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All drawings appearing in this Recommendation have been done in Autocad.
Recommendation E.411
INTERNATIONAL NETWORK MANAGEMENT - OPERATIONAL GUIDANCE
1 Introduction
Network management requires real-time monitoring of current network status
and performance and the ability to take prompt action to control the flow of
traffic when necessary (see Recommendation E.410). Operational guidance to meet
these requirements, including a description of status and performance parameters,
traffic controls and the criteria for their application are included in this
Recommendation. It should be noted that the complete range of parameters and
traffic controls are not necessary for the introduction of a limited network
management capability, however a comprehensive selection will bring substantial
benefit (see Recommendation E.410, S 5). In addition, some guidance on beginning
network management is provided, along with information on developing a network
management centre and the use of common channel signalling for network management
purposes.
2 Information requirements
2.1 Network management requires information of where and why difficulties are
occurring or are likely to occur in the network. This information is essential to
identify the source and effect of a difficulty at the earliest possible time, and
will form the basis for any network management action which is taken.
2.2 The information relating to current difficulties can be obtained from:
a) real-time surveillance of the status and performance of the network;
b) information from telephone operators as to where they are experiencing
difficulties; or where they are receiving customer complaints of
difficulties;
c) transmission system failure and planned outage reports (these reports
need not relate only to the network local to one Administration, but
should reflect the whole international network);
d) international or national exchange failures and planned outage reports;
e) news media reports detailing unforeseen events which stimulate traffic
(for example, natural disasters).
2.3 The information relating to difficulties which are likely to occur in the
future will be obtained from:
a) reports of future planned outages of transmission systems;
b) reports of future planned outages of international or national
exchanges;
c) knowledge of special events (for example, international sporting
events, political elections);
d) knowledge of national holidays and festivals (e.g., Christmas Day, New
Year's Day);
e) an analysis of past network performance.
2.4 The system availability information point, defined in Recommendation
M.721, will provide a ready source for much of the information indicated above.
3 Network status and performance data
3.1 In order to identify where and when difficulties are occurring in the
network, or are likely to occur, data will be required which will indicate the
status and measure the performance of the network. Such data will require
real-time collection and processing, and may require the use of thresholds (see S
5.1).
3.2 Data may be collected in various ways which include counters in
electromechanical exchanges which can be read manually when required (e.g.,
during periods of heavy traffic or special events), data output reports from SPC
exchanges, or computerized network management operations systems which can
collect and process data from a large number of exchanges.
Fascicle II.3 - Rec. E.411 PAGE1
3.3 Network status information includes information on the status of
exchanges, circuit groups and common channel signalling systems. This status
information can be provided by one or more types of displays. These may include
printers, video displays, and/or indicators on a display board or network
management console. To be useful, network status indicators should be available
as rapidly as possible.
3.3.1 Exchange status information includes the following:
Load measurem - These are provided by attempt
counts, usage or occupancy data, data on the percent of
real-time capacity available (or in use), blocking rates,
percentage of equipment in use, counts of second trials, etc.
Congestion measurements - These are
provided by measurements of the delay in serving incoming
calls, holding times of equipment, average call processing and
set-up time, queue lengths for common control equipment (or
software queues), and counts of equipment time-outs, etc.
Service availability of exchange equipment - This information will show when major items
of equipment are made busy to traffic. This could highlight a
cause of difficulty or give advance warning that difficulties
could arise if demand increases.
Congestion indicators - In addition to the
above, indicators can be provided by SPC exchanges which show
the degree of congestion. These indicators can show:
- moderate congestion Level 1;
- serious congestion Level 2;
- unable to process calls Level 3.
Note - While this is desirable, SPC exchanges may not be able to provide a
level 3 indicator during catastrophic failures.
The availability of specific exchange status information will depend on
the switching technology employed by each Administration. Details of exchange
measurements are found in Recommendations E.502 and Q.544.
3.3.2 group status information relates
to the following:
- status of all circuit groups available to a destination;
- status of individual circuit sub-groups in a circuit group;
- status of circuits on each circuit group.
Status indicators can be provided to show when the available network is
fully utilized by indicating:
- when all circuits in a circuit group are busy;
- when all circuits in a circuit sub-group are busy;
- when all circuit groups available to a destination are busy.
This would indicate that congestion is present or imminent. Status
information can be provided to show the availability of the network for service,
by reporting the number or percentage of circuits on each circuit group that are
made busy or are available for traffic.
