11/02/2013Bahman R. Alyaei1 Chapter 7 The Design of Long-Distance Links.

11/02/2013 Bahman R. Alyaei 1

Chapter 7

The Design of Long-Distance Links


1 Introduction1 Introduction• The network may be defined as a group of

switching nodes interconnected by links.• We may refer to a link as a transmission

highway between switches carrying one or more traffic relations.

• Links: provide connection for a multiple of switch inputs from one stage to a multiple of switch outputs in another stage.


Continue…Continue…

Examples of links.


2 The Bearer 2 The Bearer • Bearer: is what carries the information signals

(digital domain).• It could be a pair of wires or two pairs on a four-

wire basis, a radio carrier in each direction, a coaxial cable, or a fiber-optic cable.

• In the text that follows, it is assumed that the bearer will be transporting some sort of multiplex configuration, probably in a digital format (TDM).


Continue…Continue…• Modern long-distance links use either

radio or fiber-optic cable as the medium of choice.

• The decision on which one to use is driven by economics and capacity.

• Optical fiber is so far superior as transport for telecommunication digital configurations that coaxial cable must be removed from discussion.


Continue…Continue…• Coaxial cable requires many more active

repeaters per unit length than fiber optics.• Jitter: is the deviation from the ideal timing of

an event.• Jitter is a major transmission impairment on

digital systems, builds up as a function of the number of repeaters in tandem.

• Optical fiber needs no equalization, whereas coaxial cable needs equalizers.


3 Introduction to Radio 3 Introduction to Radio TransmissionTransmission

• Cable transmission medium (metallic and fiber):

1. Display little variability in performance2. Need not be shared.3. Do not require licensing (but often

require right-of-way).• Right-of-way: is a free path assigned

by the switch for a subscriber.


Continue…Continue…• Radio transmission medium:1. It displays notable variability in

performance.2. Shared with other users (not secure).3. Requires licensing (different frequency

bands are allocated for different applications and to different service providers by telecommunication authorities).



• A major factor in the medium selection process is information bandwidth.

• Fiber optics seems to have nearly an infinite bandwidth.

• Radio systems have very limited information bandwidth (bandpass).


Continue…Continue…• Then, why use radio in the first

place if it has so many drawbacks?1. Often, radio turns out to be less

expensive than fiber-optic cable.2. No requirement for right-of-way.3. Less vulnerable to vandalism.


Continue…Continue…4. Not susceptible to “accidental”

cutting of the link.5. Often more suited to crossing rough

terrain (hilly areas).6. Often more practical in heavily

urbanized areas (inside the city).7. As a backup to fiber-optic cable links.


Radio links


Continue…

Microwave link tower


Continue…Continue…• Fiber optic cable systems provide strong

competition with line-of-sight (LOS) microwave, but LOS microwave does have a place and a good market.

• Satellite communications is an extension of LOS microwave.

• It is also feeling the “pinch” of competition from fiber optic systems.



• Satellite link has two drawbacks:

1. Limited information bandwidth.2. Excessive delay when the

popular geostationary satellite systems are utilized.


4 Design Essential For LOS 4 Design Essential For LOS Microwave Systems Microwave Systems

• LOS microwave provides broadband bearer connectivity over a link or series of links in tandem.

• LOS phenomenon at frequencies from 150 MHz and upwards into the millimeter spectrum.

• Each link can be up to 30 miles (46 km) long or more depending on terrain topology.

• Some links extend over 100 miles (160 km).



Frequency bands and their applications



A sketch of an LOS microwave radio relay system

Refraction phenomena


Continue…Continue…• LOS implies that the antenna of the radio link

on one end has to be able to “see” the antenna on the other end.

• This may not necessarily be true, but it does give some idea of the problem.

• Let us suppose smooth earth (i.e. no mountains or ridges, buildings, or sloping ground of any sort).

• Here our LOS distance is limited by the horizon.


Continue…Continue…• The horizon (or skyline): is the apparent line

that separates earth from sky.• Is the line that divides all visible directions

into two categories: those that intersect the Earth's surface, and those that do not.

• At many locations, the true horizon is obscured by trees, buildings, mountains, etc., and the resulting intersection of earth and sky is called the optical or visible horizon.


