BIG-LAN Frequently Asked Questions
Last Updated: July 24, 1995
Acknowledgements: A lot of people provided information for me and I
freely admit that I have not recorded the list of names. Thanks
to all.
Contents
--------
I. About BIG-LAN
II. Explanation of this Memo
III. Sources of Information on Campus Networks
1. Must-Read Sources
2. A Few General Sources
3. LISTSERV Mailing Lists
4. Internet Mailing Lists
5. Internet Mailing Lists with automatic subscription
6. USENET/Netnews Groups
7. Anonymous FTP-based Archive Sites
8. LISTSERV-based Archive Sites
9. RFCs (Internet "Request For Comments")
10. Other Useful Online Papers
11. Sources of Protocol Documents
12. Useful Free Software
13. Books
14. Periodicals
15. Training Courses
16. Conferences
IV. Basic Glossary on Campus Networks
V. Frequently Asked Questions on Campus Networks
1. What is the difference between Ethernet and IEEE 802.3?
2. What is encapsulation? What do I have to know about it?
3. How do I know whether to use a router or a bridge?
4. How do I know whether to use a bridge or a repeater? How many
repeaters may I put on an Ethernet?
5. Should I use "manageable" hubs, concentrators, etc on my LAN?
6. Which LAN technology should I use? Arcnet? FDDI? Token Ring?
10BASE-T?
7. What is the ideal cable to install in a new building?
8. What is the ideal cable to install between buildings on a campus?
9. Whose routers are recommended?
10. Whose bridges are recommended?
11. Whose Ethernet equipment are recommended?
12. Whose Token Ring equipment are recommended?
13. Whose FDDI equipment are recommended?
14. What PC network software is recommended?
15. What protocols should run on a campus-wide LAN?
16. What software is recommended for managing a campus-wide LAN?
17. What terminal server is recommended?
18. Whose troubleshooting equipment are recommended?
19. What security products should I buy?
20. Should the names of devices on my campus LAN have subdomains?
21. Should client stations use POP? Should they use just SMTP?
Should I use some non-TCP/IP protocol for mail to/from client
stations?
22. Should I enable SQE/heartbeat?
23. If I have a thinwire network interface card, how do I connect it
to a 10BASE-T concentrator?
24. How much does a collision slow down an Ethernet packet?
25. Should I worry about Ethernet tailgating?
I. About BIG-LAN
BIG-LAN is a mailing list for discussion of issues in designing and
operating Campus-Size Local Area Networks, especially complex
nes utilizing multiple technologies and supporting multiple
protocols. Topics include repeaters, bridges, routers and
gateways; how to incorporate smaller Personal-Computer type LANs
into the campus-wide LAN; how to unify the mail systems, etc.
This is an ideal list in which to debate the relative merits of
bridges vs routers.
All requests to be added to or deleted from this list, problems,
questions, etc., should be sent to big-lan-request@listserv.syr.edu.
Those familiar with LISTSERV can subscribe with
listserv@listserv.syr.edu.
Archives are available through listserv@listserv.syr.edu and
ftp://syr.edu/information/archives/big-lan/
Coordinator: John Wobus <jmwobus@syr.edu>
II. Explanation of this Memo
Since BIG-LAN is not specific to any protocol family, it will
not cover any particular protocol family in detail, e.g. this
is not a TCP/IP/Internet FAQ Memo. Fortunately, there are some
good TCP/IP FAQ Memos which are listed in the sources of
information below.
Suggestions, corrections, and contributions welcome. Please
send them to:
jmwobus@syr.edu
An up-to-date copy of this memo may be retrieved via URL:
http://web.syr.edu/~jmwobus/comfaqs/big-lan.faq
III. Sources of Information on Campus Networks
This list favors "network" sources of information: available on
the Internet and/or BITNET and other similar networks; if you
have access to BIG-LAN then you have access to one of these
networks; and these sources are not the kind which you can
discover through vendors, books, bookstores, or libraries.
1. Must-Read Sources
These are documents that you definitely should get and read if you
have questions about Campus Networks.
a. Charles Spurgeon's reading list (see below under "Other Useful
Online Papers").
b. RFCs 1175, 1594, 1207, and 1392 (see below under "RFCs").
2. A Few General Sources
These are network resources & mechanisms for getting all kinds
of information--not just on Networking; thus we can't cover them
very thoroughly in this memo.
a. LISTSERV - mailing list servers & file servers on BITNET,
accessible via e-mail. Can be reached and used from a lot of
networks. Mail the command INFO to any LISTSERV for help.
Also have database commands (i.e. search commands) for archives
they store.
b. Usenet News/Netnews: distributed bulletin board with discussions
on lots of topics. Distributed through the Internet and through
UUCP.
c. Anonymous ftp: the main way to make files available on the
Internet. ftp to a site using username "anonymous". A
password is always demanded--sometimes a banner tells you what
to use--otherwise "guest" almost always works.
d. Archie servers - network-accessible databases of where to get
files via anonymous ftp. Access is through telnet, rlogin,
mail, or a special "archie" protocol. To use via telnet,
enter username archie. Some servers: archie.ans.net,
archie.sura.net, archie.mcgill.edu, archie.unl.edu.
e. WAIS - Internet-accessible databases on different topics.
Available via WAIS protocol (basically Z39.50). Client
(and server) software is collected on quake.think.com as
well as a WAIS database of WAIS servers.
f. ftplist.txt - collected list of anonymous ftp sites.
Stored lots of places in anonymous ftp including syr.edu.
g. Internet gopher - something like anonymous ftp only more
advanced: to get started, I suggest ftping
boombox.micro.umn.edu and getting information on gopher. A
number of sites have servers.
h. Internet List of lists: available by anonymous ftp from
ftp.nisc.sri.com, or through a mail-based file server
at mailserver@nisc.sri.com.
i. LISTSERV internal list of lists. Available by mailing the
command LIST GLOBAL to any LISTSERV.
j. news.answers - newsgroup that distributes Frequently Asked
Questions memos for lots of Netnews groups.
k. FAQ archive available via anonymous ftp on rtfm.mit.edu
From the archives of news.answers, Frequently Asked Question
memos for lots of Netnews groups.
l. news.announce.newusers - has periodic postings about how to
use Usenet/Netnews and also a lot about mailing lists.
m. BITFTP. A BITNET server that allows BITNET sites to use the
Internet's File Transfer Protocol to send/receive files to
ftpable Internet sites. For more information, send mail
to BITFTP@PUCC with HELP as the message body.
n. Database of lists managed by LISTSERV@VM1.NODAK.EDU. Use through
LISTSERV's database interface.
o. Maas files--Indexes & abstracts about various services available
via Internet & BITNET including some related to campus networks.
Available via anonymous ftp from ftp.unt.edu.
p. NETSCOUT@VMTECMEX.BITNET mailing list. A list to exchange
information on the location of network resources.
LISTSERV-based so use instructions below to subscribe, etc.
q. World Wide Web servers. You need WWW or Mosaic software to
access them. A good server to start with is www.ncsa.uiuc.edu.
Mosaic is available from ftp.ncsa.uiuc.edu.
