Ethernet hub

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4-port 10BASE-T Ethernet hub with selectable MDI-X/MDI port 4 port netgear ethernet hub.jpg
4-port 10BASE-T Ethernet hub with selectable MDI-X/MDI port
8-port Ethernet hub with one 10BASE2 connector and eight 10BASE-T ports HP EtherTwist Hub8.jpg
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 [lower-alpha 1] 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. [1] A hub works at the physical layer. [2] A repeater hub also participates 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 an Attachment Unit Interface (AUI) connector to allow connection to legacy 10BASE2 or 10BASE5 network segments.

Contents

Hubs are now largely obsolete, having been replaced by network switches except in very old installations or specialized applications.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. [3] [4] [5]

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. [6] 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. [2]

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. [6] For 10BASE-T networks, up to five segments with four repeaters are allowed between any two hosts. [7] For 100 Mbit/s 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. [2]

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 is generally more robust than coaxial cable-based Ethernet (e.g. 10BASE2), where a misbehaving device can adversely affect the entire collision domain. [6] 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.[ citation needed ]

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.[ citation needed ]

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 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.[ citation needed ]

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. [8]

Gigabit Ethernet

Repeater hubs are defined in the standards for Gigabit Ethernet [9] but commercial products have failed to appear [10] 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. [11] Hubs can still be useful in special circumstances:

One of the first Ethernet hubs, the HP Starlan for StarLAN, the first Ethernet-over-twisted-pair standard, was announced in 1986. [14] Its successor, the Starlan 10, was announced in 1987. [15] By 1994, the industry had started to shift to switching. [16]

See also

Notes

  1. Network switches are sometimes called switching hubs.

Related Research Articles

<span class="mw-page-title-main">Ethernet</span> Computer networking technology

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 1983 as IEEE 802.3. Ethernet has since been refined to support higher bit rates, a greater number of nodes, and longer link distances, but retains much backward compatibility. Over time, Ethernet has largely replaced competing wired LAN technologies such as Token Ring, FDDI and ARCNET.

100BaseVG is a 100 Mbit/s Ethernet standard specified to run over four pairs of Category 3 cable. It is also called 100VG-AnyLAN because it was defined to carry both Ethernet and Token Ring frame types.

<span class="mw-page-title-main">10BASE2</span> Once-dominant 10 Mbit/s Ethernet standard

10BASE2 is a variant of Ethernet that uses thin coaxial cable terminated with BNC connectors to build a local area network.

<span class="mw-page-title-main">10BASE5</span> First commercially available variant of Ethernet

10BASE5 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 500 meters (1,600 ft) in length. Up to 100 stations can be connected to the cable using vampire taps and share a single collision domain with 10 Mbit/s of bandwidth shared among them. The system is difficult to install and maintain.

<span class="mw-page-title-main">Ethernet over twisted pair</span> Ethernet physical layers using twisted-pair cables

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.

A network switch is networking hardware that connects devices on a computer network by using packet switching to receive and forward data to the destination device.

Carrier-sense multiple access with collision detection (CSMA/CD) is a medium access control (MAC) method used most notably in early Ethernet technology for local area networking. It uses carrier-sensing to defer transmissions until no other stations are transmitting. This is used in combination with collision detection in which a transmitting station detects collisions by sensing transmissions from other stations while it is transmitting a frame. When this collision condition is detected, the station stops transmitting that frame, transmits a jam signal, and then waits for a random time interval before trying to resend the frame.

<span class="mw-page-title-main">Fast Ethernet</span> Ethernet standards that carry data at the nominal rate of 100 Mbit/s

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.

<span class="mw-page-title-main">Gigabit Ethernet</span> Standard for Ethernet networking at a data rate of 1 gigabit per second

In computer networking, Gigabit Ethernet 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 in 1999, and has replaced Fast Ethernet in wired local networks due to its considerable speed improvement over Fast Ethernet, as well as its use of cables and equipment that are widely available, economical, and similar to previous standards. The first standard for faster 10 Gigabit Ethernet was approved in 2002.

A collision domain is a network segment where simultaneous data transmissions collide with one another as a result of more than one device attempting to send a packet on the network segment at the same time. The collision domain applies particularly in wireless networks, but also affected early versions of Ethernet. Members of a collision domain may be involved in collisions with one another. Devices outside the collision domain do not have collisions with those inside.

StarLAN was the first IEEE 802.3 standard for Ethernet over twisted pair wiring. It was standardized by the IEEE Standards Association as 802.3e in 1986, as the 1BASE5 version of Ethernet. The StarLAN Task Force was chaired by Bob Galin.

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.

