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Network structure[edit]
The structure of a GSM network
The network is structured into a number of discrete
sections:
TheBase Station Subsystem(the base stations and their
controllers).
TheNetwork and Switching Subsystem(the part of the network most
similar to afixed network). This is sometimes also just called the
core network. TheGPRS Core Network(the optional part which allows
packet based Internet
connections). TheOperations support system(OSS) for maintenance
of the network.
Base Station subsystem[edit]
Main article:Base Station subsystem
GSMcell siteantennas in theDeutsches Museum, Munich, Germany
GSM is acellular network, which means thatcell phonesconnect to
it by searching for cellsin the immediate vicinity. There are five
different cell sizes in a GSM
networkmacro,micro,pico,femtoandumbrella cells. The coverage area
of each cell varies according to theimplementation environment.
Macro cells can be regarded as cells where thebase station
antennais installed on a mast or a building above average roof
top level. Micro cells are cellswhose antenna height is under
average roof top level; they are typically used in urban areas.
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Picocells are small cells whose coverage diameter is a few dozen
metres; they are mainlyused indoors. Femtocells are cells designed
for use in residential or small businessenvironments and connect to
the service providers network via a broadband internet
connection. Umbrella cells are used to cover shadowed regions of
smaller cells and fill ingaps in coverage between those cells.
Cell horizontal radius varies depending on antenna height,
antenna gain and propagationconditions from a couple of hundred
metres to several tens of kilometres. The longestdistance the GSM
specification supports in practical use is 35 kilometres (22 mi).
There arealso several implementations of the concept of an extended
cell,[10]where the cell radiuscould be double or even more,
depending on the antenna system, the type of terrain and thetiming
advance.
Indoor coverage is also supported by GSM and may be achieved by
using an indoor picocellbase station, or anindoor repeaterwith
distributed indoor antennas fed through powersplitters, to deliver
the radio signals from an antenna outdoors to the separate
indoor
distributed antenna system. These are typically deployed when a
lot of call capacity is neededindoors; for example, in shopping
centers or airports. However, this is not a prerequisite,since
indoor coverage is also provided by in-building penetration of the
radio signals fromany nearby cell.
GSM carrier frequencies[edit]
Main article:GSM frequency bands
GSM networks operate in a number of different carrier frequency
ranges (separated intoGSM frequency rangesfor 2G andUMTS frequency
bandsfor 3G), with most2GGSMnetworks operating in the 900 MHz or
1800 MHz bands. Where these bands were alreadyallocated, the 850
MHz and 1900 MHz bands were used instead (for example in Canada
andthe United States). In rare cases the 400 and 450 MHz frequency
bands are assigned in somecountries because they were previously
used for first-generation systems.
Most3Gnetworks in Europe operate in the 2100 MHz frequency band.
For more informationon worldwide GSM frequency usage, seeGSM
frequency bands.
Regardless of the frequency selected by an operator, it is
divided intotimeslotsfor individualphones to use. This allows eight
full-rate or sixteen half-rate speech channels perradio
frequency. These eight radio timeslots (or eightburstperiods)
are grouped into aTDMAframe. Half rate channels use alternate
frames in the same timeslot. The channel data rate forall 8
channels is 270.833 kbit/s, and the frame duration is 4.615 ms.
The transmission power in the handset is limited to a maximum of
2 watts in GSM 850/900and 1 watt in GSM 1800/1900.
Voice codecs[edit]
GSM has used a variety of voicecodecsto squeeze 3.1 kHz audio
into between 6.5 and13 kbit/s. Originally, two codecs, named after
the types of data channel they were allocated,
were used, calledHalf Rate(6.5 kbit/s) andFull Rate(13 kbit/s).
These used a system baseduponlinear predictive coding(LPC). In
addition to being efficient with bitrates, these codecs
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also made it easier to identify more important parts of the
audio, allowing the air interfacelayer to prioritize and better
protect these parts of the signal.
As GSM was further enhanced in 1997[11]with theEnhanced Full
Rate(EFR) codec, a12.2 kbit/s codec that uses a full rate channel.
Finally, with the development ofUMTS, EFR
was refactored into a variable-rate codec calledAMR-Narrowband,
which is high quality androbust against interference when used on
full rate channels, or less robust but still relativelyhigh quality
when used in good radio conditions on half-rate channels.
Subscriber Identity Module (SIM)[edit]
Main article:Subscriber Identity Module
One of the key features of GSM is theSubscriber Identity Module,
commonly known as aSIM card. The SIM is a detachablesmart
cardcontaining the user's subscription informationand phone book.
This allows the user to retain his or her information after
switchinghandsets. Alternatively, the user can also change
operators while retaining the handset simply
by changing the SIM. Some operators will block this by allowing
the phone to use only asingle SIM, or only a SIM issued by them;
this practice is known as SIM locking.
