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Chapter OneIntroduction to the Training Establishment

1.1 History of the Organization

Etisalat is the first cellular network in South Asia, commenced operations in 1989 with the brand name of Celltel. It is a company owned and operated by Comvic International of United States of America. Then the company was sold to Millicom International Cellular (MIC) in the same year. In 2007 MIC changed the name of Celltel to Tigo. Etisalat acquired the Sri Lankan Operation of MIC, Tigo on 16th October 2009. In 2010, Tigo was rebranded as Etisalat and now Etisalat is the worlds 12th largest mobile service provider [2] which operates with 140 million customers with operation in 15 countries across the Middle East, Africa and Asia. Mother company of Etisalat is situated in Abu Dhabi, United Arab Emirates.

1.2 Present Performance

Etisalat has achieved the Middle Easts fastest fixed line broadband service with the speed of up to 500 Mbps which is the highest mobile broadband connectivity speeds to date. And also Etisalat has launched 4G mobile services in the UAE and Saudi Arabia and operates the Middle Easts largest LTE network with population coverage exceeding 82 per cent. Currently Etisalat is the worlds 12th largest mobile service provider which operates in 15 countries. The slogan of Etisalat is Its about you that prove its care towards customers.

Vision Statement: - A world where peoples reach is not limited by matter or distanceMission Statement: - To extend peoples reach [1]

Etisalat provide number of value added services (VAS) including Web Patashala, SMS to Email, GPRS/MMS, Give Me Balance, Credit Gifting, Call A Tune (PRBT), SMS Gift etc...

1.3 Organizational Structure

The company is structured into three main divisions, namely Financial, Marketing and Engineering. There is also the Human Resources Department to handle all HR related thing of the company. The technical organization structure of Etisalat is shown in below figure. Technical team has well organized with radio access network (RAN), network operation (NOP), rollout, switching, value added services (VAS), network operational Centre (NOC) and intelligent network (IN).

Figure 1.1: Technical Organization Structure

1.4 Strengths and Weaknesses

1.4.1 Strengths

Etisalat provides the widest coverage with high quality to their customers. The company has reached very rural areas without expecting higher revenue, but coverage for all. Not only providing coverage for all but the company also consider providing QoS to their subscribers. This has caused to company to higher position among other service provider. Another most valuable strength of the company is that it has strong, knowledgeable, and committed staff although it is comparatively small. The new recruits are also having positive attitudes with high performance to match the current work force. So the strong human resource is a solid foundation to the company to be strength. Etisalat provides number of value added services to their customers including Web Patashala, SMS to Email, GPRS/MMS, Give Me Balance, Credit Gifting, Call A Tune (PRBT), SMS Gift, SMS loaded, Cricket alerts, horoscope SMS, Daily Lottery results etc...And also Etisalat introduce new packages such as super SIM, tourist SIM pack and application such as Net Nanny to attract customers. Etisalat has a plan to enlarge the coverage annually while adding new sites in less coverage areas and rural areas. And also they always try to improve the quality of the network through analyzing the daily report in sector wise. The slogan of Etisalat, Its about you implies that its array of services to match the hearts of their customers while considering their customers as their own by giving first priority to them.

1.4.2 Weaknesses

The work load and responsibility of each engineer is bit too much in urgent and critical tasks due to staff mismatch. Since the existing staff is talented although they manage the workload it results for long working hours in stress. This will impact for the company growth heavily. Another weakness is that the lack of communication and understanding between the departments even though there is a friendly environment within each department itself.

1.5 Usefulness to the Nation

Etisalat has a wide network coverage which continuously expands its coverage to all major cities to provide the best connectivity and clarity across Sri Lanka. As a service provider Etisalat offer almost 90% of coverage while providing good quality of service to their customers. Etisalat give service to many rural areas where there are no any other sources of communication. Etisalat is consistently providing not only the widest coverage and an unprecedented service, but also a host of other Value Added Services. They are dynamic and treat their customers as their own, and consider customer service as their first priority proving the slogan Its about you.

Figure 1.2: Etisalat coverage map

Etisalat carries out range of services which range from both medium-to-long and short-term projects, namely education, community/livelihood development, environment and disaster relief. One project was the OLPC (one laptop per child) which providing laptops to children in rural schools for contributing towards improving the literacy and IT skills. In this Vesak, Etisalat organized a spectacle dansala at Galle to provide over 3000 pairs of spectacles to financially challenged Sri Lankans, unable to afford the services of an optometrist and lenses.

Etisalat donate Rs. 3.5 million to build cottage, Abimansala to disabled war heroes to live in. It is a great service that provide by Etisalat in brave heart project. Another project is that Sondurudiriya which launch to recycle marketing waste. The objective was not only to recycling marketing waste but also an opportunity to uplift the lives of rural housewives to turn out handy environmentally friendly grocery bags. And also Etisalat help to overcome the devastation caused by the floods by distributing parcels containing essentials to flood victims.

The company also provides various job opportunities in technical, financial, marketing, sales and human resource management fields with comparatively high incomes. Etisalat also helps to the local education system by offering internship opportunities to various universities and other institutes.

So Etisalat is consistently providing not only the widest coverage but also great services to Sri Lanka as above described.

1.6 Suggestions to improve

According to my point of view, Etisalat need an expansion in the staff especially in planning and optimization section due to high workload on one engineer. That is results for lack of time to do research and development areas in order to meet the increasing and competitive demand of new technology with other operators. So working with fewer amounts of employers is a huge disadvantage for the company. And also the communication should be further strength between each department in order to be successful in all aspects.

During my training period I was able to observed that the lack of tools especially in planning department. Sometimes engineers have to wait until another one return the tool. So the number of tools should be increased if there requirement is high and if it is more useful. And also Etisalat should pay their attention more on introducing new attractive packages (they are not providing couple packages which is a high profit that all other operators provide) and value added services to attract more customers.

Chapter Two Technical Details

Engineering division of Etisalat Lanka (Pvt) Ltd is function under seven main divisions which are RAN, NOP, NOC, switch, IN, VAS and roll-out. Within my training period of 12 weeks I was assigned to three divisions which are RAN, switch and NOP. It was a well-organized and well-planned training program so that I was able to get fruitful technical knowledge during that period. Although I was assigned to three divisions I was able to get a basic idea on other divisions too. Sometimes they provide us the opportunity to handle responsibilities and take technical decisions all by ourselves so that it was a massive opportunity to us doing things instead of just looking over. This is the schedule of my training at Etisalat during 12 weeks of the period from 20th January 2014 to 11th April 2014.

