2
ACKNOWLEDGEMENT
First of all I would like to thank our HOD Prof. DINESH CHANDRA , my coordinating
faculty Prof. CHAYA GROVER and Prof. ARVIND TIWARI for assigning me the
seminar topic “OPTICAL NETWORK ARCHITECTURE”.
I would also like to thank my seniors, my batch mates for helping me in every possible way
for completion of this seminar.
Lastly I would specially like to thank Mr. ARVIND TIWARI for helping and guiding me
throughout this seminar report.

3
ABSTRACT
Optical networks are high-capacity telecommunications networks based on optical
technologies and component that provide routing, grooming, and restoration at the
wavelength level as well as wavelength-based services. As networks face increasing
bandwidth demand and diminishing fiber availability, network providers are moving towards
a crucial milestone in network evolution: the optical network. Just like every other layer
defined in networking, a layer architecture has to be defined for the optical layer. A multi-
wavelength mesh-connected optical network is used to define the architecture of the optic
layer. SONET is a set of transport containers that allow for delivery of a variety of protocols,
including traditional telephony, ATM, Ethernet, and TCP/IP traffic. A passive optical
network (PON) is a telecommunications network that uses point-to-multipoint fiber to the
premises in which unpowered optical splitters are used to enable a single optical fiber to
serve multiple premises.

4
TABLE OF CONTENTS
ACKNOWLEDGEMENTS.................................................................................. ii
ABSTRACT ........................................................................................................... iii
CHAPTER 1 INTRODUCTION 5
1.1 Optical Network Architecture ......................................................................5
CHAPTER 2 Benefits and History of Optical Network................6
2.1 History............................................................................................................6
2.2 Asynchronous................................................................................................ 6
2.3 Synchronous...................................................................................................6
2.4 Optical............................................................................................................6
CHAPTER 3 Dense Wavelength Division Multiplexing 8
3.1 DWDM SYSTEM ........................................................................................8
3.2 Optical Transmission Principles....................................................................9
CHAPTER 4 Synchronous Optical Networking.............................. ...........................10
4.1 The basic unit of transmission.....................................................................11
4.2 Framing..........................................................................................................11
4.3SDH frame .....................................................................................................11
4.4 Payload .........................................................................................................12
CHAPTER 5 Passive optical network.....................................................................14
5.1 History.......................................................................................................14
5.2 Network elements.......................................................................................15
CHAPTER 6 NETWORK TOPOLOGY ...............................................................16
6.1 Types of topologies.....................................................................................16
6.1.1 BUS topology...........................................................................................16
6.1.2 STAR topology........................................................................................18
6.1.3 TREE toplogy...........................................................................................19
REFERENCES ......................................................................................................20

5
CHAPTER 1
INTRODUCTION
One of the major issues in the networking industry today is tremendous demand for more and
more bandwidth. Before the introduction of optical networks, the reduced availability of
fibers became a big problem for the network providers. However, with the development of
optical networks and the use of Dense Wavelength Division Multiplexing (DWDM)
technology, a new and probably, a very crucial milestone is being reached in network
evolution. The existing SONET/SDH network architecture is best suited for voice traffic
rather than today’s high-speed data traffic. To upgrade the system to handle this kind of
traffic is very expensive and hence the need for the development of an intelligent all-optical
network. Such a network will bring intelligence and scalability to the optical domain by
combining the intelligence and functional capability of SONET/SDH, the tremendous
bandwidth of DWDM and innovative networking software to spawn a variety of optical
transport, switching and management related products.
1.1 Optical Network Architecture
Optical networks are high-capacity telecommunications networks based on optical
technologies and component that provide routing, grooming, and restoration at the
wavelength level as well as wavelength-based services. The origin of optical networks is
linked to Wavelength Division Multiplexing (WDM) which arose to provide additional
capacity on existing fibers. The optical layer whose standards are being developed, will
ideally be transparent to the SONET layer, providing restoration, performance monitoring,
and provisioning of individual wavelengths instead of electrical SONET signals. So in
essence a lot of network elements will be eliminated and there will be a reduction of electrical
equipment.
A passive optical network (PON) is a telecommunications network that uses point-to-
multipoint fiber to the premises in which unpowered optical splitters are used to enable a
single optical fiber to serve multiple premises. A PON consists of an optical line
terminal (OLT) at the service provider's central office and a number of optical network
units (ONUs) near end users. A PON reduces the amount of fiber and central office
equipment required compared with point-to-point architectures. A passive optical network is
a form of fiber-optic access network. They do both the transmission and the switching of data
in the optical domain. This has resulted in the onset of tremendous amount of bandwidth
availability. Further the use of non-overlapping channels allows each channel to operate at
peak speeds.

