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Designing the network
1.0 INTRODUCTION
As a network administrator, you must understand the hardware and software components that make up a
local area network. This knowledge will enable you to recommend and implement network systems. It will
also help you troubleshoot problems on the network. You will increase your understanding of LANs by
studying
 network cabling systems,
 network topologies, and
 protocols.
As a network administrator, You will be asked to help make important decisions about which hardware
and software to purchase when the LAN is implemented or expanded. Hence, you will need a good
background in how computers use LANs to communicate, as well as the options and standards currently
available to accomplish this communication.
2.0 LAN COMMUNICATIONS
Computers communicate over LANs by sending blocks of data called packets. Each packet contains
 the information to be transmitted,
 along with control information used by the receiving computer to identify and process the data
contained in the packet.
In other words, a packet contains
1. Data
2. From and to address, and
3. Error checking
The term interoperability refers to the capability of different computers and applications to communicate
and share resources on a network. For this we need some standard/ rule which everyone can follow. The
two major organizations that play a role in LAN standards are the International Standards Organization
(ISO), which works on LAN communication software models, and the Institute of Electrical and
Electronic Engineers (IEEE), which works on physical cable and access method standards.
3.0 OSI (OPEN SYSTEMS INTERCONNECT) MODEL
The ISO introduced a seven – layer model in 1980, known as the open systems interconnect (OSI)
model. This seven – layer model acts as a blue print to help network designers and developers build
reliable network systems that can interoperate. As a network administrator, you need to know the basic
levels and functions of the OSI model in order to understand the LAN communication process and better
be able to select and configure network hardware and software components. In addition to helping you
implement and maintain network systems, a good understanding of the principles of network
communication provided by the OSI model will be important to help you troubleshoot and identify network
problems.
LAN Communications chart
OSI Layer
Application
Presentation
Session
Transport
Network
Action
Interaction with user
To organize data in machine readable format.
Used to compress information.
Make initial connection with receiving computer;
maintain communication during session, and
session off when complete. Control data flow by
sequencing packets. Addition of packet
sequence numbers.
Reliable data delivery. Identification and
acknowledgement fields added to the message.
Determination of the best route to the
Result
Application program executed
Message packet
Packet sequence number
added to message packet.
Segment package formed
Datagram packet formed
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Data link
Physical
destination computer and addition of network
address to the packet.
Addition of the physical address of the
destination computer
Transmission of packet one bit at a time
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Ethernet frame formed
Electronic signals representing
bits appear on the cable
system.
There is a simple phrase to help you remember the OSI layers, from the application layer to the physical
layer: “All people seem to need data processing.”
For example, the transport layer on the computer will include control information in the network packet
that can be used by the transport layer on the receiving computer to acknowledge receipt of the packet.
3.1 Application Layer
The application layer consists of software that interacts with the users and enables them to perform their
tasks without being involved with the complexity of the computer or network systems. Examples of
application software include: word processors, spreadsheets, other software products used in offices. The
function of the application layer is to provide inerface to the above application software in a network
communication.
3.2 Presentation Layer
The purpose of the presentation layer is to organize the data in machine-readable form. The resulting
block information created by the presentation layer is referred to as a message packet.
Presentation layer software can also be used to compress information in order to save space and
transmission time. For increased security, using a password or key in order to make it difficult for an
intruder to capture and access the information can also use the presentation layer (data encryption).
3.3 Session Layer
The power of the session layer is to initiate and maintain a communication session with the network
system. The session layer enables to log in to the NetWare server by providing the server with a valid
user name and password.
3.4 Transport Layer
The primary function of the transport layer is the reliable delivery of information packets from the source
to the destination. The transport layer on the sending computer provides proper address information, and
the transport layer on the receiving computer sends an acknowledgment of each packet successfully
received from the network. The transport layer creates a packet, called a segment, by surrounding the
message packet with the necessary acknowledgment and identification fields.
The transport layer on some multitasking computers can also be used to place parts of several message
packets from different application into each segment. The process of placing pieces of multiple message
packets into one segment is called multiplexing. Multiplexing can save communication from several
applications simultaneously.
3.5 Network Layer
The network layer provides the information necessary to route packets through the proper network paths
in order to arrive at the destination address. Uses network addresses, which identify each group of
computers on your network system. The network layer then creates a datagram packet by encapsulating
the information in the segment packet with the necessary packet routing information.
3.6 Data Link Layer
The data link layer is the delivery system of the computer network and is responsible for using the
destination address to send the packet through the requested network cable system. The data link layer
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creates packet, called a frame, that encapsulates the datagram packet with control information including
the source and destination physical addresses.
Physical addresses are unique NIC addresses that are permanently assigned to each NIC by the
manufacturer. Each physical address is a hexadecimal number divided into two parts: The first part
identifies the manufacturer, and the second part is a unique number to identify the card among all cards
produced by that manufacturer.
