Download Communication Systems 11th lecture - Electures

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
Number)
–
The card is an external hardware module which may store
user data like received SMS or phone book entries
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 connection
–
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 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
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 1:1 communication between end user
devices
7 | 52
Communication Systems
GSM 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
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 “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
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 too
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 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
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 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)
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)
d ata rate T C H /H 4.8 T C H /F 9,6
4,8kbit/s
1
9,6kbit/s
2
1
14,4kbit/s
3
19,2kbit/s
4
2
28,8kbit/s
3
38,4kbit/s
4
43,2kbit/s
57,6kbit/s
T C H /F 14,4
1
2
3
4
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 slots
–
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 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
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 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
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 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
22 | 52
Communication Systems
GPRS components and interfaces
–
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 (PTPCONS), 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 Service
–
Defined by several organizations for GSM and UMTS
networks
–
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
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
27 | 52
Communication Systems
GPRS 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
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 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
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 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
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 device
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 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)
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
technology
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 sheets
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 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?)
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://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)
40 | 52
Communication Systems
GSM 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
41 | 52
Communication Systems
GSM 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
42 | 52
Communication Systems
GSM data services and devices
●
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
SAY
\
\
\
\
ATD*99***#
\
\
"\nwaiting for connect...\n"
...
●
Specific AP is choosen (here wap.viaginterkom.de)
43 | 52
Communication Systems
GSM 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 <--> /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>]
...
44 | 52
Communication Systems
GSM data services and devices
...
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
45 | 52
Communication Systems
GSM 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, ...
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 IMT2000 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: GSMcompatibility should be offered, the authentication and security
will be upgraded to USIM
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 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!
51 | 52
Communication Systems
GPRS, 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/UMTSnextGeneration/UMTS-Seminararbeit-Stefan%20Nagy.pdf
52 | 52