Data Communication and Data Communication and Computer NetworksComputer Networks

Chapter 2–Transmission Media and Chapter 2–Transmission Media and Network Devices Network Devices

OverviewOverview

The term The term media is used because not all networks use media is used because not all networks use traditional cable. This term encompassestraditional cable. This term encompasses

copper-based (guided)copper-based (guided) fiber-optic cable (guided)fiber-optic cable (guided) wireless (unguided)wireless (unguided)

Choosing the correct network media is an important Choosing the correct network media is an important consideration because the media forms the foundation for the consideration because the media forms the foundation for the entire network. This can be based onentire network. This can be based on

InterferenceInterference SpeedSpeed Max LengthMax Length SecuritySecurity Installation and Repair easeInstallation and Repair ease

Media InterferenceMedia Interference

As a data signal travels through a specific media, it may be As a data signal travels through a specific media, it may be subjected to a type of interference known as subjected to a type of interference known as electromagnetic electromagnetic interference (EMI).interference (EMI).

Many different things cause EMI; common sources includeMany different things cause EMI; common sources include computer monitorscomputer monitors fluorescent lightingfluorescent lighting speakers,speakers, basically anything that creates an electromagnetic field.basically anything that creates an electromagnetic field.

If a network cable is too close to such devices, the signal within the If a network cable is too close to such devices, the signal within the cable can become corrupted.cable can become corrupted.

Some network media are more susceptible than others to the effects Some network media are more susceptible than others to the effects of EMI. Copper-based media are prone to EMI, whereas fiber-optic of EMI. Copper-based media are prone to EMI, whereas fiber-optic cable is completely resistant to it.cable is completely resistant to it.

CrosstalkCrosstalk

Data signals may also be subjected to something Data signals may also be subjected to something commonly referred to as commonly referred to as crosstalk, crosstalk, which occurs when which occurs when signals from two cables in close proximity to one another signals from two cables in close proximity to one another interfere with each other.interfere with each other.

As a result, the signals on both cables may become As a result, the signals on both cables may become corrupted.corrupted.

When you're troubleshooting intermittent network When you're troubleshooting intermittent network problems, it might be worth your time to confirm that problems, it might be worth your time to confirm that crosstalk or EMI is not at the root of your problems.crosstalk or EMI is not at the root of your problems.

Transmission SpeedTransmission Speed

One of the important media considerations is the One of the important media considerations is the supported data transmission rate or speed.supported data transmission rate or speed.

Transmission rates are normally measured by the Transmission rates are normally measured by the number of data bits that can traverse the media in a number of data bits that can traverse the media in a single second.single second.

In the early days of data communications this In the early days of data communications this measurement was expressed as bits per second (bps), measurement was expressed as bits per second (bps), but today's networks are measured in Mbps (megabits but today's networks are measured in Mbps (megabits per second) and Gbps (gigabits per second).per second) and Gbps (gigabits per second).

a network that accommodates huge amounts of data -> transmission rates are a a network that accommodates huge amounts of data -> transmission rates are a crucial consideration.crucial consideration.

in small offices where they occasionally share files and maybe a printer -> in small offices where they occasionally share files and maybe a printer -> transmission rate is not a big issue.transmission rate is not a big issue.

Media LengthMedia Length

Each media has a recommended maximum Each media has a recommended maximum length, and surpassing these recommendations length, and surpassing these recommendations can cause unusual network problems that are can cause unusual network problems that are often difficult to troubleshoot.often difficult to troubleshoot.

In some cases, the network simply will not work.In some cases, the network simply will not work. Media have maximum lengths because a signal Media have maximum lengths because a signal

weakens as it travels farther from its point of weakens as it travels farther from its point of origin. The weakening of data signals as they origin. The weakening of data signals as they traverse the media is referred to as traverse the media is referred to as attenuation.attenuation.

Copper-based media is susceptible to attenuation. Some Copper-based media is susceptible to attenuation. Some copper media, such as Shielded Twisted Pair (STP) use copper media, such as Shielded Twisted Pair (STP) use a special shielding inside the cable, which increases the a special shielding inside the cable, which increases the distance the signal travels.distance the signal travels.

