From: medici@dorm.rutgers.edu (Mark Medici)
Newsgroups: comp.dcom.lans.ethernet
Subject: RFC: comp.dcom.lans.ethernet FAQ?
Summary: Current status of FAQ
Keywords: FAQ
Date: 17 Nov 92 04:08:59 GMT
Organization: Rutgers Univ., New Brunswick, N.J.

I posted this before, but it doesn't seem to have gotten out.
Apologies if this is a duplicate.


Following is the current state of the comp.dcom.lans.ethernet FAQ.
Please take some time to read it through, and either mail me your
comments directly, or post them to this list.

This file can also be ftp'd from dorm.rutgers.edu in pub/novell/DOCS
as Ethernet.QA.

----------------------------------------------------------------------

		Ethernet Network Questions and Answers

	Summarized from UseNet group comp.dcom.lans.ethernet.

Version:	12 November 1992

References:	Rich Lawrence <rich@grebyn.com>
		Rich Seifert <seifert@netcom.com>
		Mark Medici <medici@gandalf.rutgers.edu>
		Peter Desnoyers <peterd@merlin.dev.cdx.mot.com>
		Ray Hunter <rhunter@esoc.BITNET>


Q: What is a runt?
A: A packet that is below the minimum size for a given protocol.  With
   Ethernet,  a runt is a frame shorter than the minimum  legal length
   of 64 bytes (at Data Link).

Q: What causes a runt?
A: Runt  packets  can  be  caused  accidentally  or intentionally.  If
   accidental, they are most likely  the result of  a faulty device on
   the network, or software gone awry.  If  intentional, they  may  be
   designed to be  runts for a  specific reason.  SNMP (Simple Network
   Management Protocol) is often sent as runt  packets  so  that  many
   devices will simply ignore it.

Q: What is a jabber?
A: A blanket term for a device that is behaving improperly in terms of
   electrical signalling  on  a network. In Ethernet this is Very Bad,
   because Ethernet uses electrical signal levels to determine whether
   the network is available for transmission.  A jabbering device  can
   cause the entire network to halt because all other devices think it
   is busy.

Q: What causes a jabber?
A: Typically a bad network interface card in a machine on the network.
   In  bizarre  circumstances  outside  interference  might  cause it.
   These are very hard problems to trace with layman tools.

Q: What is a collision?
A: A condition where two devices  detect  that the network is idle and
   end up  trying to send packets at exactly the  same time. (within 1
   round-trip  delay) Since  only  one device  can transmit at a time,
   both devices must back off and attempt to retransmit again.

   The retransmission algorithm requires each device to wait  a random
   amount of  time,  so the two are very likely  to retry at different
   times, and thus the second  one will sense that the network is busy
   and  wait until the packet is finished. If the two devices retry at
   the same  time  (or almost the same  time) they will collide again,
   etc.

Q: What causes a collision?
A: See above.  Ethernet is a CSMA/CD  (Carrier Sense Multiple  Access/
   Collision Detect) system. It  is possible to not sense carrier from
   a previous device and attempt  to transmit anyway, or  to  have two
   devices attempt to transmit at  the  same time;  in  either case  a
   collision  results.    Ethernet  is   particularly  susceptible  to
   performance loss from such  problems when people ignore the "rules"
   for wiring Ethernet.

Q: What is a jam?
A: When a workstation receives a collision, and it is transmitting, it
   puts out a jam so all other stations  will  see the collision also.
   When a repeater detects a collision on one port, it puts out  a jam
   on  all  other  ports, causing a collision to occur on  those lines
   that are transmitting, and causing any non-transmitting stations to
   wait to transmit.

Q: What is a broadcast storm?
A: An  overloaded  term that describes  an  overloaded  protocol. :-).
   Basically it describes a condition where devices on the network are
   generating traffic that by its nature causes the generation of even
   more  traffic.   The inevitable  result  is  a huge degradation  of
   performance or complete loss of the network as the devices continue
   to  generate  more and more  traffic.  This can  be related  to the
   physical transmission or to very high level protocols.  There is  a
   famous example of Banyan Vines bringing a huge network to its knees
   because  of the addition  of a  single  server,  which brought  the
   network to "critical mass"  (this logic error  has been corrected).
   NFS is famous for this type of failure.

