Packet and circuit switching

Packet and circuit switching
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Packet and circuit switching The Dude 08-07-2006
Posted by The Dude on August 7, 2006, 11:14 am
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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



Pure Networks
Posted by John Agosta on August 7, 2006, 1:56 pm
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> Sometime ago, someone asked about Packet and circuit switching ...

>

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



Posted by gregg johnstone on August 7, 2006, 4:10 pm
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I hear what the Dude is saying-Mr Agosta, your analogys were
excellent-it is so much easier to picture something -many thanks
btw I am undrstanding that the Circuit switched is faster but consumes
mre CPU power? wheras the packet switched is slower but can operate on
a much lower spec machine?
John Agosta wrote:
> > Sometime ago, someone asked about Packet and circuit switching ...
>
> >
>
> 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


Posted by John Agosta on August 7, 2006, 4:42 pm
If you were  Registered and logged in, you could reply and use other advanced thread options





>I hear what the Dude is saying-Mr Agosta, your analogys were
> excellent-it is so much easier to picture something -many thanks
> btw I am undrstanding that the Circuit switched is faster but consumes
> mre CPU power? wheras the packet switched is slower but can operate on
> a much lower spec machine?

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 by NO_spamm on August 14, 2006, 6:25 pm
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Packet switching is making 'direction' decision per individual
data packet. The path over which data will be flowing is in principle
variable.

Circuit switching is making 'direction' decision before any data is
transfered (apart from possible fail-over technique's). The path over
which data will be flowing is the same throughout the whole session (again
apart from possible failover).

And that's it.

There is no concept of "packet switching is faster than circuit switching"
These is a difference between:
        1. setting up the data path.
        2. the bit rate at which data flow over the path.

FW


On Mon, 07 Aug 2006 15:42:14 -0500, John Agosta wrote:

>
>
>
>>I hear what the Dude is saying-Mr Agosta, your analogys were
>> excellent-it is so much easier to picture something -many thanks
>> btw I am undrstanding that the Circuit switched is faster but consumes
>> mre CPU power? wheras the packet switched is slower but can operate on
>> a much lower spec machine?
>
> 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.


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