Motorola 68P02958C00-B - Manual (Page 237)

6-2
HealthCheck Mechanism
General
The IP Gateway system includes a HealthCheck mechanism which manages the MDLC
connectivity to the sites. Associated with each site are two links, through which the site can be
reached. A background Ping Pong mechanism in both the IP Gateway and the MOSCAD units
constantly verifies which links are available. .
The HealthCheck mechanism uses the site table as the basis for its operations. As the
IP Gateway identifies which links are available, it can reduce communication overhead by
routing frames destined to sites through their operational links.
MDLC Infrastructure
MDLC provides a frame-sequence service for healthcheck mechanism. Specifically – a
dedicated channel is allocated for this activity at both the RTU and the IP Gateway. All
HealthCheck messages are transmitted and received through this channel. Being a frame-
sequence channel, the healthcheck MDLC channel (Application ID 24) is both reliable and
low-resource consumer at the same time.
The implementation a healthcheck protocol is user-defined. Thus – the MDLC provides a
framework for two entities to maintain this mechanism but does not set any rules as to neither
the data being communicated over this channel nor to timing and policy. For a healthcheck
mechanism to operate properly, both the IP Gateway and RTU should have some protocol
implemented over this infrastructure. Activation and deactivation of this infrastructure is done
through site configuration (refer to MOSCAD site configuration/MOSCAD-L and IP Gateway
site configuration).
Mechanism
In the IP Gateway, the HealthCheck mechanism relies on the MDLC infrastructure denoted
above. Thus, once activated it captures the dedicated healthcheck channel provided by MDLC
and uses the site table as a source for all sites to be managed. For each site the HealthCheck
performs the following process:
Upon a predefined interval, it sends a Ping frame to each site, one through each of the site’s
links. It then expects to receive a Pong frame for each Ping sent. A Ping arriving from a certain
site through a certain link, will set the communication status of that link to OK A site that
possesses at least one link in OK status is considered reachable. This process, done in
background, constantly monitors the status of each site’s links, providing the IP Gateway with
an updated status of all sites in the field.
In order to preserve system resources, the HealthCheck protocol is very minimal in terms of
length of data and timing upon which it operates.
In the RTU, the HealthCheck mechanism relies on the MDLC infrastructure denoted above. It
operates in slave mode. Thus, once a Ping frame arrives at the RTU, the RTU HealthCheck
mechanism replies with an echo of that frame. The RTU transmits a Pong back to the