Layer-1 Hubs
Hubs are Layer-1 devices that physically connect network devices together
for communication. Hubs can also be referred to as repeaters.
Hubs provide no intelligent forwarding whatsoever. Hubs are incapable of
processing either Layer-2 or Layer-3 information, and thus cannot make
decisions based on hardware or logical addressing.
Thus, hubs will always forward every frame out every port, excluding the
port originating the frame. Hubs do not differentiate between frame types,
and thus will always forward unicasts, multicasts, and broadcasts out every
port but the originating port.
Ethernet hubs operate at half-duplex, which allows a host to either transmit
or receive data, but not simultaneously. Half-duplex Ethernet utilizes
Carrier Sense Multiple Access with Collision Detect (CSMA/CD) to
control media access. Carrier sense specifies that a host will monitor the
physical link, to determine whether a carrier (or signal) is currently being
transmitted. The host will only transmit a frame if the link is idle.
If two hosts transmit a frame simultaneously, a collision will occur. This
renders the collided frames unreadable. Once a collision is detected, both
hosts will send a 32-bit jam sequence to ensure all transmitting hosts are
aware of the collision. The collided frames are also discarded. Both devices
will then wait a random amount of time before resending their respective
frames, to reduce the likelihood of another collision.
Remember, if any two devices connected to a hub send a frame
simultaneously, a collision will occur. Thus, all ports on a hub belong to the
same collision domain. A collision domain is simply defined as any
physical segment where a collision can occur.
Multiple hubs that are uplinked together still all belong to one collision
domain. Increasing the number of host devices in a single collision domain
will increase the number of collisions, which will degrade performance.
Hubs also belong to only one broadcast domain – a hub will forward both
broadcasts and multicasts out every port but the originating port. A broadcast
domain is a logical segmentation of a network, dictating how far a broadcast
(or multicast) frame can propagate.
Hubs are Layer-1 devices that physically connect network devices together
for communication. Hubs can also be referred to as repeaters.
Hubs provide no intelligent forwarding whatsoever. Hubs are incapable of
processing either Layer-2 or Layer-3 information, and thus cannot make
decisions based on hardware or logical addressing.
Thus, hubs will always forward every frame out every port, excluding the
port originating the frame. Hubs do not differentiate between frame types,
and thus will always forward unicasts, multicasts, and broadcasts out every
port but the originating port.
Ethernet hubs operate at half-duplex, which allows a host to either transmit
or receive data, but not simultaneously. Half-duplex Ethernet utilizes
Carrier Sense Multiple Access with Collision Detect (CSMA/CD) to
control media access. Carrier sense specifies that a host will monitor the
physical link, to determine whether a carrier (or signal) is currently being
transmitted. The host will only transmit a frame if the link is idle.
If two hosts transmit a frame simultaneously, a collision will occur. This
renders the collided frames unreadable. Once a collision is detected, both
hosts will send a 32-bit jam sequence to ensure all transmitting hosts are
aware of the collision. The collided frames are also discarded. Both devices
will then wait a random amount of time before resending their respective
frames, to reduce the likelihood of another collision.
Remember, if any two devices connected to a hub send a frame
simultaneously, a collision will occur. Thus, all ports on a hub belong to the
same collision domain. A collision domain is simply defined as any
physical segment where a collision can occur.
Multiple hubs that are uplinked together still all belong to one collision
domain. Increasing the number of host devices in a single collision domain
will increase the number of collisions, which will degrade performance.
Hubs also belong to only one broadcast domain – a hub will forward both
broadcasts and multicasts out every port but the originating port. A broadcast
domain is a logical segmentation of a network, dictating how far a broadcast
(or multicast) frame can propagate.
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