Ethernet Networking- P3 docx

Nonetheless, in environments where many devices share network media in particular, linking servers or where severe electrical interference is a factor, fiber optic cabling is a viable ch

48 Fast and Gigabit Ethernet Media and Standards

attenuation Fiber optic media can therefore be used in situa- tions where wire media pose problems, such as on factory floors

# It is much harder for someone to tap than wire media

# It is much less susceptible to attenuation than wire media

# It has much higher bandwidth than most wire media The same fi- ber optic media can carry Ethernet signals at any standard speed

On the other hand, fiber optic cabling is more difficult to work with than wire It cannot be spliced and taped with electrical tape like wire, but in- stead requires special connectors that precisely line up the ends of two seg- ments of cable with one another In addition, fiber optic equipment is more expensive than equipment for wire media

Nonetheless, in environments where many devices share network media (in particular, linking servers) or where severe electrical interference is a factor, fiber optic cabling is a viable choice For example, in graphics and video design firms where large files move between workstations, fiber op- tic cabling can significantly speed up workflow by providing additional bandwidth

5ing/e versus Multimode Fiber Optics

There are two types of fiber optic cabling, single mode and multimode Sin- gle mode, which can transmit a single wavelength of light long distances, is used primarily for WAN connections Multimode can transmit multiple sig- nals at one tim, but is more limited in length and typically used in LANs When light is introduced into an optical fiber, it can either go straight down the middle of the optical tube or it can travel at an angle, reflecting off the side of the tube as it travels Each signal traveling down the tube at a time

is known as a mode

The diameter of the core of a single-mode fiber is very small (for example,

9 microns) A single ray of light is transmitted down the core, and it travels without reflection straight to its destination In theory, one single-mode fi- ber link can be as long as 10 kilometers

Fiber Optic Cabling 49

Multimode fiber has a larger core diameter and supports the transmission

of multiple signals Each ray of light has a different angle of reflection, making it possible for the receiving device to separate the individual sig- nals (See Figure 3-6.) However, the reflection angles disperse over dis- tances (modal dispersion), spreading the signals and ultimately making it

impossible to tell the signals apart This limits the distance of multimode fiber If the core is 62.5 microns in diameter, the maximum length is ap- proximately 275 meters; 50 micron fiber can go as far as 550 meters

Figure 3-6: Multiple signals traveling down multimode fiber

Multimode fiber is generally easier to work with than single mode Be- cause fiber optic cabling cannot be spliced, the ends of two pieces of single mode fiber must the aligned precisely when they are to be used as a single run of cable Multimode fiber, because of its shorter runs, often doesn't need to be assembled out of multiple pieces of cabling; it can use a single unbroken piece of fiber

Fiber Optic Cable Bundles

Just as UTP cable comes in several varieties, fiber optic cables come in seven basic types of bundle As you can see in Table 3-4, they vary in where they are used and their strength Each can use either single or multimode fiber There are no standards for fiber optic cable assembly; therefore, these types of cable vary somewhat from one manufacturer to another

50 Fast and Gigabit Ethernet Media and Standards

Table 3-4: Types of Fiber Optic Cable

as simplex; a two-wire cable is duplex

Made of many tight-buffered fibers No reinforcement of individual fibers Must terminate in a breakout box or patch panel

Many tight-buffered cables bundled together reinforcing fibers (e.g., aramid yarn) Because the fibers are reinforced, does not need to terminate in a breakout box or patch panel; may use quick-install connectors Although more expensive per foot than distribution cabling, may be cheaper and easier to install and maintain

A single fiber optic rod runs down the center to reinforce fibers wound around it The outside coating can be filled with a gel to protect the fibers from water Therefore, it can be buried

Made from layers of fiber optic tubes

Covered with metal for burying in areas where rodents are a problem

Designed for running on utility poles Usually hung from a

"messenger" cable or from another utility wire

Fast Ethernet Standards

There are four Fast Ethernet m e d i a specifications, three of which use U T P wire and one of which uses fiber optic cable

Fast Ethernet Standards 51

Twisted- Pair Wire

The three Fast Ethernet options that are designed for UTP wire are sum- marized in Table 3-5 Current installations, however, are almost exclusive-

ly 100BASE-TX, using Category 5 or higher wire Therefore, we will focus solely on 100BASE-TX

Table 3-5: Fast Ethernet Cabling Options

Standard Cable type

100BASE-TX 100BASE-T4 100BASE-T2

Category 5 UTP (uses 2 pairs of wire) Category 3 UTP (uses 4 pairs of wire) Category 3 UTP (uses 2 pairs of wire)

