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Orbitor ISDN Callback Setup Procedures

Article Number: H012
Article Type: HowTo
Modified: December 1, 1999

SUMMARY

The ISDN Callback feature of the Orbitor is a powerful feature to control security and ISDN call charge billing. In order to use it for your application properly, you will require an understanding of both the feature itself and the ISDN line configuration.

This article will present an overview of this feature, and will provide three (3) real-world application examples that you may use as templates for your application.

MORE INFORMATION

Overview

Callback, or as it is sometimes called, dial-back, has been used in dial-up MODEM applications for over fifteen years. The primary purpose of this feature in those applications was to control access to a secure resource. It was an effective control mechanism and it was difficult to bypass. As a result, most secure dial-in applications required some form of this dial-back feature.

The basic premise of callback in a security application was that if you called me, and you are in my calling database, I will try to call you back at the pre-configured number I have in my database. If you can be reached at this number, you must be okay.

Today, it is not accepted solely as a security mechanism, as there are now more secure methods available. However, it has found renewed application to control the call charge billing of an application, by allowing or forcing a particular location to pick up the call charges. It is this aspect of callback that will be the focus of our three applications.

Prerequisites

Before you can use the ISDN callback feature in your application, a feature called Caller ID must be enabled on the ISDN circuit provided to you by the Telephone company. The Caller ID feature presents the "caller" to the Orbitor in the form of a Telephone number. It is this Telephone number that will be used to decide who is calling and where the callback should take place.

Simple Callback Scenario

In a simple callback scenario, router (A) would like to call and connect to an Orbitor router (B). Router (A) would like the Orbitor (B) to pay for the call charges. The steps that would be executed are as follows:

  1. The router (A) would place an ISDN call to Orbitor (B)
  2. (B) will not actually answer this incoming ISDN call, but instead it will interrogate the ISDN Call Signaling Setup packet to determine who the caller is.
  3. (B) verifies that this is a number located in its callback database.
  4. When (B) decides that it is a callback number, it will reject the Call Setup (forcing a disconnect) and it will then start the callback procedure.
  5. (B) will call the number that was pre-configured in its database to reach (A).
  6. (A) answers the call and the callback procedure is complete.

Given this simple example, router (B) would now be the caller who would be billed for the ISDN call. Because router (B) did not accept the original incoming call, there are no charges incurred by (A).

Application Overview

The callback functionality can be used in many different applications and has the versatility to work well in all. There are a range a features that work in tandem with Callback to fine tune the application of the feature. In the examples provided below, Central Site Router refers to the router that is only to receive calls, while the Remote Site Router is the router that must call the central site to incur the billing charges.

These references are made for the purpose of clarifying the applications presented below and to overview the different responsibilities of each router. It is quite feasible to reverse the roles so that the Central Site Router would be the Callback device. Of course, you must select which router should perform a given role to meet your specific application and to modify the configuration accordingly.

The initial call from the Central Site Router to alert the Remote Site Router to call back is commonly referred to as a tickle. It is this tickle that starts the callback procedure.

Wait just a second!

One issue that must be dealt with in this very first scenario is that the Remote Site Router can actually call back the Central Site Router before the original tickle call has been completely disconnected by the Telephone company. If this happens, the circuit will be busy and the Remote Site Router will not be able to complete the callback call. Although the Remote Site Router will retry this connection, it only serves to delay the required result.

To solve this problem, the Orbitor provides a hold-off timer called Callback Delay. This timer can be adjusted to control the exact period that the Remote Site Router will wait from the time it receives the tickle to the time it will initiate the call back.

This time period is very important and should be considered closely. The period selected must be a period where the circuit can fully disconnect the call, but before the Central Site Router initiates another tickle call. A little experimentation will provide the exact setting needed. Normally, the default setting will be quite adequate.

Configuration Parameters

There are two menus located in the Remote Site setup of the Orbitor that control the application of Callback. Presented below are the two menus. 

