From: mrunkel@steph.admin.umass.edu (Marc A. Runkel)
Newsgroups: comp.dcom.lans.ethernet,comp.answers,news.answers
Subject: comp.dcom.lans.ethernet FAQ (Frequently Asked Questions)
Date: 13 Mar 1995 16:35:01 GMT
Organization: University of Massachusetts, Amherst
Message-ID: <3k1s7l$6p@nic.umass.edu>
Reply-To: Ethernet FAQ Coordinators <enet-faq@steph.admin.umass.edu>
    01: Introduction,  contributors, how  to  contribute  to  the  FAQ  and
    01: Introduction,  contributors, how  to  contribute  to  the  FAQ  and
        network etiquette.

    02: General information about Ethernet and standards.

    03: Ethernet Cabling Information.

    04: Ethernet Devices and Components.

    05: Errors and Related Terms.

    06: Testing and Troubleshooting.

    07: Additional Information.


01.01Q: What is this document?
     A: This  is  the official FAQ (Frequently Asked Questions) listing for
        UseNet newsgroup comp.dcom.lans.ethernet.   It is intended to  be a
        reference  to the most commonly asked questions and basic  informa-
        tion about Ethernet.

01.02Q: How is this document made available?
     A: This FAQ  is posted  monthly  to newsgroup comp.dcom.lans.ethernet,
        comp.answers  and  news.answers on UseNet.   You can  also retrieve
        this file via anonymous FTP from steph.admin.umass.edu 
        (128.119.200.10) in pub/faqs as file ethernet.faq

01.03Q: Who maintains this list?
     A: This  list  is  currently  maintained by Marc Runkel.  My preferred
        mail address is enet-faq@steph.admin.umass.edu, and I would greatly
        appreciate  it  if  you  could  use  a  Subject:  line  that starts
        "Ethernet FAQ".

01.04Q: Where does all this information come from?
     A: The questions and answers are  mostly  summarized from  the  UseNet
        newsgroup comp.dcom.lans.ethernet, many of  which are  derived from
        various  IEEE,  ISO  and   EIA/TIA  documents.   Specifically,  the
        following persons have contributed  to this FAQ.   Their  knowledge
        and experience is gratefully acknowledged.

            Doug Barr          <barr@spot.colorado.edu>
            John Breeden       <johnbr@master.cna.tek.com>
            TP Brisco          <brisco@rutgers.edu>
            Peter Desnoyers    <peterd@merlin.dev.cdx.mot.com>
            Daniel Huber       <danielh@hpber199.swiss.hp.com>
            Bob Jaques         <jaques@drbob.corp.sgi.com>
            Paul Joslin        <pjoslin@mbvlab.wpafb.af.mil>
            Dave Kapalko       <medik@attme.att.com>
            Rich Lawrence      <rich@grebyn.com>
            Nick Hennenfent    <nicholas@cton.com>
            Ray Hunter         <rhunter@esoc.BITNET>
            Mark Medici        <medici@gandalf.rutgers.edu>
            Dave Mitton        <???@dec.com?>
            Rich Seifert       <seifert@netcom.com>
            Charles Spurgeon   <C.Spurgeon@utexas.edu>
	    Andras Salamon     <andras@is.co.za)

         (Note: If you have contributed something to this FAQ but your name
                is not listed  above,  please take  no offense.  There  was
                some loss of  information in this  file a while back.  Just
                mail the current FAQ maintainer  your preferred  name  and
                mail address, and which section(s) you contributed).

01.05Q: How can I submit new contributions or corrections to the FAQ?
     A: New contributions, suggestions and corrections should  be mailed to
        the  current  FAQ  maintainer,  who is  listed in  Q&A 01.03 above.
        However,  you should note that  if you are  submitting a correction
        you must provide both  the  old  and suggested new text -- messages
        to the effect of "this is wrong, fix it" will be ignored.

01.06Q: Are there any restrictions on the distribution of this FAQ?
     A: You  may  freely distribute  this  document for non-commercial
        purposes as  long as the contents remain  unchanged (including
        credits)  and  you  do not  gain  any  direct profits from the
        distribution.

01.07Q: Are there any guidelines regarding postings on this newsgroup?
     A: The standard UseNet guidelines apply to this newsgroup.  Explaining
        these guidelines in detail requires a FAQ of  its own.  If you  are
        not familiar with standard network etiquette, you should review the
        documents posted regularly in the newsgroup  news.announce.newuser.
        A quick summary is:

        *  Never forget that the person on the other side is human
        *  Be careful what you say about others
        *  Be brief
        *  Your postings reflect upon you; be proud of them
        *  Use descriptive titles
        *  Think about your audience
        *  Be careful with humor and sarcasm
        *  Only post a message once
        *  Summarize what you are following up
        *  Use mail, don't post a follow-up unless it's of value to many
        *  Read all follow-ups and don't repeat what has already been said
        *  Double-check follow-up newsgroups and distributions.
        *  Be careful about copyrights and licenses
        *  Cite appropriate references
        *  When summarizing, summarize
        *  Spelling flames considered harmful
        *  Don't overdo signatures
        *  Limit line length and avoid control characters
        *  Identify any financial interests related to your posting

01.08Q: Are the vendors and/or models of equipment listed in this FAQ the
        only or best suited for the application described?
     A: Not  necessarily.  This document does not attempt to rate equipment
        from different manufacturers, and does not endorse nor specifically
        support any one vendor's product over another.  Any references to a
        specific vendor or product is implicitly used as an example of  all
        like devices.


