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GSM Interfaces and Protocols (Global System forMobile Communication (GSM)) Part 1
The GSM specifications define the interaction between system components through
well-defined interfaces and protocols !igure "-11 shows the interfaces between the
GSM functional entities #able "-" lists the GSM interfaces
Figure 3-12 shows the protocol architecture used for the e$change of signaling
messages on each interface #he protocols are layered according to the %SI &eference
Model It consists of the Physical 'ayer ata 'in* 'ayer and 'ayer " #his 'ayer " is
not the same as defined in %SI 'ayer " In GSM the 'ayer " functions include call
mobility and radio resource management In the %SI model these functions are
pro+ided by the higher layers GSM reuses a few established protocols such as CCS,
M#P #CP SCCP IS.P and IS/ 'P protocols #he MP and 0SSP are newprotocols to support GSM specific needs
Air interface
The air interface between the MS and the 0#S is called .m Most of the
implementations use a freuency band of 233 M45 #he other deri+ati+e of GSM is
called igital cellular system 1633 (CM1633)
Figure 3-11 GSM interfaces.
TABLE 3-3 GSM nterfaces
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Interface Description
Um
Abis
A
B
C
D
E
F
G
H
It uses a freuency band of 1633 M45 #able "-7 lists the GSM freuency bands
The use! fre"uenc# ban! is di+ided into 833-*45 carriers or &! channels in both the
uplin* and downlin* direction 9ach &! channel is then further subdi+ided into eight
different timeslots ie 3 to , by #M techniues set of these eight timeslots is
referred as a #M frame 9ach frame lasts 7:1; ms #he physical channels are
further mapped to +arious logical channels carrying user traffic and control information
between the MS and the 0#S #able "-; describes the logical channels and their
usages
#he following section describes the .m interface protocols used at the MS and the 0#S
side
$h#sica% %a#er 'ayer 1 which is a radio interface pro+ides the functionality reuired to
transfer the bit streams o+er the physical channels on the radio medium #he Physical
'ayer itself does the frame delimitation #he ser+ices pro+ided by this layer to those
abo+e include
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& 'hanne% (apping )%ogica% to ph#sica%*
& 'hanne% co!ing an! ciphering
& +igita% (o!u%ation
& Fre"uenc# hopping
& Ti(ing a!,ance an! power contro%
+ata %in %a#er. Signaling 'ayer 8 is based on the 'Pm protocol to pro+ide a reliable
signaling lin* between the networ* and the mobile station 'Pm uses no flags for
frame delimitation #his way scarce radio resources are not spent on flag bits the bit
Figure 3-12 GSM protoco% stac.
TABLE 3- GSM Fre"uenc# Ban!sSystem Direction Frequency band (MHz)
GSM 900 Uplink 890-815
Downlink 935-960
GSM/DCS1800 Uplink 110-185
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Downlink 1805-1880
/etwor %a#er. Signaling 'ayer " ta*es care of signaling procedures between an MS
and the networ* It consists of three sublayers with distinct signaling procedures
& 0a!io resource (anage(ent )00*
& Mobi%it# (anage(ent )MM*
& 'onnection (anage(ent )'M*
0a!io resource (anage(ent &adio resource management (&&) comprises
procedures reuired to establish maintain and release the dedicated radio
connections #he && sublayer functions include<
& 'hanne% assign(ent an! re%ease
& 'iphering
& Mo!ification of channe% (o!es e.g. ,oice an! !ata
& an!o,er between ce%%s
& Fre"uenc# re!efinition to enab%e fre"uenc# hopping
& MS (easure(ent reports
& $ower contro% an! ti(ing a!,ance
& $aging
& 0a!io channe% access
The (obi%e station a%wa#s initiates an 00 session. !or e$ample the && procedures
are in+o*ed to establish an && session in response to a paging message or to establish
an outgoing call s shown in !igure "-1" the && messages are transferred to 0SC
transparently through the 0#S #able "-: lists && messages
Mobi%it# (anage(ent. #he mobility management (MM) sublayer handles functions and
procedures related to mobility of the mobile user #his includes procedures for<
& Authentication
& Location registration an! perio!ic up!ating
TABLE 3- Logica% Traffic an! 'ontro% 'hanne%sTraffic
channels
(TCH)
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!CH/F "#$$i%s
s&bs"$ib%$
in'o$m#(ion)sp%%"*/+#(#,
#( # $#(% o'
.8 bpswi(* # sp%%"*
"o+in #(
#$o&n+ 13
bps.
!CH/F "#$$i%s
s&bs"$ib%$in'o$m#(ion #(
# $#(% o' 11.
bps wi(* #sp%%"* "o+in
#( #$o&n+
bps.
B$o#+"#s(
"on($ol
"*#nn%ls)BCH,
!*is "*#nn%l is
b$o#+"#s( b2(*% B!S #n+
"#$$i%s
in'o$m#(ion'o$ (*%
'$%&%n"2
"o$$%"(ion o'(*% MS. 4( is
&s%+ in
+ownlink
+i$%"(ion onl2.
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!*is "*#nn%l is
b$o#+"#s( b2
(*% B!S #n+"#$$i%s
in'o$m#(ion
'o$ '$#m%s2n"*$oni#(io
n o' (*% MS.
4n #++i(ion i(
#lso "#$$i%s (*% b#s% s(#(ion
i+%n(i(2 "o+%
)BS4C,. 4( is&s%+ in
+ownlink
+i$%"(ion onl2.
