Chap – 4 - Telecommunication System
4 Telecommunications systems 93
4.1 GSM 96
4.1.1 Mobile services 98
4.1.2 System architecture 100
4.1.3 Radio interface 105
4.1.4 Protocols 110
4.1.5 Localization and calling 113
4.1.6 Handover 117
4.1.7 Security 120
4.1.8 New data services 122
4.2 DECT 130
4.2.1 System architecture 131
4.2.2 Protocol architecture 132
4.3 TETRA 134
4.4 UMTS and IMT-2000 136
4.4.1 UMTS releases and standardization 141
4.4.2 UMTS system architecture 142
4.4.3 UMTS radio interface 143
4.4.4 UTRAN 149
4.4.5 Core network 151
4.4.6 Handover 154
4.1
GSM
q formerly:
GroupeSpéciale Mobile (founded 1982)
q now:
Global System for Mobile Communication
q Pan-European
standard (ETSI, European Telecommunications Standardisation Institute)
q simultaneous
introduction of essential services in three phases (1991, 1994, 1996) by the
European telecommunication administrations (Germany: D1 and D2)
è seamless roaming within Europe possible
è seamless roaming within Europe possible
q today
many providers all over the world use GSM (more than 184 countries in Asia,
Africa, Europe, Australia, America)
q more
than 747 million subscribers
q more
than 70% of all digital mobile phones use GSM
q over
10 billion SMS per month in Germany, > 360 billion/year worldwide
q railroad
systems is GSM-Railcontrol of trains, switches, gates, and signals
q Special
features of this system are, e.g., emergency calls with acknowledgements, voice
group call service (VGCS), voice broadcast serviceso-called advanced speech
call items (ASCI)
Communication
q mobile,
wireless communication; support for voice and data services
Total
mobility
q international
access, chip-card enables use of access points of different providers
Worldwide
connectivity
q one
number, the network handles localization
High
capacity
q better
frequency efficiency
q High
transmission quality
q high
audio quality and reliability for wireless, uninterrupted phone calls at higher
speeds (e.g., from cars, trains)
Security
functions
q access
control, authentication via chip-card and PIN
Disadvantages:
q no
end-to-end encryption of user data
q electromagnetic
radiation
q abuse
of private data possible
q high
complexity of the system
q several
incompatibilities within the GSM standards
4.1.1 Mobile services
·
GSM permits the integration
of different voice and data services and the interworking
·
with existing networks.
·
GSM has defined three
different categories of services:
o Bearer
o tele
o and supplementary services.
Figure 4.3 shows a reference model for GSM services.
·
A mobile station MS isconnected
to the GSM public land mobile network (PLMN) via the Um interface.
·
GSM-PLMN is the infrastructure
needed for the GSM network.
·
Thisnetwork is connected to
transit networks, e.g., integrated services digital network(ISDN) or
traditional public switched telephone network (PSTN).
·
Theremight be an additional
network, the source/destination network, before anotherterminal TE is
connected.
·
Bearer services now comprise all services that enablethe transparent transmission of data between the interfaces to the
network.
o connection-oriented and circuit- or packet-switched.
o These services only need the lower three layers of the ISO/OSI
reference model.
·
Tele servicesare application specific and may thus need all seven layers of the
ISO/OSI referencemodel.
4.1.1.1 Bearer services
the original GSMallowing for data rates of up to 9600 bit/s for
non-voice services.
Bearer servicespermit
transparent and non-transparent
synchronous or asynchronous
datatransmission.
Transparent bearer services
·
only use the functions of
the physicallayer (layer 1) to transmit data.
·
Data transmission has a
constant delay.
·
Mechanism to
increasetransmission quality is the use of forward error correction (FEC),
which codesredundancy into the data stream and helps to reconstruct the
original data incase of transmission errors.
·
Depending on the FEC, data
rates of 2.4, 4.8, or9.6 kbit/s are possible.
·
Transparent bearer services do not try to recover lost datain
case of, for example, interruptions due to handover.
Non-transparent bearer services
·
use protocols of layers two
and three toimplement error correction and flow control.
·
Along with the
transparentbearer services,it use a radio link protocol (RLP).
·
This protocol comprises two mechanisms
o high-level data link control (HDLC)
o andspecialselective-reject
mechanisms to trigger retransmission of erroneous data.
·
Theachieved bit error rate
is less than 10–7, but now throughput and delay may varydepending on transmission
quality.
·
Data transmissioncan be
o data service (circuit switched)
§ synchronous: 2.4, 4.8 or 9.6 kbit/s
§ asynchronous: 300 - 1200 bit/s
o data service (packet switched)
§ synchronous: 2.4, 4.8 or 9.6 kbit/s
§ asynchronous: 300 - 9600 bit/s
4.1.1.2 Tele services
GSM mainly focuses on voice-oriented tele services.
These comprise encrypted
voice transmission
message services
and basic data communication with terminalsas known from the PSTN
or ISDN (e.g., fax).
q Offered services
q mobile telephony
primary goal of GSM was to enable mobile telephony offering the traditional bandwidth of 3.1 kHz
primary goal of GSM was to enable mobile telephony offering the traditional bandwidth of 3.1 kHz
q Emergencynumber
common number throughout Europe (112); mandatory for all service providers; free of charge; connection with the highest priority (preemption of other connections possible)
common number throughout Europe (112); mandatory for all service providers; free of charge; connection with the highest priority (preemption of other connections possible)
q Multinumbering
several ISDN phone numbers per user possible
several ISDN phone numbers per user possible
Additional services
q Non-Voice-Teleservices
q group 3 fax
q voice mailbox (implemented in the fixed network supporting the
mobile terminals)
q electronic mail (MHS, Message Handling System, implemented in the
fixed network)
q Short Message Service (SMS) 160 characters
alphanumeric data transmission to/from the mobile terminal using the signaling channel, thus allowing simultaneous use of basic services and SMS
alphanumeric data transmission to/from the mobile terminal using the signaling channel, thus allowing simultaneous use of basic services and SMS
q enhanced message service (EMS) 760 characters
q multimedia message service (MMS)
4.1.1.3 Supplementary services
·
In addition to tele and
bearer services, GSM providers can offer supplementaryservices.
·
These services offer various
enhancementsfor the standard telephony service, and may vary from provider to
provider.
·
Typical services are
o user identification
o call redirection
o orforwarding ofongoing calls.
o Standard ISDN features such as closed user groups
o andmultipartycommunication may be available.
·
Closed user groups are of
specialinterest to companies because they allow, for example, a
company-specific GSMsub-network, to which only members of the group have
access.
4.1.2 System architecture
·
GSM comes with a hierarchical complex
system architecture comprising many
·
entities,
interfaces, and acronyms.
