Coaxial Cable

Author:
Artem Bykov 01045273

Statement of Originality

The work contained in this assignment, other than that specifically attributed to another source, is that of the author(s). It is recognised that, should this decleration be found to be false, disciplinary action could be taken and the assignments of all students involved will be given zero marks.


Index


Introduction / Area of Application

Coaxial Cable (often called coax for short) is high-capacity cable widely used for high-frequency transmission of telephone, television, and digital audio signals. The cable is very effective at carrying many analog signals at high frequencies.
Coaxial cables have become an essential component of our information superhighway. They are found in a wide variety of residential, commercial and industrial installations. From broadcast, community antenna television (CATV), local area network (LAN), closed circuit television (CCTV) to many other applications, coax has laid the foundation for a simple, cost effective communications infrastructure.

Coaxial Cable is also widely used in the telecommunications industry, such as the telephone system where coax reaches the pole or drop nearest to the subscriber’s house, and a twisted-pair cable comes into the house and to the telephone.

Coaxial can also be a good solution for providing residences and small business with high-speed data access because it is generally wired in a bus topology, it requires less cable than other solutions and does not require a hub, cutting down on cost.

In contrast to twisted-pair wires, coaxial has the capacity to transmit information 80 times faster, has much higher bandwidth, offers greater protection from noise and interference and can support greater cable lengths between network devices. Coaxial cable also offers relatively high immunity to interference from noise sources, so it is often used in manufacturing environments.

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Coaxial Cable Physical Properties
The inner conductor is a solid metal wire usually copper enclosed in insulating dielectric material. A thin, tubular piece of metal screen also commonly called the shield acts as outer conductor surrounds the dielectric material (insulation). Its axis of curvature coincides with that of the inner conductor. Finally, an outer plastic cover or commonly called the jacket surrounds the rest.

Coaxial cable is called "coaxial" because it includes one physical channel that carries the signal surrounded (after a layer of insulation) by another concentric physical channel, both running along the same axis. Data is transmitted through the center channel, while the outer channel serves as a line to ground. These two conductors usually carry equal currents in opposite directions.


Inner / Center Conductor

Depending upon the application, many different types of conductor constructions may be found in coaxial cables.

Solid Conductors
Solid copper conductors are popular on many CCTV installations. Solid conductors provide less chance for distortion and line loss than copper covered steel. However, copper is a soft material and will break if repeatedly flexed. It is best used on permanent installation.
Stranded Conductors
Many small strands of copper can be used to make up any gauge size. Stranded conductors provide increased flexibility over solid conductors. This design has become popular for use on pan and tilt cameras, robotics and other applications that require repeated flexing.
Copper-Covered Steel Conductors
In some applications strength is a key requirement. Steel conductors covered with copper may be needed to prevent breakage in an active environment. Copper covered steel conductors provide added strength and RF support and are often used in CATV.


Outer Conductor / Shielding

In coaxial applications, shielding is an important part of the overall composition of the cable. Shielding not only protects the loss of signal in high frequency application, but also helps to prevent EMI (electromagnetic interference) and RFI (radio frequency interference) in the circuit. There are three popular types of shielding: braid and foil/braid.

Braid Shield
Braided shields are constructed of thin strands of aluminum, tinned copper or bare copper that are interwoven. Many different coverage's are available with 40-67% aluminum and 95% copper being the most popular.
Foil / Braid Shielding
Foil/braid shielding is extremely popular with CATV and applications. The 100% aluminum foil is accompanied by a braided shield to provide improved protection from EMI and RFI.
Quad-Shield
Quad-Shield Construction consists of a foil-braid-foil-braid shield. Very popular for CATV and applications in high EMI/RFI locations such as urban areas.


Outer plastic cover / Jacket

The outer plastic cover found on most coaxial cables is commonly called the jacket. The main function of the jacket is for protection from the environment and as an additional form of insulation. The compounds used to make the jacket may have different temperature ratings. The temperature rating of a cable, along with the location rating (i.e. plenum, wet, sunlight resistant etc.) will determine the minimum or maximum operating temperature of the cable. In today’s multi-application world, many jacketing choices exist:

PVC-Polyvinyl Chloride
Very flexible thermo-plastic material commonly used on general purpose coaxial products. Temperature rating: -40 to 80 degrees celsius.
PE-Polyethylene
A thermoplastic material which exhibits excellent electrical properties. Often used in direct burial applications. Temperature rating: -55 to 85 degrees celsius.
FEP-Fluorinated Ethylene Propylene
A thermoplastic material with good insulating properties. Excellent in high temperature, plenum and chemical environments. Temperature rating: -50 to 200 degrees celsius.


