Vodafone Interview 26/10/2015 Monday 11.30 AM

The E1 signal format that carries data at a rate of 2(2.048) Mbps and can carry 32 channels of 64 Kbps

each.

An E1 can carry 32 voice channels and a T1(used in North America) can carry data at a rate of

1.5(1544)Mbps and can carry 24 voice channels each with of 64 kbps speed.

An Erlang is a unit of telecommunications traffic measurement. Strictly speaking, an Erlang represents the continuous use of one voice path. In practice, it is used to describe the total traffic volume of one hour.

For example, if a group of user made 30 calls in one hour, and each call had an average call duration of 5 minutes, then the number of Erlangs this represents is worked out as follows:

Minutes of traffic in the hour = number of calls x duration

Minutes of traffic in the hour = 30 x 5

Minutes of traffic in the hour = 150

Hours of traffic in the hour = 150 / 60

Hours of traffic in the hour = 2.5

Traffic figure = 2.5 Erlangs

BCH - Broadcasting Channel

A Broadcast Channel (BCH) is a downlink channel in a GSM system that is used by the base stations to provide signalling information to the mobile stations. The mobile station needs this

information to find a network, synchronize with it and to connect to it.

There are three types of broadcasting channels:

1. Broadcast Control Channel (BCCH), 2. Synchronisation Channel (SCH),

3. Frequency Correction Channel (FCCH). 4. Cell Broadcast Channel(CBCH)

Basic idea is:

frame = information + error protection (to be sent)

carrier = a frequency which "carries" this frame of information

channel = the medium which the carrier goes thru to get to the receiver (can be radio channel, FO,

coaxial)

GSM: Physical & logical Channels

GSM: PHYSICAL AND LOGICAL CHANNELS

GSM uses a mix of Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA). FDMA parts involves the division by frequency of the 25 MHz bandwidth in to 124 carrier frequencies (Also called ARFCN) spaced 200 KHz for GSM-900. For GSM-1800

frequency spectrum of 75 MHz bandwidth is divided in to 374 carrier frequencies spaced 200 KHz. TDMA further divides each carrier frequencies in to 8 time slots such that each carrier

frequency is shared by 8 users. So in GSM, the basic radio resource is a time slot with duration of 577 µs. 8 Time slots of 577 µs constitutes a 4.615 ms TDMA Frame. GSM uses Gaussian Minimum Shift Keying (GMSK) modulation scheme to transmit information (data and

signalling) over Air Interface.

GSM uses number of channels to carry data over Air Interface, these channels are broadly divided in to following two categories:

1. Physical Channels

2. Logical Channels

GSM-PHYSICAL_AND_LOGICAL_CHANNELS.pdf

PHYSICAL CHANNELS

A physical channel is determined by the carrier frequency.

8 Time Slots (1 Time Slot = 1 Physical Channel) of 577 µs constitutes a 4.615 ms TDMA Frame. In GSM standard data on a time slot transmitted in bursts, so time slot is often expressed in BP

(Burst Period). 1 BP represents 1 TS. TDMA frame (4.615 ms of 8 TS) further structured in to multiframes. There are two types of multiframes in the system:

26 TDMA Multiframe: Consists 26 TDMA frames with duration of 120 ms and used to

carry the Logical Channels TCH, SACCH, FACCH etc. 51 TDMA Multiframe: Consists 51 TDMA frames with duration of 234.5 ms and used to

carry the Logical Channels FCCH, SCH, BCCH, CCCH, SDCCH, SACCH etc.

These multiframes further structured in to Superframe and Hyperframe.

SUPERFRAME: Superframe consists of 51*26 TDMA frames with duration of 6.12 sec. This

is corresponding to the smallest cycle for which the organisation of all channels is repeated.

HYPERFRAME: Hyperframe consists 2048 superframes (2048*51*26 TDMA frames) with duration of 3 hrs, 28 min, 53 sec and 760 ms. It is in particular smallest cycle for frequency

hopping, cyphering.

The frame hierarchy is used for synchronisation between BTS and MS.

Multiframes in GSM

LOGICAL CHANNELS

Logical Channels are determined by the information carried within the physical channel. Logical channels used to carry data and signalling information. Different logical channels are mapped in

either direction on physical channels.

Logical channels divided in to following two categories:

Traffic Channels Signalling Channels

TRAFFIC CHANNELS

In GSM system two types of traffic channels used:

Full Rate Traffic Channels (TCHF): This channel carries information at rate of 22.8

Kbps. Half Rate Traffic Channels (TCHH): This channels carries information at rate of 11.4

Kbps.

