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rBenefits of a LAN
There are numerous benefits for having a LAN. Each network benefit is
important and even critical to an organization's success. These benefits
include:
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Resource sharing
/ Workgroup
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synergy
Management
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Centralized
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Decentralized
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Data access and integration
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Economic benefits
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LAN Architecture
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Software
An end-user appplication may use a software
protocol suite such as the TCP/IP or ISO/OSI
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Hardware
The physical network medium designed to carry
informational signals, such as coaxial, twisted-pair
cable, of fibre-optical material carrying multiband
moduated laser light.
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LAN Architecture
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Topology
A network constructed of the afore mention materials on one or more
interconnecting plans.
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Backbone (bus topology)
This has been the typical LAN topology for Ethernet since its inception.
This configuration has one large co-axial cable running thoughout the area.
IBM Compatible
IBM Compatible
IBM Compatible
IBM Compatible
IBM Compatible
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LAN Architecture
" Star configuration
This method uses a central location or hub from which a number of signal carrying
cables goes out to each individual device on the branch of the LAN. A single hub can
serve from 2 to 128 Lan devices (maybe more) on a single network domain. One other
advantage to the star configuration is that the maximum distance between any two
nodes is always only two segments long.
IBM Compatible
IBM Compatible
Workstation
Workstation
IBM Compatible
Workstation
Workstation
IBM Compatible
IBM Compatible
IBM Compatible
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LAN Architecture
" Ring configuration
A true configuration has two ports on every node: one for input and one for output
Workstation
Workstation
Workstation
Token-ring
Workstation
Workstation
Workstation
Workstation
Workstation
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LAN Methodologies
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Ethernet - IEEE 802.3
Ethernet is assumed to be the LAN method unless other
-wise stated. More than 85% of all installed network
connections were Ethernet by the end of 1996. This
means there are over 150 million interconnected
workstations, PC and servers using Ethernet today.
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ATM (Asynchronous Transfer Mode)
ATM eliminates inefficiencies by dynamically sharing
network bandwidth among multiple logical connectio
ATM has been defined at speeds of 45 Mbps, 100 Mb
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155 and up to 622 Mbps.
LAN Methodologies
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ATM (Continued)
The ATM LAN equipment includes ATM switches,
routers, hubs, bridges, and workstations.
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Token Ring - IEEE 802.5
The Token Ring network was originally developed by
IBM. It is still IBM's primary local-area network (LA
technology. Token-passing networks move a small
command frame, called a token around the (circular r
network. Possession of the token grants the possessor
the right to transmit data. To transmit data, the toke
the token is changed to a data frame and the information
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is attached.
LAN Methodologies
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FDDI - (Fiber Distrubuted Data Interface)
FDDI is frequently used as a backbone technology as
well as a means to connect high-speed computers in a
local area.
ISO (International Organization for Standardization
created an international standand for FDDI. FDDI
specifies a 100-Mbps, token-passing, dual-ring LAN
using a fiber-optic transmission medium.
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LAN Methodologies
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FDDI - (Continued)
The dual-ring fiber-optic medium allows for true bi-directional
simultaneous full duplex operation at 100Mbps on each fiber chan
FDDI uses optical fiber as a transmission medium. Optical fiber
offers several advantages over traditional copper wiring:
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Security (fiber does not emit electrical signals that can be trapped)
Reliablility (fiber is immune to electrical interface)
Speed (optical fiber has much higher throughput potential than
copper cable)
No interference from the outside EMI (electromagnetic interference)
source.
Does not produce any EME interfere with and other communication
medium (also a security advantage)
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LAN Components
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Topology - A description of the physical construct or layout of a
network.
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Backbone - The primary connectivity mechanism of an Ethernet
network. All systems that have connectivity on the backbone can
have connectivity to each other.
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Segment - A continuous length of cable commonly joined with other
segments.
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Repeater - A device that amplifies and regenerates the data signal bit
by bit in order to extend the distance of the transmission. A repeate
does not read or interpet the data.
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LAN Components
" Bridge - A device that connects two or more networks segments of the same physical media
type. A bridge examines the hardware address fields of a network packet and the filt
based on addresses from one network segment to another and vice versa.
" Router - A device that has two or more network interfaces. It examines the software proto
-col (IP) address, selects an appropriate travel path and forwards the packet accordin
between seperate networks. Routers usually forward packets belonging to a single protoco
family.
" Gateway - A device that interconnects two or more communications networks based on
different protocol suites. The gateway performs any necessary protocol conversions.