Another strategy commonly employed to compensate for Another strategy commonly employed to compensate for attenuation is signal regeneration. The cable itself does attenuation is signal regeneration. The cable itself does not perform the regeneration process; rather, network not perform the regeneration process; rather, network devices such as devices such as switches switches or or repeaters handle signal repeaters handle signal regeneration. These regeneration. These devices strengthen the signal as it devices strengthen the signal as it passes, and in doing so, they increase the distance the passes, and in doing so, they increase the distance the signal can travel.signal can travel.

Fiber-optic cable does not suffer from attenuation. Fiber-optic cable does not suffer from attenuation. Instead, it suffers from a condition called "Instead, it suffers from a condition called "chromatic chromatic dispersion." Chromatic dispersion dispersion." Chromatic dispersion refers to the refers to the weakening of the light strength as it travels over weakening of the light strength as it travels over distance.distance.

Secure TransmissionSecure Transmission

Physical media provides a relatively secure Physical media provides a relatively secure transmission medium, because to gain access to transmission medium, because to gain access to the signal on the cable, a person must be able to the signal on the cable, a person must be able to physically access it, that is, he or she must be physically access it, that is, he or she must be able to tap into the cable.able to tap into the cable.

Fiber-optic cable is more secure than copper Fiber-optic cable is more secure than copper based media because the light transmissions based media because the light transmissions and glass or plastic construction make it and glass or plastic construction make it particularly hard to tap into. When it comes to particularly hard to tap into. When it comes to security, wireless media is the weakest.security, wireless media is the weakest.

Installation and RepairInstallation and Repair

Some network media are easier to manage and Some network media are easier to manage and install than others. This might seem like a minor install than others. This might seem like a minor consideration, but in real-world applications, it consideration, but in real-world applications, it can be important.can be important.

For example, fiber-optic cable is far more For example, fiber-optic cable is far more complex to install and troubleshoot than twisted complex to install and troubleshoot than twisted pair (copper cable). It's so complicated, in fact, pair (copper cable). It's so complicated, in fact, that special tools and training are often needed that special tools and training are often needed to install a fiber-optic based network.to install a fiber-optic based network.

Network MediaNetwork Media

Network media can be divided into two distinct Network media can be divided into two distinct categories: categories: cable and wireless, cable and wireless, sometimes sometimes referred to as referred to as guided guided ((bound) bound) andand unguided unguided (unbound) media.(unbound) media.

Cable media come in three common types:Cable media come in three common types: twisted-pair,twisted-pair, coaxial, andcoaxial, and fiber-optic.fiber-optic.

Wireless media have another range.Wireless media have another range.

Cable MediaCable Media

The most widely implemented mediaThe most widely implemented media There are two types of cable media: There are two types of cable media: copper copper

and glass/plastic.and glass/plastic. Copper-based cable is widely used to connect Copper-based cable is widely used to connect

LANs and wide area networks (WANs), and LANs and wide area networks (WANs), and optical cable, which uses glass or plastic, is optical cable, which uses glass or plastic, is mainly used for large-scale network mainly used for large-scale network implementations or over long distances.implementations or over long distances.

Copper MediaCopper Media

Copper is relatively inexpensive, easy to work Copper is relatively inexpensive, easy to work with, and well suited to the needs of the modern with, and well suited to the needs of the modern network. There are two types of copper cables: network. There are two types of copper cables: coax and twisted-pair cable.coax and twisted-pair cable.

Twisted pair is divided into Twisted pair is divided into unshielded twisted unshielded twisted pair (UTP) and shielded twisted pair (STP). pair (UTP) and shielded twisted pair (STP). UTP is by far the most common implementation UTP is by far the most common implementation of twisted-pair cable, and it is used for both of twisted-pair cable, and it is used for both telephone systems and computer networks.telephone systems and computer networks.

