Hubs: what are they and how do they work?

Even though other options are now available for operating a network, hubs are still used. What is a hub, what is it used for (and how?), and what ad­vant­ages and dis­ad­vant­ages does it have, es­pe­cially compared to a switch?

A hub is a network device that connects various network nodes, e.g. in an Ethernet, in a star con­fig­ur­a­tion. In the OSI (Open Systems In­ter­con­nec­tions) reference model, hubs are clas­si­fied as level 1 devices that operate at the physical layer. Their main task is to in­ter­con­nect several computers and im­me­di­ately forward received data. Hubs are usually made of plastic, run on an external power supply and contain between 4 and 16 ports, i.e. physical con­nec­tions. Their maximum bandwidth is usually 10/100 Mbit per second.

A hub receives data and then sends it in full to all connected devices (hosts). All ports of the hub operate at the same speed and are located in a collision domain (which includes all connected network devices). Unlike other network devices, a hub does not offer the option to control or exclude only in­di­vidu­al receivers. This means that all data packets are always forwarded to all computers during a transfer. This means that even those devices the data wasn’t intended for also receive the data. Since all hosts are occupied in this way, the other devices cannot send any data them­selves in the meantime. Instead, sim­ul­tan­eous requests are processed one after the other.

If you need more hosts, you can connect one hub to another hub. This con­nec­tion is created with a simple crossover cable over one of the ports. However, the number of hosts is auto­mat­ic­ally limited by the 5-4-3 rule, or repeater rule. This states that a maximum of five segments with four repeaters can be used between two end devices. In addition, the connected hosts share the entire bandwidth when a hub is used. This in­ev­it­ably leads to speed losses, es­pe­cially when trans­fer­ring large data packets.

Hubs are an outdated tech­no­logy. In addition to the speed losses mentioned above, and the lack of flex­ib­il­ity in terms of data transfer and the selected re­cip­i­ents, a hub system is also com­par­at­ively vul­ner­able to security issues. For example, the system cannot be quar­ant­ined and data traffic does not take place in a protected manner. Any security issues or privacy concerns im­me­di­ately affect all connected hosts.

For these reasons, hubs are used less often these days. Their lack of flex­ib­il­ity is no longer up to date, and the lack of speed is a major obstacle with ever larger data volumes. One possible ap­plic­a­tion that is still more common today is the expansion of older networks. Streaming media to multiple devices is also possible. Hubs are still used for network analysis. Here, the lack of flex­ib­il­ity is actually an advantage. Since all network data can be found on all ports, no ad­di­tion­al mirror port is needed to read and analyse it.

Nev­er­the­less, the time of hubs is by and large over. According to the 802.3 standards of the IEEE (Institute of Elec­tric­al and Elec­tron­ics Engineers), hubs have been con­sidered obsolete since 2011. In the meantime, the switch has taken their place. There are many reasons for this, since the switch is strictly speaking a further de­vel­op­ment of its aged pre­de­cessor. Unlike the hub, the switch makes it possible to send and receive data sim­ul­tan­eously. The bandwidth is also not limited, but is constant. While the hub always controls all hosts and delivers all data, it is possible to select in­di­vidu­al end devices with the switch.

The switch operates at level 2 of the OSI reference model and can therefore the­or­et­ic­ally create its own collision domains for each in­di­vidu­al port. To enable precise targeting, the MAC addresses of the data frames are evaluated and, if necessary, end devices for which the re­spect­ive data is not intended are excluded. Similar to a hub, however, switches such as MPLS have a broadcast function and can serve all connected hosts sim­ul­tan­eously. Due to their distinct ad­vant­ages, switches were sig­ni­fic­antly more expensive than hubs for a long time which is slowly starting to change as they become more widely used.

To get a better picture, you can see the func­tion­al­it­ies and cap­ab­il­it­ies of the hub and switch in direct com­par­is­on here:

While hubs were the only option for setting up networks for a long time, this no longer applies today. Network analysis, expansion of existing networks (without security relevance) or possibly for streaming various media are domains in which the tech­no­logy is still suitable today. Also, since the prices of hubs and switches have also converged in the meantime, the more modern tech­no­logy is clearly a wiser option.