This information could identify the cause of difficulty or give advance
warning that difficulties may arise as the demand increases.
3.3.3 Common channel signalling system status provides information that will
indicate failure or signalling congestion within the system. It includes such
items as:
- receipt of a transfer prohibited signal (Signalling Systems Nos. 6 and
7),
- initiation of an emergency restart procedure (Signalling System No. 6),
- presence of a signalling terminal buffer overflow condition (Signalling
System No. 6),
- signal link unavailability (Signalling System No. 7),
- signal route unavailability (Signalling System No. 7),
- destination inaccessible (Signalling System No. 7).
PAGE12 Fascicle II.3 - Rec. E.411
This information may identify the cause of difficulty or give advance
warning that difficulties may arise as the demand increases.
3.3.3.1 Network management actions may help to reduce congestion in common
channel signalling systems by reducing traffic being offered to common channel
signalling circuit groups, or by diverting traffic to conventional signalling
circuit groups.
3.4 Network performance data should relate to the following:
- traffic performance on each circuit group;
- traffic performance to each destination;
- effectiveness of network management actions.
It may also be desirable to assemble performance data in terms of circuit
group and destination combinations and/or traffic class (for example,
operator-dialled, subscriber-dialled, transit). (See Recommendation E.412, S
2.1.)
3.5 Data collection should be based on a system of measurement which is either
continuous or of a sufficiently rapid sampling rate to give the required
information. For example, for common control switching equipment, the sampling
rate may need to be as frequent as every second.
Reports on network status and performance should be provided periodically,
for example, on a 3 minute, 5 minute, 15 minute, 30 minute or hourly basis, with
the more frequent reports usually being more useful. However, the more frequent
reports may produce erratic data due to the peakedness of traffic, especially on
small circuit groups. Data reports compiled by a network management operations
system take on added value in that a more global view of network performance is
provided.
3.6 The network performance data is generally expressed in parameters which
help to identify difficulties in the network. Among these parameters are:
3.6.1 percentage overflow (% OFL)
% OFL indicates the relationship between the total bids offered to a
circuit group or destination, in a specified period of time, and the quantity of
bids not finding a free circuit. It will, therefore, give an indication of the
overflow from one circuit group to another, or the bids which fail because all
circuit groups to a destination are busy.
% OFL = eq \f( Overflows bids (to another circuit group or to
circuit busy signal), Total bids for the circuit group (or all circuit groups to
a destination)) x 100
1) International networks contain one-way and both-way operated circuits, and their
traffic flow characteristics are inherently different. This difference needs to be
taken into account when calculating BCH and SCH either by:
i) multiplying the number of one-way circuits by 2 to derive an equivalent number of
both-way circuits or;
ii) dividing the number of both-way circuits by 2 to derive an equivalent number of
one-way circuits.
When analyzing BCH and SCH data, and when BCH and SCH data are exchanged between
Administrations, it is essential that the method used is understood so that erroneous
conclusions may be avoided.
Fascicle II.3 - Rec. E.411 PAGE1
3.6.2 bids per circuit per hour (BCH)1 )
BCH is an indication of the average number of bids per circuit, in a
specified time interval. It will therefore identify the demand and, when measured
at each end of a both-way operated circuit group, will identify the direction of
greater demand.
BCH = eq \f( Bids per hour, Quantity of circuits available for
service)
It is not necessary to accumulate data for an hour to calculate BCH.
However, the calculated BCH must be adjusted when data accumulation is less than
hourly. For example, the bids should be doubled if 1/2 hour data is used. The
result would be BCH for the data collection period.
3.6.3 answer seizure ratio (ASR)
ASR gives the relationship between the number of seizures that result in
an answer signal and the total number of seizures. This is a direct measure of
the effectiveness of the service being offered onward from the point of
measurement and is usually expressed as a percentage as follows:
ASR = eq \f( Seizures resulting in answer signal, Total seizures) x
100
Measurement of ASR may be made on a circuit group or on a destination
basis.
3.6.4 answer bid ratio (ABR)
ABR gives the relationship between the number of bids that result in an
answer signal and the total number of bids. ABR may be made on a circuit group or
on a destination basis.
ABR = eq \f( Bids resulting in answer signal, Total bids) x 100
ABR is expressed as a percentage and is a direct measure of the
effectiveness of traffic onward from the point of measurement. It is similar to
ASR except that it includes bids that do not result in a seizure.