As you can see from the preceding table, the predominant UTP standard for Fast Ethernet uses only two of the four pairs of wires in the cable The specific wire usage for 100BASE-TX can be found in Table 3-6 The mir- rored signals (-TD a n d - R D ) are used to help identify and eliminate crosstalk (the cancellation technique) The receiving station can compare the positive and negative polarity signals They should be the same except for the polarity Any difference can be attributed to crosstalk and stripped out

Another way to look at the same issue to to call it differential signaling

Any interference that affects one wire will almost certainly affect the other

as well The receiver can then calculate the difference between the two sig- nals, which will always be constant, regardless of interference

Fiber Optics

Fiber optic configurations for Fast Ethemet are covered by the 100BASE-

FX standard Like standard Ethernet wiring for fiber optic cables, Fast Ethernet fiber optics requires two cables, one for transmitting and one for receiving

52 Fast and Gigabit Ethernet Media and Standards

Table 3-6: Fast Ethernet UTP Wire Usage

White (paired with green) +TD

Receive data Copy of receive data signal but with the oppostie polarity

Gigabit Ethernet Standards

Like Fast Ethernet, the Gigabit Ethernet standard has been written for two types of medium: fiber optics and copper wire In this case, the fiber optic implementations came first and UTP implementations have become feasi- ble to the desktop since about 2004 (The technology was available before that, but wasn't particularly affordable.)

Gigabit Ethernet Standards 53

I O00BASE-SX: Designed for horizontal cabling using multi- mode fiber In other words, you can use this stanadard for inter- connecting network segments on a single floor or for creating a

group of servers (a server farm)

I O00BASE-LX: Designed for interconnecting network seg- ments, including vertical runs through buildings, using single- mode fiber Most small offices do not need to use media based

on this standard

As you may have already concluded, the distance that you can run a fiber optic segment depends on the diameter of the fibers in the cable and the ca- ble's bandwidth As you can see in Table 3-7, segment lengths in the pub- lished standards vary from 230 to 5000 meters There is, however, a large gap between the last two entries in the table (5000 meter maximum) and the remaining entries because the last two are single-mode fiber specifications Table 3-7: Sample Fiber Optic Cable Lengths

Standard Diameter Bandwidth Cable length

(in microns) (MHz*km) (in meters)

Twis te d- Pair Wire

1000BASE-TX, which requires Cat 5 or better cabling, uses all eight wires

in the UTP cable In addition, each wire can handle a bidirectional signal

54 Fast and Gigabit Ethernet Media and Standards

(both send and receive) The signal is then sent in four parts, mirrored on each pair of wires, as in Table 3-8

Table 3-8: Fast Ethernet UTP Wire Usage

White (paired with green) +BI_DA Bidirectional data A

White (paired with orange) +BI DB m Bidirectional data B

White (paired with blue) -BI_DC Mirror of +BI_DC

White (paired with brown) +BI_DD Bidirectional data D

Note: To be completely accurate, Gigabit Ethernet over UTP cabling doesn't really run at I Gbps It actually runs

at the same speed as Fast Ethernet, but it uses all four pairs of wires at the same time and handles two signals per wire That produces the I Gbps speed/

Creating Network

Segments

The basic building block of an Ethernet is a network segment, a group of devices whose message exchanges are controlled by a single interconnec- tion device Hubs (once also known as repeaters) and switches are inter- connection devices that create individual network segments; switches and

routers connect segments to make larger networks

Note: Routing is such a complex topic that it is covered in

a chapter of its own (Chapter 6)

Although you can still purchase hubs today, and they are in use in many ex- isting networks, the price of switches has dropped so significantly that there

is rarely any reason to install a hub in a new or upgraded network As you will see, switches provide better performance at about the price of a hub

55

56 Creating Network Segments

Hubs (Repeaters)

As you will remember from Chapters 1 and 2, Ethernet was created to al- low multiple devices to share the same wire The original topology (the layout of the devices and the wire) was a straight-through bus, as in Figure 4-1 All messages are broadcast to the network bus, where all devices can read them Such topologies are based on coaxial cable (See Appendix A for details on outdated Ethemet standards.) Such segments would often be equipped with repeaters, devices that read the broadcast signal and retrans- mitted it, thus extending the length of the bus

to the hub with a single UTP cable However, because the bus is a single wire, it can carry only one signal at a time That means that a device can either transmit or receive, but not both, at the same time Communication

is therefore half duplex (bidirectional but only one direction at a time) Transmission using a hub happens in the following sequence"

