Hubs
Hubs are a form of repeater for an Ethernet LAN which has multiple ports
(they are sometimes also known as "multi-port repeaters" or "active star
networks").
Each port of a hub has an RJ-45 connector able to accept a patch cable to
connect to a wall outlet. The other end of the cable is normally connected to a
network interface card with a 10BT outlet. Many hubs also have an AUI connector
which may be used to connect an external transceiver to which other types of
Ethernet Media may be attached. (Sometimes a 10B2 connector is also provided to
allow connection directly to a coaxial cable thin Ethernet network.) Whatever
the type of connector, a single hub is a only able to connect a group of
equipments operating at the same speed (i.e. all equipment connected to a 10BT
hub must operate at 10 Mbps).
An Ethernet Hub.
Each port (or interface) allows one piece of equipment to be connected to the
hub. In this case the system connected via the port F is sending a frame of data
to the system connected at port C. The hub is not able to recognise the
addresses in the header of a frame,and therefore is unable to identify which
port to send to. Therefore, every frame is sent to every output port. (This is
in contrast to a bridge, switch or router, each of which only forwards a packet
if the destination address of the packet corresponds to a system reachable via
the output interface.)
Typical office hub. Much larger hubs are typically used to connect
larger groups of users.
Monitoring Segment Performance
Any repeater or hub may optionally monitor the number of collisions which it
observes from each port. If the number rises above a threshold, the repeater /
hub may then disable the port (sometimes called a "partition").
A hub may also monitor the state of the transceiver at each 10BT and 10BF
Ethernet port. On these interfaces, nodes send a unique signal every 16 +/- 8
ms. This signal is called an "idle pulse". Receivers monitor the medium to check
that idle pulses are received. If neither an Ethernet frame, or an idle pulse is
received within 50-150 ms, the receiver may assume that the cable is not
functioning correctly. A hub may then disable the port (sometimes called
"partition"). A disabled port is made active again when 2-10 valid idle pulses
are received. This can protect other correctly operating ports from the
disruption caused by a faulty NIC, transceiver or hub. The idle pulse is also
used in some pieces of equipment to automatically detect the speed of
transmission (10 or 100 Mbps) and availability of the full duplex mode.
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