Q: How do I recognize a broadcast storm?

A: That depends on what level  it  is occurring. Basically you have to
   be aware of the potential for  it beforehand and be looking for it,
   because in a true  broadcast storm you  will probably be  unable to
   access  the  network.   This can  change  dramatically for a higher
   level protocol.  NFS contention can result  in a  dramatic DROP  in
   Ethernet traffic, yet no one will have access to resources.

Q: How can I prevent a broadcast storm?
A: Avoid  protocols that are prone to it. Route when it is  practical.
   Don't buy Ethernet. :-).

Q: What is *high* traffic on an Ethernet? 5%? 20%? 90%?
A: High traffic is when things  start slowing down to the  point  they
   are  no  longer acceptable.  There is  not set percentage point, in
   other words.  Xerox used to use a formula based on packet size over
   time, or something, but the issue has been significantly muddied by
   the  plethora of  protocols available and how they  react  to  wire
   usage.   I  usually start  paying attention over 40-50%,  *or  when
   things slow down*. I've seen IPX segments that  were slow with less
   than 20% usage.

Q: What means SQE? What is it for?
A: SQE is the IEEE term for a collision. (Signal Quality Error)

Q: What means "heartbeat"? What is it for?
A: Heartbeat (a.k.a. SQE Test) is a means of detecting a transceiver's
   inability  to  detect  collisions.   The  normal  operation  of  an
   Ethernet   will  test  the  transceiver's  power,  transmitter  and
   receiver; if any of these fail  the station will  not hear its  own
   loopback.  Without heartbeat, it is  not  possible to determine  if
   your  collision  detector  is  operating  properly.   Heartbeat  is
   implemented  by generating a test signal on the collision pair from
   the transceiver (or its equivalent) following every transmission on
   the network. It does not generate any signal on the common medium.

   Note  the older usage of  this  term to refer to the +-.7V  carrier
   sense wave, although I haven't  heard it used that  way in  a while
   (since SQE indicators became popular on transceivers).

Q: What means "CSMA/CD"?

A: Carrier Sense,  Multiple  Access, with Collision Detection, the MAC
   (Media Access Control) algorithm used by Ethernet to help avoid two
   devices on the same cable from transmitting at the same time, or at
   least recognize when this has happened so that  the two devices can
   back-off and try again later.

Q: What means "IPG"?
A: The  InterPacket Gap (more properly  referred to as  the InterFrame
   Gap,  or  IFG)  is  an  enforced  quiet  time  of  9.6  us  between
   transmitted Ethernet frames.

Q: Does a NEMP (Nuclear Electro-Magnetic Pulse) affect an Ethernet?
A: The Russians have done the most research into the  effects of NEMP,
   although the  US  and  various European countries  have also looked
   into  it.  I doubt that the results and  theses from this  work  is
   available.  Given my very limited understanding of the effect (as a
   layman), yes, I expect it  would.  Obviously, a fiber-optic network
   (since  it  is  non-conducting)  would  have a  greater  chance for
   surviving NEMP.   However, I suspect the  EMF would not  be signif-
   icantly retarded by most system enclosures to prevent damage to the
   network interface (as well as the  rest of the system internals) in
   spite of the lack of copper network cables acting as antennae.

Q: What means "promiscuous mode"?
A: A controller  in promiscuous  mode will receive all frames, regard-
   less of destination address.  Ethernet  is promiscuous  in  that it
   allows any device on a segment to hear every packet on that segment
   if the card is so programmed. This is an  obvious  security  issue.
   It used to be  that  there was no way  around this besides encoding
   the packets  themselves,  but Synoptics  recently released a secure
   Ethernet solution (blatant employee plug).

