Ethernet Overview

Chapter 6 - Ethernet Design© 1996, BICSI LAN Design Manual - CD-ROM, Issue 11

Ethernet Overview

This chapter is designed to illustrate basic Ethernet deployment for

various cabling types. The assumption is that all users are on a

common floor and that a structured cabling system approach has been

adopted. The design of more complex Ethernet networks will be

discussed in later chapters and will cover Ethernet backbones and

Ethernet internetworking.

Introduction

Developed in the 1970’s and popularized in the 1980’s, Ethernet is the most popular networktechnology today. Ethernet connections are available for personal computers, high-performance design and scientific workstations, minicomputers and mainframe systems.

Ethernet is an architecture that provides best-effort datagram service. It has error detectionbut not error correction. It is a multi-access, packet-switched network using a passivebroadcast medium. Ethernet has no central control unit with data packets being transmittedover the network, reaching every station. Each station is responsible for recognizing theaddress in a data unit and for accepting data units addressed to it. Access to thetransmission medium is controlled by the individual station using a probabilistic accessmethod known as contention.


Goals

Beginning about 1972, Xerox Corporation’s Palo Alto research Center (PARC) begandeveloping a LAN system known as Experimental Ethernet. Early Ethernet specificationscontributed substantially to work done later by the IEEE 802.3 committee defining the CSMA/CD access control standard.

The original Ethernet goals are consistent with what have become the telecommunicationsrequirements driving the development and increased use of LANs.

These original Ethernet specifications are as follows:

Simplicity

Features that could complicate the network design without making substantial contributionto meeting other goals have been excluded.

Low cost

The cost of connection to an Ethernet network should be minimized. Technologicalimprovements will continue to reduce the overall cost of connecting stations to Ethernet.

Compatibility

All implementations of Ethernet should be capable of exchanging data at the Data Linklayer. To eliminate the possibility of incompatible variations of Ethernet, the specificationavoids optional features.

… Goals, continued


Addressing flexibility

The addressing mechanism should provide the capability to direct data frames to a singlestation, a group of stations or broadcast the message to all stations attached to thenetwork.

Fairness

All attached stations should have equal access to the network—averaged over time.

Progress

No one station attached to the network, operating in accordance with the Ethernetprotocol, should be able to prevent the operation of other stations.

High speed

The network should operate efficiently at a data rate of 10 Mbps.

Low delay

At any given level of network traffic, as little delay as possible should be introduced in thetransfer of a data frame.

Stability

The network should be stable under all load conditions. Delivered messages should makeup a constant percentage of the total network traffic.

… Goals, continued


Maintainability

The Ethernet design should allow for network maintenance, operation and planning.

Layered architecture

The Ethernet design should be specified in layered terms so as to separate the logicalaspects of the Data Link layer protocols from the physical details of the communicationschannel.

Functionality

On an Ethernet network, all stations have equal opportunity at all times to initiatecommunications over a common transmission channel. Because of this, some mechanismmust exist to resolve the conflict when more than one station attempts to transmit at thesame instant in time.

The mechanism used in Ethernet for this function is referred to as CSMA/CD —CarrierSense Multiple Access with Collision Detection. In CSMA/CD, any station wishing totransmit must first establish that the communications channel is clear. The station thenbegins its transmission, while at the same time continuing to monitor the channel for anindication of a collision.

If a collision is detected, the stations involved will stop transmitting, send a jamming signalindication a collision, wait a random amount of time (different for each station) and attempt totransmit again. Since a station can perform this operation several times in the space of onesecond, collisions are seldom noticed by users on a well-designed Ethernet system.

… Functionality, continued


Since a transmission is heard by all stations connected to the channel, Ethernet is classifiedas a broadcast-type network. The stations on an Ethernet network listen for, detect andrecover after any collision caused during normal operations.

Critical to proper operations is the ability for a transmitting station to detect a collisionbetween its transmission and that of another station. Without this ability, the station cannotrecover and rebroadcast its message. Instead, it will assume that the transmission wassuccessful. If this occurs, error-recovery will have to be initiated by higher-level software,rather than the Data Link layer. This in turn adds greater delays to network operation.

In order for a station to detect a collision during its transmission, the time delay in thepropagation of the collision signal must be limited. The collision signal must travel over thecabling system back to the station in less time than the maximum time period the station hasfor the detection of a collision.

The Ethernet specifications allow for the use of two types of coaxial cabling, unshieldedtwisted-pair cabling or optical fiber cabling. In all cases, distance limitations have beenimposed to allow for proper operation without excessive propagation delay.

The focus of this chapter is to describe the design recommendations and limitations for eachof these cable types. It should be noted that vendor-specific enhancements to Ethernet haveproduced products which can successfully operate beyond the limits described here. Byrespecting these recommendations, however, the designs will allow for reliable Ethernetoperation regardless of the equipment used.


Ethernet vs. IEEE 802.3

Overview

The original Ethernet standard developed by Digital, Intel and Xerox was first known asExperimental Ethernet and later as DIX Ethernet, in reference to the developers. ThisEthernet was the first technology to gain wide acceptance as a local area connectivitysolution.

In the early 1980’s, DIX turned the Ethernet standard over to the IEEE, where it became themodel for what is today known as IEEE 802.3. The IEEE made improvements to the originalEthernet and published the IEEE 802.3 standard for the first time in 1983.

