PTP messages as packet frames, or what is called over-the-top. This term refers to a situation where the PTP is carried just like every other IP or L2 frame, without any special handling beyond the usual QoS mechanisms. The following is a list of some popular technologies that make over-the-top PTP difficult. They tend to be broadband access-type methods, which tend to be asymmetric because they are designed for consumer broadband, which has asymmetric traffic patterns (more download than upload). Because these systems have multiple generations, optional topologies and numerous versions, a detailed treatment would be overwhelming, SO here are just the highlights: Cable: From the beginning, cable was inherently asymmetric, given that it was originally de- signed for one-way video distribution. Earlier versions of DOCSIS data services began using time to allocate cable modem slots for upstream transmission. Over time, cable systems are re- ducing this effect, and DOCSIS 4.0 will be another step in that direction. DWDM optical: Modern optical systems have levels of buffering built into them that suffer from non-static and asymmetric latency changes. The latest versions of these systems have in- troduced techniques to become PTP/timing aware. The techniques used include turning optical nodes inside the optical network into boundary clocks, running PTP over a separate link such as the optical supervisory channel (OSC), or somehow bypassing the problematic hardware. Passive optical networking (PON): There are numerous variations on PON, but many of them feature a natural asymmetry. For example, the popular GPON is broadcast in the downstream direction, but uses a TDMA technique in the upstream. This time-based scheduling causes aging of PTP packets that are waiting. Some newer technologies and later generations of PON are moving to a more symmetrical model or have adopted a native method to carry frequency, phase, and time. DSL: In a similar fashion to PON, DSL systems tend to be asymmetric because they are de- signed 