The IEEE 802 committee, which is the IEEE group that works on network standards, divides the data link
layer into two sub layers: The logical link control (LLC) layer and the media access control (MAC) LAYER.
The LLC layer interfaces with the network layer, while the MAC layer provides compatibility with the NIC
used by the physical layer.
3.7 Physical Layer
The physical layer comprises the network cable system and connectors that are responsible for sending
the data frame packet out as a series of bits.
OSI Layer
Application
Representation
Presentation
Session
Transport
Network
Data link
Physical
Function
The NetWare SEND command is a DOS command line utility that is
the application layer in this example. The SEND command allows you
to enter and send a message to another user. Other
e-mail applications could serve the same purpose.
Presentation layer converts input to binary ASCII code and creates the
message packet.
Session layer establishes communication session with receiving
computer. Sequence number added to message packet.
Transport layer adds identification and acknowledgment fields to form
segment packet. This provides “certified delivery”.
Network layer adds routing information to network containing
destination computer. Network address added to segment packet to
form datagram packet.
Data link layer adds physical address of destination computer to
create Ethernet frame.
At the physical layer the NIC converts bits to electronic signals and
transmits them on the cable system.
The Network layer in each computer keeps table – similar to a ZIP code reference book – that contains
the correct network address of all NetWare servers.
The data link layer encapsulates the datagram packet received from the network layer with heading
information, including the addresses for the destination and source computers along with error-checking
codes. The data link layer then sends the data frame to the network card for transmission, working
closely with the network card to ensure the data frame is transmitted successfully.
The physical layer of a computer network consists of hardware devices such as NICs, connectors, and
cable systems that are responsible for transmitting the message bit by bit across the network system.
The data link layer then uses the error-checking codes to perform a cyclic redundancy check (CRC), in
which a mathematical algorithm compares bits received to the CRC code contained in the frame packet.
4.0 NETWORK COMPONETS
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We will learn about the network components that make up each layer of the OSI model.
4.1 Physical Layer Components
The two aspects of the physical network system are
 the media, the transmission systems used to send electronic signals, and
 the topology, the physical geometry of the network wiring.
4.1.1 Network Media
The network media consists of the communication systems that are used to transmit and receive bits of
information. Most network media used today are often referred to as bounded media because the
signals are contained in or ”bounded” by a wire. Another medium type, which is much less common in
LANs, involves beaming signals between computers with radio and light waves. These types of
transmission media are referred to as unbounded media. Although unbounded media are generally
used in wide area network (WAN) systems and involve satellite and microwave links over hundreds and
thousands of miles, certain specialized types of unbounded media, such as infrared, are gaining
acceptance for specialized local area network (LAN) applications.
You should consider three major factors when selecting a medium for your network system:
 bandwidth,
 resistance to electromagnetic interference, and
 cost.
The bandwidth of a network medium is a measure of the medium’s capacity in terms of the number of
bits per second that can be transmitted. A general rule is that the higher the bandwidth, the more traffic
and higher speed the network medium can support.
A. Twisted – Pair Cable
Twisted – pair cable is probably the most common form of bounded medium in use on LANs today.
Twisted – Pair Cable can be unshielded or shielded and consists of pairs of single – strand wire twisted
together.
Twisting the wires together reduces the possibility of a signal in one wire affecting a signal in another
wire. Twisting the wires eliminates the noise by canceling out the magnetic field. Fifty or more pairs of
twisted wire can be put together in one large cable, referred to as a bundle pair.
One problem of unshielded twisted – pair (UTP) cable is that external electrical voltages and magnetic
fields can create noise inside the wire.
To reduce electromagnetic interference (EMI), shielded twisted – pair (STP) cables are surrounded by
a metal foil that acts as a barrier to ground out the interference. For STP cable to work, it is important to
connect the cable ground to the building’s grounding system properly. Unfortunately, the shield of STP
cable changes the electrical characteristics of the wire, reducing the distance and the speed at which the
network’s signal can be transmitted.
Two types of connectors can be used on the ends of twisted – pair cable: RJ-45 plugs and IBM data
connectors. RJ-45 plugs are similar to the modular RJ-11 plugs commonly used to connect telephones to
wall jacks and are generally preferred for unshielded cable because of their low cost and ease of
installation.
Cable types
Wire Type
Speed Range
Typical Use
1 and 2
Up to 4 MBPS
Voice and low – speed data
3
Up to 16 MBPS
Data
4
Up to 20 MBPS
Data
5
Up to 100 MBPS
High – speed data
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The major disadvantages of twisted – pair cable, especially UTP, are its sensitivity to EMI and increased
susceptibility to wiretapping by intruders. Wiretapping involves using special equipment, called a sniffer,
to direct the signals on the cable by sensing the electrical fields.
