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PUREDATA C O N N E C T I O N
PRODUCT BULLETIN
Revised Date: / / Bulletin No. 6503
Original Date: 07/04/91 Document No. 0650000300
ECO No. NA
Product: PUREDATA 10BASE-T products
_________________________________________________________________
Ethernet 10BASE-T
The 10BASE-T protocol describes an Ethernet Local Area
Network (LAN) implementation of CSMA/CD at 10 megabits per
second, baseband, Unshielded Twisted Pair (UTP) wire media. The
establishment of an IEEE standard (IEEE 802.3i) for support of
the Ethernet protocol utilizing this wiring scheme in a
point-to-point (star/tree) topology for systems interconnect is
an important industry move.
Ethernet was originally designed as a method of providing a
high speed communications channel between mid range (mini
computers) and mainframe systems. The common bus topology did
not typically require a large amount of connection points. As
the Ethernet protocol gained acceptance in the desktop computer
environment, the requirement to interconnect large numbers of
micro computers in a local area network grew. As more and more
micro computers were added to the LAN, the cabling constraints of
a common bus topology became more evident.
Interconnecting a large number of devices into a local
area network using a common bus topology can be a logistically
difficult process. The implementation of 10BASE-T provides a
much more flexible (distributes star) type topology for Ethernet.
This type of topology historically has been much easier to
install and manage from a cabling viewpoint. The technical
benefit of a distributed star topology and the economical
benefits derived from the use of UTP type wire make 10BASE-T an
attractive network solution. This document will describe the
general characteristics of the CSMA/CD Ethernet communications
protocol and the implementation of 10BASE-T technology.
AN EXPLANATION TO ETHERNET (CSMA/CD)
Carrier Sense, Multiple Access (CSMA) is a "common
bus" media sharing scheme in which stations listen in to
what's happening on the network media (cable). If the cable is
not in use, the station is permitted to transmit its' message.
CSMA is combined in IEEE 802.3 with a means of performing
Collision Detection; hence CSMA/CD.
The multiple access feature of the protocol allows the
connection of many devices to the same common bus transmission
media. The carrier sense feature allows each station to detect
activity on the channel; this may be termed "listen before
talking". A station will refrain from transmitting if it
detects that the channel is active. At times, it is possible
that due to the time it takes for the signal from one station to
transit across the network (propagation delay) that two stations
can almost simultaneously detect that the channel is available
and initiate a transmission. In this case a collision occurs.
Identification of the fact that a collision has occurred
takes place at the interface card level. Circuitry on the
interface card senses the additive voltage on the media and
informs the higher levels that a collision has taken place. In
the event of a collision, the stations involved emit a JAM pulse
to ensure that all other stations are aware that a collision has
occurred. Subsequently, the involved stations "back off"
for a semi-random timeout period. The length of the timeout
period is controlled by the individual stations network address.
Since no two network addresses are identical, no timeout period
will be of the same length. If a station, upon re-entering
communications experiences further sequential collisions, this
back off time is extended for each successive collision to a
point that the station assumes that the channel is no longer
operative.
Collisions do occur in Ethernet and their random occurrence
cannot be predicted. However, statistically, the incidence and
frequency of collisions on an Ethernet LAN is quite low. The
Ethernet protocol experiences little loading effect until
utilization of the media approaches 80 percent. This implies
that data transmissions are occurring on the media for 80% of the
time or more. Typical implementations of Ethernet (or for that
matter, any access protocol) generally do not exceed 30 to 40
percent of media utilization at any time. In this case, the wire
is quiet (available) for 40 to 70 percent of the time.
Additionally, there is only a short time interval following
the initiation of a transmission during which there is the
possibility of a collision. This time interval is termed "slot
time". The slot time interval is represented by the time
required to transmit 512 bits (64 octets) of data. This number
represents the number of bits that must be transmitted to
completely fill the wire (end-to-end). Following transmission of
these first 512 bits, all stations on the segment are aware that
a transmission is in progress and will refrain from transmitting.
For this reason, data frames must be of a certain (512 bit)
minimum length. This length is controlled by the length of the
LLC data field in the frame.
The slot time interval is the ONLY time during
transmission of data that Ethernet is susceptible to a collision.
Once the slot time interval has passed, the station is said to
have acquired the channel, and the rest of the frame is
transmitted contention free. Contention access systems (CSMA/CD)
therefore function better when the average frame size exceeds the
slot time interval, since most of the frame will then be
transmitted without interference by collision. Note that
substituting cable of different impedances and lengths may
significantly alter the propagation delay and in consequence the
slot time interval. Collisions that occur following expiry of
the slot time interval are considered as serious "system
failures", and their cause should be investigated and
corrected immediately.
Only the stations with immediate transmission
requirements are in contention for the media. Additionally,
since the incidence of collisions on most Ethernet LAN's is low,
this fair distribution of access coupled with the high data rate
makes Ethernet one of the most efficient communications protocols
available.