Phone locking[edit]
Main article:SIM lock
Sometimesmobile network operatorsrestrict handsets that they
sell for use with their ownnetwork. This is called lockingand is
implemented by a software feature of the phone. A
subscriber may usually contact the provider to remove the lock
for a fee, utilize privateservices to remove the lock, or make use
of free or fee-based software and websites to unlockthe handset
themselves.
In some countries (e.g.,Bangladesh,Brazil,Chile,Hong
Kong,India,Lebanon,Malaysia,Nepal,Pakistan,Singapore) all phones
are sold unlocked. In others (e.g.,Singapore) it is unlawful for
operators to offer any form of subsidy on a phone's price.[12]
GSM service security[edit]
See also:UMTS security
GSM was designed with a moderate level of service security. The
system was designed toauthenticate the subscriber using apre-shared
keyandchallenge-response. Communications
between the subscriber and the base station can be encrypted.
The development ofUMTSintroduces an optionalUniversal Subscriber
Identity Module(USIM), that uses a longerauthentication key to give
greater security, as well as mutually authenticating the network
andthe userwhereas GSM only authenticates the user to the network
(and not vice versa). Thesecurity model therefore offers
confidentiality and authentication, but limited
authorizationcapabilities, and nonon-repudiation.
GSM uses several cryptographic algorithms for security.
TheA5/1,A5/2andA5/3streamciphersare used for ensuring over-the-air
voice privacy. A5/1 was developed first and is astronger algorithm
used within Europe and the United States; A5/2 is weaker and used
in
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wikipedia.org/wiki/Pre-shared_keyhttps://en.wikipedia.org/wiki/UMTS_securityhttps://en.wikipedia.org/w/index.php?title=GSM&action=edit§ion=9https://en.wikipedia.org/wiki/GSM#cite_note-Subsidize-12https://en.wikipedia.org/wiki/Singaporehttps://en.wikipedia.org/wiki/Pakistanhttps://en.wikipedia.org/wiki/Nepalhttps://en.wikipedia.org/wiki/Malaysiahttps://en.wikipedia.org/wiki/Lebanonhttps://en.wikipedia.org/wiki/Indiahttps://en.wikipedia.org/wiki/Hong_Konghttps://en.wikipedia.org/wiki/Chilehttps://en.wikipedia.org/wiki/Brazilhttps://en.wikipedia.org/wiki/Bangladeshhttps://en.wikipedia.org/wiki/Mobile_network_operatorhttps://en.wikipedia.org/wiki/SIM_lockhttps://en.wikipedia.org/w/index.php?title=GSM&action=edit§ion=8https://en.wikipedia.org/wiki/SIM_lockhttps://en.wikipedia.org/wiki/Smart_cardhttps://en.wikipedia.org/wiki/Subscriber_Identity_Modulehttps://en.wikipedia.org/wiki/Subscriber_Identity_Modulehttps://en.wikipedia.org/w/index.php?title=GSM&action=edit§ion=7https://en.wikipedia.org/wiki/Adaptive_Multi-Ratehttps://en.wikipedia.org/wiki/UMTShttps://en.wikipedia.org/wiki/Enhanced_Full_Ratehttps://en.wikipedia.org/wiki/GSM#cite_note-11
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other countries. Serious weaknesses have been found in both
algorithms: it is possible tobreak A5/2 in real-time with
aciphertext-only attack, and in January 2007,The
Hacker'sChoicestarted the A5/1 cracking project with plans to
useFPGAsthat allow A5/1 to be
broken with arainbow tableattack.[13]The system supports
multiple algorithms so operatorsmay replace that cipher with a
stronger one.
On 28 December 2009 German computer engineerKarsten
Nohlannounced that he hadcracked the A5/1 cipher.[14]According to
Nohl, he developed a number ofrainbow tables(static values which
reduce the time needed to carry out an attack) and have found
newsources forknown plaintext attacks. He also said that it is
possible to build "a full GSMinterceptor ... from open source
components" but that they had not done so because of
legalconcerns.[15]Nohl claimed that he was able to intercept voice
and text conversations byimpersonating another user to listen to
their voice mails, make calls or send text messagesusing a
seven-year-old Motorola cellphone and decryption software available
free off theInternet.[16]
New attacks have been observed that take advantage of poor
security implementations,architecture and development for smart
phone applications. Some wiretapping andeavesdropping
techniqueshijack[17]the audio input and output providing an
opportunity for a3rd party to listen in to the conversation.