Continue…

Horizon


Continue…Continue…• Given a LOS microwave antenna of a

height h in feet above ground surface, then, the distance to optical horizon d in miles to the horizon just where the ray beam will graze the rounded earth surface horizon can be calculated using


Continue…Continue…• But, the real Earth is surrounded by an

atmosphere of air, the density and refractive index of which vary considerably depending on the temperature and pressure.

• Usually, the density of the air just above the surface of the Earth is greater than its density at greater altitudes.


Continue…Continue…• This makes its refractive index greater

near the surface than higher, which causes light that is traveling roughly horizontally to be refracted downward, so it goes, to some small degree, around the curvature of the Earth's surface.

• This makes the air refract light to varying extents, affecting the appearance of the horizon.



Radio and optical horizon (smooth earth)


Continue…Continue…• This phenomena led to define a new

horizon which is called radio horizon.• In this case, at a height h in feet the

distance to radio horizon d in miles is usually calculated by


Continue…Continue…• The design of a microwave LOS link involves

five basic steps:1. Setting performance requirements.2. Site selection and preparation of a path

profile to determine antenna tower heights.3. Carrying out a path analysis, also called a link

budget .4. Running a path/site survey.5. Test of the system prior to cutover to traffic.


4.2 Setting Performance 4.2 Setting Performance RequirementsRequirements

• Often a microwave link is part of an extensive system of multiple links in tandem.

• Thus we must first set system requirements based on the output of the far-end receiver of the several or many links.

• The specification would be a bit error rate (BER) on a digital bit stream.


Continue…Continue…• The specification should be based on an

existing standard.• BER on a single link may have a 1×10−12

requirement during unfaded conditions.• For many digital links, a threshold floor of no

worse than 1×10−3 is set. • This value is related to supervisory signaling

where, if further degraded, supervisory signaling is lost and the link drops out (i.e., dial-tone is returned to the subscriber).


4.3 Site Selection and 4.3 Site Selection and Preparation of a Path ProfilePreparation of a Path Profile

• A path profile: is a graphic representation of the physical features of a propagation path in the vertical plane containing both endpoints of the path, showing the surface of the Earth and including trees, buildings, and other features that may obstruct the radio signal.

• After site selection, we will prepare a path profile of each link to determine the heights of radio towers to achieve LOS.


Continue…Continue…• For long distance (hundreds of miles or

kilometers), there will be two terminal sites, where the system begins and ends.

• Along the way, repeater sites will be required.• At some repeater sites, we may have need to

drop and insert traffic. • Other sites will just be repeaters.• These drop and insert points may just as well

be buildings or other facilities in a private/corporate network.


Simplified functional block diagram of the LOS microwave system



24 ft x 24 ft Tower Base Communication Shelter



Tower Base Communication Shelter


Continue…

Inside of the Tower Base Communication Shelter



• In gross system design1. Exchange location, particularly with

tandem/transit exchanges, must be considered in light of probable radio and cable routes.

2. Another consideration is electromagnetic interference (EMI).


Continue…Continue…• Midcity Repeater-Relay or terminal sites

have the following advantages:1. Colocation with a local or toll exchange.2. Use of tall buildings as natural towers.• And have the following disadvantages:1. Wave reflections (multipath) off buildings.2. Electromagnetic interference (EMI)

problems, particularly from other nearby emitters and industrial emission.


Continue…Continue…• Terminal sites will be in or near heavily

populated areas and preferably collocated with a toll exchange.

• The tops of modern large office buildings, if properly selected, are natural towers.

• Repeater-Relay sites are heavily influenced by intermediate terrain.

• Accessible hilltops or mountain tops are good prospective locations.



Building top and hilltop repeaters


Continue…Continue…• High towers are the rule over flat country.• The higher the tower, the longer the LOS

distance.• Thus, on a given link, fewer repeaters

would be required if towers could be higher.

• Hence there is a trade-off between tower height and number of repeaters.


ContinueContinue• A rule-of-thumb for maximum tower

height is 300 ft.• Certainly, towers can be built higher. • For example, there are broadcast towers

in excess of 1000 ft. • As a tower goes above 300 ft, the cost

of maintaining twist and sway requirements begins to escalate.


5 The Satellite5 The Satellite

• Most of the presently employed communication satellites are RF repeaters.