B. Coaxial Cable
Coaxial cable, commonly referred to as “coax,” is made of two conductors. The name coaxial is derived
from the two conductors in the cable share the same axis. At the center of the cable is a fairly stiff wire
mesh tube that also serves as a shield. A strong insulating plastic tube forms the cable’s outer covering.
Generally, thicker cable is used to carry signals longer distances but is more expensive and less flexible.
When compared to twisted-pair, coaxial cable supports higher data rates and is less susceptible to EMI
and wiretapping. On the other hand coaxial cable is generally more expensive, harder to install, and more
susceptible to damage due to linking. In the past, many networks were wired with coaxial cable.
Improvement in twisted-pair cable’s bandwidth, however, along with its flexibility and lower cost, are
causing most organizations to select UPS as a medium over coaxial cable for new network installations.
Coaxial Cable
type
RG-8
RG-58
RG-59
RG-62
Resistance
Typical Usage
50 ohms
50 ohms
75 ohms
93 ohms
Thick Ethernet networks
Thin Ethernet networks
Cable TV and IBM broad band networks
ARCnet networks
C. Fiber Optic Cable.
Fiber optic cable looks similar to coaxial cable. It consists of light-conducting glass or plastic fibers at the
center of a thick tube of protective cladding surrounded by a tough outer sheath. One or more fibers can
be bounded in the center of the fiber optic cable. Pulses of light are transmitted through the cable by
either lasers or light emitting diodes (LEDs) and received by photo detectors at the far end, from 100
million bits per second to more than 2 billion bits per second. Do not attenuate (lose strength) over
distances as quickly as electrical signals, fiber optic cables can be used to carry high-speed signals over
long distances. In addition, fiber optic transmission is not susceptible to EMI and is very difficult to tap.
The principal disadvantages of fiber optic cable are its relatively high cost, lack of mature standards, and
difficulty of locating trained technicians to install and troubleshoot it.
One common use of fiber optic cable is in connecting several high-volume NetWare servers, or
minicomputers to form a backbone network. A backbone network is a cable system used primarily to
connect a host computer to NetWare servers, each of which can have its own local network
D. Infrared
Infrared is a wireless medium that is based on infrared light from light emmitting diodes (LEDs). Infrared
signals can be detected by direct line-of-sight receivers capturing signals reflected off walls or ceiling.
Infrared signals, however, are not capable of penetrating walls or other opaque objects and are diluted by
strong light sources. These limitations make infrared most useful for small, open, indoor environments
such as a classroom or a small office area with cubicles.
Infrared transmission systems are very cost-efficient and capable of high bandwidths similar to those
found in fiber optic cables. As a result, infrared medium can be a good way of connecting wireless LANs
when computers are all located within a single room or office. Infrared eliminates the need for cables and
allows computers to be easily moved as long as they can always be pointed toward the infrared
transmitter/receiver, normally located near the ceiling.
Growth of infrared media is expected to accelerate as other radio frequencies become increasingly
congested. A large pool of potential infrared installations exists in the networking of classroom computers
and limited home or small business applications.
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4.1.2 Comparing Network Media
Medium
Cost
Installation
Unshielded
twisted-pair
cable
Shielded twistedpair cable
Coaxial cable
Fiberoptic cable
Infrared
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Low
Simple
1-100 MBPS
Immunity from EMI
and Tapping
Low
Moderate
Simple to
moderate
Simple
Difficult
1-100 MBPS
Moderate
1-1000 MBPS
100-2000 MBPS
Moderate
Very high
Simple
10-100 MBPS
Subject to
interference from
strong light sources.
Moderate
Moderate to
high
Moderate
Capacity
4.1.3 Network Topologies
An important aspect of a network system using bounded media is the method chosen to connect the
networked computers. The physical geometry or cable layout used to connect computers in a LAN
is called a network topology. Linear bus, ring, and star are the three major topologies used today to
connect computers in a LAN.
A. Star Topology
The star topology derives its name from the fact that all cables on the network radiate from a central hub.
The hub is a device that connects the network cables together and passes the signals from one cable to
the next. The type of hub we need will depend on the access system used by the network cards
(described in the section on data link components). Although star topologies entail higher costs due to the
amount of wire needed, they are generally more reliable and easier to troubleshoot than other topologies.
Because each cable in a star topology is a separate component, the failure of one cable does not affect
the operation of the rest of the network. Another advantage of the star topology is the ease of adding or
removing devices on the network without affecting the operation of other computers.
