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If my understanding is correct, circuit- or packet- switching does not happen inside the switch -- it happens on the network. It is a characteristic of a network and not of a switch. The difference on the switch level is that in circuit switching the switch fabric is occupied for the duration of the whole data transfer, whereas in packet switching the switch fabric is occupied for only the duration of a packet. So in essence, the only difference is the software, but the hardware is basically the same.

So given, for example, an Ethernet switch, can we reprogram it to act like a telephone switch, and vice versa?

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So in essence, the only difference is the software, but the hardware is basically the same.

It depends on what exactly you call "software" and what "hardware".

So given, for example, an Ethernet switch, can we reprogram it to act like a telephone switch, and vice versa?

Depending on how the switch is built, it might be possible to reprogram an FPGA-based Ethernet switch in a way that it connects two Ethernet ports permanently until a certain bit pattern is detected.

However, I doubt that there are many Ethernet switches using FPGAs.

You might argue that the wiring of a gate array is some kind of "software" and not "hardware" (*). However, this kind of software is etched into a microchip during the production and cannot be changed later on. So it would not be possible to reprogram a switch using such a type of microchip.

If the switch uses a full-custom ASIC, the functionality (packet switching) is definitely done purely in hardware and there is nothing involved that you could call "software". You cannot reprogram such a chip either.


(*) Until the mid-1990s, firmware (like the PC BIOS) but also application programs were often stored this way in "mask ROM" microchips. So the fact that some algorithm is represented by an unchangeable wiring in a microchip does not imply that this wiring is "not software".

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So given, for example, an Ethernet switch, can we reprogram it to act like a telephone switch, and vice versa?

No. A circuit-switch sets up a dedicated, end-to-end circuit for the entire conversation, and the circuit is dedicated to the conversation. Packet switching breaks up a conversation into packets to be able to share a circuit among several conversations.

Modern ethernet switching is done in hardware, and you need different hardware to circuit switch.

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  • Yes, modern Ethernet switches are ASIC-based, but some may be not, just like some PCs have dedicated graphics cards and some do not. But what if they are software-based? – Noob_Guy Mar 15 at 10:33
  • Software-based bridges are no longer competitive in pricing, and they have not been for years. Switching hardware is now a cheap commodity. In any case, ethernet is fundamentally packet-switched. – Ron Maupin Mar 15 at 10:38
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circuit- or packet- switching does not happen inside the switch -- it happens on the network

Circuit or packet switching are fundamentally different paradigms in network stack design. Any switch there is only does one and not the other. The "switching" in packet switching is the individual forwarding of packetized data, which happens inside switches and routers (based on different network layers).

With circuit switching, the forwarding is done in bulk. A logical connection is established and the channel is allocated for the duration of that connection (when you're in a phone call the line is busy). Packets usually only carry labels and no complete source and destination addresses.

In packet switching, each packet must carry complete addresses and is forwarded on an individual basis. The forwarding network is essentially stateless and oblivious to any logical connections on higher network layers.

The difference in hardware is that circuit-switching hardware may only need to switch connections at a very low frequency (every few hours in extreme) while packet-switching hardware needs to switch at packet frequency (up to 1.4 million pps for a Gigabit Ethernet link).

An Ethernet switch uses one or more ASICs that are purpose-made. You cannot reprogram that hardware for another protocol.

However, if your "telephone switch" includes a switch used for VoIP then, of course, you can use (any) Ethernet switch as a "telephone switch".

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  • Yes, modern Ethernet switches are ASIC-based, but some may be not, just like some PCs have dedicated graphics cards and some do not. But what if they are software-based? – Noob_Guy Mar 15 at 10:31
  • In a PC - or any other hardware - that uses software-based switching, the Ethernet NICs are hardware that cannot be used to do phone calls. An entirely software-based (and hypothetical) device could be reprogrammed, of course. – Zac67 Mar 15 at 10:48

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