Q: How can I test an Ethernet?
A: You  must  be  more specific.  Do you wish to  test  the electrical
   integrity of the wire (ie, will it carry a  signal properly)  or do
   you wish to  test the performance of it while running, etc?  If the
   former, a TDR  (see  below) or cable scanner that  incorporates and
   expands   on  the  capabilities  of   a  TDR  would   be  the  most
   comprehensive tool, though  a  great  deal can be determined with a
   simple  ohmmeter.   The  latter  requires special  and  often  very
   expensive  software,  usually  combined  with custom  hardware,  to
   capture, optionally filter, and analyze the  network packets.   The
   most basic test is to connect a pair of devices and see if they can
   communicate with each other, while monitoring any status indicators
   that the devices might provide.

Q: What is a "TDR"?

A: A Time-Domain Reflectometer is a tool used  to detect cable faults.
   This device operates by sending a brief signal pulse down the cable
   and looking  for its reflection to bounce back.   By  analyzing the
   reflected pulse, it is possible to make judgments about the quality
   of the cable segment.  More advanced units  can not only detect and
   identify the nature of the problem, but give a reasonably  accurate
   indication of the  problem's location  (distance from the point  of
   the  test).  There  is also a device known as an OTDR, which  is an
   Optical Time-Domain Reflectometer for fiber-optic cables.

Q: What means "BERT"?
A: Bit  Error Rate  Tester.  This  equipment  is  used to  analyze the
   amount and types of errors that occur on a cable segment.

Q: What (free) tools are there to monitor/decode/etc an Ethernet?
A: There are many built into most Unix systems.  Some cards for the PC
   come with utilities. There are several  free ones available.  Again,
   use archie.

Q: What is the difference  between  an Ethernet frame and a  IEEE802.3
   frame?  Why are there two types?  Why is there a difference?
A: Ethernet was invented at Xerox Palo Alto Research Center  and later
   became  an  international  standard.  IEEE  handled  making  it   a
   standard; and their specifications are slightly  different from the
   original Xerox ones.  Hence, two different  types.   802.3 uses the
   802.2 LLC to distinguish among multiple clients, and has a "LENGTH"
   field where Ethernet has a 2-byte "TYPE" field to distinguish among
   multiple client protocols.

   TCP/IP and DECnet  (and others) use Ethernet_II  framing, which  is
   that which Xerox/PARC originated, while NetWare defaults to 802.3.

Q: What is SNAP
A: Sub-Network Access Protocol

Q: Where  can  I  find  out  which Protocols use  which Ethernet  type
   numbers?  
A: Look at IETF RFC-1340 - Assigned Numbers RFC.

Q: What is UTP, STP?
A: Unshielded  twisted pair, shielded  twisted pair.  UTP  is what the
   phone companies  typically use, though this is  not always of high-
   enough quality for high-speed network use.  STP is mostly from IBM.
   Either  one  can  be  used for  Ethernet, but  they  have different
   electrical  characteristics (impedance)  and  can't  be  mixed  and
   matched  freely.   Some manufacturer's  hubs and concentrator cards
   can be  bought that will speak to either type of cable, so  you CAN
   hook them together in a manner.

Q: What exactly means 10Base5, 10BaseT, 10Base2, 10Broad36, etc.
A: The "10" stands for signalling speed: 10MHz. "Base" means Baseband,
   "broad" means  broadband.  Initially, the last section as  intended
   to indicate  the maximum  length  of an  unrepeated  cable segment.
   This convention  was modified  with  the  introduction  of 10BaseT,
   where the T  means twisted pair,  and  10BaseF  where  the F  means
   fiber (see the  following  Q&A for specifics).  This actually comes
   from the IEEE committee number for that media.

   In actual practice:

     10Base-2   Is 10MHz  Ethernet running  over thin,  baseband coax.
                10Base-2 is also commonly referred to as thin-Ethernet
                or Cheapernet.
     10Base-5	Is 10MHz Ethernet running over  standard (thick) base-
                band coax.
     10Base-F   Is 10MHz Ethernet running over fiber-optic cabling.
     10Base-T   Is 10MHz Ethernet  running  over  unshielded, twisted-
                pair cabling.

