A hub (specifically an Ethernet hub) lives at Layer 1 of the OSI model, as a result, a hub does not make forwarding decisions. Instead, a hub receives bits on one port and then retransmits those bits out all other ports (that is, all ports on the hub other than the ports on which the bits were received). Because of this basic function, a hub is also called a bit spitter or a repeater
Hubs are most often connected to other network devices via UTP cabling; however, some early versions of Ethernet hubs (prior to the popularization of Ethernet switches) supported fiber-optic connections.
The three basic types of Ethernet hubs are as follows:
• Passive hub: This type of hub does not amplify (that is, electrically generate) received bits.
• Active hub: This type of hub regenerates incoming bits as they are sent out all the ports on a hub other than the port on which the bits are received.
• Smart hub: The term smart hub usually implies an active hub with enhanced features, such as support for Simple Network Management Protocol (SNMP).
A significant downside to hubs - and the main reason they have largely been replaced with switches - is that all ports on a hub belong to the same collision domain. A collision domain represents an area on a LAN on which there can be only one transmission at a time. Because multiple devices can reside in the same collision domain, as is the case with multiple PCs connected to a hub, if two devices transmit at the same time, those transmissions collide and have to be retransmitted.
Because of the collision domain issue and the inefficient use of bandwidth (that is, bits being sent out all ports rather than only the port needing the bits), hubs are rarely used in modern LANs. However, they are an important piece of the history of Ethernet networks and share some of the characteristics of different areas of modern Ethernet networks. For example, a wireless access point (AP) is much like a hub, in that all the wireless devices associated with the AP belong to the same collision domain.
Figure 9-1 Ethernet Hub
In Figure 9-1, notice that the PCs depicted are interconnected using an Ethernet hub, but they are all in the same collision domain. As a result, only one of the connected PC's can transmit at any one time. This characteristic of hubs can limit the scalability of hub-based LANs.
Keep in mind that all the devices on a hub belong to the same broadcast domain, which means that a broadcast sent into the hub will be propagated out all of the ports on the hub (other than the port on which the broadcast was received).
About The Author
Anthony Sequeira, CCIE No. 15626, is a Cisco Certified Systems Instructor (CCSI) and author regarding all levels and tracks of Cisco Certification. Anthony formally began his career in the information technology industry in 1994 with IBM in Tampa, Florida. He quickly formed his own computer consultancy, Computer Solutions, and then discovered his true passion-teaching and writing about Microsoft and Cisco technologies. Anthony joined Mastering Computers in 1996 and lectured to massive audiences around the world about the latest in computer technologies. Mastering Computers became the revolutionary online training company, KnowledgeNet, and Anthony trained there for many years. Anthony is currently pursuing his second CCIE in the area of Security and is a full-time instructor for the next-generation of KnowledgeNet, StormWind.com. Anthony is also a VMware Certified Professional.
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