An Ethernet hub, active hub, network hub, repeater hub, multiport repeater, or simply hub is a

network hardware Networking hardware, also known as network equipment or computer networking devices, are electronic devices which are required for communication and interaction between devices on a computer network. Specifically, they mediate data transmission in ...

device for connecting multiple

Ethernet Ethernet () is a family of wired computer networking technologies commonly used in local area networks (LAN), metropolitan area networks (MAN) and wide area networks (WAN). It was commercially introduced in 1980 and first standardized in 198 ...

devices together and making them act as a single

network segment A network segment is a portion of a computer network. The nature and extent of a segment depends on the nature of the network and the device or devices used to interconnect end stations. Ethernet According to the defining IEEE 802.3 standards ...

. It has multiple

input/output In computing, input/output (I/O, or informally io or IO) is the communication between an information processing system, such as a computer, and the outside world, possibly a human or another information processing system. Inputs are the signals ...

(I/O) ports, in which a

signal In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The ''IEEE Transactions on Signal Processing'' ...

introduced at the input of any

port A port is a maritime facility comprising one or more wharves or loading areas, where ships load and discharge cargo and passengers. Although usually situated on a sea coast or estuary, ports can also be found far inland, such as Ham ...

appears at the output of every port except the original incoming. A hub works at the

physical layer In the seven-layer OSI model of computer networking, the physical layer or layer 1 is the first and lowest layer; The layer most closely associated with the physical connection between devices. This layer may be implemented by a PHY chip. The ...

(layer 1) of the

OSI model The Open Systems Interconnection model (OSI model) is a conceptual model that 'provides a common basis for the coordination of SOstandards development for the purpose of systems interconnection'. In the OSI reference model, the communications ...

. A repeater hub also participates in collision detection, forwarding a jam signal to all ports if it detects a

collision In physics, a collision is any event in which two or more bodies exert forces on each other in a relatively short time. Although the most common use of the word ''collision'' refers to incidents in which two or more objects collide with great fo ...

. In addition to standard 8P8C (" RJ45") ports, some hubs may also come with a BNC or an

Attachment Unit Interface The Attachment Unit Interface (AUI) is a physical and logical interface defined in the original IEEE 802.3 standard for 10BASE5 Ethernet and the previous DIX standard. The physical interface consists of a 15-pin D-subminiature connection that ...

(AUI) connector to allow connection to legacy

10BASE2 10BASE2 (also known as cheapernet, thin Ethernet, thinnet, and thinwire) is a variant of Ethernet that uses thin coaxial cable terminated with BNC connectors to build a local area network. During the mid to late 1980s this was the dominan ...

10BASE5 10BASE5 (also known as thick Ethernet or thicknet) was the first commercially available variant of Ethernet. The technology was standardized in 1982 as IEEE 802.3. 10BASE5 uses a thick and stiff coaxial cable up to in length. Up to 100 stati ...

network segments. Hubs are now largely obsolete, having been replaced by

network switch A network switch (also called switching hub, bridging hub, and, by the IEEE, MAC bridge) is networking hardware that connects devices on a computer network by using packet switching to receive and forward data to the destination device. A netw ...

es except in very old installations or specialized applications. As of 2011, connecting network segments by repeaters or hubs is deprecated by IEEE 802.3.

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

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 In physics, a collision is any event in which two or more bodies exert forces on each other in a relatively short time. Although the most common use of the word ''collision'' refers to incidents in which two or more objects collide with great f ...

affect the entire network, limiting its overall capacity. A network hub is an unsophisticated device in comparison with a

switch In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type of ...

. As a ''multiport

repeater In telecommunications, a repeater is an electronic device that receives a signal and retransmits it. Repeaters are used to extend transmissions so that the signal can cover longer distances or be received on the other side of an obstruction. Some ...

'' it works by repeating transmissions received from one of its ports to all other ports. It is aware of

packets, that is it can detect their start (

preamble A preamble is an introductory and expressionary statement in a document that explains the document's purpose and underlying philosophy. When applied to the opening paragraphs of a statute, it may recite historical facts pertinent to the subj ...

), an idle line (

interpacket gap In computer networking, the interpacket gap (IPG), also known as interframe spacing, or interframe gap (IFG), is a pause which may be required between network packets or network frames. Depending on the physical layer protocol or encoding used, ...