While IEEE 802.3 and Ethernet are similar, they are not identical. The differences betweenthem are significant enough to make the two incompatible.

All versions of Ethernet are similar in that they share the same CSMA/CD bus architecture.However, the IEEE 802.3 standard has evolved over time so that it now supports multiplePhysical Layer options—including both 50 Ω and 75 Ω coaxial cable, unshielded twisted-paircable and optical fiber. Other differences between the two include transmission speed,signaling method and maximum cable length.


Frame formats

The most significant difference between the original Ethernet and the IEEE 802.3 standard isthe difference in their frame formats. This difference is significant enough to make the twoversions incompatible.

FIGURE 6.1:IEEE 802.3 FRAMEFORMAT VERSUSETHERNET FRAMEFORMAT

Frame CheckSequence

Pad

Information

Length Count

Source Address

Destination Address

Start FrameDelimiter

Preamble7 bytes

1 byte

2 or 6 bytes

0 - n bytes

2 bytes

2 or 6 bytes

0 - n bytes

4 bytes

IEEE 802.3 frame format

Frame CheckSequence

Pad

Information

Type

Source Address

Destination Address

Preamble8 bytes

6 bytes

0 - n bytes

2 bytes

6 bytes

0 - n bytes

4 bytes

Ethernet frame format

… Frame formats, continued


One of the differences between the two frame formats is in the preamble. The purpose of thepreamble is to announce the frame and to enable all receivers on the network to synchronizethemselves to the incoming frame. It also ensures that there is sufficient time betweenframes for error detection and recovery operations—9.6 microseconds for 10 Mbps Ethernet.The preamble is 8 bytes in length for Ethernet but 7 bytes for IEEE 802.3, where the eighthbyte becomes the start frame delimiter.

The second difference in frame format is the Type field found in an Ethernet frame. A Typefield was used to specify the protocol being carried in the frame. This enabled severalprotocols to be carried independently of one another. The Type field was replaced in theIEEE 802.3 standard by a Length Count field, which is used to indicate the number of bytesfound in the following field—the Information field.

The third difference between the two frame formats is found in the Address fields—Destination and Source. While the IEEE 802.3 format permits the use of either 2- or 6-byteaddresses, the Ethernet standard permits only 6-byte address fields. This is less of an issue,since most vendor IEEE 802.3 implementations use the 6-byte length. The 2-byte addressfield was included to accommodate early LANs using 16-bit address fields.

Summary

Over time, the trend has been towards the adoption of IEEE 802.3. Vendors helped themigration to the IEEE 802.3 standard from the original Ethernet by providing dual-functionhardware, capable of using either format. Today, vendors are providing migration paths from10 Mbps Ethernet to 100 Mbps Ethernet.

The predominant frame format in today’s Ethernet environments is IEEE 802.3, but thenetwork technology continues to be referred to as Ethernet.


Designing basic 10Base-5 Ethernet networks

Introduction

10Base-5 Ethernet is also known as Thick Ethernet and is more commonly referred to asThicknet. It was formally introduced in 1980 and represents the original Ethernetspecification.

Thicknet features a thick coaxial trunk cable with attachments called transceivers. Networkdevices are connected to the transceivers using a shielded twisted-pair cable known as atransceiver cable.

10Base-5 is falling into disuse as a network technology. However, there are a large numberof existing installations that may require expansion. As well, existing installations maymigrate towards newer technologies.

Components

RG-8-type coaxial cable

RG-8-type coaxial cable is used as the main cable, known as a trunk cable in a 10Base-5Ethernet network. It is a stiff, 50 Ω , 12 AWG coaxial cable with a 10 mm (0.4 in) outsidediameter. A special stripping and crimping tool is needed to be able to mount connectorson this cable. Many vendors supply this cable, either in bulk or in precut sections. Versionsof the cable are available as plenum cable, indoors nonplenum cable, underground-ratedcable and aerial-rated cable.

… Components, continued


Transceivers

A transceiver is a small box that provides for the electrical isolation of the cable from theattached device. It acts as a junction box on the trunk cable that permits the attachment ofstations.

A transceiver has three connectors on it. They function as follows:

• Two of the connectors attach to the Ethernet trunk cable—one for the incomingcable and one for the outgoing cable.

• The third connector is used to attach the station to the trunk cable using atransceiver cable.

Transceivers can be attached to the trunk cable in two ways. This is referred to as tappingthe cable and can be done as follows:

• One method of attachment is known as a vampire tap. This is a clamping methodwhere the transceiver actually pierces the cable. This eliminates the need to cutthe cable and mount connectors.

• The second method of attachment uses a transceiver with a T-type connector. Boththe trunk and transceiver cables attach to this T-connector. In this method, thetrunk cable must be cut and connectors attached.

The transceiver is also responsible for performing a test known as the SQE (SignalQuality Error) or Heartbeat Test. This is used to confirm that the transceiver is properlyconnected to the trunk cable.



Transceiver cable

Transceiver cables are shielded twisted-pair cables and far more flexible than the trunkcable. They are usually supplied with the transceiver unit.

A DIX-type connector is mounted on either end of the transceiver cable. One connector isfemale and the other male. The female connector is used to attach to the externaltransceiver unit and the male connector is used to attach to the station. Slide locks on theconnectors are used to lock the cable into place onto the Network Interface Card.

Network Interface Card (NIC)

Most NICs are capable of supporting both 10Base-5 and 10Base-2 Ethernet. For theattachment of the transceiver cable, the NIC should have a female DIX-type connector.DIX refers to Digital-Intel-Xerox.