02.01Q: What is Ethernet?
     A: Ethernet is a type of network cabling and signalling specifications
        (OSI  Model  layers  1 [physical]  and  2  [data link])  originally
        developed by Xerox in  the late 1970.   In  1980, Digital Equipment
        Corp.  (DEC), Intel and Xerox (the origin of  the  term DIX,  as in
        DEC/Intel/Xerox)  began joint promotion  of this baseband,  CSMA/CD
        computer communications network over coaxial cabling, and published
        the "Blue Book Standard" for Ethernet Version 1.  This standard was
        later enhanced, and in 1985 Ethernet II was released.

        The  IEEE's  (Institute of  Electrical  and Electronics Engineers')
        Project 802 then  (after considerable debate) used Ethernet Version
        2  as the basis for the  802.3 CSMA/CD network  standard.  The IEEE
        802.3 standard  is generally interchangeable with Ethernet II, with
        the  greatest  difference  being the  construction of  the  network
        packet header.

        A complete description of all Ethernet  specifications is far  out-
        side the  scope of this document.  If this area interests you,  you
        are  encouraged  to obtain  (hopefully legally) copies of the  IEEE
        802.3 documents, and perhaps the ISO 8802-3 documents as well.

02.03Q: What is an 802.3 network?
     A: That's  IEEE-ish  for Ethernet, but  with a few small  differences.
        The  physical  layer  specifications  are  identical  (though   DIX
        Ethernet never  specified standards for UTP and Fiber-Optic  media)
        and the MAC sublayer  are somewhat different.  See  "What is Ether-
        net for more info.

02.02Q: What is CSMA/CD?
     A: CSMA/CD is the media access  control mechanism used by Ethernet and
        802.3 networks; in other words, it determines how a  packet of data
        is placed on the wire.   CSMA/CD stands for "Carrier Sense Multiple
        Access, with Collision Detection".  Before  an Ethernet device puts
        a  packet "on  the wire",  it listens to find if another device  is
        already transmitting.  Once the device finds the wire is clear,  it
        starts sending the  packet while  also listening to hear if another
        device  started  sending  at  the  same  time (which  is  called  a
        collision).  Refer  to the  Q&A  on collisions for more  info about
        this phenomena.

02.03Q: What is a baseband network?
     A: A baseband  network  is  one  that provides  a  single  channel for
        communciations accross the  physical medium (e.g., cable),  so only
        one device  can transmit at a time.  Devices on a baseband network,
        such  as Ethernet, are permitted to use all the available bandwidth
        for  transmission, and the signals they transmit do not  need to be
        multiplexed onto a carrier frequency.  An analogy is a single phone
        line such  as you usually  have  to your house: Only one person can
        talk at a time--if more than one  person wants to talk everyone has
        to take turns.

02.04Q: Ok, so what is a broadband network?
     A: Simplisticly,  it is  the  opposite  of  a baseband network.   With
        broadband, the physical  cabling is virtually  divided into several
        different channels, each with  its own  unique  carrier  frequency,
        using  a technique called "frequency  division  modulation".  These
        different frequencies are  multiplexed  onto the network cabling in
        such a way to allow multiple  simultaneous "conversations"  to take
        place.  The  effect is  similar to having several virtual  networks
        traversing a single piece of wire.  Network devices "tuned" to  one
        frequency  can't  hear  the  "signal"  on  other  frequencies,  and
        visa-versa.   Cable-TV  is  an  example  of  a  broadband  network:
        multiple  conversations  (channels)  are transmitted simultaneously
        over a single  cable; you  pick  which one you want to listen to by
        selecting one of the frequencies being broadcast.

02.05Q: What is an OSI Model?
     A:  The  Open  Systems  Interconnect (OSI)  reference model is the ISO
        (International  Standards Organization) structure for  the  "ideal"
        network  architecture.  This Model outlines seven areas, or layers,
        for the network.  These layers are (from highest to lowest):

          7.) Applications:  Where the  user  applications  software  lies.
              Such  issues as  file  access and transfer,  virtual terminal
              emulation,   interprocess  communication  and  the  like  are
              handled here.
          6.) Presentation:  Differences in data  representation are  dealt
              with at this level.  For example, UNIX-style line endings (CR
              only) might be converted to  MS-DOS style  (CRLF), or EBCIDIC
              to ASCII character sets.
          5.) Session:  Communications between  applications  across a net-
              work  is  controlled  at  the  session  layer.   Testing  for
              out-of-sequence  packets  and handling two-way  communication
              are handled here.
          4.) Transport: Makes sure  the lower three layers are doing their
              job  correctly,  and  provides a  transparent,  logical  data
              stream between the end user  and the network  service s/he is
              using.   This  is the lower layer  that provides  local  user
              services.
          3.) Network: This layer makes certain that a packet sent from one
              device  to another actually gets there in a reasonable period
              of time.  Routing and flow control are performed  here.  This
              is the lowest layer of the OSI model that can remain ignorant
              of the physical network.
          2.) Data Link: This layer deals with getting data packets on  and
              off   the   wire,   error  detection   and   correction   and
              retransmission.  This  layer  is generally  broken  into  two
              sub-layers: The LLC (Logical Link Control) on the upper half,
              which does  the error checking,  and  the MAC (Medium  Access
              Control) on the lower half, which deals with getting the data
              on and off the wire.
          1.) Physical: The nuts and bolts layer.  Here is where the cable,
              connector and signaling specifications are defined.

