Communication Systems 11th lecture

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Communication Systems11th lecture

• Chair of Communication Systems
• Department of Applied Sciences
• University of Freiburg
• 2006

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Communication SystemsLast lecture GSM, BSS, SIM

• GSM Global System for Mobile communication is a
  worldwide standard
• GSM introduces a park of abbreviations -)
• Defines a network infrastructure including Base
  Station Subsystem BSS, containing the BTS (Base
  Transceiver Stations) communicating over the air
  interface with the Mobile Stations (MS,
  consisting of Mobile Equipment (ME) and SIM)
• SIM is the Subscriber Identity Module which keeps
  at least the following data IMSI (International
  Mobile Subscriber Identity), IMSI (International
  Mobile Subscriber Identity) both 15-digit,
  temporarily TMSI and MSRN (Mobile Station Roaming
  Number)
• The card is an external hardware module which may
  store user data like received SMS or phone book
  entries

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Communication SystemsLast lecture GSM, logical
structure of the network
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Communication SystemsLast lecture GSM, logical
structure of the network

• The network subsystem contains the (G)MSC,
  (Gateway) Mobile Switching Centers
• In the Home Location, Visitor Location Registers
  user data (MSISDN, configuration, ...) is kept
  permanently or temporarily
• The Authentication Center (AUC) handles the user
  authentication and cryptographic routines
• GSM has some shortcomings in security User
  authenticates itself to the server, network but
  not vice-versa
• IMSI catchers may grab MS and reroute
  connection
• Eavesdropping is possible because of leaked
  crypto algorithms and unencrypted network links

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Communication SystemsLast lecture GSM, logical
structure of the network

• Operation Maintenance Subsystem (OSS) is the
  whole systems management layer
• Network measurement and control functions,
  network administration
• Security Management, e.g. Equipment Identity
  Register (EIR) management

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Communication SystemsPlan for this lecture

• Data Services on top of GSM networks
• SMS the most expensive 140Bytes ever
• HSCSD as a typical phone network inspired data
  service
• GPRS an extension to existing GSM
  infrastructure to provide packet orientated
  services while optimizing the use of the air
  interface
• MMS the SMS successor using the GPRS backend to
  offer advanced messaging services to the
  subscribers
• WAP a protocol to bring Internet (like)
  services to the mobile device
• Using special WAP and GPRS services in PPP dial-in

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Communication SystemsGSM and data services

• GSM was the first fully digital wireless
  telephony network
• structure of logical channels offers more than
  just voice
• First very popular data communication was the
  Short Message Service (the most expensive bytes
  of the communication era -))
• defined already in the GSM phase 1, first SMS was
  sent in 1992
• defined to inform users on incoming messages on
  their voice box, there was no idea to charge for
  it initially
• SMS is store-and-forward service
• a designated SMS service center (SMS-SC) stores
  the messages there is no 11 communication
  between end user devices

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Communication SystemsGSM and SMS

• SMS widespread and used for many applications
• 11 message exchange between subscribers
• traditional informing the subscriber on received
  messages on his box, it is possible to overwrite
  already received messages with updates you have
  2 new messages with e.g. you have 4 new
  messages
• traditional information services From the
  provider, subscribed services like soccer
  results, stock quotes or just error messages from
  important servers
• M-Commerce to pay services with the bill issued
  by the mobile provider
• authentication request a one time password

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Communication SystemsGSM and SMS

• For the transfer over the wireless interface SMS
  uses the packet orientated, reliable Short
  Message Transfer Protocol (SM-TP)
• if there is no active voice communication a
  separate SDCCH is used
• no reservation of a traffic channel is needed

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Communication SystemsGSM and SMS

• during voice session the SM-TP is multiplexed
  into the SACCH, enabling the MS to receive
  messages during other active connections
• within the core network the MAP (Mobile
  Application Part) and SS7 is used
• SMS allow 160 characters of a 7-bit alphabet
  (thus 140 Byte message size)
• possible to allow interpretation as binary data
  (logo and such stuff, ...), but not really
  standardized until EMS
• PDU (Protocol Description Unit) describes type,
  encoding and length of the message
• It is possible to stipulate that SMS content is
  directly passed to the SIM (for logo, device
  settings etc.)

