Circuit and Packet Switching

Question

I'm struggling to understand 1 or 2 things about packet and circuit switching.

• In circuit switching, resources are not shared and a it's guaranteed full capacity to the call right? They are "reserved" for a certain call/circuit. But in TDM aren't users sharing the link? And in FDM the capacity is divided so users can use the link at same time, but on different frequencies, isn't that sharing and capacity division?

• Why is circuit switching slower and less responsive than packet switching?

Thanks.

Answer 1

• With circuit switching, you have a 1-to-1 circuit with some predefined channel capacity which is guaranteed. For it to work, you must first establish a circuit/connection before you send any data, and break the connection after.

• With packet switching, it's somewhat like the local post office - you bring the packets, send them, and let the other devices forward them around, to one or many different destinations.

What you mentioned in the first part is called multiplexing. When you establish, for example, a phone call, you get a 1-on-1 connection with the other side, and full channel capacity — 0–3400kHz for voice (at least in some parts of Europe), which only you can use. Since using separate wires is expensive, different multiplexing technologies have appeared, such as TDM, where your channel is "compressed", but you still have a 1-to-1 connection with the same capacity. With FDM, your call is mixed up to higher frequencies, but you still get a 1-to-1 channel, with the same capacity.

Yes, the medium is shared, but you still get the full capacity you have payed for, and a 1-to-1 channel.

As to the second part, circuit switching requires a circuit to be set up (you have to dial a number), and after sending the data/voice, you have to break down (shut down) the connection. When you need to communicate to a a lot of different people/devices, this takes time, since when one connection (call) is active, you cannot use that line for anything else, even if you are still waiting for the other person to answer/recieve the data. If compared to traditional networks, it takes a lot less time to send a 100 letters by post, then calling 100 different people.

Another problem with circuit switched systems is that the channel is used up even when no data is sent through it (noone else can use that channel at that time). If you have an 8-line telephone uplink, and even if all 8 callers are waiting on hold, your channel is still used up. With packet switched networks you can even oversell your connection (due to statistical multiplex) — if you're an ISP and have a 8Mbit/s uplink, you can sell 10, 20, maybe even more 1Mbit/s connections, since it is statistically improbable, that all of the users will use the connection fully at the same time. And even if they do, their connection still works, with lower speed (compared to blocked/dropped calls on circuit switched networks).

Answer 2

A "circuit" doesn't necessarily mean the same thing as a physical electrical circuit. That's where the term started out, but when various multiplexing schemes were invented, such as TDM, "circuit" acquired the meaning of a certain channel or slot inside the physical connection. In the T-carrier system, for example, the unit called the "circuit" is the 64kbps DS0 signal, and a T1 or T3 is a logical collection of these circuits.

FDM actually can be used for circuit switching, and this was actually done in the early days of long-distance telephony. In this scheme, the "circuit" is a logical unit of a certain analog bandwidth, and each of these can be set up and disconnected individually at the multiplexer.

Circuit switching is not necessarily slower, because the capacity and delay guarantees can prevent slowdowns that result from overuse of links. The sense in which this is normally said is that a circuit-switched network requires some time to establish a connection before sending data, whereas a packet-switched one can just fire off a packet to some host without need handshaking.

Answer 3

In circuit switched connection:

• a connection is always established from end to end before transferring data.
• in the general case the connection has fixed capacity for the whole duration of the connection even if you don't transmit any useful data and at the same time other connections can;t use your capacity
• the whole capacity is guaranteed
• the speed of the connection is constant

In packet switched connection:

• the connection may not be established from end to end before start of data transmission
• the capacity of connection may be variable, different connections share the same physical link
• the connection capacity is not guaranteed
• the connection speed may be variable

Today the networks are a very complex mix of CS and PS connections. PS networks use many protocols for resource reservation, QoS, traffic prioritization. PS networks try to combine the advantages of both PS and CS connections.