as the customer portal for transmission service requests (TSR), even for affiliated power marketers wanting access to their own parent company's transmission. OASIS Nodes OASIS nodes are entirely web-based, and public access is limited. Power marketers that become signatories to a transmission provider's OATT gain more complete access so they can view existing transmission and service availability and existing service requests made by other parties. There are also market observers who have read-only access, who may view activity but not request services. Transmission facilities have power transfer limits that must be maintained to allow the power grid to operate reliably. Transmission operators perform system studies in various future time frames to determine how much transfer capacity is required to serve their own "native load", and how much capacity must remain as a buffer to prevent unscheduled or accidental overflows that can damage high voltage equipment. The difference between the capacity needed to serve load and to maintain safe flow margins can be made available for purchase on the OASIS node. Unplanned outages and other system emergencies can adversely impact the total power transfer capability across transmission systems, and it sometimes becomes necessary for transmission providers to curtail power flows across the system by revoking transmission rights given to buyers on the OASIS. Some transmission buyers are willing to pay higher rates to avoid having their transactions curtailed, and as such transmission companies offer different priorities of transmission service at varying rates. The least expensive type of transmission is generally "non-firm" and purchased on an hour to hour basis. Daily non-firm is a slightly higher priority (because the buyer committed to purchasing all day), and increments go up from there to weekly, monthly, seasonally, yearly, or longer with the cost for each also rising incrementally. "Firm" transmission services are even more expensive, but are the last transactions to be curtailed. Even before the appearance of the OASIS nodes, many groups of transmission owners had already turned over operational control of their collective bulk transmission systems to Independent System Operators of various forms. These ISO's offered OASIS access to their collective systems very early on, so that it was often possible to make a single OASIS transmission service request that could cross multiple transmission systems. Since the inception of OASIS, and under the prodding of FERC to move transmission assets under the control of ISO's, the number of OASIS nodes is decreasing