Siemens SIMATIC NET Instrucciones de operaciones descargar pdf (Pagina 18)

Industrial Ethernet Networks

2-4

SIMATIC NET Twisted-Pair and Fiber-Optic Networks

C79000-G8976-C125-02

Network Access Using the CSMA/CD Protocol

CSMA/CD (Carrier Sense Multiple Access with Collision Detect) is also known as

Listen While Talk (LWT).

This is a distributed access technique; in other words, each DTE connected to the

network has the same access rights.

If a DTE wants to send data, it first “listens” to the medium to find out whether

another DTE is already transmitting. If no other DTE is transmitting, it can start its

transmission. If the DTE detects that the transmission medium is being used by a

different DTE, it must wait until the medium is free again.

All DTEs listen to the data transmitted. Based on the destination address

information in the data, a DTE recognizes whether or not it should receive the

data.

If more than one DTE wants to send at the same time and they all detect that the

medium is free, they start to transmit. After a brief time, the transmitted data will

collide.

The DTEs have a mechanism that allows them to detect such collisions. All the

DTEs involved in the collision then stop transmitting, wait for a random time

calculated differently for each individual DTE, and then attempt to send the data

again. This is repeated until one DTE succeeds in transmitting without a collision.

The others then wait until the transmission medium is free again.

Collision Domain

To make sure that the CSMA/CD access technique functions correctly, the span of

an Ethernet network is limited by the maximum permitted propagation time of a

data packet. The distance within which the CSMA/CD protocol functions perfectly

is known as the collision domain. In the classic 10 Mbps Ethernet, the collision

domain is a span of 4520 m. The configuration rules resulting from these

restrictions can be found in the section “Network Configuration”.

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Siemens SIMATIC NET Instrucciones de operaciones Pagina 18