1

Department of Electronics & Communication Engineering

SEMINAR REPORT

ON

OPTICAL NETWORK ARCHITECTURE

SIDDHARTH SINGH

JSS MAHAVIDYAPEETHA

JSS Academy of Technical Education

Noida

2013-14

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.

19

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.

20

REFERENCES

1. " Full Serv ice Access Network" . FSAN Group of f icial 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

DATE OF ACCESS- 6/3/2014

3.IEEE SPECTRUM http://www.ieee-spectrum.org/

DATE OF ACCESS- 6/3/2014

4.USPTO http://www.uspto.gov/

DATE OF ACCESS- 6/3/2014