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.
CompTIA Network+ N10-008 Cert Guide contains proven study features that allow you to succeed on the exam the first time. Expert instructor Anthony Sequeira shares preparation hints and test-taking tips, helping you identify areas of weakness and improve both your conceptual knowledge and hands-on skills, essential for successful completion of the performance-based testing items on the exam. This complete, CompTIA-approved study package includes the following:
• A test-preparation routine proven to help you pass the exams
• Clearly defined chapter learning objectives covering all N10-008 exam topics
• Chapter-ending review questions and exam preparation exercises, which help you drill on key concepts you must know thoroughly
• The powerful Pearson Test Prep practice test software, complete with hundreds of well-reviewed, exam-realistic questions, customization options, and detailed performance reports
• 40 performance-based exercises to help you prepare for the hands-on exam questions
• A free copy of the CompTIA Network+ N10-008 Simulator Lite software, complete with meaningful lab exercises that enhance your hands-on skills
• More than 60 minutes of video mentoring
• A final preparation chapter that guides you through tools and resources to help you craft your review and test taking strategies
• An Exam Essentials appendix that quickly recaps all major chapter topics for easy reference, both in print and interactive digital format
• A key terms Glossary in both print and on the companion website, which acts as an interactive flash-card application
• Study plan suggestions and templates to help you organize and optimize your study time
• A 10% exam discount voucher (a $33+ value!)
Well regarded for its level of detail, study plans, assessment features, challenging review questions and exercises, video instruction, and hands-on labs, this approved study guide helps you master the concepts and techniques that ensure your exam success.
Master the topics on the CompTIA Network+ N10-008 exam, including:
• Network topologies and media types
• IP addressing
• Network services
• Data center architectures and cloud concepts
• Routing, Ethernet switching, and wireless networking
• Network availability and disaster recovery
• Network security
• Remote access
• Network troubleshooting
Learn more about the CompTIA Network+ N10-008 Cert Guide at amazon.com
More Networking Topologies Articles:
• The Difference Between a Hub and a Router
• The IEEE 802.3 Ethernet Standards
• A Guide to Broadband Internet Connections
• MPO Connector, MTP Connector, What's the Difference?
• Ethernet Network
• Wireless or Wired Network?
• Routing Technologies, Bandwidth Management and Traffic Shaping
• Introduction to ATM (Asynchronous Transfer Mode) Networks
• Network Storage Server Options
• How In-Row Cooling Increases Data Center Efficiency