The star topology is rapidly becoming the most popular way to wire computers together due to its
exceptional flexibility and reliability. Today star networks are usually wired with a Patch Panel, In a patch
panel system, a wire runs from each potential computer location in the building through a drop cable to a
central patch panel. A Patch cable is then used to connect a device in any given location to the hub. A
patch panel system makes it easy to move a computer to another location as well as to connect or
disconnect computers from the network for troubleshooting purposes. Star topologies are generally
implemented with twisted pair cable rather than coaxial cable because of lower cable cost combined with
the increase flexibility and smaller size of twisted pair cable. RJ45 connectors on twisted pair cable allow
easy connection of computers to wall outlets and between hubs and patch panels.
B. Linear Bus Topology
The linear bus topology connects computers in series by running a cable from one computer to the next.
The method of attaching the computers to the bus depends on the network card and cable system.
When coaxial cable is used, each computer is usually attached to the bus cable by means of the Tconnector. When twisted pair cable is used, each network card usually contains two RJ-45 female
connectors that allow twisted-pair cable to be run from one computer to the next.
Each end of a linear bus network requires some sort of terminator or “Wire-Wrap” plugs in order to
prevent echo signals from interfering with communication signals. The resistance and size of coaxial
cable is an important factor and depends on the requirements of the network cards (described in the
section on data link components)
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The primary advantages of a linear bus topology are to reduce amount of cable needed and the ease of
wiring computers that are clustered in locations such as a classroom or a computer lab. The two biggest
disadvantages of a bus network are adding or removing computers and troubleshooting.
Star networks in many organizations are gradually replacing linear bus networks because star networks
are easier to troubleshoot. A broken wire in a star network configuration affects only one workstation. In
a linear bus network all computers on the cable segment fail when the cable is disconnected or broken
anywhere in the network.
C. Ring Topology
A ring topology is similar to a liner bus topology with the single difference being that the ends of the
cable are connected instead of terminated. As a result, signals on the ring topology travel around the
network in one direction until they return to the device from which they originated. In a ring topology,
each computer in the ring receives signals and then retransmits them to the next computer in the ring.
Because the signals are regenerated at each device along the network, a ring topology allows its network
signals to traverse longer distances as long as there is another computer located within the distance limit
of each network card’s transmitter.
The disadvantage of a ring topology is the extra cable needed to complete the ring circle when computers
are spread out in serial fashion. In addition, the ring has the same disadvantage as the linear bus in
terms of interrupting network transmissions in order to add or remove workstations. An advantage of the
ring topology over the linear bus topology is that ring topology is often easier to troubleshoot. Because
each computer on the ring receives and then retransmits a signal, it is possible for the troubleshooter to
use software that quickly determines which computer is not receiving the signal.
4.2 Data Link Layer Components
As mentioned in the previous section, the data link layer components actually control the way signals are
transmitted and received on the network cable system. As a result, the components you select for the
data link level of your network will determine what network topologies and cable types can be used on the
network, conversely, when you want to use an already existing cable system, you will want to select data
link products that best support it. The data link layer components consist of the network interface cards
and card driver programs.
4.2.1 Network Interface Card
The network interface card is the component that acts as an interface between the network’s data link
and physical layers by converting the commands and data frames from the data link layer into the
appropriate signals used by the connectors on the physical cable system.
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Topology
Wiring
expansion
Fault Tolerance Troubleshooting
Star
Requires the
great amount
of wire
because a
cable must be
led from each
computer to a
central hub
Easy to expand by
using a patch panel
to plug new
computers into the
hub
Highly fault
easiest to trouble shoot by
tolerant because removing suspect computers
a bad cable or from the network
connector will
affect only one
computer
Linear bus
Usually
requires the
least amount
of cable
because the
cable is
connected
from one
computer to
the next
difficult to expand
unless a connector
exists at the
location of the new
computer
Poor fault
tolerance
because a bad
connector or
cable will disrupt
the entire
network
segment
The most difficult to
troubleshoot because all
computers can be affected
by one problem.
Ring
Wiring
requirements
are more than
those of linear
bus because
of the need to
connect the
cable ends but
are less than
those of a star
Difficult to expand
because of the
need to break the
ring in order to
insert a new
computer
Poor fault
tolerance
because a bad
connector or
cable will disrupt
the entire
network
segment
Fairly easy to troubleshoot
with proper software that can
identify which computer
cannot receive the signal
4.2.2 Driver software
It is needed to control the network card and provide an interface between the data link layer and the
network layer software. In order to provide this software interface, Novell has developed a set of driver
specifications, called the Open data interface (ODI). ODI-compatible drivers allow the network card to
be shared by multiple programs running on the workstation or on the NetWare server. For example, ODI
drivers enable the NetWare server to communicate with both Apple Macintosh and IBM PCs attached to
the same network.
Microsoft networks, on the other hand, use a driver interface called network driver interface
specifications (NDIS) to interface network card drivers to Microsoft’s network operating system. NDIS
compatible drivers allow software developer to write programs for use on Windows 95 and Windows NT
computers without requiring them to write instructions to control the network card. The NDIS drivers
perform the hardware functions for them. Microsoft’s approach results in fewer programming
requirements for applications developers as well as more standardized and reliable networking
functionality in those applications.