        There is also the undocumented but widely recognized  ninth network
        layer:

          9.) Bozone (a.k.a., loose nut behind the wheel): The user sitting
              at and using (or abusing,  as  the case may be) the networked
              device.  All the error detection/correction algorithms in the
              world cannot protect your network from the problems initiated
              at the Bozone layer.

02.06Q: What does an ethernet packet look like?  
     A. See the information below, as described  in the National  databook.
        The ethernet packet preamble is normally generated by  the chipset.
        Software  is  responsible  for  the  destiantion   address,  source
        address, type, and  data.  The chips normally will append the frame
        check sequence.

          +------------+
          |            | Preamble -
          |   62 bits  |  A series of alternating 1's and 0's used by the
          |            |  ethernet receiver to acquire bit synchronization.
          |            |  This is generated by the chip.
          +------------+
          |            |  Start Of Frame Delimiter -
          |    2 bits  |  Two consecutive 1 bits used to acquire byte
          |            |  alignment. This is generated by the chip.
          +------------+
          +------------+
          |            |  Destination Ethernet Address -
          |    6 bytes |  Address of the intended receiver.
          |            |  The broadcast address is all 1's.
          +------------+
          |            |  Source Ethernet Address -
          |    6 bytes |  The unique ethernet address of the sending
          |            |  station. 
          +------------+
          |            |  Length or Type field -
          |    2 bytes |  For IEEE 802.3 this is the number of bytes of
          |            |  data.  For Ethernet I&II this is the type of
          |            |  packet.  Types codes are > 1500 to allow both to
          |            |  coexist.  The type code for IP packets is 0x800.
          +------------+
          |   46 bytes |  Data -
          |      to    |  Short packets must be padded to 46 bytes.
          | 1500 bytes |
          +------------+
          +------------+
          |            |  Frame Check Sequence -
          |    4 bytes |  The FCS is a 32 bit CRC calculated using
          |            |  the AUTODIN II polynomial.
          |            |  This field is normally generated by the chip.
          +------------+

        The shortest packet is: 6 + 6 + 2 + 46 = 60 bytes
        The longest packet is: 6 + 6 + 2 + 1500 = 1514 bytes

02.07Q: 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.

02.08Q: What is SNAP
     A:  Sub-Network Access Protocol,  an extention to the  original  802.2
        data link level format.  (SNAP is described in  IEEE 802-1990)  The
        802.2  data link format replaced the Ethernet Protocol Type concept
        with  two   8  bit   fields;   Source  SAP,  and  Destination  SAP.
        Unfortunately that causes problems with migration of protocols, and
        the lack  of SAP space  that is available.  So one SAP as allocated
        for this scheme which greatly expands the available protocol space.
        When using  the  SNAP SAP  the first 5  bytes of data are used as a
        protocol ID.  The first 3 bytes should be a  value allocated to you
        as a vendor id, the same as you get for Source address values.  The
        is called the OUI (Organizationally  Unique ID) The  second 2 bytes
        is a protocol type.

        Note that this is 802.2 and applies across all 802 LAN media types.

        For translation bridging, there is a convention, if you set the OUI
        to zero, you are representing a mapped Ethernet frame.  So  that  a
        bridge will translate such a frame back  into Ethernet  format, and
        not into an 802.3 frame format.

        802.2 SNAP frame:

            +-------+------+------+------+-------+------+------+
            | MAC   | DSAP | SSAP |  UI  |  OUI  | Type | data |
            | Header| 0xAA | 0xAA | 0x03 | 3bytes|2bytes|      |
            +-------+------+------+------+-------+------+------+

        This  will  appear the  same  on all 802 compliant  LAN  media.  On
        802.3, there will be a Length field between the SA and the DSAP but
        not on 802.5 or FDDI.

02.09Q: Where  can I find  out which  Protocols  use  which  Ethernet  type
        numbers?
     A: Look at IETF RFC-1700 - Assigned Numbers RFC.

02.10Q: What is a MAC address?
     A: It is  the unique hexadecimal serial number assigned to each Ether-
        net  network device to identify it  on  the network.  With Ethernet
        devices  (as  with  most other  network  types),  this  address  is
        permanently set at  the time of manufacturer, though it can usually
        be changed  through  software (though this is  generally a Very Bad
        Thing to do).

02.11Q: Why must 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.

02.12Q: 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-1700 (available  via  FTP)  lists
        some of the manufacturer-assigned MAC addresses.  A more up-to-date
        listing  of  vendor  MAC   address   assignments  is  available  on
        ftp.lcs.mit.edu in pub/map/Ethernet-codes.

02.13Q: What is a preamble ?
     A: A seven octet  field of  alternating one  and zero binary bits sent
        prior to each frame to allow the  PLS circuitry to reach its steady
        state synchronization with received  frame timing. (802.3 standard,
        page 24,42).

02.14Q: What is a Start Frame Delimiter (SFD)?
     A: A binary  sequence of '10101011' immediately following the preamble
        and indicating  the  beginning of a frame.   (802.3  standard, page
        24).