3. LISTSERV Mailing Lists
Send a "SUBSCRIBE" command to LISTSERV@foo, e.g.
SUBSCRIBE BIG-LAN John Doe
a. BIG-LAN@LISTSERV.SYR.EDU
b. NOVELL@LISTSERV.SYR.EDU
c. CDROMLAN@IDBSU.BITNET/IDBSU.IDBSU.EDU
d. BANYAN-L@AKRONVM.BITNET
e. CW-EMAIL@TECMTYVM.BITNET (Campus Wide E-mail)
f. CWIS-L@WUVMD.BITNET (Campus Wide Information Systems)
g. IBM-NETS@BITNIC.BITNET
h. LWUSERS@NDSUVM1.BITNET (LANWatch User List)
i. TN3270-L@RUTVM1.BITNET
j. 3COM-L@NUSVM.BITNET
h. HELP-NET@TEMPLEVM.BITNET (Help re networking software)
i. LANWORKS@MIAMIU.BITNET (LanWorks PCSA stuff)
j. LANMAN-L@NIHLIST.BITNET (MS LAN MAN stuff)
4. Internet Mailing Lists
Send a subscription request for list foo to foo-request@blah
a. big-lan@listserv.syr.edu (gives you 2 ways)
b. cisco@spot.colorado.edu
c. p4200@comet.cit.cornell.edu (Proteon routers)
d. tcp-ip@nic.ddn.mil
e. netblazer-users@telebit.com
f. info-appletalk@andrew.cmu.edu
g. net-ops@nsl.dec.com
h. nfs@tmc.edu
i. wellfleet-l@nstn.ns.ca
j. ospf@trantor.umd.edu (OSPF IP routing protocol)
k. pop@jhunix.hcf.jhu.edu
l. bind@ucbarpa.berkeley.edu
m. pc-ip@udel.edu
n. drivers@sun.soe.clarkson.edu (Packet Drivers)
o. cell-relay@indiana.edu gatewayed to comp.dcom.cell-relay)
5. Internet Mailing Lists with automatic subscription
Send a "SUBSCRIBE" command to the listed server.
a. firewalls@greatcircle.com majordomo@greatcircle.com
(about firewall routers)
b. firewalls-digest@greatcircle.com majordomo@greatcircle.com
(same list in digested form)
6. USENET/Netnews Groups
a. comp.dcom.* lans.*, modems, sys.cisco, telecom, ...
b. comp.protocols.* appletalk, tcp-ip.*, ibm, ppp, ...
c. comp.sys.proteon
d. comp.sys.novell
e. comp.sys.mac.comm
f. bit.listserv.big-lan (Note: these groups give Netnews
g. bit.listserv.novell readers a way to read the corresponding
h. bit.listserv.cwis-l LISTSERV lists)
i. bit.listserv.cw-mail
j. bit.listserv.3com-l
k. alt.dcom.* catv, telecom, ...
7. Anonymous FTP-based Archive Sites
a. syr.edu: BIG-LAN mailing list; NOVELL mailing list; a collection
of network-oriented papers & faq memos.
b. spot.colorado.edu: cisco mailing list & some other network stuff
c. hsdndev.harvard.edu: (in ndtl/results) Results of Scott
Bradner's router benchmarks.
d. ftp.uu.net: a treasure trove of software.
e. wuarchive.wustl.edu: a treasure trove of software.
f. ftp.ftp.com: packet drivers, some Unix software, other stuff.
g. ftp.utexas.edu: collection of networking info & software--
a lot of good information about Ethernet.
h. ftp.novell.com: files Novell makes available. Mirrored at
netlab2.usu.edu, bnug.proteon.com, ftp.rg.nl, tui.lincoln.ac.nz.
i. ftp.cisco.com: files Cisco makes available & some interesting
applications.
j. gatekeeper.dec.com: a treasure trove of software & stuff
(the stuff that was on decwrl.dec.com).
k. lux.levels.unisa.edu.au: files that 3Com distributes via
Compuserve.
l. ftp.unt.edu: Maas files and other goodies.
m. oak.oakland.edu: "the simtel collection, formerly at
simtel20.army.mil"; a treasure trove of software, including
packet drivers (pd1:<msdos.pktdrvr>). Mirrored on ftp.uu.net
and wuarchive.wustl.edu.
n. osi.ncsl.nist.gov: online copies of GOSIP & related documents.
8. LISTSERV-based Archive Sites
The brave can mail the command "INFO FILES" and/or the command
"INFO DATABASE" to the LISTSERV for instructions.
a. LISTSERV@LISTSERV.SYR.EDU: BIG-LAN & NOVELL mailing list archives.
9. RFCs (Internet "Request For Comments")
Some anonymous ftp sites for RFCs: nic.ddn.mil, ftp.nisc.sri.com,
nis.nsf.net, nisc.jvnc.net, venera.isi.edu, wuarchive.wustl.edu,
ftp.salford.ac.uk.
There are also some mail-based file servers:
mailserver@nisc.sri.com, info-server@nnsc.nsf.net, and
sendrfc@jvnc.net.
a. RFC1470: FYI on a network management tool catalog: Tools for
monitoring and debugging TCP/IP internets and interconnected
devices
b. RFC1175: FYI on where to start: A bibliography of
internetworking information
c. RFC1594: FYI on Questions and Answers: Answers to Commonly asked
"New Internet User" Questions
d. RFC1178: Choosing a name for your computer
e. RFC1207: FYI on Questions and Answers: Answers to commonly
asked "experienced Internet user" questions
f. RFC1244: Site Security Handbook
g. RFC1118: Hitchhiker's Guide to the Internet
h. RFC1122 & RFC1123: Requirements for Internet Hosts
i. RFC1208: A Glossary of Networking Terms
j. RFC1180: A TCP/IP Tutorial
k. RFC1173: Responsibilities of Host and Network Managers: A
Summary of the Oral Tradition of the Internet
l. IAB Official Protocol Standards (Currently RFC1540 but it is
periodically updated & given a new RFC number)
m. Assigned Numbers (Currently RFC1340 but it is periodically
updated & given a new RFC number; Includes field-values for
protocols in the TCP/IP family as well as some others)
n. RFC1392: Internet User's Glossary
10. Other Useful Online Papers
a. Charles Spurgeon. "Network Reading List: TCP/IP, UNIX, and
Ethernet". Available via anonymous ftp from ftp.utexas.edu
in directory pub/netinfo/docs as net-read.txt and netread-ps.
Also available via electronic-mail-based archive server. Send
the word "help" in the subject header or body of a message
to archive-server@ftp.utexas.edu for more information.
Also available via www.
b. Charles Hedrick. "Introduction to the Administration of an
Internet-based Local Network". Available via anonymous ftp
from cs.rutgers.edu as runet/tcp-ip-admin.doc (also .ps).
c. Charles Hedrick. "Introduction to Internet Protocols."
Available via anonymous ftp from cs.rutgers.edu as
runet/tcp-ip-intro.doc (also .ps).
d. Unofficial lists of codes used on 802.3 & Ethernet networks.
