Troubleshooting & Selection of EtherNet

Troubleshooting & Selection of EtherNet

Abstract

             EtherNet is an application layer protocol built on the standard TCP/IP protocol suite used to communicate high level industrial devices.Ethernet was developed by Xerox Corporation’s Palo Alto Research Center (PARC) in the 1970s. Ethernet was the technological basis for the IEEE 802.3 specification, which was initially released in 1980. Shortly thereafter, Digital Equipment Corporation, Intel Corporation, and Xerox Corporation jointly developed and released an Ethernet specification (Version 2.0) that is substantially compatible with IEEE 802.3. Together, Ethernet and IEEE 802.3 currently maintain the greatest market share of any local-area network (LAN) protocol. Today, the term Ethernet is often used to refer to all carrier sense multiple access collision detect (CSMA/CD) LANs that generally conform to Ethernet specifications, including IEEE 802.3. When it was developed, Ethernet was designed to fill the middle ground between long-distance, low-speed networks and specialized, computer-room networks carrying data at high speeds for very limited distances. Ethernet is well suited to applications on which a local communication medium must carry sporadic, occasionally heavy traffic at high peak data rates.

             When it was developed, Ethernet was designed to fill the middle ground between long-distance, low-speed networks and specialized, computer-room networks carrying data at high speeds for very limited distances. Ethernet is well suited to applications on which a local communication medium must carry sporadic, occasionally heavy traffic at high peak data rates.

TCP/IP Model

The TCP/IP model consists of four layers.

1) Network Layer

The Network layer combines the Data Link layer and Physical layer, including the twisted pair

cable, the Physical layer device (PHY), and the Ethernet Media Access Controller (MAC).

2) Internet Layer

The Internet layer consists primarily of a software implementation. The IP header is

evaluated or generated by software.

3) Transport Layer

The Transport layer defines what should be done with the data. This layer is based on the

following two popular protocols:

UDP is a very simple protocol and is perfect for streaming sequences (e.g., audio or video).

TCP is a highly reliable host-to-host protocol for a controlled connection. TCP is appropriate for

applications that require guaranteed delivery.

4) Application Layer

The Application layer includes all available software implementations (e.g., FTP, HTTP, SMTP, DNS, …) that make up the lower layers.

These applications can work only in combination with the API of TCP or UDP, which form the

software implementation of the Transport layer.

Troubleshooting of EtherNet

           Ethernet networks can present many symptoms, but troubleshooting can be helped by asking some common questions. By applying the principles below, our efforts can be more effective.

1) Is proper power supplied to switch and end devices?

2) Do the link LEDs indicate proper cable connections?

3) Are you having mis-communication along a path through multiple switches and routers? If so, check all link indicators and confirm that settings are consistent on all intervening devices.

4) Is the copper cabling compliant with EIA/TIA 568A or 568B as illustrated in fig.1? Mis-wired cables could show a proper link but may still have communication problems. On straight-through cable. Both ends MUST AGREE do not wire one end 568A and the other end 568B.

RJ-45 Pin	Signal	T568A Colors	T568B Colors
1.	TX	White/Green	White/Orange
2.	TX	Green	Orange
3.	RX	White/Orange	White/Green
4.	-	Blue	Blue
5.	-	White/ Blue	White/ Blue
6.	RX	Orange	Green
7.	-	White/Brown	White/Brown
8.	-	Brown	Brown

Fig 1. T568 Wire Color Variation

5) If you are using fiber optic cables, are they properly connected? The RX port on one device must connect to the TX port on the other device and vice versa.

6) If only one of the 2 devices terminating a fiber optic link indicates a valid link, could you have a “half-break”? when device A shows steady activity or a flashing link but device B shows no link, the TX cable from A could be damaged while its RX cable is intact.

7) Have you check your fiber optic cables to make sure they do not exceed allowable attenuation? Damaged fiber can impair the passage of signal and as fiber ages, its transmission efficiency diminishes.

8) Does your fiber optic link have a 10 Mbps device will generally communicate at different light frequencies and will be incompatible. (Typically 850 nm is used for 10 Mbps data and 1300 nm for 100 Mbps data.)

9) Are you connecting a single-mode fiber optic device to a multimode fiber optic device? These are typically incompatible.

10) Do any of your cables exceed the maximum distances in fig.2? Cables which exceed these distances may provide a proper link while masking possible communication issues.

Medium	Signaling & Data Rate	Minimum Required Cable	Maximum Segment Distance
Copper	10 BASE-TX	Category 3 UTP	100 m (328 ft)
	10 Mbps
Copper	100 BASE-TX	Category 5 UTP	100 m (328 ft)
	100 Mbps
Fiber	100 BASE-FX	1300 nm, multimode	Full-Duplex:
	100 Mbps	50/125 or 62.5 µm	2 km (6562 ft)
			Half-Duplex:
			412 m(1352 ft)
Fiber	100 BASE-FX	1300 nm, single-mode	Full-Duplex:
	100 Mbps		15 km (49213 ft)
			Half-Duplex:
			412 m (1352 ft)

Fig 2: Ethernet Cabling Parameters

11) Was a cable recently moved from one switch port to another? If so, wait 5 min for the address cache to refresh or cycle power on the switch. This time interval is programmable on managed switches.

12) Have you tried forcing a device or switch port to disregard auto-negotiation and operate in a modest state such as 10 Mbps/half-duplex? To control this at the switch requires a configurable or managed switch. Some high noise environments may have better communications at 10 Mbps. Also some end devices may only communicate at 10 Mbps.

13) Have you tried disabling Auto-MDIX since some devices may not implement this feature properly? This requires a configurable or managed switch.

