Datum Systems PSM-500L Manual do Utilizador descarregar pdf (Página 9)

Datum Systems, Inc. PSM-500 Modem Remote Control Protocol

Rev 0.93 12/12/10

B-9

The modem remote binary packets and front panel menus reflect a division of control into 4 functional

areas: Unit, Modulator, Demodulator and Interface. These may be logical for human control on the front

panel but are not ideal for remote monitor and control. From that perspective modem data might best be

divided into 3 categories:

1. Fixed Data - Should never change except on power-up. (e.g. Unit Type or Serial Num.)

2. Configurable Data - Is changed on command, never by itself (e.g. Mod Data Rate)

3. Status Notification - Can change autonomously like Eb/No, Alarms, Tests, etc.

The purpose of this new Unit Polling Status packet is to allow an M&C system like the SnIP itself,

or an SNMP agent or client to efficiently manage data and not constantly resend huge volumes of

never changing data in order to transfer simple information needed. The structure of the new

0x0A packet allows that. It contains all of the "common" required current status information plus

notification of other items which have changed in a fairly concise manner. See the command

table for its contents.

The SnIP polling routines and the m500ctl use this one packet to create an internal demand

driven database of modem data. This makes most m500ctl functions 2 to 15 times faster, and

allows low latency status information to create an efficient AJAX based web that displays status

information live (within a few seconds delay)

How it works in the m500ctl program is fairly simple, and similar procedures can be used in an

external M&C system connected to the modem's remote control port. The SnIP's background

modem polling routine periodically (every 1 to 5 seconds) requests the 0x0A packet data and

stores that information in a temporary file. Then as a request for data from a particular packet

arrives a decision is made based on 1) local availability of that packet data plus 2) the polling

status information indicating if that data has changed since the last read (is fresh). If either is not

satisfied then the packet with the desired data is read from the modem and the results stored for

future use. If a future request can be satisfied from the local data copy then no request is made to

the modem.

This system is called “demand driven” because no periodic polling is done of all the data. Instead

data is stored locally only when a request demands that data. Only the Polling Status data is

periodically requested. The poll timing can also be demand driven, where repeated requests

cause the polling interval to decrease.

The Polling Status packet contains virtually all data in category 3 above. Items in category 3 are

masked within the modem from the Command Change flags in Bytes 10 to 16 of the Polling

Status packet shown below. Thus those change flags indicate that other items in packet status

has changed. This keeps you from re-reading a packet for changed data you already have. If

however configurable data in category 2 is changed by someone else that bit will be set indicating

that the packet needs to be requested again.

The Change flags in Bytes 0 to 3 of each packet and the Status flags are “latched”, that is they

hold a change in status until read. Therefore changes are not lost if they return to their previous

state between reads. The read interval does not determine if an event is lost, just the granularity

of its detection. The Unit Polling Status command has a slightly different set of change flags

because of its purpose. The Byte 0-3 change flags are reset upon reading this packet, as in all

other packets, but the command change flags in Bytes 10-16 are cleared by reading the particular

packet that the flag refers to. A separate set of individual command change flags are maintained

by the modem for each interface, the internal one used by the SnIP, the rear panel remote control

port and the MCC channel control port.

The Polling Status packet also permits easier implementation of an “event recorder” where

significant events are time stamped and stored for later review.

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