Q: Are there any restrictions on how Ethernet is cabled?
A: Yes, there are many, and they vary according to the media used.
   First of all, there are distance limitations:

     10Base-2   limited to 185 meters (607 ft) per unrepeated cable
                segment.
     10Base-5   limited to 500 meters (1,640 ft) per unrepeated cable
                segment.
     10Base-F   depends on the signaling technology and medium used
                but can go up to 2KM.
     10Base-T   generally accepted to have a maximum run of 100-150M,
                but is really based on signal loss in db's (11.5db
                maximum loss source to destination).

   Then there are limitations on  the number  of  repeaters and  cable
   segments  allowed  on a single network.  There may be no  more than
   five (5) repeated segments, nor more than four (4) repeaters on any
   Ethernet; and  of the five  cable segments, only three  (3)  may be
   populated.  This is referred to as the "5-4-3" rule (5  segments, 4
   repeaters, 3 populated segments).  It can really get messy when you
   start cascading through  10Base-T  hubs, which  are repeaters  unto
   themselves.  Just  try to  remember, that any possible path between
   two network devices on an  unbridged/unrouted  network  cannot pass
   through more than 4  repeaters  or hubs,  nor more than 3 populated
   cable segments.

   Finally, 10Base-2 is limited to a maximum of 30 network devices per
   unrepeated network segment with a minimum distance  of 0.5m (1.5ft)
   between T-connectors.  10Base-5 is limited  to  a  maximum  of  100
   network devices per  unrepeated segment, with a minimum distance of
   2.5m  (8.2ft)  between  taps/T's  (usually  indicated by  a  marker
   stamped on the cable itself every 2.5m).

   I am not  aware of any theoretical  limit on the number of 10Base-T
   devices, and don't know the limitations  for  10Base-F  yet.   (Can
   someone fill-in the blanks?)

Q: What is 10Base-F?
A: 10Base-F is an IEEE standard for 10mbps  Ethernet  over fiber-optic
   cabling.  It defines  the methodology  and standard devices  which,
   ideally, can permit one  company's 10Base-F devices to interoperate
   with any others'.

Q: What means FOIRL?
A: Fiber Optic Inter Repeater Link. A  "IEEE 802 standard" worked  out
   between many vendors  some time ago  for carrying Ethernet  signals
   across long  distances  via fiber optic  cable.  It has since  been
   adapted  to  other applications  besides  connecting  segments  via
   repeaters  (you  can  get  FOIRL  cards  for  PCs).   It  has  been
   superseded by the larger 10Base-F standard.

Q: What about wireless LAN's? Are there any?
A:  Yes.   They  typically use  reflected  or  point-to-point infrared
    light, spread-spectrum RF or microwave RF transmission as as media.
    They are  typically expensive, slow (relative to Ethernet) and are
    not yet a mature technology.  There  are  special applications for
    light based (laser) repeaters.

Q: When should I choose 10BaseT, when 10Base2 (or others)?
A: The  specific environment and  application must be considered  when
   selecting your media type.  However, there  are some general rules-
   of-thumb that you can consider:

   Avoid using copper between buildings.   The electrical disturbances
   caused by lightning, as well as naturally  occurring differences in
   ground potential over distance, can very  quickly and easily  cause
   considerable  damage to equipment  and  people.  The use of  fiber-
   optic  cabling between buildings eliminates network  cabling  as  a
   safety risk.  There are also various  wireless  media available for
   inter-building links, such as laser, spread-spectrum  RF and micro-
   wave.  However,  wireless  media  is  much  more expensive and less
   reliable than fiber-optic, and should only be considered when it is
   impossible to get right-of-way for fiber-optic cable.

