Help with theory question on network topology

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I'm trying to mark a paper that deals with networks and I'm trying to
establish whether a question is correct.

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.

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)

Data packets are sent only to the intended receiver (the ethernet
broadcasts to all nodes, so this can't be true)

All data packets are transmitted to all devices (again the token ring
networks fails this)

Printing cannot be initiated from the devices in the star network (this
must be wrong since a gateway joins the ring and star)

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)?

Is my reasoning correct?


Fran

 

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fran_beta@hotmail.com wrote:

> I'm trying to mark a paper that deals with networks and I'm trying to
> establish whether a question is correct.
>
> 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.

Are you talking only about level two, that is ethernet & token ring? Or IP?
Makes a big difference. Also, define gateway. While it's is technically
possible to bridge ethernet & token ring, it's not common. Gateways
usually refer to IP (or other routable protocol) routers.

>
> 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)

It's possible, though not likely.

>
> Data packets are sent only to the intended receiver (the ethernet
> broadcasts to all nodes, so this can't be true)

If a switch is used, only broadcast frames are sent to all nodes (assuming
the switch tables are current).
>
> All data packets are transmitted to all devices (again the token ring
> networks fails this)

Not necessarily. Can't say for sure, without further details. For example,
if IP it is possible. However, this one conflicts with the previous
question.


>
> Printing cannot be initiated from the devices in the star network (this
> must be wrong since a gateway joins the ring and star)

More info about protocols used please.

>
> 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)?

In ethernet, a hub forwards all frames to all devices and in the
conventional sense is a different device from a gateway. Also a hub cannot
connect ethernet to token ring. Ethernet and token ring can be connected
either through a special type of bridge that can convert between token ring
and ethernet or with a router which can connect different technologies.

>
> Is my reasoning correct?

You'll have to expand on the details. You also need to learn more about
level two (ethernet, token ring etc.) and level three (IP, IPX etc.)
networking. Also read up on hubs, switches (including bridges) and
routers.

 

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fran_beta@hotmail.com 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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fran_beta@hotmail.com wrote:

> Not all data packets pass through the gateway (My guess, since the
> token ring only passes packets to those with the correct token)

One other thing. There's no such thing a the "correct token". There is one
token that gets passed around the ring and grabbed by a host wishing to
transmit. When the transmission is completed, the token is put back on the
ring, for the next host etc. A token has absolutely nothing to do with
what gets passed through a "gateway.

 

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J. Clarke wrote:
> fran_beta@hotmail.com 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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fran_beta@hotmail.com wrote:

(snip)

> All data packets are transmitted to all devices (again the token ring
> networks fails this)

I haven't thought about token ring for a while, but I thought the
data packets went all the way around the ring. The sender then removes
it and sends a token to the next host.

(snip)

> 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)?

Hub is, traditionally, the device at the center of a hub and spoke
network topology.

For ethernet in the beginning it was a repeater and it is often
considered as a synonym for repeater, but that is technically incorrect.
(As are many other commonly believed ethernet rules.)

I believe that hub was used for TR even earlier than ethernet, though
I have known eight port 10base2 repeaters that could be considered
hubs, even before UTP ethernet existed.

-- glen

 

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fran_beta@hotmail.com wrote:

>
> J. Clarke wrote:
>> fran_beta@hotmail.com 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.

Generally speaking, a "bridge" connects two networks at the datalink layer
(the layer that defines "ethernet" or "token ring" or "arcnet" or
whatever), a router operates at the network or transport layer (the layer
that defines "TCP/IP", "IPX/SPX", etc), and a gateway operates above this
level.

Any literature that refers to a bridge as a gateway or vice versa is _not_
to be trusted--it is either very sloppy or very old.

>> 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?

You are correct.

> 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.

If the gateway is attached to the token ring then each packet will be seen
by the gateway--what the gateway does with it is another story.

>> > 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

AS/400? Now _that_ complicates things, and the effect depends on the
vintage--early AS/400s could not communicate directly with PC
printers--something emulating the kind of printer or print server it
expects had to be attached to the network--I don't know to what extent this
has changed in more recent models.

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

That might be Twinax--the AS/400 has its own unique network standard, called
"System Application Architecture" (different from SNA, lurkers) used
primarily to connect terminals and printers. That would make sense as SAA
can have hubs, although there may be IBM-ese for them that I don't recall.

In any case with the AS/400 a gateway makes more sense--it would likely be
an SAA gateway--a couple of examples are Novell "Netware for SAA" and
Microsoft "SNA Server", which also supports SAA. Among other things it
would probably expose PC printers to the AS/400 and provide terminal
emulation, and may provide other services.