N-series connectors

Three types of N-series connectors are used in a 10Base-5 installation. These are asfollows:

• N-series male connectors.

When T-type transceivers are used, N-series connectors are used on the two endsof the trunk cable to be attached to the transceiver. Preassembled cables comewith the N-series connectors already installed.

• N-series barrel connectors.

These connectors are used to join two cable segments.

• N-series terminators.

These are 50 Ω terminators used at both ends of a cable segment. For each cablesegment, one of these terminators must have a ground wire attached.


10Base-5 design

Basic 10Base-5 design - Trunk cable deployment

The first step in 10Base-5 design is determining where the main trunk cable will run. Thecable must be placed where it is accessible to stations needing to attach to the network.

Some considerations when determining the placement of the trunk cable include thefollowing:

• The trunk cable can be a maximum of 500 m (1640 ft) in length.

• The cable must be terminated on both ends with 50 Ω, N-series terminators.

• One of the N-series terminators must be grounded—not both.

• The maximum length of a transceiver cable is 50 m (164 ft), therefore a stationneeding to attach to the trunk cable must be within 50 m (164 ft) of it.

… 10Base-5 design, continued


FIGURE 6.2: DEPLOYING THE 10BASE-5 TRUNK CABLE

Common floor, divided into zones

TelecommunicationsCloset(TC)

Thicknet trunk cable

Terminator(grounded)

Terminator(not grounded)



Basic 10Base-5 design - Transceiver deployment

Thicknet transceivers—sometimes referred to as Media Attachment Units (MAUs)—areused to attach network devices to the coaxial trunk cable.

Some considerations when placing the transceivers include the following:

• There can be amaximum of 100transceiversattached to thetrunk cable.

• Transceiversmust be at least2.5 m (8.2 ft)apart.

FIGURE 6.3:10BASE-5TRANSCEIVERDEPLOYMENT


Common floor, divi ded into zones

TelecommunicationsCloset

(TC)

Transceiver(MAU)

At least 2.5 m (8.2 ft)between twotransceivers


Basic 10Base-5 design - Station deployment

Network devices attach to the transceivers using transceiver cable. This cable issometimes referred to as an Attached Unit Interface (AUI) cable. Transceiver cable is atwisted-pair cable consisting of four 20 AWG stranded twisted-pairs covered by a commonshield. The cable can be a maximum of 50 m (164 ft) in length. Of the four pairs, one isused for transmission (called Data Out by IEEE 802.3), one for reception (called Data Inby IEEE 802.3), one to detect collisions (called Control by IEEE 802.3) and one forpowering the transceiver from the station (called Voltage by IEEE 802.3).

A single transceiver can connect one station to the trunk cable or it may connect severaldevices to the trunk cable.

Each station is equipped with an Ethernet Network Interface Card (NIC). The NIC isconnected to the transceiver cable. This NIC connection point is sometimes referred to asa DIX (Digital-Intel-Xerox) connector.



FIGURE 6.4: 10B ASE-5 STATION DEPLOYMENT

TC

NICStation

Trunk cable

Transceivercable

Single-porttransceiver

Maximum of50 m (164 ft)

Ethernet stationwith NIC

Trunk cable

Multi-porttransceiver

Transceivercable


Ethernet stationwith NIC


Designing basic 10Base-2 Ethernet networks

Introduction

10Base-2 Ethernet is also known as Thin Ethernet, and is often referred to as Thinnet.Thinnet was formally introduced in 1985 and is based on a coaxial cable transmissionmedium. The coaxial cable is thinner, more flexible and lower in cost than the traditional thickcoaxial cable used in 10Base-5. For this reason 10Base-2 is sometimes referred to asCheapernet.

In Thinnet, the transceiver is integrated into the station NIC. This permits the Thinnet trunkcable to be attached directly to each station. This Thinnet trunk cable is a smaller diametercoaxial cable, which makes it physically easier to work with. However, this relative thinnessof the conductor imposes a more restrictive design on the network.

As is the case with 10Base-5, 10Base-2 is falling into disuse as a network technology fornew installations. However, a large number of installations do exist and they may requireexpansion and potentially, migration to a new technology.


Components

RG-58 coaxial cable

RG-58 coaxial cable is used as the 10Base-2 main trunk cable. It is a 50 Ω , 20 AWGcable with a 5 mm (0.2 in) diameter. It is commonly referred to as RG-58 A/U orRG-58 C/U cable. Many vendors supply this cable either in bulk or in precut sections. Bulkcable needs to be cut to the proper length and have connectors attached. Precut cabletypically comes with connectors attached. Versions of the cable are available as plenumcable, indoors nonplenum cable, underground-rated cable and aerial-rated cable.


The 10Base-2 NIC will have a BNC-type connector on the board. It may also have aThicknet connector or a 10Base-T connector. The trunk cable will attach to a BNCT-connector which is in turn connected to a male BNC connector on the back of the NIC.



BNC connectors

BNC connectors must be attached to all cable segment ends. Cable connector kits includea center pin, a housing and a clamp-down sleeve. A stripping and crimping tool will berequired to mount the connectors.

Three types of BNC connectors are used in a 10Base-2 installation. These are as follows:

• BNC T-connectors.

T-connectors are attached to the BNC connector on the back of the NIC. These T-connectors provide two connection points for the trunk cable—one for the incomingsignal and one for the outgoing signal. These T-connectors are required for eachstation on the network.