• It may decode and recode a digital bit stream.

• It also may have some bulk switching capability, switching to cross-links connecting to other satellites.



Satellite as an RF repeater


Continue…Continue…• Satellites in general are used

for:1. Mobile applications such as

communications to ships, vehicles, planes and hand-held terminals,

2. TV broadcasting. 3. Radio broadcasting.


Continue…Continue…• In commercial telecommunications

there are three methods of handling digital communication by satellite:

1.TDMA (Time Division Multiple Access).2.FDMA (Frequency Division Multiple

Access).3.VSAT Network (Very Small Aperture

Antenna).


Continue…Continue…• There are two types of radio station on

the earth:1. Earth Station: is a radio frequency

facility located on the earth’s surface that communicates with satellites.

2. Terrestrial Station: is a radio facility on the earth’s surface that communicates with other similar facilities on the earth’s surface.



Earth station



Earth and terrestrial stations


5.1 Applications5.1 Applications• Satellite links may prove optimum for a

variety of applications, including the following:

1. On international HU trunks country to country.

2. On national HU trunks, between switching nodes that are fairly well separated in distance [ >200 mi (320 km)] in highly developed countries.



Distance involved in satellite communications


Continue…Continue…3. In areas under development where

satellite links replace HF radio and a high growth is expected to be eventually supplemented by radio link and fiber optic cable.

4. In sparsely populated, highly rural, “out-back” areas where it may be the only form of communication.



5. On final routes for overflow on a demand-assignment basis. Route length again is a major consideration.

6. In many cases, on international connections reducing such connections to one link.

7. On private and industrial networks including VSAT networks.


5.2 Satellite Orbits5.2 Satellite Orbits


5.2.1 Low Earth Orbit (LEO)5.2.1 Low Earth Orbit (LEO)• LEO is generally defined as an orbit between

160 km (99.42 mi) and 2,000 km (1,243.742 mi) above the Earth’s surface.

• To place a satellite into a LEO, it require less energy.

• LEO satellite needs less powerful amplifiers for successful transmission.

• LEO orbits are not geostationary, therefore, a network of satellites is required to provide continuous coverage.


5.2.2 Medium Earth Orbit 5.2.2 Medium Earth Orbit (MEO) (MEO)

• Is the region of space around the Earth above LEO (altitude of 2,000 km (1,243 mi)) and below Geostationary Earth Orbit (GEO) [altitude of 35,786 km (22,236 mi)].

• The most common use for satellites in MEO is for navigation and communication.


Continue…Continue…• The most common altitude is

approximately 20,200 km (12,552 mi), which yields an orbital period of 12 hours, as used, for example, by the Global Positioning System (GPS).

• GPS satellite system consists of 24 satellites in six orbital planes with four satellites in each orbit.


5.2.3 Geostationary Earth 5.2.3 Geostationary Earth Orbit (GEO)Orbit (GEO)

• GEO is a circular orbit at 35,786 km (22,236 mi) above the Earth's equator and following the direction of the Earth's rotation.

• An object in such an orbit has an orbital period equal to the Earth's rotational period (24 hours), and thus appears motionless, at a fixed position in the sky, to ground observers.


Continue…Continue…• The orbital velocity is 11068 km/h

(6877.3 mi/h).• A total of three satellites is required to cover

the earth surface. • Telephone and television broadcast signals

are beamed up to the satellite from an earth station through a large, highly directive microwave dish antenna that is synchronized to the position of the satellite.


5.3 Frequency Bands5.3 Frequency Bands

Communication Satellite Frequency Bands Allocation


Continue…Continue…Band User Downlink

frequency (GHz)

Uplinkfrequency

(GHz)UHF

CX

KuKKa

MilitaryCommercial

MilitaryCommercialCommercial

Military

0.25-0.273.7-4.27.2-7.7

11.7-12.217.7-21.220.2-21.2

0.29-0.315.9-6.47.9-8.4

14.0-14.527.5-30.043.5-45.5

Typical Links Frequency Band Allocation


5.4 VSAT5.4 VSAT• One application that continues to

show strong growth is Very Small Aperture Terminal (VSAT) systems.

• A VSAT is a small-sized telecommunications earth station that transmits and receives via satellite.

