INTERNSHIP REPORT On In plant Training in Advanced Telecom at RTTC, BSNL
FACULTY GUIDE: Prof. Saravanan K
SUBMITTED BY: Ch. Sreekar (13BEC0611)
ABSTRACT The following is a report on the in plant training received from BSNL at RTTC, Gachibowli, Hyderabad. The training covered several concepts based on fixed line and wireless communications. Concepts such as optical fibre communication and mobile communication were extensively studied and their real world applications were analysed.
ACKNOWLEDGEMENT I would like to thank Mr. Srinivasa Rao, SDE for guiding me throughout the training programme and offering their constant support. Furthermore, I would like to thank the entire staff of the RTTC, Gachibowli for always being there to help me learn and gain the skills required for pursuing my career in the near future.
Telecommunication department provides a strong and reliable technology for Telephone Communication. The telecom services have been recognized the world over as an important tool for socio-economic development for a nation. It is one of the prime support services needed for rapid growth and modernization of various sectors of the economy. Indian telecommunication sector has undergone a major process of transformation through significant policy reforms, particularly beginning with the announcement of NTP 1994 and was subsequently re-emphasized and carried forward under NTP 1999. Driven by various policy initiatives, the Indian telecom sector witnessed a complete transformation in the last decade. It has achieved a phenomenal growth during the last few years and is poised to take a big leap in the future also.
RADIO WAVES AND SPECTRUM CRUNCH
GSM CALL FLOW
PACKET CORE NETWORK GPRS AND UMTS
PACKET CORE NETWORK LTE
THE OSI MODEL
OPTICAL FIBER COMMUNICATION
VALUE ADDED SERVICES
SITE AND ITS COMPONENTS
Radio Waves •
Radio Waves are a type of electromagnetic radiations and have wavelength higher than Infra-red light.
Frequencies range from 3kHz to 300GHz.
Travel at the speed of light (3x10⁸m/s).
Radio Waves occur naturally in the form of Cosmic rays from outer space and are also generated by lightening.
Man-made radio waves are used for fixed and mobile radio communication, broadcasting, radar and other navigation systems, communications satellites, computer networks and innumerable other applications. Propagation characteristics vary with frequency of radio waves -higher wavelengths can propagate longer distances beyond earth’s horizon whereas shorter waves are reflected off the ionosphere and can travel around the world. Very short wavelengths reflect very little and travel on a line of sight.
"Spectrum" is the range of radio frequencies available. Telecommunication systems all require a certain amount of electromagnetic bandwidth to operate. Spectrum is a scarce resource and with growing telecommunication utilities and applications, it has to be optimally used.
Each country manages the spectrum allocation in their geographical areas as per best practices described by international recommendations.
Dense utilization of spectrum increases the interference and effects network quality. Regular network planning and optimization is needed for best performance of telecom networks and interference control.
CELLULAR GENERATIONS 1G
Radio signals used were analog.
Modulated to higher frequencies 150MHz or up. Only voice traffic.
GSM standard(Global System for mobile communications).
Phone conversations are digitally encrypted. Introduced data services, SMS(Short Message Service) , picture message and MMS(Multimedia Message Service).
• • • • • •
Uses 900MHz and 1800MHz frequency bands. Supports circuit switched (CS) technology. Uses TDMA(Time Division Multiple Access ) and CDMA(Code Division Multiple Access). Speed is 14.4kbps. Channel bandwidth is 200KHZ.
2.5 G • • • •
GPRS(General Packet Radio Service). Packet oriented mobile data. Supports both circuit switch(CS) and packet switch(PS). Speed is 128kbps.
2.75 G • • • •
EDGE(Enhanced Datarates for Global Evolution). Also known as eGPRS(evolved GPRS) Supports both CS and PS modes. Speed is 286kbps.
UMTS (Universal Mobile Telecommunication Service).
Uses WCDMA (Wide band Code Division Multiple Access) for radio interface. Channel bandwidth is 5MHz. Uses 900MHz and 2100MHz frequency bands.
Speed is 2Mbps.
Mobile access to video signals. Modulation technique used is QPSK(Quadrature Phase Shift Keying).
3.5G HSDPA(High Speed Downlink Packet Access), HSUPA(High Speed Uplink Packet Access). Downlink speed is 14.4Mbps and uplink is 5.76Mbps. Combinedly it is HSPA+ ( evolved High Speed Packet Access). Speed of HSPA+ is 21Mbps. New modulation technique 16 QAM is used. 4G LTE(Long Term Evolution),EPC(Evolution Packet Core )Amended web access , video conferencing. Access technique is OFDMA(Orthogonal Frequency Division Multiple Access). Uses 1800MHz (in FDD) and 2300MHz (in TDD) frequency bandS. Only Packet Switched technique for all services including voice. Dynamic channel bandwidth. Speed is 170Mbps. IP telephony. Modulation technique used is 64QAM.
