(i) Noise on transmission link converts 0bit to 1bit

(ii) A packet is routed to wrong destination

(iii) A packet switching network delivers a data unit to a terminal attached to it out of sequence

(iv) A printer printing halfway through a line is suddenly commanded by mistake to return to the beginning of line.

(v) During half duplex mode session the transmitting user starts receiving data from the user at other end.

1. Physical Layer

• This layer deals with the transfer of bits over a communication channel, for example copper wire pairs, coaxial cable, optical fibre.
• This layer is concerned with the particular choice of system parameters such as voltage level and signal duration, also setup and release of physical connection as well as mechanical aspect such as socket type and number of pins.

2. Network Layer

• This layer combines multiple frames forming a packet while sending the data and when it receives a data it separates frames from the packet.
• This layer provides routing of packet across the communication network and when doing so it is required that the nodes through which packets are passing are always connected to the network. This makes the network layer most complex layer in the OSI model.
• This layer also deals with the congestion that occurs from time to time due to surge in the traffic in the network.

3. Transport Layer

• The role of this layer is to provide appropriate address information so that the messages can be delivered to the appropriate destination session layer.
• It also performs various functions such as error free transfer of information by the error detection and recovery techniques and sequence and flow control.
• This layer also does the segmentation and reassembly or blocking and unblocking to match the size of the message produced by the session layer to the packet sizes that can be handled by the network layer.

4. Application Layer

• This layer closet to the user. It provides user services that are frequently required by the application in communication. These services include e-mail, name services, login service, network management etc.

5. Session Layer

• This layer enhances the reliability of transfer of data over transport layer by providing means of dialogue between two applications. Such as in some communication manners data transfer is half duplex where both applications takes turn in transferring the data. This facility is supported by session layer.
• This application of session layer becomes useful while transferring large files to provide synchronization points from which error recovery can be initiated instead of transferring large files again.