" Switch - A multiport device which minimally functions as a bridge, and provides for the
logical dynamic connection and disconnection between any two cable segments witho
operator intervention. The switch is a high-speed device because multiple data paths can
be established and used simultaneously.
" Concentrator - The central device through which all hosts in a twisted pair Ethernet
installation are connected.
" Hub - A central device through which various types of network packets can flow.
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Networking Models
The three networking models that provide a framework
for network communication are:
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ISO/OSI reference model
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TCP/IP suite (TCP/IP model or TCP/IP)
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SPX/IPX (Sequenced Packet Exchange/ Internetwork
Packet Exchange).
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Networking Models
SPX/IPX
This is a propriety protocol operating on Nevell Netware local area
networks. SPX/IPX uses most Ethernet network interfaces such as IEEE 802
802.3 and Ethernet-II. SPX/IPX is not supported on the internet. If informa
destination are systems connected top the internet, the model must be converted
to TCP/IP prior to making the connection to the internet.
For a through treatment of integration SPX/IPX into TCP/IP environment, refer
to:
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Network Personal Computers With Tcp/IP, Craig Hunt, O'Rielly, 1995
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Novell's Guide to Integrating UNIX and Netware Networks, Novell Press,1994
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Networking Models
The ISO/OSI reference model uses seven layers to describe its network
communications framework. Brief description of each layer are as
follows:
Application layer
Consists of user-accessed application programs
and network sevices.
Presentation layer Defines the way in which cooperating networks
represent data.
Session layer
Manages the connections between cooperating
applications.
Transport layer
Responsible for end-to-end messaging from one
application program to another, also knows as
end-to-end communication.
Network layer
Manages data addressing and delivery between
networks. This layer fragments data into smaller
pieces that the data link layer can handle.
Data link layer
Manages the delivery of data across the physical
network. This layer provides error detection and
packet framing.
Physical layer
Describes network hardware, including electric
signal characteristics such as voltage and current.
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Networking Models
The TCP/IP Model
The TCP/IP model describes its network communications frame
-work usign five layers. Brief descriptions of each layer are as
follows:
Application Layer
Consists of user-accessed application
programs and network services. This layer
is also responsible for defining the way in
which cooperating networks represent data.
A gateway functions at this layer.
Transport Layer
Manages the transfer of data using
unacknowledged transport protocols. This
layer also manages the connections between
cooperating applications.
Internet Layer
Manages data addressing and delivery
between networks, as well as fragmenting
data for the network interface layer. A
router functions at this layer.
Network Layer
Manages the delivery of data across the
physical network. A bridge works at this
level.
Physical Layer
Describes the network hardware, including
electical signal characteristics such as
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Ethernet Definition
Ethernet is a packet-switching network that is based
on a 10 Mbytes/sec broadcast technology with distri
-buted access control.
Ethernet is composed of three major elements:
= Hardware I/O that transfers data to and from a
packet switching network of computers
=
A packet switching network allows concurrent
multiple communications among nodes on a
network.
= The Ethernet packet which is a unit of data sent
across a network
= This unit of data includes the destination address,
the source address, and a form of parity checking
all wrapped around a block of information.
= The Ethernet access method protocol (CSMA/CD),
which is used to control packet transmission and
flow over the Ethernet hardware.
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CSMA/CD
Hosts send messages on an Ethernet LAN
using a Network Interface Layer protocol
and CSMA/CD.
Multiple access
H o st h a s
m e ssa g e
Carrier sense
T r a f f ic o n
n e tw o r k ?
Y e s
N o
se n d
m e ssa g e
Collision detect
a n y
c o llis io n
Y e s
N o
su cce ss
w a it , b a c k
o ff e x p ( )
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Switched Ethenet
Switched Ethernet reduces the number of
collisions on a network by utilizing an
intelligent buffering system and control of
the backbone. By removing the physical
backbone network wire and replacing it with
a central hub device that can receive, store,
and transmit packets.
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Fast Ethenet
Fast Ethernet is capable of burst rates up
to 100 Mbytes/sec over a single duplex
connection and over 200 Mbytes/sec on a
full-duplex dual-ported configuration. In a
dual-ported full-duplex configuration, data
is always traveling one direction on a single
cable from point A to point B and coming
back on a different cable from point B to
point A.
