CCNA 1 and 2 Companion Guide, Revised (Cisco Networking Academy Program) part 10 pps

A data network is much like the highway system, with data packets analogous to automobiles, and bandwidth analogous to the number of lanes on the highway.. Digital Bandwidth Measurements

representing massive amounts of information flowing back and forth across the globe in seconds or less In a sense, it might be appropriate to say that the Inter-net is bandwidth

technol-ogies and infrastructures are built to provide greater bandwidth, new applications are created to take advantage of the greater capacity The delivery over the network

of rich media content, including streaming video and audio, requires tremendous amounts of bandwidth IP telephony systems are now commonly installed in place of traditional voice systems, adding further to the need for bandwidth The successful networking professional must anticipate the need for increased band-width and plan accordingly

Analogies That Describe Digital Bandwidth

The idea that information flows suggests two analogies that might make it easier to

visualize bandwidth in a network Because both water and traffic are said to flow,

consider the following:

pipes brings fresh water to homes and businesses and carries wastewater away

This water network is made up of pipes with different diameters A city’s main water pipe might be 2 meters in diameter, whereas a kitchen faucet might have

a diameter of only 2 centimeters The width of the pipe determines the pipe’s water-carrying capacity Thus, the water is analogous to data, and pipe width is analogous to bandwidth Many networking experts say they need to “put in big-ger pipes” when they want to add more information-carrying capacity

A network of roads serves every city or town Large highways with many traffic lanes are joined by smaller roads with fewer traffic lanes These roads lead to even smaller, narrower roads, and eventually to the driveways of homes and businesses When very few automobiles use the highway system, each vehicle can move freely When more traffic is added, each vehicle moves more slowly, espe-cially on roads with fewer lanes for the cars to occupy Eventually, as even more traffic enters the highway system, even multilane highways become congested and slow A data network is much like the highway system, with data packets analogous to automobiles, and bandwidth analogous to the number of lanes on the highway When a data network is viewed as a system of highways, it is easy

to see how low-bandwidth connections can cause traffic to become congested all over the network

Figure 2-13 Pipe Analogy for Bandwidth

Figure 2-14 Highway Analogy for Bandwidth

Network devices are like pumps, valves, fittings and taps

Packets are like water Bandwidth is like pipe width

1 Lane Unpaved Road 2 Lane Road

2 Lane Divided Highway

8 Lane Superhighway

Networking Devices Are Like On Ramps, Traffic Signals, Signs, Maps, and Police

STOP

Keep in mind that the true, actual meaning of bandwidth, in this context, is the

maxi-mum number of bits that theoretically can pass through a given area of space in a

spec-ified amount of time (under the given conditions) These analogies are only to make it

easier to understand the concept of bandwidth

Digital Bandwidth Measurements

In digital systems, the basic unit of bandwidth is bits per second (bps) Bandwidth is

the measure of how much information, or bits, can flow from one place to another in a

given amount of time, or seconds Although bandwidth can be described in bits per

second, usually some multiple of bits per second is used In other words, network

bandwidth is typically described as thousands of bits per second, millions of bits per

second, and even billions of bits per second

Although the terms bandwidth and speed are often used interchangeably, they are not

exactly the same thing You might say, for example, that a T3 connection at 45

mega-bits per second (Mbps) operates at a higher speed than a T1 connection at 1.544 Mbps

However, if only a small amount of their data-carrying capacity is being used, each of

these connection types carries data at roughly the same speed, just as a small amount

of water flows at the same rate through a small pipe as through a large pipe Therefore,

it is usually more accurate to say that a T3 connection has greater bandwidth than a

T1, because it can carry more information in the same period of time, not because it

has a higher speed

Table 2-2 summarizes the various units of bandwidth

Bandwidth Limitations

Bandwidth varies depending on the type of medium as well as the LAN and WAN

technologies used The physics of the medium account for some of the difference

Physi-cal differences in the ways signals travel through twisted-pair copper wire, coaxial cable,

optical fiber, and even air result in fundamental limitations on the information-carrying

Table 2-2 Units of Bandwidth

Bits per second bps 1 bps = fundamental unit of bandwidth

Kilobits per second kbps 1 kbps = 1000 bps = 103 bps

Megabits per second Mbps 1 Mbps = 1,000,000 bps = 106 bps

Gigabits per second Gbps 1 Gbps = 1,000,000,000 bps = 109 bps

capacity of a given medium However, a network’s actual bandwidth is determined by

a combination of the physical medium and the technologies chosen for signaling and detecting network signals

For example, current understanding of the physics of unshielded twisted-pair (UTP) copper cable puts the theoretical bandwidth limit at more than 1 Gbps But in actual practice, the bandwidth is determined by the use of a particular technology, such as 10BASE-T, 100BASE-TX, or 1000BASE-TX Ethernet Bandwidth is also determined

by other varying factors, such as the number of users in the network, the equipment being used, applications, the amount of broadcast, and so on In other words, the actual bandwidth is determined not by the medium’s limitations, but by the signaling methods, NICs, and other items of network equipment that are chosen

