Siemens SIMATIC NET Instrucciones de operaciones descargar pdf (Pagina 32)

Configuring Networks

3-6

SIMATIC NET Twisted-Pair and Fiber-Optic Networks

C79000-G8976-C125-02

Variability Value and Path Variability Value

In a local area network complying with IEEE 802.3, two data packets must have a

certain minimum gap between them. If the gap is smaller, this is known as an

interframe gap error.

The variability value of a component describes the fluctuations in the propagation

time of a data packet through a network component. If two data packets pass

through several network components one after the other, the gap between the

packets is reduced. The sum of the values of all components is the Path Variability

Value (PVV). The PVV on the path between two DTEs must not exceed 40 bit

times (BT); in other words, the gap between packets can be reduced by a

maximum of 40 bit times. This value includes a safety margin that includes, among

other things, the variability value of the first MAU (Medium Attachment Unit, for

example a twisted pair transceiver integrated in the DTE).

By maintaining this maximum value, a minimum gap between the data packets is

guaranteed allowing correct recognition of the data packets. The transceiver that is

possibly connected to the remote, second DTE does not contribute to the reduction

in the interframe gap.

Points to bear in mind when configuring a network:

1. Check your network for critical connection paths. Critical paths are those in

which the signal runs through long sections of cable and a lot of network

components between two nodes.

2. If you consider a connection path to be critical, check the permitted span (delay

equivalents). The sum of the cable lengths between two nodes + the sum of the

delay equivalents of the network components between the two nodes must not

exceed 4520 m.

3. Check any critical paths to ensure that the maximum path variability values

(PVV) are kept to. The sum of the variability values of the network components

between to stations must not exceed 40 bit times.

4. For correct configuration complying with IEEE 802.3, all the paths must satisfy

these conditions.

Note

When using Industrial Ethernet OSMs/ESMs, the delay equivalent and the path

variability value only need to be checked as far as the port of an OSM/ESM, since

the collision domain starts and ends here.

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Siemens SIMATIC NET Instrucciones de operaciones Pagina 32