Portions of the official list are not released, so various
people compile unofficial lists. One that is available via
anonymous ftp is Michael Patton's pub/map/EtherNet-Codes
on ftp.lcs.mit.edu. See also RFC: "Assigned Numbers".
e. Arthur Green: "Frequently Asked Questions for
NOVELL@LISTSERV.SYR.EDU Mailing List." Available via anonymous
ftp from midir.ucd.ie.
f. Brendan Kehoe: "Zen and the Art of the Internet: A Beginner's
Guide to the Internet." Available via anonymous ftp from
ftp.cs.widener.edu in the pub/zen directory.
g. ATM Bibliography. Available via anonymous ftp from
mythos.ucs.indiana.edu.
h. John Wobus. "Lan Mail Protocols". Available via anonymous ftp
from syr.edu under information/faqs/lan-mail-protocols
i. John Wobus. "Lan Technology". Available via anonymous ftp from
syr.edu under information/faqs/lan-technology
j. Charles Spurgeon. "Guide to Ethernet". Available via anonymous
ftp from ftp.utexas.edu in pub/netinfo/ethernet as ethernet-guide.ps.
See a above.
k. Charles Spurgeon. "Guide to Ethernet Configuration". Available via
anonymous ftp from ftp.utexas.edu in pub/netinfo/ethernet as
ethernet-config.ps.
11. Sources of Protocol Documents
a. Ethernet V2 DEC-Direct; 1-800-344-4825; DEC Part Number
AA-K759B-TK.
b. IEEE 802 (802.3, Token Ring, 10BASE-T, etc) IEEE;
1-800-678-IEEE.
c. TCP/IP RFCs. See RFCs (above).
d. AppleTalk APDA; 1-800-282-APDA. Now a book in the
"Inside" series.
e. OSI Omnicom Inc.; 1-800-666-4266.
f. DECNet DEC.
g. SNA IBM.
h. Novell(IPX) Built on XNS; rest is designed by Novell.
i. FDDI ANSI; 1-212-642-4900.
Also Global Engineering Documents; 1-800-854-7179.
2805 McGaw Avenue; PO Box 19539; Irvine, CA 92714;
1-714-261-1455.
j. CCITT United Nations book shop in New York
Some of the documents are available via ftp from
world.std.com & ftp.uu.net & other sites.
k. GOSIP NTIS Sales Dept; (703)487-4650; Document
FIPS 146-1; See also Anonymous FTP-based Archive
Sites
l. XNS Xerox.
12. Useful Free Software
(see also RFC1470; listed above)
a. CUTCP (TCP/IP client for PCs) sun.soe.clarkson.edu,
omnigate.clarkson.edu
b. NCSA Telnet (Telnet clients for PCs & Macs) ftp.nsca.uiuc.edu
c. Eudora (POP3 Client for Macs) ux1.cso.uiuc.edu
d. POPmail (POP3 Client for PCs & Macs)
boombox.micro.umn.edu
e. PCROUTE (Makes IP router out of PC) accuvax.nwu.edu
f. PCBRIDGE (Makes bridge out of PC) accuvax.nwu.edu
g. Packet Drivers (Drivers for various PC LAN cards)
oak.oakland.edu
h. WinQVT (IP clients for Windows) ftp.cica.indiana.edu
i. ka9q (TCP/IP for PCs and Macs) ucsd.edu
j. PC/IP (TCP/IP client for MS-DOS) husc6.harvard.edu
k. charon (Pegasus/smtp gateway) omnigate.clarkson.edu
l. CAP (AppleTalk for Unix systems) rutgers.edu,
munnari.oz.au, gatekeeper.dec.com
m. Popper (POP3 server for Unix systems)
ftp.cc.berkeley.edu
n. Trumpet (PC Newsreader) oak.oakland.edu
o. bootpd (Bootp Daemon for Unix) lancaster.andrew.cmu.edu
p. NUPOP (POP3 daemon for MS-DOS) ftp.acns.nwu.edu
q. NETWATCH (PC Network watching program) netlab1.usu..edu
r. iupop3 (POP3 server for VMS) mythos.ucs.indiana.edu
s. Beholder (PC Network watching program) ?
t. KarlBridge (PC-based filter bridge)
nisca.acs.ohio-state.edu
u. Mosaic (multifacited information/news client)
ftp.ncsa.uiuc.edu
v. Gopher (client/server information system) boombox?
w. Pegasus (Mail client for PCs & Macs) risc.ua.edu
x. Kermit (terminal emulator) Columbia U
y. netatalk (AppleTalk for UNIX Systems) terminator.rs.itd.umich.ed
u z. etherman (X-based Ethenet monitoring) ftp.cs.curtin.edu.au
aa. interman (X-based IP monitoring) ftp.cs.curtin.edu.au
bb. packetman (Ethernet packet analyzer) ftp.cs.curtin.edu.au
13. Books
The following books were mentioned by responders to the 12/93
BIG-LAN Reader Survey as good books for administrators of
Campus-sized LANs:
a. Douglas Comer. Internetworking with TCP/IP.
b. Albitz & Liu. DNS and BIND.
c. Mark Miller. Troubleshooting Internetworks.
d. Ed Kroll. The whole Internet.
e. Marshall Rose. The Simple Book.
f. Craig Hunt. TCP/IP Network Administration.
g. Andrew Tanenbaum. Computer Networks.
h. Nemeth, Snyder & Seebass. Unix System Administration Handbook.
i. Stevens. Unix Network Programming
j. Martin A. W. Nemzow. Keeping The Link (McGraw-Hill).
k. Interconnections. Radia Perlman
l. Inside AppleTalk.
m. Caroline Arms. Campus Networking Strategies. Digital Press.
Out of print.
Also mentioned were books published by O'Reilly in general.
14. Periodicals
The following periodicals were mentioned by responders to the 12/93
BIG-LAN Reader Survey as good periodicals for administrators of
Campus-sized LANs:
a. Network World
b. Data Communications
c. LAN Magazine
d. LAN Times
e. Communications Week
f. PC Week
g. Network Computing
h. InfoWorld
i. ConneXions
j. Byte
k. Unix World
l. Macworld
m. MacWEEK
n. PC Magazine
o. Open Systems Today
p. Network Management
q. Lightwave
15. Training Courses
The following providers of tutorials were mentioned by responders
to the 12/93 BIG-LAN Reader Survey:
a. Interop Tutorials
b. Cisco training
c. Westnet training
d. Network World: Understanding SNMP
e. Trellis training
f. TC3 Land/Wan Video
g. TC3 NetWare 3.11
h. PDA Data Communications
i. Hewlett-Packard free seminars
j. Fred Prior Project Management Seminars
k. CRAY Research training program
l. Banyan training
16. Conferences
The following conferences were mentioned by responders to the 12/93
BIG-LAN Reader Survey as good conferences for administrators of
Campus-sized LANs:
a. Interop
b. EDUCOM
c. Networld
d. Comnet
e. Association of Banyan Users International
f. ACUTA
IV. Basic Glossary on Campus Networks
Another glossary is RFC1208. See "Online Papers" above.
