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between signalling points (network elements). This layer deals with routing, re-routing
when a link fails and congestion control. It has many similarities with the IP layer on
the Internet.


Signalling connection control part (SCCP)
SCCP sits on top of the message transfer part and provides both connectionless and
connection-oriented network services. Together with MTP it is referred to as the network
service part (NSP). Signalling points can have a number of attached applications operating
simultaneously; SCCP introduces the subsystem number (SSN) to ensure that the correct
application is accessed. This layer can be seen as analogous to TCP in the Internet suite
of protocols. SCCP also provides address translation capabilities, known as global title
translation (GTT). SCCP is not required for services such as TUP and ISUP and thus in
Figure 3.25 it does not extend across the complete stack.


Telephone user part (TUP)
This deals with call setup and release but is designed for the traditional analogue circuits,
which are still dominant in some parts of the world. It has been largely superseded by
ISUP. TUP was one of the ¬rst applications to be de¬ned and as such did not have
provision for ISDN services.
72 GSM FUNDAMENTALS


ISDN user part (ISUP)
This layer also de¬nes the messages that are to be used for call setup, modi¬cation, tear
down, etc. It provides the services required by ISDN. ISUP supports basic telephony in
a similar manner to TUP; however, it has a greater variety of messages which enable
many more services to be offered. Calls that originate and terminate at the same SSP
do not require the services of ISUP. Examples of ISUP messages are: Answer, Charge
information, Connect, Identi¬cation request, Information request and Release.


Transaction capabilities application part (TCAP)
TCAP provides a structured method to request processing at a remote node. It de¬nes the
information ¬‚ow and can report on the result. Queries and responses between SSPs and
SCPs are sent via TCAP messages. Typical examples of TCAP services are registration
of roaming users in a mobile network, and intelligent network services such as free-phone
or calling card. TCAP is employed for the non-circuit-related information exchange, for
example, if a subscriber™s PIN is required for using a calling card, mobile application
part (MAP; see below) messages which are sent between mobile switches and databases
to support subscriber authentication, equipment identi¬cation and roaming are carried by
TCAP. As a mobile subscriber roams into a new MSC area, the VLR requests the sub-
scriber™s pro¬le information from the HLR using MAP information carried within TCAP
messages. Although SCCP can locate the database, the actual query for data is performed
by a TCAP message. TCAP can also be used for transporting billing information.


GSM user part
A cellular network requires additional features to a ¬xed network; for example, a mobile
subscriber needs to be tracked when roaming from location to location. This extra func-
tionality is performed by MAP and BSSAP. Figure 2.26 below shows how these GSM
components ¬t into the SS7 protocol stack.


MSC
BSSAP
MAP
DTAP
BSSMAP TCAP
SCCP SCCP
mobile
station
MTP level 3 MTP level 3
BSS
MTP




MTP




MTP level 2 MTP level 2

MTP level 1 MTP level 1


Figure 3.26 GSM positioning in SS7 protocol stack
3.7 GSM AND SIGNALLING SYSTEM 7 73


Message application part (MAP)
MAP is a key protocol in many cellular networks. Both GSM and ANSI-41 networks
use MAP for accessing roaming information, paging devices, controlling handover and
sending SMS messages. Although GSM and ANSI-41 use MAP, these messages are not
directly compatible and interworking functions are required when communication between
the two types of MAP is required. MAP messages are carried by TCAP.

Base station system application part (BSSAP)
BSSAP messages are used for signalling both between the mobile device and the MSC
and also between the BSC and the MSC. For this reason the BSSAP is divided into two
sub-layers:

• The direct transfer application part (DTAP) is used to transfer the messages between
the MSC and mobile device. These messages, which include MM and CM, are not
interpreted by the BSS and are carried transparently.
• BSS management application part (BSSMAP) messages are transferred between the
BSC and the MSC to support procedures such as resource management, handover
control and paging of the mobile device.


3.7.3 Address translation
As previously mentioned, an SCP may be required when there is no actual telephone
number for a speci¬c destination but an alias is used instead, such as for a toll-free
number. Consider the earlier example of a user dialling 1-800-FLYDRIVE. In such cases
it is necessary to look up the actual telephone number in a database. A TCAP request
is sent to an SCP, which has access to this database. Figure 3.27 illustrates how this
translation can be achieved. The SSP will receive the request and look up the destination
codepoint (DCP) address of the signal transfer point (STP) that deals with 1-800 requests.
The STP will send the request onto a service control point (SCP) with the extra SSN.
This process again requires the use of a lookup table, this time located at the STP. The
SSN identi¬er will ensure that the query not only goes to the correct device but, more
speci¬cally, to the correct application (a database in this example) within the device. The
reply is returned to the SSP, which can then send an initial address message (IAM) to set
up the connection to the destination address 12543778659.