Because there are two types of driver interfaces, ODI and NDIS, you will need to be sure the network
cards you obtain for your network contain the correct driver for the type of NOS you will be supporting.
Novell provides ODI- compatible driver programs for many popular network cards with NetWare 4.1., but
some cards are not supported. The manufacturer of an unsupported card should supply a disk with the
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ODI compatible driver program that will interface its NIC to NetWare server or workstation. Whenever
possible, try to obtain NICs that work with the standard NetWare ODI drivers to make it easier to install
and maintain your network system.
4.2.3 Access methods
In addition to controlling types of signals, data link layer standards control how each computer accesses
the network. Because only one signal can be sent on the network cable at any one time, a channel
access method is necessary to control when computers transmit in order to reduce collisions that can
occur when two or more computers attempt to transmit at the same time. Collisions cause network errors
by distorting data signals, making them unreadable.
Channel access methods used on today’s LANs are either
 token passing or
 contension based
A. Token passing method
It enables only one computer to transmit a message on the network at any given time. This access to the
network is controlled by a Token, which is a special packet passed from one computer to the next to
determine which machine can use the network. When a computer needs to transmit data, it waits until it
receives the token packet and then transmits its data frame packet on the network. After the transmission
is complete, the transmitting computer releases the token. The next computer on the network can pick it
up and then proceed to transmit. In its actual implementation, the token passing system is very complex,
involving token priorities, early release of tokens, and network monitoring and error-detection functions.
As a result, network cards based on the token passing method are generally more expensive.
The token passing technology was originally developed by IBM and has now been standardized by the
IEEE 802.5 committee.
B. Contention access method
This allows a node to transmit a message whenever it detects that the channel is not in use. On a
computer network, this contention system is referred to as carrier sense multiple access with collision
detection (CSMA/CD) and has been standardized by the IEEE 802.3 committee into several different
product types, based on speed and cable type. The two most popular IEEE 802.3 committee standards,
10BaseT and 10Base2, will be described later.
A contention system works very well when network traffic is light, but its performance drops off quickly
under heavy network transmission loads. Token-based systems perform better under heavy loads
because the performance does not drop off as abruptly.
4.2.4 Token Ring Networks
IBM originally designed the token ring system for use in industrial environments that requires reliable
high-speed communications. Today, token ring is widely considered to be the best network system in
terms of overall performance and reliability.
Standard token ring cards can transmit at 4 MBPS and 16 MBPS (million bits per second). You cannot
mix cards running at 4 MBPS with cards running at 16 MBPS on the same token ring network.
Workstations are connected by twisted pair cables to a central hub called multiple stations access unit
(MAU). Although this appears to be a star arrangement, the network signals actually travel in a ring,
which is why it is often referred to as a star ring.
If a workstation is down, the relay in the MAU will automatically pass signal on to the next workstation,
hence it is resistant to breakdown. The advantages of token ring system is the speed, expandability, fault
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tolerence, and easy to troubleshoot. The disadvantage is the extra wiring, high cost of most tokenring
system components, a MAU for every 8 computers.
4.2.5 Ethernet Networks
The term Ethernet originally applied to networks using a linear bus topology and CSMA/CD (carrier sense
multiple access/ collision detection) on coaxial cable. This system, discussed in detail below, is also
known as 10Base2. However, several variants of the specification have been created, and now the term
Ethernet is used as a general reference to the entire family of variations. The members of the Ethernet
family discussed below are 10Base2, 10BaseT, 100BaseTX, and 100BaseFX.
The term 10Base2 stands for 10-Mbps baseband using digital baseband signals over a maximum of two
100-meter coaxial cable segments. The term baseband describes a computer network that carries
digital signals; a broadband system carries analog signals, like the signals used for television and
radio transmissions. In 10Base2, thin RG- 58 coaxial cable with T-connectors enables up to 30 machines
to be attached to a single cable run, which is called a segment.
According to the 10Base2 standards, a segment cannot exceed 607 feet (200M) in length, and no more
than five segments can be joined by repeaters to form the entire network. Additionally, a maximum of
three of the five segments can have workstations attached. Network professionals often refer to 10Base2
as ThinNet because of its thin coaxial cable.
Wiring 10Base2 is simpler and more cost-effective than 10BaseT in certain environments-those in which
groups of computers are located in a small area, such as a computer lab, where one coaxial cable runs
from machine to machine. Thick coaxial cable is sometimes used instead of thin coaxial cables.
Networks using thick coaxial cable are referred to as 10Base5, Thick Ethernet, or ThickNet.
