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Hubs are now largely obsolete, having been replaced by network switches except in very old installations or specialized applications.
Physical layer function
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.
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 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 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 is generally more robust than coaxial cable-based Ethernet (e.g. 10BASE2), where a misbehaving device can adversely affect the entire collision domain. 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.
Fast Ethernet classes
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/recoding 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).
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.
Gigabit Ethernet hubs
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 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.
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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.
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