1 A device checks the bus

2 If the bus is free, the device transmits using its transmit wire If the bus

is not free, the device waits, following the CDMA/CD protocol

Hubs (Repeaters) 57

Figure 4-2: The wiring inside a hub

3 The transmitting device handles a collision if one occurs

4 When the hub receives a signal without a collision, it repeats the signal and broadcasts it out all its ports

5 All attached devices recognize that there is a signal on their receive line Each checks the MAC address of the frame to determine whether

it is the receiptient of the frame

Most hubs today can handle multiple transmission speeds For example, a Fast Ethernet hub can handle 100 Mbps connections as well as the older 10 Mbps connections The ports are said to be autosensing because they can automatically detect the maximum speed of the NIC at the other end of the UTP cable

Unmanaged Hubs

A hub of the type we have been discussing is known as an unmanaged or

passive hub and has no intelligence of its own In particular, it has no idea what devices are connected to its ports All it can do is broadcast a signal out all ports A hub is a simple device with no moving parts that is reliable,

58 Creating Network Segments

easy to set up, requires virtually no maintenance, and has traditionally been quite inexpensive

Note: A hub operates at the Physical level of the joint TCP/IP and OSI protocol stacks

All devices connected to one hub share the same bus We say that they are

in the same collision domain because they are all contending for access to

the same wire

The box containing a hub is a case with RJ-45 ports, as in Figure 4-3 Over time they have varied between 4 and 36 ports If there were 36 devices at- tempting to communicate simultaneously (something that would rarely happen in a small network), performance remains very good However, as the size of the network increases, performance suffers In fact, when a sin- gle collision domain grows to around 200 devices, the network collapses during high traffic periods, unable to transmit acceptably clear signals

Figure 4-3: A 24-port hub (Courtesy of 3Com Corporation) Most unmanaged hubs are designed to be daisy-chained together to create larger networks To make this possible, each hub has an extra port For ex- ample, an eight-port hub designed for daisy chaining will actually have nine ports The extra port is designed to be connected to another hub with

a UTP cable joining the individual network segments into a single collision domain, as in Figure 4-4 When the ninth port is used to connect to another hub, the eighth port on the hub cannot be used to connect a network device because both the eighth and ninth ports are connected to the same wiring inside the hub

I1 100 meters

Workstation

Figure 4-4: A simple daisy chain of unmanaged hubs The extra port in an unmanaged hub has a special electrical property: It is

a crossover port in which the transmit and receive wires are reversed This

is essential so that the two hubs do not attempt to send and receive on the same wires In fact, you can use any of the other ports in an unmanaged

hub to connect to another hub if you use a crossover cable, a cable where

the transmit and receive wires are reversed at one end

The maximum length of a UTP cable is about 100 meters The hub daisy- chaining technique can extend that reach The major drawback is perfor- mance: The more devices contending for a bus, the slower the access

Managed Hubs

Some hubs are equipped with the ability to capture statistics about network traffic and to accept control commands from a workstation on the network

Such managed hubs make it easier to troubleshoot and maintain a network

The type of information and control a managed hub can provide usually in- cludes the following:

60 Creating Network Segments

View status of the hub: As illustrated in Figure 4-5, the infor-

mation provided to the user includes a measure of the utiliza- tion of the hub, the percentage of time taken up by collisions, the number of packets (frames) broadcast per second, and the percentage of errors detected in the Frame Check Sequence (FCS)

Figure 4-5: Viewing the status of a managed hub

View the status of a single port: As you can see in Figure 4-6,

individual port statistics are the same as those for the entire hub

Configure the hub: In Figure 4-7, for example, you can see that

the software shows a replica of the managed hub and allows the user to use a mouse to activate and deactivate individual ports

In addition, the user can set IP addresses and choose what in- formation is gathered about the system

Manage security

0 Collect hub and port usage statistics over time

Hubs (Repeaters) 61

Figure 4-6" Viewing the status of one port on a managed hub

Figure 4-7" Configuring ports on a managed hub

62 Creating Network Segments

5?ackable Hubs

Another way to extend a collision domain, getting around the 100 meter cable length limitation, is to use stackable hubs such as those in Figure 4-8 Stackable hubs are designed not only to sit one on top of another, but also connect with special stacking cables The entire stack of hubs then looks to the network as if it were one hub This is a simple and workable solution as long as no single network device is more than 100 meters from the hubs

Figure 4-8: Fast Ethernet stackable hubs (Courtesy of 3Com Corporation)

Propaga?ion Delay

Cable length isn't the only problem that you can run into when you use hubs to build an Ethernet Another major issue is propagation delay, the time it takes for a signal to be broadcast and read by all devices on a net- work As a network grows, cable distances may become so long that a de- vice may not be able to finish transmitting before it has a chance to detect collisions from other transmissions Propagation delay is the major reason for the cable length limits on standard Ethernet installations

To prevent this on a Fast Ethernet network, you must take overall cable dis- tances into account when planning the network layout You start with a ta- ble of the average round-trip delays, expressed in bit times, for the devices

on your network (see Table 4-1) The maximum allowable bit times be-