MULTIPLE ACCESS TECHNIQUES FDMA (Frequency Division Multiple Access) The available bandwidth of the common channel is divided into bands that are separated by guard bands. One user uses one frequency. TDMA (Time Division Multiple Access) The bandwidth is just one channel that is time shared between different stations. CDMA (Code Division Multiple Access) Only one channel occupies the entire bandwidth of the link and all stations can send the data simultaneously. No time sharing
OFDMA (Orthogonal Frequency Division Multiple Access) OFDMA is the access technique used in LTE. It is Frequency Division Multiplexing used as a digital multi-carrier modulation method. A large number of closely spaced orthogonal sub carrier signals are used to carry data on several parallel data streams or channels. Each sub channel is sub channel is modulated with a conventional modulation schemes (QAM or QPSK). They have low symbol rates similar to conventional single carrier modulation schemes in the same bandwidth.
In OFDMA the primary advantage is the ability to cope with the several channel conditions. FREQUENCY DIVISION DUPLEX (FDD) •
In FDD, uplink and downlink are in a pair and they cannot be varied independently.
Both uplink and downlink transmission will be assign different frequencies. Both utilize the same time slot. Uses symmetric frequencies for the uplink and downlink.
TIME DIVISION DUPLEX (TDD) • • • •
It uses same frequency for transmit and receive but uses the alternate time slots for those operations. In TDD, uplink and downlink can be varied independently. It doesn’t have to be 50-50 symmetry in UL and DL. Transmissions are concurrent rather than simultaneous.
GSM NETWORK ARCHITECTURE
The GSM network architecture comprises of many functional blocks and interfaces. The GSM network can be broadly classified into The Mobile Station (MS) The Base Station Subsystem (BSS ) The Network Switching Subsystem (NSS)
The Operation Suppor t Subsystem (OSS)
There are 3 interfaces in GSM network •
Um Interface: Between MS(Mobile Station) and BSS(Base Station Subsystem). It allows the exchange of user and signalling information.
Abis Interface(open air interface): Between BTS(Base Transceiver Station) and BSC(Base Station Controller). It allows the exchange of traffic information.
A Interface: Between BSC(Base Station Controller) and MSC(Mobile Switching Center).
MOBILE STATION (MS) • • • •
In GSM mobile phone is known as Mobile Station. It provides access to GSM network for voice and data. It is a combination of terminal equipment and subscriber’s data. The terminal equipment is known as Mobile Equipment (ME). Each MS has a unique number called IMEI number which is used for authentication purposes.
Mobile transmits and receives voice at 12.4Kbps. • Subscribers data is fed from the SIM card. Therefore MS = ME + SIM MOBILE STATION IDENTITIES IMEI (International Mobile Equipment Identity): •
15 digit number used for equipment identification.
When the phone is switched on , the unique IMEI number is transmitted and checked against the data base in the EIR. • IMEI = TAC + FAC + SNR +sp TAC (Type Approval Code): 6 digits, determined by GSM body FAC (Final Assembly Code): 2 digits, identifies the manufacturer SNR (Serial Number): 6 digits, phone serial number Sp: 1 digit, spare for future use
A GSM number which uniquely identifies a mobile subscription in the Public Switched
Telephone Network(PSTN) • It is the telephone number • MSISDN = CC+ NDC + SN CC: Country Code NDC: National Destination Code SN: Subscriber Number SIM (Subscriber Identity Module): • •
It is a removable card that plugs into the MS.
It identifies the mobile subscriber and provides information about the service that the subscriber should receive. • SIM number is mapped to IMSI number.
The International Mobile Subscriber Identity (IMSI) is the primary identity of the subscriber within the mobile network and is permanently assigned to that subscriber.
Subscribers Authentication Key ( Ki ) is used to authenticate the SIM card. • It also contain information about TMSI and LAI. •
Temporary Mobile Subscriber Identity (TMSI) - This number also identifies the subscriber. It can be alternatively used by the system. It is periodically changed by the system to protect the subscriber from being identified by someone attempting to monitor the radio interface.
Location Area Identity ( LAI ) - Identifies the current location of the subscriber. BASE STATION SUBSYSTEM (BSS)
The Base Station Subsystem is responsible for managing the radio network, and it is controlled by an MSC. Typically, one MSC contains several BSS. The BSS consists of the following elements: BSC Base Station Controller. • BTS Base Transceiver Station. • TRAU Transcoder and Rate Adaptation Unit. BASE TRANSCEIVER STATION (BTS) • • • • •
It facilitates the wireless communication between User Equipment and network. It is connected to the cell through feeder cables. Each BTS is given several TRXs which allows it to serve different frequencies. All the BTS are connected to the BSC through Abis interface which controls them. User Equipment may be several BTS serving it at a time but only one of them will be dominant.
BTS is connected to UE through air interface.
It is referred as NodeB in 3G networks and eNodeB (evolved NodeB) in LTE.