) 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 A duplex communication system is a point-to-point system composed of two or more connected parties or devices that can communicate with one another in both directions. Duplex systems are employed in many communications networks, either to allow ...

mode. Due to a larger

collision domain In physics, a collision is any event in which two or more bodies exert forces on each other in a relatively short time. Although the most common use of the word ''collision'' refers to incidents in which two or more objects collide with great f ...

, 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 10 Mbit/s 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 10BASE-T networks, up to five segments and four repeaters are allowed between any two hosts. For 100 Mbit/s networks, the limit is reduced to 3 segments (2 Class II hubs) 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 In computer networking, Fast Ethernet physical layers carry traffic at the nominal rate of 100 Mbit/s. The prior Ethernet speed was 10 Mbit/s. Of the Fast Ethernet physical layers, 100BASE-TX is by far the most common. Fast Ethern ...

network is likely to require switches to avoid the chaining limits of hubs.

Additional functions

Most hubs detect typical problems, such as excessive collisions and jabbering on individual ports, and ''partition'' the port, disconnecting it from the shared medium. Thus, hub-based

twisted-pair Ethernet Ethernet over twisted-pair technologies use twisted-pair cables for the physical layer of an Ethernet computer network. They are a subset of all Ethernet physical layers. Early Ethernet used various grades of coaxial cable, but in 1984, StarL ...

is generally more robust than coaxial cable-based Ethernet (e.g. 10BASE2), where a misbehaving device can adversely affect the entire

. Even if not partitioned automatically, a hub simplifies troubleshooting because hubs remove the need to troubleshoot faults on a long cable with multiple taps; status lights on the hub can indicate the possible problem source or, as a last resort, devices can be disconnected from a hub one at a time much more easily than from a coaxial cable. To pass data through the repeater in a usable fashion from one segment to the next, the framing and data rate must be the same on each segment. This means that a repeater cannot connect an 802.3 segment (Ethernet) and an 802.5 segment (Token Ring) or a 10 Mbit/s segment to 100 Mbit/s Ethernet.

Dual-speed hub

In the early days of Fast Ethernet, Ethernet switches were relatively expensive devices. Hubs suffered from the problem that if there were any

10BASE-T 1 (one, unit, unity) is a number representing a single or the only entity. 1 is also a numerical digit and represents a single unit of counting or measurement. For example, a line segment of ''unit length'' is a line segment of length 1. I ...

devices connected then the whole network needed to run at 10 Mbit/s. Therefore, a compromise between a hub and a switch was developed, known as a dual-speed hub. These devices make use of an internal two-port switch, bridging the 10 Mbit/s and 100 Mbit/s segments. When a network device becomes active on any of the physical ports, the device attaches it to either the 10 Mbit/s segment or the 100 Mbit/s segment, as appropriate. This obviated the need for an all-or-nothing migration to Fast Ethernet networks. These devices are considered hubs because the traffic between devices connected at the same speed is not switched.

Fast Ethernet

100 Mbit/s hubs and repeaters come in two different classes: Class I delay the signal for a maximum of 140 bit times. This delay allows for translation/recoding between 100BASE-TX, 100BASE-FX and 100BASE-T4. Class II hubs delay the signal for a maximum of 92 bit times. This shorter delay allows the installation of two hubs in a single collision domain.

Gigabit Ethernet

Repeater hubs are defined in the standards for

Gigabit Ethernet In computer networking, Gigabit Ethernet (GbE or 1 GigE) is the term applied to transmitting Ethernet frames at a rate of a gigabit per second. The most popular variant, 1000BASE-T, is defined by the IEEE 802.3ab standard. It came into use i ...

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 A protocol analyzer is a tool (hardware or software) used to capture and analyze signals and data traffic over a communication channel. Such a channel varies from a local computer bus to a satellite link, that provides a means of communication usi ...

into a network connection, a hub is an alternative to a

network tap A network tap is a system that monitors events on a local network. A tap is typically a dedicated hardware device, which provides a way to access the data flowing across a computer network. The network tap has (at least) three ports: an ''A port ...

port mirroring Port mirroring is used on a network switch to send a copy of network packets seen on one switch port (or an entire VLAN) to a network monitoring connection on another switch port. This is commonly used for network appliances that require monitor ...

. * 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.

References

External links

* {{DEFAULTSORT:Ethernet Hub Hub Networking hardware