Mạng và viễn thông P35

Public telecommunications customers will be familiar with the arrival of their monthly or quarterly bill, comprising both subscription charges paying for the rental of their line, and a

35

Charging and Accounting

for Network Use

So far we have concerned ourselves entirely with the technical and operational side of running

networks and providing telecommunications services Network operators, if they want to stay in business, also need to think about capital and operational costs, and how to recover them from their customers For a network to be financially sound and independent its tariff structure needs

to cover the total expenditure made up of

0 provision of lineplant, exchanges and buildings (i.e capital costs)

0 service maintenance and administration costs

0 interest on capital

0 depreciation of equipment

0 research and development

0 future capital investment

0 profit and shareholders’ earnings

Government regulation in some countries set tight financial limits on profits and targets, whereas

in other countries the tariff structure is tailored to meet political ends, and financing is direct from government coffers Nonetheless, the financial practices of charging and accounting for network use are common throughout the world, and they reflect the principles explained in this chapter

Public telecommunications customers will be familiar with the arrival of their monthly

or quarterly bill, comprising both subscription charges (paying for the rental of their line), and a charge for usage (the number of calls made, o r packets of data sent, etc.) The practice of charging for network use is not confined to public networks; private network operators, especially the large corporations, are increasingly levying internal

647

Networks and Telecommunications: Design and Operation, Second Edition.

Martin P Clark Copyright © 1991, 1997 John Wiley & Sons Ltd ISBNs: 0-471-97346-7 (Hardback); 0-470-84158-3 (Electronic)

648 CHARGING AND ACCOUNTING FOR NETWORK USE

‘transfer charges’ on individual departments for their use of telecommunications services This is to keep close control of areas of company activity representing either

‘profit centres’ or ‘cost centres’ Additionally, accounting between telecommunications operators is becoming more common, as a result of the recent large increase in the interconnection of networks

ITU-T defines two different types of financial transaction, to each of which different practices and a very particular vocabulary apply The two types of financial transaction are described below, and a number of key items of vocabulary are highlighted

35.1.1 Charging (Also Called Billing)

This is the name given to the practice of recovering financial costs from end-users for their use of the network Charges, including subscription charges and usage charges, are

accumulated over a standard period, typically three months, and are billed to the cus-

tomer Monies are then collectedfrom the customer, typically a telephone line subscriber

35.1.2 Accounting

The term accounting is normally applied only to the practice of financial settlement

between different network operators, where they mutually cooperate to provide ser- vices An accounting settlement is usually made by the owner of the originating network (i.e the one collecting the customer charges) to all the other operators participating in the connection This reimburses the other operators for the use of their networks

Customer charges, also known as tarzffs, usually comprise two parts, a subscription

charge and a usage charge The subscription charge is made to offset the standing or

jixed costs, for example, the rental of a telephone or other end terminal, the provision and upkeep of the local exchange and of the access line, and the operator’s general administrative overheads The actual rate of the subscription charge depends on the

type of access line, the end equipment provided, and the services to which the customer subscribes They are easily calculable from a fixed formula and are usually charged by quarterly bill Examples of services which attract a higher subscription charge are

circuits specially designed for carriage of data, or lines with supplementary services such

as closed user group ( C U G ) , diversion of incoming calls (on customer busy or customer

absent), three-party-call facility, etc

Customer usage charges are made in addition to subscription charges, to offset the use-

dependent (the variable) costs of network provision As we have seen in the chapter on

PULSE METERING 649

teletraffic theory, the greater the amount of traffic between any two exchanges, the greater the number of trunks that are required between them The cost of these extra trunks is recovered in the usage charge, which is usually calculated on a cost-plus-mark-

up basis to reflect the network resources required to provide the service The charge for any given call (or for the carriage of a data message) thus usually depends on the following parameters

0 the type of network used

0 the duration in time of the call, or

0 the volume of data carried, and

0 the distance of the communication

In addition, more sophisticated factors may also be taken into account

0 whether operator assistance, or some other value-added service was provided

0 whether the call (or message) was conveyed during the network’s peak usage period (the charge may be time-of-day dependent)

0 how many call attempts (whether abortive or not) were made; making a charge for each call attempt dissuades users from congesting the network by incessantly re- calling busy numbers

To calculate the charge due from each individual customer, the network operator must have a means of monitoring and recording each incidence of usage In most networks charges are levied only on the call (or message) originator (so that the receiver does not pay) Usage is normally monitored by the originating exchange, which must determine the origin and destination of each call (or message), its duration, and the time-of-day when conveyed In a circuit-switched network the origin of a call can be determined simply by detecting which incoming line is in use The destination is then determined by analysis of the number dialled during call set-up The chargeable duration of the call

