Chapter 35. Hubs
Hubs can also be called either Multi port Repeaters or Concentrators. They are physical hardware devices.
Some Hubs are basic hubs with minimum intelligence (i.e. 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 dependent on the Medium Access Control Protocol. At the center of the star is the Hub, with the network nodes located on the tips of the star.
Star Topology
The Hub is installed in a central wiring closet, with all the cables extending out to the network nodes. The advantage of having a central wiring location is that it's easier to maintain and troubleshoot large networks. All of the network cables come to the central hub. This way, 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 shown below:
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 multi port repeaters, and as such they 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, determine how the Ethernet Hubs 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, then 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). A single repeater is said to exist 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:
Cascaded Hub Network
Connect Hubs together through RJ45 ports creates Cascading Hubs. 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 un populated 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 & Full-Duplex Ethernet Hubs
Normal Ethernet operation is Half-Duplex: only 1 station or node is talking at a time. The stations take turns talking on the bus (CSMA/CD -bus arbitration).
Full-Duplex Ethernet Hubs are Hubs which allow two-way communication, 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.
For example, 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 two switching Hubs (doubling the available bandwidth to 20 Mbps).
There are no official standards for Full-Duplex Ethernet (proprietary standards do exist).
Switching Hubs
Switching hubs are hubs that will directly switch ports to each other. They are similar to full duplex hubs, except that they allow dedicated 10 Mbps channels between ports.
If A wanted to communicate with B, a dedicated 10 Mbps connection would be established between the two. If C wanted to communicate with D, another dedicated 10 Mbps connection would be established.
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