Concentrator
A concentrator, also known as a hub, is a network device that allows you to centralize different nodes of a computer network. Its main function is to establish a connection between an indefinite number of computers and allow data exchange. Regarding the OSI model, they act in the physical layer (layer 1) or the media access layer in the TCP/IP model. This means that said device receives a signal and repeats this signal by emitting it through its different ports (repeater).
Nowadays, the job of hubs is often done by switches.
Technical information
An Ethernet network behaves like a shared medium, that is, only one device can transmit successfully at a time, and each is responsible for collision detection and retransmission. With 10Base-T and 100Base-T links (which generally represent most or all of the ports on a hub) there are separate pairs for transmit and receive, but they are used in half duplex mode which it still behaves as a shared link medium (see 10Base-T for pin specifications).
A hub, or repeater, is a fairly simple broadcast device. Hubs fail to direct traffic coming through them, and any incoming packet is forwarded to another port (other than the ingress port). Since every packet is being sent out every other port, packet collisions result, greatly impeding traffic flow. When two devices try to communicate simultaneously, a collision will occur between the transmitted packets, which the transmitting devices detect. Upon detecting this collision, the devices stop transmitting and pause before re-sending the packets.
The need for hosts to be able to detect collisions limits the number of hubs and the total size of the network. For 10 Mbit/s networks, up to 5 segments (4 hubs) are allowed between two end stations. For 100 Mbit/s networks, the limit is reduced to 3 segments (2 hubs) between two end stations, and even then only if the hubs were of the low delay variety. Some hubs have special (and usually manufacturer specific) ports that allow them to be combined in a way that allows simpler hubs to be daisy chained through Ethernet cables, but even then a large Fast Ethernet network will likely require switches to avoid chaining hubs.
Most hubs detect typical problems, such as excessive collisions on each port. Thus, an Ethernet-based hub is generally more robust than Ethernet-based coaxial cable. Even if partitioning doesn't happen automatically, a troubleshooting hub makes it easier as the lights can indicate the potential problem with the font. It also eliminates the need to troubleshoot a very large cable with multiple outlets.
Double Speed Hubs
Hubs suffered from the problem that as simple repeaters they could only support a single speed. While regular personal computers with expansion slots could easily be upgraded to Fast Ethernet with a new network card, machines with less common expansion mechanisms, such as printers, can be expensive or impossible to upgrade. Therefore, a middle ground between hub and switch is known as a double-speed hub.
This type of device basically consists of two hubs (one of each speed) and two bridge ports between them. Devices connect to the appropriate hub automatically, based on their speed. From the bridge you only have two ports, and only one of them needs to be 100 Mb/s.
Uses
Historically, the primary reason for purchasing hubs over switches was price. This has been largely eliminated by price reductions for switches, but hubs can still be useful in special circumstances:
- A protocol analyzer connected to a switch does not always receive all the packages, as since the switch separates the ports in the different segments. Instead, the connection of the protocol analyzer with a concentrater allows you to see all traffic in the segment. On the other hand, expensive switches can be configured to allow a port to listen to the traffic of another port (what is called port of duplicate); however, this is a much higher expense than if they are employed concentrators. It allows connecting other equipment.
- Some computer or cluster groups require each of the team members to receive all the traffic they try to go to the group. A concentrater will do this naturally; using a switch in these cases requires the application of special tricks.
- When a switch is accessible to end users to make connections, for example in a conference room, an inexperienced user can reduce the network by connecting two ports together, causing a loop. This can be avoided by using a concentrator, where a loop will break into the concentrator for other users (it can also be prevented by buying switches that can detect and cope with loops, for example by applying Spanning Tree Protocol).
- A cheap hub with a 10-Base-2 port is probably the easiest and cheapest way to connect devices that only support 10-Base-2 to a modern network (not usually coming with 10-Base-2 ports with cheap switches).