ssue is that PDV happens to a different extent in both directions, which is asymmetric. Again, this is some- thing that having PTP awareness helps avoid. Avoid layer 3 transport: L3 lends itself to useful techniques such as link bundling and path load-sharing that do not have mechanisms to ensure traffic is treated similarly in both direc- tions, which will add asymmetry. Non-dynamic routing: A routing system can quite readily calculate preferred routing solutions between two end points that take different paths in each direction. For more details of various methods that can cause asymmetry, especially in the L3 (PTPoIP) con- text with PTP-unaware nodes, consult the section "Partial Timing Support with G.8275.2 and GMs at the Edge" in Chapter 11. Static Asymmetry Static asymmetry is a form of asymmetry that the engineer can deal with; static means asymmetry that is fixed and unlikely to change without a significant event (more than reloading a router). The classic case of asymmetry is that arising from differences in the cable length between the transmit and receiver fiber in a fiber pair. Every extra meter of fiber requires about an extra 5 ns of time for the laser pulses to traverse. The case where an interface optic is patched with a 5-m cable in one dir- ection and a 10-m cable in the other direction will introduce about 25 ns of asymmetry. This is one reason why engineers building extremely accurate time transfer use bidirectional fiber (the forward and reverse paths use different As inside the same single fiber). There are two basic approaches to dealing with static asymmetry: Estimate it from first principles Measure it against an external source of time, if possible In the first approach, design engineers may understand that some optical and transport devices might have unique and fixed asymmetries. If they know those values and how many of each device is used in building the network link, then they can model an estimate of the asymmetry. Another ex- ample might be compensating for the different rates of 