Information Technology Reference
In-Depth Information
Another characteristic of twisted-pair cable is called propagation delay . 1
Propagation delay is defined as the time, usually in nanoseconds, for a signal pulse
to travel from one end to the other end of a 100 m cable.
The requirements for propagation delay are shown in Table 5.1. In multiple-
pair cables, another parameter is measured, the delay skew , defined as the difference
in propagation delay between pairs in the cable. Delay skew must be held to a min-
imum for LAN signaling methods that split up the transmitted data and send it
simultaneously on more than one pair. Examples of this technique are 100BaseT4,
1000BaseT (T4 and TX), ATM-155, and ATM-622. If the delay skew is too great,
the data pulses will arrive at improper times, and the receiver circuitry will be
unable to reassemble the data packets.
Installation practices can adversely impact twisted wire cable performance.
The impedance characteristics can be significantly disturbed by any distortion of the
cable insulation or cable twist. These disturbances can be easily observed on a
device called a time-domain reflectometer (TDR). When connected to a TDR, a
cable with a normally flat response will show all sorts of lumps and bumps when
the cable is sharply bent, stapled, tie-wrapped, and partially untwisted. These dis-
tortions in the cable may cause the installed cable to fail the final installed testing
and should therefore be avoided.
UTP/STP Insulation
Two primary types of insulation materials are traditionally used in twisted-pair
LAN wiring, polyvinyl chloride (PVC) and fluorocarbon polymers. PVC is very flex-
ible and is often used with stranded wire for LAN user cords and patch cords, where
its flammability is not a disadvantage.
TABLE 5.1
Propagation Delay and Delay Skew Requirements*
Category 5/ Category 5e Category 6 Category 7
TSB-95 Class D Class D (revised) Class E Class F
Propagation delay, ns 548 5 48 548 504
Delay skew, ns 50 50 50 20
*Category requirements are for TIA/EIA-568-C and anticipated revisions for Category 7. Class requirements are for
ISO/IEC 11801 and anticipated revisions.
1 Propagation delay is inversely related to the velocity of propagation of the cable. The velocity of
propagation is normally given as a decimal fraction (or a percentage) of the speed of light in a vacuum.
Although twisted-pair cables have this parameter, it is a more common specification for coaxial cable.
Propagation delay is more commonly found in twisted-pair specs, although either parameter can eas-
ily be calculated from the other.
 
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