Tip of the Week:
Latency is signal delay which is caused by the transmission equipment and the transmission medium (fiber for this industry). For Ethernet systems it refers to packet delay. It’s typically measured in units of milliseconds (“ms” 1E-3 seconds), microseconds (“us” 1E-6 seconds) or nanoseconds (“ns” 1E-9 second).
Light travels much slower in a glass fiber than in air and therefore signal latency in a fiber is a much greater in a length of fiber than an equal distance between microwave towers.
A 100 km (62 mile) length of standard single mode fiber has a latency of about 0.49 ms. Therefore, if a fiber communications system is looking for a response from a remote site 100km away, the response will take at least 1 ms to occur if there is no additional delay from the transmission equipment. Latency for signals passing through equipment such as repeaters, switches, routers, and SONET equipment can vary greatly from a few nanoseconds to more than 1 second.
To calculate the overall link latency, sum up all the individual equipment latencies and add it to the overall fiber latency. Equipment latency is available from the manufacturer. If you are using standard single mode fiber, its latency is 4.9 us/km.
Latency is often specified in service-level agreements (SLA).
For voice traffic, a round-trip delay of 250 ms or greater is noticeable by the speakers. Some SLA require round trip latency to be less than 60 ms.
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