Scope Xlite USA Version
XLITEUSA
Issue
3
16
Appendix A
TAP - (T)elocator (A)lphanumeric (P)rotocol:
TAP is commonly used in the wide-area paging industry, generally between remote transmitting
stations via a modem. Its' advantage lies in the two-way nature of the communications link, and
using checksums, the ability to request retransmissions in the event that an error is detected by
the receiving station.
There are two optional parameters: the
password
and a
sign-off text message
, and either or
both of these can be included.
Scope TAP implementation adheres strictly to the Version 1.8 specification, and will allow both of
these parameters to be defined or left out. If a password is not programmed into the system, it will
not expect one, the same goes for the sign-off string, if the entry is blank it will not appear.
A typical TAP transaction is as follows:
1
The caller makes sure the receiving station is connected by requesting a response.
2
The receiving station responds with a request for the identity of the caller, in this case
the enquiry consists of the text string 'ID='.
3
The caller then sends a string to identify the protocol it intends to use, and a
password if this is required.
4
If the receiving station accepts the response and password it acknowledges the fact
and then requests the sender deliver the message it is holding for transmission.
5
The sender transmits the pager number and text message to deliver, and attaches a
checksum based on the contents of the data transmitted. (see example below).
6
The receiving station computes the checksum from the received data and if it
matches the appended checksum of the received data, acknowledges the fact or
requests a re-transmission of the data if this is not the case.
7
The sending station will then either request another message block is transmitted, or
indicate to the remote station that the transaction is complete.
8
Once all the data has been sent, and the sending station indicates this fact, the
receiving station will optionally send a sign-off message, and instruct the sender to
terminate the link. If no sign-off message is included, only the link termination
command is issued.
On a modem-connected link, this would be the point at which the line is dropped by both sides.
On a permanent link, the sender and receiving station return to their normal handshake
exchanges to verify the link, or cease communication until another transaction is required.
These steps are normally limited to a set number of attempts, due mainly to the requirement that
a telephone or modem link should not be maintained indefinitely. For this reason there are also
time constraints which specify how long to wait for a given response.
These 'retry' numbers and time limits are the other 'flexible' options in the TAP protocol. The
Scope system allows the installer to specify these values for themselves to suit the application to
which the protocol is applied.
