Network Support Library/RoseWare - Network Support Library.iso/hardware/arcnet.arc/ARCNET.TXT

Text File | 1989-09-18 | 13KB | 260 lines

ARCNET

ARCNET is one of the oldest existing LAN systems, originally
developed in 1977 by Datapoint.  The primary architect of ARCNET
was John Murphy, who is now a software engineer with Performance
Technology, a developer of LAN operating software.  Standard
Microsystems Corporation (SMC) developed the first ARCNET
controller chip in 1981, and the first ARCNET controller board for
IBM type PCs in 1983.  Since then ARCNET has become one of the most
popular LAN hardware systems in the world, comprising an estimated
25% of new LAN installations. This figure includes all LAN types;
the estimate for PC LANs is much higher.  For example, it is
estimated that more than half the installations of Novell's popular
NetWare operating software are using ARCNET hardware.  According
to Ben Wolfe, chairman of the ARCNET Trade Association, there are
approximately 135 manufacturers of ARCNET products.  ARCNET is
supported by many LAN operating systems, including Novell NetWare,
Banyan Vines, Western Digital Vianet, CBIS Network OS, Performance
Technologies POWERLAN, Univation LifeNET and many others. ARCNET
is also used widely in industrial and manufacturing control
systems.  ARCNET interfaces are available for various expansion
buses, including IBM PC, PC AT, PS/2, S-100, Multibus, VME bus,
Qbus, STD bus STE Bus and Nubus (used by the Apple Macintosh). Mr.
Wolfe stated that more than 1,300,000 ARCNET nodes will be
installed by the end of 1988.  The major manufacturers of ARCNET
currently include SMC, Pure Data Corporation, Thomas Conrad
Corporation and Tiara Computer Systems.

Why is a LAN system that does not bear an IEEE stamp of approval
so popular?  There are several important reasons: reliability,
flexibility, performance, ease of installation and ease of fault
diagnosis are among them.  The consensus among LAN resellers and
installers is that ARCNET is the easiest LAN to install and
troubleshoot.  Once installed, ARCNET controllers rarely cause
trouble, and, due to the "star-cluster" topology of ARCNET,
isolating sections of the network for diagnosis is quite easy.

While ARCNET at 2.5 megabits per second is not the fastest LAN
around, due to its token-passing protocol it provides reasonable
performance that does not degrade quickly under load.  ARCNET also
provides a great deal of cabling flexibility.  Additionally,
virtually any manufacturer's ARCNET controller will function
properly in the same network with any other manufacturer's
controller.  ARCNET seems to have achieved a degree of
standardization not always available in the 802.3 Ethernet and
Starlan world.

Topology and cabling

The standard topology for ARCNET is a star-cluster scheme, with
ARCNET nodes (or stations) attached to distribution devices called
hubs.  There are two types of ARCNET hubs: active and passive.  An
active hub is a powered unit that acts as a distribution device and
signal amplifier, while a passive hub only distributes signals over
short distances.  Active hubs may have between four and 64 ports,
while passive hubs have 4 ports.  By using multiple hubs, an ARCNET
can have a diameter as large as 20,000 feet, meaning a maximum
cabling distance of 20,000 feet from any node on the network to any
other node.  Standard ARCNET cable is RG-62 coaxial cable with an
impedance of 93 ohms.  Enhancements made over the last few years
have added cabling options (more about this later).

Cable Access Scheme

ARCNET is a Token Bus network.  A 'bus' network has a common cable
set, with all signals being broadcast across the entire network
simultaneously.  The signals are only received and acknowledged by
the stations they are addressed to.  ARCNET uses a Logical Token
Passing scheme to control cable access.  In token passing systems,
there is an electronic signal called a "token" that is passed from
station to station in a pre-determined order.  If a station has
possession of the token, it can send a packet of information.  If
it does not have possession of the token, it must wait until the
token is passed to it by the previous station before it can
transmit.  Basically, token passing systems are "contention-free"
systems.  Unlike the contention access scheme of Ethernet and
others (called CSMA, for Carrier Sense Multiple Access, where any
station can transmit if the cable is free.  When two stations
transmit simultaneously, a "data collision" occurs, and data
packets must be re-transmitted by both stations.), each station
"waits its turn" until it receives the token.  In token passing
systems network access is guaranteed, while access is random in
contention systems.  Performance in heavy traffic situations is
generally better with token passing systems than with contention
systems.

Token passing functions differently in ring systems, such as the
IBM Token_Ring, than it does in bus systems, such as ARCNET.  In
a token ring system, the token is physically passed from one
station in the ring to the next station in the ring.  It is then
regenerated and passed to the following station.

In a token bus, such as ARCNET, the token acts as permission to
transmit.  In simplest terms, each ARCNET station has the address
of one other station on the network in its Next ID (NID) register.
When a station has possession of the token, it can elect to send
a data packet to another station.  After it sends a packet (and
receives acknowledgement) or if it elects not to send a packet, it
broadcasts the token across the network to the station whose
address is in its NID register.  The addressed station acknowledges
receipt of the token and begins the process over again.

