CHAPTER TWO2.0 LITERATURE REVIEW

2.1 SATELLITE NETWORKINGSatellite Communication using VSAT (Very Small Aperture Terminal) since the science fiction on radio transmission through space using geo-synchronous earth satellite, provider has progressed significantly in the field of satellite communications. The early earth stations were large and expensive. The reason for the size and complexity of the early stations was not related to inadequate performance. In fact, the antennas had very high efficiency and the noise temperatures of their receivers were low. However, the satellites at that time had a relatively poor performance providing considerably low RF (radio frequency) power per transponder and a rather high noise temperature for the on-board receivers. Additionally, satellites were then considered suitable only for very long distance communication. Gradually, satellite communications have appeared as regional systems requiring smaller coverage on the earth’s surface enabling higher gain antennas. Subsequently, increase in transponder out-put power, introduction of systems having several spot beams, development of field-effect transistor amplifier for low noise receivers as well as its availability as power amplifier have changed the satellite communication scenario. Once it was possible to envisage an all solid-state transmit and receive earth station even with a rather low power output, low price, large quantity, VSAT-based earth station design could be conceive.Satellite Communication is a technology of data transmission whether one-way data broadcasting or two-way interactive using radio frequency as a medium. It consists of:i. Space Segment or Satellite (e.g Measat, Intelsat and Inmarsat)

ii. Ground Segment or earth station which includes Antenna, Outdoor Unit, Inter Facility Link, Indoor Unit and Customer Premises Equipment.

2.1.1 Types of Satellite Services

Satellite communication provides services;

i. International Telephony – using Public Switched Telephone Network (PSTN)– Intermediate Data Rate (IDR)– Time Division Multiple Access (TDMA)

ii. Broadcasting

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Transmitting Earth Station

Receiving Earth Station

Uplink 6 GHz

Downlink 4 GHz

HPA

Up Converter

Satellite ModemCPE

PSTN

LNA

Down Converter

Satellite ModemCPE

PSTN

C Band – 6/4GHzKu Band -14/12GHzKa Band – 30/20GHz

– TV Uplink– Television Receive Only (TVRO)– Digital Satellite News Gathering (DSNG)

iii. VSAT- Very Small Aperture Terminal– Personal Earth Station (PES-TDMA)– Telephony Earth Station (TES-TDMA)– Domestic IDR/Single Channel Per Carrier (SCPC)– VSAT Dialaway– VSAT SkyStar Advantage– VSAT Faraway

Fig 1.1 A TYPICAL VSAT SYSTEMNote:HPA – High Power Amplifier, LNA- Low Noise Amplifier (Earth station equipment that amplifies the transmit RF signal.) CPE – customer premises equipment (e.g. Telephone, PABX, Ethernet hub, host server, etc.)

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2.2 VSAT (VERY SMALL APERTURE TERMINAL)

VSAT (Very Small Aperture Terminal) is a satellite-based communications service that offers businesses and government agencies flexible and reliable communications solutions, both nationally and internationally, on land and at sea.

VSAT networks provide:

1. Rapid, reliable satellite transmission of data, voice and video and an ability to allocate resources (bandwidth and amplification power) to different users over the coverage region as needed.

2. VSAT industry is offering fixed network solutions that can provide a full suite of services at reasonable price. E.g: a toll quality voice channel via VSAT is available between 3-15 naira/minute today.

3. Easy to provide point-to-multipoint (broadcast), multipoint-to-point (data collection), point-to-point communications and broadband multimedia services.

4. VSATs are serviced not only in cases where the land areas are difficult to install, say in the case of remote locations, water areas, and large volumes of air space.

5. An ability to have direct access to users and user premises.

2.2.1 Specification

VSAT is a term widely used in the satellite industry to describe an earth station that is installed on the ground to receive communications from a satellite or to communicate with other ground stations by transmitting to and receiving from satellite spacecraft. The ground station may be used only for reception, but is typically capable of both receiving and transmitting. Major components of a VSAT are generally grouped in two categories, ODU (outdoor unit) and IDU (indoor unit).

