12/05/2013 11:27:00 am
Sarkari Job
Q1-How Does a Mobile Network Work?
A mobile network, also referred to as a cellular network,
employs the use of radio frequencies that can be used simultaneously by several
callers at one and the same time. Cell-sites and mobile devices manipulate the
frequency, so that they can make use of low-power transmitters to supply their
services with the least possible interference.
Mobile
carriers use these mobile networks to
offer wide network coverage to their subscribers. Larger regions are split up
into smaller cells, all of which are connected to telephone switches or
telephone exchanges, which again help in public telecommunication while on the
move.
Different types of mobile technologies are used to provide
mobile network services to users. Most common among them are GSM (Global System
for Mobile Communication), GPRS (General Packet Radio Service), CDMA (Code
Division Multiple Access), EDGE (Enhanced Data Rates for GSM Evolution), iDEN
(Integrated Digital Enhanced Network) and EV-DO (Evolution-Data Optimized).
The signal reception and/or call service quality may be
subject to change, depending upon the current location and region of the user.
Some of the leading mobile network providers in the United States of America
are Verizon Wireless, AT&T, T-Mobile and Sprint Nextel.
Q2-Functions of
bts and bsc in a gsm architecture
Base Station Subsystem
The BSS provides the interface between the ME and the NSS.
It is in charge of the transmission and reception. It may be divided into two
parts:
- Base
Station Controller (BSC): It
controls a group of BTSs and manages their radio ressources. A BSC is
principally in charge of handoffs, frequency
hopping, exchange functions and power control over
each managed BTSs.
- Base
Transceiver Station (BTS) or Base Station: it maps to transceivers and antennas used
in each cell of the network. It is usually placed in the
center of a cell. Its transmitting power defines the size of a
cell. Each BTS has between 1-16 transceivers depending on the density
of users in the cell.
Base transceiver station
A base
transceiver station (BTS)
is a piece of equipment that facilitates wireless communication between user
equipment (UE) and a network.
UEs are devices like mobile
phones (handsets), WLL phones, computers with wireless
Internet connectivity. The
network can be that of any of the wireless communication technologies like GSM, CDMA, wireless local loop, Wi-Fi, WiMAX or other wide
area network (WAN)
technology.
BTS is also referred to as the radio base station (RBS), node B (in 3G Networks) or, simply, the base station (BS). For discussion of the LTE standard
the abbreviation eNB for evolved node B is widely used.
Though the term BTS can be applicable
to any of the wireless communication standards, it is generally associated
with mobile communication technologies like GSM and CDMA. In this regard, a
BTS forms part of the base station subsystem (BSS) developments for system
management. It may also have equipment for encrypting and decrypting communications,
spectrum filtering tools (band pass filters), etc. antennas may also be considered as components
of BTS in general sense as they facilitate the functioning of BTS. Typically a
BTS will have several transceivers (TRXs) which allow it to serve several
different frequencies and different sectors of the cell (in the case of
sectorised base stations). A BTS is controlled by a parent base station controller via the base station control function
(BCF). The BCF is implemented as a discrete unit or even incorporated in a TRX
in compact base stations. The BCF provides an operations and maintenance
(O&M) connection to the network management system (NMS), and manages operational states
of each TRX, as well as software handling and alarm collection. The basic structure and
functions of the BTS remains the same regardless of the wireless technologies
Q3-Handover
In cellular telecommunications, the term handover or handoff refers to the process of transferring
an ongoing call or data session from one channel connected to the core
network to
another channel. In satellite communications it is the process of transferring
satellite control responsibility from one earth station to another without loss or
interruption of service.
Handover is the mechanism that transfers an ongoing call from
one cell to another as a user moves through the coverage area of a cellular
system. The number of cell boundary crossings increases because smaller cells
are deployed in order to meet the demands for increased capacity.
If we minimize the expected number of handovers the
switching load minimizes as well, because each handover requires network resources to reroute the call to the new base station.
In GSM, measurement reports, which are transmitted periodically from MS to BS
on the SACCH assigned to each communication, are available for each connection.
