Unlike ADSL and SDSL, High Bit Rate Digital Subscriber Line (HDSL) is a symmetric method of transmitting data at speed rates up to 1.5 Mbps in both directions. Because of the symmetric properties, the highest transmission rates can only be supported at lengths of 15,000 foot distances of two or more twisted pair lines. HDSL is an ideal service for campus environments and digital local loops, because of these distance requirements. High-Pass Filter (HPF) A signal filter which would be installed in a customer premises ADSL modem (ATU-R), which only allow higher frequency data to be delivered to the modem. See Low-Pass Filter. Hub The hub is a critical network element that connects everything to one centralized point. A hub is simply a box with multiple ports for network connections. Each device on the network is attached to the hub via an Ethernet cable.
source: aware.com