The 10BaseT-network system is very popular in business offices today because it combines the flexibility
of the star topology with the lower cost of the CSMA/CD channel access method. The IEEE 802.3
designation of 10BaseT stands for 10-Mbps baseband network using twisted-pair cable.
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Although the 10BaseT network used the same star topology as a token ring network, the 10BaseT signals
are not sent from one station to the next as in token ring. They are broadcast to all stations
simultaneously by using the CSMA/CD method standardized by the IEEE 802.3 committee. In many
instances a cable system designed for token ring can easily be converted to support 10BaseT simply by
replacing the MAUs with concentrators. The concentrator acts as a repeater, receiving signals on one
cable port and then retransmitting those signals on all other ports. When two or more network stations
attempt to transmit at the same instant, collision occurs, and the stations must retransmit after waiting a
random period of time.
The advantages of 10BaseT include high performance under light to medium network loads and low costs
for network cards due to the relative simplicity of the CSMA/CD system. Although 10BaseT performance
can be faster than token ring under light loads, it is more easily slowed due to collisions when many
stations are transmitting on the network. Another disadvantage of the 10BaseT system is additional cost
for both concentrators and for the star topology wiring.
The 100Base TX and 100Base FX network systems are extensions of the 10Base T system and are
overseen by the IEEE 802.3 committee. They use the same star topology and the CSMA/CD channel
T
access method. The designation of 100BaseTX indicates a 100-Mbps base band network using twistedpair cable or IBM STP cable. The 100BaseFX designation indicates the use of fiber optic cable.
100Base TX and 100BaseFX networks appear identical to a 10BaseT network. A concentrator is used as
the hub to connect all machines in a star topology. The concentrator still acts as a repeater.
The advantages of 100BaseTX and 100BaseFX include higher performance for networks needing fast
data transmission, such as those using video. The disadvantages include shorter maximum cable run
lengths in some cable systems, which is a tradeoff necessary to gain the extra speed, and a higher cost
of hub and NICs capable of handling the higher speed.
4.2.6 100VG-AnyLAN networks
This system was originally developed by Hewlett-Packard and AT&T Microelectronics as the 100Base VG
Ethernet system. Hewlett-Packard then worked with IBM to create 100VG-AnyLAN,a 100-Mbps network
solution to the emerging need for higher data transmission rates on the network, which is usable as an
upgrade from either Ethernet or Token ring network. The standard is under the IEEE 802.12 committee,
and designation of 100VG-AnyLAN stands for a 100-Mbps base band network using voice grade
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(category 3) twisted-pair cable, fiber optic cable, or IBM STP cable. It differs from other Ethernet systems
and token ring by using a demand priority media access method instead of the CSMA/CD channel
access method or token passing. A demand priority system enables only one workstation to broadcast
based on a priority system, so that more important network messages are broadcast first.
The term AnyLAN refers to the ability of a 100VG-AnyLan system to use either Ethernet or token ring
NICs. One network, however, must use one or other—Ethernet and Token ring cannot be combined in
the same network. This ability makes 100VG-AnyLAN a potential upgrade path for Ethernet and token
ring network. It appears identical to a 100BaseTX network, using a hub to connect all machines in a star
topology.
The 100Base FX specification has so far proved to be more popular than 100VG-AnyLAN. However,
Hewlett-Packard and other vendors still support the technology and offer products to implement.
4.2.7 ARCnet topology
Despite its relatively slow speed (2 Mbps), ARCnet was popular for small networks because of its low
cost and flexible topology. Today, the decreased cost of Ethernet systems combined with the lack of
IEEE standards for ARCnet and its slower speed make it a poor choice for most networks. The ARCnet
system has a star topology in which an active hub acts as a signal repeater, enabling cable runs of up to
2,000 feet from the active hub to the attached workstation computers. Passive hubs are simple signal
splitters. They can be used at the end of a run to split the cable and allow up to three workstations to be
attached to a single cable run. When a passive hub is used, wire length must be limited to 100 feet.
Depending on the ARCnet card used either twisted-pair or RG-68 coaxial cable can connect computers.
The advantages of ARCnet are low card cost and flexible wiring option. Its disadvantages are slow
speed, higher cabling cost based on its topology, and lack of standardization.
4.2.8 Comparing Network Systems
Selecting a network is a complex task that depends on such variable as type and location of computers,
existing wiring, and the amount of load expected on the network. In many organizations, multiple network
systems are necessary to meet the needs of different departments. Such network systems can be
connected with bridges and routers.
