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Q1: Is a directional antenna useful for mobile phones? Why? How can the gain of antenna be improved?
Ans : Antennas couple electromagnetic energy to and from space
to and from a wire or coaxial cable (or any other appropriate conductor). A theoretical reference antenna is the isotropic radiator, a point in space radiating equal power in all directions, i.e., all points with equal power are located on a sphere with the antenna as its center. The radiation pattern is symmetric in all directions .
However, such an antenna does not exist in reality. Real antennas all
exhibit directive effects, i.e., the intensity of radiation is not the same in all directions from the antenna. The simplest real antenna is a thin, center-fed dipole, also called Hertzian dipole.
The dipole consists of two collinear conductors of equal length, separated by a small feeding gap. The length of the dipole is not arbitrary, but, for example ,half the wavelength λ of the signal to transmit results in a very efficient radiation of the
energy. If mounted on the roof of a car, the length of λ/4 is efficient. This is also known as Marconi antenna.
A λ/2 dipole has a uniform or omni-directional radiation pattern in one plane and a figure eight pattern in the other two planes . This type of antenna can only
overcome environmental challenges by boosting the power level of the signal. Challenges could be mountains, valleys, buildings etc.
Q2: What are the main problems of signal propagation? Why do radio waves not always follow a straight line? Why is reflection both useful & harmful?
Ans:
In wireless networks, the signal has no wire to determine the direction of
propagation, whereas signals in wired networks only travel along the wire (which can be twisted pair copper wires, a coax cable, but also a fiber etc.). As long as the wire is not interrupted or damaged, it typically exhibits the same characteristics at each point. One can precisely determine the behavior of a signal travelling along
this wire, e.g., received power depending on the length. For wireless transmission, this predictable behavior is only valid in a vacuum, i.e., without matter between the sender and the receiver.
Transmission range: Within a certain radius of the sender transmission is possible, i.e., a receiver receives the signals with an error rate low enough to be able to communicate and can also act as sender.
Detection range: Within a second radius, detection of the transmission is possible, i.e., the transmitted power is large enough to differ from background noise.
However, the error rate is too high to establish communication.
Interference range: Within a third even larger radius, the sender may interfere with other transmission by adding to the background noise. A receiver will not be able to detect the signals, but the signals may disturb other signals.
Radio signal propagation faces the following problems:
• Attenuation (amplitude of the wave loses strength thereby the signal power) • Refraction
• Reflection • Shadowing • Scattering • Diffraction
Radio waves do not follow a straight line because of blocking objects in its path. Reflection is useful because in non-line-of-sight environments (where there is no direct path from the transmitter to receiver for example in offices, town and cities) it allows the radio signal to reach from the transmitter to the receiver. Reflection can be harmful because multiple copies of the same signal can reach the receiver at different times.
Q3: What are the main reasons for using cellular systems? How is SDM typically realized and combined with FDM? How does DCA influnence the frequencies available in other cells?
Answer
:-Cellular systems for mobile communications implement SDM. Each transmitter, typically called a base station, covers a certain area, a cell. Cell radii can vary from tens of meters in buildings, and hundreds of meters in cities, up to tens of kilometers in the countryside. The shape of cells are never perfect circles or
hexagons , but depend on the environment (buildings, mountains, valleys etc.), on weather conditions, and sometimes even on system load. Typical systems using this approach are mobile telecommunication systems , where a mobile station
within the cell around a base station communicates with this base station and vice versa.
Advantages of cellular systems with small cells are the following:
● Higher capacity: Implementing SDM allows frequency reuse. If one transmitter is far away from another, i.e., outside the interference range, it can reuse the same frequencies. As most mobile phone systems assign frequencies to certain users (or certain hopping patterns), this frequency is blocked for other users. But frequencies are a scarce resource and, the number of concurrent users per cell is very limited. Huge cells do not allow for more users. On the contrary, they are limited to less possible users per km2. This is also the reason for using very small cells in cities where many more people use mobile phones.
● Less transmission power: While power aspects are not a big problem for base stations, they are indeed problematic for mobile stations. A receiver far away from a base station would need much more transmit power than the current few Watts. But energy is a serious problem for mobile handheld devices.
● Local interference only: Having long distances between sender and receiver results in even more interference problems. With small cells, mobile stations and base stations only have to deal with ‘local’ interference.
● Robustness: Cellular systems are decentralized and so, more robust against the failure of single components. If one antenna fails, this only influences
communication within a small area.
Small cells also have some disadvantages:
Infrastructure needed: Cellular systems need a complex infrastructure to connect all base stations. This includes many antennas, switches for call forwarding,
location registers to find a mobile station etc, which makes the whole system quite expensive.
Handover needed: The mobile station has to perform a handover when changing from one cell to another. Depending on the cell size and the speed of movement, this can happen quite often.
Frequency planning: To avoid interference between transmitters using the same frequencies, frequencies have to be distributed carefully. On the one hand,
interference should be avoided, on the other, only a limited number of frequencies is available.
Higher capacity, higher number of the users: cellular systems can reuse spectrum according to certain patterns. Each cell can support a maximum number of users. Support user localisation and location based services: Less transmission power needed. Smaller cells also allow for less transmission power ( thus less radiation). The mobile systems can enjoy longer runtime.
Typically each cell holds a certain number of frequency bands. Neighboring cells are not allowed to use the same
frequencies.
Whether or not DCA depends on the current load. It can react upon sudden increase in traffic by borrowing capacity from other cells. However the ”borrowed” frequency must then be blocked in neighboring cells.
Q4: What limits the number of simultaneous users in a TDM/FDM system compared to a CDM system?
What happens to the transmission quality of connections if the load gets higher in a cell, i.e., how does an additional user influence the other users in the cell, for both TDM/FDM and CDM systems?
Ans: FDM/TDM system have a hard upper limit of simultaneous users. The system assigns a certain time-slot at a certain frequency to a user. If all time-slots at all frequencies are occupied no more users can be accepted. Compared to this ”hard capacity” a CDM system has a so-called ”soft-capacity”. The signal-to-noise-ratio typically limits the number of simultaneous users.
The system can always accept an additional user. However, the noise level may then increase above a certain threshold where transmission is impossible. In TDM/FDM systems additional users, if accepted, do not influence other users as users are separated in time and frequency( well, there is some interference;
however,this can be neglected in this context).
In CDM systems, each additional user decreases transmission quality of all other users.
Q.5 Recall the problem of hidden and exposed terminals. What is the effect of such terminals if Aloha, slotted Aloha, reservation Aloha, or MACA is used? Ans:
• In Aloha, hidden station is a serious problem. Stations start sending their data, and because of runtime to the satellite, it needs long time till the satellite has repeated the information and sent it down to all stations. Thus there can be lots of hidden stations we will learn about only very late. On the other hand, exposed
stations don’t exist – a potential sender does not listen to the medium before sending, thus he cannot be exposed.
• For Slotted Aloha, it is the same as for traditional Aloha.
