2.3 COMMUNICATION MEDIA

 Communication media used for data transfer are basically divided into following two categories:

1) Guided media, and

2) Unguided media

Guided media are those that provide a physical connection between two devices. A signal traveling through guided media is directed and contained within the physical limits of the medium. Unguided media is used for transmitting the signal without any physical media. It will be discussed in section 2.4.

Guided media includes the following:

1) Twisted pair cable

2) Coaxial cable

3) Optic Fiber cable

We will now discuss these cables in detail. Let us take the twisted pair cable first.

2.3.1 Twisted Pair Cable

Twisted pair cable is still the most common transmission media. A twisted pair cable consists of two conductors which are normally made of copper. Each conductor has its own plastic insulation typically 1 mm thick. These cables are twisted together. The wires are twisted in a helical form, similar to a DNA molecule. Twisting is done to reduce crosstalk.

Twisted pairs (Figure 5) are superior because of reduced crosstalk. They are very effective for relatively short distances (a few hundred feet), but can be used for up to a few kilometers. A twisted pair has a bandwidth to distance ratio of about 1 MHz per kilometer. The performance of the twisted pair can be substantially improved by adding a metallic shield around the wires. Shielded wires are much more resistant to thermal noise and crosstalk effects. Twisted pairs used for long distance connections (e.g., telephone lines) are usually organized as a much larger cable containing numerous twisted pairs.

Twisting pair cabling comes in several varieties, two of which are very important: Category 3 and Category 5. Category 5 has more twist per centimeter resulting in less crosstalks and a better quality signal.

2.3.2 Coaxial Cable

It has two major advantages compared to twisted pairs: 
(i) Better Shielding 
(ii) Signal
can travel longer distances at higher speeds.

Coaxial cable has a central core conductor of a solid or stranded wire enclosed in insulating sheath. This sheath is encased in an outer conductor of metal braid and the whole is protected by a plastic cover.

The outer metallic wrapping serves two functions:
a) Acts as a shield against noise.
b) Acts as second conductor to complete the circuit.
 
Coaxial cables can carry signals of higher frequency as compared to twisted pair cable. A coaxial cable usually consists of four concentric cylinders: an inner conductor, surrounded by an insulating cylinder, surrounded by an outer conductor, surrounded by a final protective cover. This combination is called a coax as shown in Figure 6. Coaxial cables are superior to twisted pairs both in terms of bandwidth and communication distance, and can provide bandwidth to distance ratios in order of 10s of MHz per kilometer. Like twisted pairs, multiple coaxes are usually housed within one cable, which may also contain twisted pairs. Coaxial cables are extensively used in LANs and long distance telephone trunk lines. Coaxial cables are used in cable TV networks and Ethernet LANs due to higher bandwidth and data rate.

2.3.3 Optical Cable

An optical fiber consists of two concentric cylinders: an inner core surrounded by a cladding. Both the core and the cladding are made of transparent plastic or glass material as shown in Figure 7, which transmits signals in the form of light. Optical fiber use reflections to guide light through a channel. The density of core and cladding must differ sufficiently to reflect the beam of light instead of refracting.

The core is used for guiding a light beam, whereas the cladding (which has a different refractive index) acts as a reflector to prevent the light from escaping from the core. Because optical fiber uses a light signal instead of electrons, it does not suffer from the various noise problems associated with electromagnetic signals. The signal is usually generated by a laser or Light Emitting Diode (LED). Optical fibers can provide bandwidth to distance ratios in order of 100s of MHz per kilometer. Like
other cables, hundreds of optical fibers are usually housed within one cable. 

They are being increasingly used by telecommunication carriers for long distance digital trunk lines. Current trends promise that they will replace twisted pair residential loops in the near future. It is used in backbone networks. Cable TV companies use a combination of optical fiber and coaxial cable.


Advantages

1) Higher band width – It can support higher band width and hence can transfer data at a higher rate.

2) Less signal attenuation – Its transmission distance is greater than the other two and it can run for 50 KMs without regeneration.

3) Immunity to electromagnetic interference

4) These cables are much lighter than the copper cables

5) These cables are more immune to tapping then the copper cables.

Disadvantages

1) Installation or maintenance – It’s a new technology and hence needs expertise which is not available everywhere.

2) Unidirectional – Propagation of light is unidirectional and we need two fibers for bidirectional communication.

3) Costly – the cables and interfaces used are relatively expensive. 


Comments

Post a Comment

Popular posts from this blog

3.8 SECURE NETWORK DEVICES

 In this unit, we have already learnt that the firewall is only one entry point to your network. Modems, if you allow them to answer incoming calls, can provide an easy means for an attacker to sneak around, your front door (or, firewall). Just as castlesweren't built with moats only in the front, your network needs to be protected at all of its entry points. Secure Modems, DialBack Systems If modem access is to be provided, this should be guarded carefully. The terminal server, or network device that provides dialup access to your network needs to be actively administered, and its logs need to be examined for strange behavior. Its passwords need to be strong not ones that can be guessed. Accounts that aren't actively used should be disabled. In short, it's the easiest way to get into your network from remote: guard it carefully. There are some remote access systems which have the feature of a twopart procedure to establish a connection. The first part is the remote user di...
Read more

3.5 SECURITY ISSUES FOR SMALL AND MEDIUM SIZED BUSINESSES

 Small and medium sized businesses use the Internet and networked applications to reach new customers and serve their existing ones more effectively. At the same time, new security threats and legislation puts increased pressure on business networks to be reliable and secure. Business Challenges According to recent studies, security is the biggest challenge facing small and mediumsized businesses. Everchanging security threats from both inside and outside the business network can wreak havoc on business operations, affecting profitability and customer satisfaction. Small and mediumsized businesses must also comply with new regulations and laws created to protect consumer privacy and secure electronic information. Security issues for small and medium – sized businesses are classified into 5 basic categories: Worms and Viruses As per research, Computer worms and viruses remain the most common security threat, with 75 percent of small and medium businesses affected by it.. Worms and v...
Read more

3.6 TOOLS FOR NETWORK SECURITY

There are many strong tools available for securing a computer network. Tools to protect your enterprise network have been evolving for the last two decades, roughly the same amount of time that people have been trying to break into computer networks. These tools can protect a computer network at many levels, and a wellguarded enterprise deploys many different types of security technologies. The most obvious element of security is often times the most easily overlooked: physical security—namely, controlling access to the most sensitive components in your computer network, such as a network administration station or the server room. No amount of planning or expensive equipment will keep your network secure if unauthorized personnel can have access to central administration consoles. Even if a user does not have evil intent, an untrained user may unknowingly provide unauthorized outside access or override certain protective configurations. Let us briefly discuss few of these network secur...
Read more