SIGNALLING CHANNELS

Signalling channel carries control information to enable the system to operate correctly. There are three main categories of signalling channels in GSM which are further divided in several

categories:

1. BROADCAST CHANNELS (BCH)

Broadcast Control Channel (BCCH) Frequency Correction Channel (FCCH)

Synchronization Channel (SCH) Cell Broadcast Channel (CBCH)

2. COMMON CONTROL CHANNELS (CCCH)

Paging Channel (PCH)

Random Access Channel (RACH) Access Grant Channel (AGCH)

3. DEDICATED CONTROL CHANNELS (DCCH)

Standalone Dedicated Control Channel (SDCCH)

Fast Associated Control Channel (FACCH) Slow Associated Control Channel (SACCH)

satcomm: C-band : U/L-6 GHz, D/L-4 GHz

Ku band: U/L-14 GHz, D/L-12 GHz

mobcomm: GSM-900: U/L-890-915 MHz, D/L- 935-960 MHz

GSM-1800: U/L-1710-1785 MHz, D/L- 1805-1880 MHz

WHY IS THAT SO?

The answer is simple too. It's all about power considerations.

In satcomm, the signals have to cross the atmosphere which presents a great deal of attenuation. The

higher the frequency, the more is the signal loss and more power is needed for reliable transmission.

Now, a satellite is a light-weight device which cannot support high-power transmitters on it. So, it

transmits at a lower frequency (higher the frequency, higher is the transmitter power to accommodate

losses) as compared to the stationary earth station which can afford to use very high-power

transmitters. This is compensated by using highly sensitive receiver circuits on the earth station which is

in the line-of-sight (LOS) of the satellite.

In mobcomm, a similar point holds. A mobile is a portable device which cannot afford high-power

transmission as it has a small battery with limited power. The 'free space path loss' comes to play. The

higher the transmitting frequency, the higher is the loss. Since a mobile station (cellphone) cannot

afford to transmit at high power to compensate for this loss, it must transmit on a lower frequency as a

lower frequency presents lesser free space path loss. Therefore, mobile-to-base station (uplink)

frequencies are lower than base station-to-mobile(downlink) frequencies.

PIU(power interface unit) convert 3 phase ac power supply in

230 volt ac with stablised and no surge and interference

effect. piu mostly used in telecom sector for providing ac

supply to all equipment used in shelter and also control dg

and consist a lot of protection and showing alarm indication

and record events upto 500.

In a symmetric three-phase power supply system, three conductors each carry an alternating current of

the same frequency and voltage amplitude relative to a common reference but with a phase difference

of one third the period. The common reference is usually connected to ground and often to a current -

carrying conductor called the neutral. Due to the phase difference, the voltage on any conductor

reaches its peak at one third of a cycle after one of the other conductors and one third of a cycle before

the remaining conductor. This phase delay gives constant power transfer to a balanced linear load. It

also makes it possible to produce a rotating magnetic field in an electric motor and generate other

phase arrangements using transformers

PMU

PMU or Power Management Unit from PACE has features like remote management, inbuilt microwave provision, power scalability, etc that enhance the fuel

and battery efficiency in the existing sites. PMU follows a rational approach towards power management through

Integrated AMF Function

Alarm Consolidation GPRS Remote

management

AC Power measurement

DG parameter monitoring

DC power Measurement

PACE PMU has an independent server that integrates it with the set of existing processes and NOC. With excellent remote management features of passive

infrastructure and distributed intelligence, scalability and high availability is ensured. PMU manages battery

efficiently and extends its life.

The highlighting feature of PACE PMU is that it reduces the operational costs through a well-researched fuel

management system The fault management and functionality units of PMU are adaptable for Air-conditioners, Batteries, Fire alarms, Diesel Gen sets,

Security, AC Power, DC power, lightening and surge protection, etc.

PACE also offers some excellent modules of Mini PMU

which can either be standalone or wall mounted. They are equipped with micro controllers, surge protection devices, AMF controller, etc for smooth functioning.

Other features of this Mini PMU are:

AC Distribution unit Fire Alarm Module Aviation Lamp Controller Static Voltage Regulator(SVR) DG Battery Charger

Air Conditioner Controller (Optional) Auto Phase selector (Optional) Fuel Optimizer Energy Meter Box for the DG Display unit

Product Features:

Over Voltage & under Voltage protections using HVD & LVD respectively. Wide Range of AC Input (150-275VAC)

Different DC Outputs available (12V, 24V, 48V, 110V)

Modular systems, easy for maintenance & servicing.

Digital Display for Input / Output Voltage & Current.

Remote Monitoring through (4-20mA) Transducers.

(N+1) or more Redundancy available.

TEC approved systems.

Optional Microprocessor controller.

Hot Swappable Rectifier Modules.

Detachable Additional Controller to ensure break-free System operation

GSM Network Architecture

Architecture of GSM network

A GSM network is composed of several functional entities, Below diagram illustrates the layout of a generic GSM

network. The GSM network can be divided into three broad parts.

