How is Ethernet a packet-switched network?

Question

Let's say for example there is an Ethernet switch, and there are four PCs connected to this switch. PC1 and PC2 are communicating with each other, and at the same time PC3 and PC4 are also communicating with each other. Obviously there is no Statistical TDM here. Also, there is only one node - the switch, so there cannot be multiple paths from source to destination.

So how does Ethernet become packet-switched? Is it because the data being exchanged are broken into packets? What are the conditions that must be satisfied to say that a network is a packet-switched network?

Answer 1

Ethernet has always used packet switching.

Packet switching works by multiplexing a physical connection on a packet basis. Each packet (or frame on the data link layer) is atomic but consecutive frames can come from or go to completely different sources or destinations (MAC addresses).

The essential distinction of a packet-switched network is that each packet is forwarded on its own and in a stateless fashion (=the forwarder does not remember any previous packets and there are no 'connections' [*]).

In a circuit-switched network, a connection is set up before any data can be transferred. Bandwidth for the connection is reserved as long as it's "up". When you need to run multiple, simultaneous connections on a physical link you need some kind of multiplexing, e.g. FDM or TDM.

Note that "multiple paths from source to destination" can be true in the IP (network) layer but not on Ethernet's data link layer. An Ethernet network is essentially bridged, and multiple paths between nodes mean you've got bridge loops - without correcting measures like the spanning-tree protocol (STP), these loops will quickly take down your network.

[*] 'Connections' are a concept that may be present in a higher layer, e.g. the transport layer (L4). The network (L3) and the data link (L2) layers have no such concept with packet switching, they are connectionless.

Answer 2

TDM is a circuit-switched network, where the circuit (path) is set up at the beginning, and it is up until it is torn down. It does not vary, and no other devices can use that circuit until it is freed. Traffic put in one end of the circuit comes out at the other end of the circuit. TDM frames are always put in one end of the circuit and sent to the other end of the circuit, following the same path.

An ethernet switch is just that, as switch. Frames entering the switch on any of the interfaces could be sent to any other interface (switched), and that is controlled by the addressing on the frames. Each ethernet frame is switched independently, based on the addressing on the frame.

A switch is smart enough that it will build a MAC address table and maintain it as a frame enters the switch. The switch will look at the source MAC address on the frame, and it will build an entry for that MAC address that tells the switch on which interface the MAC address is connected. It only takes a single frame entering the switch on an interface to add an entry in the MAC address table for the source MAC address, and switches rapidly learn which MAC addresses are connected to which interfaces.

The switch will then look at the destination MAC address and try to find it in its MAC address table to switch the frame to the correct interface for the destination MAC address. If it does not find an entry in the table, it will flood the frame to all other interfaces.