The way in which most ISPs are connected to the gateway is normally either via Fast Ethernet (FE - 100Mbps) or Gigabit Ethernet (GigE - 1000Mbps). Obviously if your ISP has a 155Mbps gateway installed then Fast Ethernet is not up to the job and so they'll have Gigabit Ethernet connections.
However there is one ISP that does things differently - hence the need for this section. BTOpenworld does not appear to have the gateway machine(s) physically installed on their premises unlike other ISPs, so a direct FE or GigE connection is not used. Instead BTO use the ISPnet service that BT offer to carry the traffic to and from the gateways into their internal network. This shows up on a traceroute as a series of "hops" before the traffic arrives at the BTO internal network. It should be pointed out that ISPnet is used by several other "dial-up" ISPs and may be viewed as a rather less than an ideal solution.