100BASE-T - A set of proposals to the IEEE 802.3 for 100Mbps
Ethernet, called 100BASE-TX, 100BASE-TF, and 100BASE-T4. A
medium-independent interface and an adaptor is planned (to be
used like the AUI and MAU of 10Mbps 802.3). This is being
developed & promoted by the Fast Ethernet Alliance.
100BASE-T4 - Proposed IEEE 802.3 standard for a 100Mbps
Ethernet-like network. One of the flavors of "100BASE-T"
proposed by the Fast Ethernet Alliance. Uses 8B6T encoding and
25MHZ clocking, and in addition to the two pairs traditionally
used in the manner of 10BASE-T, also has two pair used in
bidirectional half-duplex fashion. Among other things, this
means that this particular kind of Ethernet cannot be made full
duplex without the use of more pair. Formerly called 4T+.
100BASE-TF - A proposal to IEEE 802.3 for a 100Mbps Ethernet-like
network. Borrows the physical characteristics of FDDI's
multimode fiber PMD, but uses Ethernet framing & CSMA/CD. One
of three flavors of "100BASE-T" proposed by the Fast Ethernet
Alliance. Formerly part of 100BASE-X proposal.
100BASE-TX - A proposal to IEEE 802.3 for a 100Mbps Ethernet-like
network. Borrows the physical characteristics of FDDI's TP-PMD,
TP-PMD, but uses Ethernet framing & CSMA/CD. One of three
flavors of "100BASE-T" proposed by the Fast Ethernet Alliance.
Formerly part of 100BASE-X proposal.
100BASE-X - Old name for 100BASE-TF and 100BASE-TX.
100Mbps Copper UNI - ATM Forum UNI specification for 100Mbps over
some sort of copper cable. I believe it is just 100MbpsUNI
making use of FDDI's TP-PMD rather than the older fiber PMD.
100Mbps UNI - ATM Forum 100Mbps multimode fiber private UNI. Same
as Fore's TAXI. Borrows optical characteristics & basic
encoding of FDDI.
100VG-AnyLAN - "100VG-AnyLAN": Originally a proposal to IEEE 802.3
for a 100Mbps Ethernet-like network, later relegated to IEEE
802.12. Formerly known as 100BASE-VG. Uses Demand Priority
media access method and Quartet Signaling. I've also seen
reference to its ability to use Category 4 UTP, Category 5 UTP,
and STP, but I don't know how many pairs.
100VG-AnyLAN Forum - Group of vendors trying to accelerate
100VG-AnyLAN acceptance & interoperability.
10BASE-F - Three variants of IEEE 802.3 which runs over multimode
fiber. See 10BASE-FB, 10BASE-FP, and 10BASE-FL.
10BASE-FB - IEEE 802.3 10BASE-FB: "Synchronous Ethernet" which is
a special-purpose multimode fiber link for linking repeaters
that allows the repeaters to communicate more efficiently, thus
enlarging the count of repeaters that can be placed in series
above the traditional 4. Described in IEEE 802.3 Section 17.
10BASE-FL - IEEE 802.3 10BASE-FL: multimode fiber Ethernet used to
attach a pair of devices (each being either a host or a
repeater) as a "Link Segment"--a lot like 10BASE-T except that
it uses fiber. It makes FOIRL obsolete. 10BASE-FL transceivers
can interoperate with FOIRL transceivers. It is described in
IEEE 802.3 Section 18.
10BASE-FP - IEEE 802.3 10BASE-FP: passive star fiber Ethernet.
Attaches a number of Ethernet devices together with a passive
star hub (i.e., the hub is not electronic--it just splits the
light travelling through each incoming fiber to go out all the
outgoing fibers). It is described in IEEE 802.3 Section 16.
10BASE-T - A variant of IEEE 802.3 which allows stations to be
attached via twisted-pair cable.
155Mbps UNI - ATM Forum 155Mbps private UNI. In two flavors:
multimode and shielded twisted-pair. The multimode version is
incompatible with STS3cUNI. This version is for private
networks only and presumably will be less expensive. I heard
that a C5 version has been proposed.
25Mbps UNI - IBM proposed copper interface for ATM that so far as
been rejected by the ATM Forum. IBM's proposal that borrows
some of Token Ring's signaling characteristics. I've read the
statement that the ATM Forum doesn't support this proposal.
4T+ - Old name for 100BASE-T4.
51Mbps UNI - I don't know the actual name. ATM Forum 51Mbps UNI
for Category 3 UTP. Uses AT&T's 16-CAP (a 16 constellation
modem-type modulation scheme) line coding to transmit the
signal. The transmission convergence layer (framing) conforms
to the STS-1 SONET standard.
802, 802.x - see IEEE 802, IEEE 802.x.
ANSI "American National Standards Institute" - A definer of
standards of all kinds, including FDDI.
ANSI X3 - ANSI group developing standards for information
processing.
ANSI X3T9 - ANSI group within X3 developing standards for I/O
interfaces.
ANSI X3T9.3 Committee - ANSI group within X3T9 standardizing HiPPI.
ANSI X3T9.5 Committee - ANSI group within X3T9 that standardized
FDDI, PMD, SMF-PMD, and is standardizing TP-PMD and LCF-PMD.
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 can be run at different speeds over different
media including T1 and DS3 as well as 51Mbps, 100Mbps, 155Mbps
and 622Mbps standards (see SONET & TAXI). 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.
ATM Forum - Non-profit international industry consortium chartered
to accelerate ATM acceptance & interoperability.
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).
Backbone - a fairly nebulous term for a part of the network that
interconnects other parts of the network. For example, a campus
might have an FDDI ring that interconnects a number of
Ethernets. The FDDI ring could be called the network's
backbone.
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.
Category 3 Unshielded Twisted Pair - standardization of unshielded
twisted pair cable for voice use. Some data communications
standards such as 10BASE-T can utilize it.
Category 4 Unshielded Twisted Pair - standardization of unshielded
twisted pair cable.
Category 5 Unshielded Twisted Pair - standardization of unshielded
twisted pair cable for data use. TP-PMD requires Category
5 cable rather than Category 3.
CDDI "Copper Data Distribution Interface" - Commonly used term
for TP-PMD, but actually a trade name of Crescendo.
Cell - An ATM 53-byte cell. Note: there are various proposals for
how typical packets will be broken into cells and restored.
Cell Switching - a term for ATM-style networks. See "ATM".
CMIP "Common Management Information Protocol" - An OSI protocol
for management of network equipment. Not widely implemented.
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.
Collapsed Backbone - a network backbone that is located in a
single room. It might be a single router or multiport bridge,
or a small LAN of some sort. A typical collapsed-backbone-
style campus LAN might consist of Ethernets in a number
of buildings, each with a repeated fiber link into a single room
at a central point where a router interconnects them. An
example of the opposite would be putting a router in each
building and interconnecting them all with a big FDDI ring.
Concentrator - a device which allows a number of stations to
be connected to a LAN. In the case of Ethernet, it is
simply a multi-port repeater. In the case of ring networks
like Token Ring and FDDI, it acts as a switch which keeps
the ring intact even if individual devices are unplugged.