BASE STATION CONTROLLER (BSC) •
Several BTS are connected to one BSC. • It deals with the allocation of the radio channels and the handover from one BTS to another BTS etc •
All the BSC are connected to the MSC through A interface. • It serves as a mediators between base stations and MSC providing a voice pathway for mobile phones. • BSC has the entire database for all cell parameters associated with the BTSs.
TRANSCODER AND RATE ADAPTION UNIT (TRAU) • • • • •
This performs the transcoding functions for the voice channels and Rate Adaption for the data channels. Transcoding is the compression of the speech data from 64kbps to 12.4kbps. Rate Adaption allows the original encoded data to be carried in 64kbps. Transcoding leads to degradation of the voice quality. It can be collocated with the BTS , BSC or MSC or it can be a separate unit.
NETWORK SWITCHING SUBSYSTEM (NSS) MSC (MOBILE SWITCHING CENTRE) • • • • •
It is a core network element which controls the Network Switching Subsystem. Communicates with BSC on MS side and PSTN on other side. It handles all the inter BSC handovers. All the MSC are connected to GMSC. Calls were established , maintained and released in MSC.
GMSC (Gateway Mobile Switching Centre) All the MSC of different operators are connected to one GMSC. It is used to route the calls outside the mobile network. HLR (Home Location Register) HLR is connected to the MSC and it has all the subscriber data. HLR contains user information including address , account status and preferences. When a call routes to MSC ,it gets all the information related to the customer whether customer is activated or issued. When the subscriber switches from one operator to other , all the information is transferred to other operator’s HLR.
VLR (Visitor Location Register) • •
It contains the exact location of the all the subscribers currently present in the service area of MSC. The additional data stored in VLR is Location Area Identity (LAI) Temporary Mobile Subscriber Identity (TMSI) Mobile Station Roaming Number (MSRN) Mobile Status (Busy/available/switched off etc)
VLR is the temporary database.
AUC (Authentication Centre) •
It carries out the verification of the SIM. It provides the security information which is required for this process. It is associated with HLR.
EIR (Equipment Identity Register) • • •
It is responsible for IMEI (International Mobile Equipment Identity) checking. It stores all IMEI numbers and checks the validity. EIR classifies the IMEI numbers into: White list (Mobile is allowed on the network) Black list (Mobile is not allowed on the network) Grey list (Mobile needs to be monitored)
IN (Intelligent Network) • • • •
Prepaid system. It does all the functions of billing. Checks whether the subscriber has enough balance to make or receive a call. It contain all the information about the plans of the subscriber and duration of the sessions.
OPERATION SUPPORT SUBSYSTEM (OSS) MSC is connected to OMC (Operations and Maintenance Centre). It focuses on the maintenance of the network. It helps the network provider to access the health of the overall telecommunication network. It gives the operator the ability to perform FCAPS. Fault Management
Configuration Management Accounting Management
Performance Management Security Management
BASIC INFRASTRUCTURE OF THE MSC For the proper functioning of the MSC and other network switching elements, proper conditions has to maintained in the switching room. The three important parameters we have to take care are 1) 2) 3)
Power supply Temperature Humidity
Power Supply The network elements are given a supply of -48V (negative sign indicates that supply is given to the negative terminal and positive is grounded) The AC supply is connected to rectifier which converts the AC power supply to DC which is supplied to the network elements If there is any problem in the power supply and it fails, the supply is drawn from the Battery Banks This is called SMPS (Switch Mode Power Supply) The supply given is Uninterruptible Power Supply (UPS). It is like a backup supply. UPS is an electrical apparatus which provides power supply when the main power supply fails Temperature and Humidity The temperature in the switching room has to be maintained at 21degrees For this purpose PAC (precession Air Conditioners) are used. The Precession Air Conditioner is to precisely control the temperature, humidity and dust The sensitivity of PAC is +/- 1degree and +/- 5% Relative Humidity Components of PAC: Evaporator, condenser, compressor and heater
If the temperature of the switch room goes down than 21degree then heater is switched on and temperature is retained If the temperature is more, then the cooler is switched on and cool air is sent to the equipment to prevent them from damage due to excess heat The basic refrigeration cycle is followed for maintaining this temperature and humidity If the humidity is less than the required, then equipment will have the cracks due to dryness If the humidity is more, then the water droplets may accumulate on the equipment. This causes the destruction of the equipment. So the excess water is condensed
LOGICAL CHANNELS IN GSM AIR INTERFACE The logical channels are divided into two groups. 1) Common
Broadcast channels and common control channels 2) Dedicated
Dedicated Control Channels and Traffic Channels BROADCAST CHANNELS FCCH: Frequency Correction Channel. It allows the MS to synchronize to the frequency. It carries 142 bit zero sequence that repeats once in every 10 frames
SCH: Synchronization channel. This is used by the MS to synchronize to TDMA frame structure within the particular cell BCCH: Broadcast Control Channel. The last information the MS must receive in order to make/receive calls is carried by BCCH CBCH: Cell Broadcast Channel. Used for the transmission of generally accessible information like Short Message Services (SMS) COMMON CONTROL CHANNELS RACH: Random Access Channel. Used by MS for requesting for a channel. RACH is also used when it wants to make a call. It is transmitted in uplink AGCH: Access Grant Channel. On requested for a signalling channel by MS the network assigns a signalling channel through AGCH. It is transmitted on the downlink PCH: Paging Channel. The information on this channel is a paging message including the MS identity (IMSI/TMSI). It is used in downlink DEDICATED CONTROL CHANNELS SDCCH: Stand alone Dedicated Control Channel. It is a signalling channel. Used for location update, authentication and assignment of traffic channels SACCH: Slow Associated Control Channel. Transmission of radio link signal measurement, power control
FACCH: Fast Associated Control Channel. Used for handover commands and during call setup and release TRAFFIC CHANNELS TCH: Traffic Channel. Carries voice data. Two blocks of 57 bits contain voice data. One TCH is allocated for every call. A full rate TCH carries 12.4kbps voice data and half rate TCH carries 6.5 kbps voice CALL FLOW IN GSM REQUEST ACCESS • • •
The MS send as a channel request message to the BSS through RACH. BSS responds with the radio resource assignment on the AGCH. Then MS sends a service request message to the BSS on the SDCCH.
