Network model description

UM1573

4/27

Doc ID 023619 Rev 1

1

Network model description

The power line modem firmware stack is designed to manage a network structure

composed of several power line modems, called nodes. These modem nodes are

distributed throughout the power line. One or more modems can be used as a data

concentrator and the others as slave modems. All the nodes and the concentrator share the

same power line, which is used as a data communication channel (physical means).

The network is logically designed to be a master-slave structure, where the data

concentrator is considered a master device and each addressed node is the slave. And, in

fact, any device can initiate the communication, thereby becoming a master, while the target

node, having a unique address ID, becomes the slave device.

Each node can also act as a data repeater, without any additional programming features,

increasing both the reliability of the network and the statistical probability that the

information from the master to the slave is delivered even under difficult network conditions.

However, the coexistence of more than one master and more repeaters introduces the need

for data collision management. Since more than one device can initiate communication at

any time, this may cause a network jam, lowering overall performance. This potential

problem can be solved using one of two main techniques. One of these - carrier sense

multiple access with collision detect (CSMA/CD) - is used when a hardware device is able to

detect any collision during the data transmission. The other - carrier sense multiple access

with collision avoidance (CSMA/CA) - the one used with this transceiver, is used when the

hardware does not have this capability.

Figure 1.

CSMA/CA

The implemented conflict avoidance mechanism uses a backoff time and the “band in use”

(BU) hardware feature of the PLM to avoid transmission conflicts. Before initiating any

communication, each device waits until the band is free by checking its own BU flag. As

soon as the band is free, a random backoff time is calculated. Once the backoff time has

elapsed, if the band is still free, the transmission is started, otherwise the loop is started

again (waiting for the BU and new backoff calculation).

t

TRANSMISSION

Node B

BACKOFF

Node A

Node C

Node D

BACKOFF

BACKOFF

BACKOFF

BAND IN USE

BAND IN USE

BAND IN USE

BACKOFF

BACKOFF

BACKOFF

BACKOFF

TRANSMISSION

BAND FREE

BAND IN USE

BACKOFF

BAND IN USE

BAND IN USE

BACKOFF

BAND IN USE

BACKOFF

BAND FREE

TRANSMISSION

TRANSMISSION

BAND FREE

BAND FREE

AM15243v1