Transport Layer

Transport Layer

Transport layer receives the entire message from the application layer ( in TCP / IP model ) or from the session layer ( in ISO / OSI model).Transport layer divide the entire message into different transmittable segments. Also it add a header to each segment.It provides service point addressing ( delivery of a message to the appropriate application of a computer running many applications).The transport layer is responsible for process to process delivery of the entire message.Hence the transport layer is responsible for end to end communication

Time Division Multiplexing (TDM)

 Time Division Multiplexing (TDM)

TDM is a digital multiplexing technique that allows several connections to share the high bandwidth of a link.Instead of sharing a portion of the bandwidth as in FDM,time is shared in TDM.  Each connection occupies a portion of time in the link. i.e, each channel has a time slice assigned to it.                                

Interleaving in TDM

TDM can be visualized as two fast rotating switches,one on the multiplexing side and the other on the demultiplexing side.The switches are synchronized and rotate at the same speed,but in opposite directions.On the multiplexing side,as the switch opens in front of a connection,that connection has the opportunity to send a unit onto the path.This process is called interleaving.On the demultiplexing side,as the switch opens in front of a connection,that connection has the opportunity to receive a unit from the path.


The implementation of TDM is not as easy as that of FDM.Synchronization between the multiplexer and demultiplexer is a major issue.If the multiplexer and the demultiplexer are out of synchronization a bit belonging to one channel may be received by the wrong channel.For this reason,one or more synchronization bits are usually added to the beginning of each frame.These bits,called framing bits,follow a pattern,frame to frame,that allows the demultiplexer to synchronize with the incoming stream so that it can separate the time slots accurately.



ISO-OSI Model in Computer Networks


International Standard Organization(ISO) standard covers all the aspects of network communication.It is the OPen Systems Interconnection(OSI) model.it follows a layered framework for the design of network systems.It consist of seven different related layers,each having specific defined functionality.


Layer 7:    Application Layer

Layer 6:    Presentation Layer

Layer 5:    Session Layer

Layer 4:    Transport Layer

Layer 3:    Network Layer

Layer 2:    Data Link Layer

Layer 1:    Physical Layer

IP Loopback Address

IP Loopback Address is the loopback address in IP.
  • To test self connectivity
  • PING
  • Loop back packets are not visible on the network
Messages sent to do not appear in the network. The network adpter loopback(send back) all messages to the sending application. That is it is used when a program access a network service running on the same computer itself.That is loopback address is used to test the self connectivity.

Example: PING
A successful ping request to verify that a computer's network interface card, it's drivers, and the operating system's TCP/IP implementation are operating perfectly.

IP address range from through  are reserved for loopback purposes. This range of IP address cannot be assigned as host ip address on a network.

Modern operating systems implement TCP/IP consider  "localhost" as equivalent to

Multiplexing FDM WDM TDM

What is the purpose of multiplexing? What are the three major multiplexing techniques?


Whenever the bandwidth of a medium linking two devices is greater than the bandwidth needs of the devices,then the link can be shared.

Multiplexing is the set of techniques that allows the simultaneous transmission of multiple signals across a single data link.

Advantages(purpose) of Multiplexing :   Efficient utilization of bandwidth

Three Major Multiplexing Techniques

   1.    Frequency Division Multiplexing  (FDM)
   2.    Wave Division Multiplexing  (WDM)
   3.    Time Division multiplexing  (TDM)

Frequency Division Multiplexing  (FDM) and Wave Division Multiplexing  (WDM) are analog techniques, where as Time Division Multiplexing is digital.