Envoy Configuration and Management Manual

Introduction to Envoy

Envoy Configuration and Management Manual—429861-001

2-9

Polling (Invitation to Send)

To solicit data transfers from the tributary stations, the supervisor station periodically

polls each tributary station in the multipoint link. Polling is performed by transmitting a

tributary’s polling sequence and then waiting momentarily for a response from the

tributary.

If the tributary has no data to transmit, the next tributary station in the multipoint link is

polled. If the tributary has data to send, it responds to the poll by transmitting the data to

the supervisor. When a tributary transmits data to the supervisor, further polling of the

data link stops while the data transmission takes place. When the transmission is

finished, polling resumes with the next tributary station.

When you initiate polling, Envoy progresses from top to bottom through the polling

sequence until either a station responds positively or until all stations respond

negatively. In the latter case, polling continues once again at the top of the list. When

you initiate polling, you can specify either continuous (infinite loop) polling or

noncontinuous (finite loop) polling. For noncontinuous polling, you specify how many

passes you want made through the polling list.

Selection (Request to Send)

To transfer data to a tributary station, the supervisor station first selects the desired

tributary stations by transmitting the appropriate selection sequence. For selection to

occur, the tributary stations must be monitoring the line and must be able to accept the

incoming data. Following a successful selection, the supervisor station transmits the data

to the tributary stations.

Figure 2-4. Multipoint Data Link

007CDT .CDD

Nonswitched

Multipoint Line

(Half-Duplex or Full-

Duplex

Tributary

Station A

Station B

Station C

Station D

Tributary

Supervisor