Short Notes: Concept of Layering | Short Notes for Computer Science Engineering - Computer Science Engineering (CSE) PDF Download

 Page 1


OSI (Open System Interconnection) Model
• The OSI model is a reference tool for understanding data communication 
between hosts with help of seven layers approach in communication process.
• Each layer performs specific functions to support the layers above it and uses 
services of the layers below it.
Page 2


OSI (Open System Interconnection) Model
• The OSI model is a reference tool for understanding data communication 
between hosts with help of seven layers approach in communication process.
• Each layer performs specific functions to support the layers above it and uses 
services of the layers below it.
7 Layer OSI MODEL
Layer 7: Application Layer
• Provides User Interface
• Specifies the usage of a network by particular application 
Layer 6: Presentation Layer
• Provides Data representation
• Handles encryption and decryption 
Layer5: Session Layer
• Provides the data distinction between separate applications
• Establishes the communication between hosts using Dialog 
control
• Example: SIP, RTP,_etc.
Layer 4: Transport Layer
• Provides the segments, Provides Reliable, or Unreliable 
communication, and end to end connections
• Example: TCP, UDP, etc.
Layer 3: Network Layer
• Provides the packets, Provides logical addressing, and path 
determination for packet forwarding
• Example: IPv4, IPv6 
Layer 2: Data Link Layer
• Provides the frames, Provides the media access and physical 
addressing (MAC, LLC)
• Example: Ethernet, token ring, etc.
Layer 1: Physical layer
• Provides the bits (digital data conversion). Moves the data 
between hosts
• Example: Coax, RF link, etc.
Physical Layer:
• The physical layer coordinates the functions required to transmit a bit stream 
over a physical medium.
• It deals with the mechanical and electrical specifications of interface and 
transmission medium.
• It also defines the procedures and functions that physical devices and 
interfaces have to perform for transmission to occur.
• Ethernet network interface card performs functions at both the physical layer 
and the data link layer.
Functions of Physical Layer:
• Physical layer defines characteristics of the interface between the devices 
and the transmission medium.
• It defines the type of transmission medium.
• It defines the transmission rate (the number of bits sent each second).
• It performs synchronisation of sender and receiver clocks.
• It is concerned with the connection of devices to the medium.
1. Point-to-point configuration: Two devices are connected together 
through dedicated link.
2. Multipoint configuration: A link is shared between several devices
• It is concerned with the physical topology.
• It defines the direction of transmission called as transmission mode (simplex, 
half duplex or duplex).
• It transmits bit stream over the communication channel.
• Hardware Used: Repeater and Hub.
Page 3


OSI (Open System Interconnection) Model
• The OSI model is a reference tool for understanding data communication 
between hosts with help of seven layers approach in communication process.
• Each layer performs specific functions to support the layers above it and uses 
services of the layers below it.
7 Layer OSI MODEL
Layer 7: Application Layer
• Provides User Interface
• Specifies the usage of a network by particular application 
Layer 6: Presentation Layer
• Provides Data representation
• Handles encryption and decryption 
Layer5: Session Layer
• Provides the data distinction between separate applications
• Establishes the communication between hosts using Dialog 
control
• Example: SIP, RTP,_etc.
Layer 4: Transport Layer
• Provides the segments, Provides Reliable, or Unreliable 
communication, and end to end connections
• Example: TCP, UDP, etc.
Layer 3: Network Layer
• Provides the packets, Provides logical addressing, and path 
determination for packet forwarding
• Example: IPv4, IPv6 
Layer 2: Data Link Layer
• Provides the frames, Provides the media access and physical 
addressing (MAC, LLC)
• Example: Ethernet, token ring, etc.
Layer 1: Physical layer
• Provides the bits (digital data conversion). Moves the data 
between hosts
• Example: Coax, RF link, etc.
Physical Layer:
• The physical layer coordinates the functions required to transmit a bit stream 
over a physical medium.
• It deals with the mechanical and electrical specifications of interface and 
transmission medium.
• It also defines the procedures and functions that physical devices and 
interfaces have to perform for transmission to occur.
• Ethernet network interface card performs functions at both the physical layer 
and the data link layer.
