Re: Help with theory question on network topology


J. Clarke wrote:
> fran_beta@xxxxxxxxxxx wrote:
>
> > I'm trying to mark a paper that deals with networks and I'm trying to
> > establish whether a question is correct.
>
> When you say you're "trying to mark a paper" do you mean "grade a test"?


Yes

> If
> so, then the "right answer" would be the one that matches what the students
> were told in class and in their texts--anything else would be unfair and if
> what they were told in class and in the texts is wrong then you need to
> address that rather than punishing the students for the faculty's error.

Correct again, but this paper contains the ambiguity described and if
there is no correct and sensible answer, or if there is more than one
sensible answer I am ethically bound to deem all such answers correct.

Originally, I examined the paer and found the term "hub" where I said
there was a gateway. I assumed this was an error and replaced it so
that the ethernet linked-devices could exchange data with the token
ring devices, but with hindsight, the "error" might have been
intentional -- an attempt to psoe this very question and test knowledge
of the difference between a gateway and a hub.

I've found this a little confusing myself as gateways and "bridges" are
often used interchangeably in terminological terms at least. A
"gateway" seems to be a more sophisticated bridge, and a "router" may
well contain hardware components that allow it to move data between
different protocols.

> If you are a grader or a TA then you really should ask the professor this
> question.
>

I'm a teacher working in a setting where this coterie here is likely to
be the best qualified to answer.

> If you mean that you are trying to answer a homework question then you
> really should learn to research this sort of thing yourself, however since
> the question seems to be ill-posed in this case I can forgive a certain
> confusion.
>


Indeed

> > The question offers a diagram of a network topology bringing together
> > ethernet (star and bus) and token rings (includes a printer). A number
> > of devices are attached to each of the networks. These networks are
> > joined by a gateway.
>
> Right here there's a problem. "Gateway" is a very fuzzy term. It could be
> a bridge, a router, or something at a higher level than that. The students
> are going to have to make some assumptions about the definition of
> "gateway"--the sharp ones will see this and state their assumptions, the
> less sharp ones will go on whatever notion they may have of its definition
> assuming that what they know is correct.
>




> > The question is whether any of the following statements is correct.
> >
> > Not all data packets pass through the gateway (My guess, since the
> > token ring only passes packets to those with the correct token)
>
> Leaving aside the nature of ring networks, since the gateway is between the
> Token Ring and something else, the nature of Token Ring has little to do
> with what gets passed through the gateway. That will be determined by the
> specific nature of the gateway--all, some, or none may be passed.
>

So does that mean that the question as posed is impossible to answer
with certainty?

I assumed that it was a feature of token rings that any data packet
passing within the ring (eg between a node and the attached printer)
would not pass through the gateway.

> > Data packets are sent only to the intended receiver (the ethernet
> > broadcasts to all nodes, so this can't be true)
>
> It can be if all the Ethernet and Token Ring ports are on a common bridge
> (such bridges do exist).
>

|||||||||||||

The diagram showed:


Area 1

Standard Star Network

Devices:

Hub; MinPC; Disk Array; IBM AS/400

a LAN cable of some sort joins it to the gateway/(hub in original)

Two LAN cables leave the gateway one to the:


Token Ring in Area 2

Devices: Laser Printer; IBM PC; Server;


and the other to:

Area 3 Standard Bus Network

Devices: iMAC; server; Printer; raid drive


||||||||||||||


> > All data packets are transmitted to all devices (again the token ring
> > networks fails this)
>
> Well, actually, you seem to have a misconception about Token Ring. In a
> Token Ring each device sends data to one and only one device, the one on
> the other end of the cable connected to the transmitter in its NIC. The
> MAU is an essentially passive device.
>

OK

> If the ring is not switched, then the data goes from one machine to the next
> around the ring until it gets to its destination, and the destination then
> retransmits the data around the ring until it returns to the originator,
> which compares what was sent with what was received. So each frame
> actually passes through _all_ devices on a non-switched Token Ring. If it
> is switched then the switch sends the frame to its destination directly.
>

I see, so technically, the data packet does go to each device but is
rejected until it reaches the one for which it was intended?


> > Printing cannot be initiated from the devices in the star network (this
> > must be wrong since a gateway joins the ring and star)
>
> Depends on the design of the gateway. If it is one that either supports
> printing directly or passes the necessary packets then printing can take
> place, otherwise it can't.
>

You seem to be saying that there isn't sufficient information to say.

> > Would the answer be different if there were a hub rather than a gateway
> > (I assume this would divide the token ring from the rest of the
> > network)?
>
> A "hub" is, compared to a "gateway", a well defined device--these days with
> many devices sold as "hubs" actually being bridges it isn't as well defined
> as it once was, but it's still much clearer than "gateway". Since there is
> no such thing as a "hub" that will join Ethernet and Token Ring, they would
> indeed be isolated.
>
>

That's what I thought.


Thanks

Fran

> > Is my reasoning correct?
> >
> >
> > Fran
>
> --
> --John
> to email, dial "usenet" and validate
> (was jclarke at eye bee em dot net)

.

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