Nov 3, 2011

Abstract of CCNA study guide-2- internetworking 2

Continue the series of  Abstract CCNA study guide book .Internetworking Models
The OSI model is the primary architectural model for networks. It describes how data and network information are communicated from an application on one computer through the network media to an application on another computer. The OSI reference model breaks this approach into layers.
Advantages of Reference Models
Advantages of using the OSI layered model include, but are not limited to, the following:
- It divides the network communication process into smaller and simpler components.
- It allows multiple-vendor development through standardization of network components.
- It encourages industry standardization by defining what functions occur at each layer .
- It allows various types of network hardware and software to communicate.
- It prevents changes in one layer from affecting other layers.

The OSI Reference Model
The OSI isn’t a physical model. it's a set of guidelines that application developers can use to create and implement applications that run on a network.
The OSI has seven different layers, divided into two groups. The top three layers define how the applications within the end stations will communicate with each other and with users. The bottom four layers define how Data is transmitted end to end.
The OSI reference model has seven layers:
- Application layer (layer 7)
- Presentation layer (layer 6)
- Session layer (layer 5)
- Transport layer (layer 4)
- Network layer (layer 3)
- Data Link layer (layer 2)
- Physical layer (layer 1)
The Application Layer
the Application layer is acting as an interface between the actual application program—which isn’t part of the layered structure—and the next layer down .
The Application layer is also responsible for identifying and establishing the availability of the communication partner and determining whether sufficient resources for the intended communication exist.
The Presentation Layer
The Presentation is responsible for data translation and code formatting.
Tasks like data compression, decompression, encryption, and decryption are associated with this layer.
The Session Layer
The Session layer is responsible for setting up, managing, and then tearing down sessions between Presentation layer entities (keeps different applications’ data separate from other applications’ data.).
Offering three different modes: simplex, half duplex, and full duplex.
The Transport Layer
- The Transport layer segments and reassembles data into a data stream.
- TCP and UDP work at the Transport layer and that TCP is a reliable service and UDP is not.
- The Transport layer is responsible for establishing sessions, and tearing down virtual circuits.
- The transport layer uses acknowledgments, sequencing, and flow control terms.
The following sections will provide the connection-oriented (reliable) protocol of the Transport layer.
Flow Control
Flow control prevents a sending host on one side of the connection from overflowing the buffers in the receiving host.
the protocols involved ensure that the following will be achieved:
- The segments delivered are acknowledged back to the sender upon their reception.
- Any segments not acknowledged are retransmitted.
- Segments are sequenced back into their proper order upon arrival at their destination.
- A manageable data flow is maintained to avoid congestion, overloading, and data loss.
Connection-Oriented Communication
Let me sum up the steps in the connection-oriented session—the three-way handshake—
pictured in previous Figure :
- The first “connection agreement” segment is a request for synchronization.
- The second and third segments acknowledge the request and establish connection.
- The final segment is also an acknowledgment. It notifies the destination host has been accepted and the actual connection has been established.

what happens when a machine receives a flood of datagrams too quickly for it to process? It stores them in a memory section called a buffer. and when buffer full the transport can issue a “not ready” indicator to the sender of the flood .
A service is considered connection-oriented if it has the following characteristics:
- A virtual circuit is set up (e.g., a three-way handshake).
- It uses sequencing.
- It uses acknowledgments.
- It uses flow control.
Window is The quantity of data segments (measured in bytes) that the transmitting machine is allowed to send without receiving an acknowledgment for them .
As you can see in Figure , there are two window sizes—one set to 1 and one set to 3.
When you’ve configured a window size of 1, the sending machine waits for an acknowledgment
for each data segment it transmits before transmitting another. If you’ve configured a window
size of 3, it’s allowed to transmit three data segments before an acknowledgment is received.
The positive acknowledgment with retransmission is a technique that requires a receiving machine to communicate with the transmitting source by sending an acknowledgment message back to the sender when it receives data.
In Figure 1.12, the sending machine transmits segments 1, 2, and 3. The receiving node acknowledges it has received them by requesting segment 4. When it receives the acknowledgment, the sender then transmits segments 4, 5, and 6. If segment 5 doesn’t make it to the destination, the receiving node acknowledges that event with a request for the segment to be resent. The sending machine will then resend the lost segment and wait for an acknowledgment, which it must receive in order to move on to the transmission of segment 7.
The Network Layer
The Network layer must transport traffic between devices that aren’t locally attached.
The Data Link Layer
The Data Link layer handles error notification, network topology, and flow control.
The Data Link layer formats the message into pieces, each called a data frame, and adds a customized
header containing the hardware destination and source address.
Next Figure shows the Data Link layer with the Ethernet and IEEE specifications. When you check it out, notice that the IEEE 802.2 standard is used in conjunction with and adds functionality to the other IEEE standards.
It’s important for you to understand that routers, which work at the Network layer, don’t care at all about where a particular host is located. They’re only concerned about where networks are located and the best way to reach them. It’s the Data Link layer that’s responsible for identification of each device that resides on a local network.
It’s really important to understand that the packet itself is never altered along the route; it’s only encapsulated with the type of control information required for it to be properly passed on to the different media types.
The IEEE Ethernet Data Link layer has two sublayers:
Media Access Control (MAC) 802.3: Defines how packets are placed on the media. Physical addressing is defined here, as well as logical topologies.
What’s a logical topology? It’s the signal path through a physical topology. Line discipline, error notification (not correction), ordered delivery of frames, and optional flow control can also be used at this sublayer.
Logical Link Control (LLC) 802.2 :Responsible for identifying Network layer protocols .The
LLC can also provide flow control and sequencing of control bits.
The Physical Layer
the Physical layer does two things: It sends bits and receives bits.
The Physical layer communicates directly with the various types of actual communication media.
This layer is also where you identify the interface between the data terminal equipment (DTE) and the data communication equipment (DCE). The DCE is usually located at the service provider, while the DTE is the attached device. The services available to the DTE are most often accessed via a modem or channel service unit/data service unit (CSU/DSU).

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