Internet Surfer 2.0/Internet Surfer 2.0 (Wayzata Technology) (1996).iso/pc/text/mac/faqs.341

Text File | 1996-02-12 | 29KB | 605 lines

Frequently Asked Questions (FAQS);faqs.341



    Other interesting looking books:

    n. Mark A. Miller.  Troubleshooting Internetworks.

  13. Periodicals

    The following periodicals were mentioned by responders to the 12/91
    BIG-LAN Reader Survey as good periodicals for administrators of Campus-
    sized LANs:

    a. LAN Times
    b. Communications Week
    c. Network Computing
    d. ConneXions
    e. Data Communications
    f. Network World
    g. LAN Magazine
    h. Info World
    i. SunExpert
    j. Telecommunications
    k. Computerworld
    l. DataCommunicationInternational
    m. Datamation
    n. Digital Review
    o. LAN Technology
    p. Lightwave
    q. MacUser
    r. MacWeek
    s. MacWorld
    t. Networking Management
    u. PC Week

  14. Training Courses

    The following providers of tutorials were mentioned by responders to
    the 12/91 BIG-LAN Reader Survey:

    a. Interop
    b. ACM SIGComm
    c. Learning Tree
    d. Novell
    e. PSI
    f. Usenix

  15. Conferences

    The following conferences were mentioned by responders to the 12/91
    BIG-LAN Reader Survey as good conferences for administrators of Campus-
    sized LANs:

    a. Interop
    b. Usenix
    c. ComNet
    d. NetWorld
    e. ACM SIGComm
    f. DECUS
    g. IETF


IV. Basic Glossary on Campus Networks

   Another glossary is RFC1208.  See "Online Papers" above.

     ANSI "American National Standards Institute" - A definer of
       standards of all kinds, including FDDI.

     Appletalk - A protocol family developed by Apple Computer to
       implement LANs serving Macintoshes.

     ATM "Asynchronous Transfer Mode" - a method for switching little
       fixed-size packets (cells) around.  Like T1 and DS3, digitized
       voice was a major consideration in its design, but it can be
       used for data.  It is designed around fixed speeds too, roughly
       150MBS and 600MBS.  The fixed cell size is 53 bytes.  Though ATM
       is really designed for voice and WANs, there are schemes to use
       it in LANs.  ATM is a big buzzword these days but it is still
       very new.

     AUI "Attachment Unit Interface" - the Ethernet/IEEE 802.3 term
       for the interface between a MAU and a station.  A special kind
       of cable known as an "AUI Cable" can attach a MAU to a station
       at a distance (up to 50 meters).

     BNC Connector "Bayonet Neill-Concelman connector" - a type of
       connector used for attaching coax cable to electronic equipment
       which can be attached or detached quicker than connectors that
       screw.  ThinWire Ethernet (IEEE 802.3 10BASE2) uses BNC connectors.

     Bridge - A network "relay" which reads, buffers, and sends
       data to relay it from one data link to another, but makes
       the two data links appear as one to levels higher than the
       data link layer.

     CDDI "Copper Data Distribution Interface" - essentially a way to
       use electrical communications cables in an FDDI network.  Several
       companies have worked out ways to do this but ANSI has yet to
       standardize one.  I think CDDI was coined by Crescendo corporation
       for their scheme, but it may well be adopted by ANSI as the name.
       So far there are schemes that work on Coax, on STP and UTP, but
       the front runners look like they will be able to run on UTP for
       about 100 meters.

     CMIP "Common Management Information Protocol" - An OSI protocol
       for management of network equipment.  Not widely implmented.
       See SNMP.

     CMOT "CMIP over TCP/IP" - A protocol consisting of CMIP running
       under TCP/IP.  An alternative to SNMP.

     Coaxial Cable - any of a number of kinds of electrical
       communications cable designed so one conductor is in the
       center and the second conductor forms a ring around it.
       Depending upon who you talk to, someone might have a specific
       kind of coaxial cable in mind.  Some well known kinds are
       various Cable TV cables, cables used by IBM 327x terminals
       and ARCnet, and cables used by Ethernet & IEEE 802.3.

     DECnet - Trade name of Digital Equipment Corporation for some
       of their networking products.  It is a kind of network
       built out of Digital Equipment Corporations own networking
       protocols (with some standard protocols also used).

