Page 75 - DCAP406_DCAP_207_Computer Networks

Computer Networks/Networks

Notes Star Topology. Looking inside an Ethernet Hub, we can see that it consists of a electronic printed
circuit board. 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.

Cascaded Hub Network

Connecting Hubs together through 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’ 10 BaseT 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’ 10 BaseT ports. All stations and segments would
appear as one logical segment, with one network Number.

!
Caution In the real world, 1024 stations are never attached to one segment; as the resulting
traffic would slow the network to a crawl.

5.5.2 Hub’s Addressing

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 one
long logical bus (wire).
Half-Duplex and 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 although
proprietary standards do exist.
5.5.3 Switching Hubs

A switch, which resembles a hub, is known as switching hub. But there is stark difference
between hub and switching hub or a switch. Hub acts as a LAN concentrators and repeaters.

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