UTPUTP

Insulated copper wires arranged in regular spiral pattern.Insulated copper wires arranged in regular spiral pattern. The oldest, least expensive, and most commonly used mediaThe oldest, least expensive, and most commonly used media reduce susceptibility to interference than straight pair wiresreduce susceptibility to interference than straight pair wires Highly susceptible to electrical noise, interference, and Highly susceptible to electrical noise, interference, and

‘tapping’ of the signal as compared to the other guided media‘tapping’ of the signal as compared to the other guided media Arrangement of twisted pairs into group used for high speed Arrangement of twisted pairs into group used for high speed

(10-100 Mbps) LAN(10-100 Mbps) LAN made up of up to four twisted pairs enclosed in a plastic made up of up to four twisted pairs enclosed in a plastic

jacketjacket

STPSTP

STP, as its name implies, adds extra shielding within the STP, as its name implies, adds extra shielding within the casing, so it copes with interferencecasing, so it copes with interference

and attenuation better than regular UTP.and attenuation better than regular UTP. Shielded twisted-pair (STP) resembles UTP except that Shielded twisted-pair (STP) resembles UTP except that

it includes a foil shield that covers the wires and adds it includes a foil shield that covers the wires and adds another layer of protection against outside magnetic another layer of protection against outside magnetic interference.interference.

Because of this shielding, cable distances for STP can Because of this shielding, cable distances for STP can be greater than for UTP; but, unfortunately, the be greater than for UTP; but, unfortunately, the additional shielding also makes STP considerably more additional shielding also makes STP considerably more costly than regular UTP.costly than regular UTP.

What's with the Twist?What's with the Twist?

To reduce interference and attenuation, it was To reduce interference and attenuation, it was discovered that twisting the wires within a cable discovered that twisting the wires within a cable resulted in greater signal integrity than running the resulted in greater signal integrity than running the wires parallel to one another.wires parallel to one another.

UTP cable is particularly susceptible to crosstalk, UTP cable is particularly susceptible to crosstalk, and increasing the number of twists per foot in the and increasing the number of twists per foot in the wire achieves greater resistance against wire achieves greater resistance against interference.interference.

The technique of twisting wires together is not The technique of twisting wires together is not limited to network cable; some internal and external limited to network cable; some internal and external SCSI cables employ a similar strategy.SCSI cables employ a similar strategy.

Categories of twisted-pair Categories of twisted-pair cablecable

Category 1 - telephone cable.Category 1 - telephone cable. susceptible to interference and attenuation and low bandwidth susceptible to interference and attenuation and low bandwidth

capability,capability, not practical for network applications.not practical for network applications.

Category 2 - capable of transmitting data up to 4Mbps.Category 2 - capable of transmitting data up to 4Mbps. Still too slow for networks.Still too slow for networks.

Category 3 - capable of transmitting data up to 10Mbps. Category 3 - capable of transmitting data up to 10Mbps. A few years ago, Category 3 was the cable of choice for A few years ago, Category 3 was the cable of choice for twisted-pair networks.twisted-pair networks.

Category 4 - has potential data throughput of 16Mbps.Category 4 - has potential data throughput of 16Mbps. often implemented in the IBM Token Ring networks.often implemented in the IBM Token Ring networks.

Category 5 - capable of transmitting data atCategory 5 - capable of transmitting data at100Mbps.100Mbps.

the cable of choice on twisted-pair networks and is associated the cable of choice on twisted-pair networks and is associated with Fast Ethernet technologies.with Fast Ethernet technologies.

Category 5e - used on networks that run at up to Category 5e - used on networks that run at up to 1000Mbps.1000Mbps.

can be used up to 350 meters, depending on the can be used up to 350 meters, depending on the implementation.implementation.

Category 6 - High performance UTP cable capable of Category 6 - High performance UTP cable capable of transmitting data at over 1000Mbps.transmitting data at over 1000Mbps.

is rated up to 550 meters depending on the implementation.is rated up to 550 meters depending on the implementation. marginally more expensive than Category 5e cable.marginally more expensive than Category 5e cable.

Coaxial CableCoaxial Cable

Coaxial cable resembles standard TV cable and is Coaxial cable resembles standard TV cable and is constructed using an outside insulation cover, braided metal constructed using an outside insulation cover, braided metal shielding, and a copper wire at the center. The shielding shielding, and a copper wire at the center. The shielding and insulation help combat attenuation, crosstalk, and EMI.and insulation help combat attenuation, crosstalk, and EMI.

Coaxial cable is rarely used anymore for network Coaxial cable is rarely used anymore for network backbones or to connect computers, but it is being used backbones or to connect computers, but it is being used today to connect cable modems to the cable provider’s today to connect cable modems to the cable provider’s connection to provide a computer with a broadband Internet connection to provide a computer with a broadband Internet connection.connection.

