Physical Layer
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.
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