Exam Essentials

Understand the advantages of reference models. Using reference models, IT personnel can focus their energies on the specific functions and components of a network. For instance, making changes to one layer in an organization’s network does not require changing other layers. Ultimately, the complexity of networking is divided into more manageable pieces, allowing for a logical troubleshooting approach.

Know the functions of each OSI layer. It’s very important that you understand what each of the seven layers of the OSI reference model does. The Application, Presentation, and Session layers are the upper layers. They are responsible for communication from a user interface to an application. The Transport layer provides segmentation, sequencing, and virtual circuits. The Network layer provides logical network addressing and routing through the internetwork. The Data Link layer provides framing and placing of the data on the network medium. The Physical layer is responsible for taking 1s and 0s and encoding them into digital signal for transmission onto the network segment.

Understand how the Transport layer functions. In the Transport layer, the segments that are delivered are acknowledged and sequenced. Non-acknowledged segments are retransmitted, and data flow is maintained to avoid congestion and data loss. Windowing controls the amount of data sent from one end of the network to another end without the receipt of an acknowledgement.

Understand how the Network layer functions. In the Network layer, path determination makes it possible for routers to decide on the best route to send packets to a given destination. Network layer addressing provides the logical map used at the source and destination for communication.

Understand the five conversion steps of data encapsulation. Data from the Application layer is encapsulated at the Presentation layer.

1. The Presentation layer data plus the Application layer data is then passed to the Session layer and encapsulated for synchronization.

2. At the Transport layer, all upper layer data is then encapsulated and segmented.

3. Once at the Network layer, data received from the Transport layer and routing information is added.

4. All data is passed to the Data Link layer for framing and finally to the Physical layer for conversion to 1s and 0s to be sent across the wire.

Remember the benefits of FDDI networking. FDDI includes many benefits for campus-type LANs. Using fiber-optic cabling, there is no risk of data loss or interference and higher speeds can be deployed. The primary ring is backed up by the secondary ring in case of a break or outage. FDDI uses a token-passing method that prevents collisions and provides for predictable delays.

Understand the functions at each of the three ATM layers. The function at each of the ATM layers is very much like the function of the bottom three layers of the OSI model. The ATM Physical layer is much like the OSI Physical layer and is responsible for sending and receiving bits on the physical level. The ATM Data Link layer provides for the virtual connections and carries the ATM cells similar to frames created at the OSI Data Link layer. The AAL layer provides translation services between upper layers of the OSI and the ATM layers.