• For Reservation-Aloha, both problems don’t exist. Because we work with
reservations, all other stations know about ours at well at their own sending times. Only in the reservation phase we can have problems with placing reservations, but because we will recognize if someone else tried to do the same reservation as we, the other stations are not really hidden to us.
• MACA was designed to avoid both, hidden and exposed stations. (Note: exposed stations in principle are avoided because a station which can hear the RTS but not the CTS could interpret it as not influencing the receiver. But in reality, also a station which cannot hear the CTS but hears a transmission begin after an RTS has to wait, because each sender also becomes a receiver – even if only for an ACK after finishing its transmission.) Nevertheless, if we have changing topology (i.e. mobile devices) or asymmetric connections, a station can miss the RTS/CTS messages and send without knowing that something is destroyed.
Q 6. Explain the term interference in the space, time, frequency, and code domain. What are countermeasures in SDMA, TDMA, FDMA, and CDMA systems?
Ans: Interference and countermeasures are:
• SDMA: Interference is overlapping of cells. Just leave a protective distance between base stations and/or devices.
• TDMA: Interference is simultaneous transmission of several stations. Synchronization and time gaps between time slots are countermeasures. • FDMA: Interference means transmission on the same carrier frequency. Countermeasures are protective gaps on the frequency band.
• CDMA: Interference is sending with correlated codes. Thus orthogonal or quasiorthogonal codes have to be used (i.e. the gap in this example is in code orthogonality).
Q 7.What is the main physical reason for the failure of many MAC schemes known from wired networks? What is done in wired networks to avoid this effect?
Ans: Stations in a wired network “hear” each other. I.e., the length of wires is limited in a way that attenuation is not strong enough to cancel the signal. Thus, if one station transmits a signal all other stations connected to the wire receive the signal. The best example for this is the classical Ethernet, 10Base2, which has a bus topology and uses CSMA/CD as access scheme. Today’s wired networks are star shaped in the local area and many direct connections forming a mesh in wide area networks. In wireless networks, it is quite often the case that stations are able to communicate with a central station but not with each other. This lead in the early seventies to the Aloha access scheme (University of Hawaii). So what is CS (Carrier Sense) good for in wireless networks? The problem is that collisions of data packets cause problems at the receiver – but carrier sensing takes place at the sender. In wired networks this doesn’t really matter as signal strength is almost the same (ok, within certain limits) all along the wire. In wireless networks CS and CD at the sender doesn’t make sense, senders will quite often not hear other stations’ signals or the collisions at the receiver.
Q9: Describe the functions of the MS and SIM. Why does GSM sperate the MS & SIM? How & where is User-related data represented/stored in the GSM system?
Mobile station (MS) : The MS comprises all user equipment and softwareneeded for communication with a GSM network. An MS consists of user independent hard- and software and of the subscriber identity module(SIM), which stores all user-specific data that is relevant to GSM.3 While and MS can be identified via the international mobile equipment identity (IMEI), a user can personalize any MS using his or her SIM, i.e., user-specific mechanisms like charging and
authentication are based on the SIM, not on the device itself. Device-specific mechanisms, e.g., theft protection, use the device specific IMEI. Without the SIM, only emergency calls are possible. The SIM card contains many identifiers and tables, such as card-type, serial number, a list of subscribed services, a personal identity number (PIN),
PIN unblocking key (PUK), an authentication key Ki, and the
internationalmobile subscriber identity (IMSI) (ETSI, 1991c). The PIN is used to unlock the MS. Using the wrong PIN three times will lock the SIM. In
suchcases, the PUK is needed to unlock the SIM. The MS stores dynamic information
while logged onto the GSM system, such as, e.g., the cipher key Kc andthe location information consisting of a temporary mobile subscriber
identity(TMSI) and the location area identification (LAI). Typical MSs for GSM 900 have a transmit power of up to 2 W, whereas for GSM 1800 1 W
isenough due to the smaller cell size. Apart from the telephone interface, an3 Many additional items can be stored on the mobile device. However, this is irrelevant to GSM.MS can also offer other types of interfaces to users with display,
loudspeaker,microphone, and programmable soft keys. Further interfaces comprise computermodems, IrDA, or Bluetooth. Typical MSs, e.g., mobile phones,comprise many more vendor-specific functions and components, such ascameras, fingerprint sensors, calendars, address books, games, and Internet browsers. Personal digital assistants (PDA) with mobile phone functions are also available. The reader should be aware that an MS could also be integrated into a car or be used for location tracking of a container.
Q10: How is user data protected from unautheraised access,especially over the air interface? How could the position of an MS be localised? Think of the MS reports regarding signal quality?
Ans: Authentication centre (AuC): As the radio interface and mobile stations are particularly vulnerable, a separate AuC has been defined to protect user identity and data transmission. The AuC contains the algorithms for authentication as well as the keys for encryption and generates the values needed for user authentication in the HLR. The AuC may, in fact, be situated in a special protected part of the HLR.
● Equipment identity register (EIR): The EIR is a database for all IMEIs, i.e., it stores all device identifications registered for this network. As MSs are mobile, they can be easily stolen. With a valid SIM, anyone could use the stolen MS. The EIR has a blacklist of stolen (or locked) devices. In theory an MS is useless as soon as the owner has reported a theft. Unfortunately, the blacklists of different
providers are not usually synchronized and the illegal use of a device in another operator’s network is possible (the reader may speculate as to why this is the case). The EIR also contains a list of valid IMEIs (white list), and a list of malfunctioning devices (gray list).
• Security: GSM offers several security services using confidential information stored in the AuC and in the individual SIM (which is plugged into an arbitrary MS). The SIM stores personal, secret data and is protected with a PIN against unauthorized use. (For example, the secret key Ki used for authentication and encryption procedures is stored in the SIM.) The security services offered by GSM are explained below:
● Access control and authentication: The first step includes the authentication of a valid user for the SIM. The user needs a secret PIN to access the SIM.The next step is the subscriber authentication (see Figure 4.10). This step is based on a challenge-response scheme as presented in section 4.1.7.1.
● Confidentiality: All user-related data is encrypted. After authentication, BTS and MS apply encryption to voice, data, and signaling as shown in section 4.1.7.2.
This confidentiality exists only between MS and BTS, but it does not exist end-to-end or within the whole fixed GSM/telephone network.
● Anonymity: To provide user anonymity, all data is encrypted before
transmission, and user identifiers (which would reveal an identity) are not used over the air. Instead, GSM transmits a temporary identifier (TMSI), which is newly assigned by the VLR after each location update. Additionally, the VLR can change the TMSI at any time. Three algorithms have been specified to provide security services in GSM.
Q11: How is localisation, location update,roaming,etc done in GSM &
reflected in the database? What are typical roaming scenarios? List the enities of mobile IP & describe data transfer from mobile node to a fixed node & vice versa? Why & where encapsulation needed?