GSM usesGeneral Packet Radio Service(GPRS) for data
transmissions like browsing theweb. The most commonly deployed GPRS
ciphers were publicly broken in 2011.[18]
The researchers revealed flaws in the commonly used GEA/1 and
GEA/2 ciphers andpublished the open source "gprsdecode" software
for sniffing GPRS networks. They alsonoted that some carriers don't
encrypt the data at all (i.e. using GEA/0) in order to detect
theuse of traffic or protocols they don't like, e.g.Skype, leaving
their customers unprotected.GEA/3 seems to remain relatively hard
to break and is said to be in use on some moremodern networks. If
used withUSIMto prevent connections to fake base stations
anddowngrade attacks, users will be protected in the medium term,
though migration to 128-bitGEA/4 is still recommended.
Standards information[edit]
The GSM systems and services are described in a set of standards
governed byETSI, where afull list is maintained.[19]
GSM open-source software[edit]
Severalopen-sourcesoftware projects exist that provide certain
GSM features:
gsmd daemon byOpenmoko[20] OpenBTSdevelops aBase transceiver
station The GSM Software Projectaims to build a GSM analyzer for
less than $1000 [21] OsmocomBB developers intend to replace the
proprietary baseband GSM stack with a
free software implementation[22]
Issues with patents and open source[edit]
https://en.wikipedia.org/wiki/Ciphertext-only_attackhttps://en.wikipedia.org/wiki/Ciphertext-only_attackhttps://en.wikipedia.org/wiki/Ciphertext-only_attackhttps://en.wikipedia.org/wiki/The_Hacker%27s_Choicehttps://en.wikipedia.org/wiki/The_Hacker%27s_Choicehttps://en.wikipedia.org/wiki/The_Hacker%27s_Choicehttps://en.wikipedia.org/wiki/The_Hacker%27s_Choicehttps://en.wikipedia.org/wiki/FPGAhttps://en.wikipedia.org/wiki/FPGAhttps://en.wikipedia.org/wiki/FPGAhttps://en.wikipedia.org/wiki/Rainbow_tablehttps://en.wikipedia.org/wiki/Rainbow_tablehttps://en.wikipedia.org/wiki/Rainbow_tablehttps://en.wikipedia.org/wiki/GSM#cite_note-13https://en.wikipedia.org/wiki/GSM#cite_note-13https://en.wikipedia.org/wiki/GSM#cite_note-13https://es.wikipedia.org/wiki/Karsten_Nohlhttps://es.wikipedia.org/wiki/Karsten_Nohlhttps://es.wikipedia.org/wiki/Karsten_Nohlhttps://en.wikipedia.org/wiki/GSM#cite_note-14https://en.wikipedia.org/wiki/GSM#cite_note-14https://en.wikipedia.org/wiki/GSM#cite_note-14https://en.wikipedia.org/wiki/Rainbow_tablehttps://en.wikipedia.org/wiki/Rainbow_tablehttps://en.wikipedia.org/wiki/Rainbow_tablehttps://en.wikipedia.org/wiki/Known_plaintext_attackhttps://en.wikipedia.org/wiki/Known_plaintext_attackhttps://en.wikipedia.org/wiki/Known_plaintext_attackhttps://en.wikipedia.org/wiki/GSM#cite_note-15https://en.wikipedia.org/wiki/GSM#cite_note-15https://en.wikipedia.org/wiki/GSM#cite_note-15https://en.wikipedia.org/wiki/GSM#cite_note-16https://en.wikipedia.org/wiki/GSM#cite_note-16https://en.wikipedia.org/wiki/GSM#cite_note-16https://en.wikipedia.org/wiki/Hijack_attackhttps://en.wikipedia.org/wiki/Hijack_attackhttps://en.wikipedia.org/wiki/Hijack_attackhttps://en.wikipedia.org/wiki/Hijack_attackhttps://en.wikipedia.org/wiki/General_Packet_Radio_Servicehttps://en.wikipedia.org/wiki/General_Packet_Radio_Servicehttps://en.wikipedia.org/wiki/General_Packet_Radio_Servicehttps://en.wikipedia.org/wiki/GSM#cite_note-18https://en.wikipedia.org/wiki/GSM#cite_note-18https://en.wikipedia.org/wiki/GSM#cite_note-18https://en.wikipedia.org/wiki/Skypehttps://en.wikipedia.org/wiki/Skypehttps://en.wikipedia.org/wiki/Skypehttps://en.wikipedia.org/wiki/Subscriber_Identity_Modulehttps://en.wikipedia.org/wiki/Subscriber_Identity_Modulehttps://en.wikipedia.org/wiki/Subscriber_Identity_Modulehttps://en.wikipedia.org/w/index.php?title=GSM&action=edit§ion=10https://en.wikipedia.org/wiki/ETSIhttps://en.wikipedia.org/wiki/ETSIhttps://en.wikipedia.org/wiki/ETSIhttps://en.wikipedia.org/wiki/GSM#cite_note-19https://en.wikipedia.org/wiki/GSM#cite_note-19https://en.wikipedia.org/wiki/GSM#cite_note-19https://en.wikipedia.org/w/index.php?title=GSM&action=edit§ion=11https://en.wikipedia.org/wiki/Open_sourcehttps://en.wikipedia.org