Additional Information
The construction of coaxial cable is designed to keep a distance between the inner conductor and the outer shield, which is one factor in determining the impedance rating of the cable. Just like all electrical components, coaxial cables have characteristic impedance. Impedances often become critically important when transmitting high volumes of data at very high rates (frequencies) and various types of coaxial cable are a common way to do this. This impedance depends on the dielectric material and the radii of each conducting material

There are many different forms of coaxial cable; however, they are usually divided into two classifications — those used for baseband transmission, and those used for broadband transmission. The primary distinction between the two cable types is the characteristic impedance. Baseband cable has a characteristic impedance of 50 ohms and used for digital transmission mainly in Ethernet networks, while broadband cable impedance is 75 ohms and used for analog transmission like in cable television (CATV) and Cable Internet. Using the wrong cable will cause network problems.

Coaxial cable provides superior noise immunity over conventional twisted pairs. As with other media, its immunity to noise is subject to the impact of variables such as the application and the environment. Baseband systems usually provide immunity of 50 to 60 decibels (dB), while broadband systems operate with 85 to 100 dB.

Many of these cables or pairs of coaxial tubes can be placed in a single outer sheathing and, with repeaters, can carry information for a great distance.

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Types of Coaxial Cables

All Coaxial Cables are classified according to the RG/U number which describes cable impedance characteristics. For full list of coaxial cables look HERE.
The most common coaxial cable used today is RG58/U. This coaxial cable will carry signals for distances of up to 300 meters and mostly used in Thinwire Ethernet.

There are also variations of traditional cable which are described below.

Dual-Shielded
Coaxial Cable
Dual shielded coaxial cables have two outer conductors, or shields, enclosing the dielectric. Dual shielding is needed for strength and abrasion resistance. Dual shields allow a decrease in attenuation and the possibility of unwanted external signals.
Twinaxial
Coaxial Cable
Twinaxial cable is composed of two insulated single conductor cables or hook-up wires twisted together, having a common shield and protective jacket.
Triaxial Cable
Triaxial cable is coaxial cable with one inner conductor and two shields all separated with dielectric material. Triaxial cable signals may be transported by both the inner conductor and the inner shield while the outer shield is at ground potential.


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Baseband Coaxial Cables

The baseband coaxial is 50 ohms cable usually referred to as Ethernet cable, because it was originally used in the Ethernet Networks. In baseband systems there is no modulation of the signal. Transceivers are used to place digital signals directly onto the cable. These signals are encoded using some of the techniques such as Manchester or Differential Manchester phase encoding. The digital signal occupies the entire bandwidth of the cable; thus, baseband cables have only one channel in operation at any moment in time. Transmission is bidirectional. That is, a signal inserted at any point on the medium propagates in both directions to the ends.

Many baseband implementations operate in the range of 10Mbps for standard Ethernet, although others, such as Token Ring at 16Mbps, operate at higher rates.

Coax comes in several sizes. Standard Ethernet cable, called Thick Coaxial or Thick Ethernet, is about the diameter of a man's thumb and Thin Coaxial or Thin Ethernet, is about as thick as a woman's pinky finger. Thicker coax is more robust, harder to damage, and transmits data over longer distances. It's also more difficult to connect.

Thick Coaxial (10BASE5)

Commonly known as thick ethernet, it is characterised by its thick coaxial cable (RG-8/U). 10Base5 refers to the specifications for thick coaxial cable carrying Ethernet signals. The 5 refers to the maximum segment length of 500 meters. Thick coaxial cable has an extra protective plastic cover that helps keep moisture away from the center conductor. This makes thick coaxial a great choice when running longer lengths in a linear bus network. One disadvantage of thick coaxial is that it is difficult to install and does not bend easily. It would be extremely unusual to find an old 10Base5 network still in place. 10Base5 is now being replaced by either Thin Coax or fiber optics.

