Network hub

An Ethernet hub, active hub, network hub, repeater hub, multiport repeater, or simply hub is a network hardware device for connecting multiple Ethernet devices together and making them act as a single network segment. It has multiple input/output (I/O) ports, in which a signal introduced at the input of any port appears at the output of every port except the original incoming. A hub works at the physical layer (layer 1) of the OSI model. Repeater hubs also participate in collision detection, forwarding a jam signal to all ports if it detects a collision. In addition to standard 8P8C ("RJ45") ports, some hubs may also come with a BNC or Attachment Unit Interface (AUI) connector to allow connection to legacy 10BASE2 or 10BASE5 network segments.

Hubs are now largely obsolete, having been replaced by network switches except in very old installations or specialized applications. As of 2011, connecting network segments by repeaters or hubs is deprecated by IEEE 802.3.

A network hub is an unsophisticated device in comparison with a switch. As a multiport repeater it works by repeating bits (symbols) received from one of its ports to all other ports. It is aware of physical layer packets, that is it can detect their start (preamble), an idle line (interpacket gap) and sense a collision which it also propagates by sending a jam signal. A hub cannot further examine or manage any of the traffic that comes through it: any packet entering any port is rebroadcast on all other ports. A hub/repeater has no memory to store any data in – a packet must be transmitted while it is received or is lost when a collision occurs (the sender should detect this and retry the transmission). Due to this, hubs can only run in half duplex mode. Consequently, due to a larger collision domain, packet collisions are more frequent in networks connected using hubs than in networks connected using more sophisticated devices.

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