╔════════════════════════════════════════════════════════════════════╗
║ Direction of Propagation ║
║ ────────────────────────────────────────>> ║
║ ╔══════════════════════════════════════════════════╗ ║
║ ║ Frame size is LESS than minimum frame size ║ ║
║ ╚══════════════════════════════════════════════════╝ ║
╟──────┬───────────────────────────────────────────────────┬─────────╢
║ │ │ ║
║ WS WS ║
║ (A) (B) ║
║ Note that station "A" has completed transmission ║
║ Prior to Station "B" sensing that the media is in use ║
║ ║
║ If Station "B" initiates a transmission in response to finding ║
║ the media to be inactive, a collision will result. Additionally, ║
║ this type of collision generally results in a FATAL ERROR ║
╚════════════════════════════════════════════════════════════════════╝
COMPONENTS OF AN ETHERNET LAN
While standard implementations of Ethernet require interface
cards and wire, 10BASE-T requires another component known as a
concentrator. Wiring concentrators are used in 10BASE-T to
provide for a common point for wire distribution. The advantage
of using a concentrator and a star type topology is that a bad
cable can be easily isolated, and its effect on the LAN can be
minimized.
10BASE-T CONCENTRATOR
In 10BASE-T each port on the concentrator connects to only
one workstation. Concentrators are connected via the 10BASE2
(thin wire) BNC attachment incorporated in the Pure Data device.
Optionally, the concentrators may be connected via the active
RJ-45 ports (assuming that the patch cable provides for inversion
of transmit and receive pairs).
Pure Data manufactures a concentrator for 10BASE-T
(unshielded twisted pair) Ethernet PDC8023A-T12. The
Ethernet concentrator is an active 10BASE-T repeater that
provides a common wiring point for 12 10BASE-T connections. The
concentrator also functions as a media converter (via the 10BASE2
port).
Major Features;
= 10 megabit per second IEEE 802.3 10BASE-T standard
= Ethernet UTP wiring. (Standard allows 100 meters of UTP
from concentrator to workstation)
= 12 RJ-45 ports
= Built-in BNC thin wire Ethernet port (10BASE2)
= Link integrity test on each port continuously checks
the line from workstation to concentrator
= Automatically disconnects ports if either the link
integrity test fails or the card transmits continuously
(jabbers), to prevent network failure
= Automatically re-connects ports when failures are
corrected
= Test port detects incorrect twisted pair cable polarity
to simplify installation (receive pairs only)
= All data connections on front panel
= Rack mount package for 19" racks. Low profile 1.75"
high
Indicators
= Link integrity LED per UTP port (Green)
= Jabber LED per port (Red)
= Power, traffic and collision LED's
= Test port polarity indication LED
How a Concentrator Works
Signalling on a 10BASE-T network is predicated upon two
factors;
- CSMA/CD
- Link integrity
Link integrity is a function of Ethernet products that
support the IEEE 802.3i 10BASE-T specifications. Every 1/60th of
a second, a pulse is asserted onto the wire between a
concentrator and the interface card (or another concentrator).
This pulse is interpreted as a link integrity check. If the
check passes, then the node is permitted to remain as a part of
the LAN. In the event of a failure of link integrity, the
concentrator will automatically disable the associated port and
will turn the green link integrity LED OFF.
As well, the card in question will also discontinue the
green LED indication of proper link integrity.
The establishment of proper link integrity is of critical
importance in 10BASE-T. It is vital that the receivers (which
are tested by link integrity) on each side of the link be
verified as operational prior to allowing the adapter to
participate in communications. Otherwise, an adapter with a
non-functional receiver may continuously detect a quiet channel.
This will result in communications disruption when this adapter
initiates a transmission without knowledge of the true state of
the transmission media.
A concentrator counts as a device on the 10BASE2 segment
to which it is connected. Additionally, since signals are
regenerated and re-timed at the concentrator level, this device
is also considered to be a repeater. The following table
illustrates the use of the concentrator LED's.
Interface Cards
In order for the microcomputer to communicate
successfully on the network, an adapter, sometimes referred to an
a Network INterface Card (NIC) is required.
This adapter fits into a bus slot on the computer and is
used to connect the computer bus to the network cabling
structure. Pure Data manufactures 8 and 16 bit multiseries
interface cards as well as a 16 bit micro channel adapter and an
Ethernet card for the Toshiba laptop.
