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OSI Reference Model & Layered Communication
Sritrusta Sukaridhoto
OSI Layer
Open Systems Interconnection for Communications
There are 7 Layers Layers 7,6,5 are oriented more toward
services to the applications Layer 4,3,2,1 are oriented more toward
the flows data from end-to-end through network
7 Layer OSI
How to Remember (CISCO)
Please Physical Do Data Link Not Network Throw Transport Sousces Session Pizza Presentation Away Application
Application
Layer 7
Function & Description
An application that communicates with other computers is implementing OSI application layer concepts. Application layer refer to communications service to application.
Example
Telnet HTTP FTP WWW Browser NFS SMTP Gateway SNMP
Presentation
Layer 6
Function & Description
This layer main purpose is defining data format such as ASCII text, EBCDIC text, binary, BCD, JPEG.
Encryption is also defined by OSI as presentation layer service.
Example
JPEG ASCII TIFF GIF PICT MPEG MIDI Encryption
Session
Layer 5
Function & Description
This session layer defines how to start, control, and end conversation (called session).
This includes the control and management of multi bidirectional messages so that application can be notified if only some of a series of messages are completed.
Example
RPC SQL NFS Netbios Names AppleTalk ASP SCP DECnet
Transport
Layer 4
Function & Description
Layer 4 includes the choice of protocols that either do or do not provide error recovery. Multiplexing of incoming data for different flows to applications on the same host.
Example
TCP UDP SPX
Network
Layer 3
Function & Description
This layer defines end-to-end delivery packets. To accomplish this, the network layer defines logical addressing so that any endpoint can be identified. It also defines how routing works and how routes are learned so that the packets can be delivered
Network layer also defines how to fragment a packet into smaller packets to accommodate media with smaller maximum transmission unit size
Example
IP IPX AppleTalk DDP
Data Link
Layer 2
Function & Description
The data link specifications are concerned with getting data across one particular link or medium. The data link protocols define delivery across an individual link. These protocols are necessary concerned with the type of media in question.
Example
IEEE 802.3/802.2 HDLC Frame Relay PPP FDDI ATM
Physical
Layer 1
Function & Description
Deal with the physical characteristics of the transmission medium, Connection pins, use of pins, electrical currents, encoding and light modulation.
Example
Ethernet RJ-45 V.35 FDDI EIA/TIA-232
Some protocol define details of multiple layers.
Example: NFS
Benefit & Concept
Humans can discuss and learn about many details of a protocol specification easier
Standardized interfaces among layers Better environment for interoperability Reduce complexity, faster production Each layer can define header and trailers
around the user data One layer use the services of the layer
immediately below it.
Interaction Between OSI Layer
On Same Computer
Step 1 = Physical Layer, ensure bit synchronization
Step 2 = Data Link Layer, Frame Check Sequence (FCS) whether error occurred in transmission
Step 3 = Network Layer, Exam address Step 4 = Transport Layer, acknowleadment Step 5 = Session Layer, to ensure that a series
of messages is completed Step 6 = Presentation Layer, manipulate data
format Step 7 = Application Layer, processes the final
header and then can examine the true end-user data
Different Computer
Data Encapsulation
5 Step Data Encapsulation
Create the data Package the data for transport Add the destination network layer
address to the data Add the destination data link address
to the data Transmit the bit
Frames, Packets and Seqments
The layer 2 PDU (Protocol Data Unit) including the data link header and trailer = Frame
The Layer 3 PDU = packet The Layer 4 PDU = seqment
OSI, TCP/IP
OSI Transport Layer Function
Connection Oriented vs Connectionless Oriented
Error Recovery
Flow Control
Flow control is the process of controlling the rate at witch a computer sends data.
Implementation: Buffering Congestion Avoidance Windowing
Buffering
Buffering simply means that computers reserve enough buffer space that bursts of incoming data can be held until processed
Congestion Avoidance
Windowing
Data Link Function
MAC = Media Access Control
CSMA/CD = Carrier Sense Multiple Access/Collision Detect
OSI Network Layer Function
Routing Sending data to a nearby Router Routing data Across Network Delivering Data to End Destination
Addressing
Summary
Application (Layer 7) User Interface Telnet, HTTP Application (Layer 6) How data is presented JPEG,
ASCII Session (Layer 5) Keeping data separate from different
application OS, application access scheduling Transport (Layer 4) Reliable or unreliable delivery,
Multiplexing TCP, UDP, SPX Network (Layer 3) Logical addressing, Routing IP, IPX Data Link (Layer 2) MAC, error detection 802.3/802.2,
HDLC Physical (Layer 1) Devices Ethernet, V.35