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1. Outdoor UnitThe ODU, so named because the components reside outdoors, includes; the antenna (typically ranging in size from 3.8 meters down to as small as 0.6m in diameter), equipped with a feed system capable of receiving and transmitting, a microwave radio, also known as a HPA High Power Amplifier, and an LNA (low noise amplifier) used to convert the signal gathered by the feed. Frequency Bands are available for use in C, Ku, or Ka frequency bands and are sold by wattage capability. A complicated calculation called a "Link Budget" is performed by the satellite operator to determine both the size of the antenna and how much power (wattage) will be required to complete the transmission link between the ground station and the satellite. Frequency Bands are sometimes combined with the LNA's which are used as part of the receiving operation. The resulting combination is called a "transceiver" and saves some integration time during the installation process.

2. IDU Indoor Unit The indoor unit is typically composed of a single unit called a modem. A satellite modem is different than a telephone modem, and is used to convert the data, video, or voice generated by the customer application for transmission over satellite. The modem takes the signals from your computer, phone or other device and changes them so they can be sent to the ODU which transmits them out to the satellite and eventually to other ground stations.

Antenna diameter : 0.6m – 3.8m Traffic Capacity : 9.6kbps – 2MbpsFrequency Bands : C-band (4-6 GHz) or

Ku-Band (12-14 GHz)Ka-Band (30/20 GHz)

Use of satellite : Geo-stationary satellite (36,000km above equator)Network : Point-to-pointConfiguration : Point-to-multipoint

Equipment List : Antenna Outdoor Unit (High Power Amplifier (HPA), Low Noise Amplifier (LNA), Solid-State

Power Amplifier (SSPA)) Indoor Unit (chassis)

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Note:- Antenna size is used to describe the ability of the antenna to amplify the signal

strength; - Outdoor unit (ODU) is connected through a low-loss coaxial cable to the indoor unit

(IDU) called IFL (Inter facility Link).

Fig 1.2 VSAT Configurations

2.2.2 VSAT Servicesi. Interactive real time application:

- Point of Sale/retail/Banking (eg. ATM)- Corporate data

ii. Telephony- Rural: individual subscribers- Corporate Telephony

iii. Intranet, Internet and IP infrastructure- Multimedia delivery (e.g. video streaming)- Interactive distance learning/ training

iv. Direct-to-home- Broadband Internet access for consumers and businesses

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2.2.3 VSAT TopologyStar The hub station controls and monitors can communicate with a large number of dispersed VSATs. Generally, the Data Terminal Equipment and 3 hub antenna is in the range of 6-11m in diameter. Since all VSATs communicate with the central hub station only, this network is more suitable for centralized data applications. Mesh A group of VSATs communicate directly with any other VSAT in the network without going through a central hub. A hub station in a mesh network performs only the monitoring and control functions. These networks are more suitable for telephony applications. Hybrid Network In practice usually using hybrid networks, where a part of the network operates on a star topology while some sites operate on a mesh topology, thereby accruing benefits of both topologies.

Fig 1.3 Star Topology

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Fig 1.4 Mesh Topology

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2.2.4 How VSAT Works1. The size of a VSAT antenna varies. The feed-horn directs the transmitted power

towards the antenna dish or collects the received power from it. 2. It consists of an array of microwave passive components. Antenna size is used to

describe the ability of the antenna to amplify the signal strength. 3. The Radio Frequency Terminal (RFT) is mounted on the antenna frame and

interconnected to the feed-horn (outdoor electronics) includes Low Noise Amplifiers (LNA) and down-converters for amplification and down conversion of the received signal respectively.

4. LNAs are designed to minimize the noise added to the signal during this first stage of the converter as the noise performance of this stage determines the overall noise performance of the converter unit. The noise temperature is the parameter used to describe the performance of an LNA.

5. Up- converters and High Powered Amplifiers (HPA) are also part of the RFT and are used for up converting and amplifying the signal before transmitting to the feed-horn. The Up/Down converters convert frequencies between intermediate frequency (IF level 70 MHz) and radio frequency.

6. Extended C band, the down converter receives the signal at 4.500 to 4.800 GHz and the up converter converts it to 6.725 to 7.025 GHz. The HPA ratings for VSATs range between 1 to 40 watts.

7. The outdoor unit (ODU) is connected through a low-loss coaxial cable to the indoor unit (IDU). The typical limit of an (Interfacility Link) IFL cable is about 300 feet. The IDU consists of modulators that superimpose the user traffic signal on a carrier signal. This is then sent to the RFT for up conversion, amplification and transmission.