6
CHAPTER 2
2. Benefits and History of Optical Networks
In the early 1980s, a revolution in telecommunications networks began that was
spawned by the use of a relatively unassuming technology, fiber -optic cable. Since then, the
tremendous cost savings and increased network quality has led to many advances in the
technologies required for optical networks, the benefits of which are only beginning to be
realized.
2.1 History
Telecommunication networks have evolved during a century-long history of
technological advances and social changes. The networks that once provided basic telephone
service through a friendly local operator are now transmitting the equivalent of thousands of
encyclopedias per second. Throughout this history, the digital network has evolved in three
fundamental stages: asynchronous, synchronous, and optical.
2.2 Asynchronous
The first digital networks were asynchronous networks. In asynchronous networks,
each network element's internal clock source timed its transmitted signal. Because each
clock had a certain amount of variation, signals arriving and transmitting could have a large
variation in timing, which often resulted in bit errors.
More importantly, as optical-fiber deployment increased, no standards existed to mandate
how network elements should format the optical signal. A myriad of proprietary methods
appeared, making it difficult for network providers to interconnect equipment from different
vendors.
2.3 Synchronous
The need for optical standards led to the creation of the synchronous optical network
(SONET). SONET standardized line rates, coding schemes, bit-rate hierarchies, and
operations and maintenance functionality. SONET also defined the types of network
elements required, network architectures that vendors could
implement, and the functionality that each node must perform. Network providers could now
use different vendor's optical equipment with the confidence of at least basic interoperability.
2.4 Optical
The one aspect of SONET that has allowed it to survive during a time of tremendous
changes in network capacity needs is its scalability. Based on its open-ended growth plan for
higher bit rates, theoretically no upper limit exists for SONET bit rates. However, as higher
bit rates are used, physical limitations in the laser sources and optical fiber begin to make the
practice of endlessly increasing the bit rate on each signal an impractical solution.
Additionally, connection to the networks through access rings has also had increased
requirements. Customers are demanding more services and options and are carrying more and

7
different types of data traffic. To provide full end-to -end connectivity, a new paradigm was
needed to meet all the high-capacity and varied needs. Optical networks provide the required
bandwidth and flexibility to enable end-to-end wavelength services
Figure 1. End-to-End Wavelength Services
Optical networks began with wavelength division multiplexing (WDM),
which arose to provide additional capacity on existing fibers. Like SONET, defined
network elements and architectures provide the basis of the optical network.
However, unlike SONET, rather than using a defined bit-rate and frame structure as
its basic building block, the optical network will be based on wavelengths. The
components of the optical network will be defined according to how the wavelengths
are transmitted, groomed, or implemented in the network.

