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1.12
SEL-2701 Ethernet Processor Instruction Manual
Date Code 20020501
Introduction
Ethernet Networks
When a collision occurs on an Ethernet network, the sending nodes stop
transmitting and insert a delay before listening for a carrier and starting the
transmission sequence again. This process is called back-off. If collisions
persist, the node will eventually abandon the outgoing message and upper
protocol layers must cope with the loss of data.
Because of CSMA/CD operation, communication response times on an
Ethernet network are not deterministic. Ethernet networks do not have
guaranteed delivery times or guaranteed performance. For small, lightly
loaded networks, CSMA/CD is efficient and fast. For large networks or during
periods of sustained high network traffic, data transport times can become
significant.
For most measurement and status data collection, the nondeterministic
performance of Ethernet networking is not a cause for concern. For time
sensitive data such as peer-to-peer protection and control, Ethernet network
performance, loading, and architecture are important network design
considerations. An Ethernet switch, for example, can greatly improve Ethernet
network determinism and performance for large networks or slow data
transmission speeds. See
Hubs, Switches, and Routers on page 1.12
for more
discussion on the components of Ethernet networks.
TCP/IP,
UDP/IP, and OSI
The three most common stacks used on Ethernet networks are TCP/IP, UDP/
IP, and OSI. TCP/IP and UDP/IP are the network stacks that have gained fame
as the basis of the Internet. Telecommunication equipment is the primary
application for the OSI stack. The OSI stack and OSI seven-layer model are
two different things. The OSI stack is a network protocol stack that can be
modeled using the OSI seven-layer model shown in
The SEL-2701 operates TCP/IP, UDP/IP, and OSI stacks in parallel to allow
future applications to use whatever stack is required. UCA2 operates either on
the OSI stack or partially on TCP/IP and OSI. FTP and Telnet are application-
layer protocols on the TCP/IP stack.
Hubs, Switches,
and Routers
Originally, Ethernet networks were multidrop networks that had a single trunk
cable with a tap at each network node. While multidrop networks are simple to
imagine, they have two principle drawbacks.
First, multidrop cable systems can fail if a single section of the trunk cable is
damaged or severed. Second, it is difficult and expensive to add new nodes.
The tap length, for example, is limited. If you want to add a new node 100 feet
from the existing trunk cable, you may have to run 100 feet of trunk cable to
the new node and 100 feet of trunk cable back to the existing trunk cable. The
resulting configuration is shown in
.
Содержание SEL-2701
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