▄▄▄▄▄▄▄ ╒══┐
███████▀▀▀▀───────────────────────────────────────────────────┤┌─┤
█████ E1 E2 └┴┐│ Diagnostic
█████ BOOT PROM ° ° │▓▓ LEDs
█████ █████████ ° ° ███████ ████ E4 ▄▄│├─┐
█████ █████████ ███████ ████ ▒▒▒▒▒ ██││ │ DB-15
█████ ███████ ████ ██││ │ Connector
█████ ████ ▀▀│├─┘
█████ E3 ° ││
█████ ° ┌─┤├─┴┐Thin-coax
▀▀▀▀▀──────────────────────────────┐▄ ▄ ┌┐▄ ▄ ▄ ▄ ▄ ▄ ┌───┐ └─┤├─┬┘Network
└▀─▀─┘└▀─▀─▀─▀─▀─▀─┘ └───┴┤ Connector
│
E3 - Link Integrity (enable/disable)
╒══┐ E4 - Reverse Polarity (enable/disable)
─────────────────────────────────┤┌─┤
E3 E4 └┴┐│
° ° ││▀ (1) Card Activity LED (red)
███████ ████ ° ° ││▀ (2) Network Activity (grn)
███████ ████ ││▀ (3) Reverse Polarity LED (red)
███████ ████ ││▀ (4) Link Integrity LED (grn)
████ ││
││
┌─┤├┐ 10 Base T
────────┐▄ ▄ ┌┐▄ ▄ ▄ ▄ ▄ ▄ ┌───┐ └─┤├┘ Network
└▀─▀─┘└▀─▀─▀─▀─▀─▀─┘ └───┴┤ Connector
│
10BASE-T ETHERNET CABLING RULES
The cable media specified for 10BASE-T is unshielded
twisted pair wire that corresponds to the specifications outlined
for IBM Type 3 wire;
- 24 AWG copper tinned, solid conductors
- 100 - 105 ohm characteristic impedance
- Minimum of two twists per foot
From a wiring installation and maintenance viewpoint, the
10BASE-T distributed star topology provides greater flexibility
than the more common bus topology that is generally associated
with Ethernet. The concept of one concentrator port to one
workstation makes isolation of individual node problems much
simpler.
Unshielded twisted pair for local area network
communications is rapidly becoming the wiring system of choice
for most LAN installations. This technology is economical and
easy to install and maintain. For additional information on the
technical issues surrounding the application of UTP type wire for
local area networks, the reader is directed to the Pure Data mini
series selection entitled "Local Area Network Cabling
issues", available through Pure Data Technical Services,
Education.
General Cabling Rules
= Maximum segment length (concentrator to interface) is
100 meters
= Maximum number of repeaters/concentrators between any
two points on the LAN is four (4)
= Repeaters/concentrators count as devices on each segment to
which they are connected
= Uses two (2) pairs of UTP wire
= Connector type is RJ-45 modular, eight position
= BNC connectors on the concentrator MUST be terminated
with a 50 ohm "T"-connector impedance
Workstations are connected to concentrator ports by no
more than 100 meters of unshielded twisted pair wire. No daisy
chaining of workstations is permitted. Concentrators are
typically located in a centralized wiring closet and are
interconnected via the 10BASE2 connectors. This will provide for
the maximum number of supported workstations on the LAN. For
example; there are three segments of 10BASE2 available to
connect concentrators (or other communicating devices). Each of
the three segments is capable of supporting 30 devices. Each
concentrator supports up to twelve (12) workstations. Therefore,
a single 10BASE2 backbone system may support up to 90
concentrators, thus rendering connection points for 1080
workstations.
RJ-45 connections for unshielded twisted pair wiring are
polarity sensitive, and considerable care must be taken during
installation of the wiring system to maintain correct polarity.
The adapters will automatically compensate for reversed polarity
on the receive pairs, as will the concentrator. If the
concentrator displays an indication that the receive pairs are
indeed reversed, attempts should be made immediately to correct
the situation, regardless of whether the hardware will compensate
or not.
The following table illustrates the proper pin connections
for the RJ-45 connector;
╔═════════════════════════════════════╗
║RJ-45 MDI (Media Dependent Interface║
╠═════╤═══════════════════════════════╣
║ 1 │ TD + ║
╟─────┼───────────────────────────────╢
║ 2 │ TD - ║
╟─────┼───────────────────────────────╢
║ 3 │ RD + ║
╟─────┼───────────────────────────────╢
║ 4 │ Not used by 10BASE-T ║
╟─────┼───────────────────────────────╢
║ 5 │ Not used by 10BASE-T ║
╟─────┼───────────────────────────────╢
║ 6 │ RD- ║
╟─────┼───────────────────────────────╢
║ 7 │ Not used by 10BASE-T ║
╟─────┼───────────────────────────────╢
║ 8 │ Not used by 10BASE-T ║
╚═════╧═══════════════════════════════╝
The Pure Data Ltd. Technical Support Hotline is available every weekday from:
8am to 8pm (EST) in Canada (Monday-Thursday, Friday 8am-5pm) (416) 731-9884
8am to 8pm (EST) in USA (Monday-Thursday, Friday 8am-5pm) (800) 661-8210
9am to 5pm (GMT) in the UK (44) 71 924-3505
A staff of knowledgeable personnel is ready to assist users with any situations
involving Pure Data Ltd. products. For late breaking news and product updates
refer to the README and .DOC files contained on the Support Disk.
PUREDATA LTD. PUREDATA INC. PUREDATA LTD.
180 West Beaver Creek Rd, 1740 South I-35, Suite 140 Unit 10, Port House
Richmond Hill, Ontario, Carrollton, Texas Plantation Wharf, York Rd.
L4B 1B4 CANADA 75006 USA London SW11 3TY, U.K
(416) 731-6444 (214) 242-2040 44-71-924-3505
FAX (416) 731-7017 FAX (214) 242-9487 FAX 44-71-585-3103
BBS (416) 492-5980 BBS (214) 242-3225 BBS 44-71-738-2910