The repetition duration of the SACCH produces a fixed time grid of 480 ms
in which the measurement reports occur.
In above Figure the measured RXLEVs from the serving BTS and
from a neighborone
(NC1), according to the measurement reports submitted during a call, are shown.
The horizontal axis represents the number of measurement reports.
Obviously the handover procedure
consider a set of parameters in such a way to avoid shortcomings. On the other
hand, it is not possible to have a safe handover execution in cases like the
one shown in Figure above, since the location and direction of user as well as
the area characteristics are not known. In many cases the execution takes place
and after a couple of measurement reports the handover procedure is triggered
again.
There are several different reasons for a handover. Each
mobile terminal attempts to use the radio channel that will provide the best connection
quality, i.e., the best C/I (carrier-to interference ratio). Co-channel interference is unavoidable because
of multiple use of the same time and frequency channels due to existing cell layouts, and
consequently quality can be poor (i.e., bit-error ratio high) despite a
high signal level.
The connection of a mobile terminal to the base stations can
be the cause of interference to other mobile stations, even if it is a
high-quality one. The interference can be minimized if the interfered station
changes to a different radio channel. It is also possible for mobile users to
have the same good receive quality from more than one cell. The service quality of the network can then be optimized if mobile
users are equally distributed over the available cells.
The following pie-chart summarizes
the handover causes, showing the percentage of the different reasons for
handover:
In order to measure the handover
performance in a cellular network several counters are used. As far as the
procedure is concerned, each counter is triggered when a Handover Required
message, containing the respective cause, is routed from the BSC to the MSC
(inter-BSC handover). When the handover is internal (intra-cell or inter-cell,
intra-BSC handover), the procedure (decision and execution, respective cause
counted) is undertaken by the responsible BSC and the MSC is informed by a
Handover Performed message.
An in-depth statistical evaluation
shows that, there are several shortcomings in the normal handover procedure.
The major ones are the following:
·
High failure of handovers, due to an
insufficient planning in certain areas.
·
“Far-away-cell”, where subscribers
are served from a BTS that is far away from the cell where the user is located
·
“Ping-pong” effect, the repeated
handover between two base stations caused by rapid fluctuations in the received
signal strengths from both base stations.
·
Unnecessary handover often leads to
increased signaling traffic, which can result in traffic congestion in the
call-setup procedure of other subscribers intending to set up calls.
Q4-What is
cdma2000
CDMA2000 (also known as C2K or IMT Multi‑Carrier (IMT‑MC))
is a family of 3G mobile technology standards, which use CDMA channel access,
to send voice, data, and signaling data between mobile
phones and cell
sites. The name CDMA2000 actually denotes
a family of standards that represent the successive, evolutionary stages of the
underlying technology. These are, in order of evolution:
·
CDMA2000 1xRTT
·
CDMA2000 1xEV-DO: Release 0,
Revision A, Revision B
·
CDMA2000 1xEV-DO Revision C or Ultra
Mobile Broadband (UMB)
·
CDMA2000 1xEVDV
All
are approved radio interfaces for the ITU's IMT-2000. CDMA2000 has a relatively long technical history and
is backward-compatible with
its previous2G iteration IS-95 (cdmaOne). In the United States, CDMA2000 is
a registered trademark of the Telecommunications
Industry Association (TIA-USA)
CDMA2000 is a code-division multiple
access (CDMA)
version of the IMT-2000 standard developed by the International
Telecommunication Union (ITU).
The CDMA2000 standard is a 3G mobile technology.
The CDMA2000 family of standards includes 1xRTT, EV-DO Rev 0, EV-DO Rev A and EV-DO Rev B
(now called Ultra Mobile Broadband -- UMB). The CDMA2000 family of standards is
deployed by Verizon Wireless and Sprint in the U.S. and uses CDMA
technology as the underlying
multiplexing scheme. CDMA2000 is often confused with CDMA technology itself.
CDMA2000 has several advantages:
Stronger signal: CDMA2000 has
the ability to use signals that arrive in the receivers with different time
delays -- known as multipath. It uses the multipath signals and combines them
to make the cellular signal stronger.
