Encapsulation

Encapsulation

Layered protocol models rely on encapsulation, which allows one protocol to be used for relaying another's messages.

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Encapsulation, closely related to the concept of Protocol Layering, refers to the practice of enclosing data using one protocol within messages of another protocol.To make use of encapsulation, the encapsulating protocol must be open-ended, allowing for arbitrary data to placed in its messages. Another protocol can then be used to define the format of that data.

Encapsulation Example

For example, consider an Internet host that requests a hypertext page over a dialup serial connection. The following scenario is likely:
First, the HyperText Transfer Protocol (HTTP) is used to construct a message requesting the page. The message, the exact format of which is unimportant at this time, is represented as follows:



Next, the Transmission Control Protocol (TCP) is used to provide the connection management and reliable delivery that HTTP requires, but does not provide itself. TCP defines a message header format, which can be followed by arbitrary data. So, a TCP message is constructed by attaching a TCP header to the HTTP message, as follows:

Now TCP does not provide any facilities for actually relaying a message from one machine to another in order to reach its destination. This feature is provided by the Internet Protocol (IP), which defines its own message header format. An IP message is constructed by attaching an IP header to the combined TCP/HTTP message:

Finally, although IP can direct messages between machines, it can not actually transmit the message from one machine to the next. This function is dependent on the actual communications hardware. In this example, we're using a dialup modem connection, so it's likely that the first step in transmitting the message will involve the Point-to-Point Protocol (PPP):

Note that I've drawn the PPP encapsulation a little differently, by enclosing the entire message, not just attaching a header. This is because PPP may modify the message if it includes bytes that can't be transmitted across the link. The receiving PPP reverses these changes, and the message emerges intact. The point to remember is that the encapsulating protocol can do anything it wants to the message - expand it, encrypt it, compress it - so long as the original message is extracted at the other end.

DoD Networking Model

DoD Networking Model

The first layered protocol model we will study is the 4-layer DoD Model. This is the model originally designed for the Internet, and is important because all of the Internet's core protocols adhere to it.

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The Department of Defense Four-Layer Model was developed in the 1970s for the DARPA Internetwork Project that eventually grew into the Internet. The core Internet protocols adhere to this model, although the OSI Seven Layer Model is justly preferred for new designs.
The four layers in the DoD model, from bottom to top, are:

1. The Network Access Layer is responsible for delivering data over the particular hardware media in use. Different protocols are selected from this layer, depending on the type of physical network.
2. The Internet Layer is responsible for delivering data across a series of different physical networks that interconnect a source and destination machine. Routing protocols are most closely associated with this layer, as is the IP Protocol, the Internet's fundamental protocol.
3. The Host-to-Host Layer handles connection rendezvous, flow control, retransmission of lost data, and other generic data flow management. The mutually exclusive TCP and UDP protocols are this layer's most important members.
4. The Process Layer contains protocols that implement user-level functions, such as mail delivery, file transfer and remote login.

Cisco Menu

1: OSI Reference Model

2: DoD Networking Model

3: Encapsulation

4:|Subnetting

5:EIGRP Routing Protocol !

6:EIGRP

OSI Reference Model

What is OSI Model?

The OSI Model is used to describe networks and network application.

Layers of OSI  Model 

Three are Sever Layers of OSI Model :

OSI Layer Diagram:

  

7)  Application Layer : The application layer provider different services to the application. Example of services provided by this layer are file transfer, electronic messaging e-mail, virtual terminal access and network management.

6) Presentation Layer : The Presentation layer is responsible for protocol conversion, date encryption/decryption, Expanding graphics command and the date compression. This layer makes the communications between two host possible.

5) Session Layer : This layer is responsible for establishing the process-to-process communication between the host in the network. This layer is responsible for establishing and ending the sessions across the network. The interactive login is an example of services provided by this layer in which the connective are re-connected in care of any interruption.

4) Transport Layer : This layer is responsible for end-to-end delivers of messages between the networked hosts. It first divides the streams of data into chunks or packets before transmission and  then the receiving computer re-assembles the packets. It also guarantee error free data delivery without loss or duplications.

3) Network Layer : This layer is responsible for translating the logical network address and names into their physical address ( MAC address). This layer is also responsible for addressing, determining routes for sending and managing network problems such as packet switching, data congestion and routines.

2) Data Link Layer : Data link layer is responsible for controlling the error between adjacent nodes and transfer the frames to other computer via physical layer. Data link layer is used by hubs and switches for their operation.

1) Physical Layer : Physical  Layer is responsible for transmitting row bit stream over the physical cable. The physical layer defines the hardware items such as cables, cards, voltages etc.

How to remember the layer of OSI model?

The easiest way to remember the different layers of OSI Model is to use the mnemonic "All people seem To need Data Processing":

Layer	Name
Mnemonic
7	Application	All
6	Presentation	People
5	Session	Seem
4	Transport	To
3	Network	Need
2	Data Link	Data
1	Physical	Processing