Table 2.1: Training Schedule

DivisionTime period

RAN (Radio Access Network)20th January to 20th February 2014

Switch21st February to 20th March 2014

NOP (Network Operation)21st March to 11th April 2014

In this chapter I have described the technical knowledge that I experienced during 3 months of my training period. I have described them in different subsection according to the knowledge got from the three sections which I was assigned. Finally I have described the Etisalat network architecture as a whole in the last sub section and it is a combination of details described in previous sub sections.

2.1 Radio Access Network (RAN)

This is the department in which I started my training and I was there for one month under the supervision of Engineer Parakrama Wijemanna who is responsible for planning and optimization of the Central area.

According to Etisalat the whole country has divided into 7 territories (CA, CB, WA, WB, NA, NB, and SA) and these regions have subdivided into clusters. There is an engineer for each and every cluster for planning, optimizing and maintaining QoS. I was assigned to the Engineer who is responsible for CA- KDY (central area- Kandy) cluster. In this period, I got the basic idea of the GSM network and familiar with several tools. These are some works carried out during planning division.

frequency planned for several sites using Map Info Completed several TSSR (technical site survey report) Site planning using RNP tool Marked routes for benchmarking drive test in Google Earth Analyzed on Independent Day traffic variation with the added TREs Check on issues regarding daily report by using NPO (Network Performance Optimizer) tool in Central area Update piano file Participate to a site survey at Padukka which was a sharing site of Airtel Drive test at Padukka using TEMS.

In these sub sections I have described about the technical knowledge gathered during these activities.

2.1.1 Site Planning

New site establishment is done as a result of coverage issues, customer complaints and RNE suggestion to expand the coverage and capacity requirements. With feedback from regional engineers and marketing section, radio network engineers (RNE) identifies the potential area where new site establishment can be performed. Then RNE check the area by using planning tools such as Aircom ASSET and Alcatel radio network planning tool (A9155RNP) to identify the best location by using coverage prediction. Drive test results also use to identify the location.

Figure 2.1: Coverage prediction map

The above figure shows a coverage prediction by using the tool A9155RNP which we can get prediction by changing azimuth, mechanical and electrical tilt, tower height etc...During my training period, I was asked to decide the best location with antenna tilt and azimuth for the given two longitudes, latitudes at Hapugastalawa. According to the TRC regulations, if there is a site near to that location then it should share with that service provider. Even though company had to pay a rental for the site, sharing site is a quick process. Otherwise it is required to do site survey to find a physical location for the site. I was able to participate to the site survey at Padduka which is an Airtel site. And also I completed number of TSSR at Kandy.

Considerations for new site establishment

Availability of line of sight to an existing site (for MW link) No obstacles in the near field of the antenna Space availability for tower construction Power availability Civil suitability (soli condition) 150m or 250m clearance from the high tension line Availability of access path to the site 15m clearance from transformers, houses Operation and maintenance facility Installation possibility Site acquisition Central Environmental Authority (CEA) Civil Aviation Authority (CAA) Urban Development Authority (UDA) Board of Investment (BOI) Local Authority

These are some approval that should be taken in order to get the TRC (Telecommunication Regulatory Commission) approval.

When the exact location is finalized, RNE make a TSSR (technical site survey report) and then roll-out department proceed with the site acquisition. Sketch of a sharing site is attached with appendix.

Physical parameters planning

Antenna height, azimuth and tilt are physical parameters that should be considered for new site establishment.

Antenna height High antenna height mean large serving area but cause for interference with other sites. Low antenna height may not interfere with other sites but may not cover the expected area. So antenna height should be decided based on both geographical nature of the area and the requirement. High towers use in rural areas to maximize coverage but in urban area use low tower height to minimize interference.

Azimuth In general one site has three sectors. Antenna position in horizontal plane is called as azimuth. Widely use antenna azimuths are 00, 1200, 2400. But these values can be changed depend on coverage requirement (population), area that is not covered by other sites, direction of highway and roads, existence of town, hotel

Antenna tilt Tilting is done in order to concentrate the beam to the desired area by reducing overshoot hence to reduce interferences from neighboring sites. In rural areas the antenna tilt is very low or zero to maximize the coverage since the distance between adjacent sites are high so it is not caused for interference. But in urban areas antenna down tilt is high to minimize interference. There are two ways of tilting an antenna.

Figure 2.2: Effect of antenna tilt

Table 2.2: Mechanical and Electrical down tilting

Mechanical down tiltingElectrical down tilting

Physically down tilt the antennaThe antenna is physical not changed but the beam is tilted

Only the main lobes are down tilted while side lobes are remain sameSame down tilt for both main lobes and side lobes

Fast adjustment is possibleAdjustment of electrical tilt mostly not possible

Accurate adjustment is difficultAccurate adjustment can perform

Problems with sites with difficult accessIntroduction of additional antenna types necessary

Figure 2.3: Mechanical tilt from clamp

Figure 2.4: Electrical tilt

2.1.2 Frequency planning

GSM uses the concept of cells. One cell covers a small part of the network. A GSM network will have several cells. The cells are normally drawn as hexagonal, but in practice they are irregularly shaped.

Figure 2.5: Cell structure

In Omni-directional cells it radiates the radio waves to 3600 and if the site has a single cell it causes for low capacity. To increase capacity within the geographical area a technique called sectorization is used. It splits a single site into a number of cells, each cell has transmit and receive antennas and behave as an independent cell. Each cell uses special directional antennas to ensure that the radio propagation from one cell is concentrated in a particular direction.

Since a cell has limited area, the frequency used in this cell can be re-used in some other cell. This is known as frequency re-uses. Frequency reuses do mainly in two ways. Reuse same frequency in sufficient distance separation in geographical area Time sharing of frequency in a defined pattern as no interference with adjacent sites

The cells which use the same frequencies are known as re-use or co-channel cells. So in order to provide enough capacity frequencies have to be reused since the GSM spectrum is limited. But it should be such that to minimize co-channel and adjacent-channel interference.

GSM spectrum

The frequency spectrum is very congested since it has allocated only narrow slots of bandwidth for cellular communication. The table shows the frequency spectrum of GSM900, EGSM 900, DCS and PCS1900.

Table 2.3: GSM Frequency Bands

BandUplink frequencyDownlink frequencyChannel bandwidth

GSM 900890 - 915MHz935-960MHz200kHz

EGSM 900880 - 915MHz

925 - 960MHz

200kHz

GSM 1800 (DCS)1710 - 1785MHz

1805 - 1880MHz

200kHz

PCS 19001850 - 1910MHz1930 - 1990MHz200kHz

Uplink refers to signal flow from MS to BTS and downlink refers to signal flow from BTS to MS. The simultaneous use of uplink and downlink frequencies enables communication in both transmits and receives so it is full duplex. Lower frequencies are used for uplink and high frequencies are used for downlink since MS has lower transmit power than antenna (high frequency means high attenuation so high transmit power is needed).

