User's Manual

338 Module - eESPA

separators, unit separators, and data that is retrieved from the message

request. Every data block also contains a specifically calculated checksum

(block check character ISO 1155). After sending the data over the serial

line, the receiving side uses the block check character (BCC) to determine

whether data has arrived properly or not. In the event of a successful

delivery, the receiving side answers with an ACK.

Figure 83

msgrpy message ACK

If an incorrect BCC is found, delivery fails, and the receiving side sends

a NACK, which is prefixed with error code 1 (Transmission error, corrupt

characters or corrupt BCC received by the station).

eESPA handles only data blocks of type 1, Call to Pager data blocks. If

another type of data block comes in, eESPA reacts by sending an ACK,

but the data block is not processed.

Delivery can also fail if a timeout occurs while sending the data block.

The temporary master station, which is always the sending side, expects

to receive an ACK within a timeout of eESPA_Timeout_n seconds. In

the event of a timeout on sending a data block, the sending side tries

to re-send the data block. This retry is attempted x times (where x is the

defined value of the field eESPA_NAK_retry_cnt_n in the eESPA table).

After retrying x times and not receiving an ACK, the temporary master

station decides that the transaction is unsuccessful.

Figure 84

msgrpy message NACK

Data flow

The ESPA4.4.4 protocol prescribes a controlling station that polls devices

on the communication line. Polling means sending out requests for data.

The polling device, which is also called controlling station or master, sends

Nortel Communication Server 1000

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