# Packet and circuit switching

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Sometime ago, someone asked about Packet and circuit switching ... I am just copying and pasting how cisco explains these terms (which are not very clear to me either :) Notes within the brackets are mine.

Packet-switched networks were developed to overcome the expense of public circuit-switched networks and to provide a more cost-effective WAN technology. When a subscriber makes a telephone call, the dialed number is used _to set switches in the exchanges_ (whatever that might be) along the route of the call so that there is a continuous circuit from the originating caller to that of the called party (How does the dialer number do the job of an operator might be even more complicated). Because of _the switching operation_ used to establish the circuit, the telephone system is called a circuit-switched network. (Very nice definition :). If the telephones are replaced with modems, then the switched circuit is able to carry computer data. (as clear as mud!)

The internal path taken by the circuit between exchanges is shared by a number of conversations. Time division multiplexing (TDM) is used to give each conversation a share of the connection in turn. TDM assures that a fixed capacity connection is made available to the subscriber. ( isn't this cool? whatever that might mean :)

If the circuit carries computer data, the usage of this fixed capacity may not be efficient. For example, if the circuit is used to access the Internet, there will be a burst of activity on the circuit while a web page is transferred. This could be followed by no activity while the user reads the page and then another burst of activity while the next page is transferred. This variation in usage between none and maximum is typical of computer network traffic. Because the subscriber has sole use of the fixed capacity allocation, switched circuits are generally an expensive way of moving data.

(Is all this stuff in CCNA exam? )

An alternative is to allocate the capacity to the traffic only when it is needed, and share the available capacity between many users. With a circuit-switched connection, the data bits put on the circuit are automatically delivered to the far end because the circuit is already established. If the circuit is to be shared, there must be some mechanism to label the bits so that the system knows where to deliver them. It is difficult to label individual bits, therefore they are gathered into groups called cells, frames, or packets. The packet passes from exchange to exchange for delivery through the provider network. Networks that implement this system are called packet-switched networks.

The links that connect the switches in the provider network belong to an individual subscriber during data transfer, therefore many subscribers can share the link. Costs can be significantly lower than a dedicated circuit-switched connection. Data on packet-switched networks are subject to unpredictable delays when individual packets wait for other subscriber packets to be transmitted by a switch.

The switches in a packet-switched network determine, from addressing information in each packet, which link the packet must be sent on next. There are two approaches to this link determination, connectionless or connection-oriented. Connectionless systems, such as the Internet, carry full addressing information in each packet. Each switch must evaluate the address to determine where to send the packet. Connection-oriented systems predetermine the route for a packet, and each packet need only carry an identifier. In the case of Frame Relay, these are called Data Link Control Identifiers (DLCI). The switch determines the onward route by looking up the identifier in tables held in memory. The set of entries in the tables identifies a particular route or circuit through the system. If this circuit is only physically in existence while a packet is traveling through it, it is called a Virtual Circuit (VC).

The table entries that constitute a VC can be established by sending a connection request through the network. In this case the resulting circuit is called a Switched Virtual Circuit (SVC). Data that is to travel on SVCs must wait until the table entries have been set up. Once established, the SVC may be in operation for hours, days or weeks. Where a circuit is required to be always available, a Permanent Virtual Circuit (PVC) will be established. Table entries are loaded by the switches at boot time so the PVC is always available.

( I do not know what kind of audience CISCO has in mind, but if this stuff is for a beginner that is trying to study for CCNA, then I am too stupid for CISCO, or to waste my time with their exams)

The Dude

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Simplified, but a fair explanation of CIRCUIT vs PACKET switching.......

CIRCUIT SWITCHING

Consider this network:

SOURCE----node----node-----node----DESTINATION

Each "node" may have hundreds of ports connected to them, but only the ports being used to carry information between the SOURCE and DESTINATION are being shown.

Imagine that within each node the circuits on either side of the node are physically 'touching' each other, as if they were patched together via a human operator. (cross-connects)

In such a scenario, any information - data or voice - generated by the SOURCE would travel down the wire and traverse each cross-connect within each node until the information ultimately reaches the DESTINATION.