Ans:One fundamental feature of the GSM system is the automatic, worldwide localization of users. The system always knows where a user currently is, and the same phone number is valid worldwide. To provide this service, GSM performs periodiclocation updates even if a user does not use the mobile station (provided that the MS is still logged into the GSM network and is not completely switched off). The HLR always contains information about the current location (only the location area, not the precise geographical location), and the VLR currently
responsible for the MS informs the HLR about location changes. As soon as an MS moves into the range of a new VLR (a new location area), the HLR sends all user data needed to the new VLR. Changing VLRs with uninterrupted availability of all services is also called roaming. Roaming can take place within the network of one provider, between two providers in one country (national roaming is,often not supported due to competition between operators), but also between different
providers in different countries (international roaming). Typically, people associate international roaming with the term roaming as it is this type of roaming that
makes GSM very attractive: one device, over 190 countries!To locate an MS and to address the MS, several numbers are needed:
● Mobile node (MN): A mobile node is an end-system or router that can change its point of attachment to the internet using mobile IP. The MN
keeps its IP address and can continuously communicate with any other system in the internet as long as link-layer connectivity is given. Mobile nodes are not necessarily small devices such as laptops with antennas or mobile phones; a router onboard an aircraft can be a powerful mobile node.
● Correspondent node (CN): At least one partner is needed for communication. In the following the CN represents this partner for the MN. The CN
can be a fixed or mobile node.
● Home network: The home network is the subnet the MN belongs to with respect to its IP address. No mobile IP support is needed within the home network.
● Foreign network: The foreign network is the current subnet the MN visits and which is not the home network.
● Foreign agent (FA): The FA can provide several services to the MN during its visit to the foreign network. The FA can have the COA (defined below), acting as tunnel endpoint and forwarding packets to the MN. The FA can be the default router for the MN. FAs can also provide security services because they belong to the foreign network as opposed to the MN which is only visiting. For mobile IP functioning, FAs are not necessarily needed. Typically,
an FA is implemented on a router for the subnet the MN attaches to.
● Care-of address (COA): The COA defines the current location of the MN from an IP point of view. All IP packets sent to the MN are delivered to the
COA, not directly to the IP address of the MN. Packet delivery toward the MN is done using a tunnel, as explained later. To be more precise, the COA marks the tunnel endpoint, i.e., the address where packets exit the tunnel. There are two different possibilities for the location of the COA:
● Foreign agent COA: The COA could be located at the FA, i.e., the COA is an IP address of the FA. The FA is the tunnel end-point and forwards packets to the MN. Many MN using the FA can share this COA as common COA.
● Co-located COA: The COA is co-located if the MN temporarily acquired an additional IP address which acts as COA. This address is now topologically correct, and the tunnel endpoint is at the MN. Co-located addresses
can be acquired using services such as DHCP (see section 8.2). One problem associated with this approach is the need for additional addresses if
MNs request a COA. This is not always a good idea considering the scarcity of IPv4 addresses.
● Home agent (HA): The HA provides several services for the MN and is located in the home network. The tunnel for packets toward the MN starts at the HA. The HA maintains a location registry, i.e., it is informed of the MN’s location by the current COA. Three alternatives for the implementation of an HA exist. ● The HA can be implemented on a router that is responsible for the
home network. This is obviously the best position, because without optimizations to mobile IP, all packets for the MN have to go through
the router anyway.
● If changing the router’s software is not possible, the HA could also be implemented on an arbitrary node in the subnet. One disadvantage of this solution is the double crossing of the router by the packet if the MN is in a foreign network. A packet for the MN comes in via the router; the HA sends it through the tunnel which again crosses the router.
● Finally, a home network is not necessary at all. The HA could be again on the ‘router’ but this time only acting as a manager for MNs belonging to a virtual home network. All MNs are always in a foreign network with this solution.
The example network in Figure 8.1 shows the following situation: A CN is connected via a router to the internet, as are the home network and the foreign network. The HA is implemented on the router connecting the home network with the internet, an FA is implemented on the router to the foreign network. The MN is currently in the foreign network. The tunnel for packets toward the MN starts at the HA and ends at the FA, for the FA has the COA in this example.
IP packet delivery
packet delivery to and from the MN using the example network
of Figure 8.1. A correspondent node CN wants to send an IP packet to the
MN. One of the requirements of mobile IP was to support hiding the mobility of the MN. CN does not need to know anything about the MN’s current location
and sends the packet as usual to the IP address of MN (step 1). This means that CN sends an IP packet with MN as a destination address and CN as a source address. The internet, not having information on the current location of MN, routes the packet to the router responsible for the home network of MN. This is done using the standard routing mechanisms of the internet.
The HA now intercepts the packet, knowing that MN is currently not in its home network. The packet is not forwarded into the subnet as usual, but encapsulated
and tunnelled to the COA. A new header is put in front of the old IP
header showing the COA as new destination and HA as source of the encapsulated packet (step 2). (Tunneling and encapsulation is described in more detail
in section 8.1.6.) The foreign agent now decapsulates the packet, i.e., removes the additional header, and forwards the original packet with CN as source and MN as destination to the MN (step 3). Again, for the MN mobility is not visible. It receives the packet with the same sender and receiver address as it would have done in the home network.
Ans:- 12 Wireless access techinques used are: 1G:- FDMA
2G:- FDMA/ TDMA
2.5G:- TDMA based GSM System/CDMA 3G:- CDMA2000/WCDMA
Three Classes of wireless data networking are: 1. Wireless Personal Area Networks (WPANs) 2. Wireless LAN
3. Wireless MAN 4. WirelessWAN
Q13: Define the roles of WPAN technology in wireless data networking? Ans: IEEE 802.15.4-2003 standard for Low-Rate Wireless Personal Area
Networks (LR-WPANs), such as wireless light switches with lamps, electrical
meters with in-home-displays, consumer electronics equipment via short-range radio needing low rates of data transfer. The technology defined by the ZigBee
specification is intended to be simpler and less expensive than other WPANs, such
as Bluetooth. ZigBee is targeted at radio-frequency (RF) applications that require a
low data rate, long battery life, and secure networking.
WPAN technologies enable users to establish ad hoc, wireless communications for devices (such as PDAs, cellular phones, or laptops) that are used within a personal operating space (POS). A POS is the space surrounding a person, up to a distance of 10 meters. Currently, the two key WPAN technologies are Bluetooth and
infrared light. Bluetooth is a cable replacement technology that uses radio waves to transmit data to a distance of up to 30 feet. Bluetooth data can be transferred
through walls, pockets, and briefcases. Technology development for Bluetooth is driven by the Bluetooth Special Interest Group (SIG), which published the
Bluetooth version 1.0 specification in 1999. Alternatively, to connect devices at a very close range (1 meter or less), users can create infrared links.
To standardize the development of WPAN technologies, IEEE has established the 802.15 working group for WPANs. This working group is developing a WPAN standard, based on the Bluetooth version 1.0 specification. Key goals for this draft standard are low complexity, low power consumption, interoperability, and
coexistence with 802.11 networks.