ARCNET dynamically and automatically reconfigures every time a
station is added to or removed from the network.  When a station
is added, or 'powered on', it transmits a signal that is
unambiguously an invalid data pattern and longer than any possible
data packet.  This "recon burst" interferes with all communication
and prevents any node from receiving the token, forcing a
reconfiguration.  At this point each station sets its NID register
to its own address, then waits up to (146 X (255-ID)) microseconds.
Since ARCNET addresses are switch selectable in a range of 1 to
255, this means that the station with highest address starts the
reconfiguration.  It sends out a signal then waits a specific
amount of time (usually approximately 75 microseconds) for a
response.  If it does not receive a response it increments the
address in the NID register (in this case its own address), and
starts over.  After reaching 255 it increments the address to 0
and continues the process until it finds the lowest addressed
station on the network.  It then puts that station's address in its
address buffer and passes it the token.  Each node then
reconfigures in order.  In a worst-case scenario, this process
takes a maximum of 61 milliseconds.

When a station is removed from the network the process is much
simpler.  The station with the address just below the removed
station attempts to pass the token and waits for a response.  If
it does not receive a response it increments the value in its NID
register by one, sends out a signal and waits for a response.  It
repeats this procedure until the next station is found, then passes
the token to it.

Diagnosis

Fault diagnosis on ARCNET is usually quite easy.  Because of the
star cluster topology, it is quite easy to disconnect whole
sections of the network for fault isolation.  ARCNET also has a
built-in diagnostic tool -- the "recon burst".  In a healthy
network, recon bursts should only occur when stations are being
added to the LAN (software, such as the Novell NetWare shell, can
also force a recon burst).  A hardware problem, however, can often
cause spurious reconfigurations.  There are several diagnostic
software products that, among other things, monitor
reconfigurations.  These include ARCMonitor from Brightwork
Development, Inc., ARCDIAG from Microlan Systems, Inc. and ARCView
from SMC.  By disconnecting sections of the network and monitoring
recon bursts most ARCNET hardware problems can be located.

Extended ARCNET

Many extensions have been added to ARCNET since its original
development.  Some of these are:

1. Fiber optics.  The maximum distance between standard active
ARCNET devices (ARCNET controllers and active hubs) is 2000'.  With
fiber optics that distance can be extended.  With single fiber
optic controllers and links that distance can be extended to 4000',
while dual fiber systems can extend the distance to 11,500 feet.

2. Linear Bus ARCNET.  Another relatively recent development is the
Linear Bus ARCNET card, originally developed by SMC and now
produced by many manufacturers.  This card uses special circuitry
to allow up to 8 cards to share the same linear cable.  This cable
can have a length of up to 1000 feet.  Attachment is made using the
same type of BNC "T" connector used with thin Ethernet cable.  One
end of this cable may be connected to an active hub, allowing
interconnection with the star-cluster topology, and multiple linear
buses may be connected using 2 port "active links".

3. Twisted-Pair ARCNET.  ARCNET boards and hubs designed to use
unshielded twisted-pair (UTP) phone wire have been developed by SMC
(and, again, now manufactured by others).  These twisted-pair
boards may be cabled (up to 400 feet) to special twisted-pair
active hubs and up to 10 boards may be daisy-chained (in Starlan
fashion).  Unlike other UTP LAN implementations, only a single wire
pair is required.

4. 16 Bit AT and Microchannel Interfaces.  First released by Thomas
Conrad Corp. (16 bit AT) and Pure Data (microchannel), similar
controllers are provided by other major ARCNET manufacturers.
These boards generally provide better performance due to the 16 bit
data path and improved packet buffering.

5. Nodal Priority.  SMC now manufactures a 16 bit board designed
especially for LAN servers. This board, the PC500FS, can send
multiple packets each time it has possession of the token.  This
means that it can respond to pending requests from multiple work
stations on the same round trip of the token.

The future of ARCNET

While many of the enhancements described above contribute to
increased performance, ARCNET is still crippled by two factors: its
relatively slow (by today's standards) 2.5 megabit transmission
speed, and its inefficient 508 byte packet size.  Several groups
are attempting to correct the situation.

SMC is developing a 5 megabit version of ARCNET that will
reportedly have a 2K packet size.  Unfortunately, this system will
not be compatible with current ARCNET products.

Another approach is being taken by the ARCNET Trade Association.
They are currently setting the specifications for an enhanced
ARCNET that will initially provide a 20 megabit per second transfer
rate, and a larger packet size, probably 2048 bytes, possibly 4096.
The advantage of this specification is that it will be supported
on existing coax cabling (and possibly UTP) and will maintain full
downward compatibility with current ARCNET products.  The design
will allow 20 megabit boards to communicate with 2.5 megabit boards
at the slower speed.  While the current specification calls for 20
megabits per second, faster speeds are planned in the future, up
to 100 megabits per second.

Acknowledgements

Many thanks to Mike Fischer of Datapoint Corp., John Murphy,
formerly of Datapoint, now with Performance Technology, Ben Wolfe
of the ARCNET Trade Association and many others for their help with
this article.

ARCNET is a registered trademark of Datapoint Corp.

ARCNET Trade Association
3413 North Kennicott Avenue
Suite B
Arlington Heights, IL 60004

Datapoint Corporation
9725 Datapoint Drive
San Antonio, TX 78284
512-699-7054

Standard Microsystems Corporation
35 Marcus Boulevard
Hauppauge, NY 11788
516-273-3100

Performance Technology
800 Lincoln Center
7800 IH 10 West
San Antonio, TX 78230
512-349-2000

Pure Data
200 West Beaver Creek Road
Richmond Hill, Ontario
Canada L4B 1B4
416-731-6444

Thomas Conrad Corporation
8403 Cross Park Drive, #1C
Austin, TX 78754
512-836-1935

Tiara Computer Systems
2700 Garcia Avenue
Mountain View, CA 94043
415-965-1700

(c) Copyright 1988 Patrick H. Corrigan
Commercial reproduction prohibited without written permission.