AUTHENTICATION • • •
Before the network provide any service to MS, it has to authenticate itself on the network. The BSC sends an authentication request message to the MS. The MS calculates SRES based on the given RAND and sends it to the BSS as authentication response message.
The BSS checks the SRES and if it is correct then the MS is authenticated and allowed on the network.
The BSS sends service accept message letting the MS know that service request is received and proceeded.
After authentication, the MS switches to cipher mode.
INITIAL CALL SETUP • • • • •
After switching to the cipher mode, MS sends a cipher mode complete message. The MS sends a call set up message to BSS. This message includes the address of the called party. The BSC assigns the TCH. It also mentions which TRX and which Timeslot to use. The BSC does not assign the TCH until the MSC sends a call proceeding message to BSC that the IAM has been sent. The MS immediately switches to assigned TCH and MS sends an assignment complete message to BTS through FACCH.
CALL SETUP •
The MSC sends an Initial Address Message (IAM). The IAM contains the MSISDN of the called party.
The MSC will also send a call proceeding message to the BSS. Based on the dailed number, the GMSC decides where to route the IAM within the PSTN. The PSTN will continue to route the IAM until it reaches the correct Switching Centre and call routing is complete.
• • • •
The PSTN will establish a call circuit and sends the address complete message to the GMSC. The GMSC forwards that to MSC indicating that the call circuit is established.
CALL ESTABLISHMENT •
Once the MSC receives the ACM, it sends an alert message to MS indicating that the call is going through.
The BSC sends that message on FACCH. Once the MS receives an alert message, it will generate the ringing sound in the earpiece. The BSS sends an alerting message the subscriber will hear the line ringing. Once the called party answers phone, the PSTN will send an answer message to the MSC. The MSC forwards this to the MS in a connection message. Once the MSC receive the message, it switches over to voice and begins the call.
• • • • •
CALL TERMINATION •
When either the caller or the called party hangs up, the call will be disconnected. Either party can initiate the disconnect.
The MS sends a disconnect message to the BTS on the FACCH. BTSforwards it to MSC. Once the MSC receives the message, it sends an release message through the GMSC to the PSTN as well as down through the BSS to the MS.
• • •
The MS responds by sending a release complete message to the BSS on the FACCH. forwards the message up to the MSC.
Once MSC receives the message the call is considered ended from the call control perspective.
The MSC sends a channel release message to the BSS to release the TCH allocated to MS. The BSC forwards it to MS and it responds with the DISC message and turns to idle mode. The BSS reallocates the channel for the other call or release the TR.
HANDOVER It is a process by which the control of a mobile is transferred from one BTS to another. When the call is in process, the change in location need special processing In hard handover, within the BSS the BSC which knows the current radio link configuration prepares a new channel in new BTS. The MS is told to switch over to the new BTS In soft handover, the MS is connected to two BTS simultaneous.
PACKET CORE NETWORK – GPRS AND UMTS
GPRS architecture works on the same procedure like GSM network, but, has additional entities that allow packet data transmission. This data network overlaps a second-generation GSM network providing packet data transport. it attempts to reuse the existing GSM network elements as much as possible, but to effectively build a packet-based mobile cellular network, some new network elements, interfaces, and protocols for handling packet traffic are required. The UMTS 3G architecture is required to provide a greater level of performance of the original GSM. It has migrated through the use of other architectures which already had the ability to carry data. Accordingly many of the elements required for the UMTS network were seen as a migration. This considerably reduced the cost of implementing the UMTS network
MS (Mobile Station)
• It is device used by end user to communicate. • Mobile Station is required to access GPRS services. These new terminals will be backward compatible with GSM for voice calls. BSS (Base Station Subsystem) •
The base station subsystems require a software upgrade and a new hardware called packet control unit (PCU).