·
Figure 4.4 gives a simplified overview
of the GSM system as specifiedin ETSI (1991b).
·
A GSM system consists of three
subsystems:
o
The radio sub system (RSS)
o
the network and switching subsystem
(NSS)
o
And the operation subsystem (OSS).
·
4.1.2.1 Radio subsystem
·
As the name implies, the radio
subsystem (RSS) comprises all radio
specific entities, i.e.,
o the mobile stations (MS) and
o thebase station subsystem (BSS).
·
Figure 4.4 shows the
connection between the RSS and the NSS via the A interface(solid lines)
and the connection to the OSS via the O interface (dashedlines).
·
The A interface is typically based on circuit-switched PCM-30 systems(2.048 Mbit/s), carrying up to 30 64
kbit/s connections
·
whereas the O interfaceuses the Signaling Systemcarryingdatato/from
the RSS.
|
●Base station subsystem (BSS):
·
A GSM network comprises many
BSSs, eachcontrolled by a base station controller (BSC).
·
The BSS performs all
functionsnecessary to maintain radio connections
to an MS,
o coding/decoding ofvoice
o and rate adaptation to/from the wireless network part.
o the BSS contains several BTSs.
●Base transceiver station (BTS):
·
A BTS comprises all radio
equipment, i.e.,
o antennas,
o signal processing amplifiers necessary for radio transmission.
·
ABTS can form a radio cell
·
connected to MS via the Um
interface (ISDN U interface formobile use)
and to the BSC via the Abisinterface.
·
Um interface containsall the
mechanisms necessary for wireless transmission (TDMA, FDMA etc.)
·
Abisinterface consists of 16 or64
kbit/s connections.
·
A GSM cell can measure
between some 100 m and35 km depending on the environment (buildings, open
space, mountainsetc.) but also expected traffic.
●Base station controller (BSC):
·
The BSC basically manages
the BTSs.
·
Itreserves radio frequencies
·
handles the handover from
one BTS to anotherwithin the BSS
·
Performs paging of the MS.
·
The BSC also multiplexesthe
radio channels
●Mobile station (MS):
o The MS comprises all user equipment and softwareneeded for
communication with a GSM network.
o An MS consists of userindependent
§ hard- and software
§ subscriber identity module(SIM),
which stores all user-specific data that is relevant to GSM.
o MS can be identified via the international mobile equipment
identity(IMEI)
o a user can personalize any MS using SIM
o user-specificmechanisms
§ like charging and authentication are based on the SIM
o Device-specific
mechanisms
§ e.g., theft protection
SIM:
·
Without the SIM, only
emergency calls are possible.
·
It contains many identifiers
and tables, such as
o card-type
o a list of subscribed services
o personal identity number (PIN)-
used to unlock the MS
§ Using the wrong PIN three times will lock the SIM
o PIN unblocking key (PUK) - needed to
unlock the SIM
o authentication key Ki
o internationalmobile subscriber identity (IMSI)
·
The MS stores dynamic informationwhile logged onto
the GSM system, such as, e.g., the cipher key Kcandthe location
information consisting of a temporary mobile subscriber identity(TMSI) and
the location area identification (LAI).
·
Typical MSs for GSM900 have
a transmit power of up to 2 W, whereas for GSM 1800 1 W isenough due to the
smaller cell size.
·
Interface:
o Apart from the telephone interface, anMS can also offer other
types of interfaces to users with display, loudspeaker,microphone, and
programmable soft keys.
o Further interfaces comprise computermodems, IrDA, or Bluetooth.
·
Typical MSs, e.g., mobile
phones,comprise many more
vendor-specific functions and components, such as
o cameras, fingerprint sensors, calendars, address books, games, and
Internetbrowsers.
4.1.2.2 Network and
switching subsystem
The “heart” of the GSM system is formed by the network and switching
subsystem(NSS).
·
connects the wireless
network with standard public networks
·
performs handovers between
different BSSs
·
comprises functions
forworldwide localization of users and
·
supports charging,
accounting
·
and roamingof users between
different providers in different countries.
The NSS consists ofthe
following switches and databases:
●Mobile services switching center (MSC):
o MSCs are high-performance digitalISDN switches.
o They set up connections to other MSCs and to the BSCs via the A
interface
o It manages several BSCs in a geographical region.
o A gatewayMSC (GMSC) has additional connections to other
fixed networks, such asPSTN and ISDN.
o Interworking functions (IWF), used
to connect to public data networks (PDN)
§ handles all signaling needed for connection setup
§ connection release andhandover of connections to other MSCs.
o The standard signaling systemNo. 7 (SS7):
§ Features of SS7 are number portability,
·
free
phone/toll/collect/credit calls, call forwarding, three-way calling etc. AnMSC
also performs all functions needed for supplementary services such as
§ call forwarding, multi-party calls, reverse charging etc.
●Home location register (HLR):
·
The HLR is the most
important database in aGSM system as it stores all user-relevant information.
·
This comprises static information, such as
o mobile subscriber ISDN number (MSISDN)
o international mobile subscriber identity (IMSI).
·
Dynamic information is
o The current location area (LA) of the MS
o the mobile subscriberroaming number (MSRN)
o the current VLR and MSC.
As soon as anMS leaves its
current LA, the information in the HLR is updated. This informationis necessary
to localize a user in the worldwide GSM network. All theseuser-specific
information elements only exist once for each user in a singleHLR, which also
supports charging and accounting. HLRs can manage data for severalmillion
customers and contain highly specialized data bases which must fulfillcertain
real-time requirements to answer requests within certain time-bounds.
●Visitor location registers (VLR):
The VLR associated to each MSC is adynamic database which stores all important information needed for
theMS users currently in the LA that is associated to the MSC (e.g.,
IMSI,MSISDN, HLR address). If a new MS comes into an LA the VLR is
responsiblefor, it copies all relevant information for this user from the HLR.
Thishierarchy of VLR and HLR avoids frequent HLR updates and
long-distancesignaling of user information.
4.1.2.3 Operation
subsystem
The third part of a GSM system, the operation subsystem (OSS),
contains thenecessary functions for
network operation and maintenance.
●Operation and maintenance center (OMC):
o The OMC monitors and controlsall other network entities via the O
interface
o OMC management functions are
§ traffic monitoring
§ status reports of network entities
§ subscriber and security management, or accounting andbilling.
OMCs use the concept of telecommunication
management network(TMN).
●Authentication centre (AuC):
·
As the radio interface and
mobile stationsare particularly vulnerable, a separate AuC has been defined to protect useridentity and data transmission.
·
The AuC contains the
algorithms forauthentication as well as the keys for encryption and generates
the valuesneeded for user authentication in the HLR.
·
The AuC may, in fact, be
situatedin 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 aremobile, 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 anMS is useless as soon as the owner has reported a theft.