02.15Q: What does CRC mean?
     A: Cyclical Redundancy Check  -  A method  of  detecting  errors in  a
        message by performing a mathematical calculation on the bits in the
        message and then sending the results  of the calculation along with
        the   message.   The  receiving  work-station  performs   the  same
        calculation on the message  data  as it receives it and then checks
        the results against those  transmitted  at the end of  the message.
        If the results don't match, the receiving  end asks the sending end
        to send again.

02.13Q: What is a broadcast address?
     A: The unique  address that identifies a packet as  appropriate to all
        receiveing stations.  In 802.3 any address in which the second byte
        is an odd number. (1,3,...F).

02.14Q: What exactly do 10Base5, 10BaseT, 10Base2, 10Broad36, etc mean?
     A: These are  the IEEE  names  for  the different  physical  types  of
        Ethernet.   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 in hundreds  of meters.  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:

            10Base2   Is 10MHz Ethernet running over  thin, 50 Ohm baseband
                      coaxial cable.  10Base2 is also commonly referred to
                      as thin-Ethernet or Cheapernet.
            10Base5   Is 10MHz  Ethernet running over  standard  (thick) 50
                      Ohm baseband coaxial cabling.
            10BaseF   Is 10MHz Ethernet running over fiber-optic cabling.
            10BaseT   Is 10MHz Ethernet  running  over unshielded, twisted-
                      pair cabling.
            10Broad36 Is 10MHz Ethernet running through a broadband cable.

02.15Q: What does FOIRL mean?
     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 10BaseF standard.

02.16Q: What is LattisNet?
     A: LattisNet is a pre-10BaseT quasi-standard for running Ethernet over
        twisted-pair cabling.   It was developed by Synoptics,  and several
        other vendors made compatible equipment for a while.  LattisNet  is
        not compatible with 10BaseT,  but you can  have LattisNet  hubs and
        10BaseT hubs  in the  same  hub  chassis  or  connected to the same
        network backbone.  The primary difference is that 10BaseT synchron-
        izes the signals at  the sending  end, while LattisNet syncrhonizes
        at the receiving end.

02.17Q: What is StarLAN-10?
     A: StarLAN-10 is AT&T's variety of Ethernet over twisted-pair cabling.
        Older StarLAN-10  is not  100% 10BaseT  compliant, as  it does  not
        provide  link   integrity  to   the  AUI.   However,  many  10BaseT
        interfaces  can   be  configured  to  work  with  StarLAN-10  hubs,
        alongside  StarLAN-10  NICs.  Beware,  though,  that  the  original
        StarLAN-10 is NOT in any  way  compatible with 10BaseT, and  worse,
        there seems to be no way to tell  other than  trying it to see what
        happens.

        The current StarLAN  products supported by AT&T/NCR are fully 802.3
        compliant. This includes the  SmartHUB  model E, SmartHUB  model B,
        SmartHUB XE, and the other fiber and wire SmartHUB models.


03.01Q: What is coax?
     A: Coaxial cable (coax)  is a  metallic electrical  cable used  for RF
        (radio  frequency) and  certain data  communications  transmission.
        The cable is constructed with a  single  solid  or stranded  center
        conductor that is surrounded by the dielectric layer, an insulating
        material of  constant thickness and high resistance.  A  conducting
        layer of aluminum foil, metallic braid  or a combination of the two
        encompass  the  dielectric  and  act  as  both   a  shield  against
        interference  (to or  from the  center conductor) and as the return
        ground for the cable.  Finally, an  overall  insulating layer forms
        the  outer  jacket  of  the  cable.   Coaxial  cable  is  generally
        superior  in   high-frequency  applications  such  as   networking.
        However, for shorter distances (up to 100 meters), UTP or STP cable
        is generally  just  as reliable when using  differential modulation
        techniques (such as with 10BaseT).

        There are three  types of RG-58 cable, as far as I can tell.  There
        are probably other  subtle differences, but for  10BASE2, impedance
        and  velocity of propagation are the  important  ones.   The  table
        below summarizes:

		Cable		Impedance		Velocity
                ----------      ----------              --------------
		RG-58A/U	50 ohms			.66 or .78
		RG-58C/U	50 ohms			.66
		RG-58/U		53.5 ohms		.66 or .695

03.02Q: What is UTP, STP?
     A: Twisted pair cables.  UTP is for  UNshielded,  twisted pair,  while
        STP  is  for  SHIELDED,  twisted  pair.   UTP  is what's  typically
        installed  by  phone companies (though  this is often not  of  high
        enough quality  for  high- speed network  use)  and is what 10BaseT
        Ethernet runs over.  UTP  is graded according  to its data carrying
        ability  (e.g.,  Level 3,  Level  4, Level  5).   10BaseT  Ethernet
        requires  at  least  Level  3 cable.  Many  sites now install  only
        Level-5  UTP,  even  though  level  4 is more  than  sufficient for
        10BaseT, because of the greater likelihood that emerging high-speed
        standards will require cable with better bandwidth capabilities.

        STP is typically used for Token-Ring networks, where it is commonly
        referred  to  IBM Type 1 (or  2, 3, 6,  8,  etc); however there are
        several  manufacturers of  Ethernet equipment and  interfaces  that
        support Ethernet  over STP.  Nevertheless, Ethernet over STP is not
        officially  defined in any  standards.  While there is a good level
        of interoperability with Ethernet over  STP,  (Lattisnet, developed
        by  Synoptics,  is the recognized de facto standard in this  area),
        one  should consider the  long-term  availability and cost of  this
        non-standard scheme before planning new networks around it.