     Dialup Modem - Modem used over ordinary dial-up telephone lines
       as opposed to private or leased lines.

     Ethernet - LAN data-link protocol developed by a consortium
       of vendors; later standardized as IEEE 802.3 with a few
       modifications.  For many applications, users have not adopted
       all the IEEE 802.3 differences.  Ethernet/802.3 now can be
       run on two types of coaxial cable as well as multi-mode
       fiber and unshielded twisted-pair.  "Raw" rate of data
       transmission is 10 megabits/second.

     FDDI "Fiber Data Distribution Interface" - LAN data-link protocol.
       Designed to run on multi-mode fiber.  "Raw" rate of data
       transmission is 100 megabits/second.  Developed by the American
       National Standards Institute.

     FDDI-2 - Same speed, same fiber, same basic protocol as FDDI.
       FDDI-2 adds a layer which allows you to allocate fixed bandwith
       to applications of your choice, making it more like broadband.
       FDDI-2 is still rather new.

     Fiber - optical fiber: a very long, narrow, flexible piece of glass.
       Used for high-speed communications.

     FOIRL "Fiber Optic Inter-Repeater Link" - a standard for running
       IEEE 802.3 over fiber.  It was originally designed to link two
       repeaters, and only supports two attachments.  Many users use it
       to attach a station to a repeater.  See 10BASE-F.

     FTP - Protocol in the "TCP/IP" family for copying files from
       one computer to another.  Stands for "File Transfer Protocol".

     Gateway - A type of "network relay" that attaches two networks
       to build a larger network.  Modern "narrow" usage is that it
       is one that translates an entire stack of protocols, e.g.,
       translates TCP/IP-style mail to ISO-style mail.  Older usage
       used it for other types of relays--in particular, in the "TCP/IP"
       world, it has been used to refer to what many now insist is
       a "router".

     GOSIP "Government Open Systems Interconnect Profile" - A subset of
       OSI standards specific to US Government procurements, designed
       to maximize interoperability in areas where plain OSI standards
       are ambiguous or allow options.  Theoretically, required of all
       US Government networking procurements since mid-1990.

     Heartbeat - In Ethernet (Version 2), a test of the collision
       functionality of the transciever.  The term "Heartbeat" is often
       (wrongly) used interchangeably with "SQE" which is a similar
       function of IEEE 802.3.  See Question on SQE/Heartbeat below.

     IPX - Novell's protocol used by Netware.  Utilizes part of XNS.
       A router with "IPX routing" purports to interconnect LANs so
       that Novell Netware clients & servers can talk through the router.

     MAU "Media Adaptor Unit" - an IEEE 802.3 or Ethernet device which
       attaches a station to the cable.  Popularly called a "transceiver".
       Can be attached by cable to the station or built into the
       station.

     MIB "Management Information Base" - the set of parameters an SNMP
       management station can query or set in an SNMP agent (e.g. router).
       Standard, minimal MIBs have been defined (MIB I, MIB II), and vendors
       often have custom entries.  In theory, any SNMP manager can talk
       to any SNMP agent with a properly defined MIB.

     Multimode fiber - A type of fiber mostly used for shorter, e.g. campus
       distances.  It can carry 100 megabits/second for typical campus
       distances, the actual maximum speed (given the right electronics)
       depending upon the actual distance.  It is easier to connect to than
       Single Mode Fiber, but its limit on speed x distance is lower.

     NFS "Network File System" - an IP-based protocol originally developed
       by Sun Microsystems which provides file services.

     OSI "Open System Interconnect" - A standard put forth by the ISO for
       communication between computer equipment and networks.

     OSI Reference Model - A model put forth by the ISO for communication
       between computer equipment and networks, which maps out 7 protocol
       layers.

            Top layer:  layer number 7:   application layer
                        layer number 6:   presentation layer
                        layer number 5:   session layer
                        layer number 4:   transport layer
                        layer number 3:   network layer
                        layer number 2:   data-link layer (e.g. IEEE 802.x)
         Bottom layer:  layer number 1:   physical layer (wire & electricity)

       This model explains what each layer does.  The model is often
       used to explain anyones protocols (not just OSI) to the point
       where many people seem to believe that true data-communications
       requires these 7 layers.