8
CHAPTER 3
Dense Wavelength Division Multiplexing(DWDM)
Dense Wavelength Division Multiplexing (DWDM) is a fiber-optic transmission
technique. It involves the process of multiplexing many different wavelength signals onto a
single fiber. So each fiber have a set of parallel optical channels each using slightly different
light wavelengths. It employs light wavelengths to transmit data parallel-by-bit or serial-by-
character. DWDM is a very crucial component of optical networks that will allow the
transmission of data: voice, video-IP, ATM and SONET/SDH respectively, over the optical
layer.
Hence with the development of WDM technology, optical layer provides the only
means for carriers to integrate the diverse technologies of their existing networks into one
physical infrastructure. For example, though a carrier might be operating both ATM and
SONET networks, with the use of DWDM it is not necessary for the ATM signal to be
multiplexed up to the SONET rate to be carried on the DWDM network. Hence carriers can
quickly introduce ATM or IP without having to deploy an overlay network for multiplexing.
3.1 DWDM SYSTEM
As mentioned earlier, optical networks use Dense Wavelength Multiplexing as the
underlying carrier. The most important components of any DWDM system are transmitters,
receivers, Erbium-doped fiberAmplifiers,DWDM multiplexors and DWDM demultiplexors.
Fig 1 gives the structure of a typical DWDM system. The concepts of optical fiber
transmission, amplifiers, loss control, all optical header replacement, network topology,
synchronization and physical layer security play a major role in deciding the throughput of
the network. These factors have been discussed briefly in this sections that follow.

9
Fig.1 Block Diagram of a DWDM System
3.2 Optical Transmission Principles
The DWDM system has an important photonic layer, which is responsible for
transmission of the optical data through the network. Some basic principles, concerning the
optical transmission, are explained in this section. These are necessary for the proper
operation of the system.
Channel Spacing
The minimum frequency separation between two different signals multiplexed in
known as the Channel spacing. Since the wavelength of operation is inversely proportional
to the frequency, a corresponding difference is introduced in the wavelength of each signal.
The factors controlling channel spacing are the optical amplifier’s bandwidth and the
capability of the receiver in identifying two close wavelengths sets the lower bound on the
channel spacing. Both factors ultimately restrict the number of unique wavelengths passing
through the amplifier.
Signal Direction
An optical fiber helps transmit signal in both directions. Based on this feature, a
DWDM system can be implemented in two ways:
 Unidirectional: All wavelengths travel in the same direction within the fiber. It is
similar to a simplex case. This calls in for laying one another parallel fiber for
supporting transmission on the other side.
 Bi-directional: The channels in the DWDM fiber are split into two separate bands,
one for each direction. This removes the need for the second fiber, but, in turn reduces
the capacity or transmission bandwidth.

10
CHAPTER 4
Synchronous Optical Networking
Synchronous Optical Networking (SONET) and Synchronous Digital
Hierarchy (SDH) are standardized protocols that transfer multiple digital bit streams
over optical fiber using lasers or highly coherent light from light-emitting diodes (LEDs). At
low transmission rates data can also be transferred via an electrical interface. The method was
developed to replace the Plesiochronous Digital Hierarchy (PDH) system for transporting
large amounts of telephone calls and data traffic over the same fiber without synchronization
problems. SONET generic criteria are detailed inTelcordia Technologies Generic
Requirements document GR-253-CORE. Generic criteria applicable to SONET and other
transmission systems (e.g., asynchronous fiber optic systems or digital radio systems) are
found in Telcordia GR-499-CORE.
SONET and SDH, which are essentially the same, were originally designed to
transport circuit mode communications (e.g., DS1, DS3) from a variety of different sources,
but they were primarily designed to support real-time, uncompressed, circuit-switched voice
encoded in PCM format.[3] The primary difficulty in doing this prior to SONET/SDH was
that the synchronization sources of these various circuits were different. This meant that each
circuit was actually operating at a slightly different rate and with different phase.
SONET/SDH allowed for the simultaneous transport of many different circuits of differing
origin within a single framing protocol. SONET/SDH is not itself a communications
protocol per se, but a transport protocol.
Due to SONET/SDH's essential protocol neutrality and transport-oriented features,
SONET/SDH was the obvious choice for transporting the fixed length Asynchronous
Transfer Mode (ATM) frames also known as cells. It quickly evolved mapping structures and
concatenated payload containers to transport ATM connections. In other words, for ATM
(and eventually other protocols such as Ethernet), the internal complex structure previously
used to transport circuit-oriented connections was removed and replaced with a large and
concatenated frame (such as STS-3c) into which ATM cells, IP packets, or Ethernet frames
are placed.
Both SDH and SONET are widely used today: SONET in the United
States and Canada, and SDH in the rest of the world. Although the SONET standards were
developed before SDH, it is considered a variation of SDH because of SDH's greater
worldwide market penetration.
The SDH standard was originally defined by the European Telecommunications
Standards Institute (ETSI), and is formalized as International Telecommunication
Union (ITU) standards G.707,[4] G.783,[5] G.784,[6] and G.803.[7][8] The SONET standard was
defined by Telcordia[1] and American National Standards Institute (ANSI) standard
T1.105.[8][9]