03.03Q: 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:

            10Base2   limited to 185  meters (607 ft)  per unrepeated cable
                      segment.
            10Base5   limited to 500 meters (1,640 ft) per unrepeated cable
                      segment.
            10BaseF   depends on the  signaling technology  and medium used
                      but can go up to 2KM.
            10BaseT   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).
            10Broad36 limited to 3,600 meters (almost 2.25 miles).

        Then there are limitations on the number  of  repeaters  and  cable
        segments allowed between  any two stations on  the  network.  There
        are two different ways of looking at the same rules:

        1. The Ethernet way:

            A remote  repeater pair  (with  an  intermediate point-to-point
            link) is counted  as  a single  repeater  (IEEE  calls  it  two
            repeaters).  You cannot  put any stations on the point to point
            link (by definition!),  and there can be two repeaters  in  the
            path between any pair  of stations.   This  seems simpler to me
            than the IEEE terminology, and is equivalent.

        2. The IEEE way:

            There may be no more than five (5)  repeated segments, nor more
            than four (4) repeaters  between any two Ethernet stations; 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 10BaseT
        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, 10Base2  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.   10Base5  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).  10BaseT and 10BaseF  are
        star-wired,  so there  is no  minimum  distance requirement between
        devices, since  devices  cannot  be connected  serially.   You  can
        install  up to the  Ethernet maximum of  1024 stations per  network
        with both 10BaseT and 10BaseF.
   

03.04Q: Can I mix 10Base2 and 10Base5 cabling on a single segment?
     A: It  is not "legal", but  the network police will not read  you your
        rights and drag  you away.   Ideally, you should use a repeater (or
        bridge,  router, etc...)   between  the  different  cabling  types.
        However, in reality, it  will work fine, as  long  as  none  of the
        other network parameters (lengths, numbers of stations,  repeaters,
        etc) are near the limit of the specification.

03.05Q: What about wireless Ethernets? Are there any?
     A: Yes, and no.  Many  vendors  offer equipment for  Ethernet across a
        variety  of   unbounded,  or wireless,  connections  using  lasers,
        microwaves, and spread-spectrum radio transmissions.  However, none
        of these  methods are organized by  any standards  body, so  it  is
        unlikely  to find equipment  from any two  different  manufacturers
        that work together.

03.06Q: 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
        microwave.  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.

        10Base2 (thin Ethernet or Cheapernet) is the least expensive way to
        cable an Ethernet network.   However, the price difference  between
        10Base2 and  10BaseT  (Ethernet  over  UTP) is rapidly diminishing.
        Still,  for small, budget-conscious installations, 10Base2  is  the
        most economical topology.  The disadvantages of 10Base2 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).

        10Base5 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 10Base5 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 10Base5 too expensive  for most LAN installations,
        and like 10Base2, a single break or bad connection in the cable can
        bring the entire network down.

        10BaseT is the most  flexible topology  for LANs,  and is generally
        the  best choice for most  network installations.  10BaseT 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 10BaseT  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
        10Base2, the cabling is cheaper and requires less skill to install,
        making  10BaseT  installation  costs   only  slightly  higher  than
        10Base2.  The  flexibility and  reliability  more  than offset  the
        marginally higher price.

        10BaseF, and  its  predecessor, FOIRL,  are  the  only  recommended
        topologies  for  inter-building  links.  However,  they need not be
        limited to  this  role.  10BaseF  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, 10BaseF) is used inside buildings to form
        backbone networks and to connect wiring closets together.


03.07Q: 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.

03.08Q: Is there an official "standard" punch down scheme for 10BaseT?
     A: Get a copy  of EIA/TIA-568, it  covers  all of  that sort of stuff:
        horizontal, vertical, connectors, patch cords, cross-connects, etc.

03.09Q: Is it safe to run Unshield Twisted Pair next to power cable?
     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
        10BaseT.   10BaseT 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.

03.10Q: Can I make a cable to connect the AUI ports of two devices directly
        to each other?
     A: Yes and  no.  You can make the  equivalent of a null modem cable by
        connecting a two-pair, twisted  pair cable  connecting pins 3/10 at
        each end to  pins 5/12 (respectively)  at the other.  This connects
        transmit-to receive (null modem).   However, this will probably NOT
        work with "standard" software because:

          - There is no  collision detect.  If  a collision  occurs neither
            device will back-off or and retry.
          - There  is  no  loopback  (stations   will  not  hear  their own
            transmissions which may cause diagnostics failures).
          - There is no heartbeat (SQE  test)  provided,  which  may  cause
            diagnostic failure.

        If you want to use standard software, buy some transceivers. An AUI
        null-modem  will work  for  a  laboratory, test  environment  under
        certain conditions.

03.11Q: Can  I  connect  the  10BaseT  interface  of  two devices  directly
        together, without using a hub?
     A: Yes, but  not more  than  2 devices, and you  also  need  a special
        jumper cable between the two 10BaseT ports:

          RJ45 pin                   RJ45 pin
          ========                   ========
             1 <--[TX+]--------[RX+]--> 3
             2 <--[TX-]--------[RX-]--> 6
             3 <--[RX+]--------[TX+]--> 1
             6 <--[RX-]--------[TX-]--> 2

03.12Q: Does my Ethernet coax have to be grounded?  How?
     A: Yes and  no.  The 10Base2 spec says the coax MAY be grounded at one
        and only  one point, while  the 10Base5 spec says the coax SHALL be
        grounded at one and only one point.