• Transmission Rate - 10 Mbps.
• Maximum Length - 500 meters/segment.
• Impedance - 50 ohm, conductor diameter - 2.17 mm.
• Uses transceivers and AUI (Attachment Unit Interface) cable.
• Up to 100 nodes per segment.
• Total Maximum Extended Length (by Repeaters) - 1500 meters.

Connectors
The network stations are connected by means of external transceivers that receive the signal through vampire connectors directly from the coax cable.


Thin coaxial (10BASE2)

Commonly known as thin ethernet, thinnet or cheapernet, it is characterised by its thin coaxial cable (RG-58/U). 10Base2 refers to the specifications for thick coaxial cable carrying Ethernet signals. The 2 refers to the approximate maximum segment length of 200 meters. Thin coaxial cable is popular in school networks, especially linear bus networks. The beginning and the end of each section must be equipped with terminator of 50 Ohm.

• Transmission Rate - 10 Mbps.
• Maximum Length - 180 meters/segment.
• Impedance - 50 ohm, conductor diameter - 0.9 mm.
• BNC connections.
• 0.5 meter between consecutive connections.
• Maximum 30 nodes per segment.

Thin-Ethernet Network


An cable can be extended by installing Repeaters, which amplify the signal:



Connectors

Connector used with coaxial cables is the Bayone- Neill-Concelman (BNC) connector. Different types of adapters are available for BNC connectors, including a T-connector, terminator, and barrel connector.


Baseband Summary

• Unmodulated digital signal.
• Single channel.
• Bidirectional propagation of signal.
• No need of modems - low cost installation.
ADVANTAGES:
1. Simplicity
2. Low cost
3. Ease of installation and maintenance
DISADVANTAGES:
1. Limited distances
2. Data and voice only

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Broadband Coaxial Cables

Most broadband coaxial cable systems use a standard CATV 75-ohm cable. In broadband systems, analog signals are transmitted on the cable using both frequency and phase-modulation techniques. Multiple frequency-derived channels are operated on a single cable simultaneously. The cable's frequency spectrum is usually divided into a forward and return spectrum. The reason broadband cables are used is to get longer distances, but amplifiers are needed.

Broadband and cable TV take advantage of coax's ability to transmit many signals at the same time. Each signal is called a channel. Each channel travels along at a different frequency, so it does not interfere with other channels. Signals are modulated into Radio Frequency (RF) channels that are 6 MHz or 7 MHz wide in bandwidth. By dividing services into separate channels, different types of signals can co-exist and travel in opposite directions "inbound and outbound" along the same coaxial cable.

Broadband systems offer significantly increased bandwidth capabilities due to the fact that there are multiple channels on each cable. Typical broadband implementations with highly saturated cables may yield aggregate bandwidths as high as 100Mbps.

The broadband, and carrier band cables transmit analog signals, therefore modem converts the digital signal to analog signal prior to transmission of the data. A modem at the receiving end of the medium will convert the analog signal to a digital signal. The analog signal that carries digital data can travel longer distances and is more immune to electrical noise interferences, and signal degradation damage.

The carrier band cable that is sometimes called single channel broadband, covers only one analog frequency signal spectrum. Therefore, carrier band coaxial cables are less expensive than broadband coaxial cables, and the modems used in carrier band systems are less expensive than the modems used in broadband systems.

Topology
Coaxial cable can be used in both a point-to-point mode and a broadcast mode. In local network bus architectures, multiple devices are usually "dropped" from a single cable. Depending on the application and the required data rates, a broadband system may support thousands of connections. Amplifiers are usually placed at intervals of 0.5 to 1.5 kilometers to regenerate the signals.

Broadband Summary

• Digital signal modulated onto RF carrier (analog).
• Channel allocation based on FDM.
• Head-End for bidirectional transmission.
• Stations connected via RF modems.
ADVANTAGES:
1. Data, voice and video
2. Greater distances
3. Greater bandwidth
DISADVANTAGES:
1. Cable design
2. Alignment and maintenance
3. High cost, requires modems
4. Lack of well developed standards

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Uses of Coaxial Cable

Cable TV

Coaxial cable is used by cable TV companies to deliver service to user homes and businesses. Cable systems were originally designed to deliver broadcast television signals efficiently to subscribers' homes. To ensure that consumers could obtain cable service with the same TV sets they use to receive over-the-air broadcast TV signals, cable operators recreate a portion of the over-the-air radio frequency (RF) spectrum within a sealed coaxial cable line.