     POP "Post Office Protocol" - A TCP/IP-based protocol designed to allow
       client-stations (e.g. micros) to read mail from a server.  There
       are three versions under the name "POP": POP, POP2, and POP3.
       Latter versions are NOT compatible with earlier versions.

     Protocol - The "rules" by which two network elements trade information
       in order to communicate.  Must include rules about a lot of mundane
       detail as well as rules about how to recover from a lot of unusual
       communication problems.  Thus they can be quite complicated.

     Relay - One terminology uses the term "relay" as a device that
       interconnects LANs, different kinds of relays being repeaters,
       bridges, routers, and gateways.

     Repeater - In the "Ethernet" world, a "relay" that regenerates and
       cleans up signals, but does no buffering of data packets.
       It can extend an Ethernet by strengthening signals, but timing
       limitations on Ethernets still limit their size.

     RFC "Request For Comments" - The name is a real red herring when
       it comes to Internet RFCs.  Some really are "Requests For Comments"
       but all Internet protocol documents are stamped with an RFC number
       that they never shake, so the acronym RFC generally refers to
       documents that describe protocols in the TCP/IP family.

     RG numbers (E.g. RG62; sometimes there are qualifiers, e.g. RG 58
       A/U) a shorthand designation for military cable.  RG58 & RG62
       designate two different types of cable used by the military.
       Some data-communications equipment was designed to work with
       a particular military standard, e.g.  IBM 3270-type terminals
       use RG62.  In other cases, people use an RG-numbered cable
       that is close to what they need: for example Thinwire
       Ethernet & IEEE 802.3 10BASE2 define the type of cable they
       need and people sometimes substitute flavors of RG58, which
       are "close".  One can't recommend this practice because you
       can get yourself in trouble.  I think "RG" originally stood
       for "Radio Guide", presumably reflecting the fact that the
       series of cables was designed to handle radio frequencies.  The
       IEEE 802.3 10BASE2 specifications define two RG numbered cables
       (RG58 A/U and RG58 C/U) as meeting the cable requirements for
       thin Ethernet.  However, cable vendors may list a range of
       cables under these same RG numbers, and some of the cables
       listed may not meet the 802.3 specs.  You need to check the
       cable specifications closely, and beware of relying on the RG
       number alone when ordering network cables.

     Router - A network "relay" that uses a protocol beyond the
       data-link protocol to route traffic between LANs and other
       network links.

     Routing Protocol - a protocol sent between routers by which
       routers exchange information own how to route to various parts
       of the network.  The TCP/IP family of protocols has a bunch,
       such as RIP, EGP, BGP, OSPF, and dual IS-IS.

     Shielded Twisted Pair - a type of twisted-pair cable with a
       metallic shield around the twisted conductors.  The shield
       reduces the noise from the cable and reduces the effects of
       noise on the communications in the cable, but changes the
       electrical characteristics of the cable so some equipment
       optimized to non-shielded cable runs worse on shielded cable.

     Single Mode fiber - a type of fiber optic cable used for longer
       distances and higher speeds, e.g.  for long-distance telephone
       lines.  See also "Multimode Fiber".

     SMTP "Simple Mail Transfer Protocol" - the protocol in the
       TCP/IP family used to transfer electronic mail between
       computers.  It is not oriented towards a client/server system so
       other protocols (see "POP") are often used in that context.
       However, servers will use SMTP if they need to transfer a
       message to another server.

     SNMP "Simple Network Management Protocol" - Originally developed
       to manage IP based network equipment like routers and bridges,
       now extended to wiring hubs, workstations, toasters, jukeboxes,
       etc.  SNMP for IPX and AppleTalk under development.  Widely
       implemented.  See CMIP.

     SQE Test "Signal Quality Error Test" - an IEEE 802.3 function
       that tests the transceiver.  The term "SQE" is often (wrongly)
       used interchangeably with "Heartbeat" which is a similar
       function of Ethernet Version 2.  See Question on SQE/Heartbeat
       below.

     TCP/IP "Transmission Control Protocol/Internet Protocol" -
       literally, two protocols developed for the Defense Data Network
       to allow their ARPANET to attach to other networks relatively
       transparently.  The name also designates the entire family of
       protocols built out of IP and TCP.  The Internet is based upon
       TCP/IP.

