Computer Networking

Transport Layer

OSI Model

  • OSI stands for Open System Interconnection. It is a referencing model that describes how information moves from a one computer to another computer over the internet.
  • OSI model was developed by ISO (International Organization for Standardization) in 1984. It is now become a popular architectural model.
  • OSI consists of seven layers, and each layer performs a particular task and each layer is independent to perform their tasks.

Characteristics of OSI Model

There are 7 layers of OSI model where the Physical is the first and Application is the last layer. List of OSI layers is given under,

  1. Physical Layer
  2. Data-Link Layer
  3. Network Layer
  4. Transport Layer
  5. Session Layer
  6. Presentation Layer
  7. Application Layer

Upper and Lower Layers

The Upper 3 layers (layer 5, 6, 7) of the OSI model mainly deals with the software and lower 3 layer (layer 1, 2, 3) deals with hardware. Intermediate layer (layer 4) deals with both hardware and software.

At Sender end the Application Layer is first layer but at Receiver end the Application layer is the 7th layer as shown below,


Data Representation

Sender sends data to Application layer.  Data converted to segment at Transport layer. Segment then converted to packets (TCP) or datagram (UDP) at Network Layer. Packets are converted into Frames at data link layer. Frames are converted in to bits at physical layer.  Detail is given under


Functionalities of Different Layers

Layer Name Devices Protocols of this layer Delivery Address
7. Application Layer Gateways, Firewalls, PC, Phone etc. SMTP, HTTP, FTP, DHCP, Telnet, POP3 etc.
6. Presentation Layer Firewalls etc. SSL, MIME, MPEG etc.
5. Session Layer Firewalls etc. RPC, PAP, SAP etc
4. Transport Layer Gateways, Firewalls etc. TCP, UDP, SCTP etc END to END Port
3. Network Layer Routers, Brouter etc IP, IPV4, IPV6, ICMP,IGMP etc Source to Destination IP address
2. Data-Link Layer Bridges, Modem, NIC etc. IEEE 802.3, IEEE 802.5, RAPA, PPP, Frame Relay, CSMA etc.  Ethernet, Wi-Fi, Node to Node MAC address
1. Physical Layer HUB, Cable, Repeaters etc. IEEE 802.11, ISDN


Delivery of Data (Process)

As we see in above table, Delivery of data which is further explained through the following diagram

Keep in mind:

  • Node to Node (Hop to Hop) frames delivery is responsibility of Data link Layer.
  • Source to destination (Host to Host) packets delivery is responsibility of Network Layer
  • End to End (Process to Process OR Port to Port) Segments delivery is responsibility of Transport Layer

Further explanation of OSI layers in details is explained in upcoming lectures




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