The least expensive and most widely used guided transmission medium is twisted pair.
Physical Description
A twisted pair consists of two insulated copper wires arranged in a regular spiral pattern. A wire pair acts as a single communication link.102 Frequency
(Hertz) 103 104 105 106 107 108 109 1010 1011 1012 1013 1014 1015
Power and telephone
Rotating generators Musical instruments Voice microphones
Microwave
Radar
Microwave antennas Magnetrons
Radio
Radios and televisions Electronic tubes Integrated circuits Cellular telephony
ELF Extremely low frequency VF Voice frequency
VLF Very low frequency LF Low frequency
MF Medium frequency HF High frequency VHF Very high frequency
UHF Ultra high frequency SHF Super high frequency EHF Extremely high frequency Twisted pair
Coaxial cable
Visible light
Optical fiber
FM radio and TV
AM radio Terrestrial
and satellite transmission Wavelength
in space (meters)
106 105 104 103 102 101 100 101 102 103 104 105
ELF VF VLF LF MF HF VHF SHF EHF
106 Infrared
Lasers Guided missiles Rangefinders
UHF
Figure 4.1 Electromagnetic Spectrum for Telecommunications
105
106
CHAPTER 4 / TRANSMISSION MEDIATable 4.1 Point-to-Point Transmission Characteristics of Guided Media [GLOV98]
Frequency Range Typical Attenuation Typical Delay Repeater Spacing Twisted pair (with 0 to 3.5 kHz 0.2 dB/km @ 1 kHz 2 km loading)
Twisted pairs 0 to 1 MHz 0.7 dB/km @ 1 kHz 2 km
(multipair cables)
Coaxial cable 0 to 500 MHz 7 dB/km @ 10 MHz 1 to 9 km Optical fiber 186 to 370 THz 0.2 to 0.5 dB/km 40 km THz =TeraHertz= 1012 Hz
5ms/km 4ms/km 5ms/km 50ms/km
Outer sheath
(b) Coaxial cable
—Outer conductor is braided shield
—Inner conductor is solid metal
—Separated by insulating material
—Covered by padding
Insulation Inner conductor Outer
conductor
(c) Optical fiber
Light at less than critical angle is absorbed in jacket
Angle of incidence
Angle of reflection
—Glass or plastic core
—Laser or light emitting diode
—Specially designed jacket
—Small size and weight Core
Jacket Cladding
(a) Twisted pair
Twist length
—Separately insulated
—Twisted together
—Often "bundled" into cables
—Usually installed in building during construction
Figure 4.2 Guided Transmission Media
4.1 / GUIDED TRANSMISSION MEDIA
107
Typically, a number of these pairs are bundled together into a cable by wrapping them in a tough protective sheath. Over longer distances, cables may contain hundreds of pairs. The twisting tends to decrease the crosstalk interference between adjacent pairs in a cable. Neighboring pairs in a bundle typically have somewhat dif- ferent twist lengths to reduce the crosstalk interference. On long-distance links, the twist length typically varies from 5 to 15 cm. The wires in a pair have thicknesses of from 0.4 to 0.9 mm.
Applications
By far the most common guided transmission medium for both ana- log and digital signals is twisted pair. It is the most commonly used medium in the telephone network and is the workhorse for communications within buildings.In the telephone system, individual residential telephone sets are connected to the local telephone exchange, or “end office,” by twisted-pair wire. These are referred to as subscriber loops. Within an office building, each telephone is also con- nected to a twisted pair, which goes to the in-house private branch exchange (PBX) system or to a Centrex facility at the end office. These twisted-pair installations were designed to support voice traffic using analog signaling. However, by means of a modem, these facilities can handle digital data traffic at modest data rates.
