Specifications

Niagara Release 2.3
Revised: May 22, 2002 Niagara Networking & Connectivity Guide
Chapter 1 Understanding Networking and IP Addressing
Introduction to Networking
1–14
the network, which in turn passes it on to the next, and so on. Eventually, the frame
returns to the (original) transmitting station. At that point, the source station
compares the received frame to that which it transmitted. If the transmission is
identical (free of errors), the station releases the token. This is the access method used
in Token Ring networks.
Polling Polling is also referred to as demand priority messaging. With polling, a host device
controls which of a collection of secondary devices has access to the communications
channel. The primary device checks each secondary device it turn to see if it has
anything to transmit. If so, the secondary device is allowed to transmit for a limited
period before the primary device continues polling. The primary device can give
priority to certain devices, if necessary. Polling is typically used in hierarchical
network designs, where there is centralized control of network communications. This
is the access method used in RS-485 master/slave networks.
Expanding Networks
Devices that are used to extend transmission distances in networks, therefore
expanding their physical reach and/or capacity include repeaters, bridges, and
routers. These devices can be used to connect otherwise independent wiring
segments, sub-networks, or complete LANs.
Repeaters Because of attenuation (loss of signal strength), each of the various types of
transmission media used in networks has a maximum suggested length. Repeaters
can be used to extend the network beyond this distance. Hubs are actually multi-port
repeaters.
Repeaters are not simply amplifiers. If repeaters just amplified a signal, they would
amplify any electrical noise on the transmission medium as well as the signal.
Instead, repeaters analyze signals, make a crisp and clean copy of the data, then pass
it onto the next segment of network cable. Although the number of repeaters that you
are allowed to use is limited, you can use repeaters to extend the distance of a network
well beyond the normal limitations of the transmission medium. Repeaters are used
to tie two LANs of the same type together (i.e., Ethernet to Ethernet, Token Ring to
Token Ring, etc.).
Advantages of repeaters:
• Repeaters are inexpensive.
• Repeaters extend the distance of a network.
• Since repeaters perform little or no processing, they are very fast.
• Repeaters can connect different types of media such as twisted pair to fiber.
Disadvantages of Repeaters
• Repeaters cannot control network traffic.
• Repeaters cannot filter problems or traffic congestion from other segments.
• After network traffic passes through a number of repeaters, eventually
synchronization is lost.