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Communication SystemsGSM and EMS

• EMS introduced around turn of century and
  available on all mobile phones by now
• allows the transfer of formatted text, sounds of
  up to 80 notes, pictures of 16x16 or 32x32 pixels
  monochrome and concatenation of pictures for
  animation
• vCard and vCalendar data
• implemented through simple chaining of SMSes,
  thus avoiding dedicated transport channel
• MMS is discussed a little bit later, because of
  totally different approach

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Communication SystemsGSM and IP data services

• GSM can be used to offer pervasive data services
  (was much more interesting in the pre WLAN era)
• Voice is encoded as digital data stream, thus GSM
  is able to offer other data services to its users
  too

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Communication SystemsGSM and IP data services

• The age of GSM is detectable in the early
  definition of data services
• the rather old standard from the end of the 80s
  offers just 9,6kbit/s (netto data rate of a full
  traffic channel (TCH))
• with advanced channel coding 14,4kbit/s are
  possible
• but that is ridiculous for todays modern Internet
  web content and multimedia applications
• In the process of improving GSM the so called
  High-Speed Circuit Switched Data (HSCSD) was
  introduced

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Communication SystemsGSM - HSCSD

• HSCSD combines several time slots to achieve
  higher bandwidth on the mobile interface
• 4 channels of 14,4kbit/s add up to 57,6kbit/s
• rather simple in setup, predictable quality
• But high demands on resources
• infrequent used data channels blocked for voice
  traffic of other users, thus the cell capacity is
  reduced
• so one data service user equals to four mobile
  voice users imagine on the charges needed to
  compensate
• Thus HSCSD is standardized for a while now, but
  not every network provider offers this service
  (only D2 and E in Germany)

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Communication SystemsGSM HSCSD data rates

• HSCSD data services are charged not for amount of
  data transferred, but connect time
• Data rates depend on the available traffic
  channel types (half rate/full rate, advanced
  coding channel)

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Communication SystemsGSM GPRS

• Primary GSM data services follow the circuit
  switching network model and reserve resources in
  advance acceptable for voice but not for IP
• Extension to GSM introduced in GSM phase 2 - GPRS
• Started in 1999
• packet orientated approach to data switching
• allocation of channels request-driven thus up
  to 115kbit/s would be possible when using 8 time
  slots
• disadvantage infrastructure has to be extended
  significantly, new components are to be installed
  in BSS (Base Station Subsystem)

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Communication SystemsGSM GPRS

• Bandwidth of 53,6 kbit/s (4 full rate traffic
  channels à 13,4 kbit/s), up to 107,2 kbit/s with
  8 channels
• GPRS usually operates asynchronous with more
  bandwidth for down than for upstream
• Capabilities of a mobile device are expressed in
  class number, e.g.
• Class 8 devices are able to use up to four down-
  and one upstream channel
• Class 10 devices handle four down- and two
  upstream channels
• Advantage of GPRS over HSCSD more flexible,
  development into direction of UMTS network

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Communication SystemsGSM GPRS

• GPRS bases on an additional infrastructure GSN
  GPRS Support Nodes as an extension to GSM

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Communication SystemsGPRS components and
interfaces

• SGSN serving GSN to support the MSC for
  localization, billing and security
• GGSN gateway GSN is the gateway to the packet
  data network usually the Internet
• GR GPRS register to support the HLR (home
  location register), used for user address mapping
• Between the different components interfaces are
  defined
• Gb between BSS and SGSN and Gn between the
  different GSNs, Gi is the Internet gateway
• GPRS defines a complete protocol architecture for
  the transport of packetized data and allow
  handover between different BTS, MSC/SGNS

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Communication SystemsGPRS sessions

• For every session a PDP (Packet Data Protocol)
  context is generated and stored in GGSN, it
  consists of
• type (usually IP v4)
• address of the MS (normally the IP address),
  which allows mapping of PDP address to GSM
  address
• QoS parameters
• address of access point to external networks
  (GGSN)
• Session setup is comparable to setup of mobile
  originated voice calls
• Channels have to be activated and the
  authentication procedure to be passed