• BNC barrel connectors.

These connectors are used to join two cable segments together.

• BNC terminators.

These are 50 Ω terminators used at both ends of a cable segment. For each cablesegment, one of these terminators must have a ground wire attached. The laststation on a trunk cable requires a BNC terminator attached to the open end of itsT-connector.


10Base-2 design

Basic 10Base-2 design

The design of 10Base-2 Ethernet networks requires running the correct type of coaxialtrunk cable from one network device to the next, in a bus configuration. RG-58 A/U orRG-58 C/U is used to connect one station to the next—no transceiver cables are needed.Stations are connected to the trunk cable using a BNC T-connector on the NIC. The cablemust be terminated at both ends with 50 Ω, BNC-type terminators. One of theseterminators must be grounded.

Some considerations when designing 10Base-2 Ethernet networks are as follows:

• The maximum length of a trunk segment is 185 m (607 ft).

• There can be a maximum of 30 devices attached to a trunk cable segment.

• The T-connectors must be at least 0.5 m (1.6 ft) apart.



FIGURE 6.5: 10B ASE-2 DEPLOYMENT


TelecommunicationsCloset(TC)

Thinnet trunk cable

Terminator(grounded)

Terminator(not grounded)

Minimum of0.5 m (1.6 ft)


Designing basic 10Base-T Ethernet networks

Introduction

10Base-T Ethernet is also known as twisted-pair Ethernet. It was formally introduced in 1990and has become popular for both new and existing installations.

Part of the 10Base-T specification is to ensure compatibility with other versions of theIEEE 802.3 standard—making the transition easier. Some of these compatibility featuresinclude the following:

• Existing Ethernet NICs can be used with 10Base-T installations through the use ofadapters.

• Twisted-pair trunk cables can be added to existing trunks by using repeaterssupporting both twisted-pair and coaxial cable.

In 10Base-T, as in 10Base-2, the transceiver is built into the station NIC. As well, the coaxtrunk cable is replaced with an electronic concentrator—often referred to as a 10Base-T hub.Each station is connected directly to a port in the hub.

The 10Base-T specification includes a cable testing feature known as Link Integrity Testing.This monitoring is done from a central point and tests the twisted-pair wires on an ongoingbasis for open (cut) wires and shorts (unintended electrical contact between wires).


Components

Unshielded twisted-pair cable

UTP cable is used to connect stations to the 10Base-T hub. The UTP cable must haveCategory 3 or better transmission characteristics—as specified in the ANSI/TIA/EIA-568-Acabling standard. The UTP can be 24 or 22 AWG.


The connection point on a 10Base-T NIC is in the form of an 8-position modular jack-typeconnection. Some Ethernet NICs are available with a Thicknet DIX, Thinnet BNC, or both,in addition to the 8-position modular jack connector for 10Base-T.

If a 10Base-5 or 10Base-2 NIC is to be connected to a 10Base-T Ethernet network, anadapter known as a 10Base-T transceiver must be used. This device converts a signalintended for transmission on a 50 W coaxial cable to a signal that can be transmitted overa 100 Ω UTP cable.

10Base-T hub

10Base-T hubs are also referred to as concentrators. Each port on the hub provides aconnection point for a UTP cable to a network station. Some models also provide coaxialcable or optical fiber connections for links to other Ethernet segments.

In essence, the 10Base-T hub represents the trunk cable of a traditional Ethernet. Itshrinks the thick coax trunk cable to a very short length and stations attach to this shortcoaxial trunk cable through the hub port via a length of UTP cable, which replaces thetraditional AUI transceiver cable.


10Base-T design

Basic 10Base-T design

While the physical appearance of a 10Base-T Ethernet network is that of a star, itcontinues to operate logically in linear bus topology. This linear bus is miniaturized andfully contained in the 10Base-T hub.

10Base-T Ethernet uses 24 or 22 AWG unshielded twisted-pair cabling with Category 3 orbetter classification as specified in the ANSI/TIA/EIA-568-A cabling standard. Two pairsare used, one for transmission (pins 1 and 2) and the other for reception (pins 3 and 6).Collisions are detected and relayed to stations by the hub, which is an active (powered)device.

Some considerations when designing a UTP-based 10Base-T Ethernet network are asfollows:

• The total distance from a hub to a station cannot exceed 100 m (328 ft).

• Two hubs can be separated by a maximum of 100 m (328 ft).

• A theoretical maximum of 1024 stations can be connected to one 10Base-T LAN.

… 10Base-T design, continued


FIGURE 6.6: BASIC 10BASE-T CONFIGURATION

10Base-T hub


Common floor, di vided into zones

10Base-T NICwith built-intransceiver


10Base-T hub

… 10Base-T design, continued


FIGURE 6.7: STRUCTURED 10BASE-T DESIGN

TC

Work Area

Telecom muni cations Closet

10Base-T hub

Patch Cord

Cross-connect Hardware

UTP Horizontal Cable

Equipment CableCross-connect Hardware


Other Ethernet networks

Combined 10Base-5 and 10Base-2

Thick and thin Ethernet trunk cable can be combined into one network. The combination ofthick and thin trunk cable requires using BNC to N-series connector adapters.

Combination thick and thin coaxial cable segments used in one combination trunk segmentcan range from 185 to 500 meters (607 to 1640 feet) in length.

