Visa USA offered a peek this week into the capacity-planning process for a system that handles a huge chunk of the nation's retail transactions.

During the annual week-long stress test, conducted at an IBM facility outside Washington, D.C., Visa ran the system at a processing load of 6,200 transaction messages per second, a 20% increase over last year's peak on Dec. 24 of 5,200 transaction messages per second. One card purchase requires about two transaction messages.

A recent survey of retailers Visa conducted projects a 22% increase in holiday sales volume this year. The company generally expects its peak processing load to increase 15% to 20% a year; credit- and debit-cards last year surpassed cash and checks as the preferred form of payment for in-store purchases.

Visa also plans to consolidate credit- and debit-card authorization systems onto one platform, called Base1. Due to be completed early next year, the project will cut maintenance costs, says John Partridge, president of Inovant, Visa's global IT subsidiary.

Base1 is part of an effort to redesign VisaNet, the company's core system, around Web services, letting Visa offer services beyond authorizing and settling transactions between banks. New services could include providing retailers with stock numbers of items purchased and companies with information about purchasing card usage. It also allows Visa to settle transactions between banks in real time instead of in batch.

The centerpiece of the IBM facility, which also is a backup site for Visa and other financial institutions, is a 105,000 square-foot room with rows of processors and storage devices. The Visa system included five high-performance processors, 65 terabytes of disk storage, and 2 Gbytes of router capacity. Visa engineers fed the system a week's worth of transactions and observed the effect of different levels of credit and debit transactions on processing volume.

Debit transactions use more processing capacity because, unlike credit transactions, they require tracking the movement of funds in real time; credit transactions require a simple yes-or-no decision. The exercise is more than academic; with the number of debit transactions exceeding the number of credit transactions, Visa can expect to experience evermore increasing processing loads.