2) International networks contain one-way and both-way operated circuits, and their
traffic flow characteristics are inherently different. This difference needs to be
taken into account when calculating BCH and SCH either by:
i) multiplying the number of one-way circuits by 2 to derive an equivalent number of
both-way circuits or;
ii) dividing the number of both-way circuits by 2 to derive an equivalent number of
one-way circuits.
When analyzing BCH and SCH data, and when BCH and SCH data are exchanged between
Administrations, it is essential that the method used is understood so that erroneous
PAGE12 Fascicle II.3 - Rec. E.411
3.6.5 seizures per circuit per hour (SCH)2 )
SCH is an indication of the average number of times, in a specified time
interval, that each circuit group is seized. When related to the expected values
of average call holding times and effective call/seizure rate for the circuit
group, it will give an indication of the effectiveness of the service being
offered.
SCH = eq \f( Seizures per hour, Quantity of circuits available for
service)
It is not necessary to accumulate data for an hour to compute SCH. (See S
3.6.2 - BCH.)
3.6.6 occupancy
Occupancy can be represented in units (for example, erlangs,
hundred-call-seconds (CCS) or as a percentage. It can be measured as a total for
a destination or for a circuit group and as an average per circuit on a circuit
group. Its use for network management purposes is to show usage and to identify
unusual traffic levels.
3.6.7 mean holding time per seizure
This is the total holding time divided by the total number of seizures and
can be calculated on a circuit group basis or for switching equipment.
3.6.8 busy-flash seizure ratio (BFSR)
BFSR gives the relationship between the number of seizures that result in
a "busy-flash" signal (or its equivalent) and the total number of seizures.
Measurement of BFSR is usually made on a circuit group basis.
BFSR = eq \f( Seizures resulting in a "busy-flash", Total seizures)
x 100
Note - The source of "busy-flash" signals, or their equivalent, will vary
with the signalling system used. Therefore, the calculated BFSR on individual
circuit groups may naturally be different, and as a result, caution should be
used when comparing BFSR among circuit groups.
3.7 The number of parameters possible or necessary for particular
Administration purposes will depend upon a variety of factors. These will
include:
a) the data available at an exchange;
b) the particular routing arrangements employed (for example, SCH and BCH
relate to circuit group performance only; ABR, ASR, and % OFL can
relate to circuit group or destination performance);
c) the interrelationships which exist between the parameters (for example,
SCH can give similar indications to ASR - see S 3.6.5 above).
4 Interpretation of parameters
The interpretation of parameters on which network management actions are
based can most conveniently be made by considering the originating international
exchange as the reference point (see Figure 1/E.411).
Figure 1/E.411 - CCITT 48150
From this reference point, the factors which affect call completion can
broadly be divided into three main components:
a) switching loss (near-end loss);
b) circuit congestion loss (near-end loss);
c) distant network loss (far-end loss).
4.1 Switching loss
Switching loss may be due to:
1) common equipment or switchblock congestion, or queue overflows or
processor overloads;
2) failures in incoming signalling;
3) subscriber/operator dependent errors, such as insufficient or invalid
digits, premature call abandonment, etc.;
4) routing errors, such as barred transit access;
5) other technical failures.
Guidance to the identification of switching loss can be obtained from S
3.3.
conclusions may be avoided.
Fascicle II.3 - Rec. E.411 PAGE1
4.2 Circuit congestion loss
This loss will depend on:
1) the number of circuits available for a destination, and:
2) the level of demand for that destination,
3) the traffic performance on the circuit groups to that destination.
Indication that circuit congestion loss may occur can be obtained from the
status information detailed in S 3.3.2 above.
Circuit congestion loss can be identified by any of the following:
- percentage overflow (see S 3.6.1),
- a difference between the "bids per circuit per hour" and "seizures per
circuit per hour" measurements on the final circuit group (see SS 3.6.2
and 3.6.5),
- a difference between the "answer bid ratio" and the "answer seizure
ratio" (see SS 3.6.3 and 3.6.4).
It should be noted that for both-way operated circuit groups, excessive
demand in the incoming direction may also cause circuit congestion loss. This can
be identified by comparing incoming and outgoing bids, seizures or occupancy.
