Table of Contents

Backup/Overflow/Dynamic Line Management (DLM)

Backup/Overflow/Dynamic Line Management (DLM)


The management of switched lines (PSTN or ISDN) can be handled by the dynamic Line Management function (DLM) incorporated in the equipment. It allows optimization of the use of switched lines for backup and for load overflow.

The DLM management function performs the following:

These functions may be supported by:


Centralized management of switched lines

The access mode to switched lines can be handled by the DLM management function; this processing takes place:

For each dedicated line of a node, the utilization conditions for the switched accesses for each DLM (or PLL) service must be specified. For this, it is necessary to indicate, for each line, the following information:

The list of parties is referenced by a single X.25 address; this address may be real or logic. The address is the "switched network" aimed point (AP) associated with the line.

The "switched network" aimed points thus defined can also be managed by the routing system. This routing with the aimed point consists in associating with a set of addresses an equivalent "aimed point" address. Routing is broken down into two phases:

To simplify the configurations, it is recommended that calls should be routed with the management of the aimed points.

Figure 12-1:

Aimed Point Management Example

The configuration of node 10 should indicate:

C22: the switched numbers A and B are associated with node 15 (node 15 = "Switched Network" aimed point).

Line number 1 must make reference to the "Switched Network" aimed point defined, thus allowing the use of its routing; to accomplish this:

The "Switched network" aimed point corresponds in this case to the address of the remote equipment and only it is defined as DNIC Z0 known in classes 9 and 22.

It is possible to indicate, for each switched number associated with the aimed point, an associated behaviour. This will make it possible to define:

The list of switched lines depends on the routing used and hence on the aimed point when routing takes place with the aimed point.

Note NOTE The combination of the switched accesses requires the identification of the use made or requested at a given time for the switched accesses.

For incoming calls, the use is identified by the standard elements of the standard ISDN access D protocol.

These are the following:

These elements make it possible:

Identification and consequently combined use for incoming calls is possible only on S0 accesses incorporated in the equipment.

General principles of load measurement

Load measurement is a service that can be used independently of load overflow. For each line, an outstanding event involving a threshold overshoot is generated when the measured load is higher than the upper threshold or lower than the lower threshold.

Outstanding events can be processed internally by the Dynamic Line Management function (DLM) and/or transmitted to a remote management centre.

The "upper threshold" outstanding event is repeated as long as the load is higher than the configured threshold.

The "lower threshold" outstanding event is repeated as long as a switched line of the "backup/overflow" (C24R5P128) type is connected.

In the case of an MLP bundle, it is sufficient that a line of this type be open for the outstanding event to be repeated with each consolidated measurement.

The transmission frequency for these outstanding events can be limited by a DLM management function parameter (C24R5P127). This parameter configures the time-out between transmission of two outstanding events concerning the same line (lower threshold and upper threshold).

Load measurement is carried out by the observation of traffic (transmit and receive directions) on the lines during a period of time. This may be configured (C24R5P128 in steps of 10 s) for all the lines.

For each line, the measurement of this period is compared with the associated thresholds. This comparison is made on the basis of the maximum incoming and outgoing traffic.

The use of a switched line to relieve the overloaded line (overflow) takes place when this comparison is positive n times consecutively. The parameter n can be configured for each line in C12RiPn.

The thresholds are expressed in occupation percentage. The calculation of the occupation percentage of a line is carried out using the speed declared during configuration.

For the integrated ISDN, the speed is fixed (64 kbytes) and not configurable; for this reason, the DLM management function does not take into account the configuration in calculating the occupation percentage of B channels.

For the cost of an S0 interface, it must be configured on the D channel.

Overflow backup by MLP procedure

The standard MLP procedure is located between layers 2 and 3 of Recommendation X.25. It enables an X.25 level 3 to use several X.25 level 2 (LAPB) and thus distribute the traffic of a call on several lines.

This procedure allows the following operations:

Bundle management

A bundle is identified by the routing line called the "principal line". The other lines are called "secondary lines". All the lines of a bundle (up to 8) must be located on the same module.

Backup and overflow of the lines of the bundle takes place without any virtual circuit (VC) break. In the case of remote equipment failure, backup is possible with VC breaks (possible use of securized virtual circuits).

Line backup

The "line backup" service makes it possible to maintain the link between equipment by means of the remaining lines of the bundle or by opening an automatic or manual line.

Start-up and stopping of the "switched network line backup" service can be automatic or manual.

The opening of a switched backup in automatic mode is accomplished without a VC break and only when VCs have already been established in the bundle. The bundle is closed when no VC is established.

When subsequent calls are transmitted to the DLM function (routing configuration), a switched backup is opened to allow the reopening of the bundle.

Upon closing of the backup via the switched network without the use of the "automatic overflow" service, the opened switched line is closed upon a level 2 event on one of the secondary lines of the bundle in service.

When the automatic backup is used with the "automatic overflow" service, the cutoff of opened switched lines takes place according to the load. Automatic line backup must be configured on a single equipment (master).

Line overflow

This service allows overflow of part of the LS traffic of a bundle onto one or more switched lines. It may be manual or automatic (e.g. remote control from a management centre).

Opening and closing of switched lines in automatic mode takes place according to the bundle load measurement (see paragraph on "General principles of load measurement").

MLP bundle load measurement takes into account all the lines of the bundle whose level 2 is in service; it is carried out with the parameters of the principal line.

Only the maximum data rate of each line must be obligatorily configured in order to deduce the maximum data rate of the bundle. This is the sum of the bundle lines in service. The upper and lower thresholds as well as the consolidation configured on the secondary lines are not taken into account.

On each transmission of an "upper threshold" outstanding event, the DLM management function opens a switched line complying with the order of declaration of the lines in the bundle (C25).

The opening of this switched line allows an increase in the maximum data rate of the bundle, leading to a drop in the overall load threshold of the bundle. Switched lines are opened one by one and their opening speed depends on the configuration of the load measurement.

On each transmission of a "lower threshold" outstanding event, the DLM management function analyzes the possibility of closing the last switched line(s) opened. These lines are not closed under the following conditions:

Double confirmation of the "lower threshold" (wait for traffic stabilization).
The DLM management function closes the switched lines one by one as long as the "lower threshold is not exceeded. This mechanism makes it possible to avoid oscillation between the opening of the "upper threshold" and the closing of the "lower threshold" of the switched lines of the bundle.
The DLM management function acts according to the type of backup configured.
The switched line is not closed when automatic backup is configured and the VCs have already been established.

Return after backup

The return after backup, i.e. the disconnection of dial-up lmines opened to back up a leased line or a PLL out of service, is performed by GDL on an event putting the leased line (or PLL) into service


The DML management function does not have a user interface.

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