2.2.5 VSAT Multiple Accessing Schemes

The primary objective of the VSAT networks is to maximize the use of common satellite and other resources amongst all VSAT sites. The methods by which these networks optimize the use of satellite capacity, and spectrum utilization in a flexible and cost-effective manner are referred to as satellite access schemes. Each of the above topologies is associated with an appropriate satellite access scheme. Good network efficiency depends very much on the multiple accessing schemes. There are many different access techniques tailored to match customer applications. Access techniques including stream, transaction reservation, slotted Aloha and hybrid mechanisms are used and are configurable on a per-port basis, enabling customers to run multiple applications simultaneously. Voice of 5.6 Kbit/s Hughes-proprietary CELP compression as well as voice

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of 8/16 Kbit/s ADPCM compression schemes, synchronous data of 1.2 to 64 Kbit/s, asynchronous data of up to 19.2 Kbit/s and G3 fax relay are some of the applications.

The satellite links are often referred to as long fat pipes – they represent paths with high bandwidth-delay product. Moreover, since they typically provide a broadcast channel, media sharing methods are needed at the MAC sublayer of the data link control layer. The traditional CSMA/CD schemes typically used in LANs cannot be used with satellite channels since it is not possible for earth stations to do carrier sense on the up-link due to the point-to-point nature of the link. A carrier-sense at the downlink informs the earth stations about potential collisions that may have occurred 270 ms ago (for GEO). Such delays are not practical for implementing CSMA/CD protocols. Most satellite MAC schemes usually assign dedicated channels in time and/or frequency for each user. This is due to the fact that the delay associated in detecting and resolving multiple collisions on a satellite link is usually unacceptable for most applications.

The VSAT services are primarily based on one of two technologies: 1. Single-carrier per channel (SCPC) and 2. Time-division multiple access (TDMA). SCPC (Single-Carrier per Channel)SCPC-based design provides a point-to-point technology, making it the VSAT equivalent to conventional leased lines.

Fig 1.5 Single Carrier per Channel

TDMA (Time-division multiple access)With TDMA networks, numerous remote sites communicate with one central hub – a design that is similar to packet-switched networks. As a leased-line equivalent, SCPC can

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deliver dedicated bandwidth of up to 2 Mbit/s. Remote sites in a TDMA network compete with one another for access to the central hub, restricting the maximum band- width in most cases to 19.2 Kbit/s. Almost all international VSAT services in Asia-Pacific are based on SCPC. Most domestic offerings are based on TDMA, although some domestic operators offer point-to-point SCPC links as well. Here, we will discuss briefly TDMA, pre-assigned or demand-assigned FDMA, CDMA and other accessing techniques featuring merits and demerits of these schemes.

In a TDMA network, all VSATs share satellite resource on a time-slot basis. Remote VSATs use TDMA channels or inroutes for communicating with the hub. There could be several inroutes associated with one outroute. Several VSATs share one inroute hence sharing the bandwidth. Typical inroutes operate at 64 or 128 Kbit/s. Generally systems with star topology use a TDMA transmission technique. Critical to all TDMA schemes is the function of clock synchronization what is performed by the TDMA hub or master earth station. The VSATs may also access the inroute on a fixed assigned TDMA mode, wherein each VSAT is allocated a specific time slot or slots.

FIG 1.6 TDM/TDMA MULTIPLE ACCESS

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FDMA (Frequency Division Multiple Access) It is the oldest and still one of the most common methods for channel allocation. In this scheme, the available satellite channel bandwidth is broken into frequency bands for different earth stations. This means that guard bands are needed to provide separation between the bands. Also, the earth stations must be carefully power-controlled to prevent the microwave power spilling into the bands for the other channels. Here, all VSATs share the satellite resource on the frequency domain only. Typically implemented in a mesh or single satellite hop topology, FDMA has the following variants:

1. PAMA (Pre-Assigned Multiple Access)It implies that the VSATs are pre-allocated a designated frequency. Equivalent of the terrestrial leased line solutions, PAMA solutions use the satellite resources constantly. Consequently, there is no call-up delay what makes them most suited for interactive data applications or high traffic volumes. As such, PAMA connects high data traffic sites within an organization. SCPC (Single Channel per Carrier) refers to the usage of a single satellite carrier for carrying a single channel of user traffic. The frequency is allocated on a pre-assigned basis in case of SCPC VSAT which is also synonymously known as PAMA VSAT.