Table 2-3 lists some common networking media types, along with their limits on dis-tance and bandwidth

Table 2-3 Maximum Bandwidths and Length Limitations

Medium

Maximum Theoretical Bandwidth

Maximum Physical Distance

50-ohm coaxial cable (10BASE2 Ethernet, Thinnet)

50-ohm coaxial cable (10BASE5 Ethernet, Thicknet)

Category 5 UTP (10BASE-T Ethernet)

Category 5 UTP (100BASE-TX Ethernet)

Category 5 UTP (1000BASE-TX Ethernet)

Multimode optical fiber (62.5/125 µm) (100BASE-FX Ethernet)

Multimode optical fiber (62.5/125 µm) (1000BASE-SX Ethernet)

Table 2-4 summarizes common WAN services and the bandwidth associated with each.

Data Throughput

Bandwidth is the measure of the amount of information that can move through the

network in a given period of time Therefore, the amount of available bandwidth is a

Multimode optical fiber (50/125 µm)

(1000BASE-SX Ethernet)

Single-mode optical fiber (9/125 µm)

(1000BASE-LX Ethernet)

Table 2-4 WAN Services and Bandwidths

and small businesses

12 kbps to 6.1 Mbps = 0.128 Mbps to 6.1 Mbps

businesses

128 kbps = 0.128 Mbps

Frame Relay Small institutions (schools)

and medium-sized businesses

56 kbps to 44.736 Mbps (U.S.)

or 34.368 Mbps (Europe) = 0.056 Mbps to 44.736 Mbps (U.S.) or 34.368 Mbps (Europe)

STS-1 (OC-1) Phone companies,

data-comm company backbones

51.840 Mbps

STS-3 (OC-3) Phone companies,

data-comm company backbones

155.251 Mbps

STS-48 (OC-48) Phone companies,

data-comm company backbones

2.488 Gbps

Table 2-3 Maximum Bandwidths and Length Limitations (Continued)

Medium

Maximum Theoretical Bandwidth

Maximum Physical Distance

critical part of the network’s specification A typical LAN might be built to provide

100 Mbps to every desktop workstation, but this does not mean that each user can actually move one hundred megabits of data through the network for every second of use This is true only under the most ideal circumstances The concept of throughput can help explain why this is so

Throughput refers to actual measured bandwidth at a specific time of day, using spe-cific Internet routes, and while a spespe-cific set of data is transmitted on the network Unfortunately, for many reasons, throughput is often far less than the maximum possi-ble digital bandwidth of the medium that is being used The following are some of the factors that determine throughput:

■ Internetworking devices

■ Type of data being transferred

■ Network topology

■ Number of users on the network

■ User’s computer

■ Server computer

■ Power conditions

■ Congestion

A network’s theoretical bandwidth is an important consideration in network design, because network bandwidth is never greater than the limits imposed by the chosen medium and networking technologies Figure 2-15 lists some of the variables that affect throughput However, it is just as important for a network designer and admin-istrator to consider the factors that might affect actual throughput By measuring throughput on a regular basis, a network administrator will be aware of changes in network performance and changes in the needs of network users The network can then be adjusted accordingly

Data Transfer Calculation

Network designers and administrators are often called on to make decisions regarding bandwidth Should the size of the WAN connection be increased to accommodate a new database? Is the current LAN backbone of sufficient bandwidth for a streaming-video training program? The answers to questions like these are not always easy to find, but one place to start is with a simple data transfer calculation

Using the formula T = S / BW (transfer time = size of file / bandwidth) lets a network administrator estimate several of the important components of network performance

If the typical file size for a given application is known, dividing the file size by the net-work bandwidth yields an estimate of the fastest time that the file can be transferred

Figure 2-15 Throughput Variables

Two important points should be considered when doing this calculation:

■ The result is an estimate only, because the file size does not include any overhead

added by the process that takes place to prepare data to be transferred over the network This process is called encapsulation Encapsulation is covered in more detail in later chapters

■ The result is likely to be a best-case transfer time, because available bandwidth

is almost never at the theoretical maximum for the network type (A more accu-rate estimate can be attained if throughput is substituted for bandwidth in the equation.)

Although the data transfer calculation is quite simple, it can be tricky if you are not

careful to use the same units throughout the equation In other words, if the

band-width is measured in Mbps, the file size must be in megabits (Mb), not megabytes

(MB) Because file sizes are typically given in megabytes, you might need to multiply

the number of megabytes by 8 to convert to megabits

Try to answer the following question using the formula T = S / BW (Be sure to convert

units of measurement as necessary.)

Would it take less time to send the contents of a floppy disk full of data (1.44 MB) over an ISDN line or to send the contents of a 10-GB hard drive full of data over

an OC-48 line?

Figure 2-16 summarizes a simple formula for file transfer time calculations

Your PC (Client)

The Server

Other Users on Your LAN

Routing Within the "Cloud"

The Design (Topology) of All Networks Involved

Type of Data Being Transferred

Time of Day

Why?