To determine the maximum amount of thin coaxial cable that can be used in such aninstallation, the following equation is used:

The constant 3.28 is used to compensate for the lower performance of thin coax cable.

(Thin coax length x 3.28) + Thick coax length = 500 meters (1640 ft) maximum


10Base-FL

10Base-FL Ethernet is also known as Fiber Link Ethernet. It was formally introduced in 1993.

In 10Base-FL, a two-fiber optical fiber cable is used in a manner similar to UTP in 10Base-T.One fiber is used for transmission and the other for reception.

In place of the 10Base-T hub, a 10Base-FL hub is used. The network also follows a physicalstar topology with all devices directly connected to the hub.

Some considerations for designing a 10Base-FL Ethernet network are as follows:

• Multimode,62.5/125 µm opticalfiber is recommendedto connect stationsand hubs.

• The maximumdistance between astation’s NIC and a porton a 10Base-FL hub is2000 m (6560 ft).

FIGURE 6.8:BASIC 10BASE-FLCONFIGURATION


10Base-FL NIC


10Base-FL hub

Two-fiber cable,62.5/125 µm opticalfiber recomm ended


Troubleshooting Ethernet networks

Introduction

The most important criteria for the success of any network is its reliability. If the LAN is notoperating consistently or worse, not operating at all, it is not meeting its primary objective—the ability for users to share devices, software programs and user-created files. Therefore, ifthe LAN experiences problems, it is important to solve those problems as quickly as possible.

The increasing use of structured cabling systems has made network troubleshooting easier.Problems will continue to occur from time to time, but structured cabling systems havehelped to eliminate some types of difficulties. Some examples of the benefits of structuredcabling systems include the following:

• A single station failure should not cause the whole network to fail.

• A failure in a single cable will not bring the whole network down.

• Fewer connection points represent fewer points of failure.

• Much of the diagnostic testing can be done from a central location.


General troubleshooting guidelines

Network problems are often blamed on one of the hardware components. That is, the cablingsystem, the Network Interface Cards, the station or one of the other connectedcomponents—which may vary with the technology employed. However, software problemsshould not be overlooked. Sometimes a new software application that has been installed mayconflict with existing applications. Also, problems can happen that are not specifically relatedto the network technology at all. Problems can occur because of inadequate groundingsystems, for example.

While some problems are specific to a certain technology, there are also network problemsthat may occur independent of the networking technology being used. Therefore, sometroubleshooting is also independent of the networking technologies. Some general items toconsider if a network problem should occur include the following:

Station/NIC problems

• The Network Interface Card may not be properly seated. That is, it is not makingcontact and is unable to communicate with the network. It may be necessary toremove the NIC, clean it and reinstall it.

• There may be conflicts between the NIC and other boards in the station. In the PCenvironment, conflicts can occur due to shared IRQs (Interrupt Request Lines),DMAs (Direct Memory Address) lines, and/or I/O (Input/Output) base addresses.

• The NIC should be checked to ensure that all jumpers and dip switches are setproperly. It is important to have the appropriate documentation at hand to checkwhat the correct settings are.

… General troubleshootingguidelines, continued


• It should be verified that all power supplies are connected and functioning properly.

• Diagnostic software should be used to check for duplicate network addresses.

Cabling

• The cable should be checked for continuity, kinks, crimps, sharp bends, opens andshorts.

• It should be verified that the cabling has been configured properly. Distancelimitations should be adhered to.

• The cable connecting the station to the network should be visually inspected tocheck for loose connections. Tripping over a cable connecting the station canloosen a connection point, causing disruptions.

• It should be verified that the correct connecting cables are being used and that theyare terminated properly.

Miscellaneous

• Ensure that the products being used adhere to the networking technology beingused.

• Some vendors offer products with options that will extend the allowabletransmission distance. These products may not work in conjunction with otherproducts. They may be proprietary and work only with other specified products.


Troubleshooting Ethernet

In the case of Ethernet networks, a distinction needs to be made between coaxial cablebased-Ethernet and twisted-pair Ethernet. The type of transmission media used significantlyimpacts the approach that needs to be taken when troubleshooting the network.

Coaxial cable Ethernet

For troubleshooting purposes, coax-based Ethernet is at a disadvantage because of its bustopology. Having all devices connected to a single length of cable makes it more difficult toisolate a fault.

A single device failure may affect the whole network, a segment of the network or only thedevice itself. Isolating the cause and location of a failure is a large part of coax Ethernettroubleshooting.

The following represent some items to be considered:

Cable

• Any damage to the trunk cable may cause the network to fail. Damage can becaused by kinks or sharp bends in the cable or be caused by a mechanical deviceentering the transmission path—new connectors or transceivers.

… Troubleshooting Ethernet,continued


• The cable may need to be checked for opens, shorts and missing terminators. Anintact, properly terminated cable should produce a reading of approximately 50 Ωwhen a DC resistance test is done. If the cable has shorted, the reading will bemuch lower, in the range of 0 to 10 Ω . If the terminator at the far end is missing orthe cable is open, the reading will exceed 50 Ω by a large margin. Time DomainReflectometry (TDR) testing will provide more details regarding the problem.

• Impedance mismatches may have occurred, either due to the use of the wrong typeof cable or because the cable is kinked, has sharp bends or is improperlyterminated.

• If a cable tests open, a portion of the trunk cable may have been disconnected fromthe network device. Moving a station may cause the cable to come loose.

