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The maximum length of the extension is equal to the quantity (slotTime -
minFrameSize). The MAC continues to monitor the medium for collisions while it is
transmitting extension bits, and it will treat any collision that occurs after the
threshold (slotTime) as a late collision.
3-4. How does a switch work?
The switch is a layer 2 Ethernet Switch equipped with 24 Fast Ethernet ports
and 2 optional modules which support Gigabit Ethernet or 100M Ethernet. Each port
on it is an independent LAN segment and thus has 26 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, that are not supported in a shared hub.
Terminology
Separate Access Domains:
As per the description in the section of “What’s the 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.3-5, 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 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.
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.
- Access contention (e.g. Collision) is not necessary. Each
micro-segment has its own access domain; however, collision
can still occur between each host and the switch port.
- When performing the full duplex, the collision vanishes.
- 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.