DPP Communication 4-3
DPP Product Guide DPP001 and up
Polling link adjustment
The DPP contains a feature that enables the user to perform maintenance-related
functions on the data polling link. For actual maintenance conditions to be
performed in response to a failed data link, refer to the DPP Maintenance
Procedures Guide. The feature is activated by a system command, detailed as
follows:
• enter LINIT to initialize a specified Data Link
• enter LNKDSC to disconnect a specified data link.
Note: For more detail on polling link adjustment commands and their
resultant system messages, refer to the DPP Administration Guide.
Link layer
Link initialization and polling procedure
1 The HOC polls the DPP through a specific port at the DPP. Dedicated Data
links (dial-up links) are used. The maximum polling frequency is one poll
per hour, although on-demand manual polling from a HOC terminal is al-
lowed at any time, depending on the capabilities of the HOC equipment.
2 The following paragraphs provide the steps required to establish a commu-
nication path, initialize a link, and poll the DPP. The steps described are
presented for example purposes. Information required to define the process
for a particular installation requires access to the detailed records and other
parameters for that installation.
3 The HOC must establish a physical transmission to the DPP. This is ac-
complished by having an automatic calling unit, or similar device, at the
HOC dial the DPP over a normal telephone line, or by having a dedicated
facility. For dial-up connection, the DPP, in conjunction with a modem and
a single number dialer, or direct connect service feature, dials the HOC
back, establishing a secure two-way communication path.
4 The HOC maintains a three minute timer that is started when it initiates the
dialing sequence. If the DPP does not respond within the allotted time, two
additional retries are made. If the two additional retries are also unsuccess-
ful, the HOC reschedules the polling session for that DPP.
5 The DPP and HOC BX.25 level 2 protocols implement the first I frame
password exchange security procedure after link set-up command/response
exchange.
Note: This step is not required for dedicated links.
6 The HOC initiates restart Request/Restart confirm exchange on Logical
Channel 0, thus establishing BX.25 level 3.
7 The HOC sends a Session Connect message containing the HOC Session
Layer Password to the DPP. This message contains the DPP password,
which the HOC must verify before proceeding.
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