• Ensure that the correct cable type has been used. For example, RG-58 A/U (50 Ω)versus RG-59 A/U (75 Ω) in 10Base-2 environments.

Terminators

• A terminator can be tested using a DC resistance test to see if it is defective. TheDC resistance between the center conductor and the outside shield should be inthe 50 Ω range. If it is not, the terminator should be replaced.

• Check that cable ends are properly terminated and one terminator is grounded.



Transceivers

• Verify that power and transceiver cables are properly connected.

• It should be verified that the transceiver, the AUI cable and any options are all setto operate with the correct Ethernet version—standard Ethernet or IEEE 802.3(important in 10Base-5).

• Ensure that the minimum distance required between transceivers has not beenviolated.

• If a transceiver appears to be faulty, removing the NIC may confirm this.Disconnecting the NIC will power down the transceiver and the network mayrecover.

• In 10Base-2 networks, the transceiver is built into the NIC and it may have to bereplaced to determine if it is causing the problem.

Connectors

• In the 10Base-2 environment it is important to check for disconnected or poorlyassembled T-connectors, used at each station.


• NICs equipped with dual connectors for both 10Base-5 and 10Base-2 require that ajumper switch be set to indicate which environment is being used. If the switch isset incorrectly, the NIC cannot communicate with the network.



Software considerations

• All addresses must be of the same format. While addresses in the IEEE 802.3standard may be 2 or 6 bytes in length, they must be the same for both source anddestination addresses across the entire network.

Miscellaneous

• Ensure that products being used adhere to the version of the standard being used.This is especially important in 10Base-5 Ethernet which is closely related to theoriginal Ethernet. However, the two technologies are not compatible.

Twisted-pair Ethernet

The star topology of 10Base-T Ethernet is an advantage for troubleshooting. It makes iteasier to isolate failures—first to a single hub and from there to a single port on the hub.

Some cabling-related problems that may be encountered on a 10Base-T network includethe following:

• Cables do not have pin-to-pin continuity.

• Terminations do not all follow the same pin configuration—both T568A and T568Bare used.

• Incorrect components are installed.


Ethernet performance

Overview

Ethernet is a contention-based technology, meaning that all attached devices have equalaccess to the transmission channel at all times. This makes the prediction of actual networkutilization somewhat more difficult.

There are two factors to consider when estimating network performance. The first is thetransmission rate—the number of Ethernet frames that can be transmitted in a given timeperiod. The second is an estimate of network traffic produced by users.

Below are some sample calculations regarding frame transmission rates and estimations ofnetwork traffic.


Frame transmission rate

The amount of information carried in an Ethernet frame can vary—it is not a predefinedamount. Therefore, an Ethernet frame can vary in size from a minimum of 72 bytes to amaximum of 1526 bytes. This in turn affects the frame transmission rate—the larger theframe, the fewer the number of frames that can be transmitted in a given time period.

Additional factors to consider are as follows:

• 10 Mbps Ethernet allows for 9.6 microseconds (µs) between frames for errordetection and recovery purposes.

• There is a bit time of 100 nanoseconds (ns)—this is the time required to transmitone bit of information.

Before performing any calculations, please be aware of the following conversions:

• 1 millisecond (ms) = 0.001 seconds (10-3) or 1,000 ms per second.

• 1 microsecond (µs) = 0.000001 seconds (10-6) or 1,000,000 µs per second.

• 1 nanosecond (ns) = 0.000000001 seconds (10-9) or 1,000,000,000 ns per second.

Also ,

• 1 ms = 1,000 µs = 1,000,000 ns.

• 1 µs = 1,000 ns.

… Frame transmission rate,continued


The Frame Transmission Rate is calculated as follows:

The calculation for Transmission Time is as follows:

Transmission Time = Time to transmit one frame + Time between frames

EXAMPLE 6.1: FRAME TRANSMISSION RATE FOR A 1526 BYTE FRAME SIZE

For a maximum frame size of 1526 bytes, the amount of time required to transmit oneframe is equal to the following:

It would take 1.2304 milliseconds to transmit one frame.

Transmission Rate = 1

Transmission Time

Transmission Time = 1526 bytes

frame x 8

bits

byte x 100

ns

bit+ 9.6 microseconds

Transmission Time = 1,220,800 ns + 9,600 ns

Transmission Time = 1,230,400 ns = 1.2304 ms



Therefore, a maximum of 812 complete frames of 1526 bytes can be transmitted in onesecond.

The equivalent transmission rate in bytes is:

The equivalent transmission rate in bits is:

Transmission Rate = 1000 ms / second1.2304 ms / frame

Transmission Rate = 812.74 frames/ second

Transmission Rate in bytes = 812 framessecond

x 1526 bytesframe

= 1,239,112 bytes

second

Transmission Rate in bits = 1,239,112 bytes

second x 8

bitsbyte

= 9,912,896 bits

second

Transmission Rate in bits = 9.912 Mbps



EXAMPLE 6.2: FRAME TRANSMISSION RATE FOR A 72 BYTE FRAME SIZE

For a minimum frame size of 72 bytes, the amount of time required to transmit one frameis equal to the following:

It would take 0.0672 milliseconds to transmit one frame.

Therefore, a maximum of 14,880 complete frames of 72 bytes can be transmitted in onesecond.

