e on the sender be synchronized with the time on the receiver. Let's look at an example. Imagine that the sender sends a packet out and timestamps it as 14.100100 seconds past 12:30. Now, this is a fast and short link, and, say, 10 us later the receiver sees the packet and timestamps it. If the receiver's clock is accurately aligned with the sender, then the correct timestamp would be 14.100110 (14.100100 + 0.000010). However, what if the clock on the receiver is 50 us slower than the clock on the sender? Then the receiver would timestamp it with 14.100060, which would show the packet arrived 40 us before it was sent! Therefore, the basic rule with delay and latency measurement is that the 2DM does not need time synchronization because the transmitted and received timestamps are from the same device with the same clock. However, the 1DM measurement method requires very accurate time synchronization between the two timestamping devices. The lower the expected delay, the more closely the clocks must be synchronized in order to obtain an accurate measure. If you want to measure a link with a delay of tens of microseconds, then the time accuracy must be in microseconds (or less) or nonsensical results are possible and inaccurate results likely. The answer to this problem lies in solutions combining GNSS time sources and a well-designed PTP network that can guarantee time synchronization to the level of microseconds. 2.2.1 Circuit Emulation Although these days most network engineers are used to dealing with optical and Ethernet transport systems, the reality is that many devices around the world are still interconnected using "legacy" TDM circuits such as those based on SDH or SONET. Not only end-users and large enterprises, but major service providers still provide a large number of data circuits based on TDM-type technolo- gies, with buildings literally full of equipment to support it. The problem with service providers using TDM technology is that much of this equipment is bey- ond its useful life and no longer sup