The media-independent interface (MII) was originally defined as a standard interface to connect a Fast Ethernet medium access control (MAC) block to a PHY chip. The MII is standardized by IEEE 802.3u and connects different types of PHYs to MACs. Being media independent means that different types of PHY devices for connecting to different media can be used without redesigning or replacing the MAC hardware. Thus any MAC may be used with any PHY, independent of the network signal transmission medium.

<span class="mw-page-title-main">Medium Attachment Unit</span> Transceiver in an Ethernet network

A Medium Attachment Unit (MAU) is a transceiver which converts signals on an Ethernet cable to and from Attachment Unit Interface (AUI) signals.

<span class="mw-page-title-main">Medium-dependent interface</span> Interface between a network device and the data link it communicates over

A medium-dependent interface (MDI) describes the interface in a computer network from a physical-layer implementation to the physical medium used to carry the transmission. Ethernet over twisted pair also defines a medium-dependent interface – crossover (MDI-X) interface. Auto–MDI-X ports on newer network interfaces detect if the connection would require a crossover and automatically choose the MDI or MDI-X configuration to complement the other end of the link.

An Ethernet crossover cable is a crossover cable for Ethernet used to connect computing devices together directly. It is most often used to connect two devices of the same type, e.g. two computers or two switches to each other. By contrast, straight through patch cables are used to connect devices of different types, such as a computer to a network switch.

<span class="mw-page-title-main">Ethernet physical layer</span> Electrical or optical properties between network devices

The physical-layer specifications of the Ethernet family of computer network standards are published by the Institute of Electrical and Electronics Engineers (IEEE), which defines the electrical or optical properties and the transfer speed of the physical connection between a device and the network or between network devices. It is complemented by the MAC layer and the logical link layer. An implementation of a specific physical layer is commonly referred to as PHY.

<span class="mw-page-title-main">Cisco Catalyst 1900</span> Ethernet switch model

The Cisco Catalyst 1900 is a 19" rack mountable, managed (configurable) 10BASE-T Ethernet switch with 100BASE-TX/100BASE-FX uplink ports. This product was popular in small office networks because of its features and price.

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-medium Ethernet backbones in a tree topology. It means that in a collision domain there should be at most 5 segments tied together with 4 repeaters, with up to 3 mixing segments. Link segments can be 10BASE-T, 10BASE-FL or 10BASE-FB. This rule is also designated the 5-4-3-2-1 rule with there being two link segments and one collision domain.

Classic Ethernet is a family of 10 Mbit/s Ethernet standards, which is the first generation of Ethernet standards. In 10BASE-X, the 10 represents its maximum throughput of 10 Mbit/s, BASE indicates its use of baseband transmission, and X indicates the type of medium used. Classic Ethernet includes coax, twisted pair and optical variants. The first Ethernet standard was published in 1983 and classic Ethernet operating at 10 Mbit/s was the dominant form of Ethernet until the first standard for Fast Ethernet was approved in 1995.

References

  1. IEEE 802.3-2012 Clause 9.1
  2. 1 2 3 Dean, Tamara (2010). Network+ Guide to Networks. Delmar. pp. 256–257.
  3. IEEE 802.3 9. Repeater unit for 10 Mb/s baseband networks
  4. IEEE 802.3 27. Repeater unit for 100 Mb/s baseband networks
  5. IEEE 802.3 41. Repeater unit for 1000 Mb/s baseband networks
  6. 1 2 3 Hallberg, Bruce (2010). Networking: A Beginner's Guide, Fifth Edition. McGraw Hill. pp. 68–69.
  7. Charles Spurgeon (2000-02-16). "Chapter 13: Multi-Segment Configuration Guidelines". Ethernet: The Definitive Guide. ISBN   978-1-56592-660-8 . Retrieved 2012-01-08. The transmission path permitted between any two DTEs may consist of up to five segments, four repeater sets (including optional AUIs), two MAUs, and two AUIs.
  8. "What is the difference between Class I and Class II hubs?" Intel. Retrieved 2011-03-16.
  9. IEEE 802.3 Clause 41
  10. Neil Allen (18 October 2009). Network Maintenance and Troubleshooting Guide. Fluke Networks. ISBN   9780321647627.
  11. Matthew Glidden (October 2001). "Switches and Hubs". About This Particular Macintosh blog. Retrieved June 9, 2011.
  12. "Sniffing Tutorial part 1 - Intercepting Network Traffic". NETRESEC Network Security Blog. 2011-03-11. Retrieved 2011-03-13.
  13. Ethernet Powerlink Standardization Group (2018), Ethernet POWERLINK Communication Profile Specification. Version 1.4.0 (PDF), p. 35, retrieved 2019-05-06
  14. "HP adopts Starlan plan". Network World . 1986-11-06. p. 6.
  15. "HP's 10Mbit/sec. LAN needs no special wiring". Computerworld . 1987-08-31.
  16. "Switching strategy will be key as internet markets collide". Network World . 1994-02-21.