Unfortunately, theblacklists of different providers are not usually
synchronized and the illegaluse of a device in another operator’s network is
possible (the reader mayspeculate as to why this is the case).
·
The EIR also contains a list
of validIMEIs (white list), and a
list of malfunctioning devices (gray
list).
4.1.3 Radio interface
·
GSM implements SDMA using cells with BTS
and assigns an MS to a BTS.
·
FDD is used to separate downlink and
uplink.
·
Media access combines TDMA and FDMA.
·
In GSM 900, 124 channels, each 200 kHz
wide, are used for FDMA
·
whereas GSM 1800 uses, 374 channels.
·
Due to technical reasons, channels 1 and
124 are not used for transmission in GSM 900.
32 channels are reserved for organizational datathe remaining 90 are
used for customers.
·
Each BTS then manages a single channel
for organizational data
·
Each of the 248 channels is additionally
separated in time via a GSM TDMA frame
·
The duration of a frame is 4.615 ms.
·
A frame is again subdivided into 8 GSM
time slots, where each slot represents a
physical TDM channel and lasts for 577 μs.
·
Data is transmitted in small portions,
called bursts.
·
In the diagram, the burst is only 546.5 μs
long and contains 148bits.
·
The remaining 30.5 μs are used as
guard space to avoid overlapping withother bursts due to different path
delays and to give the transmitter time to turnon and off. Filling the whole
slot with data allows for the transmission 156.25 bit
within 577 μs.
·
Each physical TDM channel
has a raw data rate ofabout 33.8 kbit/s,each radio carrier transmits approximately
270 kbit/s over theUm interface.
·
The first and last three
bits of a normal burst (tail) are all set to 0 and can beused to enhance
the receiver performance.
·
The training sequence
in the middleof a slot is used to adapt
o theparameters of the
receiver to the current path
propagationcharacteristics and to select
the strongest signal in case of multi-pathpropagation.
·
A flag S indicates
whether the data field contains user or networkcontrol data.
·
Four bursts :
o frequency correctionburst allows the MS to correct
thelocal oscillator to avoid interference with neighboring channels
o synchronizationburst with an
extended training sequence synchronizes the MS with theBTS in time
o access burst is used for the initial
connection setup between MSand BTS
o Dummy burst is used if no data is
available for a slot.
·
MS and BTS change the carrier
frequencyafter each frame based on a common hopping sequence. An MS changes its
frequencybetween up and downlink slots respectively.
4.1.3.1 Logical channels and frame
hierarchy
GSM
specifies two basic groups of
logical channels, i.e., traffic channels
andcontrol channels
·
●Traffic
channels (TCH): GSM uses a TCH to transmit user data
(e.g., voice,fax).
·
Two basic categories of TCHs have been
defined
o
Full-rate TCH(TCH/F) and
half-rate TCH (TCH/H).
·
A TCH/F has a data rate of 22.8
kbit/s,whereas TCH/H only has 11.4 kbit/s. With the voice codecs available at
thebeginning of the GSM standardization, 13 kbit/s were required, whereas
theremaining capacity of the TCH/F (22.8 kbit/s) was used for error correction(TCH/FS).
·
Using these TCH/HSs doubles the capacity of the GSM system forvoice
transmission. However, speech quality decreases with the use ofTCH/HS and many
providers try to avoid using them.
·
better voice quality can be provided by:
·
enhanced full rate (EFR),
provides better voice quality than FR
tandem free operation (TFO)
●Control channels (CCH):
Many different CCHs are used in a GSM systemto control medium access, allocation of
traffic channels or mobility management.Three groups of control channels
have been defined:
1.Broadcast control channel (BCCH): A BTS uses this channel to signalinformation
to all MSs within a cell.
§ frequency correction channel
(FCCH):The
BTS sends information for frequency correctionvia this.
§ Synchronization channel (SCH)time synchronization.
2.Common control channel
(CCCH):
All information regarding connectionsetup between MS and BS is
exchanged via the CCCH.
BTS->MS: For callstoward an MS, the
BTS uses the paging channel (PCH)
MS->BTS: uses the randomaccess channel (RACH)
The
BTS uses the access grant channel (AGCH) to signal an MS
3.Dedicated control
channel (DCCH): While the previous channelshave all been unidirectional,
the following channels are bidirectional.
1. stand-alone dedicated control channel (SDCCH):
As long as an MS
has not established a TCH with the BTS, it uses thestand-alone
dedicated control channel (SDCCH) with a low data rate(782 bit/s) for
signaling. This can comprise authentication, registrationor other
data needed for setting up a TCH.
2. slow associated dedicated control channel (SACCH):
Each TCH and SDCCH has aslow associated
dedicated control channel (SACCH) associated withit, which is used to
exchange system information, such as the channelquality and signal power level.
3.fast associateddedicated control channel (FACCH):
Finally, if more
signalinginformationneeds to be transmitted and a TCH already exists, GSM
uses a fast associateddedicated control channel (FACCH).
Traffic multiframe:As these logical channels
are all associated withuser traffic, the multiframe is called traffic
multiframe.
controlmultiframe:TDMA frames containing
(signaling)data for the other logicalchannels are combined to a control
multiframe.
Superframe:51 multiframes with 26 frames
Hyperframe:2,048 superframes.
4.1.4 Protocols
Layer
1:
Radio
·
The physical layer, handles all radio-specific
functions.
·
Thisincludes the creation of bursts
according to the five different formats:
o
multiplexingof
bursts into a TDMA frame
o
synchronization with
the BTS, detectionof idle channels
o
measurement of the channel quality
on the downlink.
digitalmodulation
o
encryption/decryptionof
data, i.e., encryption is not performed end-to-endbut only between MS and BSS
over the air interface.
o
pulse code modulation
(PCM)
A problematic aspect
in this context are the different round trip times(RTT).
An
MS close to the BTS has a very short RTT, whereas an MS 35 km away
alreadyexhibits an RTT of around 0.23 ms.If the MS far away used the slot
structure with-out correction, large guard spaces would be required.
The
main tasks of the physical layer comprise channel coding and errordetection/correction,
which is directly combined with the coding mechanisms.
As voice was assumed to be the main service in
GSM,the physical layer also contains special functions, such as voice
activity detection(VAD), which transmits voice data only when there is a
voice signal. Thismechanism helps to decrease
interference as a channel might be silent approximately60 per cent of the
time (under the assumption that only one personspeaks at the same time and some
extra time is needed to switch between thespeakers). During periods of silence
(e.g., if a user needs time to think beforetalking), the physical layer
generates a comfort noise to fake a connection(complete silence would
probably confuse a user), but no actual transmissiontakes place. The noise is
even adapted to the current background noise at the
communication
partner’s location.