(the call duration) is normally taken to be the conversation time, which is determined by timing the period between the answer signal and the cleardown signal at the end of the call Call duration on manual networks, or of calls established semi-automatically, can

be measured directly by the human operator, but for automatic networks different procedures are used There are two principal means for automatic customer charge

monitoring: pulse metering and electronic ticketing, as explained next

The older and simpler charging method (still used in many telephone networks) is to levy call charges on the basis of the number of pre-determined intervals of time used in

.the course of the call This method is called pulse metering In it a fixed price is charged

for a small time period or unit of the conversation phase When the call is first

answered, the first unit (of time) commences After a pre-determined period of time (the

650 CHARGING AND ACCOUNTING FOR NETWORK USE

length of which will depend on the location of origin and destination and the time of day), the first unit will expire and a second unit of time will commence The charge payable is calculated by multiplying the number of units used by the fixed price per unit The customer is usually unaware when each unit ends and the next begins, or indeed how much of a partially used unit remains

On different calls, e.g to destinations at different distances away, or on calls to the

same destination made at different times of day, the time duration of a unit may be

varied In this way, the average per-minute-charge made for different calls can be varied One caution, however: it is not generally understood that the upper limit of metering pulse rate may not actually be determined by the capability of the pulsing unit, but on the ability of the transmission line to carry the pulse correctly and the meter to clock properly

Over a given period of time (say three months) the number of units used by the cus- tomer is accumulated on a counting device (called a cyclic meter), located at the ex-

change Location at the exchange eases the job of reading the meters, and reduces the scope for customer fraud

During the course of individual calls, the cyclic meter is triggered to step onward one count at the beginning of each new unit of time This is done by sending a perodic

electrical pulse to the meter The customer’s invoice shows the total number of units

used, and a financial charge which is calculated by multiplying this figure by the fixed price per unit

While calls are being set up by the exchange, the exchange control and routing system

(common equipment) obtains the destination of the call from the number dialled, to determine the appropriate outgoing route and charge rate For routing, as we described

in Chapter 28, a fair number of digits may need to be examined to decide to which precise exchange the call should next be routed However, although the same piece of common equipment may determine both the route and the call charge rate, it is likely that fewer digits will need to be analysed for charging purposes This is because most network

operators choose to run chargeband schemes, in which destinations within a fairly wide geographical zone (or chargeband) are charged at the same rate This eases the admin-

istration of the scheme and the customer’s comprehension of it As an example of a simple chargeband scheme, a public telephone company might opt to levy charge for international calls based only on the first digit of the country code This would divide the

world into eight distinct zones, as we saw in Chapter 29 (North America, Africa, Europe, South America, Far East Asia, Russia and ex-Socialist Republics, Middle East, and Central Asia) A similar national chargeband scheme could equally well be developed corresponding to the area code digits of the national telephone number

Each of the chargebands needs a charging programme The programme divides the

day into a number of time periods, for each of which a different per-minute-charge will apply At the busiest times, we expect higher rates, whereas cheap rates at off-peak times may help to stimulate extra traffic and revenue without needing more equipment

As an example, within the United Kingdom, British Telecom sub-divides its

chargebands into times of day called peak rate (corresponding to the busiest times of

day, when the highest per-minute-charge is levied), cheap rate (corresponding to periods of low activity, when the lowest per-minute-charge is made) and standard rate

(for which a medium charge is made) Over the course of a weekday, peak rate applies from 9 a.m to 1 p.m.; standard rate from 1 p.m to 6 p.m., and from 8 a.m to 9 a.m.;

PULSE METERING 651

and cheap rate applies each evening from 6 p.m to 8 a.m The per-minute-charge during

the peak rate period is made more expensive by reducing the time duration of the unit

(remember that the units have a fixed price) Figure 35.1 illustrates the relationship between price-per-minute and unit duration on the British Telecom scheme, and shows the pulsing signal required to step the customer’s meter

Exchanges which employ pulse metering to charge their customers usually have a number of machines or electronic devices to generate the meter pulse signals, one for each of the pulse supplies A selector device, controlled by the dialled number analysis

and by the time-of-day, connects the customer meter to the appropriate pulse supply for any particular call More than one pulse meter supply may exist For example, local calls will be pulsed from the local exchange (if metered at all) However, trunk and inter- national call charge pulses will most likely be generated at the trunk or international

exchange which analyses the destination country code digits A simplified diagram of a

pulse metering device is shown in Figure 35.2

A very simple form of pulse metering, in which only one meter pulse is generated for

each call, independent of call duration, is used in some networks, particularly for meter- ing local calls The advantage of such a charging philosophy is the simplicity of the

