Ethernet hub
Encyclopedia
Ethernet
Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
devices together and making them act as a single network segment
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:...
. A hub works at the physical layer
Physical layer
The physical layer or layer 1 is the first and lowest layer in the seven-layer OSI model of computer networking. The implementation of this layer is often termed PHY....
(layer 1) of the OSI model
OSI model
The Open Systems Interconnection model is a product of the Open Systems Interconnection effort at the International Organization for Standardization. It is a prescription of characterizing and standardizing the functions of a communications system in terms of abstraction layers. Similar...
. The device is a form of multiport repeater
Multiport repeater
In packet networking, a multiport repeater is the simplest multi-port active device in use. It has multiple input/output ports, in which a signal introduced at the input of any port appears at the output of every port except the original incoming. With multiple computers, the network slows, due...
. Repeater hubs also participate in collision detection, forwarding a jam signal to all ports if it detects a collision.
Some hubs may also come with a BNC
BNC connector
The BNC connector ' is a common type of RF connector used for coaxial cable. It is used with radio, television, and other radio-frequency electronic equipment, test instruments, video signals, and was once a popular computer network connector. BNC connectors are made to match the characteristic...
and/or Attachment Unit Interface
Attachment Unit Interface
An Attachment Unit Interface is a 15 pin connection that provides a path between a node's Ethernet interface and the Medium Attachment Unit , sometimes known as a transceiver. It is the part of the IEEE Ethernet standard located between the Media Access Control , and the MAU...
(AUI) connector to allow connection to legacy 10BASE2
10BASE2
10BASE2 is a variant of Ethernet that uses thin coaxial cable , terminated with BNC connectors...
or 10BASE5
10BASE5
10BASE5 was the original commercially available variant of Ethernet.For its physical layer it used cable similar to RG-8/U coaxial cable but with extra braided shielding. This is a stiff, diameter cable with an impedance of 50 ohms , a solid center conductor, a foam insulating filler, a shielding...
network segments. The availability of low-priced network switch
Network switch
A network switch or switching hub is a computer networking device that connects network segments.The term commonly refers to a multi-port network bridge that processes and routes data at the data link layer of the OSI model...
es has largely rendered hubs obsolete but they are still seen in older installations and more specialized applications.
Technical information
A network hub is an unsophisticated device in comparison with, for example, a switch. A hub does not examine or manage any of the traffic that comes through it: any packet entering any port is rebroadcast on all other ports. Effectively, it is barely aware of frames or packets and mostly operates on raw bits. Consequently, packet collisions are more frequent in networks connected using hubs than in networks connected using more sophisticated devices.100 Mbit/s hubs and repeaters come in two different speed grades: Class I delay the signal for a maximum of 140 bit times (enabling translation between 100Base-TX, 100Base-FX and 100Base-T4) and Class II hubs delay the signal for a maximum of 92 bit times (enabling installation of two hubs in a single collision domain).
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
5-4-3 rule
The 5-4-3 rule also referred to as the IEEE way is a design guideline for Ethernet computer networks covering the number of repeaters and segments on shared-access Ethernet backbones in a tree topology. It means that in a collision domain there should be at most 5 segments tied together with 4...
must be followed: up to 5 segments (4 hubs) are allowed between any two end stations. For 10BASE-T networks, up to six segments and five repeaters are allowed between any two hosts. For 100 Mbit/s networks, the limit is reduced to 3 segments (2 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
Fast Ethernet
In computer networking, Fast Ethernet is a collective term for a number of Ethernet standards that carry traffic at the nominal rate of 100 Mbit/s, against the original Ethernet speed of 10 Mbit/s. Of the fast Ethernet standards 100BASE-TX is by far the most common and is supported by the...
network is likely to require switches to avoid the chaining limits of hubs.
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 over twisted pair
Ethernet over twisted pair technologies use twisted-pair cables for the physical layer of an Ethernet computer network. Other Ethernet cable standards employ coaxial cable or optical fiber. Early versions developed in the 1980s included StarLAN followed by 10BASE-T. By the 1990s, fast, inexpensive...