        Grounding  your coax  is  generally a  good  idea; it allows static
        electricity  to  bleed  off and,  supposedly,  makes  for  a  safer
        installation.  Further,  many local  electrical codes  will require
        your network cabling to be grounded at some point.  However, I have
        personally  seen many  Ethernet networks  work with  absolutely  NO
        ground  on the segment, and even  a few  unreliable segments become
        reliable when the one and only ground  was removed.  I'm not saying
        you  should not ground  your  networks  --  you  should  absolutely
        install cabling according to your electrical codes.

        On the other hand, if you do ground your cable, make sure you do so
        only  at  one point.  Multiple grounds on an Ethernet segment  will
        not  only cause  network errors,  but also risk damage to equipment
        and  injury to people.

        If you have a repeater on one end of the segment, this will usually
        automatically ground that end of the segment (you may want to check
        the repeater documentation and configuration  to assure this is the
        case -- most repeaters  can be set-up to NOT ground).  If you don't
        have  a repeater, you  can  get terminating resistors  with  ground
        straps attached.


04.01Q: What is a "segment"?
     A: A  piece of network wire bounded by bridges,  routers, repeaters or
        terminators.

04.02Q: 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
        enemy subs.

04.03Q: 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.

04.04Q: What means "AUI"?
     A: Attachment Unit Interface, an IEEE term  for the connection between
        a controller and the transceiver.

04.05Q: 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.
        10Base2 and 10Base5 transceivers  attach directly to the common bus
        media,  though the  former  usually  use  an  internal  transceiver
        built-onto the controller circuitry with a  "T" connector to access
        the  cable, while the latter use a  separate, external  transceiver
        and  an AUI (or  transceiver) cable  to connect to the  controller.
        10BaseF, 10BaseT and FOIRL also usually  use internal transceivers.
        Having  said  that,  there  also  also  external  transceivers  for
        10Base2, 10BaseF,  10BaseT and FOIRL that can connect externally to
        the controller's AUI port, either directly or via an AUI cable.

04.06Q: 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.

04.07Q: 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.

04.08Q: 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, 10BaseT Ethernet and
        10BaseF 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 10BaseT and active
        Arcnet)  include electronics to  regenerate and  retime  the signal
        between each hub port.  Others (such  as 10BaseF 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,
        10BaseF 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 10Base2 multi-port repeater would.
     
04.09Q: 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.

04.10Q: 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.

04.11Q: What is a remote bridge?
     A: A bridge  as described above that has an Ethernet  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.

04.13Q: Is there a maximum number of bridges allowed on a network?
     A: Per IEEE 802.1 (d), the maximum  number of concatenated brides in a
        bridged LAN is  7. This number is rather arbitrary, however, and is
        based on  simulations  of  application  performance  with  expected
        bridge delays.

        In  addition, the  number  assumes that  all bridges  are LOCAL (no
        remote WAN connections), and that the default Hold Time of 1 second
        is in place (this is  the time after which a bridge will discard  a
        frame  it is holding).  This  prevents  extra-late  frame delivery.
        (i.e, a frame should never be delivered more than  ~7 seconds after
        is it sent).

        I personally (Rich  Seifert) find  this  to  be  much  too long  an
        allowance. My "rule of thumb"  for  bridged LANs is  to  limit  the
        number of hops to  4, with not more than  one  of these being a WAN
        linked remote bridge.

04.13Q: 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.

04.14Q: 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.   Routing  would  always   be
        preferable to bridging except 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),  and that  many applications  use
        non-routable protocols (i.e., NetBIOS, DEC LAT, etc.).

        Rules of thumb:

           Bridges are usually good choices for small networks with few, if
           any, slow redundant links between destinations. Further, bridges
           may be your _only_ choice for certain protocols, unless you have
           the means to encapsulate (tunnel) the unroutable protocol inside
           a routable protocol.

           Routers are  usually much better  choices  for larger  networks,
           particularly  where  you  want to  have  a relatively clean  WAN
           backbone.   Routers are better  at  protecting against  protocol
           errors (such as  broadcast storms)  and  bandwidth  utilization.
           Since routers look  deeper inside the data packet, they can also
           make forwarding decisions based on the upper-layer protocols.

        Occasionally, a combination of the two devices are  the best way to
        go.   Bridges can  be  used  to  segment  small  networks that  are
        geographicly close to each other, between each other and the router
        to the rest of the WAN.

04.15Q: Are there problems mixing Bridging & routing?
     A: Only if you plan  on  having bridged links in  parallel with routed
        links.  You need to 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.

04.16Q: Who makes the fastest/easiest/most advanced bridges or routers?
     A: The  IETF runs  bench  marks  on a  wide  selection  of bridges and
        routers.  The results (and  much of the testing itself) is  handled
        at  Harvard University by Scott Bradner.  [ed:  anyone have the ftp
        site address and path/filename for the benchmarks?]