• The terminal size is 0.75 to 3.8 meters in diameter.



• A typical VSAT site consists of a parabolic-shaped antenna mounted on the roof of a building, connected by a cable to a chassis inside the building.



VSAT terminal



A 2.5 m parabolic dish antenna for bidirectional satellite internet access


Continue…Continue…• It is most attractive for private data

circuits as an extension of Enterprise Networks.

• VSAT is a way of providing better connectivity to the internet or for private satellite communication networks in areas where there are unreliable communication networks.


Continue…Continue…• A typical VSAT unit contains a modem

for translating satellite transmissions back into data (and vice versa) and terrestrial interfaces for connecting customer equipment.

• There are many thousands of these networks now in operation worldwide.

• All VSAT systems today operate with geostationary orbit satellites (GEO).



Cisco IP VSAT Satellite WAN Network Module



VSAT Components



Branch Office WAN Backup Using Satellite


5.5 Iridium Communication 5.5 Iridium Communication • Another type of earth-satellite system that

seemed to offer great promise is a satellite constellation in a low earth orbit (LEO).

• One such system was called Iridium and was funded and fielded by Motorola on November 1, 1998.

• The Iridium satellite constellation is a large group of satellites providing voice and data coverage to satellite phones, pagers and integrated transceivers over Earth's entire surface.



Layout of Iridium satellite orbits


Continue…Continue…• Today, Iridium Communications Inc.

owns and operates the constellation and sells equipment and access to its services.

• The new enhanced version of it, is called Iridium Next, anticipated to begin launching in 2015.

• The constellation consists of 66 active satellites in orbit, and additional spare satellites to serve in case of failure


Continue…Continue…• Satellites are in LEO at a height of

approximately 485 mile (781 km).• Orbital velocity of the satellites is

approximately 17,000 mph (27,000 km/h), and orbital period of 100 minutes.

• Satellites communicate with neighboring satellites via Ka band inter-satellite links.

• Each satellite can have four inter-satellite links, two in the same orbit and two in the adjacent orbits.



• The constellation of 66 active satellites has 6 orbital planes spaced 30 degrees apart, with 11 satellites in each plane (not counting spares).

• These 66 satellites are sufficient to cover the entire Earth's surface at every moment.



An Iridium satellite



Layout of Iridium constellation showing global coverage


6 Fiber Optic Link6 Fiber Optic Link• It has the following advantages:1. Bandwidth capacity of a fiber-optic

cable is virtually unlimited.2. It has excellent attenuation

properties, as low as 0.25 dB/km.3. Channel equalizer is not required.4. Electromagnetic immunity.



5. Small size and lightweight.6. Secure compared to microwave

links and licensing is not required.• Fiber-optic transmission is used for

links under 1 ft in length all the way up to and including transoceanic undersea cable.


Continue…Continue…• All transoceanic (submarine) cables

presently being installed are based on fiber optics.

• The fiber optics of today uses three wavelength bands: around 800 nm, 1300 nm, and 1600 nm or near-visible infrared.

• The multiplexing scheme over optical fiber is known as Wavelength Division Multiplexing (WDM).



The usable wavelengths are just above and below 1 µm


6.1 Introduction6.1 Introduction• Optical fiber consists of a core made of

Silica and a cladding made of doped Silica.


Continue…Continue…• Ray theory and Snell’s law states that• When light passes from a medium of higher

refractive index (n1) into a medium of lower refractive index (n2), the refractive ray is bent away from the normal.

• As the angle of incidence becomes more oblique, the refracted ray is bent more until finally the refracted energy emerges at an angle of 90◦ with respect to the normal and just grazes the surface.



Ray paths for several angles of incidence(n1>n2), a) incident and refracted rays, b) critical angle, c) total internal reflection.


Continue…Continue…• Critical Angle: is the incident ray angle

where the refracted ray just grazes the surface.

• Total internal reflection occurs when the angle of incidence exceeds the critical angle.

• A glass fiber, for the effective transmission of light, requires total internal reflection.


Continue…• Another property of the fiber for a given

wavelength λ is the normalized frequency V defined as

• where a is the core radius, and n1 and n2 are the index of refraction of the core and cladding respectively.

• The term is called numerical aperture.