Price per minute P e a k r a t e

Standard rate

-

Cheap rate

08 0 9 13 18 Timeof day

l 2 4 h )

‘Unit’ time duratlon

I

Tirneof day

Meter pulsing

Figure 35.1 Pulse metering

652 CHARGING AND ACCOUNTING FOR NETWORK USE

Pulse signol selector (time-of-day and number onalysls controlled)

Pulse supply equtpment

I

Figure 35.2 Pulse metering selector or ‘tariff equipment’

equipment required and the ease of administration However, as the need to recover usage-dependent costs becomes more acute with the boom in demand (e.g night-time dial-in calls to the Internet), so the method is falling into obsolescence Another draw- back is the tendency to promote network congestion and phone box queues!

35.5 ELECTRONIC TICKETING

Electronic ticketing (also called automated message accounting or toll ticketing) is far

more accurate and reliable than pulse metering, and it is becoming common in consequence of the spread of stored program control exchanges In this method the exchange control computer monitors and records information about each call, including the number dialled, the duration, the time of call set up, the time of call

clear, etc The information is stored in an electronic cull record, usually on computer

storage disk or magnetic tape, until the time when the customer is due to be invoiced The invoice is calculated by adding the amounts due from each individual call, derived

from the details on the individual call records Electronic ticketing provides the scope

for more complex chargeband structures, and the capability (much demanded by customers) for an itemized record of the details and cost of each call made

35.6 ACCOUNTING

Accounting is the method of financial settlement between network operators, when more than one network is traversed by a call Accounting has historically been necessary to allow reimbursement between the networks of different countries in recompense for their handling of international calls, but in addition accounting is also becoming common even between networks operating in the same country, as deregulation allows or even demands ever greater interconnection

Most accounting is nowadays undertaken using computer monitoring devices to record the details of individual calls in a manner resembling electronic ticketing The financial settlement is made on the basis of total usage over a given period of time, and is

ACCOUNTING 653

paid from the call originating network directly to all other network operators Alterna-

tively, the accounting settlements may be cascaded between network operators, starting

at the originating end, if more than two networks have been used (see Figure 35.3)

The direct method of accounting settlement (Figure 35.3(a)) is only possible when the

owner of network A knows the exact path taken by calls in reaching network C, after traversing network B For example, if there were also a route from network B to network C via a fourth network E, then the direct settlement method may not be

possible, because owner A may not be able to calculate an apportionment in settlement for E This is the main reason for the use of the cascade method of settlement shown in Figure 35.3(b) In this method the transit party settlements are proportioned and

cascaded in sequence

Accounting settlements depend on total usage measured in paid minutes As we learned in Chapter 3 1, the number of accountable or paid minutes on each call is equal

to the total useful time that has passed between answer and call cleardown The settle- ment is calculated by multiplying the paid minute total by the accounting rate The

accounting rate itself is fixed by negotiation directly between the interconnected net- work operators, and is intended to reflect the costs associated with

0 the destination of the call and the distance

0 the services used

( Network

1 € 1

( a ) Simple (direct) settlement

( b ) Cascaded settlement

I C a l l p a t h , Accounting settlement

Figure 35.3 Cascaded settlement of account

654 CHARGING AND ACCOUNTING FOR NETWORK USE

0 the call duration

0 the time of day

0 the type of switching

0 the route taken etc

The accounting rate may be set in any accepted international currency (e.g US Dollars,

Sterling, gold Francs, special drawing rights ( S D R s ) ) , and may either be negotiated by

the bilateral agreement of the parties, or in case of difficulty paid at standardized rates such as those recommended by the ITU-T D-series recommendations

35.7 ROUTE DESTINATION ACCOUNTING

When there is more than one route to a given destination, route destination accounting must be undertaken; an example is shown in Figure 35.4, where calls between network

A and network B may either be routed directly or may overflow via network C

Correct reimbursement of both operators B and C is possible only if operator A

records total paid minutes by route and destination (further sub-divided into peak,

standard and cheap rates according to time of day if necessary) Destination settlement

is due to the operator of network B for all calls, whether routed directly or not, but the payment may be slightly lower for calls transiting network C This would reflect an apportioning of revenue to operator C and also the effective cost saving that results

from the reduced number of direct (A-B) circuits that are required

ITU-T recommendations demand that the operator of network A periodically (e.g

monthly or quarterly) declares two Route-Destination (or RD) accounts, corresponding

