10 Mbit/s hubs

The 10Base-T connections of a 10 Mbit/s hub are implemented as MDI-X ports and therefore already crossed internally. The advantage is that the terminal devices can be directly connected using 1:1 twisted pair cables and no crossover cables are required. Some hubs additionally have a so-called uplink port which can be used to connect another hub. In order to also enable the use of a 1:1 cable for this port, it is implemented as a normal non-crossed MDI port. In many cases this port can also be switched between MDI and MDI-X or is implemented as a double port with two connections in parallel (1 x MDI, 1 x MDI-X). In this case, it has to be observed that these parallel ports may only be used alternatively and not at the same time.

Hubs are normally equipped with several LEDs for status indication. So, for example a Link LED indicates the correct connection between the terminal device and port at the hub. This way, incorrect cabling can be quickly detected. Further LEDs indicate for example the data traffic over a port or the collisions.

The maximum permitted number of 10 Mbit/s hubs within one collision domain is limited to 4. This restriction is due to two reasons. One reason is that the bit period time delay, which is inevitably increased by each hub, must not exceed 576 bit periods. The second reason is that the so-called interframe gap (IFG) must not be shorter than at least 47 bit periods. The interframe gap describes the time interval between two data packages and shall allow the receiving stations to recover from the incoming data stream. However, the regeneration of an incomplete preamble performed by the hub reduces the time between the data packages due to the completion of possibly missing bits.

One possibility to get round the restriction to four hubs is the use of stackable hubs. These hubs are connected to each other via a special interface instead of using the uplink port and therefore constitute one logic unit. As a result, they appear as one single big hub to the external.

Stackable hubs