   10Base-2 (thin Ethernet or  Cheapernet)  is the least expensive way
   to  cable  an Ethernet  network.   However,  the  price  difference
   between 10Base-2  and  10Base-T  (Ethernet  over  UTP)  is  rapidly
   diminishing.   Still,  for  small,  budget-conscious  installations,
   10Base-2  is the most  economical  topology.  The  disadvantages of
   10Base-2  is that any break in  the cable  or poor  connection will
   bring  the entire network down,  and you need repeaters if you have
   more than 30 devices  connected to the network  or the cable length
   exceeds 185 meters (607 feet).

   10Base-5 is generally used as a low-cost alternative to fiber-optic
   media for use as a backbone segment within a single building.  It's
   extended  length  (500m  or 1640ft), higher  attached device  count
   (100) and better noise resistance make 10Base-5 well suited for use
   as a  network trunk for one or more floors in a building.  However,
   the high  cost of  connecting  each  device  (in  addition  to  the
   interface, you also need an  external transceiver,  or  MAU, and an
   AUI cable) makes 10Base-5 too expensive for most LAN installations,
   and like  10Base-2, a  single break  or bad connection in the cable
   can bring the entire network down.

   10Base-T is the most flexible topology for LANs,  and  is generally
   the best choice for most network installations.  10Base-T  hubs, or
   multi-hub  concentrators, are  typically  installed  in  a  central
   location to  the user community, and inexpensive UTP cabling is run
   to each network device (which may be 100m, or 330ft, from the hub).
   The signalling technology is very reliable,  even in somewhat noisy
   environments,  and 10Base-T hubs will  usually  detect many network
   error conditions  and automatically shut-down the offending port(s)
   without affecting the rest  of the network (unless,  of course, the
   offending  port was your  server, shared printer, or  router to the
   rest  of the world).  While  the  hardware  is more expensive  than
   10Base-2,  the  cabling  is  cheaper  and  requires less  skill  to
   install,  making 10Base-T installation costs  only  slightly higher
   than 10Base-2.  The flexibility  and  reliability  more than offset
   the marginally higher price.

   10Base-F, and  its predecessor,  FOIRL,  are  the  only recommended
   topologies for inter-building  links.   However,  they need  not be
   limited to  this role.  10Base-F can also be run  to  the  desktop,
   though  the  cost  is  prohibitively  high  in  all  but  the  most
   specialized environments (generally, extremely noisy  manufacturing
   facilities,   or  very  security-conscious   installations).   More
   commonly, FOIRL  (and now,  10Base-F) is  used inside buildings  to
   form backbone networks and to connect wiring closets together.

Q: What are the advantages/disadvantages of a star like cabling?
A: Old  style  Ethernet  bus  wiring  (ie, taking the  cable from  one
   machine to the next, and then  to the next, etc) is prone  to cable
   failure and  quickly consumes allowed  distances  due to  aesthetic
   wiring needs. If the wiring connection  is broken at any point, the
   entire network  (segment) fails  - and the much greater  number  of
   connections increases the probability of a failure or break. On the
   other hand, it's  pretty  easy to do for a layman  and  may involve
   less actual wiring for small segments.

   Star  wiring  eliminates  the single point  of failure  of a common
   wire. A central hub has  many connections that radiate out to hosts,
   if  one of these hosts connections fails it  usually doesn't affect
   the others.  Obviously, however, the hub becomes a central point of
   failure itself, but  studies show a  quality hub is less likely  to
   fail before a heavily used strand of coax.

   There are a bunch of other reasons hubs are desirable, but this is
   the biggie.

Q: Is there an official "standard" punch down scheme for 10BaseT?
A: Get a  copy  of EIA-568,  it  covers  all of  that  sort  of stuff:
   horizontal, vertical, connectors, patch cords, cross-connects, etc.

Q: Is it safe to run Unshield Twisted Pair  next to power cable (it is
   shielded)?
A: According to EIA/TIA-569, the standard wiring practices for running
   data cabling and companion to the above referenced EIA/TIA-568, you
   should not run data  cable parallel  to power cables.  However,  in
   reality,  this  should  not be  a  problem  with networks  such  as
   10Base-T.  10Base-T uses differential signalling to  pick  the data
   signals off  the wire.  Since any  interference  from  nearby power
   lines will usually affect  all pairs equally, anything  that is not
   canceled-out by  the  twists in  the UTP should be  ignored by the
   receiving network interface.