Uplink and downlink frequencies are separated by a wide frequency range to minimize interference between transmission and reception. We can see that there is a 45MHz separation between uplink and downlink in both GSM 900 and extended GSM but in DCS it is 95MHz. This GSM band is divided into 124 channels with bandwidth of 25MHz for both uplink and downlink.

Figure 2.6: Channel separation in GSM900 band

This channels are given a number call ARFCN (Absolute Radio Frequency Channel Number) since it is difficult to work with real frequencies. ARFCN or RF carrier is actually a pair of frequencies, one used in each direction (transmit and receive). TRC has allocated these limited resources to each operator. Etisalat was given ARFCN from 51 to 87 which are 37 channels in GSM900 band and another 30 channels from DCS band i.e. ARFCN from 624 to 653.

Broadcast Control Channel (BCCH)

The BCCH (downlink only) is transmitted by a BTS (Base Transceiver Station) to provide the signaling information required by the MS to access and identify the network. The BCCH will include information such as MS present cell (LAI-Location Area Identity), list of neighboring cells, list of frequencies used in the cells etc... It is transmitted continuously as its signal strength is measured by all MSs on surrounding cells. So this is the frequency in which each cell broadcast information needed by a MS to be connected with the network so that each site needs one BCCH frequency per sector. Etisalat uses BCCH frequencies from ARFCN 51-72.

As the BCCH carrier has the highest interferer potential because of being on air all the time it should be planned such that the interference to be avoided mainly co-channel and adjacent-channel interference. And also BCCH frequencies in sectors of the same site should have at least 2 ARFCN difference.

Co-channel interference This occurs when RF carriers of the same frequency are transmitting in close proximity to each other; the transmission from one RF carrier interferes with the other RF carrier. Interference margin for two co-channels is -9 dBm.

Adjacent-channel interferenceThis occurs when an RF source of a nearby frequency interferes with the RF carrier.

Base Station Identification Code (BSIC)

BSIC is used to distinguish between base stations using the same BCCH frequency. So BCCH and BSIC combination is used for identifying the site, to which the subscriber is attached to and this is more important in call handovers. The network should avoid same BSIC and BCCH combination in nearby cells. BSIC (Base Station Identification Code) is composed with two parts BCC (base station color code) and NCC (network color code). For Etisalat the permitted values for NCC are 3 and 5. For the BCC the permitted values are 0 to 7. Therefore integers from 30-37 and 50-57 can be used as BSICs.

Figure 2.7: BSIC Allocation

During my training period I was asked to frequency re-planning with relevant BSIC values for several sites in Kandy to minimize interference.

Frequency Hopping

When the site has 4 - 4 - 4 configuration, if we apply static frequency plan it need at least 12 frequencies for one site. But this is difficult with the limited frequency spectrum with increasing capacity even with reuse. Frequency hopping is used to overcome this problem which is a dynamic frequency allocation mechanism. The basic idea here is to change the transmit frequency continuously. Etisalat use ARFCN from 74 to 87 (14 frequencies) that generate 64 unique HSN (Hopping Sequence Number) which is a random pattern to have minimum interference with any other HSN. Those patterns are orthogonal to each other. This value can range from 0 to 63 and 0 indicates a cyclic hopping pattern while any number from 1 to 63 give a pseudorandom hopping pattern.

When planning HSN each site should has same HSN. But this is caused for co channel interference between the sectors of the site since all sectors use same frequency to transmit at all times. To overcome this problem each TRE of the site starts the hopping sequence from a different frequency which is called as MAIO (Mobile Allocation Index Offset). MAIO should differ by two to avoid adjacent channel interference.

Table 2.4: MAIO allocation

ARFCNF1F2F3F4F5F6F7F8F9F10F11F12F13F14

MAIO012345678910111213

If we define MAIO as 0 the frequency pattern will start with F1 and if it is 7 the frequency pattern will start with F8 and the HSN number will define the hopping pattern.

HSN and MAIO allocations for a site with 4 4 4 TRE configuration can be defined as,

Table 2.5: MAIO and HSN allocation for a site with 4, 4, 4 TRE configuration

TRE numberSector 1Sector 2Sector 3

TRE 1BCCH 1BCCH 2BCCH 3

TRE 2HSN=5, MAIO=0HSN=5, MAIO=2HSN=8, MAIO=1

TRE 3HSN=5, MAIO=4HSN=5, MAIO=6HSN=8, MAIO=3

TRE 4HSN=5, MAIO=8HSN=5, MAIO=10HSN=8, MAIO=5

For one HSN there can be only 7 MAIOs in this case 0, 2, 4, 6, 8, 10, 12. Since this site has 12 TREs it is needed to define another HSN with MAIOs 1, 3, 5, 7, 9, 11, 13.

2.1.3 Handover

The handover (HO) process is one of the fundamental principles in cellular mobile radio, maintaining the call in progress (dedicated mode) whilst the mobile subscriber is moving through the network. The MS switches from channel to channel and cell to cell as it moves to maintain call continuity. There are four different types of handovers.

Figure 2.8: Handover types

1. Intra BTS handover Same BTS, only change the channel or slot. This form of HO occurs to change frequency or slot use due to interference or other reasons. So this is the switching of call from one channel/ TRX to another TRX within the same cell independently by the BSC. This type of HO usually takes place when the RxQuality on the source channel deteriorates.

2. Inter BTS intra BSC handover When mobile moves out of the coverage area of one BTS but move into another cell controlled by same BSC. Then BSC assign a new TCH and slot to the mobile before releasing the old BTS.

3. Inter BSC handover When mobile moves out of the range of cells controlled by one BSC to another, then BSC has to change. In this case not only change BTS but BSC also change.

4. Inter MSC handover When mobile moves from one MSC to another MSC this type of HO occurs.

2.1.4 Cell Selection/ Re selection

Cell Selection

When a mobile switched on it will be selected to a cell. This process is called as the Cell Selection. Mobile will be selected to the cell having Best C1 (path loss criterion). (C1 > 0) C1 is used to determine whether a cell is suitable to camp on. At start up (IMSI attach) the MS selects the cell with best C1. C1 is defined as,

C1 = A - Max [B, 0])

Where,A = (RXLEV - RXLEV_ACCESS_MIN) => Received signal level at MS Minimum received signal level the MS is allowed to access the cell

B = (MS_TXPWR_MAX_CCH P) MS_TXPWR_MAX_CCH is the maximum power level the control channel (when MS sending on RACH)

P is the maximum TX power of MS

Cell Reselection

Cell reselection is a process MS change its service cell in idle mode. When the MS selects a cell and if there are not major changes in various condition the MS will stay in the selected cell and MS will begin to measure the signal levels of the BCCH TRX of its adjacent cells, record 6 adjacent cells whose signal levels are the strongest and extract from the various types of system messages and control messages of each adjacent cell. When given condition are met, the MS will move from the current cell into another one. This process is called as cell reselection.