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Multimode Ethenet
Because of packet-switching and the Ethernet being so versatile
and adaptable, an Ethernet network can incude a variety of
network speeds and interfaces. Mixing products from various
vendors is also possible because of the standards laid out in the
IEEE 802.3 specifications. For example:
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T o k e n - r in g
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1 0 B a se T
F ib e r
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1 0 0 B a se T
F ib e r
W o r k s t a t io n
1 - G b y te s /s e c E th e r n e t
H U B
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Ethenet Address
An Ethernet address is a host's unique hardware address. It is 48 bits
long and is displayed as 12 hexadecimal digits (6 groups of 2 digits),
separated by colons (:). An example of a complete Ethernet address is
8:0:20:1e:56:7D.
Unique Ethernet addresses are administered by Xerox. The first three
octets are vendor-specific and are designated by Xerox. Sun systems
always begin with the sequence 8:0:20. Sun assigns the last three octets
to Sun products it manufactures. This method ensures that each node on
an Ethernet has a unique Ethernet address.
Sending Messages
There are two types of ethernet addresses which can be used to
communicate across the network:
= Unicast address
A host sends a message to another host on the Ethernet using a
unicast address. Individual host Ethernet addresses are used for
one-to-one, unicast transmissions.
= Broadcast address
A hsot sends a message to all hosts on the Ethernet using a broadcast
address. The Ethernet broadcast address is all one (ff:ff:ff:ff:ff:ff in
hex). When an Ethernet frame is received with a destination address
of all ones, the Network Interface layer passes it to the next layer.
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Ethenet Frame Analysis
An Ethernet frame is a single unit of data transported
through the LAN. A frame is a series of bits with a definite
beginning and end. The Ethernet specification describes
how bits are encoded on the cable and how hosts on the
network detect the beginning and end of a transmission. The
following diagram illustrates the relationship in this analogy:
Octet location: 1-6
Preamble
64 bits
7-12 13-14 15-1514(the max) last 4 octet
D addr
48 bits
S addr
48 bits
Type
16 bits
Data
(maximum 1500 bytes)
CRC
32 bits
Host in an Ethernet LAN use this information to receive and
transmit data.
Premble
The 64-bit Ethernet preamble field, composed of ones and
zeros, is used for synchronization. Synchronization helps the
network interface determine where an Ethernet frame begins.
Destination Address
The destination address field is the Ethernet address of the
destination host.
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Ethenet Frame (Continued)
Source Address
The source address field is the Ethernet address of the sending
host.
Type
The fourth field of the Ethernet frame describes the type of data
encapsulated in the Ethernet frame (such as IP, ICMP, Address
Resolution Protocol (ARP) or Reverse ARP (RARP)).
Data
The data field holds a minimum of 46 bytes and a maximum of
1500 bytes of information. The network hardware determines
the maximum number of octets of data in a frame. This is called
the MTU (maximum transfer unit). If the data to be transmitted
is less than 46 bytes, the data is padded with zeros to reach to
reach the 46-byte minimum.
Cyclical Redundancy Check (CRC)
The CRC field is used for error detection. The value is calculated
based on frame contents, by the sending host. The receiving host
uses the same algorithm to recalculate the CRC upon arrival, and
then compares it with the frame CRC value. If the two values are
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Ethenet Frame Encapsulation
A p p lic a tio n
D a ta
T ra n s p o rt h e a d e r
A p p lic a tio n
D a ta
Internet header
A p p lic a tio n
D a ta
Ethernet Header
A p p lic a tio n
D a ta
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Useful Troubleshooting Commands
Snoop
Location - /usr/sbin/snoop
You can use snoop to capture network packets and display
their contents. Packets can be displayed as soon as they are
recieved, or saved to a file.
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Useful Troubleshooting Commands
Netstat
Location - /usr/bin/netstat
Use the netstat - i command to show the state of the Ethernet
interfaces.
# netstat - i
Name - The name of the device (interface).
" Mtu - The maximum transfer rate in bytes.
" Net/Dest - The network number. This field references the
file /etc/inet/networks. This file is discussed later.
" Address - The IP address for that interface.
" Ipkts/Ierrs - Shows the input packets and errors.
" Opkts/Oerrs - Shows the output packets and erros.
" Coll - The number of collisions on this interface.
" Queue - The number of packets awaiting transmission.
"
To display the contents of the routing table for the local
system, use the netstat - r. command
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Useful Troubleshooting Commands
Ifconfig
Location - /usr/sbin/ifconfig
The ifconfig command is used to display information about
the configuration of the network interface specified. The
following example shows the configuration of a 100BaseT
interface, including its IP and Ethernet addresses.
# ifconfig hme0
hme0: flags=863<UP,BROADCAST,NOTRAILERS,
RUNNING,MULTICAST> mtu 1500 inet 128.235.35.124
netmask ffffff00 broadcast 128.235.35.255
ether 8:0:20:80:d0:a7
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