Switching and TCP/IPLayers
Switching can happen at several layers of the TCP/IP protocol suite.
Switching at Physical Layer
At the physical layer, we can have only circuit switching. There are no packets
exchangedat the physical layer. The switches at the physical layer allow signals to
travelin one path or another.
Switching at Data-Link Layer
At the data-link layer, we can have packet switching. However, the term packet in this
case means frames or cells. Packet switching at the data-link layer is normally done
using a virtual-circuit approach.
Switching at Network Layer
At the network layer, we can have packet switching. In this case, either a virtual-circuit
approach or a datagram approach can be used. Currently the Internet uses a datagram
approach, as we see in Chapter 18, but the tendency is to move to a virtual-circuit
approach.
Switching at Application Layer
At the application layer, we can have only message switching. The communication at
theapplication layer occurs by exchanging messages. Conceptually, we can say that
communication using e-mail is a kind of message-switched communication, but we do
not see any network that actually can be called a message-switched network.