Q: Why has the MAC address to be unique?
A: Each  card has a unique MAC  address,  so that it  will be able  to
   exclusively  grab  packets  off  the  wire  meant  for it.  If  MAC
   addresses are  not  unique, there is no way to distinguish  between
   two stations.   Devices on  the network watch  network traffic  and
   look for their own MAC address in each packet to  determine whether
   they  should  decode  it or  not.  Special  circumstances exist for
   broadcasting to every device.

Q: Is there a special numbering scheme for MAC addresses?

A: The MAC  addresses  are exactly 6 bytes  in length, and are usually
   written in hexadecimal  as  12:34:56:78:90:AB (the  colons  may  be
   omitted, but  generally  make  the  address  more  readable).  Each
   manufacturer of Ethernet devices applies for a certain range of MAC
   addresses  they  can  use.   The  first three bytes of the  address
   determine  the  manufacturer.   RFC-1340 (available via FTP)  lists
   some of the manufacturer-assigned MAC addresses.

Q: What is a "segment"?
A: A  piece of  wire bounded by bridges, routers, or terminators. Some
   people  consider  wires  on  either  side  of a  repeater  separate
   segments, but they aren't really.

Q: What is a "subnet"?
A: Another  overloaded term. It can  mean, depending  on the usage,  a
   segment, a set of machines grouped together by a specific  protocol
   feature  (note that these machines  do not have  to  be on the same
   segment, but  they could be) or a big nylon  thing  used to capture
   soviet subs.

Q: What is a fan-out? Is this device still used?
A: Fanout  (a.k.a  transceiver multiplexor,  a.k.a.  multiport  trans-
   ceiver,  a.k.a.  DELNI) allows multiple  stations  to connect to  a
   single transceiver  or  transceiver-like  device.   They are  still
   widely used.

Q: What means "AUI"?
A: Attachment Unit Interface, an IEEE term for  the connection between
   a controller and the transceiver.

Q: What is a transceiver?
A: A  transceiver allows a station to transmit and receive to/from the
   common medium. In addition, Ethernet transceivers detect collisions
   on the medium and provide electrical isolation between stations.

Q: What means "MAU"?
A: Medium  Access  Unit, an  IEEE term for a transceiver.  MAU is also
   commonly [mis]used  to describe  a Token-Ring  Multi-Station Access
   Unit (MSAU).  Refer to HUB for an explanation of MSAU.

Q: What exactly does a repeater?
A: A  repeater  acts  on a  purely  electrical  level  to  connect  to
   segments. All it does is amplify and reshape (and, depending on the
   type,  possibly  retime)  the  analog  waveform  to extend  network
   segment distances. It does  not  know anything about  addresses  or
   forwarding,  thus it cannot be used to reduce traffic  as a  bridge
   can in the example above.

Q: What is a "HUB"?
A: A hub is a common wiring point for star-topology networks, and is a
   common synonym  for concentrator (though the latter  generally  has
   additional features or capabilities). Arcnet, 10Base-T Ethernet and
   10Base-F Ethernet and many proprietary network topologies  use hubs
   to connect multiple cable  runs in  a star-wired  network  topology
   into  a single  network.   Token-Ring MSAUs  (Multi-Station  Access
   Units) can  also  be considered a  type of  hub,  but  don't  let a
   token-ring  bigot hear  that.   Hubs have multiple ports  to attach
   the different cable  runs.  Some  hubs (such as 10Base-T and active
   ArcNet)  include electronics to  regenerate and  retime  the signal
   between each hub port.  Others (such as 10Base-F or passive Arcnet)
   simply  act as signal  splitters, similar to the multi-tap cable-TV
   splitters  you  might use on  your home  antenna coax  (of  course,
   10Base-F  uses  mirrors  to  split  the  signals  between  cables).
   Token-Ring  MSAUs use relays  (mechanical or electronic) to reroute
   the  network  signals to each active device  in series,  while  all
   other   hubs   redistribute   received   signals  out   all   ports
   simultaneously, just as a 10Base-2 multi-port repeater would.

