Ethernet Hubs

Hubs are also called Multiport Repeaters or Concentrators. They are physical hardware devices and are used with 10 Mbps Ethernet networks or slower - basically they are obsolete. There's a lot of baggage that comes with Hubs such as CSMA/CD and the 5-4-3 rule. You will probably hear from old-timers that talk about the bus arbitration method CSMA/CD and congestion on the network. Sorry that is last decade's news.

Troubleshooting Aid

Don't go throwing out all your hubs as there still are a few good uses for them. One very good use is to aid in sniffing a network problem on an Ethernet switch. Normally, an Ethernet switch creates a circuit switched connection between just two nodes. It can be difficult for a 3rd party to listen to the conversation when troubleshooting. Connect the devices together using an Ethernet hub and you will be able to see all the traffic. It will be at a slower speed of 10 Mbps but the information will still be present for analysis.

Another use for an Ethernet Hub is troubleshooting incompatibilities between equipment. Sometimes, the physical port on an Ethernet switch will not electronically talk to another device's Ethernet port. I've run into this using a Cisco 3750 Catalyst switch and a Digium IaxY VoIP ATA device. After 2 weeks of Cisco tech support, Internet searching and trying every port configuration option I could find, I threw a hub inbetween and everything worked.

Some Hubs are basic hubs with minimum intelligence - no microprocessors. Intelligent Hubs can perform basic diagnostics and test the nodes to see if they are operating correctly. If they are not, the Smart Hubs or Intelligent Hubs will remove the node from the network. Some Smart Hubs can be polled and managed remotely.

Purpose of Hubs

Hubs are used to provide a Physical Star Topology. The Logical Topology is dependant on the Medium Access Control Protocol. At the center of the star is the Hub with the network nodes on the tips of the star.

Star Topology

The Hub is installed in a central wiring closet with all the cables extending to the network nodes. The advantage of having a central wiring location is that it is easier to maintain and troubleshoot large networks. All of the network cables come to the central hub, it is especially easy to detect and fix cable problems. You can easily move a workstation in a star topology by changing the connection to the hub at the central wiring closet.

The disadvantages to a star topology are:

  • failure of the Hub can disable a major section of the network
  • The Star Topology requires more cabling than does the ring or the bus topology because all stations must be connected to the hub, not to the next station.

Hub's OSI Operating Layer

Hubs are multiport repeaters and as such obey the same rules as repeaters (See previous section OSI Operating Layer). They operate at the OSI Model Physical Layer.

Hub's Segment to Segment Characteristics

To understand the Ethernet segment to segment characteristics of a hub, the first thing to do with Ethernet Hubs is to determine how they operate. Logically, they appear as a Bus Topology and physically as a Star Topology. Looking inside an Ethernet Hub, we can see that it consists of a electronic printed circuit board which doesn't tell us much. If we form a functional drawing, we can clearly see how the Physical and Star Topology appears:

Understanding that inside the Hub is only more repeaters, we can draw the conclusion that all connections attached to a Hub are on the same Segment and have the same Segment Number. It is considered one repeater from any port to any port even though it is indicated as a path of 2 repeaters.

The 5-4-3 Rule for Ethernet Hubs:

The 5-4-3 rule is used with the legacy CSMA/CD bus arbitration method. In a 10 Mbps Ethernet network everyone could see everyone else's traffic - it was a shared network. The time it took to go from one edge of the network to the other was critical to avoid collisions. Ethernet switches and the circuit switching technology made the 5-4-3 rule obsolete.

The 5-4-3 (-2 sometimes is included) means that a network is allowed:

  • A maximum of 5 (five) segments between any two stations on the network
  • A maximum of 4 (four) repeaters between any two stations on the network
  • A maximum of 3 (three) populated segments
  • If five segments are used, then 2 (two) must be unpopulated

Cascaded Hub Network

Cascading Hubs means to connect the Hubs together through the RJ45 ports. One Master Hub (Level 1) is connected to many Level 2 (Slave) Hubs who are masters to Level 3 (slave) Hubs in a hierarchical tree or clustered star. The maximum number of stations in a Cascaded Hub Network is limited to 128.

Backbone Networks

In a Backbone Network, there is no Master Hub. The Level 1 Hubs are connected through their AUI port to a Coax Backbone. For Thin Coax, up to 30 Hubs can be connected together. For Thick Coax, up to 100 Hubs can be connected to the backbone. The Backbone is considered to be a populated segment.

Level 2 Hubs are allowed to be connected to the Level 1 Hubs' 10BaseT ports. This connection between the 2 Hubs is considered an unpopulated segment or link segment. Up to 1024 stations or nodes can be attached to the Level 2 Hubs' 10BaseT ports.

All stations and segments would appear as 1 Logical segment with 1 Network Number. In the real world, you would never attach 1024 stations to 1 segment, the resulting traffic would slow the network to a crawl.

Hub's Addressing

Again, because a Hub is just many repeaters in the same box, any network traffic between nodes is heard over the complete network. As far as the stations are concerned, they are connected on 1 long logical bus (wire).

Half-Duplex and Full-Duplex Ethernet Hubs

Normal Ethernet operation is Half-Duplex: only 1 station or node talking at a time. The stations take turns talking on the bus (CSMA/CD -bus arbitration).

Full-Duplex Ethernet Hubs are Hubs which allow 2 way communication between Hubs thus doubling the available bandwidth from 10 Mbps to 20 Mbps. Full duplex Hubs are proprietary products and normally only work within their own manufacturer's line.

If A wanted to talk to C, a direct 10 Mbps line would be connected through the 2 switching hubs. Simultaneously, if D wanted to talk to B, another direct 10 Mbps line in the opposite direction would be connected through the 2 switching Hubs thus doubling the available bandwidth to 20 Mbps.

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Copyright July 2013 Eugene Blanchard