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 aConnecting 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 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 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.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.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 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.See also
* Router (computing) * USB hubReferences
External links
* {{DEFAULTSORT:Ethernet Hub Hub Networking hardware