When PI indicates YES, the MS will get parameters (CRO, TO and PT) from BCCH, to be used to calculate C2 (channel quality criterion), which serves as cell reselection criteria. Mobile will be re-selected to the cell with best C2 if PI = Yes. The equation is as follows.

C2C1CROTO (T); when PT 31C2C1 CRO; when PT 31

Where,

CRO: Cell Reselect Offset (The artificial influence is to encourage the MS to take the priority in accessing to some cells or prevent it from accessing to others.)PT: Penalty Time (whose main role is to avoid frequent cell reselection by MS)T is a timer. When a cell is recorded by MS as one of the six strongest cells, timer starts counting, otherwise, T is reset to zero.TO (T): Temporary Offset (to avoid locating on transient cell)

Figure 2.9: MS with serving cell A and B

Assuming MS_TXPWR_MAX_CCH = P, if we calculated C1 for both cell A and B they will be 29 and 22 respectively. So the cell A will be selected by the MS at start up (when it on).

If we calculated C2 values for both cell A and B they will be 24 and 26 respectively. So the cell B will be re-selected by the MS if, C1 criterion is too low MS cannot decode downlink messages The current cell is becoming forbidden (barred) MS cannot access the cell there is a better cell regarding C2 criterion

2.2 Switch Department

The second month of my training schedule was spent with the Switch team. Switch is the department which is responsible for the core network. During that period I was able to get knowledge on call flows, NGN concepts, MPLS and services, WCS, MGW functionalities, signaling protocols, power system of NSS, voice interconnection technologies, routing and basic idea on networking. In this section the switch Engineers conduct sessions to us on a schedule as it can cover most of the section that we want to know. During that month I was able to visit all three switch site and get hand on experience of configuring a WCS test bed at Grandpass and links were configured from remote MGW at Kotikawatta to BSC.

2.2.1 Functionality of the department

The GSM architecture mainly can divide into core network and the access network. Core network or the NSS (Network Switching System) includes the main switching function of the GSM network. So it can be considered as the heart of the network.

In order to prevent mesh topology interconnection between these components the central node is a switch in the core network. These components are interconnected through the switch as in a star topology. Etisalat has three switch sites at Grandpass, Wallampitiya and Kotikawatta for geo-graphical redundancy. I was there for a period of one month so I was able to get the concepts and theory behind the core network. Following are the main function carried in the switch engineering division.

2G and 3G core network switching (maintenance and monitoring of MSC) Operation and maintenance of HLR Implementing IDD and roaming facilities Interconnection with RAN, VAS, IN and other operators OAM of NSS power system SMS routing QoS monitoring of the core network Give solutions to the customer complaints relevant to core network OAM of the IP/MPLS core CDR (call data record) generation

In switch evolution first there were hardware switches that it requires to manually connect the path. Next it was implemented to TDM switches. Then MSC (NGN Next Generation Network) that currently use is a combination of TDM matrix and IP. IMS (IP Multimedia Subsystem) which is a fully IP based technology. When this technology is implemented all subscribers and systems will be used IP.

Mainly there are two types of switches. They are, VMSC (Visited Mobile Switching Centre) All subscribers connect with the VMSC GMSC (Gateway Mobile Switching Centre) Connect our network with other service providers network

2.2.2 Network Switching System (NSS)

NSS includes the main GSM function of the GSM network. Its main function is to manage communications between the GSM network and other telecommunication networks. The components of the NSS are as below. Mobile Service Switching Centre MSC Home Location Register HLR Visitor Location Register VLR Equipment Identity Register EIR Authentication Centre AUC

There are main network elements which connect with the MSC.

1. Home Location Register (HLR)This is the database for subscriber parameters which are included by the network provider when a new subscriber is added to the system. It has subscriber profile which including location information, services and authentication parameters The data it contains is remotely accessed by all the MSCs and the VLRs in the network. The subscriber data in the HLR can be accessed through either IMSI or MSISDN. The data can also be accessed by am MSC or VLR in a different PLMN.

2. Visitor Location Register (VLR)VLR is also a database which store temporary data that exists for only as long as the subscriber is active in the particular area covered by the VLR. So it contains some duplicate data as well as more precise data relevant to the subscriber remaining within the VLR coverage. VLR eliminates the need for excessive and time-consuming references to the HLR database which is home of the data. Additionally mobile status (busy/free/no answer), location area identity (LAI), TMSI, MSRN are also stored in the VLR.

3. Equipment Identity Register (EIR)

EIR is a database of IMEI (International Mobile Equipment Identity). This database is concerned only with MS equipment and not with the subscriber who is using MS to make or receive a call. It contains three types of lists. They are White list IMEIs which are valid MS equipment Black list IMEI which have been reported as stolen or which are to be denied service. Grey list IMEI which have problems. These are not sufficiently significant to warrant as black listing.

4. Authentication Centre (AUC)

AUC is a processor system that performs authentication function. AUC is normally co-located with the HLR. The AUC/HLR can be co-located with the MSC or located remotely from the MSC.

5. Mobile Service switching Centre (MSC)

In traditional mobile MSC (legacy switch), the intelligence and connectivity are combined in the switch. In next generation network soft switch solution, the network intelligent resides in the call server while the connectivity resides in the media gateway (MGW). Etisalat uses NGN (Next Generation Network) type switches. NGN switch has mainly two parts.

i. Wireless Call Server (WCS) This is the controlling part of the NGN. WCS only handles signaling. Capacity of the WCS depends on BCCH (Busy hour call attempts). This is also called as MSC server.

ii. Media Gateway (MGW)This handles only traffic (media). MGE communicate with the WCS on IP basis. 2.2.3 Numbering Standards

Standardized numbering format are used in telecommunication such as E164, E212, E214.

1. E164 formatMSISDN (Mobile Subscriber Integrated Digital Network Number)This is the general mobile numbered format.

MSISDN = CC + NC + SN

Country code Network code Subscriber number (For Sri Lanka 94) (For Etisalat 72)

For example 94729040275 is in E164 format.