     TELNET - a protocol in the TCP/IP family that is used for
       "remote login".  The name is also often used as the name of the
       client program that utilizes the TELNET protocol.

     Terminal Server - a network device that allows a number of
       terminals to attach to a LAN, and do remote logins across the
       LAN.

     TN3270 - A variant of the TELNET program that allows one to
       attach to IBM mainframes and use the mainframe as if you had a
       3270 or similar terminal.

     Token Ring - People often mean 802.5 when they say "Token Ring"
       (see below).  In the more general sense of the word, a type of
       LAN that has stations wired in a ring, where each station
       constantly passes a special message (a "token") on to the next.
       Whoever has the token can send a message.

     Tunnelling - An important concept in the design of many kinds of
       networks: taking some protocol-family's ability to move packets
       from user to user, or to open virtual-circuits between users,
       and use this as if it were a data-link protocol to run another
       protocol family's upper layers (or even the same protocol
       family's upper layers).  Examples: running TCP/IP over Appletalk
       instead of something like Ethernet; running Appletalk over
       DECnet instead of something like Localtalk or Ethernet.

     Twisted Pair - The type of wire used by the phone company to wire
       telephones -- at least over distances like between your house
       and the central office.  It has two conductors, which are twisted.
       The twists are important: they give it electrical characteristics
       which allow some kinds of communications otherwise not possible.
       Ordinary telephone cables are not shielded (see "Shielded twisted
       Pair").

     T1 - A phone-company standard for running 24 digitized voice circuits
       through one 1.5megabit/second digital channel.  Since phone companies
       run lots of T1, and will run T1 between customer sites, the
       standard is often used for data communications, either to provide
       24 low-speed circuits, or to provide 1 high-speed circuit, or to
       be divided other ways.

     UTP (Unshielded Twisted-Pair) -  See "Twisted-Pair" and "Shielded
       Twisted-Pair".

     X.400, X.500 - OSI protocols for mail and directory services.

     10BASE-T - A variant of IEEE 802.3 which allows stations to be attached
       via twisted-pair cable.

     10BASE-F - A variant of IEEE 802.  3 under development which
       allows stations to be attached via multimode fiber.  It will
       offer a variety of methods of using fiber in an IEEE 802.3
       network that go beyond what is currently offered in FOIRL.  The
       current 10BASE-F draft is likely to be confirmed.  draft is
       likely to be confirmed.  Sections of the draft include "Fiber
       Optic Medium and Common Elements of Medium Attachment Units and
       Star, Type 10BASE-F (Section 15)", "Fiber Optic Passive Star and
       Medium Attachment Unit, Type 10BASE-FP (Section 16)", "Fiber
       Optic Medium Attachment Unit, Type 10BASE-FB (Section 17)", and
       "Fiber Optic Medium Attachment Unit, Type 10BASE-FL (Section
       18)".

     802 - The set of IEEE standards for the definition of LAN protocols.
       A story goes that a long time ago, IEEE and ANSI decided that
       IEEE would get the slow protocols and ANSI would get the fast
       ones, thus IEEE defined the 802 protocols and ANSI defined FDDI.
       Presumably IEEE saw limited application for FDDI at the time.
       Also, the IEEE standards-making committees associated with these
       standards.

     802.1 - The IEEE 802 standard for Network Management and Network
       Bridging of IEEE 802 networks.

     802.2 - An IEEE standard for the portion of LAN data-link protocols
       that is the same for all flavors of IEEE LAN protocols, e.g.
       802.3 and 802.5.  Sometimes not used.

     802.3 - An IEEE standard for LANs--their "improved" version of Ethernet.
       See Ethernet.

     802.4 - An IEEE standard for LANs: Token Bus networks.  Basically,
       standardizes MAP, a protocol that operates a Token Bus protocol on
       broadband.

     802.5 - An IEEE standard for Token-Ring-based LANs.  See Token Ring.

     802.6 - An IEEE standard for Metropolitan Area Networks.  Also known
       as DQDB.

     802.7 - IEEE 802 technical advisory group on Broadband.

     802.8 - IEEE 802 technical advisory group on FDDI & fiber optics.

     802.9 - IEEE 802 group on integrated data & voice networks.