It
generates a delay fortransmission. The delay is about 60 ms for a TCH/FS
and 100 ms for a TCH/F.
Layer 2:LAPD
·
Link access procedure for
the D-channel (LAPD) in ISDN systems:
·
Reliable data transfer over
connections
·
Re-sequencing of data
frames, and flow control
·
segmentation andreassembly
of data and acknowledged/unacknowledged data transfer.
Layer 3:RR
Only a part of this layer, RR’, is implemented in the BTS,
the remainder is situatedin the BSC. The functions of RR’ are supported by the
BSC via the BTSmanagement (BTSM).
Tasks of RR are
setup, maintenance, andrelease of radio channels.
Layer 4:MM
Mobility management (MM) contains
functions for registration,authentication,identification, location updating,
and the provision of a temporary mobilesubscriber identity (TMSI) that
replaces the international mobile subscriberidentity (IMSI) and which
hides the real identity of an MS user over the air interface.
While the IMSI identifies a user, the TMSI is valid only in the
current
location area of a VLR.
Layer 5:CM
·
Finally, the call
management (CM) layer contains three entities:
o call control(CC)
o short message service (SMS)
o supplementary service (SS).
o Dual tone multiple frequency (DTMF)
4.1.5 Localization and calling
·
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 periodic location 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
·
VLR is 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.
o
Roaming can take place within the
network of one provider
o
between two providers in one country
(national roaming)
o
between different providers in different
countries (international roaming).
● Mobile station
international ISDN number (MSISDN):
o
The only important number for a user of GSM is the phone number.
o
This number consists of
§ the
country code (CC) (e.g., +49 179 1234567 with 49 for Germany)
§ the
national destination code (NDC) (the address of the network provider,
e.g., 179)
§ the
subscriber number (SN).
● International
mobile subscriber identity (IMSI):
o
GSM
uses the IMSI for internal unique identification of a subscriber.
o
IMSI
consists of a
§ Mobile country code (MCC) (e.g., 240 for Sweden, 208 for France)
§ the mobile network code (MNC) (i.e.,
the code of the network provider)
§ the mobile subscriber identification
number (MSIN).
● Temporary mobile subscriber identity
(TMSI):
The VLR, which is responsible for the current location of a
subscriber, can assign a temporary mobile subscriber identity (TMSI) which has
only local significance in the area handled by the VLR. It is stored on the
network side only in the VLR and is not passed to the HLR.
● Mobile station roaming number (MSRN):
o
The
Mobile Station Roaming Number ( MSRN) is a temporary location dependent ISDN
number. It is assigned by the locally responsible VLR to each mobile station in
its area. Calls are also routed to the MS by using the MSRN.
§ Visitor Country Code (CC) : of the
visited network.
§ Visitor National Destination Code (NDC):
of the visited network.
§ Subscriber Number (SN): in the current
mobile network.
The MSRN helps the HLR to find a
subscriber for an incoming call. All these numbers are needed to find a
subscriber and to maintain the connection with a mobile station.
Mobile terminated call (MTC):
The interesting case is the mobile
terminated call (MTC), i.e., a situation in which a station calls a mobile
station ,the calling station could be outside the GSM network or another mobile
station.
Figure 4.8 shows the basic steps needed
to connect the calling station with the mobile user.
1. A user dials the phone number of a GSM
subscriber.
2. The fixed network (PSTN) notices (looking
at the destination code) that the number belongs to a user in the GSM network
and forwards the call setup to the Gateway MSC (2).
3. The GMSC identifies the HLR for the subscriber
(which is coded in the phone number)
and signals the call setup to the HLR (3).
4. The HLR now checks whether the number
exists and whether the user has subscribed to
the requested
services, and requests an MSRN from the current VLR (4).
5. After receiving the MSRN (5), the HLR can
determine the MSC responsible for the MS
6. Forwards this information to the GMSC (6).
7. The GMSC can now forward the call setup
request to the MSC indicated (7).
8. From this point on, the MSC is
responsible for all further steps. First, it requests the current status of the
MS from the VLR (8).
9. If the MS is available, the MSC initiates
paging in all cells it is responsible for (i.e. the location area, LA,
10. searching for the right cell
11. The BTSs of all BSSs
transmit this paging signal to the MS
12. If the MS answers (12 and 13), the VLR
has to perform security checks (set up encryption etc.)
13. The VLR then signals to the MSC to set up
a connection to the MS.
Mobile
originated call (MOC) :
MOC: MS (A party)->BSC->MSC/VLR
MTC: MSC/VLR->BSC->MS (B party)
MTC: MSC/VLR->BSC->MS (B party)
MOC starts from A party sending service
request to core network
MTC starts when core network receives B number and start analysis for paging, or IAM sending
MTC starts when core network receives B number and start analysis for paging, or IAM sending
1. The MS transmits a request for a new
connection (1)
2. The BSS forwards this request to the MSC
(2).
3. The MSC then checks if this user is
allowed to set up a call with the requested service
4. and checks the availability of resources
through the GSM network and into the PSTN.
5. If all resources are available,
6. The MSC sets up a connection between the
MS and the fixed network.
Messages
for an MTC and MOC: In
addition to the steps mentioned above, other messages are exchanged between an
MS and BTS during connection setup (in either direction). These messages can be
quite often heard in radios or badly shielded loudspeakers as crackling noise
before the phone rings. Figure 4.10 shows the messages for an MTC and MOC.
Paging is only necessary for an MTC, then similar message exchanges follow.
1. The channel access via the random access
channel (RACH) with consecutive channel assignment; the channel assigned could
be a traffic channel (TCH) or a slower signalling channel SDCCH.
2. The next steps, which are needed for
communication security, comprise the authentication
of the MS and the switching to encrypted communication.
3. If someone is calling the MS, it answers
now with ‘alerting’ that the MS is ringing and with ‘connect’ that the user has
pressed the connect button. The same actions happen the other way round if the
MS has initiated the call.
4. After connection acknowledgement, both
parties can exchange data.
5. Closing the connection comprises a
user-initiated disconnect message (both
sides can do this), followed by releasing
the connection and the radio channel.
4.1.6
Handover:
Cellular systems require handover
procedures, as single cells do not cover the whole service area
However, a handover should not cause a
cut-off, also called call drop. GSM aims at maximum handover duration of 60 ms.
·
There
are two basic reasons for a handover:
o
The
mobile station moves out of the range of a BTS
o
The
traffic in one cell is too high and shift some MS to other cells with a lower
load (if possible).
o
Handover
may be due to load balancing.
Figure 4.11 shows four possible handover scenarios
in GSM:
●
Intra-cell handover:
Within a cell, narrow-band interference could make transmission at a certain
frequency impossible. The BSC could then decide to change the carrier frequency
(scenario 1).