Q14: List the main features of 3G systems? Ans:Main features of 3G System are:
The most significant features of the 3G technology is that is supports greater voice and data capacity and higher data transfer rate at the lowest cost both in the rural and urban areas. 3G uses the radio spectrum, which allows the transmission of 384 kb/s for the mobile systems and the 2mb/s for the stationary systems. Today more telecommunication networks in the world are being upgraded to the 3G
technologies because of its greater features, scalability, higher voice and data transfer rates and better performance than the 2G tec
Combines a mobile phone, laptop PC and TV Features includes: - Phone calls/fax
- Global roaming
- Send/receive large email messages - High-speed Web
Navigation/maps Videoconferencing - TV streaming
- Electronic agenda meeting reminder. Speed: 144kb/sec-2mb/sec
Time to download a 3min MP3 song: 11sec-1.5min
Q15. What is the role of GPRS in enhancing 2G GSM system? ANS: General Packet Radio Service (GPRS) in GSM
GPRS has been standardized to optimally support a wide range of
applications ranging from very frequent transmission of medium to large data volume and
infrequent transmission of large data volume. Services of GPRS have been developed to
reduce connection setup time and allow an optimum usage of radio resources. GPRS provides
a packet data service for GSM where time slots on the air interface can be assigned to GPRS over which packet data from several mobile stations is multiplexed.
GPRS provides a core network platform for current GSM operators not only to expand
the wireless data market in preparation for the introduction of 3G services, but also a platform
on which to build IMT-2000 frequencies should they acquire them.
GPRS enhances GSM data services significantly by providing end-to-end packetswitched
data connections. This is particularly efficient in Internet/intranet traffic, where short bursts of intense data communications activity are interspersed with relatively long
periods of inactivity. Because there is no real end-to-end connection to be established, setting
up a GPRS call is almost instantaneous and users can be continuously online. Users
have the additional benefit of paying for the actual data transmitted, rather than for connection
time.
Because GPRS does not require any dedicated end-to-end connection, it only uses network
resources and bandwidth when data is actually being transmitted. This means that a given amount of radio bandwidth can be shared efficiently and simultaneously among many
users.
The implementation of GPRS has a limited impact on the GSM core network. It simply
requires the addition of new packet data switching and gateway nodes, and an upgrade
to existing nodes to provide a routing path for packet data between the wireless terminal
and a gateway node. The gateway node provides interworking with external packet data networks
for access to Internet, intranets, and databases.
A GPRS architecture for GSM is shown in Figure…… GPRS will support all widely
used data communications protocols, including IP, so it will be possible to connect with any
data source from anywhere in the world using a GPRS mobile terminal. GPRS will support
applications ranging from low-speed short messages to high-speed corporate LAN communications.
However, one of the key benefits of GPRS—that it is connected through the existing
GSM air interface modulation scheme—is also a limitation, restricting its potential for
delivering data rates higher than 115 kbps. To build even higher rate data capabilities into
GSM, a new modulation scheme is needed.
GPRS can be implemented in the existing GSM systems. It requires only minor changes in an existing GSM network. The base station subsystem (BSS) consists of base
station controller (BSC) and packet control unit (PCU). The PCU supports all GPRS protocols
for communication over the air interface. Its function is to set up, supervise, and disconnect
packet-switched calls. PCU supports cell change, radio resource configuration, and channel assignment. The base transceiver station (BTS) is a relay station without protocol
functions. It performs modulation and demodulation.
The GPRS standard introduces two new nodes, the serving GPRS support node (SGSN) and the gateway GPRS support node (GGSN). The home location register (HLR)
is enhanced with GPRS subscriber data and routing information. Two types of services are
provided by GPRS: • Point-to-point (PTP)
• Point-to-multipoint (PTM)
Packet data transmission speeds were later increased via EDGE for 2G GSM. Enhanced
Data rates for GSM Evolution (EDGE) (also known as Enhanced GPRS
(EGPRS) is a digital mobile phone technology that allows improved data transmission rates as a backward-compatible extension of GSM. EDGE is
considered a pre-3G radio technology and is part of ITU's 3G definition. EDGE is standardized by 3GPP as part of the GSM family. EDGE can be used for any
packet switched application, such as an Internet connection.
EDGE provides an evolutionary path that enables existing 2G systems (GSM, IS-136) to
deliver 3G services in existing spectrum bands. The advantages of EDGE include fast availability,
reuse of existing GSM, IS-136, and PDC infrastructure, as well as support for gradual
introduction of 3G capabilities.
EDGE reuses the GSM carrier bandwidth and time slot structure. EDGE can be seen
as a generic air interface for efficiently providing high bit rates, facilitating an evolution of
existing 2G systems toward 3G systems.
EDGE (2.5G system) [7,8] was designed to enhance user bandwidth through GPRS.
This is achieved through the use of higher-level modulation schemes. Although EDGE
reuses the GSM carrier bandwidth and time slot structure, the technique is by no means
restricted to GSM systems; it can be used as a generic air interface for efficient provision
of higher bit rates in other TDMA systems. In the Universal Wireless Communications
Consortium (UWCC), the 136 high-speed (136 HS) radio interface was proposed as a
means of satisfying the requirements for an IMT-2000 RTT. EDGE was adopted by UWCC
in 1998 as the outdoor component of 136 HS to provide 384-kbps data service. The standardization effort for EDGE has two phases. In the first phase the emphasis
has been placed on enhanced GPRS (EGPRS) and enhanced CSD (ECSD). The second
phase is being defined with improvements for multimedia and real-time services as possible
work items.
EDGE is primarily a radio interface improvement, but it can also be viewed as a system
concept that allows GSM and IS-136 networks to offer a set of new services. EDGE has
been designed to improve S/I by using link quality control. Link quality control adapts the
protection of the data to the channel quality so that an optimal bit rate is achieved for all
channel qualities.
The EDGE air interface is designed to facilitate higher bit rates than those currently
achievable in existing 2G systems. The modulation scheme based on 8-PSK is used to
increase the gross bit rate. GMSK modulation as defined in GSM is also part of the EDGE
system. The symbol rate is 271 kbps for both GMSK and 8-PSK, leading to gross bit rates
per time slot of 22.8 kbps and 69.2 kbps, respectively. The 8-PSK pulse shape is linearized
GMSK to allow 8-PSK to fit into the GSM spectrum mask. The 8-PSK burst format is similar
to GSM
In order to achieve a higher gross rate, a new modulation scheme, quaternary offset quadrature amplitude modulation (QOQAM), has been proposed for EDGE, since it can
provide higher data rates and good spectral efficiency. An offset modulation scheme is
proposed because it gives smaller amplitude variation than 16-QAM, which can be beneficial
when using nonlinear amplifiers. EDGE will coexist with GSM in the existing frequency
plan and will provide link adaptation (i.e., modulation and coding are adapted for channel conditions).
Radio Protocol Design
The radio protocol strategy in EDGE is to reuse the protocols of GSM/GPRS whenever possible,
thus minimizing the need for new protocol implementation. EDGE enhances both GSM circuit-switched (HSCSD) and packet-switched (GPRS) mode operation. EDGE
includes one packet-switched (PS) and one circuit-switched (CS) mode, EGPRS and
ECSD, respectively.
