Hi Paolo, I worked for a few years in the part of Bell Labs that was involved with networking cable systems several years ago. I am suspicious of foil shielded cables as it is difficult to properly terminate the foil shield (360 degrees). I have had a foil shield crack and split if the cable is flexed very much.
On a related topic, a good UTP (unshielded twisted pair) system can have lower emissions and higher immunity than most shielded systems. The interface design is not hard to do to make this happen. I have seen cases where the presense of the shield causes immunity problems because a large noise current lands on the chassis of the data equipment. It there are slits and holes in the chassis (back of a PC?) problems result. Take a peek at the paper: http://www.dsmith.org/pdf/roma94.pdf which I authored several years ago and presented at the EMC Roma conference. In that paper a couple of commercial STP systems were compared with a good UTP system at a test lab in Switzerland. The UTP system won hands down on both emissions and immunity! The presense of a shield, by itself, does not guarantee good performance. I seem to remember also that the shielded systems had higher losses for signal transmission than UTP which resulted in shorter permissable cable lengths. Paolo, we can go offline if you want to discuss more details. Either call or email me. Doug Paolo Roncone wrote: > > Hi all, > > anybody out there can explain the difference between FTP (Foiled > Twisted Pair) and STP (Shielded Twisted Pair) cables used for Ethernet > links. > In Ethernet standard IEEE 802.3 (2000) I find only a definition of STP > (herebelow in copied and pasted from the standard - page 28): > In a vendor's catalog, I found a definition of FTP as > > <<1.4.249 shielded twisted-pair (STP)cable: An electrically conducting > cable,comprising one or more ele- > ments,each of which is individually shielded.There may be an o erall > shield,in which case the cable is > referred to as shielded twisted-pair cable with an o erall shield > (from ISO/IEC 11801:1995).Speci .cally > for IEEE 802.3 100BASE-TX,150 . balanced inside cable with performance > characteristics speci .ed to > 100 MHz (i.e.,performance to Class D link standards as per ISO/IEC > 11801:1995).In addition to the > requirements speci .ed in ISO/IEC 11801:1995,IEEE 802.3 Clauses 23 and > 25 provide additional perfor- > mance requirements for 100BASE-T operation o er STP.>> > > The reason of my inquiry is that we bought samples of "STP" and "FTP" > cat.5 cables for 10bT ethernet applications from different vendors and > to our surprise we discovered that both "STP" and "FTP" types have an > overall (external) shield made of aluminum foil, but no shields on > individual wires or wire couples (as per 802.3 definition above). > > Any inputs, suggestions etc. would be appreciated. > > Paolo > > ------------------------------------------------------------------------------- > Paolo Roncone > EMC Compliance Engineer - Cisco Photonics Italy > via Philips 12 - Monza (MI) 20052 > mailto:pronc...@cisco.com > phone: +39 039209 1538 > fax: +39 039209 2036 -- ------------------------------------------------------- ___ _ Doug Smith \ / ) P.O. Box 1457 ========= Los Gatos, CA 95031-1457 _ / \ / \ _ TEL/FAX: 408-356-4186/358-3799 / /\ \ ] / /\ \ Mobile: 408-858-4528 | q-----( ) | o | Email: d...@dsmith.org \ _ / ] \ _ / Website: http://www.dsmith.org ------------------------------------------------------- ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson: pstc_ad...@garretson.org Dave Heald davehe...@mediaone.net For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: No longer online until our new server is brought online and the old messages are imported into the new server.
