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Communication Systems 11th lecture


Communication Systems 11th lecture Chair of Communication Systems Department of Applied Sciences University of Freiburg 2006 * | 52 Communication Systems Last lecture ... – PowerPoint PPT presentation

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Title: Communication Systems 11th lecture

Communication Systems 11th lecture
  • Chair of Communication Systems
  • Department of Applied Sciences
  • University of Freiburg
  • 2006

1 52
Communication Systems Last 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
  • The card is an external hardware module which may
    store user data like received SMS or phone book

2 52
Communication Systems Last lecture GSM, logical
structure of the network
3 52
Communication Systems Last 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
  • Eavesdropping is possible because of leaked
    crypto algorithms and unencrypted network links

4 52
Communication Systems Last 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

5 52
Communication Systems Plan for this lecture
  • Data Services on top of GSM networks
  • SMS the most expensive 140Bytes ever
  • HSCSD as a typical phone network inspired data
  • GPRS an extension to existing GSM
    infrastructure to provide packet orientated
    services while optimizing the use of the air
  • MMS the SMS successor using the GPRS backend to
    offer advanced messaging services to the
  • WAP a protocol to bring Internet (like)
    services to the mobile device
  • Using special WAP and GPRS services in PPP dial-in

6 52
Communication Systems GSM 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

7 52
Communication Systems GSM 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
  • 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

8 52
Communication Systems GSM 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

9 52
Communication Systems GSM 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.)

10 52
Communication Systems GSM 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
  • 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

11 52
Communication Systems GSM 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

12 52
Communication Systems GSM 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
  • 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

13 52
Communication Systems GSM - 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
  • so one data service user equals to four mobile
    voice users imagine on the charges needed to
  • Thus HSCSD is standardized for a while now, but
    not every network provider offers this service
    (only D2 and E in Germany)

14 52
Communication Systems GSM 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)

15 52
Communication Systems GSM 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
  • disadvantage infrastructure has to be extended
    significantly, new components are to be installed
    in BSS (Base Station Subsystem)

16 52
Communication Systems GSM 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

17 52
Communication Systems GSM GPRS
  • GPRS bases on an additional infrastructure GSN
    GPRS Support Nodes as an extension to GSM

18 52
Communication Systems GPRS components and
  • 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
  • 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

19 52
Communication Systems GPRS 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
  • QoS parameters
  • address of access point to external networks
  • Session setup is comparable to setup of mobile
    originated voice calls
  • Channels have to be activated and the
    authentication procedure to be passed

20 52
Communication Systems GPRS 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

21 52
Communication Systems GPRS 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
  • 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

22 52
Communication Systems GPRS components and
  • the SGSN decapsulates the packet and hands it
    over to the BSS for delivery to the MS

23 52
Communication Systems GPRS 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

24 52
Communication Systems GPRS 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

25 52
Communication Systems GPRS, 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
  • Defined by several organizations for GSM and UMTS
  • Common standard for the mobile phones of
    different vendors

26 52
Communication Systems GPRS and enhanced mobile
data services
  • MMS allows the addressing via
  • MSISDN (persistent telephone number of the mobile
  • 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
  • Voice encoded with Adaptive Multi Rate codec (as
    used with UMTS)
  • Graphics in several formats

27 52
Communication Systems GPRS and enhanced mobile
data services
  • MMS uses a container format for the multimedia
  • 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

28 52
Communication Systems GPRS 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)

29 52
Communication Systems GPRS 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

30 52
Communication Systems GPRS, HSCSD and WAP
  • The Wireless Application Protocol was defined to
    bring Internet like services to the mobile
  • 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
  • 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

31 52
Communication Systems GPRS, HSCSD and WAP
  • Two types of services are defined traditional
    web like and push service

32 52
Communication Systems GPRS, HSCSD and WAP
  • Data reduction is handled by the use of optimized
  • The Internet protocols are translated into their
    counterparts in the WAP standard via translation
  • 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

33 52
Communication Systems GPRS, HSCSD and WAP
  • Hash tables translated each WSP header into its
    HTTP counterpart
  • A designated gateway is needed as translation

34 52
Communication Systems WAP 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 or 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
  • The transport layer is handled by Wireless
    Datagram Protocol (WDP)

35 52
Communication Systems WAP 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

36 52
Communication Systems WAP 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

37 52
Communication Systems WAP 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
  • 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?)

38 52
Communication Systems WAP 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

39 52
Communication Systems WAP 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//
  • 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)

40 52
Communication Systems GSM data services and
  • 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

41 52
Communication Systems GSM data services and
  • 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

42 52
Communication Systems GSM data services and
  • Snippet from a Linux GPRS modem call script
  • ...
  • SAY \ndefining PDP context...\n" \
  • OK 'ATF' \
  • OK 'ATV1E0S00D2C1' \
  • OK ATCMEE1 \
  • OK 'ATcgdcont1,"IP",""'
  • OK-AT-OK ATD99 \
  • SAY "\nwaiting for connect...\n"
  • ...
  • Specific AP is choosen (here

43 52
Communication Systems GSM data services and
  • 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
  • rcvd LCP ConfReq id0x2 ltasyncmap 0x0gt ltpcompgt
  • sent LCP ConfAck id0x2 ltasyncmap 0x0gt ltpcompgt
  • rcvd LCP ConfAck id0x2 ltasyncmap 0x0gt ltpcompgt
  • ...

44 52
Communication Systems GSM data services and
  • ...
  • sent CCP ConfReq id0x1 ltdeflate 15gt
    ltdeflate(old) 15gt ltbsd v1 15gt
  • sent IPCP ConfReq id0x1 ltcompress VJ 0f 01gt
  • 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
  • sent IPCP ConfAck id0x1 ltaddr
  • rcvd IPCP ConfRej id0x1 ltcompress VJ 0f 01gt
  • sent IPCP ConfReq id0x2 ltaddr
  • rcvd IPCP ConfNak id0x2 ltaddr
  • ...
  • local IP address
  • remote IP address
  • Thus the IP setup is easily compatible to known
    PPP implementation

45 52
Communication Systems GSM data services and
  • 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
  • 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, ...

46 52
Communication Systems From 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)

47 52
Communication Systems IMT2000 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

48 52
Communication Systems UMTS
  • 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

49 52
Communication Systems UMTS
  • 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

50 52
Communication Systems UMTS
  • 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
  • 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!

51 52
Communication Systems GPRS, WAP, UMTS literature
  • German text books
  • Jochen Schiller, Mobilkommunikation
  • Bernhard Walke, Mobilfunknetze und ihre
    Protokolle, Grundlagen GSM, UMTS, ...
  • http//
  • UMTS
  • http//

52 52