2. E212 formatIMSI (International Mobile Subscriber Identity)

IMSI = MCC + MNC + MSIN

Mobile Country Code Mobile Network Code Mobile subscriber(For Sri Lanka 413) (For Etisalat 03) identification number (10 digits)

3. E214 formatThis numbering format is mostly used in the location update process of roaming. When the subscriber is not form the providers network the IMSI convert into E214 format, which can then be used for accessing the subscribers data in remote HLR. Removing MCC + MNC add CC + NC. For example, E212 413 03 0100001201E214 94 72 0100001201 Then any PSTN in the world know this should be send to Etisalat, Sri Lanka.2.2.4 Inter-MSC Location Update

This is the way how the service provider can route calls to us by finding where we are. A GSM network is divided into cells. A group of cells is considered as a location area. A mobile phone in motion keeps the network informed about changes in the location area. If the mobile moves from a cell in one location area to a cell in another location area, the mobile phone should perform a location area update to inform the network about the exact location of the mobile phone. [8]

There are three types of location updates.

1. Normal Location update When change LAC boundary this type of LU happen 2. Periodic Location update When there are no LAC changes happen there is a timer T3212 at MS so when it timeout this type of LU happen3. IMSI attach When MS is switched OFF and ON this type of LU happen

The BCCH of the neighboring cells is monitored to determine if any of the neighbors have a better signal strength. If the old and new location areas are different then mobile initiates the location area update procedure as shown in above figure.

1. The mobile request for a control channel to establish a radio connection in order to send the location update to the network.2. A radio channel is assigned to the GSM mobile3. BSC send LU request with IMSI, new LAC, CI (cell identity), LU type...etc.4. MSC find that old location area was handled by a different MSC. So MSC need to contact HLR. Check IMSI in new VLR, it does not find it.5. Then the VLR decides to authenticate the subscriber and send authentication information message to the HLR with IMSI. HLR check whether IMSI exists in HLR. 6. If it exists, HLR passes the information about the new subscriber to the VLR including 5 sets of a 64 bit ciphering key used as a session key (Kc), a 128 bit random challenge (RAND) and a 32 bit signed response (SRES).7. VLR decides to authenticate the MS by using this given RAND.8. Then VLR send authentication request with RAND (that was sent by HLR) to the MS.9. The SIM applies secret GSM algorithm (A3) on the RAND and secret key Ki to obtain the session key Kc and SRES. Then SRES send to VLR.10. If the SRES obtained from the mobile matches the SRES obtained from the HLR then subscriber authentication is successfully completed.11. Now VLR send LU request with IMSI to the HLR. This message is needed for two reasons. HLR need to update its record to point to the new MSC The new MSC does not have information about this subscriber. So this request uses to get the subscriber information.12. HLR sends ISD (Insert Subscriber Data) including prepaid, postpaid details, call forwarding information (call forwarding if busy, call forwarding if no answer, call forwarding if not reachable, call forwarding if unconditional), call waiting, call barring, multi-party...etc.13. Send acknowledgement for ISD. At this point HLR ask the old MSC/VLR to delete the record about this subscriber.14. LU accepted.15. MSC initiate ciphering of the data being sent on the channel. So that it send cipher mode command to the mobile through BSC.16. Send cipher mode complete message indicating that ciphering has been successfully enabled.17. The new MSC replies back to the mobile via BSC by assigning a new TMSI (Temporary Mobile Subscriber Identity). Since TMSI assignment is being sent after ciphering is enabled, the relationship between TMSI and the subscriber cannot be obtained by unauthorized users. Finally the radio connection is released.

2.2.5 Mobile Originating (MO) and Mobile Terminating (MT) call

Calling party is prepaid and called party has activated PRBT.

1. After establish radio connection, BSC allocate a TCH (Traffic channel) to the mobile. In that it assigns a specific frequency and a time slot on that frequency. After that MS only use those resources to communicate with the mobile network. Then mobile send CM_SERV_REQ message indicating what type of service it requires.2. MSC asks for authentication by sending AUTH_REQ (with RAND) message to mobile via BSC.3. Mobile will send AUTH_RSP with calculated SRES. After successful authentication,4. The MSC initiate ciphering of the data being sent on the channel. Send CIPHER MODE COMMAND to the mobile. This is still a clear message. Then mobile enables ciphering.5. Then MS sends the setup message to establish a voice call with the ISDN of the called party (to whom the call is).6. The mobile is informed that the call setup is in progress. At this point the mobile phone displays a message on the screen to indicate that call setup is being attempted.7. The MSC allocate a voice circuit (CIC) between the MSC and the BSC.8. Using ISD in VLR separate whether calling party is postpaid or prepaid.9. If it is a prepaid subscriber, MSC/VLR send INITIAL_DP (Initial Detection Point) message to IN with called party address, calling party information (MSISDN & IMSI) and VMSC GT of the originator.10. Request report BCSMEvent is sent back from IN with operation code Continue (if sufficient balance is available)/Release (if sufficient balance is not available). If there is sufficient balance IN send BCSMEvent message by saying at which event it should be told to the IN (trigger when call answer).11. It the called and calling parties are not belongs to the same subscriber then it requires contacting the GMSC (If the call is from Etisalat to Mobitel). Send IAM (Initial Address Message) with MSISDN.12. IAM send to other parties GMSC13. MSISDN will be routed to the HLR asking SRI (Send Routing Information) since HLR has the MSC/VLR GT of the subscribers last location in the profile. If the terminating party is an incoming charger (TCSI) HLR send camel information of terminating party to MSC. Then TCSI generate for relevant IN. Then it send INITIAL_DP message to that IN to check availability of credit. 14. Then HLR request location information from that VLR.15. VLR replies with LAC and CI of the subscriber.16. Again MSC/VLR send SRI with suppress TCSI. Because this SRI is not requesting camel information.17. A roaming number (MSRN) is requested. MSC has MSRN pool and this use to identify the subscriber location.18. Send roaming number.19. GMSC send IAM with called number (MSRN), calling number, CLIR details (use to display the MSISDN of the calling party in the called party display) 20. Now MSC VLR needs to find the subscriber. Since one location area has several cells paging mechanism is used to locate the subscriber. PAGING message send to all BSCs that handle the relevant location area. Corresponding BSC receives the paging message and BSC send page message to all cells that serve the subscribers location area. All cells in the location area will broadcast the page message on paging channel (PCH).21. All MS listen to this channel every few seconds. MS finds that the TMSI specified in the page message matches its own TMSI. So mobile responds to the page. And request for a radio connection.22. Now MSC VLR decides to authenticate the called party by sending AUTH_REQ with RAND.23. MS send AUTH_RESP and cipher mode enable message.24. Setup message 25. CIC will allocate between MSC and BSC.26. Once channel is allocated MS send ALERT message.27. Simultaneously MSC VLR sends ACM (Acknowledgement message). 28. ACM sends back to where IAM was originated via GMSC.29. At this point MSC VLR will trigger for PRBT.30. PRBT server will respond with ACM.31. After that song will be played and calling party can hear the song.32. Once the called party answers the call Connect message sends by the MS.33. ANM (answer message) will be sent to the originating side.34. When disconnect the call all resources will be released and IN will be triggered with released call.