Counterrotating Ring - (see Ring, FDDI, Token Ring) a method of
using two ring networks going in opposite directions to provide
redundancy. The network interfaces can change the path of the
ring that the data flows around, thereby preserving the ring
(thus the operation of the LAN) even if some of the cable is
uplugged or cut, or if a device on the ring fails in such a way
that it can't transmit data around the ring.
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.
DS3 UNI - ATM Forum DS3 UNI, 44.236Mbps. Also called HSSI?
DXI - ATM Forum "Data Exchange Interface".
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.
Fast Ethernet Alliance - Group of vendors working on a 100Mbps
version of IEEE 802.3. They intend to submit their proposals
for approval by the IEEE for a new set of 802.3 standards called
100BASE-T.
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 bandwidth
to applications of your choice, making it more like broadband.
FDDI-2 is still rather new.
FDSE - Full Duplex Ethernet: a variant of Switched Ethernet which
does not use CSMA/CD, but uses slightly-modified network
interface cards to send & receive packets simultaneously.
Presumably based on 10BASE-T for most clients, and cannot be
based on ThinWire or ThickWire Ethernet.
Fiber - optical fiber: a very long, narrow, flexible piece of
glass. Used for high-speed communications.
Fibre Channel - an ANSI standard to replace HiPPI. It uses optical
fiber instead of copper cables. Speeds are up to roughly
1Gbps.
Fibre Channel Systems Initiative - Group of vendors trying to
accelerate Fiber Channel acceptance & interoperability. Members
include: HP, IBM, Sun.
Firewall Router - a router which blocks traffic according to
various criteria for security--for example a router which
allows no telnet to any host through one of its interfaces
but allows ftp to a list of authorized hosts through the
same interface.
FOIRL "Fiber Optic Inter-Repeater Link" - a standard for running
IEEE 802.3 over fiber, linking two devices (each either a host
or a repeater) as a "Link Segment". It has been replaced by
10BASE-FL.
FTP - Protocol in the "TCP/IP" family for copying files from
one computer to another. Stands for "File Transfer Protocol".
Full Duplex Switched Ethernet Consortium - Group of vendors that
are working out the details of FDSE. Cabletron is a member.
Full Duplex Token Ring - IBM scheme to add switching to token-ring
hubs that would allow full-duplex linking to individual
computers using modified token-ring adaptors. Has the same
wiring characteristics as token ring.
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.
HiPPI - "High Performance Parallel Interface", ANSI draft standard
X3T9.3.
HSSI "High Speed Serial Interface" -
Hub - a nebulous term, typically applied to a multiport repeater
or concentrator consisting of a chassis with slots to be
populated by cards, allowing it to be configured with various
numbers and combinations of LAN ports. Vendors of networking
equipment often also have other types of devices that can be
inserted in the slots such as terminal servers, bridges,
routers, gateways, etc.
IEEE - Institute of Electrical & Electronic Engineers
IEEE 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.
IEEE 802 Group within IEEE that standardizes LAN technologies.
IEEE 802.1 - The IEEE 802 standard for Network Management and
Network Bridging of IEEE 802 networks.
IEEE 802.11 - Proposed IEEE 802 group for wireless Ethernet.
IEEE 802.12 - Group within IEEE 802 working on 100VG-AnyLAN.
IEEE 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.
IEEE 802.3 - An IEEE standard for LANs--their "improved" version of
Ethernet. See Ethernet.
IEEE 802.3 - Group within IEEE 802 that standardizes CSMA/CD LANs.
IEEE 802.4 - An IEEE standard for LANs: Token Bus networks.
Basically, standardizes MAP, a protocol that operates a Token
Bus protocol on broadband.
IEEE 802.5 - An IEEE standard for Token-Ring-based LANs. There
are two types: 4Mbps and 16Mbps. See also "Token Ring".
IEEE 802.6 - An IEEE standard for Metropolitan Area Networks. Also
known as DQDB.
IEEE 802.7 - IEEE 802 technical advisory group on Broadband.
IEEE 802.8 - IEEE 802 technical advisory group on FDDI & fiber
optics.
IEEE 802.9 - IEEE 802 group on integrated data & voice networks.
IMAP "Internet Mail Access Protocol" - TCP/IP-based protocol
similar to POP, but with additional function designed to handle
storage of mail on the server rather than the client. There are
two versions in common use: IMAP2 and IMAP4.
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.
LCF-PMD - FDDI "Low-Cost Fiber" PMD. Less expensive than PMD. I
don't believe it is common nor is it finished as a standard.
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.
OCx - (e.g. OC1, OC3) variants of SONET.
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.
PMD - FDDI "Physical Layer Medium Dependent" part. When "PMD" is
used by itself, it may refer to the usual kind of FDDI physical
layer that uses multimode fiber. Note that FDDI terminology
also uses it as a more generic term, referring to different FDDI
PMD's such as TP-PMD and SMF-PMD.
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.
Ring - A classification of network technology exemplified by
Token Ring and FDDI. The interconnected devices are connected
one-to-another in the shape of a ring and data flows around
it in one direction. See also "Counterrotating Ring".
RJ numbers ("Regestered Jack" numbers, e.g. RJ11, RJ45) - numbers
applied to types of connectors often used in UTP wiring.
Borrowed from voice telecommunications industry.
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.
SDH "Synchronous Digital Hierarchy" - Similar to SONET, but used
outside North America. Some of the SDH and SONET standards are
identical. Standardized by the CCITT.
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".
SMF-PMD - FDDI "Single-Mode Fiber" PMD. Runs further than PMD.
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.
SONET "Synchronous Optical Network" - A set of standard
fiber-optic-based serial standards planned for use with ATM in
North America. Developed by Bellcore. Different types of SONET
run at different speeds (OC1 runs at 51Mbps, OC3 runs at
155Mbps, OC12 runs at about 600Mbps, OC48 runs at over 2Gbps),
and use different types of fiber (OC3 has several variants for
use with different fibers & different distances; there are
versions for both single mode and multimode fiber).
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.
STP - Shielded Twisted Pair
STS-3c UNI - ATM Forum SONET STS-3c UNI, 155.52Mbps.
Switched Ethernet - really the same as Ethernet as far as
standards go: acts like a very fast multiport Ethernet bridge
giving an Ethernet to each station. Presumably based on
10BASE-T for most stations.
Switched FDDI - really the same as FDDI as far as standards
go: acts like a very fast multiport FDDI bridge. Basically the
DEC GigaSwitch.
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.
TAXI - "Transparent Asynchronous Transmitter-Receiver Interface"
Two ATM UNI specifications developed by Fore. The slower one
ran at 100Mbps and borrowed the physical characteristics of FDDI
and has been adopted by the ATM Forum as its 100Mbps UNI
specification. The faster one ran at 140Mbps.
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.
ThickWire - "ThickWire" Ethernet or IEEE 802.3 10BASE5.
ThinWire - ThinWire Ethernet or IEEE 802.3 10BASE2.
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 use the term "Token Ring" to designate
IEEE 802.5 (see above). In the more general sense of the
phrase, a token ring is 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.
TP - "Twisted Pair".