· The Mobile Station

Performs the switching of calls between the mobile users, and between mobile and fixed network users

· The Base Station Subsystem

BSS Controls the radio link with the Mobile Station.

· The Network Subsystem

Network subsystem includes the MSC,VLR,HLR

The Mobile Station and the Base Station Subsystem communicate across the Um interface, also known as the air

interface or radio link. The Base Station Subsystem communicates with the Mobile services Switching Center across

the A interface.

Mobile Station

The mobile station (MS) consists of the mobile equipment (the terminal) and a smart card called the Subscriber

Identity Module (SIM). The SIM provides personal mobility, so that the user can have access to subscribed services

irrespective of a specific terminal. By inserting the SIM card into another GSM terminal, the user is able to receive

calls at that terminal, make calls from that terminal, and receive other subscribed services.

The mobile equipment is uniquely identified by the International Mobile Equipment Identity (IMEI). The SIM card

contains the International Mobile Subscriber Identity (IMSI) used to identify the subscriber to the system, a secret key

for authentication, and other information. The IMEI and the IMSI are independent, thereby allowi ng personal mobility.

The SIM card may be protected against unauthorized use by a password or personal identity number.

Base Station Subsystem

The Base Station Subsystem is composed of two parts, the Base Transceiver Station (BTS) and the Base Station

Controller (BSC). These communicate across the standardized Abis interface, allowing (as in the rest of the system)

operation between components made by different suppliers.

The Base Transceiver Station houses the radio tranceivers that define a cell and hand les the radio-link protocols with

the Mobile Station. In a large urban area, there will potentially be a large number of BTSs deployed, thus the

requirements for a BTS are ruggedness, reliability, portability, and minimum cost.

The Base Station Controller manages the radio resources for one or more BTSs. It handles radio-channel setup,

frequency hopping, and handovers, as described below. The BSC is the connection between the mobile station and

the Mobile service Switching Center (MSC).

Network Subsystem

The central component of the Network Subsystem is the Mobile services Switching Center (MSC). It acts like a

normal switching node of the PSTN or ISDN, and additionally provides all the functionality needed to handle a mobile

subscriber, such as registration, authentication, location updating, handovers, and call routing to a roaming

subscriber. These services are provided in conjuction with several functional entities, which together form the

Network Subsystem. The MSC provides the connection to the fixed networks (such as the PSTN or ISDN). Signalling

between functional entities in the Network Subsystem uses Signalling System Number 7 (SS7), used for trunk

signaling in ISDN and widely used in current public networks.

The Home Location Register (HLR) and Visitor Location Register (VLR), together with the MSC, provide the call -

routing and roaming capabilities of GSM. The HLR contains all the administrative information of each subscriber

registered in the corresponding GSM network, along with the current location of the mobile. The location of the mobile

is typically in the form of the signalling address of the VLR associated with the mobile station. The actual routing

procedure will be described later. There is logically one HLR per GSM network, although it m ay be implemented as a

distributed database.

The Visitor Location Register (VLR) contains selected administrative information from the HLR, necessary for call

control and provision of the subscribed services, for each mobile currently located in the geographical area controlled

by the VLR. Although each functional entity can be implemented as an independent unit, all manufacturers of

switching equipment to date implement the VLR together with the MSC, so that the geographical area controlled by

the MSC corresponds to that controlled by the VLR, thus simplifying the signalling required. Note that the MSC

contains no information about particular mobile stations --- this information is stored in the location registers.

The other two registers are used for authentication and security purposes. The Equipment Identity Register (EIR) is a

database that contains a list of all valid mobile equipment on the network, where each mobile station is identified by

its International Mobile Equipment Identity (IMEI). An IMEI is marked as invalid if it has been reported stolen or is not

type approved. The Authentication Center (AuC) is a protected database that stores a copy of the secret key stored in

each subscriber's SIM card, which is used for authentication and encryption over the radio channel.

Network Aspects

A GSM mobile can seamlessly roam nationally and internationally, which requires that registration, authentication,

call routing and location updating functions exist and are standardized in GSM networks. In additio n, the fact that the

geographical area covered by the network is divided into cells necessitates the implementation of a handover

mechanism. These functions are performed by the Network Subsystem, mainly using the Mobile Application Part

(MAP) built on top of the Signalling System No. 7 protocol (SS7 or

The signalling protocol in GSM is structured into three general layers, depending on the interface, as shown in

Figure. Layer 1 is the physical layer, which uses the channel structures discussed above over the air interface. Layer

2 is the data link layer. Across the Um interface, the data link layer is a modified version of the LAPD protocol used in

ISDN, called LAPDm. Across the A interface, the Message Transfer Part layer 2 of Signalling System Number 7 i s

used. Layer 3 of the GSM signalling protocol is itself divided into 3 sublayers.