GE QS24 UR - Manual (Page 11)

QUICKSTART GUIDE
UR SWITCH MODULE – QUICKSTART GUIDE QS–7
QS.3 Hubs and Switches
Two of the major Ethernet network components are hubs and switches. A switch has
many advantages over a hub. Hubs simply transfer information from one port to all other
ports. Since a hub has no way of handling media contention, collisions can occur,
necessitating all segments connected to a hub to work in a half-duplex mode.
Switches on the other hand, are capable of buffering messages, thus allowing full duplex
operation. A switch will also learn the MAC addresses of devices connected to each of it’s
ports and will then route messages to only that port to which the destination device is
connected, thereby reducing data traffic on the network. Switches that use configuration
software to customize both switch and traffic are called “managed switches.” Other
switches - ones that have fixed configurations - are referred to as “unmanaged switches.”
QS.3.1 Fiber Optic Ethernet:
Fiber optic Ethernet is rapidly becoming the medium of choice in applications, such as
power system applications, where longer distances and immunity to EMI are of
importance. Having stated this, the higher cost and the difficulty of terminating fiber
cables allows twisted pair to continue to play a role were appropriate.
The wavelengths of light used in fiber optic communication are 820, 1300 and 1550
nanometers (nm) because it has been found that these wavelengths are attenuated least
as they travel through the fiber optic medium. Compatible ports must operate at the same
light wavelength, and must be linked with appropriate fiber. There are two categories of
fiber optic cable: multi-mode and single-mode. Note that until recently, cable used with
820nm wavelength light was offered only in multi-mode, while cable used with 1300nm
wavelength light was offered with both single- and multi-mode compatibility. Cable
compatible with 1550nm wavelength light is offered only in single-mode.
Wavelengths of light
Note that both devices must use the same wavelength of light.
Multi-Mode Single-Mode
820nm X X
1300nm X X
1550nm N/A X