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My silly brain is unable to understand the difference between OSI's Transport Layer TCP/UDP and the Circuit/Packet Switching.

Firstly, I am failing to understand which layer should Circuit/Packet Switching be referring to? As it concerns the way the data is being sent (i.e.the "broken down" pieces of information via either different/same routes and orders to destination), isn't it also the Transport Layer?

Also, as TCP/UDP are equally relating to the way data is being sent, I do not know what exactly am I referring to when I'm talking about circuit/packet switching or TCP/UDP anymore. How do these differ? Can anyone try to explain to me the basic difference?

I'm very sorry for my silly brain.

lady rohmon
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  • OSI does not have a `transport layer TCP/UDP`. TCP/IP has that. There's no point in discussing TCP/IP, which has its own layer model, in terms of the OSI reference model, which doesn't refer to anything real. – user207421 Mar 10 '17 at 08:50

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TCP and UDP are in Layer 4 of the OSI Model. This layer is the application interface to networking. TCP and UDP package the data which an application sends, and unpackages the data which an application receives. The application data are encapsulated in TCP segments or UDP datagrams, or any other layer-4 protocol the application uses.

Packet switching is in Layer 3 of the OSI Model. This is the layer where routing happens. The layer-4 segments are encapsulated in IP packets.

Frame switching is in Layer 2 of the OSI Model. This is the layer used by switches and bridges. The layer-3 packets are encapsulated in layer-2 (e.g. ethernet) frames.

Ron Maupin
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Circuit/Packet Switching operates on Layer 3, the network layer.

TCP/UDP operates on Layer 4, the transport layer

The transport code runs entirely on the users' machines, but the network layer mostly runs on the routers... Computer Networks, Andrew S. Tanenbaum, pg 482

If it runs on a router it's layer 3. If it runs on a host it's layer 4. There are some exceptions to the rule, NAT breaks encapsulation and peeks at the TPC/UDP header (Layer 4), but it's a good rule of thumb.

The two layers provide similar services (Routing, Congestion Control, Quality of Service, etc.) to different people. The network layer is controlled by ISP's while the transport layer is controlled by the user (or programmer), since users aren't likely to have control of the network layer, and ISP's shouldn't have access to an individual host.

Thomas Zumsteg
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  • Thank you for your reply! So, in a nutshell, I can think of circuit/packet switching as a pretty much same thing as TCP/UDP, but happening on "grand" scale, right? Sorry for my lack of knowledge :( – lady rohmon Mar 22 '16 at 17:28
  • Yup same problems, different people that need to solve it. Layer 3 Circuit/Packet Switching is like the Post Office (ISP) including the address, trucks, planes, deals with foreign post offices, etc. Layer 4 TCP/UDP is like everything inside the package, "Dear/From", packing list, bubble wrap, etc. For an in depth treatment I highly recommend _Computer Networks_, by Andrew S. Tanenbaum. – Thomas Zumsteg Mar 22 '16 at 18:18
  • Thank you so much for such a great explanation! I will definitely have a look at the suggested book (I clearly need it :D) – lady rohmon Mar 22 '16 at 18:28