The PCU directs the data traffic to the GPRS network and can be separate hardware element associated with the BSS.
UTRAN (UMTS Radio Access Network) The UMTS Radio Access Network or Radio Network Subsystem comprises of two main components Radio Network Controller (RNC) •
This element of the Radio Network Subsystem controls the Node Bs that are connected to it.
The RNC undertakes the radio resource management and some of the mobile management functions, although not all.
It is also the point where the data encryption and decryption takes place to protect the user data. Node B •
Node B is the term used within UMTS to denote Base Transceiver Station.
This part of the UTRAN contains the transmitter and receiver to communicate with UEs within the cell.
It participates with RNC in resource management.
PACKET SWITCHED ELEMENTS Following two new components, called Gateway GPRS Support Nodes (GGSN) and Serving GPRS Support Nodes are added Serving GPRS Support Node (SGSN): •
As the name implies, this entity was first developed when GPRS was introduced and its uses has been carried over into the UMTS network architecture.
The SGSN provides a number of functions within the network.
MOBILE MANAGEMENT: when a UE attaches to a packet switched domain of the core domain, it generates MM information about the mobile current location SESSION MANAGEMENT: The SGSN manages the data sessions providing the required quality of service and PDP (Packet Data Protocol) contexts
INTERACTION WITH OTHER AREAS OF THE NETWORK: SGSN is able to manage its elements within the network only by communicating with the other areas of network for example MSC BILLING: The SGSN is also responsible for billing. It achieves this by monitoring the flow of user data across the network. CDR (Call Detail Records) are generated by the SGSN before being transferred to charging entities Gateway GPRS Serving Node (GGSN) Like the SGSN, this entity was also first introduced into the GPRS network. The Gateway GPRS Support Node is the central element within packet switched networks. It handles inter working between the packet switched network and external networks. It is considered as a sophisticated router. In operation, when the GGSN to a specific user, it checks if the user is active and then forwards the data to the SGSN serving the particular UE.
Shared elements The shared elements of the packet core network architecture include the following network entities •
Home Location Register (HLR): This database contains all the administrative information about each subscriber along with their last known location. In this way, the packet core network is able to route the information to the relevant node B.
Equipment Identity Register (EIR): This checks whether the UE is allowed on the network or not.
Authentication Centre (AuC): protected database that carries out the verification of the SIM.
Policy and Charging Rule Functions (PCRF)
• It is a software node designated in real time to determine policy rules. • It aggregates the information from network, other operational systems and other sources in real time. • It can be integrated to different platforms like billing, charging and rating. Domain Name System (DNS) •
Delegates the responsibility of assigning the domain names and mapping them to the internet sources.
It translates the domain names to the IP addresses.
It also specifies the functionality of the database which is at its core.
Signalling Transfer Point (STP) SGSN is connected to HLR through STP. It routes messages between signalling end points.
PACKET CORE NETWORK ARCHITECTURE -LTE LTE is different from the GPRS or UMTS. It is completely packet core, there are no circuit switched elements in LTE. All the elements are newly placed.
MOBILE MANAGEMENT ENTITY (MME) Key control node for LTE access network. It replaced MSC of the earlier architectures. Activation and deactivation of the user. Responsible for the authentication of the user.
SERVING GATEWAY NODE (SGN) It replaced SGSN of GPRS. It acts as an IP server. It contains the network internal routing information. PDN GATEWAY NODE (PGN) It replaces the GGSN of the GPRS. PUBLIC DATA NETWORK (PDN)
• It is the network established and operated by the telecommunication administrator. • It can be Circuit switches or Packet Switched network that can transmit data in the digital form. • Connects the UE and external packet data network.
• It is the point of entry and point of exit for the traffic. HOME SUBSCRIBER SERVER (HSS) It replaced the HLR. • Until now we used HLR for the access of the subscriber information but now due to the upgrade in the protocols and the other network elements, HLR can’t be used in the LTE. • So we switched to the upgraded version of HLR. • It is central database • Contains user related and subscription related information. • It has same functions of HLR and AuC. POLICY AND CHARGING RULE FUNCTIONS (PCRF) It does billing. Aggregates the information from all the sources Post paid. EVOLVED NODE B (e-nodeB) It does the same functions of BTS in GPRS and Node B in UMTS. As the name indicates the eNode B has an upgrade in its functions. It transmits the data packets from the SGW to UE - Wireless transmission
THE 7 LAYERS OF THE OSI MODEL The Open Systems Interconnection model is a conceptual model that characterizes and standardizes the communication functions of a telecommunication system without regard to the underlying internal structure and technology It partitions the communication system into abstraction layers. The original version of the model has 7 layers. A layer serves the layer above it and is served by the layer below it.
