Overlapping Functions of OSI Layers

We all know that there are 7 layers in OSI Model. All 7 layers of OSI Model forms the basis of modern communication among LANs. The basic functionality of all layers is not very difficult to understand. But due to overlapping of some functions among layers, many students become confused. For example, error handling is done both at DLL and Transport Layer.

Lets discuss functions and services of OSI layers to get a clear idea about what each layer does.

First, Transport Layer vs Network Layer->

• Transport Layer does delivery of ENTIRE message but Network Layer delivers INDIVIDUAL packets.
• Transport Layer treats entire message as a single unit and ensures the whole message arrives intact and the destination. But Network layer sees no relation b/w two data packets and treats them independent entities.
• Transport Layer does delivery in a Process-to-Process manner. (process refers to an application. e.g. email message must be delivered via source email process to destination email process). Network layer is responsible to take the message to the final system only. It is Transport Layer that takes the message to the right process. An analogy for this can be: You go to meet one of your friends at his/her office. You reached at the right office address (Network Layer). From the reception, you are directed towards the deptt. where your friend works for (Transport Layer).
• The major protocols of Transport Layer: TCP, UDP and SCTP. The major protocols of Network Layer: ARP, RARP, IGP, BGP IPv4, and IPv6.
• There are different words for same term/unit information on different layers. For example, in OSI model, we use a term-PDU (Protocol Data Unit) which is referenced differently at layers:
  
  Layer Name         PDU

          Data Link Layer    Frame
          Network Layer      Packet
          Transport Layer    Segment
         Application Layer  Message

• Sliding Window Protocol works both at DLL and TL. The difference is that at DLL, individual bits are processed but at TL, processing is done at BYTE level.
• There are many devices used in LANs which work at different layers. Many students find it hard to understand which device used at which layer. Let's understand the purpose of using a device which will help you to remember their use at right layer
  Repeater: It is installed at specified distance and amplifies the "signal". As we know signals are handled at layer 1, hence, repeater works at layer 1.
  Hub: 2 types-passive hub and active hub. Passive hub simply passes the original signal and works at layer 1. Active hub is nothing but a switch; it works at layer 2. It is also called switching hub.
  Modem: A modem deals with transmission at signal level and works at layer 1.
  Bridge and Switch: Both handle MAC addresses. Since MAC addresses are handled at layer 2. So these devices work at Layer 2.
  Router: I believe many among you know it well that routers deal with IP address and work at Layer 3.
  Gateway: They are very similar to bridge but deal with inter-protocol conversion. Therefore, gateways work at all layers, i.e. 1-7, to handle conversion at all compatible layers.

I hope this post will help to clarify some doubts about OSI model. Please share your views to let me know of the topics which should be discussed here.