Transmission Time = 72 bytes

frame x 8

bits

byte x 100

ns

bit+ 9.6 microseconds

Transmission Time = 57,600 ns + 9,600 ns

Transmission Time = 67,200 ns = 0.0672 ms

Transmission Rate = 1000 ms / second0.0672 ms / frame

Transmission Rate = 14,880.95 frames / second



The equivalent transmission rate in bytes is:

The equivalent transmission rate in bits is:

Transmission Rate in bytes = 14,880 framessecond

x 72 bytesframe

= 1,071,360 bytes

second

Transmission Rate in bits = 1,071,360 bytes

second x 8

bitsbyte

= 8,570,880 bits

second

Transmission Rate in bits = 8.571 Mbps



The maximum theoretical throughput of a traditional Ethernet network is 10 Mbps.However, when a maximum frame size is used for all transmissions, the maximumpossible throughput on an Ethernet network is 9.912 Mbps. When the minimum frame sizeis used for all transmissions, the throughput drops to 8.571 Mbps.

Since it is unlikely that all frames will be the maximum or minimum size, the actualthroughput in most cases will most likely be somewhere between 9.912 and 8.571 Mbps.

Please note that this number is not taking into consideration such factors as collisions andretransmissions nor the number of PCs on the network and the performance levels.

Taking all of these factors into account, a utilization rate of 50 percent is considered to bea heavy load. When the transmission load reaches 60 to 70 percent, network performancebegins to deteriorate significantly. The problem is compounded by the fact that as thetransmission load increases, the number of collision and the number of retransmissionsincrease proportionally.


Estimating network traffic

While it is not possible to determine exactly what the amount of network traffic will be, it ispossible to make a reasonable estimate. To estimate network traffic, the following steps canbe taken in order:

1. Group network stations according to the general activities they perform—managerial,clerical, engineering, design, etc.

2. Estimate the amount of network activity for one station in each group.

3. Multiply the number of stations in a group by the estimated activity for a single station inthat group.

4. Add together the total network activity over all the groups connected to the Ethernetsegment.

5. The value calculated in step 4 is compared to network throughput to determine percentutilization.

… Estimating network traffic,continued


EXAMPLE 6.3: ESTIMATING NETWORK TRAFFIC

In order to illustrate how network traffic load can be estimated in an Ethernet environment,the following scenario is presented.

An Ethernet network has a total of 63 users connected. Of these, 20 are in managerialpositions, 25 are in clerical positions and 18 are involved in design work.

The average amount of network activity for one station in each group is estimated as follows:

Managerial Transmission Number of Total bytessize in bytes transmissions per hour

File requests 2,500 4 10,000Loading application programs 400,000 2 800,000Loading data files 250,000 3 750,000Saving files 300,000 4 1,200,000Sending/receiving E-mail 4,000 8 32,000

Total 2,792,000



Clerical Transmission Number of Total bytessize in bytes transmissions per hour

File requests 2,000 3 6,000Loading application programs 350,000 2 700,000Loading data files 150,000 2 300,000Saving files 100,000 3 300,000Sending/receiving E-mail 3,500 12 42,000

Total 1,348,000

Design Transmission Number of Total bytessize in bytes transmissions per hour

File requests 3,000 5 15,000Loading application programs 550,000 2 1,100,000Loading data files 800,000 5 4,000,000Saving files 850,000 4 3,400,000Sending/receiving E-mail 2,000 2 4,000

Total 8,519,000



Totals Total per person Number of people Total per group

Managerial 2,792,000 20 55,840,000Clerical 1,348,000 25 33,700,000Design 8,519,000 18 153,342,000

Total 242,882,000

Therefore, network traffic is estimated to be 242,882,000 bytes per hour.

Dividing this number by 3600 gives the traffic in bytes per second:

To determine the value in bits per second, the above value is multiplied by 8.

Dividing this value by 1,000,000 gives a value in Megabits per second (Mbps).

Therefore, the estimated network traffic in Mbps is quite low—network utilization is only ata little more than 5 percent of capacity.

242,882,000 bytes / hour3,600 seconds / hour

= 67,467.222 bytes / second

67,467.222 x 8 = 539,737.8 bits / second

539,737.8 bits/ second = 0.54 Mbps



EXAMPLE 6.4:ESTIMATING NETWORK TRAFFIC WHEN LARGE FILES NEED TO BE ACCESSED

In this extension of the above example, the only change is in the work done by the designers.They produce multimedia output resulting in much larger files to transport across thenetwork. It is not unusual to have a multimedia file that is 10 MB (10,000,000 bytes) in size.

Recalculating, the values in the above example become:

Managerial Transmission Number of Total bytessize in bytes transmissions per hour

File requests 2,500 4 10,000Loading application programs 400,000 2 800,000Loading data files 250,000 3 750,000Saving files 300,000 4 1,200,000Sending/receiving E-mail 4,000 8 32,000

Total 2,792,000

Clerical Transmission Number of Total bytessize in bytes transmissions per hour

File requests 2,000 3 6,000Loading application programs 350,000 2 700,000Loading data files 150,000 2 300,000Saving files 100,000 3 300,000Sending/receiving E-mail 3,500 12 42,000

Total 1,348,000



Design Transmission Number of Total bytessize in bytes transmissions per hour

File requests 3,000 5 15,000Loading application programs 550,000 2 1,100,000Loading data files 10,000,000 5 50,000,000Saving files 10,000,000 4 40,000,000Sending/receiving E-mail 2,000 2 4,000

Total 91,119,000

Totals Total per person Number of people Total per group

Managerial 2,792,000 20 55,840,000Clerical 1,348,000 25 33,700,000Design 91,119,000 18 1,640,142,000

Total 1,729,682,000

This translates to 480,467.22 bytes per second or 3,843,737.8 bits per second—3.84 Mbps. At a network traffic level of 3.84 Mbps, the utilization figure is close to40 percent of maximum possible capacity. At this point, users will begin to notice adeterioration in network speed.

