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CCENT™ (Cisco Certified Entry Network Technician) Certification Exam Notes

A. OSI Model

1. The 7 layers of OSI model are:

1. The Application Layer: Application layer is responsible for identifying and establishing the availability of intended communication partner and verifying sufficient resources exist for communication. Some of the important application layer protocols are: WWW, SMTP, FTP etc.

2. The Presentation Layer: This layer is responsible for presenting the data in standard formats. This layer is responsible for data compression, decompression, encryption, and decryption. Some Presentation Layer standards are: JPEG, MPEG, MIDI, PICT, Quick Time, TIFF.

3. The Session Layer: Session Layer is responsible for co-ordinating communication between systems/nodes. The following are some of the session layer protocols and interfaces: a) Network File System (NFS), SQL, RPC (Remote Procedure Call), X-Windows, ASP, DNA SCP.

4. The Transport Layer: The Transport Layer is responsible for multiplexing upper-layer applications, session establishment, and tearing-down of virtual circuits. This layer does "flow control" to maintain data integrity. Flow Control prevents the problem of a sending host on one side of the connection overflowing the buffers in the receiving host.

5. The Network Layer: There can be several paths to send a packet from a given source to a destination. The primary responsibility of Network layer is to send packets from the source network to the destination network using a pre-determined methods. Routers work at Network layer.

6. The Data Link Layer:
Data Link Layer is layer 2 of OSI reference model. This layer is divided into two sub-layers:
A. Logical Link Control (LLC) sub-layer.
B. Media Access Control (MAC) sub-layer.
The LLC sub-layer handles error control, flow control, framing, and MAC sub-layer addressing.
The MAC sub-layer is the lower of the two sub-layers of the Data Link layer. MAC sub-layer handles access to shared media, such a Token passing or Ethernet.

7. Physical Layer: The actual flow of bits take place through Physical layer. At Physical layer, the interface between the DTE and DCE is determined. The following are some of the standard interfaces are defined at Physical layer: A> EIA/TIA-232, EIA/TIA-449,V.24,V.35,X.21,G.703,HSSI (High Speed Serial Interface).


2. Few Important things to note about OSI layer functions:

1. Application layer is responsible for ensuring the availability of intended communication partner. This layer is also responsible for synchronizing sending and receiving applications.
2. Presentation Layer is responsible for negotiating data transfer syntax.
3. Session layer is responsible for co-coordinating communication between systems
4. Transport layer is responsible for multiplexing upper-layer applications. It is also responsible for establishment and tear down of virtual circuits.

3. Data Link Layer is layer 2 of OSI reference model. This layer is divided into two sub-layers:

1. Logical Link Control (LLC) sub-layer.
2. Media Access Control (MAC) sub-layer.
The LLC sub-layer handles error control, flow control, framing, and MAC sub-layer addressing.
The MAC sub-layer is the lower of the two sub-layers of the Data Link layer. MAC sub-layer handles access to shared media, such a Token passing or Ethernet.
4. DOD Model maps to OSI model as below:
Process/ Application maps to OSI's Application, Presentation, Session layers (layers 7,6,5).
Host-to-Host maps to ISO's Transport layer (layer 4).
Internet maps to ISO's Network layer (layer 3).
Network Access maps to ISO's Data Link and Physical Layers (layers 2, 1).
5. Switches work at layer 2 of ISO model, which is Data Link Layer. A switch looks at the destination MAC address before forwarding the frame.
6. TCP and UDP work at transport layer of OSI model or the Host-to-Host layer of DOD Model. SNMP uses UDP over IP.
7. The following are frequently used OSI layer 2 (data link) protocols:
a. PPP
b. SLIP
c. Frame Relay
d. HDLC
e. Ethernet
f. Token Ring
8. The following are some Presentation Layer standards:
Graphic and Visual Image: PICT, TIFF, JPEG
Movies and Sound: MIDI, MPEG, Quick Time
The following are Session layer standards:
NFS, SQL,RPC,X-Windows.
9. Physical layer is responsible for bit synchronization.
· Repeaters work at Physical layer (Layer 1),
· Bridges and simple switches work at Data Link Layer (Layer 2),
· Routers work at Network Layer (Layer 3) of ISO Reference Model.
10. Segments is associated with Transport layer, Packets is associated with Network Layer and
Frames is associated with Data Link Layer

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