2. DAMA (Demand Assigned Multiple Access) The network uses a pool of satellite channels, which are available for use by any station in that network. On demand, a pair of available channels is assigned so that a call can be established. Once the call is completed, the channels are returned to the pool for an assignment to another call. Since the satellite resource is used only in pro-portion to the active circuits and their holding times, this is ideally suited for voice traffic and data traffic in batch mode. DAMA offers point-to-point voice, fax, and data requirements and supports video-conferencing. The ability to use on-board signal processing and multiple spot beams will enable future satellites to reuse the frequencies many times more than today’s’ system. In general, channel allocation may be static or dynamic, with the latter becoming increasingly popular. DAMA systems allow the number of channels at any time be less than the number of potential users. Satellite connections are established and dropped only when traffic demands them.

3. CDMA (Code Division Multiple Access)Under this, a central network monitoring system allocates a unique code to each of the VSATs enabling multiple VSATs to transmit simultaneously and share a common frequency band. The data signal is combined with a high bit rate code signal which is independent of the data. Reception at the end of the link is

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TDMATime-

division Multiple Access

FDMAFrequenc

y Division Multiple Access

SCPCSingle-carrier

per Channel

VSAT TECHNOL

-0GY

DAMA CDMA

PAMA

FDMA

accomplished by mixing the incoming composite data/code signal with a locally generated and correctly synchronized replica of the code. Since this network requires that the central network management system co-ordinates code management and clock synchronization of all remote VSATs, star topology is, by default, the best one. Although this is best applicable for very large networks with low data requirements, there are practical restrictions in the use of spread spectrum. It is employed mainly for interference rejection or for security reasons in military systems.

FIG 1.7 VSAT Accessing Schemes

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2.2.6 VSAT NETWORK CHARACTERISTICSModern satellites are often equipped with multiple transponders. The area of the earth’s surface covered by a satellite’s transmission beam is referred to as the “footprint” of the satellite transponders. The up-link is a highly directional, point-to-point link using a high-gain dish antenna at the ground station. The down-link can have a large footprint providing coverage for a substantial area or a “spot beam” can be used to focus high power on a small region thus requiring cheaper and smaller ground stations. Moreover, some satellites can dynamically redirect their beams and thus change their coverage area. The received microwave power involved in satellite links is typically very small (of the order of 100 picowatts). This means that specially designed earth stations that keep carrier-to-noise ratio to a minimum are used to transmit/receive satellite communications. The front-end receiver is the most crucial part of a transceiver and contributes to the overall cost of the satellite earth station in a significant way. Here, we describe some of the characteristics of a VSAT network:

1. Flexibility

The VSAT networks offer enormous expansion capabilities; it factors in changes in the business environment and traffic loads that can be easily accommodated on a technology migration path. There are limitations faced by terrestrial lines in reaching remote and other difficult locations. On the other hand, VSATs offer unrestricted and unlimited reach. Additional VSATs can be rapidly installed to support the network expansion to any site, no matter however remote.

2. Network Management

Network monitoring and control of the entire VSAT network is much simpler than a network of leased lines, involving multiple carriers at multiple locations. A much smaller number of elements need to be monitored in case of a VSAT network and also the number of vendors and carriers involved in between any two user terminals in a VSAT network is typically one. This results in a single point of contact for resolving all your VSAT networking issues. A VSAT network management system easily integrates end-to-end monitoring and configuration control for all network subsystems.

3. Reliability

A single-point contact for operation, maintenance, rapid fault isolation and trouble-shooting makes things very simple for a client, using VSAT services. VSATs also enjoy a

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low mean-time to repair (MTTR) of a few hours, which extends up to a few days in the case of leased lines. Essentially, lesser elements imply lower MTTR. Uptime of up to 99.5 percent is achievable on a VSAT network. This is significantly higher than the typical leased line uptime of approximately 80-85%.

