Transcript A Big Test Result - Knowledge Systems Institute

Introduction to Computer
Networks
Token Ring
This lesson presents an overview of the major
Token Ring components, features, and functions.
Overview of Token Ring
• IBM's version of Token Ring was introduced in
1984 for the entire range of IBM computers and
computing environments.
• The goal of IBM's version of Token Ring was to
facilitate a simple wiring structure using twistedpair cable that connects a computer to the
network through a wall socket, with the main
wiring located in a centralized location.
• In 1985, the IBM Token Ring became an American
National Standards Institute (ANSI)/IEEE standard.
Token Ring Features
• A Token Ring network is an implementation of
IEEE standard 802.5.
• Their token-passing
ring access
method,
distinguishes Token
Ring networks from
other networks.
Architecture
• The architecture of a typical Token Ring network
begins with a physical ring.
• However, in its
implementation, a
star-wired ring,
computers on the
network are
connected to a central
hub. The logical ring
represents the token's
path between
computers.
Token Ring Basics
• A Token Ring network includes the following
features:
• Star-wired ring topology
• Token-passing access method
• Shielded and unshielded twisted-pair (IBM
Types 1, 2, and 3) cabling
• Transfer rates of 4 and 16 Mbps
• Baseband transmission
• 802.5 specifications
Frame Formats (1)
• Access control indicates the frame's priority and
whether it is a token or a data frame.
• Frame control
contains either
MAC
information for
all computers
or "end station"
information for
only one
computer.
Frame Formats (2)
• Frame check sequence contains CRC error-checking
information.
• Frame status
tells whether
the frame was
recognized,
copied, or
whether the
destination
address was
available.
How Token Ring Networking Works
• When the first Token Ring computer comes online,
the network generates a token.
• The token is a
predetermined
formation of
bits that
permits a
computer to
put data on
the cables.
How Token Ring Networking Works
• The token travels around the ring polling each
computer until one of the computers signals that it
wants to transmit data and takes control of the
token.
• A computer
cannot
transmit
unless it has
possession of
the token.
How Token Ring Networking Works
• After the computer captures the token, it sends a
data frame out on the network.
• The frame
proceeds around
the ring until it
reaches the
computer with
the address that
matches the
destination
address in the
frame.
How Token Ring Networking Works
• The destination computer copies the frame into its
receive buffer;
• and marks the
frame in the
frame status
field to indicate
that the
information was
received.
How Token Ring Networking Works
• The frame continues around the ring until it arrives a
the sending computer, where the transmission is
acknowledged as successful.
• The sending
computer then
removes the
frame from the
ring and
transmits a new
token back on
the ring.
How Token Ring Networking Works
• Token passing is deterministic, which means that a
computer cannot force its way on to the network
as it can in a CSMA/CD environment.
• If the token is available, the computer can use it to
send data.
• Each computer acts as a unidirectional repeater,
regenerates the token, and passes it along.
Monitoring the System
• The first computer to come online is assigned by
the Token Ring system to monitor network activity.
• The monitoring computer makes sure that frames
are being delivered and received correctly.
• It does this by checking for frames that have
circulated the ring more than once and ensuring
that only one token is on the network at a time.
Beaconing
• The process of monitoring is called beaconing.
• The active monitor sends out a beacon
announcement every seven seconds.
• If a station does not receive an expected
announcement from its upstream neighbor, it
attempts to notify the network of the lack of contact
• It sends a message that includes its address, the
address of the neighbor that did not announce, and
the type of beacon. From this information, the ring
attempts to diagnose the problem and make a
repair.
Recognizing a Computer
• When a new computer comes online on the
network, the Token Ring system initializes it so that
it can become part of the ring.
• This initialization includes:
• Checking for duplicate addresses.
• Notifying other computers on the network of its
existence.
Hardware Components: The Hub
• Hardware for Token Ring networks is centered on
the hub, which houses the actual ring.
• A Token Ring network can have multiple hubs.
• In a Token Ring network, the hub is known by
several names as:
• MAU (Multistation Access Unit).
• MSAU (MultiStation Access Unit).
• SMAU (stands for Smart Multistation Access Unit).
Hardware Components: The Hub
• Cables attach the individual clients and servers to
the MSAU, which works like other passive hubs.
• The internal ring
automatically
converts to an
external ring at
each connection
point when a
computer is
connected.
Hub Capacity
• An IBM MSAU has 10 connection ports. It can
connect up to eight computers. Each ring can have
up to 33 hubs.
Hub Capacity
• Each MSAU-based network can support as many
as 72 computers that use unshielded wire; or up
to 260 computers that use shielded wire.
Built-in Fault Tolerance
• MSAUs were designed to detect when a NIC fails,
and to disconnect from it.
• This procedure bypasses the failed computer so that
the token can continue on.
• Therefore, a faulty computer or connection will
not affect the rest of the Token Ring network.
Cabling
• The STP or UTP cable to a hub connects
computers on a Token Ring network.
The Fate of Token Ring Networks
• Token Ring was losing market share to Ethernet.
• Even though Ethernet is more popular, Token Ring
is good at supporting mission-critical applications.
• Token Ring users today face lots of challenges:
Complexities, manageability, cost, space, congestion
and upgrading to high-speed technologies.
• The idea of switching is to provide highperformance, low-cost alternatives.