



          {A Guided Tour of 10BASE-T


          The recent certification of the 10BASE-T specification churned up
          a wave of  new products and   a flood  of information --  company
          position  papers and magazine  articles explaining what  it's all
          about.  While some people see 10BASE-T simply as a newer and more
          flexible  implementation of Ethernet,  others are uncertain about
          what it  is and  does. And  technicians  faced with  implementing
          10BASE-T   are    justifiably  hesitant  to  proceed  without  an
          understanding of the underpinnings of the specification. 
          10BASE-T  gives Ethernet  network installers  the  option to  use
          unshielded twisted-pair  (UTP)   wiring.  It  provides a  way  to
          install networks using one of the least expensive and most common
          types   of wiring  without compromising network  performance. The
          new specification has established UTP as a  viable alternative to
          higher-cost  cabling  options  and  provides  a  touch-stone  for
          interoperability among  different vendors' products.

          Actually, UTP has  always been a wiring option.  The problem with
          pre-10BASE-T UTP  solutions,  however,  is that they've  all been
          proprietary.  Few  of   these  products  provide  any   level  of
          interoperability   with one  another. The  10BASE-T specification
          resolves these  incompatibilities, specifying  how devices   that
          communicate on a 10BASE-T segment are to perform.

          10BASE-T  is  actually   a  supplement  to  the   standard  802.3
          specification,  which established the  requirements for both thin
          and thick coax  networks. This  means that  the underpinnings  of
          802.3 - the  data transmission  clock rate, for example -- remain
          unchanged while the ability to operate with  UTP  wiring has been
          added.

          To support reliable  transmission of Ethernet signaling  over UTP
          wiring, four  problems needed  to   be resolved:  electromagnetic
          emissions (from  transmitting devices), susceptibility  (to other
          devices   transmitting in  the same  frequencies as  the 10BASE-T
          signal), crosstalk (between the UTP wires) and  jitter (explained
          later).  The  10BASE-T  committee  was  able  to  overcome  these
          problems by  leveraging off   the experience  of vendors  who had
          already  provided  UTP networking  solutions.  However, having  a
          specification alone didn't  necessarily ensure that the  products
          from   all  vendors  providing  10BASE-T    products  would  work
          together.  Like  most  IEEE  specifications,  10BASE-T  does  not
          actually tell a vendor  how to design compliant devices; instead,
          the  specification  simply  describes how  such  a  device should
          operate.

          {10BASE-T Defined


          10BASE-T defines two basic network components: the wiring and the
          devices that terminate  the  ends of  a wiring segment, known  as
          media attachment  units (MAUs). 10BASE-T cable segments use  four
          unshielded  wires, typically  at 22  to 26  AWG (between  0.6 and
          0.4mm thick). These wires are   attached to lines 1, 2, 3,  and 6
          of an  ISO-compliant physical interface  -- i.e., an  RJ-45 jack.
          These lines   correspond to "tip and ring"  receive and transmit,
          respectively ("tip and ring" are telephone terms for  essentially
          plus and  minus). Wiring  lengths run from  zero distance  -- two
          RJ-45 jacks wired together --  to some theoretical limit.

          10BASE-T provides a list  of specifications for a cable  segment,
          concluding with  the statement   that  these specifications  "are
          generally met by  100 meters  of 0.5mm  telephone twisted  pair."
          It's  theoretically  possible for a higher grade  of wiring under
          ideal  conditions  to  support  greater  segment    lengths.  For
          example,  AT&T's network  interface  cards  and  hubs  can  drive
          segments  up to  150  meters.   With  special  AT&T cabling  that
          distance  can be  extended to  200 meters.  The downside  of such
          segment-extension schemes is that they are proprietary, operating
          solely with a  single vendor's  equipment, which  is exactly what
          10BASE-T was designed to avoid.    If you're  not using  a scheme
          like  AT&T's,  however, you'll  probably need  to make  a careful
          analysis of your existing UTP  wire before using it for 10BASE-T.