11
4.1 The basic unit of transmission
The basic unit of framing in SDH is a STM-1 (Synchronous Transport Module, level
1), which operates at 155.520 megabits per second (Mbit/s). SONET refers to this basic unit
as an STS-3c (Synchronous Transport Signal 3, concatenated). When the STS-3c is carried
over OC-3, it is often colloquially referred to as OC-3c, but this is not an official designation
within the SONET standard as there is no physical layer (i.e. optical) difference between an
STS-3c and 3 STS-1s carried within an OC-3.
SONET offers an additional basic unit of transmission, the STS-1 (Synchronous
Transport Signal 1) or OC-1, operating at 51.84 Mbit/s—exactly one third of an STM-1/STS-
3c/OC-3c carrier. This speed is dictated by the bandwidth requirements for PCM-encoded
telephonic voice signals: at this rate, an STS-1/OC-1 circuit can carry the bandwidth
equivalent of a standard DS-3 channel, which can carry 672 64-kbit/s voice channels.[3] In
SONET, the STS-3c signal is composed of three multiplexed STS-1 signals; the STS-3c may
be carried on an OC-3 signal. Some manufacturers also support the SDH equivalent of the
STS-1/OC-1, known as STM-0.
4.2 Framing
In packet-oriented data transmission, such as Ethernet, a packet frame usually consists
of a header and a payload. The header is transmitted first, followed by the payload (and
possibly a trailer, such as a CRC). In synchronous optical networking, this is modified
slightly. The header is termed the overhead, and instead of being transmitted before the
payload, is interleaved with it during transmission. Part of the overhead is transmitted, then
part of the payload, then the next part of the overhead, then the next part of the payload, until
the entire frame has been transmitted.
In the case of an STS-1, the frame is 810 octets in size, while the STM-1/STS-3c
frame is 2,430 octets in size. For STS-1, the frame is transmitted as three octets of overhead,
followed by 87 octets of payload. This is repeated nine times, until 810 octets have been
transmitted, taking 125 µs. In the case of an STS-3c/STM-1, which operates three times faster
than an STS-1, nine octets of overhead are transmitted, followed by 261 octets of payload.
This is also repeated nine times until 2,430 octets have been transmitted, also taking 125 µs.
For both SONET and SDH, this is often represented by displaying the frame graphically: as a
block of 90 columns and nine rows for STS-1, and 270 columns and nine rows for
STM1/STS-3c. This representation aligns all the overhead columns, so the overhead appears
as a contiguous block, as does the payload.
The internal structure of the overhead and payload within the frame differs slightly
between SONET and SDH, and different terms are used in the standards to describe these
structures. Their standards are extremely similar in implementation, making it easy to
interoperate between SDH and SONET at any given bandwidth.