TP-PMD - FDDI "Twisted Pair Physical Layer Medium". ANSI
specification for FDDI-like service over UTP. Being
standardized by ANSI X3T9.5. Was X3T9/93-130 X3T9.5/93-022
TP-PMD/306 Rev 2.0, now there is a Rev 2.1. Uses MLT-3 encoding
instead of CDDI's NRZI encoding.
Tunneling - 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").
Type1 - IBM Type 1 STP. The most usual type of Shielded Twisted
Pair in LAN communications.
UNI - ATM Forum "User to Network Interface". See ATM.
UTP (Unshielded Twisted-Pair) - See "Twisted-Pair" and "Shielded
Twisted-Pair".
X.400, X.500 - OSI protocols for mail and directory services.
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 "tunneling", 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 12/93 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: 13 said yes; 45 said no; 10 said some of each.
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?
[Note: with the advent of 10BASE-F, this section needs updating.
-ed]
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".
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).
6. Which LAN technology should I use? Arcnet? FDDI? Token Ring?
10BASE-T?
A controversial question. Some questions & answers from the 12/93
BIG-LAN Reader Survey:
"When you install a LAN, which "Technology" (e.g. Ethernet,
Token Ring) do you prefer?"
All respondents said Ethernet through three also said FDDI
is good.
"If you have experience with two or more LAN technologies, which
have you found works better?"
Answers received:
Ethernet works best 18
10BASE-T is best 6
Ethernet & FDDI work best 3
Ethernet is better than Token Ring 2
Ethernet costs less than FDDI 2
Localtalk better than 10BASE-T 1
FDDI is best 1
Ethernet is better than Pronet-10 1
Ethernet is better than ARCNet 1
Ethernet is better than PhoneNet 1
Ethernet followed by FDDI 1
Ethernet & Token Ring equal 1
Depends on how they are maintained 1
7. What is the ideal cable to install in a new building?
Distribution runs, i.e., phone closet to room: Best possible thing
to do is to leave usable pathways for future expansion. Whatever
you do, install at least 2 pair and probably 4 pair of data grade
unshielded twisted pair. It will always have uses. Install
something else too if you are tied to a particular vendor.
Multimode fiber might become popular in the future but that is a
gamble.
Riser runs, i.e., phone closet to phone closet: it is imperative
to leave usable pathways for future expansion. For Ethernet,
ThinWire is a usable riser cable, multimode fiber is possible
too.
8. What is the ideal cable to install between buildings on a campus?
Trunks, i.e., cables into the building: pathways for future
expansion very valuable. Multimode fiber is useful, run 24 fibers
if you can. Use cable with some single mode too. Run several
times what you need initially and leave a lot of the unused fiber
unterminated for the time being. Cable pulling & termination are
much more costly than the cable itself.
9. Whose routers are recommended?
Question & answer from the 12/93 BIG-LAN Reader Survey:
"Name some router vendors whose routers you have used and
recommend:"
Cisco got 55 mentions; Wellfleet 9; Proteon 8; 3Com 3; Novell 3;
Xyplex 3; Network Systems 2; DEC 2; HP 2; NAT 2; Retix 1; NAC 1;
GatorBox 1; Alantec 1; Telebit 1; Fibronics 1; Shiva 1;
PCRoute 1.
10. Whose bridges are recommended?
Question & answer from the 12/93 BIG-LAN Reader Survey:
"Name some bridge vendors whose routers you have used and
recommend:"
DEC got 11 mentions; 3Com 8; Cabletron 5; Retix 5; Xyplex 5; HP
4; Cisco 3; Gandalf 3; Wellfleet 2; D-link 1; Asante 1; ODS 1;
Synernetics 1; PlainTree 1; Alantec 1; Artel 1; Develcon 1;
Gandalf 1; karl-bridge 1; Allied Telesis 1; Vitalink 1; ATT 1.
11. Whose Ethernet equipment are recommended?
Question & answer from the 12/93 BIG-LAN Reader Survey:
"Name some Ethernet concentrator/transceiver/repeater vendors
whose Ethernet equipment you have used and recommend:"
Cabletron got 30 mentions; 3Com 15; Allied Telesis 15; HP 13;
Synoptics 11; Asante 9; Chipcom 8; DEC 7; SMC 7; David Systems
4; Xyplex 3; Milan 2; Lantronix 2; Gandalf 2; D-Link 2; Canary
2; ATT 2; BlackBox 2; Hughes 2; Fibermux 2; St. Clair 1;
Pirelli-Focom 1; Pilkington 1; ODS 1; Networth 1; LANNET 1;
Kalpana 1; Isolan 1; Interphase 1; Intel 1; IMC 1; Hirschmann 1;
Fibercom 1; BICC 1.
12. Whose Token Ring equipment are recommended?
Query and answers from the 12/93 BIG-LAN Reader Survey:
"Name some Token Ring equipment vendors whose Token Ring
equipment you have used and recommend:"
IBM was mentioned by 12 responders; Proteon 3; ODS 2; UB 1;
Thomas-Conrad 1; Startek 1; Madge 1; HP 1; Cabletron 1; CSP 1.
13. Whose FDDI equipment are recommended?
Query and answers from the 12/93 BIG-LAN Reader Survey:
"Name some FDDI equipment vendors whose FDDI equipment you have
used and recommend:"
Cisco was mentioned by 8 responders; Crescendo 7; DEC 5;
Synoptics 3; Interphase 3; 3Com 3; Fibronics 2; Cabletron 2;
Synernetics 1; Sun 1; SGI 1; Proteon 1; PlainTree 1; ODS 1;
Network Peripherals 1; IBM 1; Fibermux 1; Chipcom 1.
14. What PC network software is recommended?
Query and answers from the 12/93 BIG-LAN Reader Survey:
"Name some PC network software vendors whose PC network software
you have used or recommend:"
Novell was mentioned by 32 responders; FTP Software 21; Apple 7;
SunSelect 6; Microsoft 5; NCSA 4; IBM 4; Banyan 4; DEC 4;
NetManage 3; Clarkson 3; 3Com 3; Word Perfect 2; WinQVT 2;
Reflection 2; Qualcomm 2; Brightworks 2; Beame & Whiteside 2.
15. What protocols should run on a campus-wide LAN?
Query and answers from the 12/93 BIG-LAN Reader Survey:
"Name some protocols that you use to interconnect your campus
that you would recommend:"
TCP/IP was mentioned by 63 responders; IPX 26; AppleTalk 17;
DECNet 7; LAT 3; VINES 2; SNA 2; CLNS 1.
16. What software is recommended for managing a campus-wide LAN?
Queries and answers from the 12/93 BIG-LAN Reader Survey:
"Name some network management system that you use for the
management of a campus LAN, that you recommend:"
SunNet Manager was mentioned by 13 respondents; HP OpenView 8;
Cabletron Spectrum 3; Cabletron Remote LanView 3; PSI SNMP 2;
Netlabs 2; CiscoWorks 2.
"Name other software that you use for the management of a campus
LAN that you recommend:"
Ping was mentioned by 4 respondents; Traceroute 3; SunNet
Manager 2; Network General Sniffer 2; Neon Software NetMinder 2;
CMU SNMP 2.