          Another   misunderstanding   about  10BASE-T   is   its  supposed
          susceptibility to  electromagnetic  emissions. A  10BASE-T signal
          transmits in  a frequency  range of 10  to 200  Mhz. In  a common
          office   environment, few devices emit  electromagnetic radiation
          in that range with enough power to affect a  signal on a 10BASE-T
          UTP segment.

          While  it's theoretically possible that an FM radio transmitter's
          80 Mhz to 108 Mhz broadcast  could affect 10BASE-T transmissions,
          your LAN  would have to  be within  touching range  of the  radio
          transmitter to be affected.

          {Establishing Connections


          Included  in   the  10BASE-T   specification  are   the  required
          capabilities for  MAUs. Any device   intended to transmit  over a
          10BASE-T segment must interface to  the segment as a MAU. Typical
          10BASE-T devices that act as MAUs include network interface cards
          (NICs) and hubs. the 10BASE-T  specification  also has a name for
          devices  that can initiate  the transmission of  information over
          cable  segments: DTEs, for Data Terminating Equipment. A  typical
          DTE is a LAN PC with a NIC.   Physically,   10BASE-T  NICs   look
          almost exactly like  the 10BASE-2 and 10BASE-5 NICs  you've  used
          previously, because  the manufacture  of a  10BASE-T NIC  doesn't
          represent any unusual  redesign  or production  changes. Standard
          Ethernet NICs  usually provide  both BNC and  AUI interfaces  for
          10BASE-2 and  10BASE-5 connections,  respectively. Most  10BASE-T
          vendors have  simply removed  the  NICs  BNC connector and  a few
          chips, replacing these with an MAU chipset and RJ-45 connector. 

          The  network hub is  what actually supports  the interoperability
          between  different  DTE's.   Different  vendors  call the  hub  a
          multiport repeater, concentrator or wiring center.  10BASE-T hubs
          are  active devices; that is  they interact with the signal on  a
          10BASE-T  segment,   performing  vital  functions     for  packet
          retransmissions.  In  this role,  the  hub  serves  as  a  signal
          repeater.      To  work as a repeater port, the MAUs phase-locked
          loop (PLL) circuitry  senses a series of   changes in  voltage on
          the 10BASE-T  wire. This  set  of signals,  called the  preamble,
          tells the MAU  that a  packet  is to follow.  The PLL starts  its
          clock, knowing that the timing  for the packet signals will match
          the  preamble   timing.  The   MAU  also   knows  the   signaling
          requirements  for the  packet header,  as  well as   the  maximum
          length a packet can be.

          If the timing for the preamble signals and the packet signals are
          out of  phase, a condition known  as  "jitter" exists. The hub is
          expected to remove any jitter, regenerate the preamble and clock,
          amplify  signal symmetry and send the packet on its way.

          The hub  must also serve  as an active filter,  rejecting packets
          that are severely distorted. The  10BASE-T specification mentions
          a  "jitter-budget", which defines  the jitter limits  for various
          components  on each 10BASE-T segment.  If the aggregate amount of
          jitter generated by  all these  components exceeds  the budget, a
          device may not be able to transmit on that segment.    How     do
          MAUs  know when  they can  transmit?  10BASE-T MAUs  use a  state
          machine    architecture.  They  determine  whether  the  10BASE-T
          segment   to   which   they   are   connected   is   viable   for
          communications,   and  they   know  whether   there   is  another
          functioning MAU at the other end  of the  segment. During traffic
          idle states, a MAU  sends out a "link integrity" pulse  to ensure
          that there's  another active MAU at the other end of the link.

          External  conditions can  cause a  MAU  to determine  that it  is
          unable to  transmit under the  required  conditions. For example,
          if an  active telephone  cable carrying a  phone line  voltage is
          inadvertently connected to a 10BASE-T segment, the MAUs at either
          end  of  the segment  should shift  in   to  a "link  fail" state
          because they "know" they can't transmit on such a line.     If  a
          DTE gets stuck in transmit for a period longer than 150 ms (known
          as "jabber"),  a MAU is   required  to disable transmit  and loop
          back, shift  status to "link  bad" wait for  a period of  0.25 to
          0.75   seconds, then  re-enable the link.  Most vendors  put link
          integrity  LEDs on  the backplane  connector of   their  NICs and
          above the  connector ports for  their hubs to allow  diagnosis of
          problem situations.      Moving to a  hub-based wiring scheme has
          a number of  advantages, the greatest of which is  the  potential
          for  network management.  A variety  of  vendors offer  different
          schemes   for  gathering   network     transmission   statistics,
          monitoring problems and isolating faults. Some of these solutions
          are  proprietary, while others  use more widely adopted protocols
          such as SNMP.


