As mentioned previously, usage accounting is the ability to account for the consumption of a system's resources. Usage accounting can be performed in support of internal "customers" in a cost center service or in support of a fully-commercial service. In either case it is important to know which users are consuming which resources for both cost allocation and capacity planning purposes.
Usage accounting functions must be more robust and flexible in a commercial environment, where customers are unlikely to tolerate inaccuracies in billing statements. Usage accounting is even more challenging in the case of large systems because of the larger quantities of usage data which are captured and must be managed.
Usage accounting is not equivalent to billing, but it provides the data necessary for billing. Typically billing is proprietary and handled by billing services providers. The capability for rendering large numbers of bills is a highly specialized activity which could be distracting to a networking services provider.
The CDPD usage accounting services provide information on how the resources behind CDPD network services are used and by whom. The overall goal of this usage accounting service was to provide automatic, near real-time collection and dissemination of resource usage data; because CDPD spans service provider domains, a standardized usage accounting information exchange mechanism is essential.
The CDPD system collects network layer airlink usage data for each subscriber to support billing for this resource usage. The accounting service collects this data with supporting information necessary to provide accurate computation of charges. The data collected includes packet count, packet size, source and destination addresses, geographic location of M-ESs, time of transmission and so forth.
The key resource accounted for in CDPD systems is the utilization of the airlink bandwidth at the network layer. Because the CDPD network provides connectionless, datagram services to M-ESs, the accounting approach is to accumulate statistics about network layer protocol data units (NPDUs) that cross the airlink. Retransmissions at the transport layer and above are not recognized as being redundant and could be double-counted from the perspective of an end-user. It is possible that this retransmission was caused by Layer 4 time-outs resulting from contention or bad RF conditions in CDPD.7.7
Because the accounting mechanisms capture mobile data link utilization, traffic between two M-ESs gets measured twice-once when it traverses the mobile data link from an M-ES to its serving MD-IS, and again when it is transmitted from (possibly) another serving MD-IS to the second M-ES. However, the airlink usage for this M-ES to M-ES correspondence is associated with each M-ES' respective airlink (MDLP link).
The CDPD accounting mechanisms do not measure the distance that a particular NPDU travels between its source and destination. They also do not measure traffic that does not cross a mobile data link (e.g., they do not measure traffic between two F-ESs). But then, CDPD is assumed to provide mobile data services, not conventional data services.
CDPD services are offered by a variety of service providers who interact with one another in a variety of relationships. Because no single CDPD service provider covers all of North America, it will often be the case that a CDPD service provider provides service to a visiting M-ES. This is particularly true for nationwide CDPD customers7.8 .
To efficiently account for nationwide services spanning multiple CDPD service providers, the CDPD accounting service distinguishes between home CDPD network service providers and home accounting service providers. A subscriber of CDPD service provider A might be served most of the time by service provider B (perhaps in a subcontractor arrangement for service provider A); CDPD accounting mechanisms support this relationship. There is no need for network service relationships to dictate business (accounting) relationships.
CDPD usage accounting mechanisms, in themselves, do not address pricing, billing, the reconciliation of usage claims among CDPD network service providers or receivables processing. These functions are accomplished by applications that are outside the scope of the CDPD System Specification. CDPD usage accounting mechanisms exist to capture and distribute CDPD resource usage information in support of the business applications of service providers.
The CDPD accounting service is defined in Part 630 of the CDPD System Specification and Part 1023 of the CDPD Implementor Guidelines.
The CDPD accounting service relies on resource consumption data initially collected in serving MD-ISs. This accounting data is distributed amongst CDPD accounting correspondents via a simple one-way peer protocol. This accounting data exchange protocol is layered on top of an X.400-based message transfer system. The accounting protocol has been designed to be relatively independent of the accounting transfer mechanism so that migration to additional transfer mechanisms in the future is possible.
The accounting mechanisms are described by first providing a top-level description of all of the correspondents that participate in the CDPD accounting model, and then establishing the relationships between them. A relationship between two correspondents simply means that the correspondents may send messages to one another. The model is depicted in Figure 7.2.
The entities in the CDPD accounting model include the following components:
¥ Accounting Meter
An Accounting Meter in a serving MD-IS captures NPDU statistics and delivers these statistics in the format of a traffic matrix to a Serving Accounting Distributor.
