[asbo dog]

is this why vm are so bad compared with adsl/fttc

Upstream slots: Unlike an ethernet card, a cable modem is not allowed to transmit on demand. A single upstream channel frequency has to be shared by hundreds of cable modems that cannot hear each other's transmissions. If more than one cable modem transmitted at once, the UBR would not be able to understand either of them. So all cable modems must remain silent until they are allocated a time slot (measured with a precision of microseconds) by the UBR. The time-slot allocations are broadcast by the UBR on the downstream as MAP packets, so called because they map time slots to individual cable modems. So a typical sequence might be:

PC transmits a packet to the cable modem.
The cable modem stores the packet, and waits for an opportunity to transmit a bandwidth request to the UBR.
The UBR receives the bandwidth request, and schedules an upstream time-slot in the near future. This is added to a list of such scheduled slots for all other cable modems, and transmitted as a MAP packet at intervals on the downstream channel.
The cable modem monitors all MAP packets on the downstream. When it sees a timeslot allocated to itself, it waits until that allocated time, and then transmits the waiting packet.
All of this takes time. If the upstream is congested, the UBR might not be able to allocate a time-slot until some considerable time in the future, and poor latency will be seen. If signal conditions on the cable are poor, the bandwidth request or the MAP packet might not be heard, and nothing will happen until the cable modem retries the bandwidth request.