Network
System
Cable
Types
Topology
Token ring
star
10Base2
UTP, STP,
fiber
Coaxial
10BaseT
UTP
Star
linear bus
Maximum IEEE
Speed
Number of Standard
Nodes
96
802.5 4-16
mbps
30 per
802.3 10 mbps
segment
with
maximum
of 3
segments
512
802.3 10 mbps
Access
Method
Distance
Token
150' per cable run
CSMA/CD
607' per segment
CSMA/CD
100 meters per
cable run on UTP
Cat 3 & 4: 150
meters on UTP cat
5
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100BaseTX
UTP,STP
Star
512
802.3 100 mbps CSMA/CD
100 meters per
cable run on UTP
Cat 3 & 4: 150
meters on STP
type 1
100BaseFx
fiber
star
512
802.3 100 mbps CSMA/CD
412 meters on
fiber
100VGAnyLAN
UTP, STP,
fiber
star
240
ARCnet
RG-62
coaxial
UTP
star
255 NONE
802.12 100 Mbps Demand
priority
2 Mbps
Token
standard
100 meters per
cable run on UTP
Cat 3 & 4: 150
meters on STP
type 1; 2000 M on
fiber
2000' from active
hub, 100' from
passive hub
4.3 Repeaters, Bridges and Routers
Each network system presented in this chapter has unique limitations. In some cases, you will want to
take advantage of certain features found in two different products. For example, in a school environment
you might want to implement the Ethernet system in computer labs to take advantage of the economical
coaxial wiring arrangement. If other computers in the building are located many feet apart in completely
separate areas, however, you will not want to connect them this way. You can solve this problem by
creating two separate networks: Ethernet for the lab and token ring for the office. You then connect the
networks so they share access to the same NetWare server. In other cases, it might be necessary to
break a large network into two or more smaller networks to overcome performance problems or cabling
distances, or to accommodate large numbers of users.
Within a network system, you use repeaters to maintain a strong, reliable signal throughout the network.
To connect separate network systems you use bridges or routers, and the resulting connected networks
are called an internetwork.
4.3.1 Repeaters
Network cable systems consist of one or more cable lengths, called segments, that have termination
points on each end, Repeaters are hardware devices that allow you to link network segments together.
Repeaters work at the physical layer of the OSI model. This means that the repeater simply receives
signals from one network segment and then retransmits them to the next segments.
The hub of a star network topology, for example, can act as a repeater, receiving a signal from one
computer cable and broadcasting it on the other cables. Each computer in a ring topology acts as a
repeater, receiving the signal from the “upstream” computer and retransmitting it to the next computer on
the ring. Repeaters are also used to connect two linear bus segments. This use of repeaters increases
the fault tolerance of a linear bus network because a bad connector or cable on one segment does not
prevent computers on other segments from communicating.
4.3.2 Bridges
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A bridge operates at the data link layer of the OSI model. This means that the bridge sees only the
packet’s frame information, which consists of the addresses of the sender and receiver along with errorchecking information. During network operation, the bridge watches packets on both networks and builds
a station of workstation node addresses for each network. When it sees a packet on one network that
has a destination address for a machine on the other network, the bridge reads the packet, builds new
frame information, and sends the packet out on the other network. Bridges work at the data link level,
and are used to connect networks of the same type. For example, a bridge can connect two different
token ring networks and allow more than 100 users to access the same NetWare server.
Another use for a bridge might be to break a heavily loaded Ethernet or 10BaseT network into two
separate networks in order to reduce the number of collisions occurring on any one-network system. A
bridge is often contained in a separate black box but can also consist of specializing software running on
a microcomputer that simply contains two network cards.
4.3.3 Routers
Routers are needed to create more complex internetworks. A router operates at the network layer of
the OSI model and therefore has access to the datagram information containing the logical network
address along with control information. When a router is used, each network must be given a separate
network address. The router information contained in the datagram packet enables a router to find the
correct path and, if necessary, break up a datagram for transmission on a different network system. Two
disadvantages of routers are that they require a little more processing time than bridges and that network
packets must use a datagram format that the router can interpret.
Generally, networks with different network topologies are connected with routers, whereas
networks of the same topology are connected with bridges. Novell uses routers in its NetWare
servers to allow up to eight different network cards to be installed in a single NetWare server computer.
This enables you to use the NetWare server to connect networks of different types and topologies in
order to form an internetwork.
5.0 PROTOCAL STACKS
The network’s protocol stack is responsible for formatting requests to access network services and
transmit data. While the delivery of the data packets throughout a network system is the
responsibility of its data link and physical layer components, the functions of the network,
transport, and session layers are built in to a network operating system’s protocol stack
5.1 IPX/SPX
The IPX/SPX Protocol is Novell’s proprietary system that implements the session, transport, and network
OSI layers, as shown in Figure. Notice that IPX/SPX is not a true implementation of OSI model because
IPX and SPX functions overlap layers. This is true of many older protocol stacks that were developed
before the OSI model was developed and standardized.
