OSI Model


• Not a network architecture, because it does not specify the exact services and
protocols to be used in each layer, it just formally defines and codifies the concept
of layered network architecture
• Each layer describe what happens at each stage in the processing of data for
transmission
• Layers help to reduce complexity
• Each layer relies on the next lower layer to perform more primitive functions
• Each layer provides services to the next higher layer
• Changes in one layer should not require changes in other layers
The functions of different layers
Physical
• responsible for transmitting raw bits over a communication path
• concerned with issues such as
-mechanical interfaces, e.g. design of a network connector
-electrical interfaces, e.g. voltage level of bits
-procedural interfaces, e.g. whether transmission may
proceed simultaneously in both directions
Data Link
• Responsible for the transfer of data between the ends of a physical link
• Provides for error detection, "framing", and flow control
• Resolves problems due to damaged, lost, or duplicate frames
• Formatted messages are referred to as frames rather than packets
Network
• Responsible for the source to destination routing
• Addresses and resolves all inherent problems related to the transmission of data
between heterogeneous networks
• Formatted messages are referred to as packets
• In broadcast networks the network layer is often thin or nonexistent, because of
easy to solve routing problems
• Sometimes no need for a network layer if using point-to-point link
Transport
• Provides for error-free delivery of data
• Accepts data from the session layer and splits data into smaller packets if
necessary
• passes these packets to the network layer, and ensures that packets arrive in
sequence, with no losses or duplications, at their destination
Session
• Provides for coordination between communicating processes between nodes.
• Manages dialog control (e.g. Can allow traffic to go in both direction at the same
time, or in only one direction at time.)
• Responsible for synchronizing the flow of data, and reestablishing a connection
in the event a failure occurs.
Presentation
• Provides for data formats, and code conversions
• Concerned with syntax and semantics of data being transmitted
• Encodes messages in a form that is suitable for electronic transmission
• Data compression and encryption is done at this layer
Application
• Consists of protocols that define specific user-oriented applications such as email,
file transfer, and virtual terminal
Notes
Differences between a computer network (CN) and a distributed system(DS)
• CN collection of computers connected by single technology
• DS collection independent computers appears as one coherent system
• Middleware responsible for the DS
• WWW is the example of DS
• DS software system built on top of network
The two services a network offers
Connection oriented
• A connection is established between ES’s (end System) that is used for duration
of call
Call setup
Data transfer
Call termination
E.g: Virtual circuits at this layer
• IS’s ( intermediate system) connect two or more networks
IS appear as ES to each network
Logical connection set up between ESs
-Concatenation of logical connections across networks Individual network virtual circuits
joined by IS
• Advantages
– Fixed path
– Order of message preserved
– No loss of data
• Reliable
– But the process of acknowledgement adds overhead and delay
– Example: telephone, ftp
Connectionless
• Each packet sent independently
• Routing decisions made at every IS
• Corresponds to datagram service in packet switched network
• Network layer protocol common to all ES’s and routers
– Known generically as the internet protocol
• Internet Protocol
– One such internet protocol developed for ARPANET
– Example: Telegraph systems, email, remote login
– Advantages
– Flexibility
– Robust
– No unnecessary overhead
– Unreliable
– Not guaranteed delivery
– Not guaranteed order of delivery
– Packets can take different routes
– Reliability is responsibility of next layer up (e.g. TCP)

The following table shows an example of 6 different services

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