The OSI Reference Model is a structured set of guidelines for communication between two end users in a network. It’s used in application development and understanding how an internetwork actually works. The OSI model is broken down into seven layers.
- Application
- Presentation
- Session
- Transport
- Network
- Data Link
- Physical
Clue to remember it : "All People Seem To Need Data Processing"
The layers that will be primarily working with in Cisco career and exam praparation are the first three layers. Switching occurs at Layer 2 and Routing at Layer 3. However, it’s vital that understand what all seven layers do for Intro and ICND exams.
Application Layer : Layer that defines the interface between the communications software and any applications that nieed to communicate outside the computer on which the application resides. The layer that the end user actually interacts with. The application layer ensures that the remote communication partner is available, that the needed communication resources exist, and that both ends of the communications agree on procedures involving data integrity, privacy and error recovery. The sender or receiver can also be authenticated at this level.
When trying to decide if a protocol is an Application layer protocol, just remember that protocols required the end user to enter a request.
Protocol example : Telnet, POP3, SMTP, HTTP, WWW, NFS, FTP, SNMP
Presentation Layer : the layer’s main purpose is to define data format, just what it sounds like, it’s layer that concerned with properly presenting the information. The communication systems must agree on how the data is to be presented. This layer concerned with :
- compatibility with the operating system
- proper encapsulation of data for network transmission
- data formatting (ascii, binary)
- data encryption and compression.
Protocol Example : JPEG, ASCII, GIF, MPEG, MIDI, EBCDIC, TIFF, PICT, encryption
Session Layer : layer that defines how to start, control and end conversations (called sessions). This layer handles the construction and teardown of the conncetion between the two end points involved in the communication. Some sessions last just long enough to send a unidirectional message, where other sessions will be of longer duration. Remember, this layer is consider it as the Manager of the communication between a local and a remote system.
Protocol Example : RPC, SQL, NFS, ZIP, AppleTalk Zone Information Protocol, NetBIOS named, DECnet SCP
Transport Layer : this layer’s purpose is to establish a connection between end-to-end systems, segment data received from the upper layer of the OSI model, and to make sure the data gets to the destination in the correct order and free of errors. At this layers, there are two methods for transporting data : connection-oriented, referring to TCP and connectionless referring to UDP.
Protocol example : TCP, UDP, SPX
Network Layer : also known as Layer 3 and IP Address referred to as Layer 3 Address. This layer is aware of the address of neighbor nodes in the network and responsible for selecting the best route to transmit data between devices that are not locla to each other. The end-to-end delivery of packets is defined at this layer. Router operates at this layer.
Protocol example : IP, IPX, AppleTalk DDP
Data Link Layer : This layer concerns itself with the actual media that will be used to transport the data. The protocol is determined by the media type. This layer is generally referred to as Layer 2, and MAC address as Layer 2 Address. Error control and notification are performed on frames at this level. Switch operate at this layer, as do bridges.
The data link layer has two sub layers of its own :
- The Logical Link Control (LLC) tells the data link layer how to handle an incoming packet. For example, a host would look in the LLC header to see which layer 3 protocol the frame is destinated for.
- The MAC sub layer, deals with the physical media. MAC addressed are defined her.
Protocol example : IEEE 802.3/802.2, HDLC, Frame Relay, PPP, FDDI, ATM, IEEE 802.5/802.2
Physical Layer : also known as the bit layer, physical characteristic and specifications for cabling, pins, electrical currents, and connectors are all defined at this layer.
Protocol example : EIA/TIA-232, V.35, EIA/TIA-449, RJ-45, Ethernet, 802.3, 802.5, B8ZS
OSI layering Concepts and Benefits :
- Easier to learn
- Easier to develop
- Multivendor interoperability
- Modular engineering
(Source : Chris Bryant and Wendell Odom)
ada versi murahnya CCNA Certification Library. Kemudian satu buku lagi untuk melengkapi latihan soal : Flash Cards and Exam Practice. Untuk praktek kedua buku tersebut dilengkapi NetSIM LE Network Simulator, tapi hasil googling disarankan untuk bisa membuat Home Labs sehingga kita bisa latihan dengan alat yang sebenarnya. Bung Himawan Nugroho menyarankan untuk menggunakan Router seri 2610 (3 buah) dan 2522 (1 buah) untuk switchingnya kalo bisa pake seri 2950, dengan pertimbangan lab ini akan dipakai terus untuk sertifikasi selanjutnya…kembali harganya cukup lumayan… router bekas bisa cari di eBay dengan harga cukup competitive….tapi yo tetep wae larang…:) 
Akhirnya setelah timbang sana timbang sini, meskipun sedikit terlambat kalo di ukur dari segi usia….. 