Device: DEV802915                                  V 05P4.2.1.3
  REMOTE SITE 2 (CSR) CONNECTION SET-UP ISDN CALL SET-UP MENU

Option                 Value           Description
 1. Advanced settings   menu      - Advanced ISDN call settings
 2. ISDN number	       [none]     - Set ISDN number
 3. Alt. ISDN number   [none]     - Set alternate ISDN number
 4. Group	       [1]        - Specify dial group
 5. Wildcard	       [none]     - Set wildcard ISDN number
 6. Call you	       [none]     - Set call you prefix
 7. Call me	       [none]     - Set call me prefix
 8. Callback	       [disabled] - Enable/disable callback




Device: DEV802915                                 V 05P4.2.1.3
   CONNECTION SET-UP ISDN CALL SET-UP ADVANCED SETTINGS MENU

Option                  Value	       Description
 1. Callback delay	[2 s]	  - Time to wait until callback
 2. Redial timer	[10 s]    - Time to wait until redial
 3. Redial count	[5]	  - Number of redials to try

Scenario #1

Callback to the same number that called you

This is the simplest of the callback scenarios. In this example, when the Central Site Router calls the Remote Site Router, the calling number presented in the Caller ID field is the number you need to call back.

The parameters that need to be adjusted on the Remote Site Router are:

  1. ISDN number - This must be set to the ISDN number that will be sending the tickle (in our example, Central Site Router)
  2. Alt. ISDN number - If a 128Kbps access rate is required this would be the number to establish the second link to the Central Site Router.
  3. Call you - If the number you are calling is long distance this would be where you can include the long distance prefix such as the long distance access code, the country code or the area code. If you are calling long distance, it is suggested that you use this feature and not include the area code in the ISDN number parameter.
  4. Callback - Enabled This it the parameter that enables Callback on for this remote site.
  5. Callback delay - This may need to be adjusted to a larger value than the default 2 seconds. The symptoms of this parameter being set to a value smaller than required would be that when the Remote Site Router calls the Central Site Router back, it will receive a busy signal and be disconnected.

Scenario #2

Callback to a different number

This scenario presents the situation where the tickle is actually originated on a different, perhaps reserved, telephone circuit. Although this calling number will be consistent, the Remote Site Router must call a different number to reach the intended location.

In order to deal with this example, we are now going to establish a Wildcard that will be used to match the incoming call. The Wildcard entry is the number that calls and it will be used to select the correct Remote Site profile in the Orbitor to establish the return call.

The parameters that need to be adjusted are:

  1. ISDN number - This must be set to the ISDN number of the Central Site Router.
  2. Alt. ISDN number - As above
  3. Wildcard - This must be set to the ISDN number that would be seen in the Caller ID field of the call setup message during the tickle.
  4. Call you - As above.
  5. Callback - Enabled - This turns Callback on for this remote site.
  6. Callback delay - As above.

Scenario #3

Callback to a different number

In this scenario, the Central Site Router is connected to a PBX rather than directly to an ISDN line. As a result, any call from the Central Site Router will actually pass through the PBX and be seen to originate from a different outside ISDN line. This outside line may differ since the PBX would choose an available line automatically.

The Remote Site Router, then, would actually see calls from a range of ISDN numbers. The Caller Line ID presented to the Remote Site Router would not actually correspond to the Central Site Router that is calling, but would instead correspond to the chosen outside line at the PBX.

Fortunately, the group of ISDN numbers that will be used by the PBX for this call will be very similar and perhaps even in consecutive order. For example, there may be ten ISDN numbers (931-9300 through 931-9309) that might be allocated at the PBX for outgoing calls.

The parameters that need to be adjusted are:

  1. ISDN number - This must be set to the ISDN number of the Central Site Router.
  2. Alt. ISDN number - As above
  3. Wildcard - "931-930X" This would match the range 931-9300 through 931-9309.
  4. Call you -As above
  5. Callback - Enabled - This turns Callback on for this remote site.
  6. Callback delay - As above.

A final note on Wildcards

When using Wildcard entries, the 'X' is used as a placeholder. When you specify an 'X', the Orbitor will match any number that is valid in that position. Any number of 'X' may be placed in the Wildcard number, but care should be exercised that the proper constraints are placed on the incoming number so not to open up a large block of unwanted incoming numbers.

If the range of ISDN numbers used by the PBX are not contiguous, you may have to create multiple remote sites each with their own unique Wildcards to match the desired range of calling numbers.

REFERENCES

For further information on the Callback feature, please refer to the Orbitor Reference manual for details.§
Keywords: Callback, ISDN, Wildcard, security
Product: Orbitor
Model: All

Copyright © 1999 Develcon Electronics Ltd.  All Rights Reserved.