Path: utzoo!utgpu!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!bloom-beacon!think!ames!pasteur!agate!tehran.berkeley.edu!glass
From: glass@tehran.berkeley.edu (Brett Glass)
Newsgroups: comp.dcom.lans
Subject: Re: Token Ring (was: Re: Info on LANs)
Message-ID: <18659@agate.BERKELEY.EDU>
Date: 31 Dec 88 20:18:21 GMT
References: <12786@cup.portal.com> <920001@hposdl.HP.COM> <10777@s.ms.uky.edu>
Sender: usenet@agate.BERKELEY.EDU
Lines: 139
Disorganization: University of California, Berkeley

In <10777@s.ms.uky.edu>, David Herron writes:

> Yeah, I just got done reading the current BYTE this morning and
> was especially intrigued with the token ring article ...

Thank you.

> but I had a couple of questions..  mainly about assumptions of the author.

> the style of daisy chaining -- The IBM design (he didn't make it clear
> if he was talking about just the IBM design or token rings in general
> at this point)...

All IEEE 802.5 Token Rings have the same topology.

> has some sort of central ring off which you have two
> cables going out to the node.  The central ring has, at the point where
> the cable-going-out-to-the-node (I'm going to call it "drop cable")
> connects in there's some circuitry which detects whether the node is up
> or not and if it isn't simply passes packets (frames) along.

The "circuitry" consists of a simple relay and a few passive
components.

> Why have all this extra cabling?  Yeah it's a good idea, if you're going
> to have a fancy net like token ring, to have some smarts to allow packets
> to go through when nodes go down or are disconnected.  BUT .. I think you
> could do it without having to use two cables to reach the node ... one
> should be able to do the job.

There is only one cable. It contains two pairs (four conductors):
A transmit pair and a receive pair. Since the transmitted and
received signals go to different nodes, it would be messy and
expensive to use the same pair for both.

> To guard against nodes being turned off you put some stuff on the board
> which is somehow always powered.

The Token Ring was designed for the absolute maximum degree of
reliability. A circuit which relies on a power source that
must always be present (a lithium battery, for instance) will
eventually fail. This is why IBM's MAUs are entirely passive.

> Or .. the broadband cabling we have on campus proves that you
> can have multiple transmitters on a cable all transmitting at the
> same time.  Have the central box transmit on one frequency and
> have the node transmit on another.

Using broadband transmission, rather than baseband, would make
the system MUCH more expensive than adding a pair of wires to
each cable. FCC certification would be harder and reliability
could suffer. IBM's decision seems to be the right one.

> Lastly .. it was suggested in the article that we use the normal wiring
> conduit and closets as we do now for phones and ether cable.  I don't
> think that would work unless these cables are very very very thin.

The standard Token Ring cable isn't much thicker than good 50-ohm
coax.

> At least not compared to ethernet where we pull just one cable
> all around the floor to serve all our workstations.

Pulling one cable all around the floor is fine for a "techie"
environment, but won't cut it in the business world. And it's a
nightmare if you need to add stations in unanticipated places.
IBM takes a more mature view. It envisions the LAN as a permanent
part of the building, like the telephone system, and recommends
that cables and wiring centers be put in when the building is
constructed. A number of companies have followed this advice, and
it has worked.

> only one talker at a time -- I see that this helps to avoid collisions
> but it seems a big waste of the available bandwidth.  Ether also allows
> only one talker at a time so neither has an advantage over the other
> in this regard.  There should be some way of having more than one packet
> circling the ring at a time.  Or did I misread the article?

You did. The article mentioned Early Token Release, implemented
as part of the new 16-Mbps standard.

> size limits -- He said a few things about token rings not being as size
> limited as ethernets.  First off if you were to have a ring of any appreciable
> number of stations and each station were the maximum distance apart
> the delays for the one-packet-at-a-time to go around the ring would be
> very hideous, at least in computer terms.

Early Token Release reduces this latency. Also, remember that
there is only 1.5 bit of delay in each adapter except the Active
Monitor. The time for a token to circulate is comparable to the
time an Ethernet node must wait to make sure the line is clear
before transmitting.

> broadcast packets -- how do you do one?  Off hand I'd think you could
> do it much as now, put a special TO-ADDR (all one's?) and your own
> FROM-ADDR ...

This is how it's done. It's standard IEEE addressing.

> all the nodes around the ring see the packet and actually
> receive it but don't set that special bit saying it was received.
> Eventually the original transmittor will receive it again and set
> the special bit and pass it on.

In the Token Ring, every recipient attempts to set the FCI bit.

> extra overhead beyond normal ethernets .. both the extra cabling and
> also the two watchdog boxes.

A "star" wiring configuration is no more inefficient than phone
wiring. And it is especially worthwhile in the case of the Token
Ring because it allows the ring to be fault-tolerant. The
"watchdog boxes" (the MAUs) are worth the money not only for
reliability purposes but because they allow nodes to be inserted
and removed easily. The net can be reconfigured without bringing
it down (Try to do that with Ethernet!). You can't afford to shut
down a business to add or remove a node on a network.

> As I think it over I just see all this extra stuff tossed in
> that makes it more expensive than ether. And you're not gaining
> anything in speed.

A 4-Mbps Token Ring will outperform a 10-Mbps Ethernet under
heavy loads because of the absence of collisions. And the
priority scheme guarantees that important messages arrive
quickly. Read the section titled "Bravery Under Fire" in the
article.

> OOooh  AAaah, they've raised the speed up to 16Mbps ... that's
> still the same ball-park as ether.

That's a 60% improvement, not counting the additional gains from
the lack of collisions. Perhaps this is why Sun Microsystems was
one of the original test sites for the 16-Mbps ring.

-- Brett Glass

(Note: I don't usually read Usenet news, so please direct responses to
 my electronic mailbox.)