Functions of Physical Layer:
• Physical layer defines characteristics of the interface between the devices 
and the transmission medium.
• It defines the type of transmission medium.
• It defines the transmission rate (the number of bits sent each second).
• It performs synchronisation of sender and receiver clocks.
• It is concerned with the connection of devices to the medium.
1. Point-to-point configuration: Two devices are connected together 
through dedicated link.
2. Multipoint configuration: A link is shared between several devices
• It is concerned with the physical topology.
• It defines the direction of transmission called as transmission mode (simplex, 
half duplex or duplex).
• It transmits bit stream over the communication channel.
• Hardware Used: Repeater and Hub.
• Data Unit: Bit stream
Data Link Layer:
• The data link layer transforms the physical layer, a raw transmission facility, to 
a reliable link.
• It is responsible for Node-to-Node delivery.
• It makes the physical layer appear error free to the Network layer.
Functions of Data Link Layer:
• Data Framing: Division of stream of bits received from network layer into 
manageable data units called frames. Segmentation of upper layer datagrams 
(packets) into frames.
• Flow Control: It is to manage communication between a high speed 
transmitter with the low speed receiver.
• Error Control: It provides mechanism to detect and retransmit damaged or 
lost frames and to prevent duplication of frames. To achieve error control, a 
trailer is added at the end of a frame.
• Access Control: Gives mechanism to determine which device has control over 
the link at any given time, if two or more devices are connected to the same 
link.
• Physical Addressing: Adding a header to the frame to define the physical 
address of the sender (source address) and/or receiver (destination address) 
of the frame.
• Hardware Used: Bridges and switches.
• Data Unit: Frames
• Protocol Used: Simplex protocol, stop and wait protocol, sliding window, HDLC 
(High Level Data Link Control), SDLC, NDP, ISDN, ARP, PSL, OSPF, NDP.
Network Layer:
• Network layer is responsible for source to destination delivery of a packet 
possibly across multiple networks (links).
• If the two systems are connected to the same link, there is usually no need for 
a network layer.
• If the two systems are attached to different networks (links) with connecting 
devices between networks, there is often a need of the network layer to 
accomplish source to destination delivery.
Functions of the Network Layer:
• Logical Addressing: If packet passes the network boundary, we need 
logical addressing system to distinguish the source and destination systems.
• Routing: Independent networks or links are connected together with the help 
of routers or gateways. Routers route the packets to their final destination. 
Network layer is responsible for providing routing mechanism.
• Hardware Used: Routers
• Data Units: Packets
• Protocols Used: IP (Internet, Protocol), NAT (Network Address Translation), 
ARP (Address Resolution Protocol), ICMP (Internet control Message Protocol), 
BGP (Border Gateway Protocol), RARP (Reverse Address Resolution Protocol), 
DHCP (Dynamic Host Configuration Protocol), BOOTP, OSPF.
Page 4


OSI (Open System Interconnection) Model
• The OSI model is a reference tool for understanding data communication 
between hosts with help of seven layers approach in communication process.
• Each layer performs specific functions to support the layers above it and uses 
services of the layers below it.
7 Layer OSI MODEL
Layer 7: Application Layer
• Provides User Interface
• Specifies the usage of a network by particular application 
Layer 6: Presentation Layer
• Provides Data representation
• Handles encryption and decryption 
Layer5: Session Layer
• Provides the data distinction between separate applications
• Establishes the communication between hosts using Dialog 
control
• Example: SIP, RTP,_etc.
Layer 4: Transport Layer
• Provides the segments, Provides Reliable, or Unreliable 
communication, and end to end connections
• Example: TCP, UDP, etc.
Layer 3: Network Layer
• Provides the packets, Provides logical addressing, and path 
determination for packet forwarding
• Example: IPv4, IPv6 
Layer 2: Data Link Layer
• Provides the frames, Provides the media access and physical 
addressing (MAC, LLC)
• Example: Ethernet, token ring, etc.
Layer 1: Physical layer
• Provides the bits (digital data conversion). Moves the data 
between hosts
• Example: Coax, RF link, etc.
Physical Layer:
• The physical layer coordinates the functions required to transmit a bit stream 
over a physical medium.