Enhanced GPRS (EGPRS). The EDGE radio link control (RLC) protocol is somewhat
different from the corresponding GPRS protocol. The main changes are related to improvements
in the link quality control scheme.
A link adaptation scheme regularly estimates the link quality and subsequently selects
the most appropriate modulation and coding scheme for transmission to maximize the user
bit rate. The link adaptation scheme offers mechanisms for choosing the best modulation
and coding alternative for the radio link. In GPRS, only the coding schemes can be changed
between two consecutive link layer control (LLC) frames. In the EGPRS, even the modulation
can be changed. Different coding and modulation schemes enable adjustment for the
robustness of the transmission according to the environment.
Services Offered by EDGE
PS Services. The GPRS architecture provides IP connectivity from mobile station to an
external fixed IP network. For each service, a quality of service (QoS) profile is defined.
The QoS parameters include priority, reliability, delay, and maximum and mean bit rate. A
specified combination of these parameters defines a service, and different services can be
selected to suit the needs of different applications.
CS Services. The current GSM standard supports both transparent and nontransparent
services. Eight transparent services are defined, offering constant bit rates in the range of
9.6 to 64 kbps.
Thus, EDGE CS transmission makes the high-bit-rate services available with fewer time slots, which is advantageous from a terminal implementation perspective. Additionally,
more users can be accepted since each user needs fewer time slots, which increases the
capacity of the system.
Q16..show how CDMA IS-95 systems are moving to provide 3G services.? Soluiton16..
2G CDMA Cellular (IS-95)
GSM uses TDMA, but who uses CDMA in 2G? While some systems have appeared, IS-95 is
the best-known example of 2G with CDMA. Recall that in the case of CDMA, each user is
assigned a unique code that differentiates one user from others. This is in contrast to TDMA
where each user is assigned a time slot. Why use CDMA for cellular? Although the debate
between CDMA versus TDMA has been raging for a while (see Section 8.5.5), there are
several advantages of CDMA for cellular networks. The main advantage of CDMA is that
many more users (up to 10 times more) can be supported as compared to TDMA. Although
this leads to some complications (see Section 8.5.5), the advantage of supporting more users
far outweighs the disadvantage of added complexity.
The IS-95 cellular system has different structures for its forward (base station to mobile
station) and backward links. The forward link consists of up to 64 logical CDMA channels,
each occupying the same 1228 kHz bandwidth. The forward channel supports 4 different
types of channels:
� Traffic channels (channels 8 to 31 and 33 to 63) – these 55 channels are used to carry the
user traffic (originally at 9.6 Kbps, revised at 14.4 Kbps).
� Pilot (Channel 0) – used for signal strength comparison, among other things, to determine
handoffs
� Synchronization (Channel 32) – a 1200 bps channel used to identify the cellular system
(system time, protocol revision, etc.).
All these channels use the same frequency band – the chipping code (a 64-bit code) is used to
distinguish between users. Thus 64 users can theoretically use the same band by using
different codes. This is in contrast to TDMA where the band has to be divided into slots – one
slot per user. The voice and data traffic is encoded, assigned a chipping code, modulated and
sent to its destination. The data in the reverse travels on the IS-95 reverse links. The reverse
links consist of up to 94 logical CDMA channels, each occupying the 1228 kHz bandwidth.
The reverse link supports up to 32 access channels and up to 62 traffic channels. The reverse
links support many mobile unit-specific features to initiate calls, and to update location during
handoffs.
The overall architecture of 2G CDMA-based systems are similar to the TDMA-based GSM
systems (see Figure 8-10). The main difference is that the radio communication between the
Base Station Subsystem and Mobile System uses CDMA instead of TDMA. Of course, the
MSC now has to worry about handling soft handoffs, but the overall structure stays the same.
There are conflicting performance claims for CDMA and TDMA. The debate is raging
because hardware vendors have chosen sides and consequently the standardizing bodies have
been lobbied hard. The primary motivation for this level of debate is that vendors want their
selection to become the industry standard. Since both TDMA and CDMA have become TIA
(Telecom Industry Association) standards – IS-54 and IS-95, respectively – the debate goes
on to determine which standard is better. Technically speaking, CDMA has the following
advantages over TDMA.
� Network capacity: In CDMA, the same frequency can be reused in adjacent cells
because the user signals differentiate from each other by a code. Thus frequency reuse
can be very high and many more users (up to 10 times more) can be supported as compared to TDMA.
� Privacy: Privacy is inherent in CDMA since spread spectrum modulates data to signals
randomly (you cannot understand the signal unless you know the randomizing code).
� Reliability and graceful degradation: CDMA-based networks only gradually degrade
as more users access the system. This is in contrast to the sudden degradation of TDMAbased
systems. For example, if the channel is divided between ten users, then the eleventh
user can get a busy signal in a TDMA system. This is not the case with CDMA because
there is no hard division of channel capacity – CDMA can handle users as long as it can
differentiate between them. In case of CDMA, the noise and interference increases gradually as more users are added because it becomes harder to differentiate between
various codes.
� Frequency diversity: CDMA uses spread spectrum, thus transmissions are spread over a
larger frequency bandwidth. Consequently, frequency-dependent transmission impairments that occur in certain frequency ranges have less effect on the signal. � Environmental: Since existing cells can be upgraded to handle more users, the need for
But, there are some drawbacks of CDMA cellular also:
� Relatively immature. As compared to TDMA, CDMA is a relatively new technology;
but it is catching up fast.
� Self-jamming. CDMA works better if all mobile users are perfectly aligned on chip
(code) boundaries. If this is not the case, then some interference can happen. This situation is better with TDMA and FDMA because time and frequency guard bands can
� Soft handoff. An advantage of CDMA is that it uses soft handoff (i.e., two cells can own
a mobile user for a while before the handoff is complete). However, this requires that the
mobile user acquires the new cell before it relinquishes the old – a more complex process
than hard handoff used in FDMA and TDMA schemes.
The main advantage of CDMA is that the frequency reuse can be very high and many more
users can be supported in a cell as compared to TDMA. Although this leads to a soft handoff
that is more complicated than the hard handoff used in TDMA, the advantage of supporting
more users far outweighs the disadvantage of added complexity
Q17..show how 2G GSm systems are moving to achieve 3G services? ANS:
: A Step-by-Step Towards IMT-2000 (UMTS) GSM at 9.6 Kbps HSCSD: dial-up access at up to 57.6 Kbps GPRS: variable speeds, depending on configuration. ~ 14 and 28 Kbps by mid-2001 EDGE: up to 384 Kbps UMTS: at 384 Kbps and a max speed of 2 Mbps GSM at 9.6 Kbps HSCSD: dial-up access at up to 57.6 Kbps GPRS: variable speeds, depending on configuration. ~ 57 and 114 Kbps by mid-2001 EDGE: up to 384 Kbps UMTS: at 384 Kbps and a max speed of 2 Mbps 2G 2.5G 3G GSM at 9.6 Kbps HSCSD: dial-up access at up to 57.6 Kbps GPRS: variable speeds, depending on configuration. ~ 14 and 28 Kbps by mid-2001 EDGE: up to 384 Kbps UMTS: at 384 Kbps and a max speed of 2 Mbps GSM at 9.6 Kbps HSCSD: dial-up access at up to 57.6 Kbps GPRS: variable speeds, depending on configuration. ~ 57 and 114 Kbps by mid-2001 EDGE: up to 384 Kbps UMTS: at 384 Kbps and a max speed of 2 Mbps 2G 2.5G 3G 2G SYSTEMS
The development of the digital technology, on one hand, and frequent cases when analog systems reached their full capacity, especially in big cities, on the other hand, led to the development of the second-generation (2G) systems.