Twisted pair is also the most common medium used for digital signaling. For connections to a digital data switch or digital PBX within a building, a data rate of 64 kbps is common. Twisted pair is also commonly used within a building for local area networks supporting personal computers. Data rates for such products are typ- ically in the neighborhood of 100 Mbps. However, twisted-pair networks with data rates of to 10 Gbps have been developed, although these are quite limited in terms of the number of devices and geographic scope of the network. For long-distance applications, twisted pair can be used at data rates of 4 Mbps or more.
Twisted pair is much less expensive than the other commonly used guided transmission media (coaxial cable, optical fiber) and is easier to work with.
Transmission Characteristics
Twisted pair may be used to transmit both ana- log and digital transmission. For analog signals, amplifiers are required about every 5 to 6 km. For digital transmission (using either analog or digital signals), repeaters are required every 2 or 3 km.Compared to other commonly used guided transmission media (coaxial cable, optical fiber), twisted pair is limited in distance, bandwidth, and data rate. As Figure 4.3a shows, the attenuation for twisted pair is a very strong function of fre- quency. Other impairments are also severe for twisted pair. The medium is quite susceptible to interference and noise because of its easy coupling with electromag- netic fields. For example, a wire run parallel to an ac power line will pick up 60-Hz energy. Impulse noise also easily intrudes into twisted pair. Several measures are taken to reduce impairments. Shielding the wire with metallic braid or sheathing reduces interference. The twisting of the wire reduces low-frequency interference, and the use of different twist lengths in adjacent pairs reduces crosstalk.
For point-to-point analog signaling, a bandwidth of up to about 1 MHz is pos- sible. This accommodates a number of voice channels. For long-distance digital point-to-point signaling, data rates of up to a few Mbps are possible; for very short distances, data rates of up to 10 Gbps have been achieved in commercially available products.
(a) Twisted pair (based on [REEV95])
(b) Coaxial cable (based on [BELL90])
(c) Optical fiber (based on [FREE02])
(d) Composite graph 107
106 105
104 103
102
Frequency (Hz)
Attenuation (dB/km)
0 5 10 15 20 25 30
26-AWG (0.4 mm) 24-AWG (0.5 mm) 22-AWG (0.6 mm) 19-AWG (0.9 mm)
Attenuation (dB/km)
0 0.5 1.0 1.5 2.0 2.5 3.0
800 900 1000 1100 1200 1300 1400 1500 1600 1700
Wavelength in vacuum (nm)
1015 1012
1 THz 109
1 GHz 103
1 kHz
106 1 MHz
0.5 mm twisted pair
Attenuation (dB/km)
0 5 10 15 20 25 30
Frequency (Hz)
typical optical fiber 9.5 mm
coax
108 107
106 105
Frequency (Hz)
Attenuation (dB/km)
0 5 10 15 20 25 30
3/8" cable (9.5 mm)
Figure 4.3 Attenuation of Typical Guided Media
108
4.1 / GUIDED TRANSMISSION MEDIA
109 Unshielded and Shielded Twisted Pair
Twisted pair comes in two varieties:unshielded and shielded. Unshielded twisted pair (UTP) is ordinary telephone wire.
Office buildings, by universal practice, are prewired with excess unshielded twisted pair, more than is needed for simple telephone support. This is the least expensive of all the transmission media commonly used for local area networks and is easy to work with and easy to install.
Unshielded twisted pair is subject to external electromagnetic interference, including interference from nearby twisted pair and from noise generated in the envi- ronment. A way to improve the characteristics of this medium is to shield the twisted pair with a metallic braid or sheathing that reduces interference.This shielded twisted pair (STP) provides better performance at higher data rates. However, it is more expensive and more difficult to work with than unshielded twisted pair.
Category 3 and Category 5 UTP
Most office buildings are prewired with a type of 100-ohm twisted pair cable commonly referred to as voice grade. Because voice-grade twisted pair is already installed, it is an attractive alternative for use as a LAN medium. Unfortunately, the data rates and distances achievable with voice- grade twisted pair are limited.In 1991, the Electronic Industries Association published standard EIA-568, Commercial Building Telecommunications Cabling Standard, which specifies the use of voice-grade unshielded twisted pair as well as shielded twisted pair for in-building data applications. At that time, the specification was felt to be adequate for the range of frequencies and data rates found in office environments. Up to that time, the prin- cipal interest for LAN designs was in the range of data rates from 1 Mbps to 16 Mbps.