Because because each of the wires are "nailed up" to each other within the cross-connect ( almost as if they were soldered together ), there is no need for any type of "address overhead" to accompany the data/voice information which is traveling down the wires.

Any information generated by the SOURCE simply passes down the physical path which has been established by each of the cross-connects. None of this information needs to be inspected in any way by the nodes, because a physical path has been established between the two end points. Information entering on one side of each node is simply moved to the other side of the node.

The cross-connects within each noide can be established in one of two ways:

1 - They are manually established and available 24x7x365. The term for this is a DEDICATED line. Some people use the term "nailed-up." Point-to-point leased lines such as a 56k, T1, etc., are examples of this arrangement.

2 - They can be established and dissolved "on demand." The telephone operator moving patch chords around when making a telephone call is an example of this. Of course, we do not employ telephone operators any more to perform this function. Instead, we have intelligent "switches" that can be directed via a signaling mechanism to establish the physical connections for us. The signaling mechanisms could be rotary dial pulses, DTMF tones from a telephone touch pad, ISDN Q921/Q931, on hook / off hook conditions, and many others.

Regardless as to if the circuit switched connections were established using case (1) or case (2), information being passed through each node is never inspected by the node because there is no need to, as a 'physical' connection has been estabished and there is no need to inspect the data content for address / routing information.

What comes in on one side of the node simply goes out the other side of the node.

==============================================================================

PACKET SWITCHED

There is no physical 'cross-connect' established within each node. Instead, each parcel of information the SOURCE generates must contain some_form_of_addressing information. The nodes will inspect this addressing information and make a forwarding DECISION based upon that addressing information. Small 'store and forward' delays are always present within each node. This is because of the time it takes to hold the parcel of information (packet), inspect the address information found within the packet, and then ultimately forward the packet along the correct path to the DESTINATION.

There are two generic manners in which to perform packet switching:

1- Datagram service. IP packets fall into this category. Each packet contains the SOURCE and DESTINATION addresses of the communication end stations. Because each packet contains the source and destination addresses, each packet can be 'independently' routed through each node within the network - they do not have to follow each other. An analogy would be what happens after a wedding. Everyone goes there own different ways, but, somehow some time later, they all wind up at the same party to eat, drink, and in some cases, make asses out of themselves. Perhaps too much 'drink' involved.....

2 - Virtual Circuit (VC) mode. VCs operate like a funeral prosession. The driver of the herse knows where he is going, and all the cars follow the leader to the final resting place. VCs tend to preserve the sequential order of packet flow. Think of a garden hose. If you rolled red, white, and blue marbles into a garden hose, then they will show up in the same order - red, white, and blue. (unlike the 'datagram / IP' service where the marbles can possibly show up blue, white, red.)

There are two kinds of Virtual Circuits. "Switched Virtual Circuits / SVC" and also "Permanent Virtual Circuits / PVC." In BOTH cases, each packet contains some_form_of_addressing information to enable to nodes along the path to inspect, decide, and forward the packets along the correct path associated with the DESTINATION. Frame Relay Data Link Connection Identifiers (DLCI), X25 Logical Channel Numbers (LCN) are examples of the address information found within each parcell of information being switched through each node.

In the PVC mode, the path between SOURCE and DESTINATION is pre-determined by the network node administrators, and the DLCI/LCN information is also pre-determined for that path. The SOURCE station simply adds the DLCI/PVC information to each parcel being presented to the network. The nodes along the path then inspect each parcel, decide where to send them, and forward them on accordingly.

In the SVC mode (rare with frame relay, common with X25) the path between SOURCE and DESTINATION is not pre-determined. Instead there is a form of 'signaling' which is used. The 'signaling' mechanism provides a way for the SOURCE user to inform the network that a connection is desired, and what the desired DESTINATION for the connection is. Each node takes this 'call request' and establishes a temporary 'on-demand' VC / garden hose between the SOURCE and DESTANTIONS. When the signaling mechanism informs the SOURCE user that the (switched / on-demand) VC has been established, the network will inform the SOURCE and DESTINATION users what DLCI/LCN to use for the connection. Packet forwarding will commence with each packet having the appropriate DLCI/LCN information appended to them. The nodes will once again inspect this addressing information, decide what (temporary) connection is associated with these DLCI/LCNs, and then forward the parcells along their way.

