Packet and circuit switching

Just to one thing picked from below:

Could you give an example of an implemented OSI model infrastructure?

Regards, Frank

GM / Boeing Implemented an OSI system back in the late 80s. It was called MAP/TOP. There were a few companies selling OSI protocol suites back then, too. A company called "Tocuh OSI" from Scott's Valley, California sold such software. I have installed it. The US Government had also implemented some OSI systems as defined within its "USGOSIP"(United States Government Open Systems Interconnect Profile) parameters.

Most, if not all of the efforts to migrate towards OSI ended after Internet use became 'commercially' available.

-ja

Funny, that the same (?) US government, called the Department of Defense developed the DARPA net, which is the Internet based on...

One of the reasons the the OSI model never made it, is because of the long specifications of the interlayer communications within the software.

Nice, but not practical...

Anyway, the discussion was on mis-information, right?

So, that brings us to the fundamental question: What is truth? And that question is is not supposed to be answered in this news-group, I guess.

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

John Agosta wrote:

thanks, but what does the word "switching" mean in telecommunications (no doubt it's an old term - I suppose predating cisco's naming their products "switches"?

Passing the "packet"(as in the whole lot) from one telecommunications device to another? I doubt it's called switching and at layer 2, and for that reason. If that were so, surely they'd use the old word for that function - bridging. And if it were at layer 3 it'd be called routing.

I don't quite understand your question/statement in the first two sentences of the last paragraph, but I will attempt to discuss.....

In the old telcomm world, the word "switching" usually involved layer 1 issues. IE - mapping bits from one channel into another. For simplicity's sake, we could call this a "cross connect," or a "DCS." ( digital cross connect system). Sometimes called "DACS." (A for "access")

Assuming the DCA is not in some alarm state, your typical DCS would not "inspect" the contents of a bit stream to determine any intelligent (address/protocol/content) information. Bits would simply come in one side of the DCS and exit out the other side. The mapping of input-to-output channles would be done on an administrative staic basis, or, with some signaling mechanism - which in itself is another topic.

Enter the world of 'computer communications,' and for instance, ethernet driven network devices.

Here, the word "switching" includes some "inspection" of the content / frame.. Layer 2 inspection - like MAC addresses.

The term "routing" would infer the inspection of the 'layer 3' contents carried within the layer 2 'frame.'

Suggest you read slowly the elaboration about "circuit vs packet" switching in the previous post because it is pretty well spot-on.

I think you should let go the mapping between the OSI layers and the naming. Why? How would you handle/name the following technologies?

Label switching, lambda switching, L3 switching, L4 switching, cable routing?

Every technology should be considered in its context. You can have TCP over IP over GRE inside IPsec over a MPLS infrastructure over IP over Frame Relay over SDH over lambda (colors) over fibre. So, how are you naming each of the encapsulations?

And if I add RSVP for bandwidth allocation inside the MPLS infrastructure, I am bringing in a 'kind of circuit switching technology' into the story.

If you look at history out technologies started out to be circuit switched (Bell started with this :-)), we moved to packet switching, but we like to have bandwidth guarantees (for voice, for example), so, we build technologies (algorithms like Marking, WRED, CBWFQ, LLQ, etc) to simulate circuits switch technologies on top of a packet switched technology.

FW

it looks like you did !

great, as I expected

thanks, I googled around but didn't find anything. Is there basic resource on this?

it's obviously a network of nodes and the packet gets switched between them.

talking telecommunications- So, The whole system is called DCS or DACS What is the name of the nodes(i.e. it's not a router or bridge, it's a what?), and is it correct to say that the packet is "switched between them". i.e. "..the packet is being switched between the ____ (the what?!) "

i'm sure cisco doesn't go into this at that level. But at the level of what is the whole system, and what are the nodes that the packet passes through, I think that's a minimal level for a thinking individual!

yeah, i know that one a bit, lots of info available for that one!

thanks

Perhaps the following would be a good starting point to bone up on some the circuit switched (T1) physical layer issues......

thanks, I will study that

q_q_

No_spamm wrote:

I wasn't really trying to put it in an OSI layer. I was really determining that in this context, switching wasn't xyz.

That is a great demonstration of where forcing the OSI layer terminology can be inappropriate. If I want to look into all those technologies, to get a good overview, what is a good route - any particular resource(s) or cert track?

So far i'm only really aware of the CCNA for a broad view of networking. CCNP then going more specific in it. What would cover the kind of things you mention?

many thanks

I am not really familiar with the latest certifications contents. But I do remember that CCNA talks about some general architectures, like the OSI, model. On CCNP, I am not sure (skipped that one...). I believe CCNP gives you some more details on how several routing protocols work. However, I once met a CCNP certified instructor. And he was convinced that in order to have BGP in your network, you do not need any underlying IGP (OSPF, EIGRP, RIPv2, ISIS, whatever). That's a bad instructor...

Anyway, I suggest doing the following:

Try to understand the OSI model, just for the context. I mean the relationship between the layers. Then try to understand how for example a 'common' TCP/IP stack is built and where it (relatively) fits into the OSI model. There are probably some drawings out there that show the relationship of the several protocols, just an example:

UTP, 802.3, IP, TCP, telnet fiber, 802.3, IP, TCP, telnet fiber, SDH/SONET, IP, TCP, telnet T1, Frame-relay, IP, TCP, telnet T1, HDLC, IP, TCP, telnet

Then try to understand some individual protocols (fitting your CCNA cert.) and fit them into the picture. When you look at the header of these protocols, you'll see some "fields" that refer to the next encapsulation layer. Don't let the architecture itself scare you of: MPLS labels are sitting between the MAC header and IP header (or layer 2 and layer 3). But how labels are actually distributed (together with label binding, label stacking, etc) is another story and may depend on the underlying technology (Cell-mode MPLS on ATM versus Frame-mode MPLS on non ATM, but you can have frame-mode MPLS over ATM :-). BTW: the difference is whether you use VPI/VCI cell identifiers in ATM instead of Labels and use a "VPI/VCI - Label mapping" at the head-ends). Anyway, that's why MPLS is sometimes called a layer 2.5 protocol.

FW

thanks, I know the OSI and have seen fields that refer to the protocl above e.g. Ethernet has Type , which can refer to IP or IPX etc. But CCNA doesn't offer an overview of so many technologies.

I think CCNA is probably a bit weak on the technology section.In that it's not broad or deep. Especially AWN technologies.

CCNP i've heard isn't much broader than CCNA. Just more depth. So it won't have the breadth either. Which might explain that bad instructor.

OSI isn't a problem.

Any ideas on how to see/run into/learn about those technologies though

- getting at least a good overview?

Sounds silly but:

1. Google
2. Draw pictures yourself. That's what I did.
3. Read a lot: Cisco website (not always clear, btw), RFCs (yes, sometimes very very useful if you want to know exactly how it works) Cisco Press books (sometimes many errors)

FW

Technically, you 'could' have BGP running within a network without an IGP. Neighbors just need to be 'found,' and directly connected neighbors are easily found. Remote neighbors can be reached with a series of static routes. In the majority of cases, this is a silly arrangement, but entirely possible.

Perhaps the instructor made some clarifying statement* to that effect ?

• See the reference to silly arrangement. ;-)

Yes, that's is possible, and in a triangle you do not even need static routes unless you are using loopback addresses as peering addresses... Anyway, I was surprised by the fact this instructor said you do not need an IGP..