Re: Help with theory question on network topology

fran_beta@xxxxxxxxxxx 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.
.

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