• It deals with the mechanical and electrical specifications of interface and 
transmission medium.
• It also defines the procedures and functions that physical devices and 
interfaces have to perform for transmission to occur.
• Ethernet network interface card performs functions at both the physical layer 
and the data link layer.
Functions of Physical Layer:
• Physical layer defines characteristics of the interface between the devices 
and the transmission medium.
• It defines the type of transmission medium.
• It defines the transmission rate (the number of bits sent each second).
• It performs synchronisation of sender and receiver clocks.
• It is concerned with the connection of devices to the medium.
1. Point-to-point configuration: Two devices are connected together 
through dedicated link.
2. Multipoint configuration: A link is shared between several devices
• It is concerned with the physical topology.
• It defines the direction of transmission called as transmission mode (simplex, 
half duplex or duplex).
• It transmits bit stream over the communication channel.
• Hardware Used: Repeater and Hub.
• Data Unit: Bit stream
Data Link Layer:
• The data link layer transforms the physical layer, a raw transmission facility, to 
a reliable link.
• It is responsible for Node-to-Node delivery.
• It makes the physical layer appear error free to the Network layer.
Functions of Data Link Layer:
• Data Framing: Division of stream of bits received from network layer into 
manageable data units called frames. Segmentation of upper layer datagrams 
(packets) into frames.
• Flow Control: It is to manage communication between a high speed 
transmitter with the low speed receiver.
• Error Control: It provides mechanism to detect and retransmit damaged or 
lost frames and to prevent duplication of frames. To achieve error control, a 
trailer is added at the end of a frame.
• Access Control: Gives mechanism to determine which device has control over 
the link at any given time, if two or more devices are connected to the same 
link.
• Physical Addressing: Adding a header to the frame to define the physical 
address of the sender (source address) and/or receiver (destination address) 
of the frame.
• Hardware Used: Bridges and switches.
• Data Unit: Frames
• Protocol Used: Simplex protocol, stop and wait protocol, sliding window, HDLC 
(High Level Data Link Control), SDLC, NDP, ISDN, ARP, PSL, OSPF, NDP.
Network Layer:
• Network layer is responsible for source to destination delivery of a packet 
possibly across multiple networks (links).
• If the two systems are connected to the same link, there is usually no need for 
a network layer.
• If the two systems are attached to different networks (links) with connecting 
devices between networks, there is often a need of the network layer to 
accomplish source to destination delivery.
Functions of the Network Layer:
• Logical Addressing: If packet passes the network boundary, we need 
logical addressing system to distinguish the source and destination systems.
• Routing: Independent networks or links are connected together with the help 
of routers or gateways. Routers route the packets to their final destination. 
Network layer is responsible for providing routing mechanism.
• Hardware Used: Routers
• Data Units: Packets
• Protocols Used: IP (Internet, Protocol), NAT (Network Address Translation), 
ARP (Address Resolution Protocol), ICMP (Internet control Message Protocol), 
BGP (Border Gateway Protocol), RARP (Reverse Address Resolution Protocol), 
DHCP (Dynamic Host Configuration Protocol), BOOTP, OSPF.
Transport Layer:
• The transport layer is responsible for- source to destination (end-to-end) 
delivery of the entire message.
• Network layer does not recognise any relationship between the packets 
delivered.
• Network layer treats each packet independently, as though each packet 
belonged to a separate message, whether or not it does. The transport layer 
ensures that the whole message arrives intact and in order.
Functions of Transport Layer:
• Service Point Addressing: The transport layer header must include a type of 
address called service point address (or part address).
• Segmentation and Reassembly: A message is divided into transmittable 
segments, each segment containing a sequence number.
• Flow Control Flow: control at this layer is performed end to end rather than 
across a single link.
• Error Control: This layer performs an end to end error control by ensuring that 
the entire message at the receiving transport layer without error (damage, 
loss or duplication). Error correction is usually achieved through 
retransmission.
• Connection Control: Transport layer can deliver the segments using either 
connection oriented or connection less approach. Hardware Used: Transport 
Gateway Data Unit: Segments Protocol Used: TCP (Transmission Control 
Protocol) for connection oriented approach and UDP (User Datagram 
Protocol) for connection less approach.