Continue…Continue…• Fiber optic communication system designer

must take into account, minimum bending radius and fiber strength, which are specified by the manufacturer.

• Minimum bending radii vary from about 2 cm to 10 cm, and as a rule of thumb, about 10 times the cable diameter.

• Fiber cable strength includes maximum pulling tension, maximum permissible compression load, and a maximum permissible impact force.


6.3 Types of Optical Fiber6.3 Types of Optical Fiber• There are three categories of optical fiber1. Step index (multimode): is characterized

by an abrupt change in refractive index.2. Graded index (multimode): is

characterized by a continuous and smooth change in refractive index.

3. Single mode (monomode): is designed such that only one mode can propagate (V ≤ 2. 405).



Construction and refractive index properties for (a) step-index fiber, (b) graded-index fiber


Optical fiber transmission modes (a) step-index fiber, (b) graded-index fiber and (c) single mode


6.4 Splices and Connectors6.4 Splices and Connectors• There are two methods of coupling, in fiber

optic systems, namely1. Splices.2. connectors.• The objective in either case is to transfer as

much light as possible through the coupling.• A good splice can have an insertion loss as

low as 0.09 dB, whereas the best connector loss can be as low as 0.3 dB.


Continue…Continue…• There are two types of splice:1. Mechanical splice: in which an optical

matching substance is used to reduce splicing losses.

2. Fusion splice: also called a hot splice, is where the fibers are fused together. The fibers to be spliced are butted together and heated with a flame or electric arc until softening and fusion occur.


Continue…

Fiber optic connectors



Splices



• Connectors are nearly universally used at the source and at the detector to connect the main fiber to these units.

• Splices require special splicing equipment and trained technicians.


6.5 Light Sources6.5 Light Sources• Also called photon sources.• Convert efficiently electrical energy (current)

into optical energy (light) in a manner that permits the light output to be effectively launched into the optical fiber.

• Three types light sources:1. LED — Light-Emitting Diode2. VCSEL — Vertical Cavity Surface Emitting

Laser3. LD — Laser Diodes


Continue…Continue…• All three are semiconductor devices.• LEDs are less efficient than LDs but are

considerably more economical.• VCSEL costs less than a LD and more

than an LED.• It is easier to couple light to a fiber from a

VCSEL than from either an LED or an LD.


6.6 Light Detectors6.6 Light Detectors• The most commonly used detectors

(receivers) are:1. PIN: the terminology PIN derives from

the semiconductor construction of the device where an intrinsic (I) material is used between the p–n junction of the diode.

2. Avalanche Photodiode (APD)


Continue…Continue…• APD is a gain device displaying gains on the

order of 15–20 dB. Where the PIN diode is not a gain device.

• PIN is more economical and requires less complex circuitry than does its APD counterpart.

• The response time of the APD is far better than that of the PIN diode.

• Bias voltages for APDs are much higher than for PIN diodes.


6.8 Fiber-Optic Link Design6.8 Fiber-Optic Link Design• The basic parameters in designing a

fiber optic communication system are:1. Type of signal to be transmitted (e.g.,

CATV analog, or PCM)2. Bit rate and format (e.g., SONET/SDH or

8-bit data, digital TV and WDM).3. System length, fiber portion, end-to-end.4. SNR or BER (1×10−10) at the electrical

output of the terminal-end detector.


6.9 Wavelength-Division 6.9 Wavelength-Division Multiplexing (WDM)Multiplexing (WDM)

• In Fiber-optic communication, WDM is a technology which multiplexes a number of optical carrier signals onto a single optical fiber by using different wavelengths (i.e. colors) of laser light.

• This technique enables bidirectional communications over one strand of fiber, as well as multiplication of capacity.


Continue…Continue…• The term WDM is commonly applied to an

optical carrier (which is typically described by its wavelength), whereas FDM typically applies to a radio carrier (which is more often described by frequency).

• Since wavelength and frequency are tied together through a simple directly inverse relationship, the two terms actually describe the same concept.



Block diagram of the wavelength division multiplexing (WDM) system

11/02/2013Bahman R. Alyaei1 Chapter 7 The Design of Long-Distance Links.

Documents

Transcript of 11/02/2013Bahman R. Alyaei1 Chapter 7 The Design of Long-Distance Links.