/wiki/Open_sourcehttps://en.wikipedia.org/wiki/Open_sourcehttps://en.wikipedia.org/wiki/Openmokohttps://en.wikipedia.org/wiki/Openmokohttps://en.wikipedia.org/wiki/Openmokohttps://en.wikipedia.org/wiki/Openmokohttps://en.wikipedia.org/wiki/OpenBTShttps://en.wikipedia.org/wiki/OpenBTShttps://en.wikipedia.org/wiki/Base_transceiver_stationhttps://en.wikipedia.org/wiki/Base_transceiver_stationhttps://en.wikipedia.org/wiki/Base_transceiver_stationhttps://en.wikipedia.org/wiki/GSM#cite_note-21https://en.wikipedia.org/wiki/GSM#cite_note-21https://en.wikipedia.org/wiki/GSM#cite_note-22https://en.wikipedia.org/wiki/GSM#cite_note-22https://en.wikipedia.org/w/index.php?title=GSM&action=edit§ion=12https://en.wikipedia.org/w/index.php?title=GSM&action=edit§ion=12https://en.wikipedia.org/w/index.php?title=GSM&action=edit§ion=12https://en.wikipedia.org/w/index.php?title=GSM&action=edit§ion=12https://en.wikipedia.org/wiki/GSM#cite_note-22https://en.wikipedia.org/wiki/GSM#cite_note-21https://en.wikipedia.org/wiki/Base_transceiver_stationhttps://en.wikipedia.org/wiki/OpenBTShttps://en.wikipedia.org/wiki/Openmokohttps://en.wikipedia.org/wiki/Openmokohttps://en.wikipedia.org/wiki/Open_sourcehttps://en.wikipedia.org/w/index.php?title=GSM&action=edit§ion=11https://en.wikipedia.org/wiki/GSM#cite_note-19https://en.wikipedia.org/wiki/ETSIhttps://en.wikipedia.org/w/index.php?title=GSM&action=edit§ion=10https://en.wikipedia.org/wiki/Subscriber_Identity_Modulehttps://en.wikipedia.org/wiki/Skypehttps://en.wikipedia.org/wiki/GSM#cite_note-18https://en.wikipedia.org/wiki/General_Packet_Radio_Servicehttps://en.wikipedia.org/wiki/Hijack_attackhttps://en.wikipedia.org/wiki/Hijack_attackhttps://en.wikipedia.org/wiki/GSM#cite_note-16https://en.wikipedia.org/wiki/GSM#cite_note-15https://en.wikipedia.org/wiki/Known_plaintext_attackhttps://en.wikipedia.org/wiki/Rainbow_tablehttps://en.wikipedia.org/wiki/GSM#cite_note-14https://es.wikipedia.org/wiki/Karsten_Nohlhttps://en.wikipedia.org/wiki/GSM#cite_note-13https://en.wikipedia.org/wiki/Rainbow_tablehttps://en.wikipedia.org/wiki/FPGAhttps://en.wikipedia.org/wiki/The_Hacker%27s_Choicehttps://en.wikipedia.org/wiki/The_Hacker%27s_Choicehttps://en.wikipedia.org/wiki/Ciphertext-only_attack
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7/28/2019 Network structure - GSM.docx
5/5
Patents remain a problem for any open-source GSM implementation,
because it is notpossible for GNU or any other free software
distributor to guarantee immunity from alllawsuits by the patent
holders against the users. Furthermore new features are being added
tothe standard all the time which means they have patent protection
for a number of years.[citationneeded]
The original GSM implementations from 1991 may now be entirely
free of patentencumbrances, however patent freedom is not certain
due to the US "first to invent" systemthat was in place until 2012.
The "first to invent" system, coupled with "patent termadjustment"
can extend the life of a US patent far beyond 20 years from its
priority date. It isunclear at this time whetherOpenBTSwill be able
to implement features of that initialspecification without limit.
As patents subsequently expire, however, those features can beadded
into the open source version. As of 2011, there have been no law
suits against users ofOpenBTS over GSM use.[citation needed]
https://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttps://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttps://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttps://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttps://en.wikipedia.org/wiki/OpenBTShttps://en.wikipedia.org/wiki/OpenBTShttps://en.wikipedia.org/wiki/OpenBTShttps://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttps://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttps://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttps://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttps://en.wikipedia.org/wiki/OpenBTShttps://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttps://en.wikipedia.org/wiki/Wikipedia:Citation_needed