Session Layer:
• The session layer is the network dialog controller.
• It establishes, maintains and synchronises the interaction between 
communicating systems.
• It also plays important role in keeping applications data separate.
Functions of Session Layer:
• Dialog Control: Session layer allows the communication between two 
processes to take place either in half duplex or full duplex. It allows 
applications functioning on devices to establish, manage and terminate a 
dialog through a network.
• Synchronization: The session layer allows a process to add check points 
(synchronization points) into a stream of data.
Presentation Layer:
• It is responsible for how an application formats data to be sent out onto the 
network.
• It basically allows an application to read and understan the message.
Functions of Presentation Layer:
• Translation: Different systems use different encoding system, so the 
presentation layer provides interoperability between these different encoding
Page 5


OSI (Open System Interconnection) Model
• The OSI model is a reference tool for understanding data communication 
between hosts with help of seven layers approach in communication process.
• Each layer performs specific functions to support the layers above it and uses 
services of the layers below it.
7 Layer OSI MODEL
Layer 7: Application Layer
• Provides User Interface
• Specifies the usage of a network by particular application 
Layer 6: Presentation Layer
• Provides Data representation
• Handles encryption and decryption 
Layer5: Session Layer
• Provides the data distinction between separate applications
• Establishes the communication between hosts using Dialog 
control
• Example: SIP, RTP,_etc.
Layer 4: Transport Layer
• Provides the segments, Provides Reliable, or Unreliable 
communication, and end to end connections
• Example: TCP, UDP, etc.
Layer 3: Network Layer
• Provides the packets, Provides logical addressing, and path 
determination for packet forwarding
• Example: IPv4, IPv6 
Layer 2: Data Link Layer
• Provides the frames, Provides the media access and physical 
addressing (MAC, LLC)
• Example: Ethernet, token ring, etc.
Layer 1: Physical layer
• Provides the bits (digital data conversion). Moves the data 
between hosts
• Example: Coax, RF link, etc.
Physical Layer:
• The physical layer coordinates the functions required to transmit a bit stream 
over a physical medium.
• It deals with the mechanical and electrical specifications of interface and 
transmission medium.
• It also defines the procedures and functions that physical devices and 
interfaces have to perform for transmission to occur.
• Ethernet network interface card performs functions at both the physical layer 
and the data link layer.
Functions of Physical Layer:
• Physical layer defines characteristics of the interface between the devices 
and the transmission medium.
• It defines the type of transmission medium.
• It defines the transmission rate (the number of bits sent each second).
• It performs synchronisation of sender and receiver clocks.
• It is concerned with the connection of devices to the medium.
1. Point-to-point configuration: Two devices are connected together 
through dedicated link.
2. Multipoint configuration: A link is shared between several devices
• It is concerned with the physical topology.
• It defines the direction of transmission called as transmission mode (simplex, 
half duplex or duplex).
• It transmits bit stream over the communication channel.
• Hardware Used: Repeater and Hub.
• Data Unit: Bit stream
Data Link Layer:
• The data link layer transforms the physical layer, a raw transmission facility, to 
a reliable link.
• It is responsible for Node-to-Node delivery.
• It makes the physical layer appear error free to the Network layer.
Functions of Data Link Layer:
• Data Framing: Division of stream of bits received from network layer into 
manageable data units called frames. Segmentation of upper layer datagrams 
(packets) into frames.
• Flow Control: It is to manage communication between a high speed 
transmitter with the low speed receiver.
• Error Control: It provides mechanism to detect and retransmit damaged or 
lost frames and to prevent duplication of frames. To achieve error control, a 
trailer is added at the end of a frame.
• Access Control: Gives mechanism to determine which device has control over 
the link at any given time, if two or more devices are connected to the same 
link.
• Physical Addressing: Adding a header to the frame to define the physical 
address of the sender (source address) and/or receiver (destination address) 
of the frame.
• Hardware Used: Bridges and switches.
• Data Unit: Frames
• Protocol Used: Simplex protocol, stop and wait protocol, sliding window, HDLC 
(High Level Data Link Control), SDLC, NDP, ISDN, ARP, PSL, OSPF, NDP.
