What Is Coaxial Cable ? – How It Is Used In The Physical Layer ?

The physical layer is the lowest layer in the OSI reference model. Data transmission in this layer is one of its topmost priority (by first converting the analog signals into digital bits). We have already seen that there are two types of transmission mediums. In this post, we will explore one of them i.e Coaxial cable in the wired transmission medium category.

A signal traveling along any of the wired media is directed and contained by the physical limits of the medium. Twisted-pair and the coaxial cable basically use the metallic (copper) conductors that accept and transport signals in the form of electric current.

What is Coaxial Cable in the Physical Layer ?

The construction of the coaxial cable is as shown in the below figure.  It consists of two concentric conductors namely an inner conductor and a braided outer conductor separated by a dielectric material

The external conductor is in the form of a metallic braid and used for the purpose of shielding. The coaxial cable may contain one or more coaxial pairs.

In a coaxial cable , a solid center conductor is located coaxially within a cylindrical outer conductor. The conductors are separated by a solid dielectric material and the outer conductor is covered with a plastic sheath.

What Is Coaxial Cable That Is Used In The Physical Layer ?

Normally , the shield is kept (maintained) at ground potential and a voltage is then applied to the center conductor to carry electrical signals. Interference from the external sources and leakage from the center core can be prevented with a metallic shield.

Center core may be copper-plated steel , silver-plated copper wire. The dielectric insulator may be solid plastic (PE, polyethylene) , foam plastic (PF), solid teflon.

The metallic shield may be braided copper wire , foil shield and braided wire (double layer shield) , foil braided wire (quad shield), or solid metal tube. 

The construction of a coaxial cable with other accessories such as connector, jacket etc. is shown in fig below :



What Is Coaxial Cable That Is Used In The Physical Layer ?

The wire mesh (braided conductor) protects the inner conductor from Electromagnetic Interference (EMI). It is often called a shield. A  tough plastic jacket forms the cover of the cable as shown above providing insulation and protection.

The coaxial cable was initially developed for analog telephone networks. A single coaxial cable would be used to carry more than 10,000 voice channels at a time. The digital transmission systems using the coaxial cable were developed in 1970’s. These systems operated in the range of 8.5 Mb/s to 565 Mb/s.

The most popular application of a coaxial cable is in the cable TV system. The existing coaxial cable system has a range from 54 MHz to 500 MHz. Other important application is cable modem with the cable modem termination system (CMTS).

One more application is Ethernet LAN using the co-axial cable. The coaxial cable is normally used for its large bandwidth and very high noise immunity.

Features of Coaxial Cable 

1. Two types of cables having 75 Ω and 50 Ω impedance are available. ( 50Ω cable is used for digital transmission and 75Ω cable is used for analog transmission  )

2. Due to the shield provided, this cable normally has excellent noise immunity.

3. Coaxial cable (or coax) carries signals of higher frequency ranges than those in case of twisted pair cable, in part because the two media are constructed quite differently . 

4. It has a large bandwidth and low losses.

5. This cable is suitable for point to point and multipoint applications. This is the most widely used medium for local area networks.

6. Although the coaxial cable has a much higher bandwidth, the signal weakens rapidly and it requires the frequent use of repeaters .

7. It has a data rate of 10 Mbps which can be further increased with the increase in diameter of the inner conductor.

8. The specified maximum number of nodes is upto 100.

9. The attenuation is less as compared to the twisted pair cable.

10. Coaxial cables are generally inexpensive (as compared to optical fiber cables)

Coaxial Cable Standards used in practical life

 The coaxial cables are categorized by their RG ratings where RG stands for Radio Government .  Each RG number represents a unique set of physical specifications, including the wire gauge of the inner conductor, the thickness and the type of the inner insulator, construction of the shield, and the size and type of the outer casing. 

What Is Coaxial Cable That Is Used In The Physical Layer ?

Advantages and applications of the Coaxial Cables

Coaxial cables generally have excellent noise immunity due to the shield. They can be used for high data rates and they offer less attenuation. Further, it has a larger bandwidth than twisted-pair cables .

A common application of coaxial cable is in traditional Ethernet LANs .Because of its high bandwidth, and consequently high data rate, coaxial cable was chosen for digital transmission in early Ethernet LANs

Baseband Coaxial Cable  !!

The baseband coaxial cable is the one that makes use of digital signaling. The original ethernet scheme generally makes use of baseband coaxial cable.  The baseband coaxial cables are used to allow digital signaling for the data.

The digital signal used for data transfer on these cables is encoded using Manchester or Differential Manchester coding . The digital signals need larger bandwidth. Hence the entire frequency spectrum of the cable is consumed. So it is not possible to transmit multiple channels using FDM.

The transmission of digital signals on the cable is bi-directional . The baseband co·axial cable was originally used for the Ethernet system that operates at 10 Mbps.

The maximum length of baseband coaxial cable between two repeaters is dependent on the data rates. Lower the data rate longer is the cable. The length has to be reduced with increased data rates so as to reduce the probability of errors getting introduced.

Broadband Coaxial Cable !!

This is the coaxial cable that is used in the cable TV system. It has higher bandwidth compared to the baseband cable. This cable has certain disadvantages such as it is more expensive, more difficult to install, and maintain as compared to the baseband co-axial cable.

Connector for Coaxial Cable

Coaxial cable is another important type of guided transmission media It has higher bandwidth as compared to that of twisted pair cable. The coaxial cable connectors are required for connecting a coaxial cable to a computer or any other device.

Below figure shows the various types of BNC connectors. The BNC connectors are available in three different types :


What Is Coaxial Cable That Is Used In The Physical Layer ?

1. BNC connector
2. BNC-T connector
3. BNC terminator

The BNC connector connects the end of the cable to a device such as a TV set. The BNC – T connector is used in Ethernet networks for branching out a cable for connection to a computer or other devices. The BNC terminator is used at the end of the cable as a termination. This avoids the reflection of signal.

See you in the next post with complete details about the twisted pair cables used in the physical layer . Stay tuned.

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Techie Aric

Aric is a tech enthusiast , who love to write about the tech related products and 'How To' blogs . IT Engineer by profession , right now working in the Automation field in a Software product company . The other hobbies includes singing , trekking and writing blogs .

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