Q: What exactly does a bridge?
A: A bridge will connect to distinct segments  (usually referring to a
   physical length  of wire) and transmit traffic between  them.  This
   allows you to extend the  maximum size of  the network  while still
   not  breaking the maximum  wire  length,  attached device count, or
   number of repeaters for a network segment.

Q: What does a "learning bridge"?  
A: A  learning bridge monitors  MAC  (OSI layer  2) addresses  on both
   sides of its  connection and  attempts to learn which addresses are
   on which  side.   It  can  then decide when it  receives  a  packet
   whether it should cross the  bridge or stay local (some packets may
   not need to  cross the bridge  because the source  and  destination
   addresses are both on one side).   If  the bridge receives a packet
   that  it  doesn't  know  the addresses of, it  will  forward it  by
   default.

Q: What is a remote bridge?
A: A  bridge as described  above that has  an Ethernet (or token-ring)
   interface  on  one side and  a  serial interface on the  other.  It
   would connect to a  similar device on  the other side of the serial
   line.  Most commonly  used in  WAN links where it  is impossible or
   impractical  to install network cables.  A high-speed modem  (or T1
   DSU/CSU's, X.25 PAD's,  etc)  and  intervening  telephone  lines or
   public data network would be used to connect the two remote bridges
   together.

Q: What exactly does a router?  

A: Routers work much like bridges, but they pay attention to the upper
   network layer protocols  (OSI  layer  3) rather than physical layer
   (OSI layer 1) protocols.  A router will decide whether to forward a
   packet by looking at  the  protocol level  addresses (for instance,
   TCP/IP addresses) rather than  the  MAC  address.  Because  routers
   work  at layer 3 of  the  OSI stack, it  is possible  for  them  to
   transfer packets between different media types (i.e., leased lines,
   Ethernet, token ring,  X.25, Frame Relay  and FDDI).  Many  routers
   can  also  function as bridges.  Routing would always be preferable
   to bridging except for the fact that routers are slower and usually
   more  expensive (due  to the amount of processing required  to look
   inside  the  physical packet  and  determine  which interface  that
   packet needs to get sent out).
   
Q: So should I use a router or a bridge?
A: There is no absolute answer to this.  Your network layout, type and
   amount of hosts and traffic, and  other issues (both  technical and
   non-technical) must be considered.  The following are the pros  and
   cons of each:

     Routing:
       + Can route between different media (although  FDDI to Ethernet
         bridges  are becoming common  via  the  Translation  Bridging
         standard).
       + There is isolation of  Multicast &  Broadcast  packets at the
         MAC layer which helps to reduce broadcast storms.
       + Can run multiple active paths between sites in a mesh network
         to use  links  efficiently  (bridging  uses spanning tree  to
         decide if a link is forwarding or in a back up state).
       + Takes  part in  higher  level  protocol  so can provide  more
         features (examples = logical zones in Appletalk, proxy ARP on
         IP).
       + Provide a clean  cut off when connecting  multiple management
         domains.
       + Only needs to know 'where  next?' and so  hides the detail of
         remote networks, whereas bridges  must  understand the  whole
         topology of the net.

     Bridging:
       + Much cheaper boxes.
       + Learning bridges virtually autoconfigure themselves.
       + Works with any protocol that conforms to the  MAC level spec.
         some protocols such as DEC LAT & MOP can only be bridged.
       + Within a site uses IP  address space more  efficiently whilst
         providing some traffic segregation (address space is becoming
         a real scarce resource!).
       + Bridges  are  generally less complex devices,  which  usually
         translates to higher reliability.
       + Easy  inter-vendor working via spanning tree standard (802.1d
         or DEC STP)

Q: Are there problems mixing Bridging & routing?
A: You should be very careful about running bridges providing links in
   parallel to a  router. Bridges may forward broadcast requests which
   will confuse the router  there are lots of  protocols you  may  not
   think  of  filtering (e.g. ARP,  Apple ARP over  802.3  etc. etc.).
   Also, DECnet routers have the  same MAC address on all ports.  This
   will  probably cause the  bridge to think it  is seeing an Ethernet
   loop.