●
Inter-cell, intra-BSC handover: This is a typical handover scenario. The mobile station moves
from one cell to another, but stays within the control of the same BSC. The BSC
then performs a handover, assigns a new radio channel in the new cell and
releases the old one (scenario 2).
●
Inter-BSC, intra-MSC handover: As a BSC only controls a limited number of cells; GSM also has
to perform handovers between cells controlled by different BSCs. This handover
then has to be controlled by the MSC (scenario 3).
This situation is also shown in Figure
4.13.
●
Inter MSC handover: A
handover could be required between two cells belonging to different MSCs. Now
both MSCs perform the handover together (scenario 4).
To provide all the necessary information
for a handover due to a weak link, MS and BTS both perform periodic measurements of the downlink and uplink quality respectively.
(Link quality comprises signal level and bit error rate.)
Measurement reports are sent by the MS
about every half-second and contain the quality of the current link used for
transmission as well as the quality of certain channels in neighboring cells
(the BCCHs).
Ping-pong effect may occur in GSM – a value which is
too high could cause a cut-off, and a value which is too low could cause too many
handovers.
Figure 4.13 shows the typical signal flow
during an inter-BSC, intra-MSC handover.
·
The
MS sends its periodic measurements reports, the BTS old forwards these reports
to the BSC old together with its own measurements.
·
Based
on these values and, e.g., on current traffic conditions, the BSCold may decide
to perform a handover and sends the message HO_required to the MSC.
·
The
task of the MSC then comprises the request of the resources needed for the
handover from the new BSC.
·
This
BSC checks if enough resources (typically frequencies or time slots) are
available and activates a physical channel at the BTS new to prepare for the
arrival of the MS.
·
The
BTS new acknowledges the successful channel activation, BSC new acknowledges
the handover request.
·
The
MSC then issues a handover command that is forwarded to the MS. The MS now
breaks its old radio link and accesses the new BTS.
·
The
next steps include the establishment of the link (this includes layer two link
establishment and handover complete messages from the MS).
·
Basically,
the MS has then finished the handover, but it is important to release the
resources at the old BSC and BTS and to signal the successful handover using
the handover and clear complete messages as shown.
4.1.7
Security
The
SIM stores personal, secret data and is protected with a PIN against
unauthorized use.
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.
●Confidentiality: All
user-related data is encrypted. After authentication, BTSand MS apply
encryption to voice, data, and signaling. This confidentiality exists only
between MS and BTS, but it does notexist end-to-end or within the whole
fixedGSM/telephone network.
●Anonymity: To
provide user anonymity, all data is encrypted before transmission,and user
identifiers (which would reveal an identity) are not usedover the air. Instead,
GSM transmits a temporary identifier (TMSI), which isnewly assigned by the VLR
after each location update. Additionally, the VLRcan change the TMSI at any
time.
Three
algorithms have been specified to provide security services in GSM.
Algorithm A3 is
used for authentication
A5 for encryption
A8 for thegeneration
of a cipher key.
4.1.7.1 Authentication
Authentication
is based on the SIM, which stores
the individual authentication key Ki, the user identification IMSI,
and the algorithm used for authentication A3.
Authentication uses a
challenge-response method:
1. The
access control AC generates a random number RAND as challenge, and the
SIM within the MS answers with SRES (signed response) as response.
2. The AuC performs the basic generation of
random values RAND, signed responses SRES, and cipher keys Kc for each IMSI,
and then forwards this information to the HLR.
3. The
current VLR requests the appropriate values for RAND, SRES, and Kc from the
HLR.
4. For
authentication, the VLR sends the random value RAND to the SIM.
5. Both
sides, network and subscriber module, perform the same operation with RAND and
the key Ki, called A3.
6. The
MS sends back the SRES generated by the SIM;
7. The
VLR can now compare both values. If they are the same
8. The
VLR accepts the subscriber, otherwise the subscriber is rejected.
4.1.7.2 Encryption
To
ensure privacy, all messages containing user-related information are encrypted
in GSM over the air interface.
1. After authentication, MS and BSS can start
using encryption by applying the cipher key Kc (the precise location of security
functions for encryption, BTS and/or BSC are vendor dependent).
2. Kc
is generated using the individual key Ki and a random value by applying the
algorithmA8.
3. Note
that the SIM in the MS and the network both calculate the same Kc based on the
random value RAND.
4. The
key Kc itself is not transmitted over the air interface.
5. MS
and BTS can now encrypt and decrypt data using the algorithm A5 and the cipher
key Kc.
6. Kc
should be a 64 bit key – which is not very strong, but is at least a good
protection against simple eavesdropping.
4.3
TETRA
TETRA - Terrestrial
Trunked Radio
Trunked radio systems
·
many different radio carriers
·
assign single carrier for a short period
to one user/group of users
·
taxi service, fleet management, rescue
teams
·
interfaces to public networks, voice and
data services
·
very reliable, fast call setup, local
operation
TETRA
- ETSI standard
o
formerly: Trans European Trunked Radio
o
offers Voice+Data and Packet Data
Optimized service
o
point-to-point and point-to-multipoint
§ call
forwarding, call barring, identification, call hold, call priorities, emergency
calls and group joins
§ channel
has a bandwidth of 25 kHz and can carry
36
kbit/s.
o
several frequencies: 380-400 MHz,
410-430 MHz
TETRA
Architecture
Similar to GSM, but
simpler:
-
MS (mobile station)
-
SwMI (Switching and management
Infrastructure)
-
Um radio interface
-
HDB (Home Database)
-
VDB (Visitor Database)
-
Base station (not always needed)
-
No Handover
-
Main services:
-
V+D: Circuit switched up to four TDMA
channel
-
PDO: connection less, statistical mux
-
Additional services:
-
Group call, acknowledged group call,
broadcast call
TDMA
frame structure of
TETRA:
·
Each frame consists of four slots
(four channels in the V+D service per carrier), with a frame duration of 56.67
ms.
·
Each slot carries 510 bits within
14.17 ms, i.e., 36 kbit/s.
·
16 frames together with one control
frame (CF) form a multiframe, and finally, a hyperframecontains
60 multiframes.
·
To avoid sending and receiving at the same
time, TETRA shifts the uplink for a period of two slots compared to the
downlink.
Services offered by TETRA compared
to GSM:
·
TETRA offers traffic channels (TCH) and
control channels (CCH) similarto GSM.
o
Typical TCHs are TCH/S for voice
transmission
o
and TCH/7.2, TCH/4.8, TCH/2.4 for data
transmission (depending on the FEC mechanisms required).
·
However, in contrast to GSM, TETRA
offers additional services like
·
group call, acknowledged group call,
broadcast call, and discreet listening.