12
4.3 SDH frame
The STM-1 (Synchronous Transport Module, level 1) frame is the basic transmission
format for SDH—the first level of the synchronous digital hierarchy. The STM-1 frame is
transmitted in exactly 125 µs, therefore, there are 8,000 frames per second on a 155.52 Mbit/s
OC-3 fiber-optic circuit.[nb 1] The STM-1 frame consists of overhead and pointers plus
information payload. The first nine columns of each frame make up the Section Overhead
and Administrative Unit Pointers, and the last 261 columns make up the Information Payload.
The pointers (H1, H2, H3 bytes) identify administrative units (AU) within the information
payload. Thus, an OC-3 circuit can carry 150.336 Mbit/s of payload, after accounting for the
overhead.[nb 2]
Carried within the information payload, which has its own frame structure of nine
rows and 261 columns, are administrative units identified by pointers. Also within the
administrative unit are one or more virtual containers (VCs). VCs contain path overhead and
VC payload. The first column is for path overhead; it is followed by the payload container,
which can itself carry other containers. Administrative units can have any phase alignment
within the STM frame, and this alignment is indicated by the pointer in row four.
An STM-1 frame. The first nine columns contain the overhead and the pointers. The
frame is shown as a rectangular structure of 270 columns and nine rows but the
protocol does not transmit the bytes in this order.
4.4 Payload
User data (774 bytes for STM-0/STS-1, or 2,340 octets for STM-1/STS-3c)
For STS-1, the payload is referred to as the synchronous payload envelope (SPE),
which in turn has 18 stuffing bytes, leading to the STS-1 payload capacity of 756
bytes.[11]
The STS-1 payload is designed to carry a full PDH DS3 frame. When the DS3 enters
a SONET network, path overhead is added, and that SONET network element (NE) is
said to be a path generator and terminator. The SONET NE is line terminating if it

13
processes the line overhead. Note that wherever the line or path is terminated, the section
is terminated also. SONET regenerators terminate the section, but not the paths or line.
An STS-1 payload can also be subdivided into seven virtual tributary groups (VTGs).
Each VTG can then be subdivided into four VT1.5 signals, each of which can carry a
PDH DS1 signal. A VTG may instead be subdivided into three VT2 signals, each of
which can carry a PDH E1 signal. The SDH equivalent of a VTG is a TUG-2; VT1.5 is
equivalent to VC-11, and VT2 is equivalent to VC-12.
Three STS-1 signals may be multiplexed by time-division multiplexing to form the
next level of the SONET hierarchy, the OC-3 (STS-3), running at 155.52 Mbit/s. The
signal is multiplexed by interleaving the bytes of the three STS-1 frames to form the
STS-3 frame, containing 2,430 bytes and transmitted in 125 µs.
Higher-speed circuits are formed by successively aggregating multiples of slower
circuits, their speed always being immediately apparent from their designation. For
example, four STS-3 or AU4 signals can be aggregated to form a 622.08 Mbit/s signal
designated OC-12 or STM-4.
The highest rate commonly deployed is the OC-768 or STM-256 circuit, which
operates at rate of just under 38.5 Gbit/s.[12] Where fiber exhaustion is a concern, multiple
SONET signals can be transported over multiple wavelengths on a single fiber pair by
means of wavelength-division multiplexing, including dense wavelength-division
multiplexing (DWDM) and coarse wavelength-division multiplexing (CWDM). DWDM
circuits are the basis for all modern submarine communications cable systems and other
long-haul circuits.
Next-generation SONET/SDH:
SONET/SDH development was originally driven by the need to transport multiple
PDH signals—like DS1, E1, DS3, and E3—along with other groups of multiplexed
64 kbit/s pulse-code modulatedvoice traffic. The ability to transport ATM traffic was another
early application. In order to support large ATM bandwidths, concatenation was developed,
whereby smaller multiplexing containers (e.g., STS-1) are inversely multiplexed to build up a
larger container (e.g., STS-3c) to support large data-oriented pipes.
One problem with traditional concatenation, however, is inflexibility. Depending on
the data and voice traffic mix that must be carried, there can be a large amount of unused
bandwidth left over, due to the fixed sizes of concatenated containers. For example, fitting a
100 Mbit/s Fast Ethernet connection inside a 155 Mbit/s STS-3c container leads to
considerable waste. More important is the need for all intermediate network elements to
support newly introduced concatenation sizes. This problem was overcome with the
introduction of Virtual Concatenation.