2.2.6 SMS MO and MT

1. No need to request for a traffic channel to send message since message also send in a signaling channel. SMS is transferred to MSC VLR.2. Assuming the subscriber has authenticated. Cipher mode command send to MS to enable ciphering.3. Cipher mode complete message sent by MS.4. According to the IMSI, SMS is forward to the relevant SMSC.5. Then MSC VLR send ack message to MS. This is not the delivery report.6. SMSC also send ACK message for SUBMIT_SM.7. SMSC request SRI from MSC VLR.8. MSC VLR request SRI from HLR.9. HLR send the VMSC GT of the end party.10. MSC VLR forwards it to SMSC.11. SMS is forward to the new MSC VLR.12. To find the end party paging message send to all BSC.13. The relevant MS will respond to the paging message.14. MSC send CP_DATA to MS via BSC.15. MS send ACK for CP_DATA.16. MSC VLR sends ACK for forward SM.17. Delivery report is sent by SMSC to MS via MSC and BSC.

2.2.7 MPLS (Multiprotocol Label Switching)

In conventional layer 3 IP forwarding, when a packet traverses through the network, each and every router extracts all the information of the packet header that relevant to forwarding it. In the most common case, the only relevant field in the header is the destination address field, but in some cases, other header fields might also be relevant. Then routing table lookup determines the next hop of the packet. All other routers which pass the packet should analysis the header independently to determine the next hop. In addition, a complicated table lookup must also be done at each router. In conventional IP forwarding mechanism, it is needed to look at the table two times. RIB (Router Information Base) is used to check whether destination is exists or not. And FIB (Forwarding Information Base) is used to find the best forwarding path of the packet. So it has a complicated process due to this double lookup.

But in MPLS it is used label switching mechanism which analysis layer 3 header only one time. The Layer 3 header is then mapped into a fixed length, unstructured value called as label. Once a label is assigned, a short label header is added at the front of the Layer 3 packet. This header is carried across the network as part of the packet. At subsequent hops through each MPLS router in the network, labels are swapped and forwarding decisions are made by means of MPLS forwarding table (LFIB-Label Forwarding Information Base) lookup for the label carried in the packet header. Because the label is of fixed length and unstructured, the MPLS forwarding table lookup process is fast than normal IP forwarding. So MPLS is used for speeding up network traffic flow in an IP network compared to normal IP based routing network.

MPLS network Architecture

MPLS operate at a layer between layer 2 (data link layer) and layer 3 (network layer), so it is referred to as layer 2.5 protocol.

Figure 2.14: MPLS network routers

Label edge router (LER)LER is operating at the edge of an MPLS network so it acts as the entry and exit point for an MPLS network. The function of LER is to push an MPLS label at the entry point and pop an MPLS label at the exit point. In that case, when IP datagram forward into an MPLS network, LER use routing information to determine which label to be assigned and then forward that labeled packet to the network. When packet is destined to exit the MPLS network, what LER do is strip off (pop) the label and forward the IP packet to the destination by using normal IP forwarding. The LER which first prefixes the MPLS header to a packet is called as ingress router and the LER which pops the label from the packet is called as egress router. And also LER that function as ingress or egress are often called as PE (Provider Edge) router.

Label switch router (LSR)LSR is operating at the middle of an MPLS network which performs routing based only on the label. So these routers are responsible for switching the labels (swap). The function of LSR is to determine the next hop by using the label included in the packet header and old label is then removed from the header and replaced with the new label before the packet is routed forward. They are also called as P (Provider) routers since they are function as transit routers. Label-switched path (LSP)A label-switched path (LSP) is a path through an MPLS network that set up by using a signaling protocol. Normally MPLS routers use BGP (Boarder Gateway Protocol) as routing protocol due to high MTU (Maximum Transmission Unit). The LSPs are shown in above figure with blue and red color lines. LDP (Label Distribution Protocol) is used to distribute labels between LERs and LSRs.

MPLS network operationIn MPLS not only the IP lookup is replaced with a label lookup in the label forwarding information base (LFIB), it use different label operations as below. Push-The label is added to the packet header. Swap-The old label is removed and a new label is replaced with a new label. Pop-The label is removed.

Figure 2.15: Example of MPLS network

Firstly ingress router (R1) inserts a label (push) into the header and forwards the packet. According to the LFIB R1 knows it should forward to R2. Then R2 switches (swap) the label by simply looking at the LFIB. R3 also switches the label. Finally the egress router (R4) removes (pop) the label and forwards the original IP packet to the CE (customer edge) router.

Figure 2.16: Push, pop and swap operation in MPLS network

MPLS services

Epipe

Epipe is a point to point Ethernet bridging service that forward traffic from one site to another. When an epipe is required between two sites, only one pseudo wire is required. By making a tunnel through PE routers this service can implemented in an MPLS network. For example if a subscriber wants to get a constant speed data link by making an epipe, it can give priority to that subscriber. It inserts a service label to identify this special subscriber.

Figure 2.17: Epipe service

VPLS (Virtual Private LAN Service)

VPLS is a multipoint layer 2 services that enables to connect geographically dispersed sites. After implementing the VPLS service those site appear to be on the same LAN. VPLS can provide point to point and multipoint services. As an example in order to communicate between all Ceylon bank branches within the country it can implement a VPLS based network in MPLS network.

Figure 2.18: VPLS service

VPRN (Virtual Private Router Network)

Figure 2.19:VPRN network

Without VPRN, when router 1 send a packet to router 3 source IP is 10.0.0.0/8 and the destination is 192.168.10.0/24 so PE router cannot distinguish whether it send to router 3 or 4. So that router discard it or send to either router 3 or 4. So ISP router need two virtual routers (one for Ceylon bank and other one for Commercial bank) at both PE to separate that two networks. So VPRN is configured virtual routers in PE routers. For example, when router 1 (Ceylon bank) sends a packet to router 3 it add a tag of 10 and when router 2 (Commercial bank) sends a packet to router 4 it add a tag of 20 which is called as router distinguisher. Now PE router knows to which virtual router it should forwards. If router 1 send a packet with tag a label of 10 then PE router forward it to the virtual router. Then it add a tag of 40 which is called as route target and it send through the link. When packet reaches to the other side PE router knows that it should pass to the virtual router of ceylon bank. So the packet successfully forwards to router 3.

CpipeCpipe is used to carry TDM frames between two nodes on IP/MPLS network. This is a point to point service that is used when the end nodes are TDM based equipments.

