Network latency and network bandwidth correlation

This sounds like some form of "bufferbloat", probably on the part of the DSLAM/LNS that's performing the 6Mb rate limiting.

It might be your CPE box, but that's a little less likely.

Ping is a utility to test network connectivity. It uses ICMP echo request/reply to do this. The originating host sends an ICMP echo request, and the receiving host sends an ICMP echo reply. There are several messages which may be returned:

• The target host could reply, and you have a successful ping.
• If a certain time period passes before receiving a message, you get a timeout. This is caused by the host on a different network not replying (firewall, target down, unidirectional routing, etc.) during the allotted time period.
• You could get a destination unreachable message, meaning that the layer-3 address could not be resolved to the layer-2 address. This implies that either the host is on the same network, and it did not respond to ARP, or if the host is on a different network, the configured gateway for your host is down or misconfigured.
• You could get a network unreachable message, meaning the host is on a different network, and a router in the path has no route to the network for the target host.

Most ping programs run the ping several times, and they return a round-trip ICMP network latency. The Ping program usually gives you the number of packets sent, received, lost, and the loss percent, along with the lowest, highest, and average ping latency. Unfortunately, this is often confused with a real network latency, but the two may not be related on a network with multiple hops, especially the Internet where ICMP is often delayed or rerouted, unlike other traffic. ICMP is not the protocol used for most data communications, and the latency returned by ping may not reflect any other application latency.

Bandwidth is only indirectly related to latency. The bandwidth on an unloaded network has nothing at all to do with the latency. The real network latency is due mostly to network congestion, and bandwidth plays the largest part in that for busy networks. Latency reflect the aggregation of distance latency, and the latency added by network devices in the path. The largest component of the latency is by the network devices, due to congestion on the network, and a busy connection causes congestion.