GE MDS SD4A Technical Manual page 54

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Figure 26. Store and Forward System Example
Store and Forward is available in Packet with MAC mode and supports
all MAC features including collision avoidance, retries, and acknowl-
edgements. Furthermore, all major radio features are supported in SAF
networks such as diagnostics, over the air programming, bridging,
VRCs, etc.
Dual Duty of SAF Remotes: Since an SAF device doubles as a Remote
radio it may participate in direct exchanges with the Master/AP. By
design, an SAF device is always within the primary coverage area and
has direct communication to the AP. Therefore, all SAF devices act as
both a standard Remote for direct exchanges, as well as an SAF unit for
Store and Forward exchanges. An SAF exchange is defined as any
exchange between the AP and a Remote through an SAF device. In all
cases, the AP still controls the operation of the network.
After sending an SAF data frame, the AP remains idle until the frame
has been delivered to the Remote, at which time it will return to direct
or SAF operation, depending on the message type (unicast or broadcast).
Direct vs. SAF Traffic: Depending on network topology, it is possible
for some Remotes to hear both direct polls from the AP and SAF mes-
sages from an SAF-designated Remote. It is more desirable to use the
direct traffic to avoid the latency of going through the SAF link, and the
SD firmware handles this situation automatically. SAF frames are
ignored when the direct path is available. The system is dynamic; if the
direct path becomes unavailable for any reason, it quickly switches to
the SAF path to provide continued operation.
Traffic Routing: Typically, network radios are immobile, but fading
may cause paths to come and go. Because of this, the AP device imple-
ments routing to each Remote device. This ensures that only SAF traffic
passes through SAF Remotes. Direct traffic is terminated at the local
coverage area, keeping efficiency high. The SD radio achieves these
steps automatically, and does not require user input. The following is an
explanation of how this routing scheme is achieved.
46 SD Series Technical Manual MDS 05-4846A01, Rev. F