Q: What is a Kalpana EtherSwitch?
A: A device that  works  sort of like  a  bridge,  but off a different
   principle.   It's advantages are that it is extremely fast  and can
   "bridge" more than one packet at a time (it  is not limited  to two
   interfaces  as a  traditional bridge is). Disadvantages are that it
   does not understand spanning  tree and doesn't work well in many to
   one networks. You probably don't understand that, so ignore it.

Q: What is a driver?
A: Typically  the software  that allows an Ethernet card in a computer
   to decode packets and send them to the operating  system and encode
   data from the operating  system for  transmission  by  the Ethernet
   card through the  network.  By  handling the nitty-gritty  hardware
   interface chores, it provides a device-independent interface to the
   upper layer  protocols,  thereby  making  them  more universal  and
   [allegedly]  easier  to  develop  and  use.  There  are many  other
   meanings to this word,  but this is probably  what  you are looking
   for.

Q: What is NDIS, packet driver, ODI.?
A: NDIS is a Microsoft/3com puppy  that  allows "stacking" of multiple
   protocols for a single underlying driver.  Essentially  it allows a
   single  Ethernet card in  a PC  (it's not limited  to  Ethernet) to
   speak many different  network "languages", and usually  at the same
   time.

   A packet driver is another method of allowing multiple protocols to
   access  the network interface  at the  same  time.   Developed  and
   supported  by FTP  Software Inc, Clarkson University, BYU and, more
   recently, Crynwr Software, the packet driver spec  (PDS) is used to
   provide a device independent interface  to  various TCP/IP applica-
   tions,  and often  in  combination  with  concurrent Novell  access
   (IPX/SPX).

   ODI is Novell and Apple's equivalent of  NDIS.   There are  differ-
   ences between the two specs, but not so much as to warrant descrip-
   tion in this text.

   The next logical question is "which one should I use?"  There is no
   simple or obvious answer, except that you should  use the one  most
   commonly required by your software.

Q: Is there a troubleshooting guide for Ethernet?
A: Many. I suggest you check your local technical bookstore.
   (Recommendations needed)

Q: What books are good about Ethernet LAN's?
A: There are  many.  The following are recommended  by readers on this
   list:

   "The  Ethernet  Management  Guide  - Keeping  the Link"  by  Martin
   Nemzow.   This  book  has  good coverage  of  most  of  the average
   considerations  of Ethernet, from what  Manchester encoding is down
   to production segment traffic analysis.

Q: Where can I get IEEE803.x docs online?
A: Nowhere.  IEEE documents must be ordered  from the IEEE themselves.
   You can contact them at:

     Institute of Electrical and Electronic Engineers
     445 Hoes Lane
     P.O. Box 1331
     Piscataway, NJ 08855-1331
     U.S.A.
     (800) 678-IEEE

Q: Where can I get EIA/TIA docs online?
A: Nowhere?  Must be ordered from...?

Q: Where can I find the specifications of Ethernet equipment?
A: From the manufacturer of the product, probably.

Q: Where can I find IETF (Internet Engineering Task Force) documents?
A: These are available for anonymous FTP from a number of sites.  One
   known  location is athos.rutgers.edu in /ietf.  Drafts are also on
   athos in /internet-drafts.

-- 
_________________________________________________________________________
RUCS     | Mark A. Medici, Systems Programmer III, User Services Division
User     | Rutgers University Computing Services, New Brunswick, NJ 08903
Services | [medici@gandalf.rutgers.edu]                    [908-932-2412]
Newsgroups: comp.dcom.lans.ethernet,comp.sys.ibm.pc.hardware