LAYER 1: PHYSICAL •
It defines the electrical and physical specifications of the data connection and voltages , frequency and cable specifications for the wireless devices.
It defines the relation between the device and the physical transmission medium. It defines the transmission mode and the network topology. It is responsible for the transmission of the unstructured data in the physical medium.
LAYER2: DATA LINK • •
The data link provides the node to node data transfer i.e. a link between two directly connected nodes. It detects and corrects the errors that may occur in the physical layer.
It defines the protocols to establish and terminates the connection between two physically connected devices.
The data link layer has two sub layers :Logic Link Control layer (LLC): responsible for identifying network layer and encapsulating them and control error checking and frame synchronization Media Access Control (MAC): responsible for controlling how devices in the network gain access to medium and permission to transmit it
LAYER3: NETWORK •
The network layer provides the functional and procedural means of transferring variable length data sequence from one node to another connected to the same network.
It translates logical network address into physical machine address. A network layer may provide reliable message delivery. If the message is too large to be transmitted from one node to another, then network layer may implement message delivery by splitting the message into several fragments at one node, sending them independently and reassembling the fragments at another node.
LAYER4: TRANSPORT •
The transport layer segments data from the sending host’s system and reassembles the data into a data stream on the receiving host’s system.
It controls the reliability of the given link through flow control, segmentation and error control.
It can keep the track of segments and retransmit if any fails. It also provide the acknowledgement for the successful data transmission and sends the next data if no error occurred.
OSI has 5 classes of transport protocols.
LAYER5: SESSION • •
It controls the dialogues between the two points. It establishes, manages and terminates the connection between the connection between the remote and local applications.
It provides simplex, half duplex and full duplex operations and controls termination, reestablishment processes.
It is responsible for the graceful close of sessions, session check pointing and recovery. Commonly implemented explicitly in the application environment.
LAYER6: PRESENTATION •
It provides context between application layer entities, in which it may use different syntax and semantics, the presentation layer provides an mapping between them.
The layer provides independence from data representation by translating application and network formats.
The presentation layer transforms the data into the form that the application layer accepts. Sometimes called as syntax layer.
LAYER7: APPLICATION •
It is closest layer to the user end.
It interacts with the software applications and implement the communicating component.
Identifying communication partners, resource availability and synchronizing communication. • While identifying, it determines the identity and availability for data transmission.
In resource availability, it is checked whether sufficient network resource available are not.
In synchronization, all communication between the application require communication. • Everything in this layer is application specific.
SS7: SIGNALLING SYSTEM 7 SS7, Signalling System number 7, supports the PSTN (Public Switched Telephone Network) in: Call setup Call management Call release and Between telephone office and CPE (Customer Premises Equipment) to simply transport voice traffic and data/video traffic. SS7 uses •
In-band signalling mechanism:-
The signalling information and the user data are carried on the same channel •
Out-of-band signalling mechanism:Some application require a high quality transfer of the signalling information so this band is used.
In the out-of-band signalling, the signalling information is carried in a separate and dedicated physical channel, and the user data is carried in other channels
MTP1: MESSAGE TRANSFER PART LEVEL 1 The MTP1 is the physical layer. It provides a transport for an individual signalling data link. It defines the physical, electrical and functional characteristics of a signalling data link layer and the means to access it. MTP2: MESSAGE TRANSFER PART LEVEL 2 •
The MTP2 is the data link layer.
It provides the functions and procedures to transfer signalling messages over an individual signalling data link.
It is primarily responsible for error free transmission of data for some basic communication functions.
Once information enters the network, it must be transferred in the proper sequence and without error between network nodes over each segment of the transmission path.
MTP3: MESSAGE TRANSFER PART LEVEL 3 • • •
The MTP3 is the network layer . MTP3 provides the means to establish, maintain and terminate connections between systems. It provides the services that transmit the data through the network from originator to its final destination.
MTP3 performs two functions:Signalling Message Handling (SMH): Delivers incoming messages to their intended User Part and routes outgoing messages toward their destination. MTP3 uses the PC to identify the correct node for message delivery. Each message has both an Origination Point Code (OPC) and a DPC. The OPC is inserted into messages at the MTP3 level to identify the SP that originated the message. The DPC is inserted to identify the address of the destination SP. Routing tables within an SS7 node are used to route messages. Signalling Network Management (SNM): Monitors link sets and route sets, providing status to network nodes so that traffic can be rerouted when necessary. SNM also provides procedures to take corrective action when failures occur, providing a self-healing mechanism for the SS. SCCP: SIGNALLING CONNECTION CONTROL PART SCCP is a combination of parts of network layer and parts of transport layer. It relies on the MTP level 1,2,3 to provide additional functions for both connectionless and connection oriented network services to transfer circuitrelated and non circuit related signalling information between switches. When coupled with the MTP layers, SCCP provides specialized routing and management and control functions for the transfer of higher level messages between the MTP layers and the SCCP users. TCAP: TRANSACTION CAPABILITIES APPLICATION PART
• It provides application level functions for special SS7 services. • Service information exchanged between the SSP and the network database would typically be defined with the TCAP.