·
Emergency services need a sub-second
group-call setup in harsh environments which possibly
·
lack all infrastructure. These features
are currently not available in GSM or other typical mobile telephone network
4.2
DECT
(Digital European Cordless Telephone)
standardized by ETSI (ETS 300.175-x) for cordless telephones
- standard describes air interface
between base-station and mobile phone
- DECT has been renamed for
international marketing reasons into „Digital Enhanced Cordless
Telecommunication“
- Characteristics
- frequency: 1880-1990 MHz
- channels: 120 full duplex
- duplex mechanism: TDD (Time
Division Duplex) with 10 ms frame length
- multplexing scheme: FDMA with 10
carrier frequencies,
TDMA with 2x 12 slots - modulation: digital, Gaußian
Minimum Shift Key (GMSK)
- power: 10 mW average (max.
250 mW) range: approx. 50 m in buildings, 300 m open space
4.2.1
System architecture
A
DECT system may have various different physical implementations depending on
its actual use.
A
global network connects the local communication to the outside world and
offers its services via the interface D1.
·
Global networks could be
o
integrated services digital networks
(ISDN)
o
public switched telephone networks
(PSTN)
o
public land mobile networks (PLMN)
o
packet switched public data network
(PSPDN).
·
The services offered by these networks
include
o
transportation of data
o
translation of addresses
o
routing of data between the local
networks.
Local
networks:
·
DECT context offer local telecommunication
services
·
simple switching to intelligent
callforwarding
·
address translation etc
o
Home data base (HDB) and
visitor data base (VDB):
§ Both
databases support mobility with functions that are similar tothose in the HLR
and VLR in GSM systems. Incoming calls are automatically forwardedto the
current subsystem responsible for the DECT user, and the currentVDB informs the
HDB about changes in location.
The
DECT core network consists of the fixed radio termination (FT) and the portable
radio termination (PT), and basically only provides a multiplexing service.
FT and PT cover layers one to three at the fixed network side and mobile
network side respectively. Additionally, several portable applications (PA) can
be implemented on a device.
Protocol Architecture DECT
reference Model:
Follows
the OSI reference model
The physical layer, medium access control, and
data link control8 for both the control plane (C-Plane) and the user
plane (U-Plane).
·
An additional network layer has been
specified for the C-Plane, so that user data from layer two is directly
forwarded to the U-Plane.
·
DECT
layers I
- Physical
layer
- modulation/demodulation GMSK
- generation of the physical channel
structure with a guaranteed throughput
- controlling of radio transmission
- channel assignment
on request of the MAC layer
- detection
of incoming signals
- sender/receiver
synchronization
- collecting
status information for the management plane
- MAC
layer
- maintaining basic services,
activating/deactivating physical channels
- multiplexing of logical channels
- e.g., C:
signaling, I: user data, P: paging, Q: broadcast
- segmentation/reassembly
- error control/error correction
·
DECT
layers II
- Data
link control layer
- creation and keeping up reliable
connections between the mobile terminal and basestation
- two DLC protocols for the control
plane (C-Plane)
- connectionless
broadcast service:
paging functionality - point to
point service
- in-call
signaling (similar to LAPD within ISDN), adapted to the underlying MAC
service
- several services specified for the
user plane (U-Plane)
- null-service:
offers unmodified MAC services
- frame
relay: simple packet transmission
- frame
switching: time-bounded packet transmission
- error
correcting transmission: uses FEC, for delay critical, time-bounded
services
- bandwidth
adaptive transmission
- „Escape“
service: for further enhancements of the standard
·
DECT
layers III
- Network
layer
- offers services to request, check,
reserve, control, and release resources at the basestation and mobile
terminal
- resources
- necessary
for a wireless connection
- necessary
for the connection of the DECT system to the fixed network
- main tasks
- call
control: setup, release, negotiation,
control
- call
independent services: call
forwarding, accounting, call redirecting
- mobility
management: identity management,
authentication, management of the location register
·
Enhancements of the standard
- GAP (Generic Access Profile)
standardized by ETSI in 1997
- assures
interoperability between DECT equipment of different manufacturers
(minimal requirements for voice communication)
- enhanced
management capabilities through the fixed network: Cordless Terminal
Mobility (CTM)
- DECT/GSM Interworking Profile
(GIP): connection to GSM
- ISDN Interworking Profiles (IAP,
IIP): connection to ISDN
- Radio Local Loop Access Profile
(RAP): public telephone service
- CTM Access Profile (CAP): support
for user mobility
4.4
UMTS and IMT-2000
o
UTRA (was: UMTS-Universal Mobile
Telecommu
o
nications System, now: Universal
Terrestrial Radio Access)
o
enhancements of GSM
§ EDGE
(Enhanced Data rates for GSM Evolution):
·
enhanced modulation schemes for data rates of
up to 384 kbit/s
§ CAMEL
(Customized Application for Mobile Enhanced Logic)
·
introduce intelligent network
§ VHE
(virtual Home Environment)
·
QoS aspects, roaming, services, billing,
accounting, radio aspects, core networks, access networks, terminal
requirements, security, application domains, operation and maintenance, and
several migration aspects.
o
fits into GMM (Global Multimedia
Mobility) initiative from ETSI
Basic
requirements for UMTS and for UTRA:
Key
requirements are
§ min.
144 kbit/s rural (goal: 384 kbit/s)
§ min.
384 kbit/s suburban (goal: 512 kbit/s)
§ up
to 2 Mbit/s urban
UMTS should also provide several bearer services:
·
circuit and packet switched
transmission
·
Handover should be possible
between UMTS cells, but also between UMTS
·
and GSM or satellite
networks.
·
The system should be
compatible with GSM,ATM, IP, and ISDN-based networks.
·
To reflect the asymmetric
bandwidth needsof typical users, UMTS use
o For thepaired band (using FDD as a duplex mechanism)
§ used forpublic mobile network providers (wide area GSM)
§ ETSI adopted the W-CDMA(Wideband CDMA)
o For the unpaired band (using TDD as duplexmechanism)
§ used for local and indoor communication ( DECT).
§ the TD-CDMA (Time Division CDMA)
Five groups of 3Gradio access technologies:
●IMT-DS: The direct spread technology comprises wideband CDMA (WCDMA)systems.
This is the technology specified for UTRA-FDD and used
by all
European providers and the Japanese NTT DoCoMo for 3G wide area
services.
●IMT-TC: Initially, this family member, called time code, contained
only the
UTRA-TDD
system which uses time-division CDMA (TD-CDMA). Later on,
the Chinese
proposal, TD-synchronous CDMA (TD-SCDMA) was added.
●IMT-MC: cdma2000 is a multi-carrier technology standardized by
3GPP2
(Third
generation partnership project 2, 3GPP2, 2002), which was formed
shortly
after 3GPP to represent the second main stream in 3G technology.
