I hereby explain the SDU (service data unit )and PDU ( protocol data unit) in the view of transmitter. Let us understnad the PDUs at different layers Application layer - PDU is data Presentation layer - PDU is data Session layer - PDU is data Transport layer - PDU segment Network layer - PDU packet Data link layer PDU frames Physical layer PDU bits. while transmission application, presentation and session layer data is incapsulated and forwarded to Transport layer. At transport , TCP segments are created as layer 4 PDUs. When passed to IP, they are treated as layer 3 SDUs. The IP software packages these SDUs into messages called IP packets or IP datagrams, which are layer 3 PDUs. These are in turn passed down to a layer 2 protocol, say Ethernet, which treats IP datagrams as layer 2 SDUs, and packages them into layer 2 PDUs (Ethernet frames) which are sent on layer 1 . On the receiving device, the process of encapsulation is reversed. The Ethernet software inspects the layer 2 PDU (Ethernet frame) and removes from it the layer 2 SDU (IP datagram) which it passes up to IP as a layer 3 PDU. The IP layer removes the layer 3 SDU (TCP segment) and passes it to TCP as a layer 4 PDU. Consclusion is the PDU from upper layer added and encapsulated at sender end and removes and decapsulated at receiving end. This is the way communication happens. Hope it makes sense. ThanksName used on OSIName Used in TCP/IP NetworksLayer 1 - PhysicalLayer 1 - Network Access/InterfaceLayer 2 - Data linkLayer 2 - NetworkLayer 3 - NetworkLayer 3- TransportLayer 4 - TransportLayer 4 - ApplicationLayer 5 - sessionThere are only four layers defined in TCP/IP model Solution I hereby explain the SDU (service data unit )and PDU ( protocol data unit) in the view of transmitter. Let us understnad the PDUs at different layers Application layer - PDU is data Presentation layer - PDU is data Session layer - PDU is data Transport layer - PDU segment Network layer - PDU packet Data link layer PDU frames Physical layer PDU bits. while transmission application, presentation and session layer data is incapsulated and forwarded to Transport layer. At transport , TCP segments are created as layer 4 PDUs. When passed to IP, they are treated as layer 3 SDUs. The IP software packages these SDUs into messages called IP packets or IP datagrams, which are layer 3 PDUs. These are in turn passed down to a layer 2 protocol, say Ethernet, which treats IP datagrams as layer 2 SDUs, and packages them into layer 2 PDUs (Ethernet frames) which are sent on layer 1 . On the receiving device, the process of encapsulation is reversed. The Ethernet software inspects the layer 2 PDU (Ethernet frame) and removes from it the layer 2 SDU (IP datagram) which it passes up to IP as a layer 3 PDU. The IP layer removes the layer 3 SDU (TCP segment) and passes it to TCP as a layer 4 PDU. Consclusion is the PDU from upper layer added and encapsulated at sender end and removes and decapsulated at receiving end. This is .

1. I hereby explain the SDU (service data unit )and PDU ( protocol data unit) in the view of
transmitter.
Let us understnad the PDUs at different layers
Application layer - PDU is data
Presentation layer - PDU is data
Session layer - PDU is data
Transport layer - PDU segment
Network layer - PDU packet
Data link layer PDU frames
Physical layer PDU bits.
while transmission application, presentation and session layer data is incapsulated and forwarded
to Transport layer. At transport , TCP segments are created as layer 4 PDUs. When passed to IP,
they are treated as layer 3 SDUs. The IP software packages these SDUs into messages called IP
packets or IP datagrams, which are layer 3 PDUs. These are in turn passed down to a layer 2
protocol, say Ethernet, which treats IP datagrams as layer 2 SDUs, and packages them into layer
2 PDUs (Ethernet frames) which are sent on layer 1 .
On the receiving device, the process of encapsulation is reversed. The Ethernet software inspects
the layer 2 PDU (Ethernet frame) and removes from it the layer 2 SDU (IP datagram) which it
passes up to IP as a layer 3 PDU. The IP layer removes the layer 3 SDU (TCP segment) and
passes it to TCP as a layer 4 PDU.
Consclusion is the PDU from upper layer added and encapsulated at sender end and removes and
decapsulated at receiving end.
This is the way communication happens.
Hope it makes sense.
ThanksName used on OSIName Used in TCP/IP NetworksLayer 1 - PhysicalLayer 1 - Network
Access/InterfaceLayer 2 - Data linkLayer 2 - NetworkLayer 3 - NetworkLayer 3- TransportLayer
4 - TransportLayer 4 - ApplicationLayer 5 - sessionThere are only four layers defined in TCP/IP
model
Solution
I hereby explain the SDU (service data unit )and PDU ( protocol data unit) in the view of
transmitter.
Let us understnad the PDUs at different layers
Application layer - PDU is data

2. Presentation layer - PDU is data
Session layer - PDU is data
Transport layer - PDU segment
Network layer - PDU packet
Data link layer PDU frames
Physical layer PDU bits.
while transmission application, presentation and session layer data is incapsulated and forwarded
to Transport layer. At transport , TCP segments are created as layer 4 PDUs. When passed to IP,
they are treated as layer 3 SDUs. The IP software packages these SDUs into messages called IP
packets or IP datagrams, which are layer 3 PDUs. These are in turn passed down to a layer 2
protocol, say Ethernet, which treats IP datagrams as layer 2 SDUs, and packages them into layer
2 PDUs (Ethernet frames) which are sent on layer 1 .
On the receiving device, the process of encapsulation is reversed. The Ethernet software inspects
the layer 2 PDU (Ethernet frame) and removes from it the layer 2 SDU (IP datagram) which it
passes up to IP as a layer 3 PDU. The IP layer removes the layer 3 SDU (TCP segment) and
passes it to TCP as a layer 4 PDU.
Consclusion is the PDU from upper layer added and encapsulated at sender end and removes and
decapsulated at receiving end.
This is the way communication happens.
Hope it makes sense.
ThanksName used on OSIName Used in TCP/IP NetworksLayer 1 - PhysicalLayer 1 - Network
Access/InterfaceLayer 2 - Data linkLayer 2 - NetworkLayer 3 - NetworkLayer 3- TransportLayer
4 - TransportLayer 4 - ApplicationLayer 5 - sessionThere are only four layers defined in TCP/IP
model