Chapter Four Transmission Basics and Networking Media

Objectives

• Explain data transmission concepts including full-duplexing, attenuation, and noise

• Describe the physical characteristics of coaxial cable, STP, UTP, and fiber-optic media

• Explain the benefits and limitations of different networking media

• Identify the best practices for cabling buildings and work areas

• Describe the methods of transmitting data through the atmosphere

Transmission Basics

Transmission has two meanings:

Refers to process of issuing data signals on a medium

Refers to progress of data signals over a medium

On a data network, information can be transmitted via one of two methods:

Analog

Digital

Both analog and digital signals are generated by electrical current, pressure of which is measured in volts

In analog signals, voltage varies continuously

In digital signals, voltage turns off and on repeatedly

Amplitude

Measure of a signal’s strength

Frequency

Number of times a signal’s amplitude changes over a period of time

Expressed in hertz (Hz)

Wavelength

Distances between corresponding points on a wave’s cycle

Phase

Refers to progress of a wave over time in relationship to a fixed point

Binary system encodes using 1s and 0s

Bits can only have a value of either 1 or 0

Eight bits together form a byte

Noise or any interference that may degrade signals affects digital signals less than analog signals

Data Modulation

Modem

Name reflects function as modulator/demodulator

Modulation

Technique for formatting signals

Frequency modulation (FM)

Method of data modulation in which frequency of carrier signal is modified by application of a data signal

Amplitude modulation (AM)

Modulation technique in which amplitude of carrier signal is modified by application of a data signal

Transmission Direction

Simplex

Signals travel in only one direction

Half-duplex

Signals may travel in both directions over a medium but in only one direction at a time

Full-duplex

Signals are free to travel in both directions over a medium simultaneously

Also referred to just as duplex

Channel

Distinct communication path between two or more nodes

Multiplexing

Allows multiple signals to travel simultaneously over one medium

To accommodate multiple signals, single medium is logically separated into subchannels

For each type of multiplexing:

Multiplexer (mux) is required at sending end of channel

Demultiplexer (demux) separates the combined signals and regenerates them in original form

Time division multiplexing (TDM)

Divides channel into multiple intervals of time

Statistical multiplexing

Similar to TDM

Assigns slots to nodes according to priority and need instead of in succession

Wavelength division multiplexing (WDM)

Used only with fiber-optic cable

Data is transmitted as pulses of light

Fiber-optic modem (FOM) is a demultiplexer used on fiber networks that employ WDM

Relationships Between Nodes

Point-to-point

Transmission involving one transmitter and one receiver

Broadcast

Transmission involving one transmitter and multiple receivers

Webcasting

Broadcast transmission used over the Web

Throughput and Bandwidth

Throughput is amount of data the medium can transmit during a given period of time

Also called capacity

Bandwidth measures difference between highest and lowest frequencies a media can transmit

Range of frequencies is directly related to throughput

Throughput Baseband and Broadband

Baseband

Transmission form in which (typically) digital signals are sent through direct current (DC) pulses applied to the wire

Broadband

Transmission form in which signals are modulated as radiofrequency (RF) pulses that use different frequency ranges

Transmission Flaws

Electromagnetic interference (EMI)

Interference that may be caused by motors, power lines, television, copiers, fluorescent lights, or other sources of electrical activity

Radiofrequency interference (RFI)

Interference that may be generated by motors, power lines, televisions, copiers, fluorescent lights, or broadcast signals from radio or TV towers

Attenuation

Loss of signal strength as transmission travels away from source

Analog signals pass through an amplifier, which increases not only voltage of a signal but also noise accumulated

Regeneration

Process of retransmitting a digital signal

Repeater

Device used to regenerate a signal

Media Characteristics

Throughput

Perhaps most significant factor in choosing a transmission medium is throughput

Cost

Cost of installation

Cost of new infrastructure versus reusing existing infrastructure

Cost of maintenance and support

Cost of a lower transmission rate affecting productivity

Cost of obsolescence

Media Characteristics

Size and scalability

Specifications determining size and scalability:

Maximum nodes per segment

Maximum segment length

Maximum network length

Latency is the delay between the transmission of a signal and its receipt

Connectors

Connects wire to network device

Noise immunity

Thicker cables are generally less susceptible to noise

Possible to use antinoise algorithms to protect data from being corrupted by noise

Conduits can protect cabling from noise

Coaxial Cable

Consists of central copper core surrounded by an insulator, braiding, and outer cover called a sheath

Coaxial Cable Thicknet (10Base5)

Also called thickwire Ethernet

Rigid coaxial cable used on original Ethernet networks

IEEE designates Thicknet as 10Base5 Ethernet

Almost never used on new networks but you may find it on older networks

Used to connect one data closet to another as part of network backbone

Thicknet Characteristics

Throughput

According to IEEE 802.3, Thicknet transmits data at maximum rate of 10 Mbps

Cost

Less expensive than fiber-optic but more expensive than some other types of coaxial cable

Connector

Can include a few different types of connectors, which are very different from those used on modern networks

In Thicknet networking, the transceiver is a separate device and may also be called a media access unit (MAU)

Attachment Unit Interface (AUI)

Ethernet standard establishing physical specifications for connecting coaxial cables with transceivers and networked nodes

An AUI connector may also be called a DIX or DB-15 connector

N-series connector (or n connector)

Screw-and-barrel arrangement securely connects coaxial cable segments and devices

Noise immunity

Because of its wide diameter and excellent shielding, has the highest resistance to noise of any commonly used types of network wiring

Size and scalability

Because of its high noise resistance, it allows data to travel longer than other types of cabling

Thinnet (10Base2)

Also known as thin Ethernet

Characteristics:

Throughput

Can transmit at maximum rate of 10 Mbps

Cost

Less expensive than Thicknet and fiber-optic cable

More expensive than twisted-pair wiring

Connectors

Connects wire to network devices with BNC T-connectors

A seen in Figure 4-19, BNC barrel connectors are used to join two Thinnet cable segments together

Size and scalability

Allows a maximum of 185 m per network segment (see Figure 4-20)

Noise immunity

More resistant than twisted-pair wiring

Less resistant than twisted-pair wiring

Signal bounce

Caused by improper termination on a bus network

Travels endlessly between two ends of network

Prevents new signals from getting through

Twisted-Pair (TP) Cable

Color-coded pairs of insulated copper wires twisted around each other and encased in plastic coating

Twists in wire help reduce effects of crosstalk

Number of twists per meter or foot known as twist ratio

Alien Crosstalk

When signals from adjacent cables interfere with another cable’s transmission

Shielded Twisted-Pair (STP)

STP cable consists of twisted wire pairs that are individually insulated and surrounded by shielding made of metallic substance

Unshielded Twisted-Pair

Consists of one or more insulated wire pairs encased in a plastic sheath

Does not contain additional shielding

To manage network cabling, it is necessary to be familiar with standards used on modern networks, particularly Category 3 (CAT3) and Category 5 (CAT5)

10BaseT

Popular Ethernet networking standard that replaced 10Base2 and 10Base5 technologies

Enterprise-wide network

Spans entire organization

Often services needs of many diverse users

100BaseT

Enables LANs to run at 100-Mbps data transfer rate

Also known as Fast Ethernet

Two 100BaseT specifications have competed for popularity as organizations move to 100-Mbps technology:

100BaseTX

100BaseT4

100BaseVG

Cousin of Ethernet 100 Mbps technologies

VG stands for voice grade

Also called 100VG-AnyLAN

Originally developed by Hewlett-Packard and AT&T

Now governed by IEEE standard 802.12

Comparing STP and UTP

Throughput

Both can transmit up to 100 Mbps

Cost

Typically, STP is more expensive

Connector

Both use RJ-45 connectors (see Figure 4-27) and data jacks

Noise immunity

STP is more noise-resistant

Size and scalability

Maximum segment length for both is 100 meters

Fiber-Optic Cable

Contains one or several glass fibers at its core

Surrounding the fibers is a layer of glass called cladding

Single-mode fiber

Carries light pulses along single path

Multimode fiber

Many pulses of light generated by LED travel at different angles

Throughput

Reliable in transmitting up to 1 gigabit per second

Cost

Most expensive type of cable

Connector

You can use any of 10 different types of connectors (see Figure 4-30)