When the user no longer needs the on-demand garden hose, the signaling mechanism being utilized informs the network nodes that the connection can be cleared. The DLCI / LCN assignments which were used for the prior call can then be put back into an available pool of address space for other users to take advantage of.

The only bad thing about SVC services is that it's a real bithc rolling up all the garden hoses and putting them back where they belong after using them. They tend to get tangled up.

-ja

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I wouldn't make those generalizations. I would say there are usually more 'latencies" involved with packet switching within each node due to the requirement to accept, inspect, and then forward the packets. In a circuit switched environment, things are more like a continuous 'flow,' resulting in lower latencies.

• posted

Well, for now (the exam) I have memorized (something I always hated to do) that Circuit Switched ( which is used in ISDN) is _much slower_ than Packet Switched ( which is used in Frame Relay). Cell Switched is the fastest and is used in ATM .... Obviously, you think the opposite because the explainations are not right. I do not have much time to look at them today ( as my exam is tomorrow), but I still appreciate Mr Agosta's reply.

The Dude

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I think you are confusing the issue of transport speed vs the switching technology. ISDN is "slow" because (in BRI) there is 128,000 bps of bearer capability. That 'speed' is not a limitation of "circuit switching," but a limitation of ISDN. An ISDN connection at 64kb is still faster than say, a 9600 bps packet switched connection. ATM can be very "fast" indeed, but this is due to the combination of hardware-based cell switching, and the (usually) high speed transport facilities that carry these cells. A cicuit switched connection at "any" bit rate offers better throughput/speed than a packet or cell switched connection over the same medium.

CCNA testing aside, it would be helpful to research/study some of these basics with respect to telecom/communications.

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By faster, are you referring that it uses less CPU/memory or that it is capable of transferring more data per second? If you are going by the CPU/memory idea, then circuit switching is a little bit simpler, and consumes less resources. As for the transferring of data, packet switching can be more efficient because it can use multiple links, make better decisions on which links to use, etc etc. But this comes at a cost, which is higher processor usage, more memory, and the added complexity of making an IP network. Like for instance if you're using OSPF as a routing protocol, it can use a lot of resources. It also doesn't have to establish and set up a dedicated circuit to the destination, which causes some overhead.

I'm still studying for the CCNA exam, so take my words with a grain of salt.

• posted

Hmm, ok, not the best example of CCNA curriculum material - someone was trying very hard to show how to verbosely complicate a reasonably straightforward difference.

(1) Circuit Switched: The "circuit", the "communications channel", the "connection" is established before any data is transferred. So when you make a "traditional" telephone call you dial the number, the system connects you to the destination phone, if it's not busy it rings, but you don't start to talk until the other person answers. Then for the duration of that phone call your speech follows the same electronic pathway until one party hangs up.

(2) Packet Switched: "Connectionless, best effort" - the system starts to transfer data from the source in packets as soon as it is ready to go, whether or not the destination is ready. Furthermore each packet could take a different route to the destination. Other layer protocols manage the reliability/control issues.

This is fine for "data" of course, but demonstrates the issues that have to be resolved when you want to implement Voice/Telephony over IP.

Aubrey

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Thank you! At least someone understands where I come from!

I really like this simple explaination!

Is this an anology with sending an e-mail?

The Dude

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KISS is nearly always best :-)

But remember connectionless best-effort packet switching occurs at the Layer

3 Network (or Internet) Layer. Transport and Application Layer protocols ensure a session or virtual circuit has been established first if one is required, such as for email or FTP. But TFTP, which uses UDP doesn't bother.

Aubrey

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So, you are saying that " Circuit switched " is similar to TCP and " packet switched" is similar to UDP, correct?

The Dude

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Well, maybe, sort off :-)

Review the operation of the TCP/IP suite of protocols.