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Communication SystemsGPRS sessions

• After the session setup as shown below
• SGSN encapsulates the IP packet and routes it
  over the GPRS backbone with the help of the PDP
  context defined

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Communication SystemsGPRS sessions

• depending on the routing decision the packet
  leaves the GPRS network on a designated GGSN as a
  normal Internet routeable IP packet
• at this point normally NAT/IP masquerading takes
  place (most GPRS providers offer only addresses
  from the private IP ranges to mobile
  subscribers
• the packet reaches the destination machine with
  standard IP routing
• The destination machine (usually) answers the
  request from the MS and sends a packet back to
  the GGSN
• the GGSN looks up the position of the MS,
  encapsulates the packet and routes the packet
  within the GPRS backbone to the SGSN

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Communication SystemsGPRS components and
interfaces

• the SGSN decapsulates the packet and hands it
  over to the BSS for delivery to the MS

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Communication SystemsGPRS services and QoS

• GPRS offers several services
• Point-to-Point connection orientated network
  service (PTP-CONS), which keeps connections open
  even when cell handovers occur
• Point-to-Point connectionless network service
  (PTP-CLNS), similar to UDP in the IP world, no
  handovers are required, provided
• Point-to-Multipoint is planned in Phase 2 and
  offers group communication (conferences, ...,
  comparable to IP multicast)
• QoS profiles could be requested by the user

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Communication SystemsGPRS services and QoS

• Three QoS profiles available low, medium, high
• They define reliability class
• loss probability of standard data units (SDU)
  ranges from 10-9 in class 1 to 10-2 in class 3,
  same for corrupt SDU probability
• duplicate and out of sequence packet probability
  ranges from 10-9 in class 1 to 10-5 in class 3
• delay class
• delays range from 0.5s in best up to 250s in
  worst class
• and user data throughput class
• No idea if really in use or theoretical option
  like QoS fields in IP header, of course the
  enforcement of classes is much easier than in the
  IP world

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Communication SystemsGPRS, HSCSD and enhanced
mobile data services

• The introduction of high bandwidth data
  services allows more than SMS or EMS services
• Mobile service providers have to find additional
  way to earn revenues from their networks in a
  market environment with sinking fees they can
  charge for voice services
• SMS was a really successful offering, so a
  successor was defined
• MMS is the abbreviation for Multimedia Messaging
  Service
• Defined by several organizations for GSM and UMTS
  networks
• Common standard for the mobile phones of
  different vendors

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Communication SystemsGPRS and enhanced mobile
data services

• MMS allows the addressing via
• MSISDN (persistent telephone number of the mobile
  subscriber)
• Or just an email address defined in RFC822
• IP should be supported in near future
• MMS is able to handle
• Formatted text, different fonts and text
  encodings
• Voice encoded with Adaptive Multi Rate codec (as
  used with UMTS)
• Graphics in several formats

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Communication SystemsGPRS and enhanced mobile
data services

• MMS uses a container format for the multimedia
  content
• SMIL (Synchronized Multimedia Integration
  Language), XML based, which defines several
  modules for layout, timing, synchronization (of
  graphics, animation, text and speech or sound
  ...)
• WML (Wireless Markup Language) for the
  presentation like in WAP browser
• A MMS Center (MMS-C) or MMS relay/server handles
  the messages basically in a similar way like SMS
• Store-and-forward architecture which sends and
  receives messages to and from a mobile subscriber

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Communication SystemsGPRS and enhanced mobile
data services

• MMS Center may exchange data with external (MMS,
  email, FAX, value-added services) servers
• It looks up user settings and preferences from
  the Home Location Register (HLR)

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Communication SystemsGPRS and enhanced mobile
data services