to the total originated paid minutes sent

0 to Destination B; by Route direct

0 to Destination B; by Route via C

Figure 35.4 ‘Route destination’ accounting

CHARGING AND ACCOUNTING ON DATA NETWORKS 655

Declaration of accounts between network operators consists of exchanging informa- tion about each other’s total originated paid minutes of traffic, categorized as Route/ Destination and peak/standard/cheap time-of-day (as necessary) The due payment is calculated as the difference between outpayments due for outgoing traffic and revenue due from incoming calls; and one of the operators pays the outstanding balance accordingly When incoming and outgoing traffic are roughly balanced, very little settlement is required, but usually there is a net financial gain for settling accounts with

an incoming balance of traffic, and a net outpayment when most of the traffic is outgoing For some developing countries the net balance can either provide a significant service of hard currency, influencing the speed of development of their networks For others, the debt is a severe problem and embarrassment

35.8 CHARGING AND ACCOUNTING ON DATA NETWORKS

When data are carried by a circuit-switched network, customer charges and inter- operator accounts are usually calculated according to the total time for which the

circuit was available to the end users The usage time, or paid time, is the time that elapses between the answer signal and the circuit cleardown This period is equivalent to the conversation time of circuit-switched telephone network, and the same principles apply as have already been described In addition a subscription charge is usually applied to the use of each customer network port Data network types charged and accounted in this way include telex, circuit-switched data networks and ISDN

In cases where data is carried by a packet-switched, frame relay or ATM (i.e cell-

switched) network, the accounting and charging principles are usually different The

subscription charges are typically on a per-port basis, but in addition, further charges may apply according to the number of simultaneous logical channels (i.e virtual

channels) which may be set up Subscription charges may also apply for supplementary

services including, for example, the closed user group ( C U G ) facility which allows only

members of the C U G to communicate with one another This may be a useful feature in

a corporate data network, where the corporation does not wish outsiders to be able to access their computers

Usage charges in data networks are usually based on one or more of the following factors

0 the speed of the connection established (i.e the bitrate, typically the committed information rate, CIR) is the determinor

0 the duration of the connection in time

0 the number of packets, frames or cells sent or some other measure of the transmitted volume of data

In packet-switched (i.e X.25-based) networks it is usual to charge and account according only to the total volume of data carried by the network The volume is quoted

in segments The standard segment size is 64 octets (or bytes), equivalent to 64 X 8 = 512

bits (Confusingly, however, a segment of SMDS data is only 48 bytes long.)

656 CHARGING AND ACCOUNTING FOR NETWORK USE

Most frame relay networks were built up offering only PVC (permanent virtual cir-

cuit) service A frame relay PVC can be set up only by subscription, and is a permanent connection between two defined endpoints So far as the end customer is concerned, a

frame relay PVC appears much like a leaseline with a bitrate equal to the committed

information rate (CZR), but at times of low network usage the customer may be able to

send much higher speed data for short bursts (excess bursts) Frame relay connections

have thus to date been charged in a similar manner to leaselines, based on a fixed

monthly subscription charge according to the CIR As switched virtual circuit ( W C ) or

virtual call ( V C ) frame relay connections become possible, these are more likely to be charged on a volume-related basis similar to the approach discussed above which is used

in X.25 packet networks

ATM networks are only just emerging, and no fixed pattern is yet established As ATM

is expected to be a ‘universal’ technology capable of providing many different types

of voice, data and video transport services, I expect the emergence of different types of customer-charging principles, mimicking the charge structures of the equivalent services provided using ‘legacy’ technologies

Methods of charging and invoicing customers, and for accounting and settling the inter-network account, are in general similar to the electronic ticketing technique used

in circuit-switched networks

35.9 CHARGING AND ACCOUNTING FOR MANUAL

(OPERATOR) ASSISTANCE

Calls set up on behalf of the end user, and controlled by the network operator (i.e

manual or semi-automatic calls), are normally charged and accounted according to the

details of the call, as recorded on the operator’s paper ticket or electronic equivalent The chargeable (and accountable) time on an operator call is usually less than the

total conversation time (the time between the called-subscriber-answer and the call- clear) This is because some time is taken up by the operator before the two end parties can speak According to the nature of the call, the unchargeable conversation time may have been used by the operator

to confirm that a particular person has answered the call, if a personal call has been

requested

0 to check that the receiver is willing to accept call charges if a collect (i.e reverse

charge) call has been requested

0 for some other similar reason

Operator-controlled calls often carry a premium charge This offsets the cost of the

operator’s time, and the extra unchargeable network time required at the commence- ment of each call Operator-assisted calls are also accounted between network operators

at a slightly higher rate than automatic calls, for similar reasons The manual

accounting rate is used when a second operator is also used in the terminating country,

otherwise the semi-automatic accounting rate applies