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1.11
Date Code 20020501
SEL-2701 Ethernet Processor Instruction Manual
Introduction
Ethernet Networks
As long as there is a defined interface between layers, one layer can be
replaced by another that conforms to the interface specification. For example,
Ethernet networks can operate over many different media, from wire cables to
fiber optics to wireless radio connections. The physical layer can be replaced
as long as the interface remains unchanged.
A group of layers designed for a specific application can be defined together
and called a “stack.” The stack may coexist with other stacks on the network
or may operate in a way that requires networks to be segregated based on
stacks. See
TCP/IP, UDP/IP, and OSI on page 1.12
for more information on
Ethernet stacks.
Ethernet Physical
and Data Link Layers
Ethernet networks operate over many different physical layers. Each standard
physical layer and corresponding data link layer has a designator (e.g.,
10BASE-T) that identifies the layer specifications. The most popular physical
layers for LANs within a single building are fiber optics (10BASE-FL and
100BASE-FX) and twisted-pair (10/100BASE-T). For general use networks,
10 and 100 Mbps are the most popular data transmission speeds.
The data transmission speed defines how many bits of information can travel
past a certain point on the cable within a second. A bit is the smallest unit of
binary data and is either a 1 or a 0. While data transmission speed indicates the
relative performance of various networks, it is not a measure of throughput–
how quickly useful data travel across the network.
Media Access
With Ethernet networks, the time required for data to move across the network
is not guaranteed. This lack of predictable timing must be considered for time-
critical applications such as peer-to-peer protection and control messages. In a
deterministic system, all events occur with completely predictable timing and
sequence. A basic understanding of Ethernet media access rules is important
for understanding why Ethernet networks are not considered deterministic.
High-speed bus and multidrop networks can operate over many different
physical connections or media (for example, fiber-optic cable or twisted-pair
cable). Network operation requires that all devices are connected to a common
medium. All network nodes use the same signaling method.
Because only one node at a time can successfully send data, multidrop and bus
networks must have media access rules to move data effectively across the
network. In networks similar to Modbus
®
, there is a single master device. All
network traffic is either the master requesting information or a response to the
master.
A second method of media access control is token rotation. A special
message, or token, is controlled by a master or forwarded from each peer to
the next. Each node gets the token, acts as the network master, and sends
messages to other devices. For lightly loaded networks, token rotation is
inefficient because nodes that have no pending network operations still receive
the token. A network error may also corrupt or destroy the token message,
causing the network to generate a new token. Token generation is very slow,
compared to normal network operations.
In order to overcome both the drawbacks of master-based and token rotation
media access control, Ethernet networks use a system called CSMA/CD
(carrier sense multiple access/collision detection). In this system, all nodes
can send data at any time. In order for a node to send data, it must first listen
for a carrier to determine that no other node is transmitting. Collisions occur
when two nodes both transmit data at the same time. Ethernet network nodes
have mechanisms to detect collisions.
Содержание SEL-2701
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