3-5. How does a switch work?
The switch is a layer 2 Ethernet Switch equipped with 24 Gigabit Ethernet ports and 4 optional
modules which support Gigabit Ethernet or 100M Ethernet. Each port on it is an independent LAN
segment and thus has 24 LAN segments and 26 collision domains, contrast to the traditional shared
Ethernet HUB in which all ports share the same media and use the same collision domain and thus limit
the bandwidth utilization. With switch’s separated collision domain, it can extend the LAN diameter farther
than the shared HUB does and highly improve the efficiency of the traffic transmission.
Due to the architecture, the switch can provide full-duplex operation to double the bandwidth per
port and many other features, such as VLAN, bandwidth aggregation and so on, not able to be supported
in a shared hub.
3-5-1. Terminology
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Separate Access Domains:
As per the description in the section of “What is Ethernet?”, Ethernet utilizes CSMA/CD to arbitrate
who can transmit data to the station(s) attached in the LAN. When more than one station transmits data
within the same slot time, the signals will collide, referred to as collision. The arbitrator will arbitrate who
should gain the media. The arbitrator is a distributed mechanism in which all stations contend to gain the
media. Please refer to “What’s the Ethernet” for more details.
In Fig.3-6, assumed in half duplex, you will see some ports of the switch are linked to a shared
HUB, which connects many hosts, and some ports just are individually linked to a single host. The hosts
attached to a shared hub will be in the same collision domain, separated by the switch, and use
CSMA/CD rule. For the host directly attached to the switch, because no other host(s) joins the traffic
contention, hence it will not be affected by CSMA/CD. These LAN segments are separated in different
access domains by the switch.
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Micro-segmentation:
To have a port of the switch connected to a single host is referred to as micro-segmentation. It has
the following interesting characteristics.
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There is no need the access contention (e.g.Collision). They have their own access
domain. But, collision still could happen between the host and the switch port.
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When performing the full duplex, the collision vanishes.
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The host owns a dedicated bandwidth of the port.
The switch port can run at different speed, such as 10Mbps, 100Mbps or 1000Mbps. A shared hub
cannot afford this.
