Bill's Blog on Industrial Ethernet

Our apologies to our visitors for today's outage. For about 3 hours, access was spotty and sometimes impossible. Our ISP informed us that our server had a "bent backplane" and required a replacement. As of this posting, things seemed to have returned to normal, but don't be surprised if slow access persists for some time yet.

Please be patient. Thank you!

In Ethernet, cut-through switching allows a switch to start forwarding a frame before the whole frame is received. This occurs as soon as the destination address is processed. Cut-through reduces latency (the time to get through the switch), but communication reliability suffers as a result.

Cut-through switching is a disadvantage for reliability because the frame checksum (FCS) must be checked by every device in the path. Since the checksum cannot be checked until the entire packet has been received, a corrupted packet can be forwarded.

Recently a customer called to ask which products from Contemporary Controls offer cut-through switching. He had been searching for such a switch to satisfy an IT departmental demand for cut-through functionality, but his search was largely unsuccessful. I had to answer that only one of our product lines (the EIS Series) offers this feature.

Why is cut-through functionality becoming hard to find? This has happened for a number of reasons.

Many years ago when Shared Ethernet was the principal Ethernet technology, cut-through was of more concern due to the 10 Mbps data rate. As Fast Ethernet and 100 Mbps operation grew in popularity, cut-through became less important. The duration of 100 Mbps packets is one-tenth that of 10 Mbps packets, and latency is proportionately better in Fast Ethernet. As a minor issue, faster signaling exposes the data stream to greater reliability threats -- and since cut-through already degrades reliability, store-and-forward operation appeared more attractive.

Probably the main reason for the decline of cut-through switching is the queuing backlog. This occurs when a packet that is ostensibly cut-through, nevertheless must be stored due to traffic already passing through the same port. The queue bottleneck could worsen when several devices must communicate through a single channel to a server.

Cut-through switching is of greater concern in certain critical real-time networks such as PowerLink or in some non-Ethernet networks such as InfiniBand. In common Industrial Ethernet applications, however, cut-through has become an obsolete method of packet switching. Because of its limited use in somewhat specialized markets, cut-through availability in the future will likely be an alternative that is hard to find.

The ODVA CIP Networks Conference and 11th Annual Meeting was held at the beautiful Pointe Hilton Tapatio Cliffs Resort (see photo) in Phoenix on February 22, 2006. Two Contemporary Controls attendees were R & D Manager Bennet Levine (right) and Software Engineer Harpartap Parmar (left). A good time was had by all.

Among the several presentations were ones on Real-Time Ethernet and on Designing Ethernet for Harsh Environments. Both of these papers have important insights into resolving tough issues. More photos and downloadable presentations and papers can be accessed at:

www.odva.org/10_2/09_down/09_present.htm

I have received many calls about problems that were due to one of the most common issues facing network technicians: improperly installing twisted-pair cable. The problem of improper pairing presents mystifying symptoms that arise from crosstalk. The problem can be difficult to detect and cause corrupted or even total failure of data transfer. Perhaps the most common symptom is the failure of two ports to properly link with the Auto-MDIX feature.

Today, most twisted-pair LAN wiring is Category 5 (CAT5) unshielded twisted pair (UTP) cable. CAT5 UTP has 4 pairs, making a total of 8 wires. Each wire will have a single strand (solid) for running through walls and ceilings or multiple strands (flex) for patch and drop cables.

Pairs are color coded to make it easier to identify the same wire at each end of the cable. But more importantly, each pair uses the same color so pairs can be more readily identified from end to end.

Each of the 4 pairs has a wire of a solid color and its mate has the same color applied as a stripe over white insulation. EIA/TIA Standard 568B calls for the colors to be blue, orange, green and brown.

The most commonly used connectors with CAT5 UTP are RJ-45. The "RJ" stands for Registered Jack and "45" specifies the pin-numbering scheme.

Signals for 10BASE-T are carried on just two pairs: orange on pins 1/2 and green on pins 3/6. The other pairs are tied to pins 4/5 and 7/8.

The chart below illustrates the "right" and "wrong" way to wire an RJ-45 connector. The column entitled "Wrong Pins" depicts the common-sense pattern where each wire is placed adjacent to its pair partner. The "Right Pins" column shows the proper (but counter- intuitive) arrangement with the green pair split between pins 3 & 6. That is, you would expect the green pair to occupy pins 3 and 4 -- not 3 and 6!

Note that a simple continuity check will not reveal improper pairing!

Welcome to Bill's Blog, launched March 7, 2006.

Hi, I am Bill Greer, Senior Product Specialist at Contemporary Controls where we are celebrating our 30th year as a leading manufacturer of Industrial Networking Equipment. My purpose in creating and moderating this blog is to share my thoughts, experiences and occasional insights on Industrial Ethernet.

Working our Help Desk is one of my responsibilities, and I consider this blog to be an extension of that effort -- presenting information that I glean from my daily job experience. The topics will range from simple (but tricky) issues to those that were downright baffling at the outset but fascinating once our team of engineers solved them.

If you have encountered some noteworthy Industrial Ethernet issues -- whether mind-bending or merely humorous -- feel free to post a comment and thus share it with our readers.

I will also post news and developments from the world of Industrial Ethernet when these items seem particularly relevant to our readers.

Check back often -- here you just might find the solution that has been eluding you! I hope you find these posts both enjoyable and helpful.

Thanks for reading -- Bill Greer.