• The TCAP is supported by SCCP. TCAP messages are encapsulated into the SCCP header part. ISUP AND TUP
• • • • •
TUP and ISUP sit on top of MTP to provide circuit-related signalling to set up, maintain, and tear down calls. TUP has been replaced in most countries because it supports only POTS calls. Its successor, ISUP, supports POTS and ISDN calls as well as a host of other features and added flexibility. Both TUP and ISUP are used to perform inter switch call signalling. ISUP also has inherent support for supplementary services, such as automatic call back, calling line identification etc OPTICAL FIBER COMMUNICATION Transmitting information from one place to another by sending by sending pulses of light through an optical fiber using light intensity modulation.
It uses laser as light source.
Advantages : less attenuation(0.2Db/km), very high i/f carrying capacity, greater safety and immune to EMI & RFI, moisture and corrosion.
An optical fiber consists of core, cladding and buffer.
The cladding guides the light along the core by using the method of TIR ( Total Internal Reflection).
TIR: For all angles of incidence greater than the critical angles, the incident ray gets reflected back into the denser medium itself.
Fiber optics with the glass fiber, it uses light wavelengths around 850, 1310, 1550nm(IR).
Transmission losses or attenuation of the signal in an optical fiber is measured in Db/km
Losses: material absorption, Rayleigh scattering, radiative losses.
Basic elements to determine transmission system performance: Fiber loss factor Type of fiber, Transmitter Receiving sensitivity, Number of splices Types of splices and Margin (minimum sensitivity of signal that receiver can take) SINGLE MODE AND MULTI MODE FIBERS
Multi mode fiber
It has a very large core which allows multiple modes of light to travel
simultaneously. Large core allows the use of low cost optical transmitters (LEDs) and
connectors. Primarily used in systems with short transmission distances.
Single mode fiber
Has a smaller core diameter, allowing only one mode of light to propagate at a time.
Maintain spatial and spectral integrity of optical signals over long distances, allowing more information to be transmitted.
SPLICERS AND CONNECTORS
Splicing and connectorizing is done to precisely match the core on one optical fibre cable to that of the other.
This is done to produce a smooth junction through which light signals can pass without alteration or interruption.
Fibres are joined in two ways: Splices, which form permanent connections between fibres in the system. Connectors, which form remateable connections, typically at termination points. MUX and WDM
In fibre-optic communications, wavelength-division multiplexing (WDM) is a technology which multiplexes a number of optical carrier signals onto a single optical fibre. This technique enables bidirectional communications over one strand of fibre, as well as a multiplication of capacity.
PDH vs SDH
PDH SDH PDH
Digital multiplexer’s inputs are of
Digital multiplexer’s inputs are of
same bit rate and are derived from different clocks from different oscillators. Each will differ within tolerance of few clock periods.
same bit rate and are derived from common clock, hence are in phase.
Bit interleaving is used to combine
Word (group of bits) interleaving is
used to combine digital signals.
Rates derived from 2.048 Mbps
basic rate include bit stuffing in 30 channels: 2.048 x 4 gives 8.448 Mbps (120 channels) 8.848 x 4 gives 34.368 Mbps (480 channels) 34.368 x 4 gives 139.264 Mbps (1920 channels) 139.264 x 4 gives 564.992 Mbps (7680 channels)
Uses basic bit rate of 155.52Mbps
(STM-1) and four time multiples of this, i.e., 622.08 (STM-4) Mbps, 2488.32 Mbps (STM-16) and 9953.28 Mbps (STM-64). STM stands for Synchronous Transport Module.
Value Added Services (VAS) Term for non -core services. All the services apart from voice calls and data are Value Added Services. Major value added services SMS Ringback tones and caller tunes Missed call alerts and voicemail box Devotional services Location based services etc SHORT MESSAGE SERVICE (SMS) It is a text message component service component of phone, web or mobile communication It uses standardized communication protocols to allow fixed line or mobile phone devices to exchange short text messages The SMS are sent trough SMSC (Short message Service Centre) The SMSC provides “store and forward mechanism”. It attempts to send the message to the SMSC recipients, if the recipient is not available, then it stores the message for the later retry The delivery of the SMS is ‘best effort’ but there is no guarantee that the message is actually delivered to the recipient
Each subscriber has a unique number called “message sender number” in the mobile handset which is also present in the SMSC by which the senders are identified and message is routed The size of the message is restricted and different for each handset or operator TSG OPERATIONS The complaints are divided into two categories 1 FTR: 2
First Time Resolutions. Complaints which are resolved in the first time
SR: Service Request. Complaints which are not resolved by the first time are sent to the technical service group (TSG)
DIFFERENT TYPES OF COMPLAINTS Coverage problems Connectivity issues Data complaints IN/VAS Coverage problem Coverage is checked in 3 ways 1. out door coverage 2. indoor coverage 3.while moving Depends upon the distance between the site and complaint location
Call drops while travelling, lack of voice quality etc come under connectivity issues Connectivity issues Customer profile is checked for any changes in the settings If channels are blocked or if the site reaches the maximum number of calls Data complaints Data issues can occur due to the same reasons 1.
