//ini hasil migrasi blog, 7 Juli 2015. Pindahan dari unisa81.net//
Lagi iseng-iseng mencoba men-summarize
mengenai 7 layer OSI (Open System
Interconnection ) dari berbagai sumber. Semoga rangkuman yang dibawah ini cukup
bermanfaat untuk memberikan sekilas gambaran mengenai 7 layer OSI dan functional aspect-nya
OSI model adalah bagian dari OSI initiative, yang
merupakan standar guidelines yang dibuat untuk membantu vendor, engineers, developers dll dalam
memahami network architecture di setiap layer
beserta fungsi-fungsinya dalam proses aliran komunikasi (ini pengertian ala Unisa, mungkin berbeda
dengan sumber-sumber lain tapi maksudnya kira-kira sama)
Rangkuman di bawah tetap dipertahankan dalam
bahasa inggris karena sumber-sumber yang dijadikan rujukan juga berbahasa
inggris, selain itu Unisa kesulitan untuk menerjemahkan beberapa istilah ke
dalam Bahasa Indonesia. Namun jika memungkinkan suatu saat akan diterjemahkan
ke dalam Bahasa Indonesia.
1. Application
layer (layer 7)
a. Closest
to the user and provides file transmission, message exchanges, terminal
sessions and much more
b. It
includes the protocols that support the application but not include the actual applications.
c. Example
of the protocol working at this layer
SMTP, HTTP, LDP, FTP, Telnet and TFTP
d. Applications
communicate with the underlying protocols through API (application programming
interface)
e. For
example, email message through email client Outlook will be sent to SMTP. SMTP
adds its information to the user’s information and passes it down to the
presentation layer
2. Presentation
layer (layer 6)
a. Receives
information from the application layer protocols and puts it in a format all
computer following the OSI model can understand
b. This
layer provides a common means of representing data in a structure that can be
properly processed by the end system
c. The
presentation layer on the sender’s system converted the file to ASCII (American
standard code for Information Interchange), and the receiver’s computer knows
how to open all specific files
d. The
presentation layer adds information to
tell the destination computer the file type and how to process and present it
e. For
example, if the user sends graphic created in Corel application to another user
who does not have Corel application, the user’s operating system can still
present the graphic because it has been saved into a standard format.
f. The
layer also handles data compression and encryption issues (if a program
requests a certain file to be compressed and encrypted before being transferred
over the network, the presentation layer provides the necessary information for
the destination computer)
g. Instructions
are added to the data package that tell the receiving system how to decrypt or
decompress the data properly
3. Session
layer (layer 5)
a. Responsible
for establishing a connection between two applications, maintaining it during
the transfer of data, and controlling the release of this connection
b. It
works in 3 phases : connection establishment, data transfer and connection
release (provides restart and recovery if necessary), and provides overall
maintenance of the session
c. Some
protocols that work at this layer : NFS, SQL, NetBIOS and RPC
Note : Session layer
protocols control application-to-application communication, whereas the
transport layer protocols handle computer-to-computer communication
4. Transport
Layer (layer 4)
a. The
transport layer provides end-to-end data transport services and establishes the
logical connection between two communicating computers
b. Two
computers agree on establishing connection through a handshaking process. It
helps provide more reliable data transfer, error detection, correction,
recovery and flow control, and optimizes the network services needed to perform
these tasks
c. The
main protocol : TCP, UDP and SPX. TCP provide reliable data transmission when
compared with UDP.
d. The
transport layer receives data from many
different applications and assembles the data into a stream to be properly
transmitted over the network
e. Analogy
: the bus can carry many people, the transport layer protocol can carry a
variety of application data types
Note
: The
difference between layer 4 and layer 5 is that protocols that work at the
session layer set up connections between application,
whereas protocols that work at the transport layer set up connections
between computer systems. We can have
6 different applications on comp A communicating to 6 applications on comp B.
The session layer protocols keep track of these different sessions. Transport
layer does not know or care what applications are communicating with each
other. It just provide mechanism to get the data from one system to another
5. Network
layer (layer 3)
a. The
network layer determines the most efficient path for each packet to take. The
protocols at the network layer must determine the best path for the packet to
take (many routes can lead to one destination). Routing protocols build and
maintain their routing tables at
this layer
b. Network
layer insert information into the packet’s header so it can be properly
addressed and routed
c. The
protocols in network layer do not ensure the delivery of the packets. They
depend on the protocols at the transport layer to catch any problems and resend
packets if necessary
d. Some
of protocols work in Network layer : IP, ICMP, RIP, OSPF, BGP, IGMP
6. Data
Link Layer (layer 2)
a. The
data link layer converts the data into bits for the physical layer
b. The
data packet being translated into local area network (LAN) or wide area network
(WAN) technology binary format for proper line transmission at Data Link layer
c. The
Data link layer is where the network stack knows what format the data frame
must be in to transmit properly over Token
Ring, Ethernet, ATM or Fiber Distributed Data Interface (FDDI) networks
d. Data
link layer also manage reorder frames (out of sequence), and notify upper-layer
protocols when there are transmission error conditions
e. The
data link layer is divided into 2 functional sub layer :
i. Logical
Link Control (LLC), communicates with the protocol immediately above it
ii. Media
Access Control (MAC), will have the appropriately loaded protocol to interface
with the protocol requirements of the physical layer
f. Examples
of data link protocols are Ethernet for local area networks (multi-node) and
PPP, HDLC and ADCCP for point-to-point (dual-node) connections
Note
: Depending on the network technology being
used (Ethernet, token ring, FDDI, and so on), the network card driver encodes
the bits at the data link layer, then turned into electricity states at the
physical layer and placed onto the wire for transmission
7. Physical
Layer (layer 1)
a. Physical
Layer converts bits from data link layer into
electrical signals (It translates communications requests from the Data
Link Layer into hardware-specific operations to affect transmission or
reception of electronic signals)
b. This
layer controls synchronization, data rates, line noise, and medium access
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