4.3 Distant network loss
Distant network loss may be divided into:
1) technical loss : due to distant exchange and national circuit faults,
2) subscriber dependent loss : due to subscriber B busy, no answer,
invalid distant number, number unavailable, etc.,
3) traffic dependent loss : these losses are due to lack of distant
network capacity to meet traffic demand.
Under normal conditions, and for a large sample measured over a long
period, distant network loss can be said to have a fixed or ambient overhead loss
(this value depends on destination with some hour-by-hour and day-by-day
variations).
Under abnormal situations (heavy demand, failures, etc.) distant network
losses can be significantly affected. Variations in distant network loss can be
identified by any of the following:
- answer seizure ratio (see S 3.6.3) (this is a direct measurement),
- seizures per circuit per hour (see S 3.6.5) (this is an indirect
measurement),
- mean holding time per seizure (see S 3.6.7) (this is an indirect
measurement),
- busy-flash seizure ratio (see S 3.6.8) (this is a direct measurement).
5 Criteria for action
5.1 The basis for the decision on whether any network management action should
be taken will depend upon real-time information on the status and performance of
the network. It is advantageous if the output of this information can be
initially restricted to that which is required to identify possible difficulties
in the network. This can be achieved by setting threshold values for performance
parameters, and for the number or the percentage of circuits and common control
equipment which are in service, such that when these threshold values are
crossed, network management action can be considered. These threshold values will
represent some of the criteria by which decisions are reached.
5.2 Indications that a threshold has been crossed and "all circuits on a
circuit group are busy" and "all circuit groups to a destination are busy" may be
used to direct attention to the particular area of the network for which detailed
performance information will then be required.
5.3 The decision on whether or not to take network management action, and what
action will be taken, is the responsibility of the network management personnel.
In addition to the criteria mentioned above, this decision will be based on a
number of factors, which could include:
- a knowledge of the source of the difficulty;
- detailed performance and status information;
- any predetermined plans that exist (see Recommendation E.413);
- experience with and knowledge of the network;
- routing plan employed;
- local traffic patterns;
- ability to control the flow of traffic (see Recommendation E.412).
This personnel is responsible for ensuring that conventional network
management controls, once activated, are not left unsupervised.
6 Network management actions
PAGE12 Fascicle II.3 - Rec. E.411
6.1 General
Network management actions fall into two broad categories:
a) "expansive" actions, which are designed to make available lightly
loaded parts of the network to traffic experiencing congestion;
b) "protective" actions, which are designed to remove traffic from the
network during congestion which has a low probability of resulting in
successful calls.
Normally, the first choice response to a network problem would be an
expansive action. Protective actions would be used if expansive actions were not
available or not effective.
Network management actions may be taken:
- according to plans which have been mutually agreed to between
Administrations prior to the event (see Recommendation E.413);
- according to ad hoc arrangements agreed to at the time of an event (see
Recommendation E.413);
- by an individual Administration wishing to reduce its traffic entering
the international network, or to protect its own network.
6.2 Expansive actions
Expansive actions involve the rerouting of traffic from circuit groups
experiencing congestion to other parts of the network which are lightly loaded
with traffic, for example, due to differences in busy hours.
Examples of expansive actions are:
a) establishing temporary alternative routing arrangements in addition to
those normally available;
b) in a country where there is more than one international exchange,
temporarily reorganizing the distribution of outgoing (or incoming)
international traffic;
c) establishing alternative routings into the national network for
incoming international traffic;
d) establishing alternative routings to an international exchange in the
national network for originating international traffic.
The protective action of inhibiting one direction of operation of both-way
circuits [see S 6.3 a)] can have an expansive effect in the other direction of
operation.
6.3 Protective actions
Protective actions involve removing traffic from the network during
congestion which has a low probability of resulting in successful calls. Such
traffic should be removed as close as possible to its origin, thus making more of
the network available to traffic which has a higher probability of success.
Examples of protective actions are:
a) Temporary removal of circuits from service (circuit busying). This
action may be taken when a distant part of the network is experiencing
serious congestion.
Note - In the case of both-way circuits, it may only be necessary to
inhibit one direction of operation. This is called directionalization.
b) Special instructions to operators. For example, such
instructions may require that only a limited number of
attempts (or none at all) be made to set up a call via a
congested circuit group or exchange, or to a particular
destination experiencing congestion.
c) Special recorded announcements. Such announcements may be
connected at an international or national exchange and,
when there is serious congestion within part of the
network, would advise customers (and/or operators) to take
appropriate action.
d) Inhibiting overflow traffic. This action prevents traffic
from overflowing onto circuit groups or into distant
exchanges which are already experiencing congestion.
e) Inhibiting direct traffic. This action reduces the traffic
accessing a circuit group in order to reduce the loading on
the distant network.
f) Inhibiting traffic to a particular destination (code
blocking or call gapping). This action may be taken when it
is known that a distant part of the network is experiencing
congestion.