• MMS data exchange is handled directly over GPRS
• Using e.g. IP/TCP/HTTP
• Or indirectly linking in a WAP gateway before
  then using HTTP
• The MMS relay/server may transform data format
  into mail format or vice versa
• So the same service is charged differently (GPRS
  data services uses simply another Access Point
  (AP) than MMS) ... as long as the user can be
  maked to believe ...
• Two years ago German computer magazine ct
  demonstrated a charge free data connection over
  the MMS gateways within the GPRS backbone

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Communication SystemsGPRS, HSCSD and WAP

• The Wireless Application Protocol was defined to
  bring Internet like services to the mobile
  platform
• GPRS data rate is rather restricted as usually
  the display and compute power of the MS is
• Thus a specific protocol was defined by Ericsson,
  Motorola, Nokia Unwired Planet in 1997
• WAP 1.0 was released in 1998, but nobody really
  used it (to expensive for to restricted services
  offered)
• The initial standard was extended to WAP version
  1.1, 1.2, 1.2.1 (not really compatible and
  available on every mobile device)
• After long series of failures WAP 2.0 was defined
  in 2001 integrating well defined and agreed upon
  Internet standards

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Communication SystemsGPRS, HSCSD and WAP

• Two types of services are defined traditional
  web like and push service

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Communication SystemsGPRS, HSCSD and WAP

• Data reduction is handled by the use of optimized
  protocols
• The Internet protocols are translated into their
  counterparts in the WAP standard via translation
  tables
• HTTP-Header Accept application/vnd.wap.wmlc
• WSP-Header 0x80,0x94
• HTTP-Header Accept-Language enq0.7
• WSP-Header 0x83,0x02,0x99,0x47
• HTTP-Header Accept-Language en,sv
• WSP-Header 0x83,0x99,0x83,0xF0

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Communication SystemsGPRS, HSCSD and WAP

• Hash tables translated each WSP header into its
  HTTP counterpart
• A designated gateway is needed as translation
  device

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Communication SystemsWAP 1.X helper protocols

• Of course the webserver has to offer WAP user
  agent (UA), the so called Wireless Application
  Environment (WAE) optimized content
• try out the www.google.de or www.bahn.de with a
  WAP UA to see two good examples
• The OSI session layer is presented by WSP, the
  Wireless Session Protocol, a transaction layer by
  WTP (Wireless Transaction Protocol)
• A security layer is provided with WTLS, the
  Wireless Transport Layer Security (thus a secure
  connection of a WAP UA and a secure website may
  consist of two parts with unpacking at the WAP
  gateway)
• The transport layer is handled by Wireless
  Datagram Protocol (WDP)

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Communication SystemsWAP 1.X helper protocols

• We see A whole new protocol stack was invented
  to translate the existing protocols in optimized
  ones in mobile phone networks
• The reduction rate compared to the existing
  internet protocols is rather good
• When connections get faster and devices get
  better displays nobody cares so much
• The whole design was rather complex, error prone
  and the gateway software proprietary
• There are only few content providers (of course
  the mobile providers with their community
  portals) which made bigger investments (for a
  rather small user group) and thus use of the
  technology

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Communication SystemsWAP 2.0 standard

• WAP 2.0 simply replaces the complex architecture
  with a WAP proxy which is mostly HTTP compatible
• The standard protocol methods like GET, POST,
  CONNECT, HEAD OPTIONS are supported
• Content is formatted with WAP optimized style
  sheets

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Communication SystemsWAP 2.0, GPRS and cool
add-on packages

• Thus the mobile service provider offered a HTTP
  like service over their GPRS infrastructure
• Trying to push the mobile Internet special
  tariffs were introduced (understanding pricing in
  mobile communication is as easy as understanding
  the German tax system)
• O2 (aka viag interkom) offers a WAP package for
  just 5EUR flat compared to a GPRS MB charged with
  9EUR
• Of course they use another AP than for normal
  GPRS (same like with MMS)
• Of course other protocols than WAP are forbidden
  to use (but how to distinguish?)