●IMT-SC: The enhancement of the US TDMA systems, UWC-136, is a single
carriertechnology originally promoted by the Universal
WirelessCommunications Consortium (UWCC).
●IMT-FT: As frequency time technology, an enhanced version of the
cordless
telephone
standard DECT has also been selected for applications that do not
require
high mobility.
4.4.1 UMTS releases and
standardization
Release 4
introduces quality of service in the
fixed network plus several execution
environments.
Release 5
specifies a radically different core network. The GSM/GPRS
basednetwork will be replaced by an almost all-IP-core
Additional
features are end-to-end QoS messaging and several data compressionmechanisms.
Release 6
Comprises the use of
multipleinput multiple output (MIMO) antennas, enhanced MMS, security
enhancements,WLAN/UMTS interworking, broadcast/multicast services, enhanced
IMS,IP emergency calls, and many more management features.
4.4.2 UMTS system architecture
The UTRA network (UTRAN) handlescell level mobility and
comprises several radio network subsystems (RNS).
Thefunctions of the RNS
include radio channel ciphering and deciphering, handover
control, radio resource managementetc. The UTRAN is connected to
the userequipment (UE) via the radio interfaceUu(which is
comparable to the Um interface
in GSM). Via the Iuinterface (whichis similar to the A
interface in GSM),UTRAN communicates with the core network (CN).
The CN contains
functionsfor inter-system handover, gateways to other networks (fixed or
wireless), andperforms location management if there is no dedicated connection
between UEand UTRAN.
UMTS further subdivides the above simplified architecture into
so-called domains :
User equipment domain & Mobile equipment domain.
The user equipment domain :
·
is assigned to a singleuser
and comprises all the functions that are needed to access UMTS services.
·
Within this domain are the USIM
domain and the mobile equipment domain.
o TheUSIM domain:
§ contains the SIM for UMTS which performs functions for
encryptionand authentication of users, and stores all the necessary
user-related data forUMTS. USIM belongs to a service provider and contains a microprocessor
for an enhanced program execution environment (USAT, UMTS SIMapplication
toolkit).
o The mobile equipment domain:
§ functions for radio
The infrastructure domain:
·
is shared among all users
and offers UMTS servicesto all accepted users.
·
consists of the access network domain &core networkDomain.
o Access network domain
§ which contains access network independent functions.
o Core networkdomain can be separated
into three domains.
§ The serving - currently used by a user for accessingUMTS
services.
§ home network - data look-up
§ Transit network – used when the serving
network cannot directly
§ contact the home network.
4.4.3 UMTS radio
interface
·
The biggest difference
between UMTS and GSM comes with the new radio interface
·
(Uu).
·
The duplex mechanisms are
already well known from GSM (FDD) andDECT (TDD).
·
The direct sequence (DS)
CDMA used in UMTS is new.
·
This technology multiplies a stream of bits with a chipping
sequence. Thisspreads the signal and, if the chipping sequence is unique,
can separate differentusers.
·
To separate differentusers,
the codes used for spreading should be (quasi) orthogonal, i.e.,
theircross-correlation should be (almost) zero.
·
UMTS uses a constant
chipping rate of 3.84 Mchip/s.
The first step in a sender is spreading
of user data (datai) using
orthogonalspreading codes. Using orthogonal codes separates the different
data streams of a
sender. UMTS uses so-called orthogonal
variable spreading factor (OVSF) codes.
Orthogonal codes are generated bydoubling a chipping sequence X
with and without flipping the sign of the chips.
This results in X and –X, respectively. Doubling the chipping
sequence also results
in spreading a bit twice as much as before. The spreading factor
SF=n becomes 2n.
Starting with a spreading factor of 1, Figure 4.27 shows the
generation of orthogonal
codes with different spreading factors. Two codes are orthogonal as long as one codeis never a part of the
other code.
Looking at the coding tree
in Figure 4.27 and consideringthe construction of the codes, orthogonality is
guaranteed if one code hasnot been generated based on another.
4.4.3.1 UTRA-FDD (W-CDMA)
Figure
4.28 shows a radio frame comprising 15 timeslots.
·
Time slots in W-CDMA are not used for
user separation but to support periodic functions (note that this is in
contrast to, e.g., GSM, where time slotsare used to separate users!).
·
A radio frame consists of 38,400 chips
and has aduration of 10 ms.
·
Each time slot consists of 2,560 chips,
which roughly equals666.6 μs.11
Dedicated
physical data channel (DPDCH): This channel conveys
user orsignaling data. The spreading factor of this channel can vary between 4
and256. This directly translates into the data rates this channel can offer:960
kbit/s (spreading factor 4, 640 bits per slot, 15 slots per frame, 100
framesper second), 480, 240, 120, 60, 30, and 15 kbit/s (spreading factor 256).
Thisalso shows one of the problems of using OVSF for spreading: only certain
multiplesof the basic data rate of 15 kbit/s can be used.
Dedicated physical control
channel (DPCCH): This channel conveys control data for the physicallayer only and
uses the constant spreading factor 256. The pilot is usedfor channel
estimation. The transport format combination identifier(TFCI) specifies
the channels transported within the DPDCHs. Signaling fora soft handover is
supported by the feedback information field (FBI). Thelast field, transmit
power control (TPC) is used for controlling the transmissionpower of a
sender.
Dedicated physical channel
(DPCH): The
downlink time multiplexes controland user data. Spreading factors between 4 and
512 are available. While no collisions can occur on the downlink (only the base
station sendson the downlink), medium access on the uplink has to be
coordinated. A physicalrandom access channel (PRACH) is used
for this purpose.
A UE has to perform the
following steps during the search for a cell afterpower on:
Primary synchronization: A UE has to synchronize with
the help of a 256chip primary synchronization code. This code is the same for
all cells andhelps to synchronize with the time slot structure.
Secondary synchronization: During this second phase the
UE receives asecondary synchronization code which defines the group of
scramblingcodes used in this cell. The UE is now synchronized with the frame
structure.
Identification of the
scrambling code: The UE tries all scramblingcodes within the group of codes to find
the right code with the help of acorrelator. After these three steps the UE can
receive all further data overa broadcast channel.
4.4.3.2 UTRA-TDD (TD-CDMA)
·
The second UTRA mode, UTRA-TDD,
separates up and downlink in time using aradio frame structure similar to FDD.
·
15 slots with 2,560 chips per slot form
aradio frame with a duration of 10 ms.
·
The chipping rate is also 3.84 Mchip/s.
·
Toreflect different user needs in terms
of data rates, the TDD frame can be symmetricalor asymmetrical
·
midampleis
used for training and channel estimation.
·
To loosen the tightsynchronization a
little bit, a guard period (GP) is used.
·
4.4.4 UTRAN
This consists of several radio network subsystems (RNS).
