This topic describes the concept of an Access Point in the Communications Framework.
A Layer can make a service available to the Layer immediately above it. The service is called an Access Point.
Each Layer except the highest Layer can have an Access Point. The highest Layer is called the Application Layer. The Communications Framework can limit the use of an Access Point to selected clients.
The following diagram shows five Access Points. Each Access Point offers a service to the Layer above it.
In this diagram the Link Layer has three bearers. Each bearer has an Access Point. An application or a Layer above the Link Layer does not need to know which bearer in the Link Layer is in use during the connection. The Communication Framework can replace the bearer with another bearer. The Communication Framework does not need to tell the application when the bearer changes. An application can continue to send and receive data when the change occurs.
For example, consider an email application on a device. The device connects over a GPRS network. A short time later the device detects that a Wi-Fi connection is available. This event can occur if the device has moved from an office location to a coffee shop. The device can have a selection policy. The policy can state that the device switch to WiFi without the creation of a new connection.
The selection policy contains a list of Access Points in order of priority. Device manufacturers, Network Operators and users can configure the order of priority. The priority can be based on items like available bandwidth and cost. An alternative to selection policy is the concept of connection preference. For example, a device connects with a specific bearer (for example, GPRS) unless the application changes the bearer explicitly.
It is useful to note that an Internet Access Point (IAP) is a specialisation of the Access Point concept for a Link Layer. IAPs identify a service to be accessed through a concrete link-layer bearer. Each IAP has an associated Access Point with the same physical value.
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