2.2.8 NSS power system

Figure 2.20: NSS power system block diagram

The basic block diagram of the NSS is shown in above figure. -45V DC is required for switches (MGW, WCS), HLR, SGSN, IN, VLR etc. 230V AC is required for VAS servers, air conditioners, lights, PCs etc.

Function of each blocks are described below.

ATS (Automatic Transfer Switch) - Automatically switches on the generator when CEB power is not available.

MCCB (Moulded Case Circuit Breaker) - Isolates the circuit when there is a fault (earth fault, overcurrent)

Surge Protector To protect electrical devices from voltage spikes (lightning). Uses filter to filter out high frequencies.

Battery bank Rectifier is connected to a battery bank in order to store DC power. When CEB and generator power is out of service battery power is discharged.2V batteries are connected series in order to have 48V.

Figure 2.21: Battery bank

Rectifier Rectifiers are used to convert AC into DC.

Figure 2.22: Rectifier bank

Inverter Inverter is used to convert DC into AC. When AC power is out of service inverter is used to convert stored power in battery bank to AC.Diesel Generator Diesel generator will automatically switch on when the commercial power unavailable.

Figure 2.23: Main Distribution Panel

Power factor is the ratio between real power and apparent power. Etisalat switch power factor is around 0.94 (0.93-0.94). Power factor is reduced mainly due to inductive loads such as air conditioner units. It increases due to capacitive load such as rectifiers. By using a capacitor bank power factor can be improved. But Etisalat does not use a capacitor bank because comparatively the cost for capacitor bank is higher than to the cost of reactive power.

2.3 Network Operations (NOP)

I spent the last month of my training in this department and it is responsible for the operation and maintenance of the BSS. NOP Engineers are always on-call and usually not in their seats since most of the maintenance is done during night. All NOP Engineers are not at the head office, some of them are regional engineers which responsible for a set of sites in there region and they also on call duty. I was given the chance to visit sites with Galle region Engineer for two days and they were BTS site at Imaduwa and BSC, BTS site at Rumassala. We were able to participate to several BTS, BSC site visits with Engineers when there are issues on those sites when we were at head office. I was able to get knowledge on BSS equipment and also about the MW transmission from those site visits as described in these sub sections.

2.3.1 BTS (Base Transceiver Station)

The area covered by a BSS is divided into cells and each cell is managed by one or more BTS. Each BTS consists of radio transmission and reception devices, including antenna and signal processing equipment for the Air Interface. So BTS can be considered as a GSM network element that implements the radio (air) interface between the network and the mobile station.

Three types of BTSs used in Etisalat, Indoor BTS Outdoor BTS Micro BTS

BTS performs Telecommunication functions

The coverage of the radio transmission The management of air interface with the mobileModulate and encode traffic and signaling data from the BSC. This data is then sent to the Mobile Stations over the air Interface as a radio signal. Traffic and signaling received from the Mobile Stations is demodulated and decoded to recover the baseband data.

O&M functions

O&M functions monitor and control the correct operation of the BTS and its external interfaces. These functions are shared between the BTS and BSC. There are three categorized of O&M function: Configuration Management Fault Management External Alarm Handling

Transmission functions

This is a function of managing the transfer of traffic and control data between the BTS and BSC. To minimize operating costs, all data passed between the BTS and BSC in time division multiplexed onto a single physical interface. This is the Abis Interface, which carries all the data sent between the BSC and BTS. One Abis can carry 12 TRE traffic. Abis interface connection can be dedicated cabling or microwave or fiber optic.

Figure 2.24: BTS BSC communication

Figure 2.25: BTS architecture

2.3.2 Transceiver Equipment (TRE)

TRE transmits and receives the signals with MS using TDM. There are many types of TREs mainly two types according to the two frequency bands GSM900 and GSM1800 (DCS). Half rate, full rate and frequency hopping supported and non-supported are also available. And also there are twin TRE which has two TREs in a single module and single TRE which has single TRE in a single module are used in Etisalat. One TRE has 8 time slots.

Figure 2.26: Twin type TRE

2.3.3 Antenna Network Combiner (ANC)

The ANC is the intermediate stage between the GSM antennas and the TRE. Up to 4 TRE and 2 GSM antennas (there are two antennas inside a GSM panel antenna) can be connected to one ANC. It also allows the same antenna to be used both for transmission and reception of radio signals. This is done using an internal duplexer. Even though it is possible to connect two TREs per antenna this operation will cause a power loss of 3dB (air-combining).

Figure 2.27: Antenna Network Combiner

2.3.4 SUM (Station Unit Module)

This is the module that performs center controlling and management of all modules of the BTS. This card consists with a micro-processor which can be configured by logging into. Generating the clocks for all other BTS modules, handling operation and maintenance link (OML to communicate with the BSC), bundling channels into Abis are some duties.

Figure 2.28: SUMA card (Station Unit Module Advance)

2.3.5 BSC (Base Station Controller)

BSC acts as the network controller of the BSS. BSC provides telecommunication functions, transmission functions and operation and maintenance functions.

Telecommunication functions

GSM and GPRS Radio Frequency Management (Managing the broadcast and common control channels, managing the signaling channels) Traffic Channel Resources Management (Selecting TCH, establishing and releasing radio resources in response to requests from the MSC, the MFS and the MS) Short Message Service Cell Broadcasting (Broadcasting messages to all the MSs of one or more cells) BSSAP Protocol Management (Handling messages between the MS and the MSC for circuit service) BSCGP Protocol management (Handling messages between MS and MFS for packet service)

Transmission function

The transmission functions consist for the 9130 BSC Evolution in providing BTS to TC access for CS Services and MFS access for PS services.

O & M function

Database Management Software Management Logical and Hardware Configuration Management Fault Management Performance Management (counters use to check performance)During my training period I was able to identify the hardware architecture of Alcatel 9130 BSC at several sites. It comprises the ATCA (Advanced Telecom Computer Architecture) shelf and the LIU (Line Interface Unit) shelf with LIU cards which interface A-bis and A-terMux traffic E1s to Ethernet.

Figure 2.29: ATCA and LIU shelf in 9130 BSC

2.3.6 TC (Transcoder)

The Transcoder is required to convert the speech or data output from the MSC (64 kbps PCM), into the form specified by GSM specifications for transmission over the air interface, that is, between the BSS and MS (64 kbps to 16 kbps and vice versa) The 64 kbps Pulse Code Modulation (PCM) circuits from the MSC, if transmitted on the air interface without modification, would occupy an excessive amount of radio bandwidth. This would use the available radio spectrum inefficiently. The required bandwidth is therefore reduced by processing the 64 kbps circuits so that the amount of information required to transmit digitized voice falls to the rate of 16 kbps.