14
CHAPTER 5
Passive optical network (PON)
A passive optical network (PON) is a telecommunications network that uses point-
to-multipoint fiber to the premises in which unpowered optical splitters are used to enable a
single optical fiberto serve multiple premises. A PON consists of an optical line
terminal (OLT) at the service provider's central office and a number of optical network
units (ONUs) near end users. A PON reduces the amount of fiber and central office
equipment required compared with point-to-point architectures. A passive optical network is
a form of fiber-optic access network.
In most cases, downstream signals are broadcast to all premises sharing multiple
fibers. Encryption can prevent eavesdropping.
Upstream signals are combined using a multiple access protocol, usually time division
multiple access (TDMA).
5.1 History
Two major standard groups, the Institute of Electrical and Electronics
Engineers (IEEE) and the Telecommunication Standardization Sector of the International
Telecommunication Union (ITU-T), develop standards along with a number of other industry
organizations. TheSociety of Cable Telecommunications Engineers (SCTE) also
specified radio frequency over glass for carrying signals over a passive optical network.
Starting in 1995, work on fiber to the home architectures was done by the Full Service
Access Network (FSAN) working group, formed by major telecommunications service
providers and system vendors.[1] The International Telecommunications Union (ITU) did
further work, and standardized on two generations of PON. The older ITU-T G.983 standard
was based on Asynchronous Transfer Mode (ATM), and has therefore been referred to as
APON (ATM PON). Further improvements to the original APON standard – as well as the
gradual falling out of favor of ATM as a protocol – led to the full, final version of ITU-T
G.983 being referred to more often as broadband PON, or BPON. A typical APON/BPON
provides 622 megabits per second (Mbit/s) (OC-12) of downstream bandwidth and 155
Mbit/s (OC-3) of upstream traffic, although the standard accommodates higher rates.
The ITU-T G.984 Gigabit-capable Passive Optical Networks (GPON) standard represented
an increase, compared to BPON, in both the total bandwidth and bandwidth efficiency
through the use of larger, variable-length packets. Again, the standards permit several choices
of bit rate, but the industry has converged on 2.488 gigabits per second (Gbit/s) of
downstream bandwidth, and 1.244 Gbit/s of upstream bandwidth. GPON Encapsulation
Method (GEM) allows very efficient packaging of user traffic with frame segmentation.
By mid-2008, Verizon had installed over 800,000 lines. British Telecom, BSNL, Saudi
Telecom Company, Etisalat, and AT&T were in advanced trials in Britain, India, Saudi
Arabia, the UAE, and the USA, respectively. GPON networks have now been deployed in
numerous networks across the globe, and the trends indicate higher growth in GPON than
other PON technologies.