Start from the top, let's say a web server has received a request for HTTP data to be sent to a host somewhere. So the Application layer presents the HTTP data to the Transport layer. TCP, says ok, let's see if the host which requested the HTTP data is ready to receive it. The 3 way establishment process occurs, with protocol datagrams containing Sequence Numbers and Acknowledgements being exchanged - ok, good to go. The HTTP data is chopped up into segments with TCP fields added, including source & destination ports, so that the data will end up in the destination host web browser and not its email client. The Internet layer encapsulates these segments into packets with the destination (and source) IP address and off they go with routers directing the packets towards the destination network.

Now not all of these packets may make it to the requesting host. The TCP acknowledging process notifies the source of those packet(segments) that did arrive, so lost segments (packets) are re-sent and maybe the window size is also adjusted. Halfway through delivering the webpage one of the intermediate links goes down. So using the destination network address the routers will re-route the packets along other routes. If significantly more packets are lost and time delays occur TCP will say, whoa, time to re-establish the "link". And then it tries to start the process again.

So I guess you can say TCP tries to set up a "circuit switched" communication process while IP is using connectionless packet switched communications.

Remember, though, the above is a grossly simplified scenario and explanation.

Aubrey

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I am sorry, but any attempt to relate TCP, which is a host-resident software process to explain how L1 circuit switching works is going a couple steps over the edge for me. TCP and UDP and Absolutely_Nothing to do with the issue of circuit or packet switching. Trying to make that connection is like trying to say there is a similarity between a Porsche 911 and a jar of apple sauce. Two totally unrelated issues. Sorry to be harsh, but attempting to make a connection between these processes is probably going to add to the level of confusion instead of clarify things.

• posted

Finally, you were being honest. Please, next time, do not try to explain things that you do not understand to the others.

TCP and UDP and

The only level of confusion so far has been added by your posts only. Yes, you should be sorry for being harsh too. Mr. Adams seems to understand the subject very well, and the way he explains things has been excellent! Thank you Mr. Adams!

The Dude

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Hey John I think you missed this --->> "Well, maybe, sort off :-)"

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Hey Dude, thanks for the kind words, but I think you're close to being a bit a harsh too. John knows his stuff, I just explain things differently to him.

Aubrey

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What in heaven's name are you talking about?

You either have me confused with someone else, or just confused.

• posted

That's extremely dangerous. And wrong to push your philosophy (which is of that nature) onto others. Some people , including myself, would rather not learn wrong information, and would prefer no explanation to a wrong one.

Even if you make him aware, it's still bad to do that.

It shouldn't be wrong or contain bits of confusion to make a "story" of how it works tell better. But actually I think your explanation wasn't so distorted.

I've actually heard far more dangerous story tellers. Stories that I won't repeat because I'd like to keep them forgotton.

Ok, i'll tell one of them. , one of the stories was that a company making their own NICs will throw away the first and last NIC becuase they're all 0s and all 1s , and those are broadcasts!!!!!!

Of course, the reason why that one is wrong is becasue the company wouldn't make the full range. The first 3 bytes of the MAC are issued, and wouldn't be all 0s, and they'd start with those first 3 bytes. I don'tk now if they'd throw away the last one or just avoid making it. I suppose it depends how sophisticated their equipment is. And the All 0 MAC isn't and wasn't ever a broadcast. It may have been issued to somebody- or not - more likely not because it could be confused with all 0s MAC displayed on a computer screen because of a problem reading MAC address.

I used to hear many such stories. After that and another similar one , I didn't take a word seriously anymore.

They're ok when you know it already and want to correct misinformation.

I'm not comparing your explanation to that story, but you see why I have disdain for stories. And even minor sentences can create huge confusion.

I had a book that says that the OSI is a reference model, that's all it is, it's conceptual . Which is not true at all. It's a network architecture and a reference model. And that was supposed to be a good book. So please, no misinformation. Misinformation takes 10* longer to unlearn than it does to learn.

Though of course it's better when you warn people of misinformation first, so they don't hang on every word. The worst misinformers are those that you think probably know what they're talking about and are serious, and cause you to hang onto every word.

And i've had some pretty bad ones that answer "Yes" when the answer is "No"(but they didn't understand the question). Perhaps if they'd said "Sort Of". I wouldn't have taken it so seriously and learnt the wrong thing. But still it's not good to do that.

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