Shielded Twisted-Pair (STP) | Types of Communications Media

Shielded twisted-pair (STP) cabling was first made popular by IBM when it introduced type classification for data cabling. Though more expensive to purchase and install than UTP, STP offers some distinct advantages. The current ANSI/TIA-568-C cabling standard recognizes IBM Type 1A horizontal cable, which supports frequency rates of up to 300MHz, but does not recommend it for new installations. STP cable is less susceptible to outside electromagnetic interference (EMI) than UTP cabling because all cable pairs are well shielded.

Not All UTP Is Created Equal!

Though two cables may look identical, their supported data rates can be dramatically different. Older UTP cables that were installed to support telephone systems may not even support 10Base-T Ethernet. The ANSI/TIA-568-C standard helps consumers choose the right cable (and components) for their application. The standard has been updated over the years and currently defines four categories of UTP cable: Categories 3, 5e, 6, and 6A. Here is a brief rundown of categories past and present:

• Category 1 (not defined by ANSI/TIA-568-C) This type of cable usually supports frequencies of less than 1MHz. Common applications include analog voice telephone systems. It was never included in any version of the 568 standard.
  
  
• Category 2 (not defined by ANSI/TIA-568-C) This cable type supports frequencies of up to 4MHz. It's not commonly installed, except in installations that use twisted-pair ARCnet and Apple LocalTalk networks. Its requirements are based on the original, proprietary IBM Cabling System specification. It was never included in any version of the 568 standard.
  
  
• Category 3 (recognized cable type in ANSI/TIA-568-C) This type of cable supports data rates up to 16MHz. This cable was the most common variety of UTP for a number of years starting in the late 1980s. Common applications include 4Mbps UTP Token Ring, 10Base-T Ethernet, 100Base-T4, and digital and analog telephone systems. Its inclusion in the ANSI/TIA-568-C standard is for voice applications.
  
  
• Category 4 (not defined by ANSI/TIA-568-C) Cable belonging to Category 4 was designed to support frequencies of up to 20MHz, specifically in response to a need for a UTP solution for 16Mbps Token Ring LANs. It was quickly replaced in the market when Category 5 was developed, as Category 5 gives five times the bandwidth with only a small increment in price. Category 4 was a recognized cable in the 568-A Standard, but was dropped from ANSI/TIA/EIA-568-B and also does not appear in ANSI/TIA-568-C.
  
  
• Category 5 (was included in ANSI/TIA/EIA-568-B for informative purposes only) Category 5 was the most common cable installed, until new installations began to use an enhanced version. It may still be the cable type most in use because it was the cable of choice during the huge infrastructure boom of the 1990s. It was designed to support frequencies of up to 100MHz. Applications include 100Base-TX, FDDI over copper, 155Mbps ATM over UTP, and, thanks to sophisticated encoding techniques, 1000Base-T Ethernet. To support 1000Base-T applications, the installed cabling system had to pass performance tests specified by TSB-95 (TSB-95 was a Telecommunications Systems Bulletin issued in support of ANSI/TIA/EIA-568-A, which defines additional test parameters). It is no longer a recognized cable type per the ANSI/TIA-568-C standard, but for historical reference purposes, Category 5 requirements, including those taken from TSB-95, are specified in ANSI/TIA-568-C.2.
  
  
• Category 5e (recognized cable type in ANSI/TIA-568-C) Category 5e (enhanced Category 5) was introduced with the TIA/EIA-568-A-5 addendum of the cabling standard. Even though it has the same rated bandwidth as Category 5, that is, 100MHz, additional performance criteria and a tighter transmission test requirement make it more suitable for high-speed applications such as Gigabit Ethernet. Applications are the same as those for Category 5 cabling. It is now the minimum recognized cable category for data transmission in ANSI/TIA-568-C.
  
  
• Category 6 (recognized cable type in ANSI/TIA-568-C) Category 6 cabling was officially recognized with the publication of an addition to ANSI/TIA/EIA-568-B in June 2002. In addition to more stringent performance requirements as compared to Category 5e, it extends the usable bandwidth to 250MHz. Its intended use is for Gigabit Ethernet and other future high-speed transmission rates. Successful application of Category 6 cabling requires closely matched components in all parts of the transmission channel, that is, patch cords, connectors, and cable.
  
  
• Category 6A or Augmented Category 6 (recognized cable type in ANSI/TIA-568-C) Category 6A cabling was officially recognized with the publication of ANSI/TIA/EIA-568-B.2-10 in February 2008. In addition to more stringent performance requirements as compared to Category 6, it extends the usable bandwidth to 500MHz. Its intended use is for 10 Gigabit Ethernet. Like Category 6, successful application of Category 6A cabling requires closely matched components in all parts of the transmission channel, that is, patch cords, connectors, and cable. 
  
  
• Category 7 (recognized cable type in ISO 11801) Category 7 is an ISO/IEC category suitable for transmission frequencies up to 1GHz. It is widely used in Europe and is gaining some popularity in the United States. It is not presently recognized in ANSI/TIA-568-C.

Some STP cabling, such as IBM types 1 and 1A cable, uses a woven copper-braided shield, which provides considerable protection against EMI. Inside the woven copper shield, STP consists of twisted pairs of wire (usually two pairs) wrapped in a foil shield. Some STP cables have only the foil shield around the wire pairs.

New Nomenclature for Twisted-Pair Cables

TIA is addressing the potentially confusing nomenclature for different types of twisted-pair cables:

• Shielded twisted-pair (STP) will be called U/FTP.
• Screened twisted-pair (ScTP or FTP) will be called F/UTP.
• Category 7 screened shielded twisted-pair (S/STP or S/FTP) will be called ScFTP.

Figure 1 shows a typical STP cable. In the IBM design, the wire used in STP cable is 22 AWG (just a little larger than the 24 AWG wire used by typical UTP LAN cables) and has a nominal impedance of 150 ohms, but category versions can have a nominal impedance of 100 ohms.

 

Figure 1: STPcable

Constructions of STP in 24 AWG, identical in copper conductor size to UTP cables, are more commonly used today.

Simply installing STP cabling does not guarantee you will improve a cable's immunity to EMI or reduce the emissions from the cable. Several critical conditions must be met to achieve good shield performance:

• The shield must be electrically continuous along the whole link.
• All components in the link must be shielded. No UTP patch cords can be used.
• The shield must fully enclose the pair, and the overall shield must fully enclose the core. Any gap in the shield covering is a source of EMI leakage.
• The shield must be grounded at both ends of the link, and the building grounding system must conform to grounding standards (such as J-STD-607-A).

If even one of these conditions is not satisfied, shield performance will be badly degraded. For example, tests have shown that if the shield continuity is broken, the emissions from a shielded cabling system increase by 20dB on the average.