The main aim in the design of the 2G systems was the maximization of the
system capacity measured as the number of users per spectrum per unit area.
2G networks are digital, both systems use digital signaling to connect the radio towers (which listen to the handsets) to the rest of the telephone system.
Three primary benefits of 2G networks over their predecessors were that
phone conversations were digitally encrypted, more efficient on the
spectrum allowing for far greater mobile phone penetration levels; and data services for mobile, starting with SMS.
CAPACITY OF 2G SYSTEM
Digital voice data can be compressed and multiplexed much more
effectively than analog voice encodings through the use of various codecs, allowing more calls to be packed into the same amount of radio bandwidth. The digital systems were designed to emit less radio power from the
handsets. The cells are smaller, so more cells could be placed in the same amount of space.
DRAWBACKS OF 2G
There are drawbacks to the current GSM:
The GSM is a circuit switched, connection oriented technology, where the
end systems are dedicated for the entire call session. This causes inefficiency in usage of bandwidth and resources.
The GSM-enabled systems do not support high data rates. They are unable to handle complex data such as video.
These devices have small hardware configurations with less powerful
CPUs, memory and display units, and support simple functionality.
Only basic messaging services such as SMS can be supported.
The GSM networks are not compatible with the current TCP/IP and other
common networks because of differences in network hardware, software and protocols
Evolution of Wireless Sys. (2.5G) • 2G telephony is highly successful • Enhancement to 2G on data service
– GSM: HSCSD and GPRS – IS-95: IS-95b
• The improved data rate is still too low to support multimedia traffic • ITU initiated 3G standardization effort in 1992, and the outcome is
IMT-2000.
Goals of 3G Systems More services
Web browsing VoD
Video phone call Mobile computation Improved quality
Higher rates: 2.048 Mbps for low speed users, 384 Kbps for modest speed users and 144 Kbps for high speed users
More reliable and larger capacity Compatible with 2G systems
More flexible
Support both circuit-switching and packet-switching Work in hierarchical mode with pico-/micro-/macro-cells Support asymmetric services
Q18:what are the data rate requirements for 3g system
Third generation cellular systems are being designed to support wideband services like high speed Internet access, video and high quality image transmission with the same
quality as the fixed networks. The primary requirements of the next generation cellular
systems are [1]:
• Voice quality comparable to Public Switched Telephone Network (PSTN).
• Support of high data rate. The following table shows the data rate requirement of the 3G systems
Table 1.1: 3G Data Rate Requirements
Mobility Needs Minimum Data Rate
Vehicular 144 kbps
Outdoor to indoor and pedestrian 384 kbps
Indoor Office 2 Mbps
• Support of both packet-switched and circuit-switched data services. • More efficient usage of the available radio spectrum
• Support of a wide variety of mobile equipment
• Backward Compatibility with pre-existing networks and flexible introduction of new services and technology
Internet co
Q 19: define ip wireless tecnology
IPWireless is the broadband technology based upon UMTS(Universal Mobile Telecommunications System). It uses either 5 or 10 MHz TDD carriers and QPSK(quadrature phase shift keying ) modulation. The theoretical peak
transmission speeds for a 10MHz deployment are 6 Mbps downlink and 3 Mbps uplink. The IPWireless system only uses QPSK modulation and no advanced
antenna technologies. With the inclusion of advanced antenna technologies and the development of High-Speed Downlink Packet Access (HSDPA), IPWireless has signifi cant potential.
SOMA networks has also developed a wireless broadband technology based on UMTS. Like
UMTS, SOMA’s technology uses 5 MHz FHSS carriers. Peak throughput is claimed to be as
high as 12 Mbps, making SOMA one of the faster wireless broadband technologies.
mmunications: a much greater bandwidth for the downlink than the uplink.
Q20: compare 3g and 4g.
Motivation for 4G Research Before 3G Has Not Been Deployed?
1. 3G performance may not be sufficient to meet needs of future high-performance applications like multi-media, full-motion video, wireless teleconferencing. We need a network technology that extends 3G capacity by an order of magnitude.
2. There are multiple standards for 3G making it difficult to roam and
3. 3G is based on primarily a wide-area concept. We need hybrid networks that utilize both wireless LAN (hot spot) concept and cell or base-station wide area network design.
4. We need wider bandwidth
5. Researchers have come up with spectrally more efficient modulation schemes that can not be retrofitted into 3G infrastructure
6. We need all digital packet network that utilizes IP in its fullest form with converged voice and data capability.
Comparing Key Parameters of 4G with 3G
3G (including 2.5G, sub3G) 4G Major Requirement Driving Architecture Predominantly voice driven - data was always add on
Converged data and voice over IP
Network Architecture
Wide area cell-based Hybrid - Integration of Wireless LAN (WiFi, Bluetooth) and wide area
Speeds 384 Kbps to 2 Mbps 20 to 100 Mbps in mobile mode Frequency Band Dependent on country
or continent (1800-2400 MHz)
Higher frequency bands (2-8 GHz)
Bandwidth 5-20 MHz 100 MHz (or more)
Switching Design Basis
Circuit and Packet All digital with packetized voice
Access Technologies
W-CDMA, 1xRTT, Edge
OFDM and
MC-CDMA (Multi Carrier CDMA) Forward Error Correction Convolutional rate 1/2, 1/3 Concatenated coding scheme Component Design Optimized antenna design, multi-band adapters Smarter Antennas, software multiband and wideband radios IP A number of air link
protocols, including IP 5.0
All IP (IP6.0)
Ques21 What is multi input multi output (MIMO) system? Explain.
Answer Systems with more than one input or more than one output are known
as Multi-Input Multi-Output systems. MIMO is the use of multiple antennas at both the transmitter and receiver to improve communication performance. It is one of several forms of smart antenna technology. The terms input and output refer to the radio channel carrying the signal, not to the devices having antennas. MIMO technology has attracted attention in wireless communications, because it offers significant increases in data throughput and link range without additional
bandwidth or transmit power. It achieves this by higher spectral efficiency have more bits per second per hertz of bandwidth and link reliability or diversity. Because of these properties, MIMO is an important part of modern wireless communication standards such as IEEE 802.11n (Wifi), 4G, WiMAX etc.
