Re: Help with theory question on network topology
- From: "J. Clarke" <jclarke.usenet@xxxxxxxxxxxxxxxx>
- Date: Sun, 21 Aug 2005 10:32:07 -0400
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"? 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.
If you are a grader or a TA then you really should ask the professor this
question.
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.
> 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.
> 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).
> 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.
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.
> 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.
> 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.
> Is my reasoning correct?
>
>
> Fran
--
--John
to email, dial "usenet" and validate
(was jclarke at eye bee em dot net)
.
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