4. Cost

A comparison of costs between a VSAT network and a leased line network shows that a VSAT network offers significant savings over 2-3 years timeframe. This does not take into account the cost of downtime, inclusion of which would result in the VSAT network being much more cost-effective. Pay-by-mile concept in case of leased line sends the cost spiraling upwards. More, so if the locations to be linked are dispersed all over the country. In case of VSATs, the service charges depend on the bandwidth which is allocated to the network in line with customer requirements. With a leased line, a dedicated circuit in multiples of 64 Kbit/s is available whether the customer needs that amount of bandwidth or not.

5. Link Budgets

It ascertains that the RF equipment would cater to the requirements of the network topology and satellite modems in use. The link Budget estimates the ground station and satellite EIRP required. Equivalent isotropically radiated power (EIRP) is the power transmitted from a transmitting object. Satellite ERP can be defined as the sum of output power from the satellite’s amplifier, satellite antenna gain and losses. Calculations of signal levels through the system (from originating earth station to satellite to receiving earth station) to ensure the quality of service should normally be done prior to the establishment of a satellite link. This calculation of the link budget highlights the various aspects. Apart from the known losses due to various cables and inter-connecting devices, it is advisable to keep sufficient link margin for various extraneous noise which may affect the performance. It is also a safeguard to meet eventualities of signal attenuation due to rain/snow. As mentioned earlier a satellite provides two resources, bandwidth and amplification power. In most VSAT networks, the limiting resource in satellite Trans-ponder is power rather than bandwidth.

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2.2.7 BROADBAND TECHONLOGIES VSAT – BT VSAT

BT VSAT offers highly reliable, flexible support of integrated multimedia communications. Compared to alternative technologies, BT VSAT offers customers the following features and benefits:

1. Star network topology - offers end-to-end shared hub services for network requirements that cannot economically support a dedicated hub and operated by experienced staff to ensure optimum service levels.

2. Full mesh connectivity - provides a hubless network using only one satellite hop, offers lower delay and better response times. Smaller networks can be implemented at lower costs than traditional hub-based systems.

3. Bandwidth-on-Demand - architecture automatically allocates a pool of bandwidth to meet customer requirements of any site. The customer can move or add host computers and PABX's without having to re-engineer or re-size the network. Servers may be centralized or distributed. The bandwidth automatically "follows" the new traffic patterns.

4. Scalability of network capacity - The aggregate network capacity can be increase from time to time as the number of sites and volume per site grows.

5. Modularity and open system architecture - supports modular and open system architecture. The customer can expand the number of interfaces at the indoor unit as he requires.

6. Economics of statistical multiplexing - Multiple applications share the same bandwidth. The customer uses and pays for less total bandwidth than with the more traditional multiple dedicated network approaches.

7. Network Management and Control - operating on global standards and operational 24 hours a day, 7 days a week for some of major Earth Stations.

8. Cost effective solution - provides cost-effective communication solutions with high level functionality and performance since the pricing is distance independent. It offers a competitive alternative even for countries, which have a high degree of communication infrastructure.

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The dish is small, easily transportable and installation lead-time is much shorter if compared to terrestrial links. In addition, VSAT network allows rapid, low-cost network re-configuration and expansion to meet new or unexpected business requirements. Cost effective transmission and network operations are made possible by use of the C-band satellite frequency and frequency times division multiple access (FTDMA), Frequency division multiple access (FDMA) or Time division multiple access (TDMA) transmission techniques. VSAT offers a wide of protocols and features, providing extraordinary flexibility and virtually unlimited expansion capabilities. In addition, VSAT network is typically engineered to achieve a minimum of 99.5% end-to-end availability for all locations.

2.2.8 BT VSAT Architecture.

VSAT can customize and implement select topology of network that is best suited to the customers' requirement.

Hub type (VSAT HubStar)

VSAT HubStar is a private network designed for data, multimedia and voice applications, providing highly reliable communications between a central hub and almost any number of geographically dispersed sites. It integrates both high-speed Internet access and video multicasting capabilities. The network is suitable for point to multi-point communication for customers having a single data center requiring connectivity to its branches in geographically dispersed locations. This service supports transmission bandwidth ranging from 9.6 kbps to 128 kbps duplex. One of the advantages of Star topologies is that the hub can maintain effective control of the network through centralized processing. It is well suited for business traffic from the hub at the company headquarters and individual VSATs located at field offices, retail outlets or branches.