Two types of coax are used in networking: Two types of coax are used in networking: thin coax thin coax and and thick coax. Neither is particularly popular anymore, thick coax. Neither is particularly popular anymore, but but you are most likely to encounter thin coax.you are most likely to encounter thin coax.

Fiber-Optic CableFiber-Optic Cable

Unlike standard networking cables, which use electric signals to send Unlike standard networking cables, which use electric signals to send data transmissions, fiber uses light.data transmissions, fiber uses light.

As a result, fiber-optic transmissions are not susceptible to EMI or As a result, fiber-optic transmissions are not susceptible to EMI or crosstalk, giving fiber cable an obvious advantage over copper-based crosstalk, giving fiber cable an obvious advantage over copper-based media.media.

In addition, fiber-optic cable is highly resistant to the signal In addition, fiber-optic cable is highly resistant to the signal weakening, referred to as chromatic dispersion.weakening, referred to as chromatic dispersion.

Further advantages of fiber cable include the facts that it's small in Further advantages of fiber cable include the facts that it's small in diameter, it's lightweight, and it offers significantly faster transmission diameter, it's lightweight, and it offers significantly faster transmission speeds than other cable media.speeds than other cable media.

So, why aren't all networks using fiber cable?So, why aren't all networks using fiber cable? The drawback of fiber is that it can be more complex to install than The drawback of fiber is that it can be more complex to install than

UTP. Creating custom lengths of fiber-optic cable requires trained UTP. Creating custom lengths of fiber-optic cable requires trained professionals and specialized tools.professionals and specialized tools.

A fiber-optic cableconsists of severalcomponents, includingthe optic core at thecenter, an opticcladding, insulation,and an outer jacket.The optic core isresponsible forcarrying the light signaland is commonlyconstructed of plasticor glass.

Single-mode and MultimodeSingle-mode and Multimode

Two types of optical fiber are commonly available: singlemode and Two types of optical fiber are commonly available: singlemode and multimode.multimode.

Multimode fiber (MMF) has a larger core than single-mode. This larger Multimode fiber (MMF) has a larger core than single-mode. This larger core allows hundreds of light rays to flow through the fiber core allows hundreds of light rays to flow through the fiber simultaneously.simultaneously.

Single-mode fiber (SMF), on the other hand, has a small core that Single-mode fiber (SMF), on the other hand, has a small core that allows only a single light beam to pass. The light transmissions in single-allows only a single light beam to pass. The light transmissions in single-mode fiber pass through the core in a direct line, like a flashlight beam.mode fiber pass through the core in a direct line, like a flashlight beam.

The numerous light beams in multimode fiber bounce around inside the The numerous light beams in multimode fiber bounce around inside the core, inching toward their destination. Because light beams bounce core, inching toward their destination. Because light beams bounce within the core, the light beams slow down, reduce in strength, and take within the core, the light beams slow down, reduce in strength, and take some time to travel along the cable.some time to travel along the cable.

For this reason, single-mode fiber's speed and distance are superior to For this reason, single-mode fiber's speed and distance are superior to those of multimode.those of multimode.

Single-mode fiber is used for long runs because it can Single-mode fiber is used for long runs because it can transmit data 50 times further than multimode fiber and transmit data 50 times further than multimode fiber and at a faster rate. For example, single-mode fiber might be at a faster rate. For example, single-mode fiber might be used on an organization’s corporate campus between used on an organization’s corporate campus between buildings.buildings.

Multimode fiber provides high bandwidth at high speeds Multimode fiber provides high bandwidth at high speeds over medium distances (up to about 3000 feet) but can over medium distances (up to about 3000 feet) but can be inconsistent for very long runs.be inconsistent for very long runs.

Electromagnetic SpectrumElectromagnetic Spectrum

Wireless Transmission Wireless Transmission

Wireless methods do not use electrical (cables) or Wireless methods do not use electrical (cables) or optical (fiber optics) conductors. It uses the earth’s optical (fiber optics) conductors. It uses the earth’s electromagnetic frequency spectrum.electromagnetic frequency spectrum.

The different type of wireless communications systems The different type of wireless communications systems includeinclude

Microwave link: directionalMicrowave link: directional Radio link: omnidirectionalRadio link: omnidirectional InfraredInfrared BluetoothBluetooth

Satellite Point to Point LinkSatellite Point to Point Link