          {Installation

          If  you've installed  Ethernet NICs  before,  you'll find  little
          difference in the installation of NICs for  10BASE-T. As a matter
          of fact, aside from the different connectors, you may have a hard
          time telling   the 10BASE-T  boards apart from their  10BASE-2 or
          10BASE-5 counterparts.   Note that you don't have to discard your
          investment in 10BASE-2  and 10BASE-5 Ethernet NICs   to implement
          10BASE-T. Interface devices known as "micro-MAUs" can be attached
          directly  to the  AUI  port  on most  Ethernet NICs that  allow a
          10BASE-T connection through the existing NIC.     You  will  also
          find that  the cost for  wiring new connections will  probably be
          lower. The  wire   itself is less  expensive and  more plentiful.
          Vendors for the installation of telephone-type wiring abound  and
          you'll no longer  have to explain patiently to  the installer the
          differences among grades of coax  cable.

          When it  comes to hooking  the wiring together, however,  you may
          need to do some adapting.   Unless you've used multiport repeater
          configurations  for your  10BASE-2 or  10BASE-5  Ethernet network
          before,  you'll find that the requirements of star-wiring include
          a significant mental shift. You have to  redesign your network to
          support  a  hub-based  layout, pull  workgroup  connections  to a
          wiring closet,  then run cables between closets. 

          In addition,  you'll have to  take into account  the cost of  the
          hubs themselves. Also,  every time   you need to  add a new  hub,
          your per-station wiring costs goes up. You'll also  increase your
          costs  as you   add  newer capabilities  to your  hubs, including
          network   management  and  bridging  to  other  topologies    and
          locations. 

          To help  decide whether  to  use 10BASE-T,  answer the  following
          questions: 

          {* How long are  your existing wiring segments?}
          In most  cases,  you  won't  have to worry:  AT&T reports that 95
          percent of all phone connections have a telephone well within 100
          meters of a wiring closet.

          {* What's the quality of your wiring?}
          Not all UTP wire  is created equal and it will affect the maximum
          length of cable segments.

          {* Are you sure you'll be using  RJ-45 jacks?}
          Even though 10BASE-T needs only four wires,don't make the mistake 
          of using RJ-11 jacks. An RJ-11 connector  will fit into an  RJ-45
          slot, but the wires won't correspond.

          {* Does your  wiring run through many series  of punchdown blocks?}
          If so, make sure  the blocks  have been wired  correctly. In many
          buildings with  excess telephone  wiring, the  connections   have
          never been used. Be careful to test the wiring you intend  to use
          for 10BASE-T  to make   sure that no  active telephone lines  are
          connected. If line  segments carry tinging or  battery  voltages,
          devices  using them  will  be  unable  to  communicate.  Document
          everything.

          When designing your  network, remember that  10BASE-T is still  a
          CSMA/CD  system. The  hubs   are not  bridges; they  don't filter
          traffic,  except at a  very low level. And  you'll still have all
          the attendant   issues to deal with  that you do on  10BASE-2 and
          10BASE-5   networks,  including  the  potential  for    waves  of
          collisions, packet fragmentation and broadcast  storms. If you've
          already installed a UTP   network that uses a pre-10BASE-T scheme
          (StarLAN 10, for  example), get your vendor  to tell you how   to
          link to a compliant system.

          In some  cases you'll find  that your existing wiring  can't meet
          the standards imposed by   10BASE-T. Ideally, you'll end up using
          10BASE-T in  situations  where  you have  control  over  the  new
          wiring of a workgroup, say in a "cubicle  jungle" where new phone
          and other  wiring must  be run   anyway. This  will allow  you to
          control the  quality and segment  lengths of your UTP  wiring, so
          you can  design your 10BASE-T network from scratch.

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