!*is "*#nn%l
"#$$i%s
b$o#+"#s(in'o$m#(ion
$%l#(%+ (o (*%
B!S #n+ (*%n%(wo$k. !*%
in'o$m#(ion
in"l&+%s"on'i&$#(ion
+%(#ils o'"ommon
"on($ol"*#nn%ls
)CCH,
+%s"$ib%+ b%low.
4( is &s%+ in+ownlink
+i$%"(ion onl2.
Common"on($ol
"*#nn%ls
)CCH,
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!*is is &s%+ (o
p#% #n MS. 4(is &s%+ in
+ownlink
+i$%"(ion onl2.
!*% MS &s%s(*is "*#nn%l (o
$%&%s( (*%
#llo"#(ion o' #SDCCH. 4( is
&s%+ in &plink
+i$%"(ion onl2.
!*% B!S
#llo"#(%s #
SDCCH o$!CH in
$%spons% (o (*%
#llo"#(ion$%&%s( b2 (*%
MS &sin (*is
"*#nn%l. 4( is&s%+ in
+ownlink
+i$%"(ion onl2.
D%+i"#(%+
"on($ol
"*#nn%ls)DCH,
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!*is "*#nn%l is
&s%+ 'o$
"#$$2insin#lin
in'o$m#(ion
b%(w%%n (*%B!S #n+ # MS
b%'o$%
#llo"#(ion o' #
!CH. Fo$%#mpl%7
SDCCH is
&s%+ 'o$"#$$2in
sin#lin
m%ss#%s
$%l#(%+ (o&p+#(%
lo"#(ion #n+
"#ll%s(#blis*m%n(.
!*is is #
bi+i$%"(ion#l"*#nn%l.
!*is "*#nn%l is#lw#2s &s%+ in
"on&n"(ionwi(* # !CH o$
# SDCCH. !*%MS #n+ (*%
B!S &s% i( (o
m#in(#in #nSDCCH o$ #
!CH. 4n (*%
&plink7 (*% MSs%n+s
m%#s&$%m%n(
$%po$(s (o (*%B!S &sin (*is
"*#nn%l. 4n (*%
+ownlink7 (*%
B!S ($#nsmi(sin'o$m#(ion (o
k%%p (*%
mobil%
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&p+#(%+ on
$%"%n( "*#n%s
in s2s(%m"on'i&$#(ion.
!*is "*#nn%l is
#lw#2s#sso"i#(%+ (o #
!CH #n+ is
&s%+ (o($#ns'%$
sin#lin
m%ss#%sw*%n # mobil%
is #l$%#+2
inol%+ in #"#ll.
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Figure 3-13 Air interface signa%ing protoco%s.
& Securit#
& TMS rea%%ocation
& MS !etach4attach
As shown in Figure 3-13 the CM layer from the transmitting side uses the MM layer to
establish && connection and then transfers messages transparently across to the
recei+ing side that is MSC #able "-: lists MM messages
'onnection (anage(ent. #he connection management (CM ) sublayer contains thefunctions and procedures for call control #his includes procedures to establish release
and access ser+ices and facilities #he CM consists of three sublayers namely call
control (CC) supplementary ser+ices (SS) and short message ser+ices (SMS)
The ca%% contro% sub%a#er pro+ides procedures for IS/ call control #hese procedures
are based on IS/ call control procedures defined in the I#.-# =2"1 specification
4owe+er the minor modifications are done to adopt these to mobile en+ironment
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#he supplementary ser+ice sublayer pro+ides the procedures to support non-call-related
supplementary ser+ices such as call forwarding and call waiting
TABLE 3-5 La#er 3 Messages
messa!es MM messa!es CM messa!es
C*#nn%l %s(#blis*m%n( m%ss#%s :%is($#(ion m%ss#%s C#ll %s(#blis*m%n( m%ss#%s
ADDi(ion#l ASSinm%n(
4MS4 DE!#"* A;E:!in
4D#(in C=#in m%ss#%s
Conn%"(ion C=
>AGin :E&%s( !2p% CM SE:i"% :E@%"( EME:G%n"2 SE!U>
>AGin :E&%s( !2p% 3 CM SE:i"% :E&%s( >:=G:ESS
>AGin :%S>ons% CM SE:i"% AB=$( C#ll p*#s% m%ss#%s
H#n+o%$ m%ss#%s CM :E%S(#blis*m%n( M=Di'2
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:E&%s(
ASSinm%n( CoMm#nD S%"&$i(2 m%ss#%s M=Di'2 :E@%"(
ASSinm%n( C=Mpl%(% AU!H%n(i"#(ion :E@%"( M=Di'2 C=M>l%(%
ASSinm%n( FA4;&$% AU!H%n(i"#(ion USE: 4
HA
>Hsi"#l 4
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>A:!i#l :E;%#s% :E;%#s% C=Mpl%(%
>A:!i#l :E;%#s% C=Mpl%(% =(*%$ m%ss#%s
S2s(%m in'o$m#(ion m%ss#%s C=SS(%m 4
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=(*%$ m%ss#%s
CHA # P % S # < GSM Interfaces and Protocols (Global System for Mobile Communication
(GSM)) Part 8
P & 9 ? I % . S P % S # < System rchitecture (Global System for Mobile Communication (GSM))
Part 8
• R E L AT E D L I N K S
o &oaming in @ireless /etwor*s
&oaming and @ireless /etwor*s
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