04.17Q: What is a Kalpana EtherSwitch?  Are there other devices like it?
     A: A device that works sort of like a multisegment bridge, but with  a
        complicated  internal bus that allows full crosspoint switching.  A
        Kalpana  or  other such  switch is  exactly  equivalent to a  fully
        connected mess of simple bridges among  the  Ethernets.   A 12-port
        Kalpana or  similar switch is obviously  rather easier  to use  and
        cheaper than the equivalent mesh of  132 simple  bridges.  However,
        the  EtherSwitch does  not  use  the Spanning Tree  Algorithm  and,
        therefore, cannot be used in situations where a bridging loop might
        occur.

        There  are competing devices  from  other manufacturers,  including
        some that do implement  the  Spanning Tree Algorithm.  For example,
        Alantec has a multi-port bridge/router supporting  12 segments with
        full spanning tree and snmp and it runs at about ethernet speeds.

04.18Q: 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.

04.19Q: 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
        applications,  and  often in  combination  with  concurrent  Novell
        access (IPX/SPX).

        ODI  is  Novell   and  Apple's  equivalent   of  NDIS.   There  are
        differences between the two specs,  but not so  much as  to warrant
        description 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.


05.01Q: What means SQE? What is it for?
     A: SQE is the IEEE term for a collision. (Signal Quality Error)

05.02Q: What means SQE Test? What means heartbeat?  What are they for?
     A: SQE Test (a.k.a. heartbeat) is a means of detecting a transceiver's
        inability  to detect  collisions.   Without SQE  Test,  it  is  not
        possible to  determine if  your  collision  detector  is  operating
        properly.  SQE Test 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.

        The problem with SQE  Test is  that it is not part of the  Ethernet
        Version  1.0 specification.  Therefore,  Version 1.0  equipment may
        not function  with transceiver that  generates the SQE Test signal.
        Additionally,  IEEE  802.3  specifications  state that  IEEE  802.3
        compliant  repeaters  must  not be attached  to  transceivers  that
        generate heartbeat.   (This  has  to  do with  a  jam  signal  that
        prevents  redundant collisions  from  occurring  on  the  network).
        Therefore, you must usually turn-off  SQE  Test (heartbeat) between
        the transceiver and an 802.3 repeater.

05.03Q: 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.

05.04Q: What means "promiscuous mode"?
     A: Promiscuous  mode is  a condition where the network interface  con-
        troller will pass all frames, regardless of destination address, up
        to   the  higher  level  network  layers.    Normally  the  network
        controller  will only  pass  up  frames  that  have  that  device's
        destination address.  However,  when put  in  promiscuous mode, all
        frames are passed on up the network stack regardless of destination
        address.   Promiscuous  mode is  usually used by network monitoring
        tools and transparent bridges (and, frequently, by network crackers
        trying to snatch passwords, or other data they're normally not able
        to see, off the wire).

05.05Q: 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 60 bytes (at Data Link).

05.06Q: What causes a runt?
     A: Runt packets are most  likely  the  result of a collision, a faulty
        device on the network, or software gone awry.

05.07Q: 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.

05.08Q: 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.

05.09Q: 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,
        and the process repeats until  either the  packet finally  makes it
        onto the network without  collisions,  or 16 consecutive  collision
        occur and the packet is aborted.

05.10Q: 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.

05.11Q: How many collisions are too many?
     A: This  depends  on your application  and protocol.   In many  cases,
        collision rates of 50% will not cause a large decrease in perceived
        throughput.  If  your network is  slowing down and you  notice  the
        percentage of  collisions is  on  the  high  side, you may want try
        segmenting your network with either a bridge  or  router to see  if
        performance improves.

05.12Q: How do I reduce the number of collisions?
     A: Disconnect devices from  the  network.  Seriously, you need to cut-
        down on the number of devices on the network segment  to affect the
        collision  rate.  This is  usually  accomplished  by splitting  the
        segment into two  pieces and putting a bridge  or router in between
        them.

05.13Q: What is a late collision?
     A: A late collision occurs when two devices transmit at the same time,
        but due to cabling errors (most commonly, excessive network segment
        length or repeaters between  devices) neither detects  a collision.
        The reason this happens is because the time to propagate the signal
        from one end of the network to another is  longer than the time  to
        put the entire packet on the network, so the two devices that cause
        the late collision  never see that  the other's sending until after
        it  puts the  entire  packet on  the network.  Late  collisions are
        detected by the transmitter after the first  "slot time" of 64 byte
        times.  They  are only  detected  during  transmissions of  packets
        longer than 64 bytes.  It's detection is exactly  the same as for a
        normal collision; it just happens "too late."

        Typical causes  of late  collisions are  segment cable  lengths  in
        excess  of  the  maximum  permitted  for  the  cable  type,  faulty
        connectors  or  improper cabling,  excessive  numbers of  repeaters
        between  network devices,  and defective Ethernet  transceivers  or
        controllers.

        Another  bad thing about  late  collisions  is  that they occur for
        small packets also,  but cannot be detected by the transmitter.   A
        network  suffering a  measurable rate of late collisions (on  large
        packets)  is  also  suffering  lost   small  packets.   The  higher
        protocols do  not cope well with such losses.  Well, they cope, but
        at much  reduced speed.  A 1% packet  loss is enough  to reduce the
        speed of NFS by 90% with the default retransmission timers.  That's
        a 10X amplification of the problem.

        Finally, Ethernet  controllers  do not retransmit packets  lost  to
        late collisions.

05.14Q: 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.

05.15Q: 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.  

05.16Q: 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.

