Basecx

Gigabit Ethernet over copper cabling

25 m

Extending a LAN Segment

You can add devices to an Ethernet LAN to extend segments. This topic describes how adding repeaters or hubs can overcome the distance limitation in an Ethernet LAN.

A repeater is a physical layer device that takes a signal from a device on the network and acts as an amplifier. Adding repeaters to a network extends the segments of the network so that data can be communicated successfully over longer distances. There are, however, limits on the number of repeaters that can be added to a network.

A hub, which also operates at the physical layer, is similar to a repeater. Figure 2-1 shows two users connected to a hub, each 100 meters from the hub and effectively 200 meters from one another.

Figure 2-1 Extending the Segment Link with a Hub

100 Meters 100 Meters

Figure 2-1 Extending the Segment Link with a Hub

100 Meters 100 Meters

When a hub receives a transmission signal, it amplifies the signal and retransmits it. Unlike a repeater, however, a hub can have multiple ports to connect to a number of network devices; therefore, a hub retransmits the signal to every port to which a workstation or server is connected. Hubs do not read any of the data passing through them, and they are not aware of the source or destination of the frame. Essentially, a hub simply receives incoming bits, amplifies the electrical signal, and transmits these bits through all its ports to the other devices connected to the same hub.

A hub extends, but does not terminate, an Ethernet LAN. The bandwidth limitation of a shared technology remains. Although each device has its own cable that connects to the hub, all devices of a given Ethernet segment compete for the same amount of bandwidth.

Collisions

Collisions are part of the operation of Ethernet, occurring when two stations attempt to communicate at the same time. Because all the devices on a Layer 1 Ethernet segment share the bandwidth, only one device can transmit at a time. Because there is no control mechanism that states when a device can transmit, collisions can occur as shown in Figure 2-2.

Figure 2-2 Ethernet Collision

Figure 2-2 Ethernet Collision

I can't get to the payroll server.

Collision

I could have walked to Finance by now.

Collision

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