Each standard television channel in Australia occupies 7 MHz of RF spectrum. Thus a traditional cable system with 400 MHz of downstream bandwidth can carry the equivalent of about 60-100 analog TV channels

Circuits in coax network usually wired in bus topology, and need to have signal amplifiers installed at points along the network. The amplifiers can be one-way or two-way; the two-way amplifiers permit the provision of interactive services over the network, allowing consumers to select from a menu of services (for example, home shopping, videos on demand, or educational services).


Cable Internet

Cable Internet access is delivered through coaxial cable by a Cable Modem at the user's end. A Cable Modem sends and receives data to and from the Internet by using the existing coaxial cable TV network. Cable TV Networks are high band-with networks i.e. 550 to 750 MHz by their very nature of design.

Digital data signals are transmitted over radio frequency (RF) carrier signals on a cable system. For two-way communication, there is one carrier signal that carries data in the “downstream” direction (from the cable network to the customer), and another that carries data in the “upstream” direction (from the customer to the cable network). Higher-frequency signals flow toward the subscriber and lower-frequency signals flow toward the broadcasting head-end. This upstream and downstream bandwidth is shared by the active data subscribers connected to a given cable network segment, typically 500 to 2,000 homes on a modern HFC network.

To deliver data services over a cable network, one television channel (in the 85 - 750 MHz range) is typically allocated for downstream traffic to homes and another channel (in the 5 - 65 MHz band) is used to carry upstream signals. Each downstream band can carry one analog video channel, up to 27 Mbps data stream. Typically, cable internet access provide has maximum of 1.5 – 6 Mbps of bandwidth on the system. However, due network sharing that bandwidth on the network segment performance can be much lower.

The cable modem access network operates at Layer 1 (physical) and Layer 2 (media access control/logical link control) of the Open System Interconnect (OSI) Reference Model. Thus, Layer 3 (network) protocols, such as IP traffic, can be seamlessly delivered over the cable modem platform to end users.

A Cable Modem Termination System (CMTS) communicates through these channels with cable modems located in subscriber homes to create a virtual local area network (LAN) connection. All cable modems can receive from and send signals only to the CMTS, but not to other cable modems on the line. A typical CMTS consists of an Input interface, Router, Cable Modem card and a powerful Microprocessor. Most cable modems are external devices that connect to a personal computer (PC) through a standard Ethernet card or Universal Serial Bus (USB) connection

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History


Coaxial cable was invented in 1929 and first used commercially in 1941. AT&T established its first cross-continental coaxial transmission system in 1940.

Government and military applications summoned in the further development of coaxial cables. Built to military specifications and classified according to Radio Guide Utility (RG/U) numbers, these products were developed to help support high frequency radio transmissions. The steady growth of the computer industry further developed the need for coaxial cables for commercial use. Manufacturers of “proprietary” system demanded a variety of unique cable designs.

The first major installation in Australia of coaxial cable was in the early 1960s, when the first broadband link joining Sydney, Canberra and Melbourne was constructed. The link opened for traffic in April 1962.
The cable laid between these centres was made up six coaxial tubes and 32 pairs of other wires, and had a potential capacity of thousands of simultaneous telephone calls, in addition to being able to relay television programs.
Presently, coaxial cables are installed in underground ducts in metropolitan areas. However, in country areas, and over long-distance routes, the cable is buried about a metre deep along its route. Rather larger excavations have to be made for the underground housings for the repeater equipment, into which the cable is led and jointed.

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Multiple Choice Questions

Question 1. A transmission medium consisting of thickly insulated copper wire describes:
a) coaxial cable.
b) twisted wire.
c) fiber-optic cable.
d) microwave.

Question 2. The range of frequencies that can be accommodated on a particular telecommunications channel is the channel’s:
a) byte count.
b) bandwidth.
c) topology.
d) bit speed.

