"There are two definitions for wireless LAN roaming:
Internal Roaming (1): The Mobile Station (MS) moves from one access point (AP) to another AP within a home network because the signal strength is too weak. An authentication server (RADIUS) performs the re-authentication of MS via 802.1x (e.g. with PEAP). The billing of QoS is in the home network. A Mobile Station roaming from one access point to another often interrupts the flow of data among the Mobile Station and an application connected to the network. The Mobile Station, for instance, periodically monitors the presence of alternative access points (ones that will provide a better connection). At some point, based on proprietary mechanisms, the Mobile Station decides to re-associate with an access point having a stronger wireless signal. The Mobile Station, however, may lose a connection with an access point before associating with another access point. In order to provide reliable connections with applications, the Mobile Station must generally include software that provides session persistence.
External Roaming (2): The MS (client) moves into a WLAN of another Wireless Internet Service Provider (WISP) and takes their services (Hotspot). The user can independently of his home network use another foreign network, if this is open for visitors. There must be special authentication and billing systems for mobile services in a foreign network." [Wireless LAN. Wikipedia]
This Cisco roaming wireless local area network diagram example was created using the ConceptDraw PRO diagramming and vector drawing software extended with the Cisco Network Diagrams solution from the Computer and Networks area of ConceptDraw Solution Park.

"An invoice, bill or tab is a commercial document issued by a seller to a buyer, relating to a sale transaction and indicating the products, quantities, and agreed prices for products or services the seller has provided the buyer.
Payment terms are usually stated on the invoice. These may specify that the buyer has a maximum number of days in which to pay, and is sometimes offered a discount if paid before the due date. The buyer could have already paid for the products or services listed on the invoice.
In the rental industry, an invoice must include a specific reference to the duration of the time being billed, so in addition to quantity, price and discount the invoicing amount is also based on duration. Generally each line of a rental invoice will refer to the actual hours, days, weeks, months, etc., being billed.
From the point of view of a seller, an invoice is a sales invoice. From the point of view of a buyer, an invoice is a purchase invoice. The document indicates the buyer and seller, but the term invoice indicates money is owed or owing. ...
Payment of invoices.
Organizations purchasing goods and services usually have a process in place for approving payment of invoices based on an employee's confirmation that the goods or services have been received.
Typically, when paying an invoice, a remittance advice will be sent to the supplier to inform them their invoice has been paid." [Invoice. Wikipedia]
The flow chart example "Invoice payment process" was created using the ConceptDraw PRO diagramming and vector drawing software extended with the Flowcharts solution from the area "What is a Diagram" of ConceptDraw Solution Park.

This payment process flowchart example was created on the base of the diagram of payment process using Global Payment Processing Services from the U.S. Securities and Exchange Commission website.
"Global Payment Processing Services. CyberSource Advanced enables merchants to accept payments made by all major credit and debit cards including American Express®, Discover®, Diners Club International®, JCB, MasterCard®, and Visa® cards. Our customers can also accept payment by corporate procurement cards, electronic checks, PayPal® Express Checkout, and the Bill Me Later® service. Merchants that have business models based on subscriptions can utilize the CyberSource recurring billing service with automated account updating services. For merchants selling internationally, we support direct debit, and bank transfers, as well as regional card brands such as Carte Bleue, Carta Si, Dankort, Laser, Solo, and Visa Electron. We provide these services for online, call center, kiosk, integrated voice response (“IVR”), and IP-enabled point of sale (“POS”) transactions."
[www.sec.gov/ Archives/ edgar/ data/ 934280/ 000119312510042764/ d10k.htm]
The flowchart example "Global Payment Solutions" was created using the ConceptDraw PRO diagramming and vector drawing software extended with the Sales Flowcharts solution from the Marketing area of ConceptDraw Solution Park.