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Communication SystemsWAP 2.0, GPRS and cool
add-on packages

• OpenVPN is an open source VPN software which is
  able to offer services over HTTP CONNECT proxies
• Invented to get a pass-through on rather
  restricted firewalls
• The OpenVPN has just to present the correct UA
  identifier the provider expects to see

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Communication SystemsWAP 2.0, GPRS and cool
addon packages

• Even normal web traffic can pass the provider
  proxy, if the correct identifier string is
  presented, e.g.
• Mozilla/1.22 (compatible MSIE 5.01 PalmOS 3.0
  EudoraWeb 2.1
• Profile http//wap.sonyericsson.com/UAprof/P800R1
  02.xml
• The Internet forums are full of discussions on
  pass through, lists of allowed user agents are
  easily available
• Disclaimer Use this information for
  demonstrations on suboptimal firewall setup and
  offered services issues only
• Setup was developed and proved as a
  Studienarbeit at the professorship (will be
  published in Linux Magazine soon)

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Communication SystemsGSM data services and
devices

• Each modern mobile phone can be used as a modem
  to connect TE (any Terminal Endpoint) to the
  wireless data service
• Term modem is not correct, because the digital
  data stream has not to be modulated onto an
  analogous signal
• Other devices like PCMCIA cards available too

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Communication SystemsGSM data services and
devices

• Not all phones or PCMCIA cards may offer HSCSD
  and several services classes for GPRS
• But device handling is rather similar to
  traditional modem or ISDN dial-in connections
• A hayes compatible AT command set is used to
  setup and close the data connection, there are
  GSM specific commands to enter the PIN (for
  enabling the access to the SIM card plugged into
  PCMCIA) or to get information on signal strength
• When the connection is established the PPP
  (Point-to-Point protocol) is used to pass IP and
  DNS configuration

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Communication SystemsGSM data services and
devices

• Snippet from a Linux GPRS modem call script
• ...
• SAY \ndefining PDP context...\n" \
• OK 'ATF' \
• OK 'ATV1E0S00D2C1' \
• OK ATCMEE1 \
• OK 'ATcgdcont1,"IP","wap.viaginterkom.de"'
  \
• OK-AT-OK ATD99 \
• SAY "\nwaiting for connect...\n"
• ...
• Specific AP is choosen (here wap.viaginterkom.de)

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Communication SystemsGSM data services and
devices

• The dial command does not use a typical
  telephone number (to reach a certain service) but
  addresses a stored profile in the mobile phone
  for the GPRS access
• Connect ppp0 lt--gt /dev/rfcomm1
• sent LCP ConfReq id0x1 ltasyncmap 0x0gt ltmagic
  0x71179e05gt ltpcompgt ltaccompgt
• rcvd LCP ConfReq id0x1 ltasyncmap 0x0gt ltpcompgt
  ltaccompgt ltauth papgt
• No auth is possible
• sent LCP ConfRej id0x1 ltauth papgt
• rcvd LCP ConfRej id0x1 ltmagic 0x71179e05gt
• sent LCP ConfReq id0x2 ltasyncmap 0x0gt ltpcompgt
  ltaccompgt
• rcvd LCP ConfReq id0x2 ltasyncmap 0x0gt ltpcompgt
  ltaccompgt
• sent LCP ConfAck id0x2 ltasyncmap 0x0gt ltpcompgt
  ltaccompgt
• rcvd LCP ConfAck id0x2 ltasyncmap 0x0gt ltpcompgt
  ltaccompgt
• ...

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Communication SystemsGSM data services and
devices

• ...
• sent CCP ConfReq id0x1 ltdeflate 15gt
  ltdeflate(old) 15gt ltbsd v1 15gt
• sent IPCP ConfReq id0x1 ltcompress VJ 0f 01gt
  ltaddr 0.0.0.0gt
• rcvd LCP ProtRej id0x4 80 fd 01 01 00 0f 1a 04
  78 00 18 04 78 00 15 03 2f
• rcvd IPCP ConfReq id0x1 ltaddr 10.49.48.62gt
• sent IPCP ConfAck id0x1 ltaddr 10.49.48.62gt
• rcvd IPCP ConfRej id0x1 ltcompress VJ 0f 01gt
• sent IPCP ConfReq id0x2 ltaddr 0.0.0.0gt
• rcvd IPCP ConfNak id0x2 ltaddr 10.45.48.63gt
• ...
• local IP address 10.49.48.66
• remote IP address 10.49.48.67
• Thus the IP setup is easily compatible to known
  PPP implementation

