What is delay skew?

What is delay skew?

To gain an understanding of delay SKEW and why it is so important, it is first necessary to understand the concept of propagation delay. Measured in nanoseconds per meter of cable, propagation delay is an expression of the amount of time a signal is delayed when propagated through a length of wire. Delay SKEW is the result obtained when comparing the propagation delay between any two wiring pairs within the same cable.

The concept of delay SKEW was first introduced in 1997 in an addendum issued by the Telecommunications Industry Association (TIA), which referenced the existing commercial building cabling standard. This announcement was intended to establish performance requirements for screened twisted pair cables and unshielded twisted pair for applications using multiple pairs in parallel transmission within a cable.

Why delay SKEW is so important

The reason delay SKEW became an issue which needed to be addressed at this time was that some cable manufacturers had begun the practice of using different jacketing materials for some wire pairs within the same cable sheath. Since insulating materials affect propagation delay along the copper wire, it then became possible for wire pairs sheathed with different insulating materials to introduce transmission errors into networks, although only within those networks which used more than one wire pair in the transmission and reception of signals.

Key factors in the calculation of delay SKEW are the jacketing materials used in conductors and the cable geometry, which refers to how wiring pairs are twisted within a cable. It is standard practice for manufacturers to twist each wiring pair in a multi-pair cable at a slightly different rate so as to reducecrosstalk. Since wiring pairs are twisted at slightly different rates, they end up with slightly different lengths and slightly different delay SKEWs.

Due to the differing propagation delay rates between these wiring pairs, it was thus possible for signals transmitted sequentially over different wiring pairs to be received at the other end out of sequence. Obviously, the introduction of network errors by the very equipment used to enable network transmissions would be a critical configuration flaw.

The TIA requirement

In an effort to avoid these kinds of errors, the TIA addendum established that the delay SKEW between the fastest and the slowest pair of wires in a cable must be less than 45 nanoseconds per hundred meters. Once this standard was established, it then became necessary to develop tools which could measure the delay inherent in network cabling.

Net Chaser

Because it is so important to know the delay SKEW for the wiring pairs of a cable, many types of measuring devices have been developed for just that purpose. One of the best of these is Net Chaser Ethernet Speed Certifier, which tests and speed-certifies the data transmission of ethernet cables.

Net Chaser was designed for cable testing in accordance with the requirements of TIA568A/B, which was the 1997 addendum issued by TIA, and it detects noise in the network, as well as faults in cable wiring, and it ensures that cables can support speed capabilities of active equipment in the network. Net Chaser Ethernet Speed Certifier is proudly made available to network cable installers and home automation integrators by the T3 Innovation company.