MIMO technology leverages multipath behavior by using multiple, “smart” transmitters and receivers with an added “spatial” dimension to dramatically increase performance and range. MIMO allows multiple antennas to send and receive multiple spatial streams at the same time. This allows antennas to transmit and receive simultaneously.
MIMO makes antennas work smarter by enabling them to combine data streams arriving from different paths and at different times to effectively increase receiver signal-capturing power. Smart antennas use spatial diversity technology, which puts surplus antennas to good use. In order to implement MIMO, either the station (mobile device) or the access point (AP) need to support MIMO. Optimal
performance and range can only be obtained when both the station and the AP support MIMO.
Legacy wireless devices can’t take advantage of multipath because they use a Single Input, Single Output (SISO) technology. Systems that use SISO can only send or receive a single spatial stream at one time.
MIMO technology takes advantage of a radio-wave phenomenon called multipath where transmitted information bounces off walls, ceilings and other objects, reaching the receiving antenna multiple times via different angles and at slightly different times.
Question 22 What is the software defined radio system?
Answer Software Defined Radio (SDR) refers to the technology wherein
and general purpose microprocessors are used to implement radio functions such as generation of transmitted signal (modulation) at transmitter and
tuning/detection of received radio signal (demodulation) at receiver. A radio that includes a transmitter in which the operating parameters of the transmitter
including the frequency range, modulation type or maximum radiated or
conducted output power can be altered by making a change in software without making any hardware changes. A basic SDR system may consist of a personal computer equipped with a sound card, or other analog-to-digital converter,
preceded by some form of RF front end. Significant amounts of signal
processing are handed over to the general-purpose processor, rather than being
done in special-purpose hardware. Such a design produces a radio which can receive and transmit widely different radio protocols (sometimes referred to as a waveforms) based solely on the software used.
Software radios have significant utility for the military and cell phone services, both of which must serve a wide variety of changing radio protocols in real time. Software-defined radios are expected by proponents like the SDRForum (now The Wireless Innovation Forum) to become the dominant technology in radio
communications. SDRs, along with software defined antennas are the enablers of
the cognitive radio.
Motivation of SDR
Commercial wireless communication industry is currently facing problems due to constant evolution of link-layer protocol standards (2.5G, 3G, and 4G) existence of incompatible wireless network technologies in different countries inhibiting deployment of global roaming facilities
problems in rolling-out new features due to wide-spread presence of legacy subscriber handsets.
Applications
Military, Real-time flexibility, Secure International connectivity, Portable
command for crisis management, Bluetooth, WLAN, GPS, Radar, WCDMA, GPRS, GSM, AM, FM, etc.
Features
Reconfigurability, future-proof, service, mode, multiband, multi-standard terminals and infrastructure equipment, Ubiquitous Connectivity, Interoperability, SDR facilitates implementation of open architecture radio
systems.
Programmability
Hardware radio, no software changes, Software controlled radio in PDR, BB operations and link layer protocols are implemented in software.
Question 23 List some of the new technologies that will be used in the 4G
system?
Answer The new technologies used in the 4G system are 4G mobile systems focus on seamless integration of existing wireless technologies including WWAN,
WLAN and Bluetooth. 4G standards setting peak speed requirements for 4G
service at 100 Mbit/s for high mobility communication (such as from trains and cars) and 1 Gbit/s for low mobility communication (such as pedestrians and stationary users). The 4G systems will encompass all systems from various networks, public to private, operator-driven broadband networks to personal areas, and ad hoc networks. The 4G intends to integrate from satellite broadband to high altitude platform to cellular 2G and 3G systems to wireless local loop (WLL) and broadband wireless access (BWA) to WLAN, and wireless personal area networks (WPANs). A 4G system is expected to provide a comprehensive and secure all-IP based mobile broadband solution to laptop computer wireless
modems, smartphones and other mobile devices. Facilities such as ultra-broadband Internet access, IP telephony, gaming services and streamed
multimedia may be provided to users. A 4G system can provide a comprehensive IP solution where voice, data and streamed multimedia can be provided to users on an "Anytime, Anywhere" basis. The data transfer rates are also much higher than previous generations.
The main objectives of 4G
are:-1) 4G will be a fully IP-based integrated system.
2) This will be capable of providing 100 Mbit/s and 1 Gbit/s speeds both indoors
and outdoors.
3) It can provide premium quality and high security. 4) 4G offer all types of services at an affordable cost.
4G is developed to provide high quality of service (QoS) and rate requirements set by forthcoming applications such as wireless broadband access, Multimedia
Messaging, Video Chat, Mobile TV, High definition TV content, DVB, minimal service like voice and data and other streaming services. 4G technology allow high-quality smooth video transmission. It will enable fast downloading of
full-length songs or music pieces in real time. 4G mobile data transmission rates are planned to be up to 20 megabits per second which means that it will be about 10-20 times faster than standard ASDL services.
Technologies
Multicarrier Modulation Multicarrier modulation (MCM) is a derivative of
frequency-division multiplexing. It is not a new technology. Forms of multicarrier systems are currently used in DSL modems and digital audio/video broadcast (DAB/DVB). MCM is a baseband process that uses parallel equal bandwidth sub channels to transmit information and is normally implemented with fast Fourier transform (FFT) techniques. MCM’s advantages are better performance in the inter-symbol-interference environment and avoidance of single-frequency interferers.
Smart Antenna Techniques Smart antenna techniques, such as multiple-input
multiple-output (MIMO) systems, can extend the capabilities of the 3G and 4G systems to provide customers with increased data throughput for mobile high-speed data applications. MIMO systems use multiple antennas at both the transmitter and receiver to increase the capacity of the wireless channel.
Single-input, multiple-output There are N antennas at the receiver. If the signals
received on the antennas have on average the same amplitude, then they can be added coherently to produce an N2 increase in signal power.
Multiple-input, single-output We have M transmitting antennas. The total power
is divided into M transmitter branches.
Multiple-input, multiple-output MIMO systems can be viewed as a combination of
MISO and SIMO channels.
OFDM-MIMO Systems OFDM and MIMO techniques can be combined to
achieve high spectral efficiency and increased throughput. The OFDM-MIMO system transmits independent OFDM modulated data from multiple antennas simultaneously. At the receiver, after OFDM demodulation, MIMO decodes each subchannel to extract data from all transmit antennas on all the subchannels.
Q.no 24 What is the basic requirement & entity involed for the mobility management?
Mobility management contains two components:Location management and handoff management [2].