¥ Serving Accounting Distributor
A Serving Accounting Distributor sorts traffic matrix segment rows into home accounting segments which it then distributes to the appropriate Home Accounting Distributors.
¥ Serving Accounting Collector
A Serving Accounting Collector stores and processes home accounting segments for the serving CDPD service provider; the data contained in these segments could be used for billing and planning purposes.
¥ Home Accounting Distributor
A Home Accounting Distributor sorts home accounting segment flows into consolidation accounting segments and distributes them to Consolidation Accounting Collectors, with a copies to a Home Accounting Collector.
¥ Home Accounting Collector
A Home Accounting Collector stores and processes home accounting segments.
¥ Consolidation Accounting Collector
A Consolidation Accounting Collector stores and processes consolidation accounting segments.
The following subsections will describe these CDPD usage accounting entities.
Datagrams (Network PDUs) to and from M-ESs flow through serving MD-ISs and are counted by Accounting Meters located in the MD-ISs. Information about each packet is gathered by the Accounting Meter and accumulated in a traffic matrix, typically located in a volatile storage location in the serving MD-IS. This information includes the number of NPDUs and NPDU-bytes conveyed successfully across the airlink.
The traffic matrix information also includes the identifier of the cell that served the M-ES and the Home Tariff Code provided to the serving MD-IS by the home MD-IS during registration. A new matrix row is created whenever a new M-ES NEI and correspondent NEI pair begin communicating. The traffic matrix is periodically flushed to the Serving Accounting Distributor, a more permanent store for this data.
The primary purpose of a Serving Accounting Distributor is to build home accounting segments orHASs, one for each different home accounting area encountered in the received traffic matrix segments. At some mutually agreed upon interval, the SAD sends these HASs to the appropriate target home accounting areas.7.9 The O/R (originator/recipient) address of the HADs are statically preconfigured in the SAD; since there are a limited number of CDPD service providers, each with a single HAD, which has an unvarying address, scaling and flexibility are not concerns.
On receipt of a traffic matrix segment from an Accounting Meter, the Serving Accounting Distributor may choose to add Serving Tariff Code information to the rows of the traffic matrix segments. This information could be used by the Consolidation Accounting Collectors to generate the proper bills to end customers/subscribers. The CDPD specifications define entities and protocols but not implementation nor how this information should be used by service providers.
A Home Accounting Distributor accepts home accounting segments from potentially multiple Serving Accounting Distributors with the purpose of delivering consolidation accounting segments to the various Consolidation Accounting Collectors and to a Home Accounting Collector in that accounting area.
The Home Accounting Distributor finds the O/R Address of the appropriate Consolidation Accounting Collectors from the subscriber profile maintained by Directory Services. The O/R Address of the Home Accounting Collector is provided to the Home Accounting Distributor by network management.
Having a single HAD (address) defined for each home accounting area means that only one address per service provider needs to be shared with other service providers. Thus each CDPD service provider has flexibility in their ability to configure and evolve their internal accounting architecture.
There are actually two kinds of HAD-the P-HAD and the R-HAD-defined by CDPD System Specification Release 1.1. These two types of HAD reflect the inter-service provider relationships possible in CDPD. The P-HAD or primary HAD is located in the domain of the CDPD service provider owning the business relationship with the customer receiving service. The R-HAD or routing HAD is located in the domain of the CDPD service provider providing the mobile home function (MHF), i.e., the mobility support for the subscriber.
This separation of mobility and accounting is necessary to support situations in which one service provider's customer has subscribers which are homed in another service provider's area. For efficiency reasons, there is no requirement that the mobility home be the same as the accounting home. Whenever this situation prevails, the SAD must transmit identical HASs to both HADs for a given subscriber.
The Home Accounting Collector is the final repository of CASs, which have been forwarded by the HAD. The HAC is where CASs are stored and processed by the various applications using accounting data, most notably billing and planning. This is also where the home service provider can verify the activities of its subscribers visiting other service provider domains and audit the corresponding service charges of the serving service providers.
The Consolidation Accounting Collector defines the storage and processing location for large CDPD customers whose subscribers (mobiles) commonly receive service from multiple CDPD service providers. Large nationwide customers would be expected to operate their own CACs to verify the activities of their subscribers and audit the corresponding service charges of the serving service providers.