Subsequently, as users migrated to higher-performance workstations and applica- tions, there was increasing interest in providing LANs that could operate up to 100 Mbps over inexpensive cable. In response to this need, EIA-568-A was issued in 1995. The new standard reflects advances in cable and connector design and test meth- ods. It covers 150-ohm shielded twisted pair and 100-ohm unshielded twisted pair.
EIA-568-A recognizes three categories of UTP cabling:
• Category 3:UTP cables and associated connecting hardware whose transmis- sion characteristics are specified up to 16 MHz
• Category 4:UTP cables and associated connecting hardware whose transmis- sion characteristics are specified up to 20 MHz
• Category 5:UTP cables and associated connecting hardware whose transmis- sion characteristics are specified up to 100 MHz
Of these, it is Category 3 and Category 5 cable that have received the most attention for LAN applications. Category 3 corresponds to the voice-grade cable found in abundance in most office buildings. Over limited distances, and with proper design, data rates of up to 16 Mbps should be achievable with Category 3. Category 5 is a data-grade cable that is becoming standard for preinstallation in new office buildings. Over limited distances, and with proper design, data rates of up to 100 Mbps are achievable with Category 5.
A key difference between Category 3 and Category 5 cable is the number of twists in the cable per unit distance. Category 5 is much more tightly twisted, with a typical twist length of 0.6 to 0.85 cm, compared to 7.5 to 10 cm for Category 3. The
110
CHAPTER 4 / TRANSMISSION MEDIAtighter twisting of Category 5 is more expensive but provides much better perform- ance than Category 3.
Table 4.2 summarizes the performance of Category 3 and 5 UTP, as well as the STP specified in EIA-568-A. The first parameter used for comparison, attenuation, is fairly straightforward. The strength of a signal falls off with distance over any transmission medium. For guided media attenuation is generally exponential and therefore is typically expressed as a constant number of decibels per unit distance.
Near-end crosstalk as it applies to twisted pair wiring systems is the coupling of the signal from one pair of conductors to another pair. These conductors may be the metal pins in a connector or wire pairs in a cable. The near end refers to coupling that takes place when the transmit signal entering the link couples back to the receive conductor pair at that same end of the link (i.e., the near transmitted signal is picked up by the near receive pair).
Since the publication of EIA-568-A, there has been ongoing work on the development of standards for premises cabling, driven by two issues. First, the Gigabit Ethernet specification requires the definition of parameters that are not specified completely in any published cabling standard. Second, there is a desire to specify cabling performance to higher levels, namely Enhanced Category 5 (Cat 5E), Category 6, and Category 7. Tables 4.3 and 4.4 summarize these new cabling schemes and compare them to the existing standards.