     802.11 - Proposed IEEE 802 group for wireless Ethernet.


V. Frequently Asked Questions on Campus Networks

     It is hard to answer typical BIG-LAN questions in advance for
     two reasons.  Answers are often long and they are often
     controversial.  To provide some sort of objective information
     relevant to the controversies, a survey of BIG-LAN readers was
     taken on answers to various questions, so this memo could offer
     a sampling of opinions.  Note that the opinions below are
     extracted from the 41 responses received for the survey.  We
     can't say these 41 responses represent a fair sampling of campus
     LAN administrators, but they do show some of the answers that
     you would get if you posed some of these questions to the
     BIG-LAN readership.

  1. What is the difference between Ethernet and IEEE 802.3?

     Ethernet ran through an evolution starting with some
     experimenting at Xerox, and ending with a standard
     published by Xerox, DEC, and Intel, which they offered to
     the world (with minimal royalties) as a standard technology
     for building LANs.  The Institute of Electrical &
     Electronic Engineers took this as a proposed standard, and
     rewrote the protocol description making some clarifications
     and a few changes.  Some of the changes have been
     universally adopted, and others have not.  After the first
     go round of IEEE standard defining, Ethernet version 2 was
     introduced which brought it more into line with standards.
     The basic differences are:

         - Heartbeat vs SQE (see below)
         - Which pin in the MAU & AUI connectors carry the ground
            conductor
         - Packet Length Field vs Type Field

     The latter issue is the one in which IEEE 802.3 has not
     displaced Ethernet.  Ethernet had a 16-bit field which
     defined the type of packet (examples: IP, XNS, Appletalk).
     The IEEE committee decided to use that field to specify the
     length of the packet, and have the data-portion of the
     packet define itself through the next higher level of
     protocol (e.g., IEEE 802.2).  However, the sets of possible
     values for that field used by the two different protocols
     are completely separate, and both protocols are designed to
     deliberately ignore packets with fields outside their own
     sets of values.  Thus Ethernet and IEEE 802.3 packets can
     coexist on the same cable, though a computer which expects
     to get packets belonging to just one of the protocols won't
     notice any packets sent according to the rules of the other
     (the expression used is "they pass by each other like ships
     in the night").

     These days, LANs use both.  There is a way to send TCP/IP
     packets via 802.3, but when 802.3 was introduced, there
     were already so many systems using the Ethernet rules that
     the use of Ethernet-style packets for TCP/IP has persisted
     now for years.

  2. What is encapsulation?  What do I have to know about it?

     One encapsulation issue on LANs is whether IEEE 802.3
     packets are used or Ethernet packets are used to
     encapsulate your traffic on your IEEE 802.3/Ethernet LAN.
     See previous question for more explanation.  Most TCP/IP
     systems use Ethernet, any that uses IEEE 802.3 by default
     might surprise you by not interoperating with the rest of
     your TCP/IP network.

     A second encapsulation issue on IEEE 802.3/Ethernet
     networks is whether your Novell (IPX) packets use Novell's
     default encapsulation or whether they use Ethernet-style
     encapsulation.  Novell, at least for a long time, had the
     distinction of using IEEE 802.3 as if it were the only
     protocol on the network, not following the rules for
     avoiding other protocols running under IEEE 802.3 rules.
     They offered a utility called ECONFIG that changed Netware
     to use Ethernet rules, and use them properly, so Novell IPX
     packets could utilize the same LAN as other protocols.  In
     no case would the Novell traffic bother Ethernet traffic--
     only any other IEEE 802.3 traffic if ECONFIG wasn't used.
     In any case, a single Ethernet segment, or bridged
     segments, had to have all Novell servers and clients
     configured the same, in order to interoperate.

     A third encapsulation issue stems from Berkeley Unix 4.2,
     from which many versions of Unix and many TCP/IP
     implementations have been modeled.  It used, by default,
     its own encapsulation rules (i.e., manner of putting IP
     packets within Ethernet packets) which is termed "Trailer
     Encapsulation".  When an Ethernet had some computers using
     Trailer Encapsulation and some not, TCP/IP connections
     would often work, but hang when large data transfers were
     taking place.  The next version of Berkeley Unix, version
     4.3, remedied this by avoiding Trailer Encapsulation except
     when it was guaranteed to work correctly.