3.7.4 Example of routing of a call to a mobile subscriber
When a call is made to a mobile subscriber, the number dialed is the mobile subscriber™s
MSISDN and not the IMSI. An ISUP IAM, which contains this MSISDN, is routed to
the gateway MSC (G-MSC) in the mobile user™s home network. The G-MSC will send
a MAP request to the HLR containing the MSISDN number, and the HLR will use this
to identify the corresponding IMSI for the particular mobile device. In smaller mobile
74 GSM FUNDAMENTALS


Routing Table
Query Send to DPC
Global Title Request STP
0800 10-30-50 (SSN 345)
10-30-50
1-800-35937483
.. ..
.. ..




Global Title Request
1-800-35937483
Connect Request
1-800-FLYDRIVE Global Title Reply




Global Title Reply
SSP




12543778659
12543778659

Routing Table
Query Send to DPC
SCP
0800 10-30-50
10-30-70
.. ..
.. .. SSN 345
1-800 nos.


Figure 3.27 Global title translation

networks, there may only be one HLR and a routing information request will be sent to
this HLR by the MSC. In larger networks where there are a vast number of subscribers,
there may be multiple HLRs. In this case, the MSC needs to determine the correct HLR
for this particular subscriber and may utilize global title (GT) translation to assist in this
procedure. The HLR record also contains the address of the current VLR serving this
subscriber; of course, if the mobile device is switched off or out of coverage there will
not be a VLR address entry. The HLR can now contact the VLR, again using the MAP
protocol, to request a mobile station roaming number (MSRN). This number, generated by
the VLR, is essentially an ISDN number at which the mobile subscriber can be reached.
The HLR can now respond to the initial routing information request from the G-MSC.
This response will include the MSRN. The G-MSC can now attempt to complete the call
to the mobile subscriber using the MSRN, again using an ISUP IAM message. It should
be noted that the allocated MSRN is only valid for the duration of the call and the next
call to this subscriber may use a completely different MSRN.
The network now has to ¬nd which group of cells the mobile is in and send a paging
signal asking that particular mobile device to respond. Even though the mobile device is
only positioned in a single cell, the page will normally be transmitted across the whole
LA, causing extra signalling traf¬c and noise in the cells that comprise the LA. Once
paged, the mobile device will respond on the random access channel (RACH), indicating
which speci¬c cell it is in and also requesting resources. The network will respond on the
AGCH and offer resources for the mobile device to use. The mechanism used for GPRS
is very similar to the above.
Once the call enters the network it is sent over E1 or T1 links using the TDM structure.
These links connect the BTSs to the BSC and from the BSC through a transcoder (TRAU)
to the MSC. From there the calls are routed via a G-MSC to the PSTN or ISDN. The
TDM system is restrictive and can be inef¬cient since it is circuit switched and the TDM
slots are reserved for a user even if no voice/data is being transferred. Figure 3.28 shows
an example of the overall procedure.
3.7 GSM AND SIGNALLING SYSTEM 7 75


Home PLMN Visited PLMN
Mobile
G-MSC HLR VLR MSC BSS
Device

IAM
SRI
PRN
PRN Ack
SRI Ack
IAM
SIFIC
Page Mobile
Page
Page
IAM Initial Address Message
Channel Request
PRN Provide Roaming Number
SIFIC Send Information for incoming call Immediate Assign
SRI Send Routing information Page Response
Connection
Process Established
Request

Security Procedures - Authentication etc
Call Arrived
Call
Setup
Complete
Call Confirm
Allocate
Assignment
Channel
Command
Assignment
Allocation Complete
Complete
Alerting
Address Complete Message (ACN)
Connect
Answering Message (ANM)
Connect Acknowledge
Complete
Call Ack.

Figure 3.28 Mobile terminated call


3.7.5 Example of routing of an SMS message to a
mobile subscriber

When an SMS message is sent to a mobile device it will arrive at a service centre
(SC), which is responsible for relaying and store-and-forwarding the SMS to the mobile
device. The SC will identify the G-MSC for SMS (SMS-GMSC) for the particular mobile
device and will forward the message. The SMS-GMSC will interrogate the HLR using
the MSISDN number of the mobile device requesting routing information, and any further
76 GSM FUNDAMENTALS


PLMN
Home or visited PLMN
Mobile
Service Centre
Device
HLR
G-MSC MSC
VLR


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