Ethernet Hub - Technical Information

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 or symbols. Consequently, due to the larger collision domains, 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 must be followed: up to 5 segments (4 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.

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 frames, regenerates the electrical signal on the bit (more precisely the symbol) level, and broadcasts these symbols 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.