Fascicle II.3 - Rec. E.411 PAGE1
g) Circuit reservation. This action reserves the last few idle
circuits in a circuit group for a particular type of
traffic.
6.4 Information on the network management controls (and their method of
activation) which can be used for expansive and protective actions is found in
Recommendation E.412.
6.5 Actions during disasters
6.5.1 Disasters whether man-made or natural can result in damage to the
telephone network, they can give rise to heavy calling, or both.
6.5.2 A single point of contact for network-related information should be
established to prevent confusion, duplication of effort, and to ensure an orderly
process of returning communications to normal. It is recommended that the single
point of contact be the network management implementation and control point (see
Recommendation E.414, S 4) within the Administration affected by the disaster.
6.5.3 The role of the network management implementation and control point may
vary depending on the size or impact of a disaster. However, the following are
functions which may be required:
- assess the impact of the disaster on the network (transmission systems,
exchanges, circuit groups, destination codes, isolated destinations);
- provide status information, as appropriate, to:
i) operator services
ii) public relations and media
iii) government agencies
iv) other network management implementation and control points;
- develop and implement control strategies (expansive and protective);
- assist in determining the need for, and locating, technical equipment
to restore communications.
7 Exchange of information
7.1 Effective network management requires good communications and cooperation
between the various network management elements within an Administration and with
similar elements in other Administrations (see Recommendation E.414). This
includes the exchange of real-time information as to the status and performance
of circuit groups, exchanges and traffic flow in distant locations.
7.2 Such information can be exchanged in a variety of ways, depending on the
requirements of the Administrations. Voice communications can be established
between or among network management centres using dedicated service circuits or
the public telephone network. Certain operational signals, such as switching
congestion indicators, may be transported directly by the common channel
signalling system. (See Recommendation Q.297 for Signalling System No. 6 and
Recommendations Q.722, Q.723, Q.724, Q.762, Q.763 and Q.764 for Signalling System
No. 7.) Larger data exchange requirements on a regular basis may be supported by
the Telecommunications Management Network (TMN) (see Recommendation M.30) or by
use of a packet switched network capability. The transfer of smaller amounts of
data on an infrequent basis may be supported by telex or similar media, or by
facsimile.
7.3 Guidance on the use of common channel signalling for
management
7.3.1 Common channel signalling systems provide a fast and reliable means of
transfering network management operational signals between exchanges. An example
is the transfer of exchange congestion status signals for the Automatic
Congestion Control (ACC) system (see Recommendation E.412, S 3.1). These signals
should be given a high priority in common channel signalling flow control.
Specific details on the application of network management operational signals in
Signalling System No. 6 are found in Recommendation Q.297. In the case of
Signalling System No. 7, the details for the Telephone User Part (TUP) are found
in Recommendations Q.722, Q.723 and Q.724, and the ISDN User Part (ISUP) are
found in Recommendations Q.762, Q.763 and Q.764.
7.3.2 Signalling System No. 7 may also be used to transfer network management
data and status information between an exchange and its network management
operations system, and between network management operations systems. It should
be noted that in these applications, the volume of data to be transferred can be
quite large and its frequency of transmission can be as high as every three
minutes. When this data is transferred over signalling links which also handle
user signalling traffic, stringent safeguards must be adopted to minimize the
risk of signalling system overloads during busy periods when both user signalling
PAGE12 Fascicle II.3 - Rec. E.411
traffic and network management data transmissions are at their highest levels.
These safeguards include the following:
- limiting the amount of network management information to be transferred
on signalling links which also carry user signalling messages;
- using dedicated signalling links for network management purposes;
- using the telecommunications management network (TMN), or the
Operations and Maintenance Application Part (OMAP) in Signalling System
No. 7 (for further study);
- developing appropriate flow control priorities for network management
information (for further study);
- equipping the network management operations system in such a way that
it can respond to signalling system flow control messages.