     A fourth encapsulation issue is "tunnelling", which
     consists of one of the layers in the protocol stack
     mimicking another layer to provide a way of running a
     different set of upper layers than would otherwise be
     possible.  This is rather widely used and seldom explained
     to beginners.  It is perhaps best explained with an actual
     example:

[Here put an example, perhaps Appletalk over IP]

[Include "encapsulated bridging" as a second example]

  3. How do I know whether to use a router or a bridge?

     (Note that the answer to this question is oriented to
     Ethernet-based LANs).  Few administrators of networks doubt
     that a network can be large enough to require routers nor
     that there are situations where a bridge is an effective
     solution.  However, there is controversy as to where to
     draw the line.  Campus-sized networks involving distances
     of up to a mile and possibly thousands of stations, can be,
     and have been built solely out of one or the other.  The
     BIG-LAN Survey of 12/91 showed the following opinion among
     respondents:

       Survey question: "When you build a campus network, do you
       tend to use bridges as opposed to routers?"

       Answers: 9 said yes; 26 said no; 2 said "brouters"
       (combination bridge/routers) would be the best solution.

     Some clear tradeoffs: routers generally have to be set up
     no matter what whereas bridges can be plug-and-play on a
     network without too much total traffic; bridges generally
     have a higher speed-to-cost ratio and the low-end bridge is
     less expensive than the low-end router; routers handle huge
     networks with links of different speeds better.

  4. How do I know whether to use a bridge or a repeater?  How many
     repeaters may I put on an Ethernet?

     You cannot keep plugging more repeaters and add more cables to
     an Ethernet indiscriminately and expect it to work.  With too
     large a networks, the protocol which keeps the number
     of collisions down (known as CSMA/CD) fails to do that.  The
     protocol documents supply rules-of-thumb which, if followed,
     prevent this from occurring.  If you break them, you may be risking
     large performance degradations.

     The latest version of the rules-of-thumb (which have been updated
     over time as new features like 10BASE-T have been added to the
     protocol) are in the IEEE 802.3 document describing 10BASE-T,
     specifically IEEE Std 802.ei-1990 in the section called "System
     Considerations for Multisegment 10 Mb/s Baseband Networks"
     (When 10BASE-F is released later, this section will be updated again).
     The rules refer to the piece of the LAN that is between repeaters
     as a segment and refer to 4 kinds: 10BASE5 (i.e. "classic" Ethernet)
     and 10BASE2 (i.e., ThinWire Ethernet) both classified as "Coax"
     segments and FOIRL (fiber inter-repeater links) and 10BASE-T, both
     classified as "Link" segments, and both of which have the property
     that you can attach things only to their ends.  The basic repeater
     rule is that between any two stations on the LAN, there may be
     at most 4 repeaters and three coax segments.  In addition, there
     are length restrictions on the segments which are designed to
     keep CSMA/CD working properly:

        10BASE5         500 meters
        10BASE2         185 meters
        FOIRL           500 meters (1000 meters in some cases)
         10BASE-T        100 meters (or more)

     FOIRL links can be 1000 meters if you have at most 3 repeaters
     between stations instead of 4.  10BASE-T links can be longer
     if the cable will support it: CSMA/CD is not the limiting factor
     on 10BASE-T.  For the purposes of this discussion, bridges, routers,
     and gateways are "stations" since the CSMA/CD protocol does not
     pass through them.  Thus if you discover these rules prevent
     you from putting a repeater in the network where you need one, then
     you can put a bridge there instead, or perhaps split the LAN
     somewhere else using a bridge.

  5. Should I use "manageable" hubs, concentrators, etc on my LAN?

     This is a controversial question also.  Vendors have
     attempted to make hubs and concentrators that require
     little training & manpower to manage & troubleshoot, and
     they will attempt to convince you that they have succeeded.
     You pay a premium for "manageability".  Those who remain
     skeptical wonder how much the management features are ever
     used: for example, management allows you to turn on & off
     ports from an operator's console; how often do you need to
     do such a thing?  Also, some of the benefits attributed to
     management packages are simply due to good record keeping,
     something which the administrator must find the manpower to
     accomplish with a management package or without one
     (presumably with a simple dbms, which can often be tailored
     more to the administrators needs).