Although the reason for transcoding was to reduce the data rate over the air interface, the number of terrestrial links is also reduced approximately on a 4:1 ratio since TC has located in the MSC site.

Figure 2.30 : Transcoder function [12]

2.3.7 BTS to antenna connectivity

Figure 2.31: BTS to antenna connectivityThe main feeder is normally a 7/8-inch (diameter) cable or larger. This size cable has good transmission efficiency to minimize signal loss. Jumper cables, at each end of the feeder, are 1/2-inch (diameter) or smaller but signal loss is high. They are typically five to 10 feet in length and have a smaller bend radius than the main feeder to ease attachment of the transmission line to the equipment and antenna.

When all the cables are fixed, install the connector at the bottom, and, if recommended, the surge arrestor can be installed to protect from lightning. Then the bottom jumper can be connected as shown in the below figure.

Figure 2.32: Feeder-jumper connectivity

2.3.8 Transmission technologies

Transmission link play a crucial role in transporting large volumes of data from the BTS to the BSC, from the BSC to the MSC and between MSCs. MW transmissions is used to connect BTS to BSC via Abis interface and BSC to TC (transcoder) via Ater-Mux interface. Optical fiber links are used to carry critical, high capacity links such as A-interface from TC to MSC.

Figure 2.33: 2G transmission network

Figure 2.34: 3G transmission network

Comparison of microwave and fiber optic transmission

Microwave transmission Less time for deployment compared to fiber. MW link planning and deploying can be done within a week Both capital and operational expenditure are low compared to fiber MW transmission is the best solution for high traffic planning within less time Allocating a frequency for a new link is a big issue since MW spectrum has fully utilized MW equipment damage due to lightning MW links fail due to rainfall Interference, fading issues

Fiber optic transmission Require more time for laying (about several months) Capital and operational expenditure are high Fiber optic transmission is best for long term planning High reliability with respect to MW transmission If there is a fiber cut it is very difficult to find it Higher data rate with respect to MW transmission

2.3.9 Microwave link planning

Due to coverage issues, customer complaints, RNE suggestions require to establish a new site. In MW planning it should try to connect to the BSC in shortest path. And also it should try to avoid site overloading from antennas. Those are some main considerations when planning MW links.

1. MW frequency planningSelecting frequency for a link is decided by TRC. Type of the link depends on the usage of that link on the network. High frequencies use for short distance and low frequencies use for long distance due to high attenuation in high frequencies. Frequency should be selected such that it is matched with the distance as shown in the below table.

Table 2.6: MW frequency with distance

Distance (km)MW frequency (GHz)

0-2.523

2.5-6.518

6.5-1015

10-1513

>158

And also frequency planning should be such that to avoid high-low conflict as shown in below figure.

Figure 2.35: MW frequency selection with minimum interference

2. Line of sight (LOS)Line of sight must have to consider in MW link planning. Not only the LOS at least 1st Fresnel zone should be clear without any obstacles. Planning tool can be used to check the LOS. But LOS cannot determine by only using tool (due to buildings) so path survey (mirror test) is done to check LOS. Loss of LOS is a big issue in MW transmission due to trees, buildingsLOS with no any obstacles in 1st Fresnel zone is shown in below figure.

Figure 2.36: Line Of Sight

3. Capacity of the linkBased on expected traffic number of E1 links needed must be determined. The capacity of the other end of the microwave link should be able to carry this new traffic to the BSC. This is the reason, to identify a nearest microwave point having an adequate amount of capacity as the first step. The MW links support the most commonly used digital communication hierarchies.

PDH (Plesiochronous Digital Hierarchy) SDH (Synchronous Digital Hierarchy)

The required capacity of the link is determined based on the number of TREs in the site. A single time slot in GSM is 16kbps and one TRE contains 8 time slots. So the data rate of a TRE is (16*8 = 128 kbps) 128 kbps. And also one E1 link consists of 32 channels of 64 kbps each and so one TRE requires 2 channels of an E1 link. Other than this traffic channels, a TRE require the service of signaling channel named RSL and channels for synchronization and OAM.

PDH (Plesiochronous Digital Hierarchy)

PDH links more suitable for access networks.

Figure 2.37: PDH Hierarchy

Capacity of a PDH link may vary from 4, 16 and 64 E1s etc.

SDH (Synchronous Digital Hierarchy)

SDH links widely use for backbone networks.

Figure 2.38: SDH Hierarchy [13]

So STM-1 (Synchronous Transport Module 1) means 63E1 => 155.52 Mbps STM-4 (Synchronous Transport Module 4) means 252E1 => 622.08 Mbps

Due to overhead these values are greater than to the exact value.

2.3.10 Main equipment in MW link

Figure 2.39: Equipment of MW link

Antenna

This is the component which transmits the microwave signal to the air. It is usually a highly directional antenna resulting in high antenna gain. Polarization is another important characteristic. Links can either be horizontally polarized or vertically polarized or both depending on the antenna. The input to the antenna from the ODU is always a RF signal which is Omni directional hence the antennas task is to direct the signal it transmit thereby increase its dBm level. And on the other hand receive radio transmission signals. Antenna comes in various frequency bands, types and sizes.

IDU (Indoor Unit)

This is where all the baseband signal processing is carried out. IDU is responsible to modulating and multiplexing signals into a predefined IF (intermediate frequency) and also add power to this modulate signal and that compound signal is sent to ODU through the IF cable. IDU is also responsible to de-multiplex the captured signal by ODU and give the relevant output in mode of E1 or STM. PDH IDU has capable of multiplexing and de-multiplexing the E1s as required. But for SDH link a separate MUX is required.

Figure 2.40: Indoor Unit

ODU

The outdoor unit contains all the equipment that are needed to generate the microwave signal on the transmit frequency and amplify the weak received signal before it is sent to the IDU. The ODU has a High power amplifier to amplify the transmit signal Low noise amplifier to amplify the weak received signal Band pass filters reject any frequency which does not belong to its operational range. Modulators and demodulators

The ODU receive the signal that is required to transmit at an intermediate frequency via the IF cable. An internal oscillator generates the microwave signal and modulates it. Then it feeds to the high power amplifier to amplify the transmit signal. ODU has capability of Tx and Rx of both vertical and horizontal polarization (with a dual polarized antenna) which enables maintain 2 logical links over single physical link.

Figure 2.41: MW Antenna with ODU

IF cable

Carry signals in intermediate frequency from IDU to ODU and vice versa. Signals includes traffic, power (-48V DC for ODU) and control information. Coaxial cables are used.

Figure 2.42: Block diagram of IDU and ODU MW link requirements

-32dBm < Rx level < -25dBm Link availability 99.999% Fade margin 36 dBm Interference power < -95dBm