2.2.8 BT VSAT Reachability.

VSAT is a satellite-based service covering national and regional telecommunication needs. The service is served from small parabolic dishes (1.8m/2.4m/3.8m) accessing to the satellite directly from the customer premises. That explains the capability of the service reaching out to challenging areas of the country and region. This means of communication can also serve as part of company's network diversity.

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2.3 SYNCHRONOUS DIGITAL HIERARCHYSDH (Synchronous Digital Hierarchy) is an international standard for high speed telecommunication over optical/electrical networks which can transport digital signalsin variable capacities. It is a synchronous system which intends to provide a more flexible, yet simple network infrastructure. SDH (and its American variant- SONET) emerged from standard bodies somewhere around 1990.These two standards create a revolution in the communication networks based on optical fibers ,in their cost and performance.

2.3.1 BACKGROUND OF SDHThe development of digital transmission systems started In the early 70s, and was based on the Pulse Code Modulation (PCM) method. In the early 80's digital systems became more and more complex, yet there was hugedemand for some features that were not supported by the existing systems. The demand was mainly to high order multiplexing through a hierarchy of increasing bit rates up to 140 Mbps or 565 Mbps in Europe.The problem was the high cost of bandwidth and digital devices. The solution that wascreated then was a multiplexing technique, allowed for the combining of slightly non synchronous rates, referred to as plesiochronous, which lead to the term plesiochronous digital hierarchy (PDH). Plesiochronous - "almost synchronous , because bits are stuffed into the frames as padding and the calls location varies slightly - jitters - from frame to frame".

Fig 1.8 Multiplexing with PDH

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2.3.2 WHY USING SDH? Although PDH was A breakthrough in the digital transmission systems, it has a lot ofweaknesses:

1. No world standard on digital format (three incompatible regional standards - European, North American and Japanese).

2. No world standard for optical interfaces. Networking is impossible at the optical level.

3. Rigid asynchronous multiplexing structure.

4. Limited management capability.

Because of PDH disadvantages, It was obvious that a new multiplexing method is needed.The new method was called SDH.

Fig 1.9 Multiplexing with SDH

SDH has a lot of advantages:

1. First world standard in digital format.

2. First optical Interfaces.

3. Transversal compatibility reduces networking cost. Multivendor environment drives price down

4. Flexible synchronous multiplexing structure.

5. Easy and cost-efficient traffic add-and-drop and cross connect capability.

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6. Reduced number of back-to-back interfaces improves network reliability and serviceability.

7. Powerful management capability.

8. New network architecture. Highly flexible and survivable self healing rings available.

9. Backward and forward compatibility: Backward compatibility to existing PDH Forward compatibility to future B-ISDN, etc. SDH is based on byte interleaving and not bit interleaving, as PDH was based on.The bit rate increased from 64 Kbps in PDH to 1.5 - 2 Mbps in SDH.

SDH/SONET Vs. PDH rates

2.3.3 When do we use SDH?

1. When networks need to increase capacity, SDH simply acts as a means of increasing transmission capacity.

2. When networks need to improve flexibility, to provide services quickly or to respond to new change more rapidly.

3. When networks need to improve survivability for important user services.

4. When networks need to reduce operation costs, which are becoming a heavy burden.

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2.3.4 LAYERS MODEL OF SDH The following scheme describes the different layers of SDH , according to the OSI model :

Fig 2.0 Synchronous Digital Hierarchy Layers

2.3.5 SDH Elements The most common SDH elements are :

Fig 2.1 Terminal Mutiplexer

The terminal multiplexer is used to multiplex local tributaries (low rate) to the stm-N (high rate) aggregate. The terminal is used in the chain topology as an end element.

The REGENERATOR is used to regenerate the (high rate) stm-N in case that the distance between two sites is longer than the transmitter can carry.

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The ADD AND DROP MULTIPLEXER (ADM) passes the (high rate) stm-N through from his one side to the other and has the ability to drop or add any (low rate) tributary. The ADM used in all topologies.

Figs 2.2 ADD-AND DROP MULTIPLEXER

The SYNCHRONOUS DIGITAL CROSS CONNECT receives several (high rate) stm-N and switches any of their (low rate) tributaries between them. It is used to connect betweenseveral topologies.