8 Beginning network management
The introduction of network management into an existing network should be
viewed as a long-term project. This long period is required:
- to gain knowledge and experience of network management;
- to carry out studies on the requirements of an individual network;
- to write specifications for network management requirements in present
and future telephone exchanges and to hold discussions with
manufacturers;
- to oversee the introduction of facilities and to organize and train
suitable network management staff;
- to introduce limited facilities in existing older technology exchanges.
A rational approach would consist in first using existing limited
facilities to manage the network, while at the same time developing full network
management facilities with the introduction of modern stored program control
(SPC) exchanges.
8.1 Utilizing existing resources and capabilities
8.1.1 Responsibility
As an important first step, the responsibility for network management
should be identified and assigned within an organization. This initial
organization can then be expanded, as required, in accordance with Recommendation
E.414.
8.1.2 Telephone operators
Operators are usually aware of problems as they occur in the network, and
this information can reveal the need to control traffic. The operators can then
be directed to modify their procedures to reduce repeated attempts, or to use
alternative routings to a destination. They can also provide special information
and/or instructions to customers and distant operators during unusual situations.
8.1.3 Exchange capabilities
Exchanges may have been provided with certain features which can be
adapted for network management purposes. Data already available for maintenance
or traffic engineering purposes could be used for network management, or could be
made available through the addition of an interface unit. In addition, manually
operated switches or keys can be provided in electro-mechanical exchanges to
block certain destination codes or to change alternate routing. They can be
provided separately for each item of common control equipment, thereby allowing
flexible control of traffic to a destination.
The scope for network management in a telecommunications network may
depend on the technology of the exchanges in that network. However, close
examination of the manufacturers' specifications for SPC exchanges may reveal
that certain network management functions may be available, for example, via a
maintenance terminal.
8.1.4 Circuits
Both-way circuits can be made busy to one direction of operation to
improve the flow of traffic in the other direction. In addition, both-way and
one-way circuits can be removed from service, when necessary. Both of these
actions may be taken by verbal direction to the responsible maintenance
organization.
8.2 Improving capabilities
From the experience gained through the use of these simple tools, more
sophisticated network management facilities can be specified. In the interest of
cost reduction, these up-graded network management capabilities should be planned
for introduction as a part of a planned addition or modification to an exchange,
and should be specified as a part of the initial installation of new systems.
Fascicle II.3 - Rec. E.411 PAGE1
Before purchasing a new exchange, attention should be paid to the ability of the
exchange to provide network management requirements as specified in
Recommendations Q.542 and Q.544.
In some cases, certain off-line network management information storing and
processing needs may be accommodated by the use of personal computers.
9 Considerations for the development of network management
9.1 Network management can be provided on a distributed basis, where network
management functions are provided "on-site" at the exchange, or on a centralized
basis, where network management functions for a number of exchanges are provided
at a single location. Each approach provides certain advantages which should be
recognized when deciding which one would be appropriate for an Administration's
situation. In general, the decentralized distributed approach may be more
appropriate where activity levels are relatively low. It may also be an
appropriate way to get started in network management. The centralized approach
may be more appropriate in networks where activity levels are high. In some
networks, a combination of these approaches may be most effective.
9.2 Advantages of the decentralized (distributed) approach
The decentralized (distributed) approach provides certain advantages,
which include the following:
- locally available features and capabilities can be developed and used
(see S 8.1.3);
- a more detailed analysis and assessment of localized problems are
possible;
- survivability of network management functions is improved, since a
problem or outage at one location will not usually result in the loss
of all network management capabilities;
- network management functions may be assigned to existing staff,
eliminating the need to develop a dedicated, specialized staff;
- it may provide an interim capability while a long-term plan is being
developed and deployed.
9.3 Advantages of the centralized approach
A centralized network management centre provides a number of operational
benefits when compared with a distributed approach, where network management
functions are provided "on-site" at the exchange. These include:
- more effective network management operations. A centralized approach is
inherently more effective in dealing with complex, interrelated network
problems in the SPC-common channel signalling environment, and will
become more so during the transition to ISDN. In many cases, the most
effective response to a problem in the international network might be
to take action in the national network, and vice-versa. A centralized
approach simplifies the problem of coordination of activities in these
cases;
- a more "global" view of network performance. This, in turn, will permit
faster and more accurate problem identification, and the development of
more effective control strategies which can be implemented with less
delay;
- a central point of contact for network management, both internally and
with other Administrations (see Recommendation E.414);
- more efficient network management operations. The cost of staffing and
training is reduced, and staff expertise is enhanced through
specialization.
