thumb|300px|4-port [[10BASE-T Ethernet hub with selectable MDI-X/MDI port]]
thumb|300px|8-port Ethernet hub with one [[10BASE2 connector and eight 10BASE-T ports]]
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.
Physical layer function
A layer 1 network device, such as a hub, transfers data but does not manage any of the traffic coming through it. Any packet entering a port is repeated to the output of every other port except for the port of entry. Specifically, each bit or symbol is repeated as it flows in. A repeater hub can therefore only receive and forward at a single speed. Dual-speed hubs internally consist of two hubs with a bridge between them. Since every packet is repeated on every other port, packet collisions affect the entire network, limiting its overall capacity.
A network hub is an unsophisticated device in comparison with a switch. As a multiport repeater it works by repeating transmissions 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. A hub has no memory to store data and can handle only one transmission at a time. Therefore, hubs can only run in half duplex mode. Due to a larger collision domain, packet collisions are more likely in networks connected using hubs than in networks connected using more sophisticated devices.
Connecting multiple hubs
The need for hosts to be able to detect collisions limits the number of hubs and the total size of a network built using hubs (a network built using switches does not have these limitations). For networks built using repeater hubs, the 5-4-3 rule must be followed: up to five segments (four hubs) are allowed between any two end stations. For networks, the limit is reduced to three segments between any two end stations, and even that is only allowed if the hubs are of Class II. Some hubs have manufacturer-specific stack ports allowing them to be combined in a way that allows more hubs than simple chaining through Ethernet cables, but even so, a large Fast Ethernet network is likely to require switches to avoid the chaining limits of hubs.
Gigabit Ethernet
Repeater hubs are defined in the standards for Gigabit Ethernet but commercial products have failed to appear due to the industry's transition to switching.
Uses
Historically, the main reason for purchasing hubs rather than switches was their price. By the early 2000s, there was little price difference between a hub and a low-end switch. Hubs can still be useful in special circumstances:
- For inserting a protocol analyzer into a network connection, a hub is an alternative to a network tap or port mirroring.
- A hub with both 10BASE-T ports and a 10BASE2 port can be used to connect a 10BASE2 segment to a modern Ethernet-over-twisted-pair network.
- A hub with both 10BASE-T ports and an AUI port can be used to connect a 10BASE5 segment to a modern network.
- As hubs have lower latency and jitter compared to switches – as long as there are no collisions – they may be better suited for real-time networks, e.g. Ethernet Powerlink.
One of the first Ethernet hubs, the HP Starlan for StarLAN, the first Ethernet-over-twisted-pair standard, was announced in 1986. Its successor, the Starlan 10, was announced in 1987. By 1994, the industry had started to shift to switching.
See also
- Router (computing)
- USB hub