A. Location Management
Location management is a two-stage process that enablesthe network to discover the current attachment point of themobile user for call delivery, as shown in Fig. 1. The firststage is location registration (or location update). In this stage,the mobile terminal periodically notifies the network of its newaccess point, allowing the network to authenticate the user andrevise the user’s location profile. The second stage is call delivery. Here the network is queried for the user location profile and the current position of the mobile host is found
Fig.1. Location management operations
B. Handoff Management
Handover (or handover) management enables the network tomaintain a user’s connection as the mobile terminal continues Mobility Management in Next-GenerationWireless Systems
Fig. 2. Handoff management operations
To move and change its access point to the network. The threestage process for handoff first involves initiation, where either the user, a network agent, or
changing network conditions identify the need for handoff. The second stage is new connection generation, where the network must find new resources for the handoff connection and perform any additional routing operations. Under network-controlled handoff (NCHO)[11], or mobile-assisted handoff (MAHO), the network generates a new connection, finding new resources for the handoff and performing any additional routing operations. For mobile-controlled handoff (MCHO)[10], the mobile terminal finds the new resources and the network approves. The final stage is data-flow control, where the delivery of the data from the old connection path to the new connection path is maintained according to agreed upon service mobile terminal finds the new resources and the network approves. The final stage is data-flow control, wherethe delivery of the data from the old connection path to the new connection path is maintained according to agreed-upon service guarantees. The handoff management operations are presented in Fig. 2.
Intracell handoff and intercell handoff.
Intracell handoff occurs when the user moves within a service area (or cell) and experiences signal strength deterioration below a certain threshold that results in the transfer of the user’s calls to new radio channels of appropriate strength at the same base station (BS) [11].
Intercell handoff occurs when the user moves into an adjacent cell and all of the terminal’s connections must be transferred to a new BS. While performing
handoff, the terminal may connect to multiple BS’s simultaneously and use some form of signaling diversity to combine the multiple signals. This is called soft handoff. On the other hand, if the terminal stays connected to only one BS at a time, clearing the connection with the former BS immediately before or after establishing a connection with the target BS, then the process is referred to as hard handoff.
MOBILITY MANAGEMENT FOR THE PLMN
In ordinary wire line networks, such as the telephonenetwork, there is a fixed relationship between a terminal andits location[8]. Changing the location of a terminal generally involves the network administration and it cannot easily be performed by a user. Incoming calls for a particular terminal are always routed to its associated location as there is no distinction between a terminal and its location [9]. In contrast, mobile terminals (MT’s) are free to travel and thus the network access point of an MT changes as it moves around the network coverage area. As a result, the ID of an MT does not implicitly provide the location information of the MT and the call delivery process becomes more complex. In order to perform the registration, update, and call delivery operations described above, the network stores the location information of each MT in the location databases. Then the information can be retrieved for call delivery.Current schemes[3] for PLMN
location management arebased on a two-level data hierarchy such that two types of network location database, the home location register (HLR) and the visitor
location register (VLR), are involved in tracking an MT. In general, there is an HLR for each network and a User is permanently associated with an HLR in his/her subscribed network. Information about each user, such as the types of services subscribed and location information are stored in a user profile located at the HLR. The number of VLR’s and their placements vary among networks. Each
VLR stores the information of the MT’s (downloaded from the HLR) visiting its associated area.
Fig. 3. SS7 signaling network.
Network management functions, such as call processing and location registration, are achieved by the exchange of signaling messages through a signaling network. Signaling System 7 (SS7) [5] is the protocol used for signaling exchange, and the signaling network is referred to as the SS7 network the type of CSS currently implemented for the PLMN is known as a mobile switching center (MSC). Fig. 3 shows the SS7 signaling network which connects the HLR, the VLR’s, and the MSC’s in a PLMN based network. The signal transfer points (STP’s) as shown in Fig. 3 are responsible for routing signaling. Messages within the SS7 network. For reliability reason, the STP’s are installed in pairs. As mentioned previously,
location management includes two major tasks: location registration (or update) and calldelivery (see Fig. 1). For PLMN, the location registrationprocedures update the location databases (HLR and VLR’s)and authenticate the MT when up-to-date location informationof an MT is available. The call delivery procedures locate the MT based on the information available at the HLR and the VLR’s when a call for an MT is initiated. The IS-41 and the GSM MAP location management strategies are very similar to each other. While GSM MAP is designed to facilitate personal mobility and to enable user selection of network provider, there are a lot of
1. Location Registration: In order to correctly deliver calls, the PLMN must keep track of the location of each MT. As described previously, location information is stored in two types of databases, VLR and HLR. As the MT’s move around the network coverage area, the data stored in these databases. To ensure that calls can be delivered successfully, the databases are
periodically updated through the process called location
registration.Location registration is initiated by an MT when it reports its current location to the network. We call this reporting process location update. Current systems adopt an approach such that the MT performs a location update whenever it enters a new LA. Recall that each LA consists of a number of cells and, in general, all BTS’s belonging to the same LA is connected to the same MSC. When an MT enters an LA, if the new LA belongs to the same VLR as the old LA, the record at the VLR is updated to record the ID of the new LA [14]. Otherwise, if the new LA belongs to a different VLR, a number of extra steps are required to: 1) register the MT at the new serving VLR; 2) update the HLR to record the ID of the new serving VLR; and3) deregister the MT at the old serving VLR. Fig. 4 shows the location registration procedure when an MT moves to a newLA. The following is the ordered list of tasks that areperformed during location registration. The MT enters a new LA and transmits a location
updatemessage to the new BS. The BS forwards the location update message to the MSC which launches a registration query to its associated VLR. The VLR updates its record on the location of the MT. If the new LA belongs to a different VLR, the new VLR determines the address of the HLR of the MT from its mobile identification number (MIN) [10]. This is achieved by a table lookup procedure called global title translation. The new VLR then sends a location registration message to the HLR. Otherwise, location registration is complete.The HLR performs the required procedures to authenticate the MT and records the ID of the new serving VLR of the MT.
Fig. 4. Location registration procedures.
The HLR then sends a registration acknowledgment messageto the new VLR. The HLR sends a registration cancellation message to the old VLR. The old VLR removes the record of the MT and returns a cancellation acknowledgment message to the HLR. Depending on the distance between the current and the home locations of the MT, in steps 3)–6), the signaling messages may have to go through several intermediate STP’s before reaching their destinations. For example, a user who subscribes to wireless services in Atlanta will normally be assigned to an HLR located in the Atlanta area. When this user is roaming in London, each location update performed by his/her mobile phone will result in the transmission of four transatlantic SS7 messages [messages (3)–(6) as shown in Fig.6]. These messages may transverse a number of STP’s in the SS7[10] network before reaching their destinations, which generate additional load to the network elements and the transmission links. The location registration may, therefore, result in significant traffic load to the SS7 network. As the number of mobile subscribers keeps increasing, the delay for completing a location registration may increase.
Fig.6. Mobile IP location registration.
Call Delivery: Two major steps are involved in calldelivery: 1) determining the serving VLR of the called MT and
2) locating the visiting cell of the called MT.
Locating the serving VLR of the MT involves the following database ookup procedure (see Fig. 5).
Fig. 5. Mobile IP architecture.
1. The calling MT sends a call initiation signal to the serving MSC of the MT through a nearby BS.
2. The MSC determines the address of the HLR of the called MT by global title translation and sends a location request message to the HLR.
