9.1
Evaluative Concepts
As the perspicacious reader of the previous section will have already noted, the definitions of completions there do not consider the possibility of misroutes, which are those instances in which a distant station answers, but the station answering was not the station called. Thus, for example, under the definitions in Table 8.1, any call attempt that is answered is considered to be a normal completion even though the call was routed to the ‘‘wrong number’’. The definitions of quantifiers of connection reliability similarly exclude misroutes from counts of connection failures, except possibly for those test configurations in which the identity of the answering station is testable, e.g. by recognition of a signature response.
There are two reasons for not explicitly recognizing misroutes in analyses of connection reliability. One is practical; the other is theoretical. The prac- tical reason is that there is no easy way to determine whether a connection to the wrong station occurred because of the way that the system routed the call or because there was an error in the dialing. Voice service users can make that determination only when the call is answered, and even then will not know for sure what happened, tending to attribute sporadic misroutes to errors in their dialing rather than in the system’s handling of the call attempt. This uncer- tainty also spills over into data from automatically-dialed call attempts. With- out mechanisms that enable the unambiguous identification of the station answering a call, it is nearly impossible to identify misroutes even when the expected station answer is distinctive. For example, even when the expected answer from a station called is a test progress tone, it cannot be readily inferred that a voice answer reflects a misroute when it is impossible to distinguish William C. Hardy Copyright q 2001 John Wiley & Sons, Ltd ISBNs: 0-471-49957-9 (Hardback); 0-470-84591-0 (Electronic)
between a human voice answer from a station, a recorded voice message announcing that the station called is out of service, and a recorded voice announcement to the effect that the call attempt could not be completed as dialed or could not be routed due to congestion in the network.
The practical alternatives, then, are to simply treat misroutes as call attempts that were successfully routed across the network, albeit to the wrong station, or to develop and deploy capabilities for discriminating misroutes from accurately completed calls, which would enable determination of:
1. Whether each number dialed was, in fact, the right number, correctly dialed; and
2. Whether the destination station answering the call was the requested desti- nation, or an acceptable alternative, such as the answer center to which calls for that station are directed when the station does not answer, a different call center programmed to handle overflow from the center called, etc.
Because misroutes are more likely to be perceived by users to be conse- quences of misdialing and construed to indicate that the system is completing calls, and because misroutes are rarely the cause of completion failures, it makes a lot more sense to count the as perceived connection successes where necessary than to try to distinguish them.
The theoretical reason for treating misroutes as connection successes is equally compelling. When we consider the possible reasons for users’ concerns as to how well their telecommunications services handle connection requests, there are two distinctly different perceived risks that may be driving the concern. The first, addressed in the preceding discussion of connection reliability, is the possibility of encountering a situation in which connection failures are frequent enough or persistent enough to become a source of frustration. The associated concern is whether a properly executed connection request will result in an origin/destination connection, or fail en route, which naturally leads to development of quantifiers that accurately reflect ways that the system may fail to set up a connection.
The other perceived risk driving user concerns as to how well the system will handle connection requests is whether information transmitted over a connection will get to the intended recipient. The risk in this case is that misrouting may result in delivery of confidential or sensitive material to the wrong destination. Such consequences are not often contemplated for voice services, because speaker recognition and conversational exchanges usually suffice to assure the caller that the correct station has been reached. However, in any telecommunications service, such as fax or e-mail, in which correct
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specification of the desired end-to-end connection and the accurate set up of the associated connection are the only guarantors that information transmitted will reach the intended recipient, there is always the nagging question as to whether what was transmitted really got to the person(s) that were supposed to get it, or some unauthorized, perhaps unpleasant or threatening, someone is: † Having great sport with that painfully mushy Valentine/love letter e-mailed
to someone who would appreciate its sentiment;
† Eagerly transcribing account numbers and tracing signatures from that funds transfer request faxed to a stockbroker; or
† Wondering why someone whose name they do not recognize would leave a message on their answering machine to bring $500 cash for bail money, leaving no number to call back.
9.2
Concern
The recognition of the potentially disastrous consequences of misroutes when confirmation of the accuracy of the connection is not possible, then, engenders a general user concern with routing reliability, expressed by the question:
When I request a connection and one is set up, can I be sure that it has been set up to the right destination?
Unlike the concern with connection reliability, which is pretty much limited to intermittently-used voice services, concerns with routing reliability can extend to any telecommunications (or postal) service where there is no means of directly confirming that the destination receiving the information is the one intended.
9.3
Measure
The generic measure of routing reliability is the conditional probability, Pr|c,
that a connection will be completed to the intended recipient of information or an acceptable alternative, given that it is completed to any recipient.
9.4
Quantifiers
9.4.1 Perceived QoS
From the perspective of the user, there is no possibility of detecting misrouting unless the call attempt is answered. Consequently, the appropriate quantifier
for perceived QoS with respect to routing reliability is the direct estimate of
Pr|c, obtained by setting:
Pruc¼ ðNrÞ=ðNaÞ ð32Þ
where Na is the number of call attempts answered, and Nris the number of
answered call attempts for which the right station or an acceptable alternative answered.
Since a call must be answered before the information can be passed to any
recipient, Pr|ccan also be estimated indirectly from the answer–seizure ratio
(ASR) defined in Eq. (29), calculated from a sample of Ntcall attempts and a
count, Nm, representing the number of misroutes observed among those Ntcall
attempts by setting:
Pruc¼ 1 2 ðNmÞ=½ðNtÞðASRÞ ð33Þ
9.4.2 Intrinsic QoS
While the users’ proximate concern with routing reliability is the exposure to having information transmitted to the wrong recipient, which can occur only when the end-to-end connection for transmitting that information is estab- lished, the potential for such an untoward event is created by misdirection that occurs somewhere in the process of setting up a requested end-to-end connection. The quantifier that most appropriately reflects that potential is the
unconditional misroute probability, Pm, estimated by the ratio CM/CA, where
CA denotes the number of call attempts observed in a sample and CM denotes those in which misrouting occurred, regardless of the ultimate disposition of the call attempt.
As described in the discussion of evaluative concepts for routing reliability, there may be no easy way of discriminating misroutes to produce the count CM, even when data acquisition is fully automated. However, wherever an
estimate of Pmis required for evaluation, and Pr|cis known, Pmcan be quanti-
fied indirectly by setting:
Pm¼ 1 2 Pruc ð34Þ
9.5
Evaluation
One of the important reasons for characterizing and analyzing routing relia- bility separately from connection reliability is that users have very little toler- ance for detected misroutes. Users may forgive an occasional brief period of
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frustration with a temporary inability to complete calls, or tolerate low call completion rates into particular destinations when it is known that there is nothing that the service provider can do about it, for example, because the destination is in a underdeveloped area. However, the first time that a user discovers, even by hearsay, that a correctly addressed fax, e-mail message, or data file was delivered to the wrong recipient due to a fault in the routing system for a service will probably be the last time that service will be described as being ‘‘satisfactory’’, and the advent of pressures on the service provider for assurances that it will never happen again!
For this reason, any but negligibly small values of Pm, on the order of 1025
to 1026or less should be considered to be exhibiting unsatisfactory QoS with