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Communication SystemsGSM data services and
devices

• The GPRS or HSCSD data rate is comarable to
  traditional wired modem connections
• You might end up with download rates up to
  5-6kByte/s, the upload is often much slower
• GSM, GPRS is not able to cope with fast movement
  of the MS very well
• The round trip times of packets are rather awful
  a small ping packet can take around 600-1000ms to
  travel (lot of protocols, stacks and devices are
  included)
• Useable for traditional asynchronous services
  like email and web (at least for low footprint
  sites), but not for interactive, high traffic
  services, like TV, video conferences, ...

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Communication SystemsFrom GSM to 3rd generation
mobile networks

• The short comings of GSM led to the development
  of a next generation mobile network
• The new network
• Should use the scarce resources of the shared
  medium air more efficiently
• Should be really international (GSM had a
  primarily scope on Europe first)
• Much higher data rates should be offered with
  reduced delays
• Preferring the packet orientated approach over
  the circuit switched one data services play an
  increasing role in mobility and voice could be
  just seen as data too (in reality is voice is
  digitized and sent in packets in GSM already)

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Communication SystemsIMT2000 and UMTS

• International Telecommunication Union (ITU)
  defined demands for third generation mobile
  networks with the IMT-2000 standard
• 3GPP (3G Partnership Project) continued that work
  by defining a mobile system that fulfils the
  IMT-2000 standard
• Resulting system is called Universal Mobile
  Telecommunications System (UMTS)
• Release '99 defined the bearer services with 64
  kbit/s circuit switched and up to 384 kbit/s
  packet switched data rates
• Location services and call services were defined
  GSM-compatibility should be offered, the
  authentication and security will be upgraded to
  USIM

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Communication SystemsUMTS

• Several different paths from 2G to 3G defined
• In Europe the main path starts from GSM when GPRS
  was added to the system
• From this point it is possible to go to the UMTS
  system as we will see in core network structure
  of UMTS next lecture
• In North America the system evolution will start
  from TDMA going to EDGE and from there to UMTS
• In Japan (the blind spot of GSM) two different 3G
  standards used
• W-CDMA (which is compatible with UMTS) by NTT
  DoCoMo, Vodafone KK, and by new entrants

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Communication SystemsUMTS

• cdma2000 (not compatible to European standards)
  which is very successfully used by KDDI
• Transition to 3G is being largely completed in
  Japan during 2005/2006
• UMTS system bases on layered services, like IP
  but unlike GSM
• top is the services layer, which will give
  advantages like fast deployment of services and
  centralized location
• In the middle layer is control layer, which will
  help upgrading procedures and allow the capacity
  of the network to be dynamically allocated

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Communication SystemsUMTS

• bottom layer is handled by the connectivity layer
  where any transmission technology can be used and
  the voice traffic will transfer over ATM/AAL2 or
  IP/RTP
• UTMS will converge the mobile phone networks
  towards the IP world
• Thus ATM is just the old existing traditional
  infrastructure used
• Using IP in UMTS might push the IP world toward
  IP v6, because there will be a huge number of
  mobile phone subscribers (which might even exceed
  the number of IP dial-in Internet users)
• A lot of GSM infrastructure will be reused in
  UMTS networks nevertheless, more on radio
  network, W-CDMA next lecture!

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Communication SystemsGPRS, WAP, UMTS literature

• German text books
• Jochen Schiller, Mobilkommunikation
• Bernhard Walke, Mobilfunknetze und ihre
  Protokolle, Grundlagen GSM, UMTS, ...
• GPRS, WAP
• http//www.wikipedia.org/
• UMTS
• http//www.ks.uni-freiburg.de/download/papers/tels
  emWS05/UMTS-nextGeneration/UMTS-Seminararbeit-Stef
  an20Nagy.pdf

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