Layering

Layering one common technique in which the software designer uses it to break down a complex system into parts simpler. 

Example: Networking: layers of the OSI layer and TCP / IP. 

When the system is divided into layers, then the principal part of the system software is arranged in layers, where each upper layer depends on lower layers.
Higher layer uses the services defined by the lower layer, but the lower layer does not need to know the higher layers. Each layer is usually menyembunyikanlower layernya from upper layer, so layer 4 using the services of layer 3, which use the services of layer 2, layer 4 but did not know anything about 2.

Excess layering :

• We only know that the application consists of one course without singlelayer know layers other.
For example, we can find out how to make the FTP service without padaTCP must know how to work a physical Ethernet Card.
• We can change the layers with other applications that implement the same basic services.
An FTP service may vary can continue running without having to replace the Ethernet, PPP, or the cables.
• We can minimize the dependence between layers.
If we replace the network cable, we do not need to also change the FTP service.
• Layer strongly supports standardization.
TCP / IP is the standard because they define how the layers they must beoperasi.
• After the layer is formed, we can use it for a variety of other services.
For example, TCP / IP is used by FTP, telnet, SSH, and HTTP. All of these protocols, the program had lower levels of each protocol as well.


Weakness layering :

• Use of lead and adds layers of complexity level of the process.
Because it consists of several layers, each layer must have their respective functions, and a process must pass through each of the first layer can only produce output.
So each layer must have a different process capability.
• Layer-mengenkapsulasifungsi their respective functions so that we can not know details of the function of a layer.
Layers work together as one so that all layers must work optimally.