15
G.987 defined 10G-PON with 10 Gbit/s downstream and 2.5 Gbit/s upstream – framing is
"G-PON like" and designed to coexist with GPON devices on the same network.
5.2 NETWORK ELEMENTS
A PON takes advantage of wavelength division multiplexing (WDM), using one
wavelength for downstream traffic and another for upstream traffic on a single non-zero
dispersion-shifted fiber (ITU-T G.652). BPON, EPON, GEPON, and GPON have the same
basic wavelength plan and use the 1,490 nanometer (nm) wavelength for downstream traffic
and 1,310 nm wavelength for upstream traffic. 1,550 nm is reserved for optional overlay
services, typically RF (analog) video.
As with bit rate, the standards describe several optical budgets, most common is
28 dB of loss budget for both BPON and GPON, but products have been announced using
less expensive optics as well. 28 dB corresponds to about 20 km with a 32-way split. Forward
error correction (FEC) may provide another 2–3 dB of loss budget on GPON systems. As
optics improve, the 28 dB budget will likely increase. Although both the GPON and EPON
protocols permit large split ratios (up to 128 subscribers for GPON, up to 32,768 for EPON),
in practice most PONs are deployed with a split ratio of 1x32 or smaller.
A PON consists of a central office node, called an optical line terminal (OLT), one or
more user nodes, called optical network units (ONUs) or optical network terminals (ONTs),
and the fibers and splitters between them, called the optical distribution network (OD0N).
“ONT” is an ITU-T term to describe a single-tenant ONU. In multiple-tenant units, the ONU
may be bridged to a customer premises device within the individual dwelling unit using
technologies such as Ethernet over twisted pair, G.hn (a high-speed ITU-T standard that can
operate over any existing home wiring - power lines, phone lines and coaxial cables) or DSL.
An ONU is a device that terminates the PON and presents customer service interfaces to the
user. Some ONUs implement a separate subscriber unit to provide services such as telephony,
Ethernet data, or video.
An OLT provides the interface between a PON and a service provider′s core network.
These typically include:
 IP traffic over Fast Ethernet, Gigabit Ethernet, or 10 Gigabit Ethernet;
 Standard TDM interfaces such as SDH/SONET;
 ATM UNI at 155–622 Mbit/s.
The ONT or ONU terminates the PON and presents the native service interfaces to the
user. These services can include voice (plain old telephone service (POTS) or voice over IP
(VoIP)), data (typically Ethernet or V.35), video, and/or telemetry (TTL, ECL, RS530, etc.)
Often the ONU functions are separated into two parts:
 The ONU, which terminates the PON and presents a converged interface—such
as DSL, coaxial cable, or multiservice Ethernet—toward the user;
 Network termination equipment (NTE), which inputs the converged interface and outputs
native service interfaces to the user, such as Ethernet and POTS.
A PON is a shared network, in that the OLT sends a single stream of downstream
traffic that is seen by all ONUs. Each ONU only reads the content of those packets that are
addressed to it. Encryption is used to prevent eavesdropping on downstream traffic.

16
CHAPTER 6
NETWORK TOPOLOGY
Network topology is the arrangement of the various elements (links, nodes, etc.) of
a computer network. Essentially, it is the topological structure of a network, and may be
depicted physically or logically. Physical topology refers to the placement of the network's
various components, including device location and cable installation,
while logical topology shows how data flows within a network, regardless of its physical
design. Distances between nodes, physical interconnections, transmission rates, and/or signal
types may differ between two networks, yet their topologies may be identical.
A good example is a local area network (LAN): Any given node in the LAN has one or more
physical links to other devices in the network; graphically mapping these links results in a
geometric shape that can be used to describe the physical topology of the network.
Conversely, mapping the data flow between the components determines the logical topology
of the network.
Topology
There are two basic categories of network topologies:[4]
1. Physical topologies
2. Logical topologies
The shape of the cabling layout used to link devices is called the physical topology of the
network. This refers to the layout of cabling, the locations of nodes, and the interconnections
between the nodes and the cabling.[1] The physical topology of a network is determined by
the capabilities of the network access devices and media, the level of control or fault
tolerance desired, and the cost associated with cabling or telecommunications circuits.
The logical topology in contrast, is the way that the signals act on the network media, or
the way that the data passes through the network from one device to the next without regard
to the physical interconnection of the devices. A network's logical topology is not necessarily
the same as its physical topology. For example, the original twisted pair
Ethernet using repeater hubs was a logical bus topology with a physical star topology layout.
6.1 Types of Topologies
6.1.1 Bus
In local area networks where bus topology is used, each node is connected to a single
cable. Each computer or server is connected to the single bus cable. A signal from the source

