PDH signals like DS1(1.544 Mbps), E1 (2.048 Mbps), DS3 (44.736 Mbps) and E3 along with other groups of multiplexed 64 kbit/s pulse-code modulated voice traffic.

The ability to transport ATM traffic was another early application.

In order to support large ATM bandwidths, the technique of concatenation was developed, whereby smaller SONET multiplexing containers (eg, STS-1) are multiplexed to build up a larger container (eg, STS-3c) to support large data-oriented pipes.

SONET/SDH is therefore able to transport both voice and data simultaneously.

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Next-generation SONET/SDH

One problem with traditional concatenation, however, is inflexibility.

Depending on the data and voice traffic mix that must be carried, there can be a large amount of unused bandwidth left

over, due to the fixed sizes of concatenated containers. For example, fitting a 100 Mbit/s Fast Ethernet connection

inside a 155 Mbit/s STS-3c container leads to considerable waste.

Virtual Concatenation (VCAT) allows for a more arbitrary assembly of lower order multiplexing containers, building larger containers of fairly

arbitrary size (e.g. 100 Mbit/s).

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Next-generation SONET/SDH

Virtual Concatenation increasingly leverages X.86 or Generic Framing Procedure (GFP) protocols in order to map payloads of arbitrary bandwidth into the virtually concatenated container.

Link Capacity Adjustment Scheme (LCAS) allows for dynamically changing the bandwidth via dynamic virtual concatenation, multiplexing containers based on the short-term bandwidth needs in the network.

The set of next generation SONET/SDH protocols to enable Ethernet transport is referred to as Ethernet over SONET/SDH (EoS).