Throughput <= Digital Bandwidth of a Medium

Figure 2-16 File Transfer Time Calculation

Digital Bandwidth Versus Analog Bandwidth

Until recently, radio, television, and telephone transmissions were sent through the air

and over wires using electromagnetic waves These waves are called analog because

they have the same shapes as the light and sound waves produced by the transmitters

As light and sound waves change size and shape, the electrical signal that carries the transmission changes proportionately In other words, the electromagnetic waves are

analogous to the light and sound waves.

Analog bandwidth is measured by how much of the electromagnetic spectrum is

occu-pied by each signal The basic unit of analog bandwidth is hertz (Hz), or cycles per

sec-ond Typically, multiples of this basic unit of analog bandwidth are used, just as with digital bandwidth Units of measurement that are commonly seen are kilohertz (kHz), megahertz (MHz), and gigahertz (GHz) These are the units used to describe the band-width of cordless telephones (which usually operate at either 900 MHz or 2.4 GHz) 802.11a and 802.11b wireless network frequencies usually operate at 5 GHz and 2.4 GHz

Although analog signals can carry a variety of information, they have some significant disadvantages compared to digital transmissions The analog video signal that requires

a wide frequency range for transmission cannot be squeezed into a smaller band There-fore, if the necessary analog bandwidth is unavailable, the signal cannot be sent The same goes for digital bandwidth; however, it is less common for digital bandwidth to experience this because of its much-larger bandwidth capabilities

In digital signaling, all information is sent as bits, regardless of the kind of information

it is Voice, video, and data all become streams of bits when they are prepared for trans-mission over digital media, which gives digital bandwidth an important advantage over analog bandwidth Unlimited amounts of information can be sent over the smallest (lowest-bandwidth) digital channel Regardless of how long it takes, when the digital

BW =

P =

T =

S =

Best Download T = S

BW Typical Download T = SP

Maximum theoretical bandwidth of the Òslowest linkÓ between the source host and the destination host (measured in bits per second).

Actual throughput at the moment of transfer (measured in bits per second).

Time for file transfer to occur (measured in seconds).

File size in bits.

information arrives at its destination and is reassembled, it can be viewed, listened to,

read, or processed in its original form

It is important to understand the differences and similarities between digital and analog

bandwidth Both types of bandwidth are regularly encountered in the field of

informa-tion technology However, because this course is concerned primarily with digital

net-working, the term bandwidth refers to digital bandwidth.

Networking Models

Learning about networking is easier when you start with theory and concepts and then

move on to the more concrete aspects of implementation As a network professional,

you need to learn the theory of how networks communicate before you design, build,

and maintain networks Learning the concept of layers can help you understand the

action that occurs during communication from one computer to another

This section covers the concepts of layering and how it applies specifically to

commu-nication models Two specific models, OSI and TCP/IP, are described, as well as

peer-to-peer layer communication and encapsulation

Using Layers to Analyze Problems in a Flow of Materials

The concept of layers helps you understand the action that occurs during

communica-tion from one computer to another The following quescommunica-tions involve the movement of

physical objects, such as highway traffic or electronic data:

■ What is flowing?

■ What are the different forms of the object that is flowing?

■ What rules govern flow?

■ Where does the flow occur?

This motion of objects, whether physical or logical, is called flow Layers help describe

the details of the flow process Examples of systems that flow are the public water

sys-tem, the highway syssys-tem, the postal syssys-tem, and the telephone system

Now examine Table 2-5 What network is being examined? What is flowing? What are

the different forms of the object that is flowing? What are the rules for flow? Where

does the flow occur? The networks listed in this chart give analogies to help you

under-stand data flow in computer networks

The network communication process is complex The data, in the form of electronic signals, must travel across media to the correct destination computer and then be con-verted back into its original form to be read by the recipient Several steps are involved

in this process For this reason, the most efficient way to implement network commu-nications is a layered process In a layered communication process, each layer per-forms a specific task

In the next few sections, you’ll see how the network communication process is broken down using a layered model You’ll also see how data is sent over the network and how it reaches its intended destination As you learn about the network communica-tion process, it is important for you to understand the various steps, components, and protocols of the network communication process This understanding provides you with valuable troubleshooting information when the communications process does not proceed smoothly

Using Layers to Describe Data Communication

The difficulty in dealing with network communications is that it is a very complex pro-cess It would be extremely difficult for someone to understand this process if he or she looked only at network communication as a whole The solution to this issue was to break down the total network communication system into a series of layers Each layer

Table 2-5 Network Comparisons

Network

What Is

drinkable, wastewater/

sewer

Access rules (turning taps), flushing, not put-ting certain things

in drains

Pipes

Highway Vehicles Trucks, cars,

cycles

Traffic laws and common courtesy

Roads and highways Postal Objects Letters (written

information), packages

Rules for packag-ing and attachpackag-ing postage

Postal service boxes, offices, trucks, planes, delivery people Telephone Information Spoken

languages

Rules for access-ing phone and rules for politeness

Phone system wires, EM waves, and so on