FIG/3-23
NCP
SPX
IPX
Ethernet
Token
ring
ARC net
Others
Physical
Data link
X
X
X
X
X
X
OSI Model Layers
Network Transport Session
X
X
X
X
X
Presentation
X
Application
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IPX (internetwork packet exchange) is the NetWare protocol that manages packet routing and
formatting at the network layer. To function, IPX must be loaded on each network workstation and on the
NetWare server. In addition to IPX, each workstation and NetWare server must have loaded a network
card driver in order to transmit the frames containing the packets. IPX software and the network card
driver are brought together during the network installation process.
In addition to IPX, NetWare uses two protocols, SPX and NCP, to provide network services.
SPX (sequential packet exchange) operates at the OSI transport level and provides guaranteed
delivery of packets by receiving an acknowledgement for each packet sent.
NCP (NetWare Core Protocol) provides the session and presentation levels at the workstation through
DOS requester workstation clients. On the NetWare server, NCP provides network services such as
login, file sharing, printing, security, and administrative functions.
5.2 TCP/IP
Transmission control protocol /internet protocol covers the network and transport OSI layers, as does
IPX/SPX. Unlike IPX/SPX, however, TCP and IP don’t overlap in the transport layer. Like the IPX
protocol, TCP/IP is responsible for formatting packets and then routing them between networks using IP
(internet protocol).
IP is more sophisticated than IPX in fragmenting packets and transmitting over wide area network links.
When IP is used, each workstation is assigned a logical network and node address. IP allows packets to
be sent out over different routers and then reassembled in the correct sequence at the receiving station.
TCP (transport control protocol) operates at the transport level and provides the guaranteed delivery of
packets by receiving acknowledgements. The acknowledgment system allows the sender and receiver to
establish a window for the number of packets to be acknowledged. This allows for better performance
over WANs because each packet does not need to be individually acknowledged before another packet is
sent.
Today TCP/IP is commonly used on UNIX operating systems as well as the Internet. NetWare servers
can use the TCP/IP protocol to communicate with UNIX-based computers to provide Internet services,
and to route TCP/IP packets between network cards.
FIG/3-24
TCP
IP
Ethernet
Token
Ring
Others
Physical
Data Link
X
X
X
X
X
X
OSI Model Layers
Network Transport Session
X
X
Presentation
Application
NetWare 4.1 provides a TCP/IP module that can be loaded on the NetWare Server. NetWare 4.1 also
allows you to use TCP/IP as the NetWare protocol. If you choose to use TCP/IP, IPX packets are placed
within TCP/IP packets—the IPX structure is not totally eliminated.
5.3 NetBEUI
The NetBEUI protocol is Microsoft’s own protocol stack and is integrated into Windows for Workgroups,
Windows 95, and Windows NT products. Of the three protocols described in this section, NetBEUI is the
smallest, fastest, and easiest to use. It has few features, however, and cannot be used in large internet
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work environments because it does not support the network layer needed for routing packets between
networks. As a result, the NetBEUI protocol is limited to communicating with other computers attached to
the same network cable system. Another disadvantage of the NetBEUI protocol is that it was developed
specifically to support peer-to-peer networking on small networks comprising 30 to 50 workstations.
Physical
NetBIOS
NBForNBT
Ethernet
Token
Ring
Others
Data
Link
Network
Transport
X
X
Session
Presentation
Application
X
X
X
X
X
X
X
The NetBEUI protocol stack consists of NetBIOS and service message blocks (SMBs) at the session
layer and NetBIOS frames (NBF) at the transport layer.
Because NetBIOS-based applications are popular, Novell has provided a NetBIOS interface to work with
its IPX/SPX protocol. This allows workstations to run peer-to-peer applications while still accessing
service from NetWare servers. The LANtastic peer-to-peer network product also uses NetBIOS to
establish communication among DOS-based computers.
5.4 AppleTalk
The AppleTalk protocol suite was originally developed to allow Macintosh computers to communicate in
peer-to-peer networks. It currently provides connectivity for a variety of computer systems including IBM
PCs running MS-DOS, IBM mainframes, and various UNIX-based computers. The AppleTalk protocol
suite was developed after the OSI model was conceived and therefore can be mapped reasonably well to
the OSI layers.
On the data link level, the Apple Address Resolution Protocol (AARP) connects the AppleTalk protocol
stack to the Ethernet, 10BaseT, or token ring protocol. AppleTalk supports the routing of packets
between networks by using the Datagram Delivery Protocol (DDP).
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Local area network (LAN)
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Physic
al
Apple
Filing
Protocol
(AFP)
Apple
Session
Protocol
(ASP)
Apple
Transition
Protocol
(ATP)
Datagram
Delivery
Protocol
(DDP)
AARP
(Apple
Address
Resolution
Protocol)
Local Talk
Ethertalk
(Ethernet)
Token Talk
(Token
Ring)
Data
Link
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Network
Session
Transpor
t
Presentati
on
Applicati
on
X
X
X
X
X
X
X
X
X
X
X
X
___________
17