Network Layer:
• Network layer is responsible for source to destination delivery of a packet 
possibly across multiple networks (links).
• If the two systems are connected to the same link, there is usually no need for 
a network layer.
• If the two systems are attached to different networks (links) with connecting 
devices between networks, there is often a need of the network layer to 
accomplish source to destination delivery.
Functions of the Network Layer:
• Logical Addressing: If packet passes the network boundary, we need 
logical addressing system to distinguish the source and destination systems.
• Routing: Independent networks or links are connected together with the help 
of routers or gateways. Routers route the packets to their final destination. 
Network layer is responsible for providing routing mechanism.
• Hardware Used: Routers
• Data Units: Packets
• Protocols Used: IP (Internet, Protocol), NAT (Network Address Translation), 
ARP (Address Resolution Protocol), ICMP (Internet control Message Protocol), 
BGP (Border Gateway Protocol), RARP (Reverse Address Resolution Protocol), 
DHCP (Dynamic Host Configuration Protocol), BOOTP, OSPF.
Transport Layer:
• The transport layer is responsible for- source to destination (end-to-end) 
delivery of the entire message.
• Network layer does not recognise any relationship between the packets 
delivered.
• Network layer treats each packet independently, as though each packet 
belonged to a separate message, whether or not it does. The transport layer 
ensures that the whole message arrives intact and in order.
Functions of Transport Layer:
• Service Point Addressing: The transport layer header must include a type of 
address called service point address (or part address).
• Segmentation and Reassembly: A message is divided into transmittable 
segments, each segment containing a sequence number.
• Flow Control Flow: control at this layer is performed end to end rather than 
across a single link.
• Error Control: This layer performs an end to end error control by ensuring that 
the entire message at the receiving transport layer without error (damage, 
loss or duplication). Error correction is usually achieved through 
retransmission.
• Connection Control: Transport layer can deliver the segments using either 
connection oriented or connection less approach. Hardware Used: Transport 
Gateway Data Unit: Segments Protocol Used: TCP (Transmission Control 
Protocol) for connection oriented approach and UDP (User Datagram 
Protocol) for connection less approach.
Session Layer:
• The session layer is the network dialog controller.
• It establishes, maintains and synchronises the interaction between 
communicating systems.
• It also plays important role in keeping applications data separate.
Functions of Session Layer:
• Dialog Control: Session layer allows the communication between two 
processes to take place either in half duplex or full duplex. It allows 
applications functioning on devices to establish, manage and terminate a 
dialog through a network.
• Synchronization: The session layer allows a process to add check points 
(synchronization points) into a stream of data.
Presentation Layer:
• It is responsible for how an application formats data to be sent out onto the 
network.
• It basically allows an application to read and understan the message.
Functions of Presentation Layer:
• Translation: Different systems use different encoding system, so the 
presentation layer provides interoperability between these different encoding
methods. This layer at the sender end changes the information from sender 
dependent format into a common format. The presentation layer at receiver 
end changes the common format into its receiver dependent format.
• Encryption and Decryption: This layer provides encryption and decryption 
mechanism to assure privacy to carry sensitive information. Encryption 
means sender transforms the original information to another form and at the 
receiver end, decryption mechanism reverses the new form of data into its 
original form.
• Compression: This layer uses compression mechanism to reduce the number 
of bits to be transmitted. Data compression becomes important in the 
transmission of multimedia such as text, audio and video.
Application Layer:
• This layer enables the user, whether human or software, to access the 
network.
• It provides user interfaces and support for services such as electronic mail, 
remote file access and transfer shared database management and other types 
of distributed information services.
• Examples: Telnet, FTP, etc
Functions of Application Layer:
• Network Virtual Terminal: It is a software version of a physical terminal and 
allows a user to logon to a remote host. To do so, the application creates a 
software emulation of a terminal at the remote host.
• File Transfer, Access and Management: It allows a user to access files, 
retrieve files, manage files or control files in a remote computer.
• Mail Services: It provides Electronic messaging (e-mail storage and 
forwarding).
• Directory Services: It provides distributed database sources and access for 
global information about various objects and services.