Question 3. Which of the following best describes the Cable TV line that is used in most homes today?
a) twisted-pair wire
b) coaxial cable
c) fiber-optic cable
d) modem cable

Question 4. What Cable is commontly known as Thinet?
a) 10Base2.
b) 10Base5..
c) 10BaseF.
d) 10BaseT.

Question 5. What is the most common coaxial cable used today?
a) RG6/U
b) RG45/U
c) RG15/U
d) RG58/U

Question 6. What type of signals can be transmited on broadband coaxial cable?
a) analog
b) digital
c) radiol
d) microvave

Question 7. What topology does Cable Internet use?
a) Star
b) Ring
c) Bus
d) Hybrid

Question 8. Which of the following is not a coaxial cable type?
a) Monoaxial Coaxial Cable
b) Dual-Shielded Coaxial Cable
c) Twinaxial Coaxial Cable
d) Triaxial Cable

Question 9. Mbps repserents?
a) Megabytes per second
b) Milibytes per second
c) Megabits per second
d) Milibits per second

Question 10. The speed of nomal Ethernet is?
a) 10 MBps
b) 100 MBps
c) 100 Mbps
d) 10 Mbps

Question 11. Most commonly used for Broadband Intert is:
a) twisted wire.
b) coaxial cable.
c) fiber-optic cable.
d) microwave.

Question 12. What year was coaxial cable invented?
a) 1929
b) 1939
c) 1989
d) 1949

Question 13. Standard television channel in Australia occupies:
a) 6 MHz of RF spectrum
b) 85 - 750 MHz of RF spectrum
c) 6 MHz of RF spectrum
d) 7 MHz of RF spectrum

Question 14. Cable modem access network uses:
a) OSI Reference model
b) TCP/IP Reference model
c) IP Referencemodel
d) Network Layer

Question 15. BNC connectors does NOT include:
a) T-connector
b) terminator
c) barrel connector
d) X-connector

Question 16. CCTV stands for?
a) Closed Circuit Telephone
b) Cummunity Circuit Television
c) Closed Circuit Television
d) None of the following

Question 17. Another name for outer plastic cover found on coaxial cable is:
a) Tube
b) Jacket
c) Wire
d) Cable

Question 18. Impedance of Broadband cable is?
a) 75 ohm
b) 50 ohm
c) 100 ohm
d) 0 ohm

Question 19. Which of the following is NOT advantage of baseband coaxial cable
a) Simplicity
b) Low cost
c) Ease of installation and maintenance
d) Large RF spectrum

Question 20. Whats the maximum bandwidth can u expect from Australian Cable ISP?
a) 1.5 – 6 Mbps
b) 28 kbps
c) 10 Mbps
d) 600 kbps

 

Answers

1. a 6. a 11. b 16.c
2. b 7. c 12. a 17.b
3. b 8. a 13. d 18. a
4. a 9. c 14. a 19. d
5. d 10. d 15 d 20. a

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Summary


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Glossary

10Base2 coaxial cable: The IEEE specifications for thin wire or thin net Ethernet network cable with a maximum segment length of 185 meters.
10Base5 coaxial cable: The IEEE specifications for thick wire Ethernet network cable with a maximum segment length of 500 meters.
Attenuation: A decrease in a signal's strength (measured in decibels) as it transmits over wires or cables. The shorter the wire or cable the less attenuation occurs.
Bandwidth: The amount of transmission capacity that is available on a network at any point in time. Available bandwidth is dependent on factors such as the rate of data transmission speed between networked devices and the type of device used to connect to a network.
Center Conductor: The solid or stranded wire in the middle of the coaxial cable.
Dielectric: The insulating material that separates the center conductor and the shielding.
Electromagnetic Interference (EMI): Electrical or electro-magnetic energy that disrupts electrical signals.
Frequency: The number of times a periodic action occurs in one second. Measured in Hertz.
Ethernet: International standard networking technology for wired implementations with a speed of 10 Mbps
Impedance: Measure of how much voltage must be applied to the cable to achieve a given signal strength. Measured in Ohms.
Megahertz (MHz): A measure of frequency in one million cycles per second.
RG/U: Symbols used to represent coaxial cable specifications
Shielding: Conductive envelope made of wires or metal foil that covers the dielectric and the center conductor

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References / Acknowledgements top of page
This document is 2002 Artem Bykov.