14) Are your devices utilizing DHCP to assign their IP addresses? If so, is your DHCP server functioning properly so that your devices have properly receive their IP address assignments from your DHCP server?  Can you confirm that they have received IP address assignments?

15) If using TCP/IP for communication, have you checked for proper IP communication? A simple check is to connect computer to the switch & “ping”  those devices. If using a DHCP server, can you ping the server? 

16) Is your ping failing to communicate to all IP devices? If so, look at the counts of port transmissions and receptions for the ports in question (this requires a managed switch). This will help identify a bad port or device.

17) Are you certain your IP addresses allow proper communications between your devices? All devices must be in the same subnet unless a router is present to permit communication between different subnets.

18) Are VLANs active on the switch? All devices must be in the same VLAN if they are to communicate.

19) Is a trunk active on your switch that could be interfacing with your ability to communicate? End devices should not be connected to trunk ports.

20) Are ports disabled on your switch? Although a disabled port may indicating a proper link, it will not pass messages.

21) On a managed switch have you checked the error counts for relevant ports? High noise environments or poorly wired cables could cause errors that might interface with communications. If your cables are routed near high noise sources, try rerouting the cables away from high noise sources.

22) Can you successfully ping devices but failed to communicate at a higher level? Most managed switches allowed you to monitor traffic of specific switch ports. This can be done with port mirroring and application (such as Ethereal) to view the traffic.

Table1: Troubleshooting procedures for common Ethernet media problems

Selection of EtherNet

            Any Category 5 or higher cable will support 10/100BaseT traffic. However, Category 5

cabling will not necessarily provide the level of performance needed or survive long term

in a harsh high noise industrial environment. One of the most important factors in selecting the correct cable is having a thorough understanding the environment where the cables will installed. The planner should already understand the application needs (i.e. 10Mb/s, 100Mb/s…) and will need to focus on environmental and mechanical aspects of the cabling. To aid in understanding the environment, this guide is based on a concept called MICE (Mechanical, Ingress, Climatic/Chemical and Electromagnetic). Cabling attributes and performance can be defined for the industrial areas based on the environments and conditions as defined by MICE severity levels called classifications. This guide will focus on the minimum cabling requirements to provide adequate performance in areas of E1, E2, and E3 Electromagnetic interferences. Harsh attributes

such as Mechanical, Ingress and Chemical (M,I,C) will be relatively straight forward. Given this, it is not adequate to simply specify cabling as Cat 5e, Cat 6, shielded, or unshielded. This document will help the reader to select connectivity components based on zero mitigation. Mitigation is a method of converting one environment into a less harsh environment. Further, this document provides guidance in the selection of cabling components to minimize performance degradation due to effects from EMC.

Steps in Selecting Cables:

1. Determine the channel bandwidth requirements to suit the applications for example Channel Class or Category, see Table 3 below.

2. Determine cable type (shielded or unshielded)

3. Determine additional electrical attributes needed based on E1, E2 or E3 noise types and levels.

4. Select two pair or four pair cabling

5. Determine the M,I,C severity levels where the cables will be deployed

6. Determine additional attributes for the cabling, for example high flex, weld splatter etc.

    Each of the 6 steps will be described in detail in the following paragraphs. To help clarify each step, a cable specification will be determined based on an example justification. At the end of each step, the example will be specified in a table like the one shown here:

Table: Channel Bandwidth VS Cabling Categories

Example Cable Specification
Step 1 Channel	Step 2 Cable type	Step 3 EMI Performance	Step 4 2/4 pair	Step 5 M.I.C. severity classifications	Step 6 Additional Mech. attributes
Justification	Justification	Justification	Justification	Justification	Justification
Specification	Specification	Specification	Specification	Specification	Specification

There are a total of 6 steps in defining the correct cable(s) for the application(s) and environment(s). A network planner may have a need to specify any where from one or more cables dependent on the applications and number of environmental areas within the communications coverage area. The final number of cables will be based on cost and complexity of the network and level of mitigation. For example, cost may not be as important as having one cable construction in the entire system. In this case, a super cable, “one cable does all” may be specified. The planner may choose to reduce the number of cables needed by protecting the cables through mitigation techniques (Isolation and Separation). Any special attributes (defined in step 6 below), such as high flex, is certain to increase the cable count. Regardless of how many different cables are identified, the planner must consider the environment.

In general, all cabling and cabling components need to be complaint with TIA 568B and or ISO/IEC 11801 with the additional requirements.

Industrial EtherNet Cable Selection

Advantages of Ethernet

1..Conceptually Simple :

Ethernet is simply daisy-chained together with coax cable and "T" adapters. There are usually no hubs, transcievers, or other devices used.
2.Relatively Inexpensive: Due to the simplicity inherent in the design of Ethernet, it can be an inexpensive technology to implement. 
3.Noise Immunity :The coaxial cable used in a Ethernet network is very well shielded, and has a very high immunity from electrical noise caused by outside sources. 

Disadvantages of Ethernet

1.Difficult To Change: 
2.Fault Intolerant :
If any device or cable section attached to the network fails, it will most likely make the entire network go down.
3.Difficult Troubleshooting 
Ethernet networks are very difficult to troubleshoot. There is no easy way to determine what node or cable section is causing a problem, and the network must be troubleshoot by a "process of elimination." This can be very time consuming.
4.Specialized Cable 
The RG-58A/U coaxial cable used in Ethernet networks can not be used for any other purpose. In the event that the network is changed to another type, then the cable will have to be replaced.

Reconfiguring a ethernet is somewhat difficult to do once it is in place. Any changes to the network will result in at least some "down time," as the bus must be broken and a new section spliced in at the point of the break.