Ethernet is generally more robust than coaxial cable-based Ethernet (e.g. 10BASE2), where a misbehaving device can adversely affect the entire collision domain
Collision domain
A collision domain is a section of a network where data packets can collide with one another when being sent on a shared medium or through repeaters, in particular, when using early versions of Ethernet. A network collision occurs when more than one device attempts to send a packet on a network...
. 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.
Hubs are classified as physical layer devices in the OSI model. At the physical layer, hubs support little in the way of sophisticated networking. Hubs do not read any of the data passing through them and are not aware of their source or destination addressing. A hub simply receives incoming Ethernet frame
Ethernet frame
A data packet on an Ethernet link is called an Ethernet frame. A frame begins with Preamble and Start Frame Delimiter. Following which, each Ethernet frame continues with an Ethernet header featuring destination and source MAC addresses. The middle section of the frame is payload data including any...
s, regenerates the electrical signal, and broadcasts these packets out to all other devices on the network.
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-T10BASE-T
Ethernet over twisted pair technologies use twisted-pair cables for the physical layer of an Ethernet computer network. Other Ethernet cable standards employ coaxial cable or optical fiber. Early versions developed in the 1980s included StarLAN followed by 10BASE-T. By the 1990s, fast, inexpensive...
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 consisted of an internal two-port switch, dividing the 10 Mbit/s and 100 Mbit/s segments. The device would typically consist of more than two physical ports. 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 prevented the need for an all-or-nothing migration fast Ethernet networks. These devices are considered hubs because the traffic between devices connected at the same speed is not switched.
Uses
Historically, the main reason for purchasing hubs rather than switches was their price. This motivator has largely been eliminated by reductions in the price of switches, but hubs can still be useful in special circumstances:- For inserting a protocol analyzerProtocol analyzerA "Protocol analyzer" is a tool used to capture and analyze signals and data traffic over a communication channel. Such a channel differs from a local computer bus to a satellite link, that provides a means of communication using a standard communication protocol...
into a network connection, a hub is an alternative to a network tapNetwork tapA network tap is a hardware device which provides a way to access the data flowing across a computer network. In many cases, it is desirable for a third party to monitor the traffic between two points in the network. If the network between points A and B consists of a physical cable, a "network...
or port mirroringPort mirroringPort Mirroring is used on a network switch to send a copy of network packets seen on one switch port to a network monitoring connection on another switch port. This is commonly used for network appliances that require monitoring of network traffic, such as an intrusion-detection system...
. - When a switch is accessible for end users to make connections, for example, in a conference room, an inexperienced or careless user (or saboteurSabotageSabotage is a deliberate action aimed at weakening another entity through subversion, obstruction, disruption, or destruction. In a workplace setting, sabotage is the conscious withdrawal of efficiency generally directed at causing some change in workplace conditions. One who engages in sabotage is...
) can bring down the network by connecting two ports together, causing a loop. This can be prevented by using a hub, where a loop will break other users on the hub, but not the rest of the network. This hazard can also be avoided by using switches that can detect and deal with loops, for example by implementing the spanning tree protocolSpanning tree protocolThe Spanning Tree Protocol is a network protocol that ensures a loop-free topology for any bridged Ethernet local area network. The basic function of STP is to prevent bridge loops and ensuing broadcast radiation...
. - A hub with a 10BASE2 port can be used to connect devices that only support 10BASE2 to a modern network. The same goes for linking in an old 10BASE5 network segment using an AUI port on a hub. Individual devices that were intended for thicknet can also be linked to modern Ethernet by using an AUI-10BASE-T transceiverTransceiverA transceiver is a device comprising both a transmitter and a receiver which are combined and share common circuitry or a single housing. When no circuitry is common between transmit and receive functions, the device is a transmitter-receiver. The term originated in the early 1920s...
.