Noise immunity

Unaffected by either EMI or RFI

Size and scalability

Network segments made from fiber can span 100 meters

Signals transmitted over fiber can experience optical loss

Two popular connectors used with fiber-optic cable:

ST connectors

SC connectors

10BaseF and 100BaseFX

10BaseF

Physical layer standard for networks specifying baseband transmission, multimode fiber cabling, and 10-Mbps throughput

100BaseFX

Physical layer standard for networks specifying baseband transmission, multimode fiber cabling, and 100-Mbps throughput

Physical Layer Networking Standards

Cable Design and Management

Cable plant

Hardware comprising enterprise-wide cabling system

Structured cabling

Method for uniform, enterprise-wide, multivendor cabling systems

Entrance facilities

Backbone wiring

Backbone cabling that provides vertical connections between floors of a building are called risers

Equipment room

Telecommunications closet

Punch-down block is a panel of data receptors

Patch panel is a wall-mounted panel of data receptors

Horizontal wiring

Work area

Patch cable is a relatively short section of twisted-pair cabling with connectors on both ends that connect network devices to data outlets

Installing Cable

Straight-through cable

Terminations at both ends are identical

Crossover cable

Terminations locations of transmit and receiver wires on one end of cable are reversed

Do not untwist twisted-pair cables more than one-half inch before inserting them

Do not strip off more than one inch of insulation from copper wire in twisted-pair cables

Watch bend radius limitations for cable being installed

Test each segment of cabling with cable tester

Use only cable ties to cinch groups of cable together

Avoid laying cable across floor where it may sustain damage

Install cable at least three feet away from fluorescent lights or other sources of EMI

Always leave slack in cable runs

If running cable in plenum, area above ceiling tile or below subflooring, make sure cable sheath is plenum-rated

Pay attention to grounding requirements

Atmospheric Transmission Media

Infrared transmission

Infrared networks use infrared light signals to transmit data through space

Direct infrared transmission depends on transmitter and receiver remaining within line of sight

In indirect infrared transmission, signals can bounce off of walls, ceilings, and any other objects in their path

RF transmission

Radio frequency (RF) transmission relies on signals broadcast over specific frequencies

Two most common RF technologies:

Narrowband

Spread spectrum

Choosing the Right Transmission Media

Areas of high EMI or RFI

Corners and small spaces

Distance

Security

Existing infrastructure

Growth

Chapter Summary

• Information can be transmitted via analog or digital methodology

• Throughput is the amount of data a medium can transmit during a given period of time

• Noise is interference that distorts an analog or digital signal

• Costs depend on many factors

• There are three specifications that dictate size and scalability of networking media

• Connectors connect wire to the network device

• Coaxial cable consists of central copper core surrounded by an insulator and a sheath

• Thicknet cabling is a rigid coaxial cable used for original Ethernet networks

• Both Thicknet and Thinnet coaxial cable rely on bus topology and must be terminated at both ends with a resistor

• Twisted-pair cable consists of color-coded pairs of insulated copper wires, twisted around each other and encased in plastic coating

• STP cable consists of twisted pair wires individually insulated and surrounded by a shielding made of metallic substance

• UTP cabling consists of one or more insulated wire pairs encased in a plastic sheath

• Fiber-optic cable contains one or several glass fibers in its core

• On today’s networks, fiber is used primarily as backbone cable

• Best practice for installing cable is to follow the TIA/EIA 568 specifications and manufacturer’s recommendations

• Wireless LANs can use radio frequency (RF) or infrared transmission

• RF transmission can be narrowband or spread spectrum

• Infrared transmission can be indirect or direct

• To make correct media transmission choices, consider throughput, cabling, noise resistance, security/flexibility, and plans for growth