Each RNS iscontrolled by a radio
network controller (RNC) and comprises several componentsthat are called
node B.
An RNC in UMTS can be compared with the BSC;
Anode B is similar to a BTS.
Each node B can control several antennas whichmake a radio
cell.The mobile device, UE, can be connected to one or moreantennas. EachRNC is
connected with the core network (CN) over the interface Iu(similar tothe
role of the A interface in GSM) and with a node B over the interface Iub.
Anew interface, which has no counterpart in GSM, is the interface Iurconnectingtwo
RNCs with each other.
4.4.4.1
Radio network controller
An
RNC in UMTS has a broad spectrum of tasks as listed in the following:
●Call admission control:
It
is very important for CDMA systems to keep theinterference below a certain
level. The RNC calculates the traffic withineach cell and decides, if
additional transmissions are acceptable or not.
●Congestion control: During
packet-oriented data transmission, several stations
share
the available radio resources. The RNC allocates
bandwidth toeach station in a cyclic fashion and must consider the QoS
requirements.
●Encryption/decryption: The
RNC encrypts all data arriving from the fixednetwork before transmission over
the wireless link (and vice versa).
●ATM switching and
multiplexing, protocol conversion: Typically,
theconnections between RNCs, node Bs, and the CN are based on ATM. AnRNC has to
switch the connections to multiplex different data streams.
●Radio resource control:
The
RNC controls all radio resources of the cellsconnected to it via a node B. This
task includes interference and load measurements.
The
priorities of different connections have to be obeyed.
●Radio bearer setup and
release: An RNC has to set-up, maintain, andrelease a logical
data connection to a UE (the so-called UMTS radio bearer).
●Code allocation: The
CDMA codes used by a UE are selected by the RNC.
These
codes may vary during a transmission.
●Power control: The
RNC only performs a relatively loose power control
(the
outer loop).
●Handover control and
RNS relocation: Depending on the signal strengths
received
by UEs and node Bs, an RNC can decide if another cell would bebetter suited for
a certain connection. If the RNC decides for handover itinforms the new cell.
If a UEmoves further out of the range of one RNC, a new RNC responsible for
theUE has to be chosen. This is called RNS
relocation.
●Management: Finally,
the network operator needs a lot of informationregarding the current
load, current traffic, error states etc. to manage its network.The RNC
provides interfaces for this task, too.
4.4.4.2 Node B
An important task of a node B is the inner loop power
control to mitigatenear-far effects. This node also measures
connection qualities and signalstrengths. A node B can even support
a special case of handover, a so-calledsofter handover which takes
place between different antennas of the same node
4.4.4.3 User equipment
●As the counterpart of a node B, the UE performs signal quality
measurements,
inner loop power control, spreading and modulation, and rate
matching.
●As a counterpart of the RNC, the UE has to cooperate during
handover and
cell selection, performs encryption and decryption, and
participates in theradio resource allocation process.
●As a counterpart of the CN, the UE has to implement mobility
management
functions, performs bearer negotiation, or requests certain
services fromthe network.
This list of tasks of a UE, which is not at all exhaustive,
already shows thecomplexity such a device has to handle. Additionally, users
also want to haveorganizers, games, cameras, operating systems etc. and the
stand-by time shouldbe high.
4.4.5 Core network
The
circuit switched domain (CSD) comprises theclassical circuit switched
services including signaling. Resources are reserved atconnection setup and the
GSM components MSC, GMSC, and VLR are used.The CSD connects to the RNS via
apart of the Iu interface called IuCS
The
packet switched domain (PSD) uses the GPRS components SGSN andGGSN and
connects to the RNS via the IuPSpart of the Iu interface.
Bothdomains need the
data-bases EIR for equipment identification and HLR for locationmanagement
The CSD uses the ATMadaptation layer 2 (AAL2) for
user data transmission.
TheAAL2 segmentation and reassembly layer (SAR) is, for
example, used to segmentdata packets received from the RLC into small chunks
which can betransported in ATM.
In the PSD several more protocols are needed.
UE are encapsulated using the GPRS tunnelingprotocol (GTP).
Packet data convergence protocol (PDCP).
header compression to avoid redundant
data transmission
Theradio link control (RLC) layer offers three different
transport modes.
Theacknowledged mode transfer uses ARQ for error correction
and guarantees onetimein-order delivery of data packets.
The unacknowledged mode transferdoes not perform ARQ but
guarantees at least one-time delivery of packets withthe help of sequence
numbers.
The transparent mode transfer simply forwardsMAC data
without any further processing.
4.4.6
Handover
UMTS
knows two basic classes of handovers:
● Hard handover: Switching
between different antennas or different systems is performed at a certain point
in time. UTRA TDD can only use this type. Switching between TDD cells is
done between the slots of different frames.
Eg:
Inter frequency handover, i.e., changing the carrier frequency, is a
hard handover.
Inter
system handovers (handovers to and from GSM or other
IMT-2000 systems)
are
hard handovers in UMTS. This includes
● Soft handover: This
is the real new mechanism in UMTS compared to GSM and is only available in the
FDD mode. Soft handovers are well known from traditional CDMA networks as they
use macro diversity, a basic property of CDMA.
The
fact that a UE receives data from different antennas at the same time makes a
handover soft. Moving from one cell to another is a smooth, not an abrupt
process. Macro-diversity makes the transmission more
robust with respect to fast fading, multi-path propagation, and shading. If one
path is blocked by an obstacle the chances are good that data can still be
received using another antenna.
If the UE moves in the example from the upper cell to the lower cell,
the upper RNC acts as a serving RNC (SRNC) while the other is the drift
RNC (DRNC).
Intra-node
B, intra-RNC: UE1 moves from one antenna of node B1 to
another antenna. This type of handover is called softer handover. In
this case node B1 performs combining and splitting of the data streams.
● Inter-node B,
intra-RNC: UE2 moves from node B1 to node B2. In
this case RNC1 supports the soft handover by combining and splitting data.
● Inter-RNC: When
UE3 moves from node B2 to node B3 two different types of handover can take
place. The internal inter-RNC handover is not visible for the CN, as
described in Figure 4.34. RNC1 can act as SRNC, RNC2 will be the DRNC. The CN
will communicate via the same interface Iu all the time. As soon as a relocation of the interface Iu
takes place (relocation of the controlling RNC), the handover is called an external
inter-RNC handover.
Communication
is still handled by the same MSC1, but the external
handover
is now a hard handover.
● Inter-MSC: It
could be also the case that MSC2 takes over and performs a hard handover of the
connection.
● Inter-system: UE4
moves from a 3G UMTS network into a 2G GSM network. This hard handover is
important for real life usability of the system due to the limited 3G coverage
in the beginning.
ue
to the limited 3G coverage in the beginning.
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