17. What terminal server is recommended?
Query and answers from the 12/93 BIG-LAN Reader Survey:
"Name vendors of terminal servers that you use and recommend:"
Cisco was mentioned by 21 respondents; Xylogics 12; Xyplex 11;
DEC 9; Emulex 4; Spider 2; Equinox 2; Netblazer 1; Livingston 1;
Lantronix 1; HP 1; Datability 1; Digiboard 1; Allied Telesis 1;
3Com 1.
18. Whose troubleshooting equipment are recommended?
Query and answers from the 12/93 BIG-LAN Reader Survey:
"Name some vendors of network troubleshooting equipment that you
use and would recommend:"
Network General was mentioned by 30 respondents; HP 11;
MicroTest 4; Tektronix 3; Spider 3; Fluke 3; FOTEC 3; W&G 2;
Novell 2; FTP 2; Exfo 2; Van Jacobson 1; Pentascanner 1; NCC 1;
NAT 1; LM-1 1; Consultronics 1; Antel 1; AG Group 1.
19. What security products should I buy?
Query and answers from the 12/93 BIG-LAN Reader Survey:
"Name some security products that you use to maintain security
on your campus LAN that you recommend:"
COPS was mentioned by 5 respondents; tcpwrapper(s) 3; SecurID 3;
Crack 3; Cisco access control 2; xtacacs 1; npassword 1;
Tripwire 1; Socks 1; Netware 1; Native VINES security 1; McAffee
Anti-Virus NLM 1; HP 1; Bridges 1; Beame and Whiteside 1.
20. Should the names of devices on my campus LAN have subdomains?
Example of name without subdomain: bigvax.sequoia.edu; example
with subdomain: bigvax.acs.sequoia.edu. It is possible to run
networks of thousands of computers without the bother of
subdomains, but they have some advantages.
Queries and answers from the 12/93 BIG-LAN Reader Survey:
"For Internet names of nodes on a campus network that supports
TCP/IP, do you prefer the use of subdomains?"
49 responders said yes, 11 said no, 3 said it depends.
"If you have worked on a campus that utilizes subdomains and one
that does not, which does your experience tell you is the better
way to administer names in a campus network?"
13 responders said the LAN with subdomains worked better; 1 said
the LAN without subdomains worked better; 2 said it doesn't
matter and 3 said it depends.
21. Should client stations use POP? Should they use just SMTP?
Should I use some non-TCP/IP protocol for mail to/from client
stations?
Query and answers from the 12/93 BIG-LAN Reader Survey:
"For client station's mail, which do you prefer: SMTP;
TCP/IP-based client-server protocols (e.g. POP, POP2, etc);
other LAN protocols?"
22 responders preferred TCP/IP-based client-server protocols
(e.g. POP, IMAP, PCMAIL); 20 preferred SMTP; 5 preferred other
LAN protocols; 3 said "use all three"; 3 said "SMTP and
TCP/IP-based client-server protocols"; 3 said "SMTP and other
LAN protocols"; 1 said "TCP/IP-based Client-server Protocols and
other LAN protocols".
22. Should I enable SQE/heartbeat?
SQE Test (often labeled "SQE" by vendors) is part of IEEE 802.3
that is designed to test part of the the MAU (transceiver)
hardware. It basically consists of the MAU trying out the
collision signal line immediately after each packet it sends.
Thus a station on the network can verify that the MAU is working
by watching for this signal and can log an error for you if the
signal is not present. Correct practice is to turn SQE Test off
on any MAU that is attached to a repeater and turn it on on any
MAU attached to a station. Not doing this can lead to incorrect
repeater operation and/or a lack of logging of serious network
errors when they occur.
However, many vendors of networkable stations take no advantage of
SQE Test (it was new to IEEE 802.3 & Ethernet Version 2, not being
present in earlier Ethernet) and there have been many reports of
stations that won't even work properly when it is enabled. Thus
your dilemma: some of your users may have stations that won't work
unless you set your MAU's wrong. Maybe some day all vendors will
fall into line, or the IEEE will revise its standard to get rid of
SQE Test. In the mean time you are forced to know which stations
log errors without it and which ones work poorly with it on.
Examples of computers/networking equipment sensitive (one way or
the other) to SQE test:
Definitely can't handle SQE Test:
No convincing confirmations
Mixed & inconclusive reports saying they can't handle SQE Test:
Some Sun workstations
Cisco routers
Needs SQE Test or it reports errors (i.e., uses SQE Test as
intended):
VAX/VMS
Alpha/VMS
23. If I have a thinwire network interface card, how do I connect it
to a 10BASE-T concentrator?
Ethernet standard provides only one way to do interconnect
thinwire (10BASE2) and 10BASE-T: using a repeater (e.g. a
concentrator). Since this is expensive and it increases the
repeater count, thus limiting the expanse of the rest of the
network, customers want, and several vendors provide adaptors that
are not real repeaters. Typically, these allow a 10BASE-T segment
to end in a shorter-than-usual thinwire segment. One depends upon
the vendor to provide instructions as to how its use affects the
limitations on segment lengths and repeater counts.
24. How much does a collision slow down an Ethernet packet?
Perhaps you've noticed the phenomena that you might ask otherwise
intelligent & knowledgeable network professionals how many
collisions indicate too much load, and they immediately divert the
conversation to the question of whether your network is broken.
The implication is that they're more inclined to believe your
Ethernet is performing poorly due to being broken than due to load.
Here's an explanation, probably more than you ever wanted to
know:
Coaxial Ethernet was designed so that everyone shares the same
single cable. Electrical characteristics of transmission were
chosen so that when more than one station places bits on the
network, the voltages in effect "add" and the transceiver can
sense the "unusual" voltage as a collision.
Transceivers detect the collisions, and signal the stations by
raising a "collision detect" line to the station. According to
the standard, transceivers signal any collision that occurs when
it is sending a packet, and also any triple collision.
The Network Interface hardware takes care of retransmissions and
reports the collision to the driver. It might not report complete
information on the number of collisions--for example, one Ethernet
chip will report after each packet it sends, whether there were 0,
1, 2, <16, or >16 Collisions. The driver usually keeps a count
that it updates from the information it gets from the card.
Repeaters do not "recreate" electrical collisions on other
networks. Any time the repeater detects a collision, it is, by
definition, in the midst of transmitting a packet. It can no
longer pick up valid data off the net to continue sending the
packet. The Ethernet spec says it should start sending 32 bits of
made-up data (called a JAM) that will make the packet terminate
early, with a CRC error. None receiving stations on the other
side of the repeater will see "collision" signaled by their
transceiver. Instead, they will receive just the beginning of a
packet. This is called a "runt". The network interface hardware
could, theoretically, report a runt as a collision, which might be
useful for some kinds of monitoring. Or the software, might
consider a runt a collision and increment the same count. Or it
can count them separately, or not count them at all. Software
that reports these separately from collisions usually refers to
them as runts or JAMs.
Link segments like 10BASE-T, FOIRL and 10BASE-FL attach only two
devices and have separate paths in each direction. Thus
collisions are superfluous, but must still be detected and
reported since Ethernet interfaces cannot be assumed to have the
ability to send and receive packets at the same time. Thus the
transceivers watch for packets flowing in both directions at the
same time, and signal collision to the station as well as produce
a JAM signal on the line so that the stations trying to send the
packets will get the message that this was a collision and the
packet needs to be resent.
