Open Systems Interconnection Model

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Open Systems Interconnection Model
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CIRCUIT SWITCHING

• Connection establishment occurs at the beginning
  of session
• Exclusive use of channel established for the rest
  of the session
• No buffering at the intermediate switching
  facilities
• Highly efficient when high transfer rates need to
  be supported on continual basis
• Addressing information sent only upon call
  establishment

Source Warren HiokiTelecommunications, Fourth
Edition
3
PACKET SWITCHING

• Messages are divided in to discrete units
  (packets) and these are routed independently to
  their final destination.
• Each packet includes Addressing and control
  information
• At each node a routing decision is made based on
  the destination address
• Each packet is routed independently of the other
  and they all may arrive at the destination node
  using different paths across the network
• The packets may not arrive in the same sequence
  at the destination node
• The destination node is responsible for
  re-arranging the packets in sequence.

Source Warren HiokiTelecommunications, Fourth
Edition
4
The Open Systems Interconnections (OSI) Standard
was developed in 1984 by the International
Organization for Standardization (ISO)
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The OSI Model
Application
End user applications
Transmission Codes, Encryption, compression, file
formats
Presentation
Management of user session
Session
End-to-End reliable transport, Error Correction,
Flow Control, multiplexing of end user info.
Transport
Routing of packets across the network/s
Network
Error control, flow control, framing and
sequencing of data
Data Link
Electrical and Mechanical characteristics
Physical
The above figure describes the function of each
layer of OSI in general. In any given system, the
actual capabilities of a specific layer depend
on the type of protocol or application
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HTTP,FTP, SMTP,DHCP, NDS
Application
XDR, JPEG, MPEG (compression code)
Presentation
LDAP, RPC, SAP
Session

Transport
TCP, SPX, UDP
IP, IPX, X.25 packet layer, ISDN Call control
Q.931, RIP, OSPF, EIGRP
Network
802.x, X.25 LAPD, Frame Relay, HDLC, ATM, PPP,
Data Link
RS-232, V.35, X.21, RS-449, SONET
Physical
MPLS resides at layer 2.5!
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Network Scenario 1
Host 1
Host 2
Router
Router
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Network Scenario 2
Host A
Host B
Router
Router
Switch
Switch
Network
Switch