Drop-offs and breakups:
Drop-offs occur only when the mobile device is two times further from the
cellular base station. CDMA networks use a scheme called soft handoff, which
minimizes signal breakup as a handset passes from one cell to another.
Analog capabilities: In rural
areas of the U.S., CDMA2000 offers analog capabilities that GSM does not.
Capacity: CDMA2000 has a very
high spectral capacity, so it can accommodate more users per MHz of bandwidth.
Noise reduction: CDMA2000 uses
an exclusive technology called vocoder
EVRC which reduces background
noise.
CDMA2000 has a few disadvantages:
Channel pollution: One major
problem with CDMA2000 is channel pollution, where there are too many signals
from cell sites in the subscriber's phone, but none is dominant -- degrading
call quality.
International roaming: Another
disadvantage of this technology is the lack of international roaming
capabilities and the only CDMA2000 devices that can be used internationally
must also have a GSM radio. If you have mobile users who travel overseas you
may want to consider a dual-mode
mobile device, because it offers the most flexible solution for
international mobile users.
Remote activation: CDMA2000
devices are activated remotely, by the carrier, using the phone's electronic
serial number (ESN). Since each carrier has a database of all the ESNs that are
approved for its network, this lets most CDMA carriers refuse to activate
phones not originally intended for their network.
In the U.S., CDMA2000 and GSM are
currently the competing cellular phone standards. They are about equal in the
U.S. in terms of users; but, internationally, 85% of the mobile users employ
GSM. The future battle in cellular communications will
be between WiMAX and Long Term Evolution (a GSM technology).
It is important for mobile managers
to realize that -- in the long term -- the CDMA2000 family will be phased out.
Verizon is abandoning CDMA2000 and moving to the GSM family for its 4G LTE
network, and Sprint is using WiMAX for its 4G rollout.
Mobile managers need to consider such
factors as coverage, performance, international roaming, mobile device
selection and price when planning or modifying their mobile strategy. This is a
major consideration for enterprises with distributed offices, employees and
international business travelers when planning their mobile communications strategy.
Q5-Diversity
techniques
In telecommunications,
a diversity scheme refers to a method for improving the
reliability of a message signal by using two or more communication channels with different characteristics. Diversity plays an
important role in combatting fading and co-channel interference and avoiding error
bursts. It is based on the fact that
individual channels experience different levels of fading and interference.
Multiple versions of the same signal may be transmitted and/or received and
combined in the receiver. Alternatively, a redundant forward error correction code may be added and different parts of the message
transmitted over different channels. Diversity techniques may exploit the multipath propagation, resulting in a diversity
gain, often measured in decibels.
The
following classes of diversity schemes can be identified:
·
Time
diversity: Multiple versions of the same
signal are transmitted at different time instants. Alternatively, a
redundant forward error correction code is added and the message is spread in time by means
of bit-interleaving before it is transmitted. Thus, error
bursts are avoided, which simplifies
the error correction.
·
Frequency diversity: The signal is transmitted using several frequency channels
or spread over a wide spectrum that is affected by frequency-selective fading. Middle-late 20th century microwave radio relay lines often used several regular wideband radio channels, and one protection channel for
automatic use by any faded channel. Later examples include:
·
OFDM modulation
in combination with subcarrier interleaving and forward error correction
·
Spread
spectrum, for example frequency
hopping or DS-CDMA.
·
Space
diversity: The signal is transmitted over
several different propagation paths. In the case of wired transmission, this
can be achieved by transmitting via multiple wires. In the case of wireless
transmission, it can be achieved by antenna
diversity using multiple transmitter
antennas (transmit diversity)
and/or multiple receiving antennas (reception diversity).
In the latter case, a diversity combining technique
is applied before further signal processing takes place. If the antennas are
far apart, for example at different cellular base station sites or WLAN access
points, this is called macrodiversity or site
diversity. If the antennas are at a distance
in the order of one wavelength, this is called microdiversity. A special case is phased antenna
arrays, which also can be used for beamforming, MIMO channels
andspace–time coding (STC).