At one time, before IBM repositioned their midrange and mainframe systems as
servers, this was one of the the standard ways to connect an AS/400 to a PC
network, the other being to put 5250 (the standard AS/400 terminal was the
IBM 5250) emulation boards in the PCs that were to be used as terminals.

I'm no AS/400 expert by the way, you might want to try to find a more
specific newsgroup for advice on that end--it's really outside of the
Ethernet world and the PC world as well--it's an IBM Midrange system, which
used to be considered "heavy iron", and it's better to think of it as a
mainframe than as a PC.

> 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

"Standard bus network" doesn't mean anything. In fact there isn't any bus
network to which an iMac can be attached directly--its ports are 100TX
IIRC--you'd need a converter to attach to 10base2 and I don't know if it
works with localtalk at all.


> ||||||||||||||
>
>
>> > 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?

Not rejected, just passed along and otherwise ignored.

>> > 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.

Exactly.

>> > 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

--
--John
to email, dial "usenet" and validate
(was jclarke at eye bee em dot net)

 

[Guest]

Archived from groups: comp.dcom.lans.ethernet (More info?)

glen herrmannsfeldt wrote:

> fran_beta@hotmail.com wrote:
>
> (snip)
>
>> All data packets are transmitted to all devices (again the token ring
>> networks fails this)
>
> I haven't thought about token ring for a while, but I thought the
> data packets went all the way around the ring. The sender then removes
> it and sends a token to the next host.
>
> (snip)
>
>> 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)?
>
> Hub is, traditionally, the device at the center of a hub and spoke
> network topology.
>
> For ethernet in the beginning it was a repeater and it is often
> considered as a synonym for repeater, but that is technically incorrect.
> (As are many other commonly believed ethernet rules.)
>
> I believe that hub was used for TR even earlier than ethernet, though
> I have known eight port 10base2 repeaters that could be considered
> hubs, even before UTP ethernet existed.

On Token Ring it's a "Multistation Access Unit", aka "MSAU" or "MAU", the
latter pronounced like the name of the late Premiere of the People's
Republic of China.


> -- glen

--
--John
to email, dial "usenet" and validate
(was jclarke at eye bee em dot net)

 

[kr]

Archived from groups: comp.dcom.lans.ethernet (More info?)

fran_beta@hotmail.com wrote:
>
> 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 think you were correct in assuming that the term "hub" was an error;
otherwise, the network diagram becomes totally meaningless. See below.

> 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.

No, we're talking about different concepts here.

A hub is a passive ethernet repeater. It sends everything it receives on
one port out all the other ports, and that's it. Some consider a hub a
layer 1 device, but it's mostly referred to as a layer 2 device.

A bridge is _always_ a layer 2 device, meaning it forwards frames to and
from two different network segments. A bridge can only function between
segments made up of the same network topology, with one exception: It is
possible to bridge between Ethernet and Token Ring. A switch is a bridge
with more than two ports.

The main difference between a bridge/switch and a hub, is that a bridge
automatically "learns" which layer 2 addresses are at which end of the
bridge, by simply watching the source MAC addresses of the frames. It
then stops forwarding frames that are not relevant to the segment.

A "gateway" is a rather loose term that usually means "a router", but it
could conceivably also mean an application level proxy, and all sorts of
other things as well. A gateway relates to a hub and a bridge a bit like
"public transport" relates to a train and a taxi.

> 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.

Token Ring is a type of layer 2 topology. It is highly unlikely that
this "gateway", whatever it is, operates at layer 2.

Token Ring works roughly like this: All nodes in a ring are connected to
a passive repeater unit called a MAU. The MAU is basically just a bunch
of relays.

A short data frame, called "a token", is passed from node to node around
the ring. Whenever a node needs to send data, it first waits for the
token. It then attaches its data to the token, and sends it to its
neighbour.

The neighbour recognizes that the token has data attached to it, and
passes it on. It does this whether it is the intended recipient of the
data or not. When the originating node gets its own token+data back, it
generates a new token, and sends it to its neighbour.

Some newer versions of Token Ring supports "Early Release Token", which
means that the transmitting node generates a new token immediately after
sending the token+data frame.

But I digress.

> 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

The first thing this diagram tells me, it that the person who made it,
has very limited knowledge about networks. First of all, there's no such
thing as a "standard star network", and much less a "standard bus
network". Also, "RAID drives" and "disk arrays" are not network units,
they are either part of a server or inside a NAS unit.

But the diagram is relatively clear about one thing: We're talking about
three different network topologies. The "star network" is probably
Ethernet (although star-based ArcNet also exists), and the "bus network"
is probably supposed to mean Apple LocalTalk (but could also be ArcNet
or coax-based ethernet). Token Ring is the only unambiguous term here.

This means that your "gateway" must be a router. It can certainly not be
a hub, since no such device exists that would allow one to connect
Ethernet, Token Ring and LocalTalk.