Of the types of network delays that can occur, few affect speeds over large networks more than network cable propagation delay. Propagation delay is the length of time it takes for a signal to propagate from one end of a circuit to another. The primary factor influencing it is the length of the cable, although other variables can affect it. Limiting delay to within a reasonable threshold keeps large networks functioning at as close to an optimal level as possible.
What is Propagation Delay?
Propagation delay is the length of time it takes for a signal to pass from one end of a network circuit to another. One expresses the measurement in nanoseconds. This is the critical reason for the limitation on network cable lengths, as the longer a cable is, the greater the potential delay. When the delay becomes too overpowering, it can result in the loss of control over communications, which renders a network effectively nonfunctional. Propagation delay is also affected by signal frequency. For this reason, technicians take network propagation delay measurements at 10 Mhz.
While this article focuses on network cable propagation delay, there are other sources and media in which propagation delay may occur. For instance, it is often associated with radio transmissions, and logic gates.
Network cable propagation delay should not be confused with the Nominal Velocity of Propagation (NVP), a different type of propagation measurement. Unlike propagation delay, which is a measurement of time, Nominal Velocity of Propagation is a measurement of velocity or speed. It measures the inherent velocity of signal travel relative to the speed of light in a vacuum. The letter “c” abbreviates the speed of light in formulae. NVP uses a percentage of c to express its value. For instance, it might be expressed as 75% or .75c. Structured wiring cables have NVP values of .6c to .9c.
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Causes of Propagation Delay
The root cause of network cable propagation delay is cable length. This fact is the reason that guidelines limit cable lengths during the network installation. In some cases, a cable up to 25 percent longer can be used in network applications when the installer cannot adjust the location of the nodes. In these cases, the cable may still offer a propagation delay measurement that falls within an acceptable range. However, it is advisable to shorten the distance or get a waiver from the customer in these cases, as using longer cables can result in network instability.
Temperature increases can also increase propagation delay, which is why a climate-controlled environment is important for computing equipment. Temperature increases can result in increased resistance in the transmission medium, which subsequently increases the signal’s delay between two points.
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Effects of Propagation Delay
Propagation delay contributes to the overall effect known as the “interconnect bottleneck.” The interconnect bottleneck limits performance based on the connections between components rather than the internal speed of the components themselves. The components of a network may process signals internally at a rate much higher than that which is achievable over the medium of the cable. Therefore, minimizing delay is a key factor in maximizing the speed of a network.
Any delay on a network results in decreased responsiveness and communication speed. Excess propagation delay will be noticeable in the daily operations of a network that suffers from it. When the propagation delay is extreme, it will result in a loss of control of communications entirely.
Guidelines for Propagation Delay
There is a variety of rules regarding acceptable network cable propagation delays and the measures taken to limit them. The first is that field testers must take measurements with the signal operating at 10 Mhz. This is because higher frequencies can decrease delay, thus skewing the results.
Also, structured wiring standards anticipate a maximum horizontal delay of 570 nS. Delays are variable between pairs in a network cable. Delay skew, which is the term for the difference in delay between pairs in a network cable, should not exceed 50 nS on any link segment up to 100 meters. All pairs must meet the requirement.
Other Types of Network Delay
While network cable propagation delay is a key component in determining network speeds, there are various types of network delay that may affect a given system. A few are outlined below.
Transmission delay is how long it takes put a packet onto a link. In other words, it’s the length of time required to put bits of data onto the wire or other communication medium. This delay is dependent on the amount of data to be transferred (length of the packet) and the bandwidth of the network. Bandwidth is the maximum rate of data transfer across a given path.
A queuing delay is the amount of time a job waits in line before the processing unit can execute it. Queuing delay is dependent on congestion. It is the difference in time between the arrival of a packet at the destination and its execution. There are three primary causes for this delay: originating switches, intermediate switches, or call receiver servicing switches.
Processing delay is the amount of time it takes a router to process an information unit. High-speed routers commonly experience delays on the order of microseconds or less. When coupled with other delays, they may cause a noticeable problem but on their own are largely negligible.
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Network cable propagation delay can cause several possible serious malfunctions on a network. As a result, it’s important to understand it and work to minimize it by following installation standards and testing each link segment during installation. Because the delay represents the interconnect bottleneck in a network setting, it’s a very important issue to address and keep tabs on.
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