ZyXEL Communications PLA-470 V2 - V3.0.5 Guia de Instalação descarregar pdf (Página 94)

Access to Physical Frames

The data exchanged over a PLC network is carried in PLC frames known as “physi-

cal frames.”

The PLC frames circulate over the electrical network between all the outlets in

encrypted form. As explained above, it is difficult to have access to the physical

medium. Because of this, the frames are relatively protected from attacks intended

to accumulate enough frames to try them out with a brute forcing tool intended to

try out all the combinations or using various decryption algorithms.

In addition, the PLC frames are carried in several frequency bands; each of these

bands may use various information transport techniques, i.e., binary data modula-

tion techniques over the transmission channel.

As we have seen in Chapters 2 and 3, the various network PLC devices perma-

nently adapt their digital transmission technique according to the quality of the PLC

links, i.e., the capacity of the transmission channel in terms of bit rate. For this pur-

pose, the tone map indexes the links between the PLC device storing it and all the

other network PLC devices.

To have access to the physical frames, it is therefore necessary to continually

know this tone map in order to identify the technique used to transport information

between the network PLC devices.

Authentication

The authentication of a PLC device consists in knowing the NEK that identifies the

network to which it belongs. If a PLC device does not have the right NEK, it cannot

exchange data with the devices of the PLC network to which it wishes to connect.

Figure 4.12 illustrates the main steps relating to the access of a PLC device to a

network identified by the NEK (network encryption key) of HomePlug 1.0 and

Turbo. This NEK, called here NEK2, is the identifier of the PLC network since only

the PLC devices that have a configuration with this key belong to this network.

Certain more advanced PLC devices, like those of the Asoka brand, are used to

create an authentication of the devices concerning the MAC address in addition to

the NEK key. This authentication is managed from the network administration

interface by means of a list of MAC addresses which may belong to the PLC net-

work.

Network Keys

In a computer network, the network keys are used to protect the exchanged data by

encrypting it before sending this data over the network. In a PLC network, the data

flows over the electrical network, which is a shared network. Therefore, it is impor-

tant to encrypt the data to avoid data recovery. For this purpose, the PLC networks

use keys that make it possible to identify a network and all the PLC devices belong-

ing to it.

In HomePlug 1.0, there are two encryption keys, NEK and DEK, stored in a reg-

ister specific to each device and accessible via the EKS (encryption key select)

parameter.

Security for PLC Networks 75

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ZyXEL Communications PLA-470 V2 - V3.0.5 Guia de Instalação Página 94