The purpose of this example is to show how applications being introduced today willquickly overload network bandwidth that was considered sufficient for applications usedhistorically.


Ethernet growth

Overview

An Ethernet network can connect more than a single group of users on a common floor.Ethernet can be used to link hundreds of users throughout a building—or multiple buildingson a campus—using one of the following designs:

• Multiple Ethernet trunks or hubs (called segments) can be linked to each otherusing devices called repeaters to form a single Ethernet network, on which allstations on all segments share one transmission channel. This is the leastelaborate method of network growth.

• Multiple Ethernet networks can be linked together using devices called bridges toform an Ethernet internetwork. Each network continues to have its own distincttransmission channel available only to attached stations. A bridge device connectsto two networks simultaneously, allowing messages to travel between networks.

• Finally, multiple Ethernet networks can be linked to a common backbone network,which acts as a transmission channel for all communications between bridges onan internetwork. Each bridge connects both to a network and to the backbone.

These methods of expanding Ethernet are discussed in a later chapter.


Ethernet Overview .................................................................. 1Introduction ....................................................................................... 1

Goals .................................................................................................. 2Simplicity ............................................................................................................2Low cost .............................................................................................................. 2Compatibility .......................................................................................................2Addressing flexibility ...........................................................................................3Fairness .............................................................................................................. 3Progress ............................................................................................................. 3High speed ..........................................................................................................3Low delay ............................................................................................................3Stability ...............................................................................................................3Maintainability .....................................................................................................4Layered architecture ...........................................................................................4

Functionality ..................................................................................... 4

Ethernet vs. IEEE 802.3 .........................................................6Overview ............................................................................................ 6

Frame formats .................................................................................. 7

Summary ........................................................................................... 8

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Designing basic 10Base-5 Ethernet networks.................... 9Introduction ....................................................................................... 9

Components ..................................................................................... 9RG-8-type coaxial cable .................................................................... 9Transceivers .................................................................................... 10Transceiver cable .............................................................................11Network Interface Card (NIC) ..........................................................11N-series connectors ........................................................................ 12

10Base-5 design ............................................................................ 13Basic 10Base-5 design - Trunk cable deployment .......................... 13Basic 10Base-5 design - Transceiver deployment .......................... 15Basic 10Base-5 design - Station deployment ................................. 16

Designing basic 10Base-2 Ethernet networks.................. 18Introduction ..................................................................................... 18

Components ................................................................................... 19RG-58 coaxial cable ........................................................................ 19Network Interface Card (NIC) ......................................................... 19BNC connectors .............................................................................. 20

10Base-2 design ............................................................................ 21Basic 10Base-2 design .................................................................... 21

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Designing basic 10Base-T Ethernet networks ................. 23Introduction ..................................................................................... 23

Components ................................................................................... 24Unshielded twisted-pair cable ......................................................... 24Network Interface Card (NIC) ......................................................... 2410Base-T hub .................................................................................. 24

10Base-T design ............................................................................ 25Basic 10Base-T design ................................................................... 25

Other Ethernet networks ..................................................... 28Combined 10Base-5 and 10Base-2 ............................................ 28

10Base-FL ....................................................................................... 29

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Troubleshooting Ethernet networks .................................. 30Introduction ..................................................................................... 30

General troubleshooting guidelines .......................................... 31Station/NIC problems.................................................................... 31Cabling .......................................................................................... 32Miscellaneous ................................................................................ 32

Troubleshooting Ethernet ............................................................ 33Coaxial cable Ethernet .................................................................... 33

Cable ............................................................................................. 33Terminators ................................................................................... 34Transceivers .................................................................................. 35Connectors .................................................................................... 35Network Interface Card (NIC) ....................................................... 35Software considerations ................................................................ 36Miscellaneous ................................................................................ 36

Twisted-pair Ethernet ...................................................................... 36

Ethernet performance .......................................................... 37Overview .......................................................................................... 37

Frame transmission rate .............................................................. 38

Estimating network traffic ............................................................ 44

Ethernet growth .................................................................... 50Overview .......................................................................................... 50

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Example 6.1: Frame transmission ratefor a 1526 byte frame size ................................ 39

Example 6.2: Frame transmission ratefor a 72 byte frame size .................................... 41

Example 6.3: Estimating network traffic ................................. 45

Example 6.4: Estimating network traffic whenlarge files need to be accessed ....................... 48

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Figure 6.1: IEEE 802.3 frame format versusEthernet frame format ............................................. 7

Figure 6.2: Deploying the 10Base-5 trunk cable .................. 14

Figure 6.3: 10Base-5 transceiver deployment ...................... 15

Figure 6.4: 10Base-5 station deployment .............................. 17

Figure 6.5: 10Base-2 deployment .......................................... 22

Figure 6.6: Basic 10Base-T configuration ............................. 26

Figure 6.7: Structured 10Base-T design ................................ 27

Figure 6.8: Basic 10Base-FL configuration ........................... 29

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Documents

Ethernet Overview