Table 4.2 Comparison of Shielded and Unshielded Twisted Pair
Attenuation (dB per 100 m) Near-End Crosstalk (dB) Frequency Category 3 Category 5 150-ohm Category 3 Category 5
(MHz) UTP UTP STP UTP UTP 150-ohm STP
1 2.6 2.0 1.1 41 62 58
4 5.6 4.1 2.2 32 53 58
16 13.1 8.2 4.4 23 44 50.4
25 — 10.4 6.2 — 41 47.5
100 — 22.0 12.3 — 32 38.5
300 — — 21.4 — — 31.3
Category 3 Category 5 Category 5E Category 6 Category 7
Class C Class D Class E Class F
Bandwidth 16 MHz 100 MHz 100 MHz 200 MHz 600 MHz
Cable Type UTP UTP/FTP UTP/FTP UTP/FTP SSTP
Link Cost 0.7 1 1.2 1.5 2.2
SSTP= Shielded screen twisted pair FTP= Foil twisted pair
UTP = Unshielded twisted pair 1Cat 512
Table 4.3 Twisted Pair Categories and Classes
4.1 / GUIDED TRANSMISSION MEDIA
111
Table 4.4 High-Performance LAN Copper Cabling Alternatives [JOHN98]
Name Construction Expected Performance Cost
Cable consists of 4 pairs of 24 AWG Mixed and matched cables and (0.50 mm) copper with thermoplastic connecting hardware from various Category 5 polyolefin or fluorinated ethylene manufacturers that have a reasonable 1 UTP propylene (FEP) jacket. Outside sheath chance of meeting TIA Cat 5 Channel
consists of polyvinylchlorides (PVC), a and ISO Class D requirements. No fire retardant polyolefin or fluoropolymers. manufacturer’s warranty is involved.
Cable consists of 4 pairs of 24 AWG Category 5 components from one (0.50 mm) copper with thermoplastic supplier or from multiple suppliers polyolefin or fluorinated ethylene where components have been
Enhanced Cat 5 propylene (FEP) jacket. Outside sheath deliberately matched for improved 1.2 UTP (Cat 5E) consists of polyvinylchlorides (PVC), a fire impedance and balance. Offers ACR
retardant polyolefin or fluoropolymers. performance in excess of Cat 5 Higher care taken in design and Channel and Class D as well as a
manufacturing. 10-year or greater warranty.
Cable consists of 4 pairs of 0.50 to 0.53 mm Category 6 components from one copper with thermoplastic polyolefin or supplier that are extremely well fluorinated ethylene propylene (FEP) jacket. matched. Channel zero ACR point Category 6 Outside sheath consists of polyvinylchlorides (effective bandwidth) is guaranteed
UTP (PVC), a fire retardant polyolefin or to 200 MHz or beyond. Best available 1.5 fluoropolymers. Extremely high care taken UTP. Performance specifications
in design and manufacturing. Advanced for Category 6 UTP to 250 MHz
connector designs. are under development.
Cable consists of 4 pairs of 24 AWG Category 5 components from one (0.50 mm) copper with thermoplastic supplier or from multiple suppliers polyolefin or fluorinated ethylene propylene where components have been
Foil Twisted (FEP) jacket. Pairs are surrounded by a deliberately designed to minimize 1.3 Pair common metallic foil shield. Outside sheath EMI susceptibility and maximize
consists of polyvinylchlorides (PVC), a fire- EMI immunity. Various grades may retardant polyolefin or fluoropolymers. offer increased ACR performance.
Cable consists of 4 pairs of 24 AWG Category 5 components from one (0.50 mm) copper with thermoplastic supplier or from multiple suppliers polyolefin or fluorinated ethylene where components have been propylene (FEP) jacket. Pairs are deliberately designed to minimize
Shielded Foil surrounded by a common metallic foil EMI susceptibility and maximize EMI 1.4 Twisted Pair shield, followed by a braided metallic immunity. Offers superior EMI
shield. Outside sheath consists of protection to FTP.
polyvinylchlorides (PVC), a fire retardant polyolefin, or fluoropolymers
Also called PiMF (for Pairs in Metal Foil), Category 7 cabling provides positive SSTP of 4 pairs of 22-23AWG copper with a ACR to 600 to 1200 MHz. Shielding thermoplastic polyolefin or fluorinated on the individual pairs gives it Category 7 ethylenepropylene (FEP) jacket. Pairs are phenomenal ACR.
Shielded-Screen individually surrounded by a helical or 2.2
Twisted Pair longitudinal metallic foil shield, followed by a braided metallic shield. Outside sheath of polyvinylchlorides (PVC), a fire-retardant polyolefin, or fluoropolymers.
EMI= Electromagnetic interference ACR =Attenuation to crosstalk ratio