05.17Q: How can I prevent a broadcast storm?  
     A: Avoid protocols that are prone to it.  Route when it is practical.

05.18Q: What is an Alignment Error ?
     A: A received frame that does not contain  an integer number of octets
        and contains a frame check  sequence validation error.  A  frame in
        which the number of bits received is not an  integer  multiple of 8
        and  has a FCS (Frame Check Sequence) error.  (802.3 standard, page
        41)

05.19Q: 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*. 


06.01Q: How can I test an Ethernet?
     A: This depends on what  level  you want to test.  The most basic test
        (a.k.a., "the fire  test") is to connect a pair  of devices to  the
        network  and see  if they can communicate  with each other.  If you
        want  to test  the  electrical integrity of the wire (i.e., will it
        carry  a signal properly), a TDR or cable scanner that incorporates
        TDR  and  other  functions, would  be  the most comprehensive  tool
        (though a great deal cab be determined with a simple ohmmeter).  If
        you   need  to  test   the  performance  or  troubleshoot  protocol
        transmission  problems, you will  need  special  and  usually  very
        expensive  software,  usually  coupled  with  custom  hardware,  to
        capture, optionally filter, and analyze the network packets.

06.02Q: Is there a troubleshooting guide for Ethernet?
     A: Yes, many.  I suggest you  check  your  local technical  bookstore.
        (Recommendations from the list would be appreciated!)

        There are also some common sense  steps  you can  take.  [Volunteer
        needed to fill this section out -- I think it's important but I'm a
        little short on time at the moment to do it myself -mm]

06.03Q: 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.

06.04Q: 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.

06.05Q: What (free) tools are there to monitor/decode/etc an Ethernet?
     A: There are many built into most Unix systems.  For example, the ping
        command can be used to determine if a given host is alive, and will
        also tell you the round trip transmission time.  ifconfig will tell
        you the status of the network  interfaces.  netstat will  summarize
        statistics  for  network usage.  spray will  allow  you to generate
        network  traffic  directed   at  a  particular   host.   Use   "man
        command-name" to  learn  more about a unix command.   Using "man -k
        network" may also provide leads to the tools provided by your  unix
        vendor.

        Many  more public domain  tools  are  available  for  unix systems.
        These include:
           traffic:    allows systems to graphically display network load
           tcpdump:    collect statistics and display individual packets
           etherfind:  ????
           nfswatch:   summarize/display traffic, particularly nfs packets
           traceroute: determine the route between two hosts

        Some cards for the PC come with utilities.  There are several  free
        ones available,  including  ping (Clarkson University  and others),
        The Beholder (packet capture and display) and others.


07.01Q: Are there any other sources of information about Ethernet?
     A: There  are  a LOT of information sources.   Try  to get the BIG-LAN
        FAQ.   One known  anonymous  FTP location is icarus.cns.syr.edu  in
        /information/big-lan.   The big-lan.faq  file is a kind of superset
        to this ethernet faq.   Another excelleny  document is the  network
        reading   list,  "net-read.txt"   (or   "net-read.ps")  by  Charles
        Spurgeon, available  via  anonymous FTP from ftp.utexas.edu.  Cisco
        Systems has a useful document: DOC-GLOSS part number 78-0888-01.


07.02Q: What books are good about Ethernet LAN's?
     A: The IEEE 802.3 documents are considered the definitative source for
        information on  Ethernet.   However, these may  not be suitable for
        all  levels  of  users.  Surprisingly,  there  are few  good  books
        specifically dealing with  Ethernet LANs,  but  here are a few that
        you might find useful:

            Local Area Networks, An introduction to the technology
            by John E. McNamara, published by Digital Press, 1985
            165 pps. with index and glossary, $29.00
            ISBN 0-932376-79-7,  Digital Press part number EY-00051-DP.

            Network Troubleshooting Guide
            by Digital Equipment Corporation, August 1990
            Approx. 278 pps. with index and glossary, $95.00
            Digital Press part number EK-339AB-GD-002.

        These books and others are recommended in the network reading list,
        net-read.txt, from ftp.utexas.edu.

07.03Q: Where can I get IEEE802.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

        You can also get order information via e-mail to askieee@ieee.org.

07.04Q: Where can I get EIA/TIA docs online?
     A: Nowhere.  They must be ordered from:

          Global Engineering
          800-854-7179

        (I am still trying to contact this vendor for more details.)

07.05Q: Where can I find the specifications of Ethernet equipment?
     A: From the  manufacturer of  the product [hopefully!].   In the  IEEE
        802.3 documents for standard devices.

07.06Q: Where can I find IETF (Internet Engineering Task Force) documents?
     A: These are available for anonymous FTP  from a number of sites.  The
        cannonical location is ietf.cnri.reston.va.us in /ietf. Drafts are
        at the same site in /internet-drafts.

07.07Q: Where can I get the current version of this document?
     A: Check  in  newsgroups  comp.dcom.lans.ethernet,   comp.answers  and
        news.answers.   It  is   also  available  via  anonymous  ftp  from
        steph.admin.umass.edu in path pub/faqs as ethernet.faq.
-- 
Marc A. Runkel                              marc.runkel@registrar.umass.edu
Network Analyst                             Of course, this is just my
Registrar's Office * Systems Support Group  tiny, insignificant, humble
University of Massachusetts, Amherst        opinion.  If you don't like it....