17
travels in both directions to all machines connected on the bus cable until it finds the intended
recipient. If the machine address does not match the intended address for the data, the
machine ignores the data. Alternatively, if the data matches the machine address, the data is
accepted. Since the bus topology consists of only one wire, it is rather inexpensive to
implement when compared to other topologies. However, the low cost of implementing the
technology is offset by the high cost of managing the network. Additionally, since only one
cable is utilized, it can be the single point of failure. If the network cable is terminated on
both ends and when without termination data transfer stop and when cable breaks, the entire
network will be down.
Linear bus
The type of network topology in which all of the nodes of the network are connected
to a common transmission medium which has exactly two endpoints (this is the 'bus', which
is also commonly referred to as the backbone, or trunk) – all data that is transmitted between
nodes in the network is transmitted over this common transmission medium and is able to
be received by all nodes in the network simultaneously.
Distributed bus
The type of network topology in which all of the nodes of the network are connected
to a common transmission medium which has more than two endpoints that are created by
adding branches to the main section of the transmission medium – the physical distributed
bus topology functions in exactly the same fashion as the physical linear bus topology (i.e.,
all nodes share a common transmission medium).
Bus network topology

18
6.1.2 Star
In local area networks with a star topology, each network host is connected to a
central hub with a point-to-point connection. In Star topology every node (computer
workstation or any other peripheral) is connected to central node called hub or switch. The
switch is the server and the peripherals are the clients. The network does not necessarily have
to resemble a star to be classified as a star network, but all of the nodes on the network must
be connected to one central device. All traffic that traverses the network passes through the
central hub. The hub acts as a signal repeater. The star topology is considered the easiest
topology to design and implement. An advantage of the star topology is the simplicity of
adding additional nodes. The primary disadvantage of the star topology is that the hub
represents a single point of failure.
Extended star
A type of network topology in which a network that is based upon the physical star
topology has one or more repeaters between the central node (the 'hub' of the star) and the
peripheral or 'spoke' nodes, the repeaters being used to extend the maximum transmission
distance of the point-to-point links between the central node and the peripheral nodes beyond
that which is supported by the transmitter power of the central node or beyond that which is
supported by the standard upon which the physical layer of the physical star network is based.
If the repeaters in a network that is based upon the physical extended star topology are
replaced with hubs or switches, then a hybrid network topology is created that is referred to
as a physical hierarchical star topology, although some texts make no distinction between the
two topologies.
Distributed Star
A type of network topology that is composed of individual networks that are based
upon the physical star topology connected in a linear fashion – i.e., 'daisy-chained' – with no
central or top level connection point.

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Star network topology
6.1.3 Tree
This particular type of network topology is based on a hierarchy of nodes. The highest
level of any tree network consists of a single, 'root' node, this node connected either a single
(or, more commonly, multiple) node(s) in the level below by (a) point-to-point link(s). These
lower level nodes are also connected to a single or multiple nodes in the next level down.
Tree networks are not constrained to any number of levels, but as tree networks are a variant
of the bus network topology, they are prone to crippling network failures should a connection
in a higher level of nodes fail/suffer damage. Each node in the network has a specific, fixed
number of nodes connected to it at the next lower level in the hierarchy, this number referred
to as the 'branching factor' of the tree.
Advantages
 It is scalable. Secondary nodes allow more devices to be connected to a central node.
 Point to point connection of devices.
 Having different levels of the network makes it more manageable hence easier fault identification
and isolation.
Disadvantages
 Maintenance of the network may be an issue when the network spans a great area.
 Since it is a variation of bus topology, if the backbone fails, the entire network is crippled.

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REFERENCES
1. "Full Service Access Network" . FSAN Group official web site. 2009. Archived
from the original on October 12, 2009. Retrieved September 1, 2011
DATE OF ACCESS- 6/3/2014
2. IEEE Explore http://ieeexplore.ieee.org/Xplore/DynWel.jsp
3.IEEE SPECTRUM http://www.ieee-spectrum.org/
4.USPTO http://www.uspto.gov/

Optical network architecture