·
Polarization diversity: Multiple versions of a signal are transmitted and received
via antennas with different polarization. A diversity combining technique
is applied on the receiver side.
·
Multiuser diversity: Multiuser diversity is obtained by opportunistic user
scheduling at either the transmitter or the receiver. Opportunistic user
scheduling is as follows: at any given time, the transmitter selects the best
user among candidate receivers according to the qualities of each channel
between the transmitter and each receiver. A receiver must feed back the
channel quality information to the transmitter using limited levels of
resolution, in order for the transmitter to implement Multiuser diversity.
·
Cooperative diversity: Achieves antenna diversity gain by using the cooperation
of distributed antennas belonging to each node.
There
are several types of receiver diversity methods
Time
Diversity
Frequency
Diversity
Multiuser
Diversity
Space
Diversity
Q6-what is wmax
WiMAX is a wireless technology put forth by the WiMAX Forum that is one of the technologies that is being used for 4G networks. It can be used in both point to point and the typical WAN type configurations that are also used by 2G and 3G mobile network carriers. Its formal name is IEEE standard 802.16. Sprint owns a WiMAX based network that is marketed under the name XOHM, though that will eventually be merged with Clearwire's network and sold under the Clearwire name. LTE is a competing technology that has the support of far more carriers worldwide.Architecture of wimax
The IEEE
802.16e-2005 standard provides the air interface for WiMAX but does not define
the full end-to-end WiMAX network. The WiMAX Forum's Network Working Group
(NWG) is responsible for developing the end-to-end network requirements,
architecture, and protocols for WiMAX, using IEEE 802.16e-2005 as the air
interface.
The WiMAX
NWG has developed a network reference model to serve as an architecture
framework for WiMAX deployments and to ensure interoperability among various
WiMAX equipment and operators.
The
network reference model envisions a unified network architecture for supporting
fixed, nomadic, and mobile deployments and is based on an IP service model.
Below is simplified illustration of an IP-based WiMAX network architecture. The
overall network may be logically divided into three parts:
- Mobile Stations (MS) used by
the end user to access the network.
- The access service network
(ASN), which comprises one or more base stations and one or more ASN
gateways that form the radio access network at the edge.
- Connectivity service network
(CSN), which provides IP connectivity and all the IP core network
functions.
The
network reference model developed by the WiMAX Forum NWG defines a number of
functional entities and interfaces between those entities. Fig below shows some
of the more important functional entities.
- Base station (BS): The BS is responsible for providing the air
interface to the MS. Additional functions that may be part of the BS are
micromobility management functions, such as handoff triggering and tunnel
establishment, radio resource management, QoS policy enforcement, traffic
classification, DHCP (Dynamic Host Control Protocol) proxy, key
management, session management, and multicast group management.
- Access service network gateway
(ASN-GW): The ASN gateway typically
acts as a layer 2 traffic aggregation point within an ASN. Additional
functions that may be part of the ASN gateway include intra-ASN location
management and paging, radio resource management, and admission control,
caching of subscriber profiles, and encryption keys, AAA client
functionality, establishment, and management of mobility tunnel with base
stations, QoS and policy enforcement, foreign agent functionality for
mobile IP, and routing to the selected CSN.
- Connectivity service network
(CSN): The CSN provides
connectivity to the Internet, ASP, other public networks, and corporate
networks. The CSN is owned by the NSP and includes AAA servers that
support authentication for the devices, users, and specific services. The
CSN also provides per user policy management of QoS and security. The CSN
is also responsible for IP address management, support for roaming between
different NSPs, location management between ASNs, and mobility and roaming
between ASNs.
The WiMAX
architecture framework allows for the flexible decomposition and/or combination
of functional entities when building the physical entities. For example, the
ASN may be decomposed into base station transceivers (BST), base station
controllers (BSC), and an ASNGW analogous to the GSM model of BTS, BSC, and
Serving GPRS Support Node (SGSN).
