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The physical layer defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between communicating network systems.  Physical layer specifications define characteristics such as voltage levels, timing of voltage changes, physical data rates, maximum transmission distances, and physical connectors.  Physical-layer implementations can be categorized as either LAN or WAN specifications.

-The Physical layer has two responsibilities, send and receive bits (bits have a value of 1 or 0).

-The interface between DCEs and DTEs is defined at the Physical layer.

-The DCE is on the service provider side.

-The DTE is the attached device, the services available to a DTE are accessed through a CSU/DSU.

-HSSI Peer-based communications assumes intelligence in DCE and DTE devices.

Hubs and Repeaters

-Hubs are multiple port repeaters.  A repeater receives a signal, regenerates the digital signal, and forwards it on all active ports.  An active hub does the same thing.  All devices plugged into a hub are on the same collision and the same broadcast domains.  Hubs don't look at any traffic that enters, it just forwards all traffic to all ports.  Every device connected to the hub must listen if a device transmits.