9.4 Network management operations systems
A computer-based network management operations system can provide
considerable benefits to a network management centre due to its ability to
process large volumes of information and to present that information in a common
format. The functions of a network management operations system include the
following:
- collecting alarms, status information and network management traffic
data from exchanges (see S 3 and Recommendation E.502);
- processing the collected data and calculating network management
parameters (see S 3 and Recommendation E.502);
- providing performance reports (see S 9.4.1);
- comparing network management parameters with thresholds to identify
unusual conditions;
- applying controls in exchanges based on input commands;
PAGE12 Fascicle II.3 - Rec. E.411
- calculating hard-to-reach status of destinations and providing this
information to exchanges;
- interfacing with network management centre visual displays, and work
station terminals and printers;
- preparing administrative reports;
- maintaining a database of network statistics and information.
Note - Many of these functions can also be provided to the Network
Management Centre by each SPC exchange, however, the provision of these functions
in a network management operations system may reduce the requirements placed on
the exchanges.
9.4.1 Performance reports
Performance reports can be provided in the following ways:
i) automatic data - this data is provided automatically as specified in
the operations system software, and cannot be readily changed by the
network manager;
ii) scheduled data - this data is provided according to a schedule
established by the network manager;
iii) demand data - this data is provided only in response to a specific
request by the network manager. In addition to performance data, demand
data includes reference data, such as the number of circuits provided
or available for service, routing information, assigned threshold
values, numbers of installed switching system components, etc.;
iv) exception data - this data is provided when a data count or calculation
crosses a threshold established by the network manager.
Data reports can be provided on a regular basis, for example, every 3
minutes, 5 minutes, 15 minutes, 30 minutes, or hour. The specific interval for
any data report will be determined by the network manager. Historic data relating
to at least the previous two or three periods should also be available.
9.4.2 Other considerations
It should be noted that shorter collection intervals increase the
usefulness of the data to the network manager, but also increase the size and
cost of the operations system and may increase the volatility of the data.
It should also be noted that it is important that network management
controls should not become completely unavailable due to the failure or
malfunction of the network management operations system or of its communications
links with exchanges. Therefore, network management operations systems should be
planned with a high degree of
Fascicle II.3 - Rec. E.411 PAGE1
reliability, survivability and security. This could be achieved through the
provision of certain essential capabilities (such as controls and automatic
routing protection mechanisms) on-site in the exchange, or by redundancy in
computers and data links, or through the provision of alternative stand-by
centres.
The failure of a network management operations system should not have an
adverse impact on normal traffic flow in the network.
ANNEX A
(to Recommendation E.411)
Terminology for network management
A.1 circuit
A circuit connects two exchanges. A national circuit connects two
exchanges in the same country. An international circuit connects two
international exchanges situated in different countries. (Based on
Recommendation D.150 and Recommendation F.68.)
A.2 circuit group
The set of all switched circuits which directly interconnect one exchange
with another.
A.3 circuit sub-group
A set of circuits within a circuit group which are uniquely identifiable
for operational or technical reasons. A circuit group may consist of one or more
circuit sub-groups.
A.4 destination
A country in which the called subscriber is located or an area or other
location that may be specified within that country. A destination can be
identified by the digits used for routing the call.
A.5 bid
An attempt to obtain a circuit in a circuit group or to a destination. A
bid may be successful or unsuccessful in seizing a circuit in that circuit group
or to that destination.
A.6 seizure
A seizure is a bid for a circuit in a circuit group which succeeds in
obtaining a circuit in that circuit group.
A.7 answer signal
A signal sent in the backward direction indicating that the call is
answered. (Based on Recommendation Q.254.)
A.8 holding time
The time interval between seizure and release of a circuit or switching
equipment.
A.9 busy-flash signal (sent in the backward direction)
This signal is sent to the outgoing international exchange to show that
either the circuit group, or the called subscriber, is busy (Signalling Systems
No. 4 and No. 5, see Recommendations Q.120 and Q.140).
Note - In Signalling Systems No. 6 and No. 7, there is no busy-flash
signal. However, the equivalent of busy-flash can be roughly approximated through
the aggregation of specific backward failure signals such as circuit group
congestion, national network congestion and subscriber busy.
PAGE12 Fascicle II.3 - Rec. E.411