2.3.6 SDH Topologies

Fig 2.3 LINEAR BUS TOPOLOGY

The LINEAR BUS (CHAIN) topology used when there is no need for protection and the demography of the sites is linear. The RING TOPOLOGY is the most common and known of the SDH topologies it allows great network flexibility and protection.

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Fig 2.4 MESH TOPOLOGY

The MESH TOPOLOGY allows even the most paranoid network manager to sleep well at nights because of the flexibility and redundancy that it gives.

Fig 2.5 STAR TOPOLOGY

The STAR TOPOLOGY is used for connecting far and less important sites to the network.

2.3.7 Usage of SDH elements in SDH Topologies

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The Terminal multiplexer can be used to connect two sites in a high rate connection.

Fig 2.6 SDH CONNECTION

The Add and Drop Multiplexer (ADM) is used to build the chain topologies in the above picture. At the ends of the chain usually a Terminal Multiplexer is connected.

The Add and Drop Multiplexer (ADM) is used to build the ring topology. At each site we have the ability to add & drop certain tributaries.

2.3.8 SDH Protection The SDH gives the ability to create topologies with protection for the data transferred.Following are some examples for protected ring topologies.

Fig 2.7 DUAL UNIDIRECTIONAL RING

At this picture we can see Dual Unidirectional Ring. The normal data flow is according to ring A (red). Ring B (blue) carries unprotected data which is lost in case of breakdown or it carries no data at all.

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FIG 2.8 BREAKDOWN OF RING A & B

In case of breakdown rings A & B become one ring without the broken segment.

Fig 2.9 Bi-Directional RingThe Bi-directional Ring allows data flow in both directions. For example if data from one of the sites has to reach a site which is next to the left of the origin site it will flow to the left instead of doing a whole cycle to the right.

Fig 3.0 Breakdown of both Rings

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In case of breakdown some of the data is lost and the important data is switched. For example if data from a site should flow to its destination through the broken segment, it will be switched to the other side instead.

2.3.9 SDH Management SDH has enhanced management capabilities:

1. Alarm/Event Management

2. Configuration Management

3. Performance Management

4. Access and Security Management

Fig 3.1 Network Management

Depicted above is a Management Station connected to a SDH ring through site 1 which contains the gateway element. The Gateway elements receive the status of all the other elements in the net through the special fields that exists in the SDH protocol (in band).

2.4 SDH vs. PDH

Few years ago the common way to build a backbone network that supplies broadband communication to the suppliers (BT, Bezeq etc.) was a PDH network. The topology of a PDH network is the Mesh topology where every multiplexer in each site worked with its own clock. In order to synchronize between two multiplexers that work together, usually the transmission was made according to the local clock and the reception was made according to the recovered clock that was recovered from the received data.The PDH contains 4 basic bit rates:

1. E1 - 2.048 Mbit/Sec

2. E2 - 8.448 Mbit/Sec

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3. E3 - 34.368 Mbit/Sec

4. E4 - 139.264 Mbit/Sec

The En is the result of multiplexing of 4 En-1. The fact that each of the multiplexers transmits according to its own clock creates a problem when we need to multiplex several transmitted data streams, the problem is that we can't decide which clock to choose for the multiplexing. If we will choose a fast clock we will not have enough data to put in the frame from a slower incoming data stream (we will get empty spaces in the frame), from the other hand if we will choose a slow clock the data at the faster incoming stream will be lost.

This problem was solved with a stuffing algorithm, which is implemented by using a fast clock that allows transmission of indication bits and stuff bits. In case that the data is slower than "expected", the indication bits indicate that the following stuff bits are "garbage" and if the data is faster than "expected" the indication bits indicate that the following stuff bits are data. This is the reason why 4 * En-1 < En.

There are two common ways to connect between two PDH sites. The first is by Radio Frequency (RF) and the other is by Electrical Signal over copper cable. Since we can’t afford too many cables or frequencies usually E3 or E4 is used. In order to transmit E1 (a very common data rate) we need 2 or 3 levels of multiplexing, this means that in a full E4 constellation 1+4+16=25 multiplexers are needed.

Furthermore there is no inbound management in the PDH protocol if we need to know the status of one of the multiplexers, or if we need to change the route of one of the trails we have to go to the site or build an outside network that allows us to manage the PDH network.

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