1 | 52 Communication Systems 11 th lecture Chair of Communication Systems Department of Applied...

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Communication Systems11th lecture

Chair of Communication SystemsDepartment of Applied Sciences

University of Freiburg2006

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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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● 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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● 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 1:1 communication between end user devices

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Communication SystemsGSM and SMS

● SMS widespread and used for many applications– 1:1 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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● 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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– 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)

data rate TCH/H4.8 TCH/F9,6 TCH/F14,44,8kbit/s 19,6kbit/s 2 1

14,4kbit/s 3 119,2kbit/s 4 228,8kbit/s 3 238,4kbit/s 443,2kbit/s 357,6kbit/s 4

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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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– 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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● 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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● 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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– 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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● 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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● 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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● 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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● Two types of services are defined: traditional web like and push service

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● 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: en;q=0.7

WSP-Header: 0x83,0x02,0x99,0x47

HTTP-Header: Accept-Language: en,sv

WSP-Header: 0x83,0x99,0x83,0xF0

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● 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)

http://www.google.de/

http://www.bahn.de/

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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/P800R102.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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– 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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● Snippet from a Linux GPRS modem call script

... SAY „\ndefining PDP context...\n" \

OK 'AT&F' \

OK 'ATV1E0S0=0&D2&C1' \

OK AT+CMEE=1 \

OK 'AT+cgdcont=1,"IP","wap.viaginterkom.de"' \

OK-AT-OK ATD*99***# \

SAY "\nwaiting for connect...\n"

...● Specific AP is choosen (here wap.viaginterkom.de)

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● 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 <--> /dev/rfcomm1

sent [LCP ConfReq id=0x1 <asyncmap 0x0> <magic 0x71179e05> <pcomp> <accomp>]

rcvd [LCP ConfReq id=0x1 <asyncmap 0x0> <pcomp> <accomp> <auth pap>]

No auth is possible

sent [LCP ConfRej id=0x1 <auth pap>]

rcvd [LCP ConfRej id=0x1 <magic 0x71179e05>]

sent [LCP ConfReq id=0x2 <asyncmap 0x0> <pcomp> <accomp>]

rcvd [LCP ConfReq id=0x2 <asyncmap 0x0> <pcomp> <accomp>]

sent [LCP ConfAck id=0x2 <asyncmap 0x0> <pcomp> <accomp>]

rcvd [LCP ConfAck id=0x2 <asyncmap 0x0> <pcomp> <accomp>]

...

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...

sent [CCP ConfReq id=0x1 <deflate 15> <deflate(old#) 15> <bsd v1 15>]

sent [IPCP ConfReq id=0x1 <compress VJ 0f 01> <addr 0.0.0.0>]

rcvd [LCP ProtRej id=0x4 80 fd 01 01 00 0f 1a 04 78 00 18 04 78 00 15 03 2f]

rcvd [IPCP ConfReq id=0x1 <addr 10.49.48.62>]

sent [IPCP ConfAck id=0x1 <addr 10.49.48.62>]

rcvd [IPCP ConfRej id=0x1 <compress VJ 0f 01>]

sent [IPCP ConfReq id=0x2 <addr 0.0.0.0>]

rcvd [IPCP ConfNak id=0x2 <addr 10.45.48.63>]

...

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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● 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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– 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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– 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, ...● UMTS:

http://www.ks.uni-freiburg.de/download/papers/telsemWS05/UMTS-nextGeneration/UMTS-Seminararbeit-Stefan%20Nagy.pdf

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