profile : if the user is blocked or has the balance for the session
quota: if the quota for using the data is finished or the date is over
throttling issues 4. Third party server problems
IN/VAS SMS are not being due to change in the message sender number or any other settings Prepaid and post paid billing issues etc TOOLS USED IN TSG 1. Quick HLR 2.
FSSAT ICRM PACS Supervisor
7. 8. • • • •
CELL SITE The cell site or cell tower is a cellular telephone site where antenna and electronic communication equipment are placed. The site is connected to the BTS through the feeder cables. It is responsible for the transmission and reception of the signals. each site has two types of antennas GSM antenna and Microwave antenna.
• • • •
The number of GSM antenna depends upon on the number of sectors. Each cell site has some sectors on which the GSM antenna are placed through which the signal transmission takes place. The microwave antenna is used for the connection between two towers. The sited are divided into different types based on the construction and number of sectors/coverage. GSM ANTENNA
The GSM antenna or a sector antenna is a directional microwave antenna with a sector shaped radiation pattern. At the bottom, there are RF connectors and coaxial cables and adjustment mechanics. The main reflector screen is produced from aluminium and all the internal parts are housed into fibreglass enclosure to keep its operation stable regardless of the weather conditions Typically the BTS contains 3 sector antennas will have the 66 degrees of horizontal beamwidth. This means the signal strength at the ±33 degrees is half of its peak value at the centre. There is usually a downward beam tilt so that the base station can more effectively cover its immediate area and not cause RF interference The coverage area which is equal to the square of the sector’s projection to the ground can be effectively adjusted by changing electrical and mechanical tilt. Electrical tilt is set by using a special control unit, which is usually built into the antenna case, through different remote control devices. Mechanical tilt is set manually by adjusting an antenna fastener. To increase or widen the coverage area, several sector antennas are installed on the same supporting structure. Once it is attached, then it has to be positioned for good data rates and the consistency of the good signal strength with the coverage area. Prior to the positioning, the grounding and lightening protection is required.
MICROWAVE ANTENNA Microwave antenna is used for point to point communications because their small wavelength allows them to direct in narrow beams, which can be pointed directly at the receiving antenna. Due to high frequencies, the microwave band has the very large information carrying capacity. Microwave radio transmission is used to transmit and receive the analog or digital signals using a line of sight radio path. But the line of sight limits the distance. It cannot pass through the mountain and hilly areas
Line Of Sight: it is a type of propagation that can transmit and receive data only where transmit and receive stations are in view of each other without any sort of an obstacle between them. FM radio, microwave and satellite transmission are examples of line-of-sight communication The cell sites are classified into three types based on the type of tower Ground Base Towers. The structure or the tower is built on the ground. Generally seen in the rural areas where there are no high buildings
Roof Top Towers. They are present on the already existing buildings. The tower is built on the roof to match the required height
Roof Top Poles. They are placed on the buildings where the height increment is not required. The GSM antenna is installed on the pole and placed on the building
DRIVE TESTING Drive testing is a method of measuring and assessing the coverage, capacity and Quality of Service (QoS) of a mobile radio network. The technique consists of using a motor vehicle containing mobile radio network air interface measurement equipment that can detect and record a wide variety of the physical and virtual parameters of mobile cellular service in a given geographical area. By measuring what a wireless network subscriber would experience in any specific area, wireless carriers can make directed changes to their networks that provide better coverage and service to their customers. Drive testing requires a mobile vehicle outfitted with drive testing measurement equipment. The equipment is usually highly specialized electronic devices that interface to OEM mobile handsets. This ensures measurements are realistic and comparable to actual user experiences. The data set collected during drive testing field measurements can include information such as: Signal intensity, Signal quality, Interference, Dropped calls, Blocked calls, Anomalous events, Call statistics, Service level statistics, Quality of Service information, Handover information, Neighboring cell information, GPS location co-ordinates.
In short, the internship I had at BSNL Regional Telecom Training Centre was very much helpful in learning the concepts of 1. GSM technology 2. Call flow in GSM 3. packet core of GPRS, UMTS and LTE 4. Optical fiber communications 5. Intelligent Network and Value Added Services 6. TSG operations 7. Cell site and types of sites 8. Drive test. It has been a great opportunity to know about the various techniques which are used in networking. The concept of how cellular generations are evolved, how a call is established and how it terminates are very much interesting. I am grateful to those people who helped me in the successful completion of the internship and this will surely be helpful to me in my academics.