Ethernet interfaces retransmit packets up to 16 times with an
exponential backoff for the first 10. The minimum retransmission
time is relatively quick and the detection process takes a fixed
amount of time, so 75% of all times that two stations are
contending for a net are resolved with one station starting a
successful transmission within 250 microseconds. It is important
to realize that Ethernet's collisions are a normal part of
scheduling the use of the LAN, that it is used only when carrier
sensing doesn't do the trick, and that Ethernet uses a
third-generation scheme that handles collisions very smoothly when
when the hardware works & is properly assembled, even under high
loads. A lot of mis-information is spread about collisions, often
from people dealing with Ethernet's competitors, but also often
from Ethernet users who simply haven't studied it too closely, or
listened to the wrong people.
A collision is always detected & taken care of (to the point of
starting the backoff) within the first 50 microseconds of a
packet's transmission on a correctly functioning Ethernet. Aside
from helping to limit the time spent dealing with collisions, this
insures that collisions of even the smallest legal packets are
always detected. Some interface hardware reports late collisions,
i.e. collisions signaled after this time: unlike collisions,
which are normal, late collisions are a type of error. Note that
on the other side of a repeater, the late collision simply looks
like a CRC error perhaps with an alignment error. There are two
causes of late collisions: faulty hardware; or the network being
too large. In either case, it tells you that the network is
having a problem, and packets are almost surely being lost
sometimes, causing unnecessary & occasionally severe performance
penalties. If the network is too large, properly placed routers,
bridges (or some switches) can subdivide it into two
properly-sized Ethernets.
Can random collisions cause packets to be lost? The exponential
backoff algorithm yields a probability of 50% that a pair of
colliding packets require more than one retransmission to get
through if two stations are contending for the net at exactly the
same time, and only 25% of the ones that still haven't succeeded
fail to get through after the second retransmission. For the
16-retry limit, the calculation of the faction not making it is:
1/2 x 1/4 x .... 1/(2*10) x (1/(2*10))**6
or (1/2)**115
or about (1/10)**34.
I conclude that on every Ethernet ever installed, for every packet
sent, that this has never happened (give me a billion LANs that
transmit a billion packets every day for a billion days and the
odds are still a million to one against even one lost packet).
When more than two stations are involved (i.e., more than two
stations have something to send at exactly the same time), these
odds aren't so overwhelming--thus I conclude that there have
indeed been packets lost on correctly functioning Ethernets
somewhere (Note: also the randomness of the backoff is probably
not perfect and I've heard of network interfaces that illegally
stop before 16 retries!). Recall also that stations do sense
carrier: collisions only resolve the problem of what happens when
the packets start at almost the same time. Probably the most
usual time for a collision is when two stations simultaneously see
the end of a packet, both having a packet to send. In this case,
there will be more than one collision on average, but as stated
above, 75% of the time, one of them will have started a successful
transmission within 250usec.
In contrast to the smooth handling of properly detected
collisions, an undetected collision causes a packet to be lost,
which must be retransmitted by software: for example NFS is often
set to time out at .5 seconds, so a lost packet (for example, the
result of an undetected collision) causes a delay typically 2000
times longer. Networks with problems that cause undetected
collisions, frequent unnecessary collisions, or lose packets for
other reasons are much worse performance killers than collisions
caused by an increase in load.
How many packets can you tolerate an Ethernet losing? 1 in 100?
1 in 1000? 1 in 10,000? 1 in 100,000? Depends. 1 in 100 is
very bad. Where do you draw the line? Back-of-an envelope
example of the effects: NFS often transmits blocks of 6 Ethernet
packets, the loss of any one of which results in the
retransmission of all 6. The loss of one packet in 12,000 means
that every 2,000th block takes on the order of 2000 times longer
to complete than normal, or performance is decreased to 50% of
that on a working Ethernet.
The Ethernet's packet loss problems are relative to those of your
router, bridge, or switch. Routers, bridges, and switches lose
packets when their buffers fill up, so if your
router/bridge/switch is losing one packet in 10,000, then for
traffic passing through the router/bridge/switch, addressing an
Ethernet packet loss rate of 1/100,000 would have little effect,
and addressing an Ethernet packet loss rate of 1/10,000 would help
no more than addressing your router/bridge/switch problem.
25. Should I worry about Ethernet tailgating?
Tailgating is a phenomena resulting from bugs in the design
of Ethernet interfaces, which some vendors claim are due
to ambiguities or changes in the Ethernet specification. There
was indeed a change in the IEEE 802.3 specification's wording
designed to eliminate misunderstanding.
Tailgating problems consist of packets following close after
packets, collisions, and/or noise: so close that some network
interfaces aren't ready to receive them yet.
The standard says network interfaces should wait a minimum of
9.6us after the end of a packet before sending another (the
"interpacket gap"). Network interfaces typically don't start
detecting the beginning of packets for a while after the end of a
packet (i.e. carrier goes to idle) to avoid trying to treat the
typical noise at the end of a packet as the beginning of the next
packet. This has been called its "blind time". The standard
doesn't specify how long the blind time should be, but naturally
it must be less than the 9.6us interpacket gap. However on real
products, the blind times vary between a fraction of 1us and 4us
or longer.
Another element is that some network interfaces sometimes send 24
bits of data while the line is idle: not a real packet: somehow
this causes short interpacket gaps. My guess is that it makes
some interfaces go blind while not stopping other interfaces
from sending immediately.
Some interfaces don't wait 9.6us after a collision before sending
a packet.
There have been interfaces that cheat on the 9.6us interpacket
gap after a packet. This is so explicitly against the standard
that vendors of such products have been quick to fix them.
Some products:
Tailgate Tailgate
Blind 24Bit after after
Time Garbage Collisions Packets
------- ------- ---------- --------
IBM PCMCIA 0.6us
(Notebook Sniffer)
Intel 82596 4.6us x
(Desktop Sniffer)
SEEQ 8003 x x
(Cisco, oldSGI)
AMD Lance AM7990 >4us
(Sun)
Intel 82586 long x
(oldSun)
oldKalpana x
------- ------- ---------- --------
Tailgate Tailgate
Blind 24Bit after after
Time Garbage Collisions Packets
(Notes: Information from InfoWorld, 11/93 and 3/94; IBM PCMCIA
cards are highly immune to the problems; Kalpana has fixed its
switches)
Example: If a network has two Suns that have Intel 82596 Ethernet
chips (A and B) and two other stations (C and D), you can have the
following situation:
C and D send packets which collide.
A sends a packet to B too soon after the collision.
B remains blind too long to receive the packet.
Thus TCP, NFS, or whatever, must retransmit. Typical NFS
retransmission time would be in the .5 to 1 second range, thus one
lost packet translates into .5-1 second of waiting. TCP
retransmission time adjusts itself to the network & is typically
shorter between stations on the same LAN, but, for example, can be
long if the packet is lost between a station and a router while
the station is talking over a WAN.
End of Memo: BIG-LAN Frequently Asked Questions
