| circuit switcing vs packet switching... Is it correct to say that all the telecommunication, phone, data, voice, video is now based on packet switching?
Is circuit switching now completely obsolete or is it still used?
Circuit switching can be either digital or analog. Packet switching is instead always and only digital, correct?
Thanks!
student25 |
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 tschmidtPremium,MVM
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| The public switched telephone network (PSTN) is a switched network. Except for local loop it is almost entirely digital.
The carriers are using VoIP to some extent but circuit switching in the telephone network is far from dead.
Packet switching is inherently digital.
/tom |
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| hello Tom,
thanks again. Computers work with digital signals (1s and 0s). Modems convert those electric signals to analog ones for transmission over the infrastructure.
DSL, cable are digital services, aren't they? That leads me to think that the local loop, internet-service wise, is mostly digital.
Unless you intend the local loop in the sense of the telephone service. Those 4khz are used on copper wire using an analog modulation.... In that case it is still analog (VoIP aside).....
I guess the whole distinction between analog and digital is simply referring to the type of modulation that gets applied to the carrier signal that carries the message signal, correct?
Both in analog and digital signals, a carrier signal properties ( amplitude, phase, polarization) get modified according to a 1s and 0s pattern in one case or reflecting the intelligent message to be transmitted.
It seems to me that ASK modulation and AM modulation are very very similar....
thanks |
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tschmidtPremium,MVM
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| said by student25: Modems convert those electric signals to analog ones for transmission over the infrastructure. Just to nit pick a little the terminal side of modem is digital. Modem is a contraction of modulation demodulation. The purpose of a modem it to interface a digital signal (designed for short distance) to some sort of long distance connection.
said by student25: DSL, cable are digital services, aren't they? That leads me to think that the local loop, internet-service wise, is mostly digital. Local loop is telephone terminology for the copper circuit between Central Office and subscriber. Initially it was designed to support Plain Old Telephone Service (POTS). It has been pressed into service to support Internet access. It is called a loop because the Central Office supplies power (battery) the subscriber station has no power of its own. Telephone instrument is a current, not voltage, driven device.
said by student25: I guess the whole distinction between analog and digital is simply referring to the type of modulation that gets applied to the carrier signal that carries the message signal, correct? The distinction between analog and digital can get pretty muddy. Digital information is quantified into discrete steps, analog is infinitely variable. However if you look at complex signaling it can be very difficult to tell what is analog and what is digital.
said by student25: Both in analog and digital signals, a carrier signal properties ( amplitude, phase, polarization) get modified according to a 1s and 0s pattern in one case or reflecting the intelligent message to be transmitted. It depends, as processing power has increased modulation schemes have gotten more complex to eak out ever more capacity of a bandwidth limited channel, the Shannon limit.
If we go back to your earlier question about T-1 it was developed in the late 1950, early 60s. Semiconductor technology was very primitive. Initial T-1 signalizing was done using AMI, Alternate Mark Inversion. Transmitting a zero resulted on no change, whereas a 1 caused opposite. If the previous 1 was a plus value the next 1 was transmitted as a negative. This causes problems if a string of zeros needs to be transmitted but was not a huge issue for voice. Modern T-1 uses a much more complex modulation scheme that requires a DSP to encode and decode. The benefit is only a single pair is needed, distance between repeaters is longer, and much less noisy then more primitive signaling.
/tom |
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| Tom,
you know too much  I am impressed with your answers.
I looked at your website. There is a paper that I am starting to study: »www.tschmidt.com/writings/Broadb···Mile.pdf
What is your background? i will soon make a donation.
Since we are at it, I have been bugged by another question....let me open another post so others will benefit,
thanks,
student25 |
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| reply to tschmidt
Hi Tom,
I read this on wikipedia:
"The public switched telephone network (PSTN) also referred to as the plain old telephone service (POTS) is the network of the world's public circuit-switched telephone networks. It consists of telephone lines, fiberoptic cables, microwave transmission links, cellular networks, communications satellites, and undersea telephone cables all inter-connected by switching centers which allows any telephone in the world to communicate with any other. Originally a network of fixed-line analog telephone systems, the PSTN is now almost entirely digital in its core and includes mobile as well as fixed telephones."
So currently, when we place a phone call in the US to a phone in Europe, our conversation is all digital in nature, aside maybe from the last mile, i.e. our home to the nearest central office....
Doesn't that mean that the circuit switching methodology is not necessarily existing anymore and our conversation travel over packet switching? Am I wrong in thinking that?
It seems that the PSTN is the old dinosaur and the "switchd" is not true anymore... ???
thanks!
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nonymousPremium
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| reply to tschmidt
said by tschmidt:The public switched telephone network (PSTN) is a switched network. Except for local loop it is almost entirely digital.
The carriers are using VoIP to some extent but circuit switching in the telephone network is far from dead.
Packet switching is inherently digital.
/tom
Not necessarily the last mile but I thought the backbone was going more packet switched? |
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tschmidtPremium,MVM
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| reply to student25
said by student25:So currently, when we place a phone call in the US to a phone in Europe, our conversation is all digital in nature, aside maybe from the last mile, i.e. our home to the nearest central office....
Doesn't that mean that the circuit switching methodology is not necessarily existing anymore and our conversation travel over packet switching? Am I wrong in thinking that?
You are confusing analog vs digital with routing.
You are correct except for first-mile wire telephone network is digital. That simply refers to how information is transferred.
Circuit vs Packet switching is about how information traverses the network.
In a circuit switched network at the beginning of the session an end-to-end path is established. That path remains open for the duration of the session.
In a packet switched network each packet has a source and destination address. As each packet arrives at a router the router needs to decide how to forward it toward its ultimate destination.
MPLS muddies the water from this nice clear distinction. Multiprotocol label switching is a circuit switched protocol used extensively in packet networks.
Circuit switched networks have lower overhead then packet switched. Remember until the 1970's telephone switching logic was implemented with electromechanical relays. The down side it channel capacity is consumed whether or not it is being used.
The advantage of packet switching is it is stateless. The network itself has no knowledge of anything other then the specific packet it is carrying. The downside is at each hop complex routing decisions need to be made.
/tom |
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