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management


Figure 3.22 Example MM messages

Type Messages

ALERTING

SETUP

Call establishment PROGRESS

AUTHENTICATION REQUEST AUTHENTICATION RESPONSE

CONNECT CONNECT ACKNOWLEDGE

RELEASE RELEASE COMPLETE
Call clearing
DISCONNECT

Supplementary service HOLD HOLD ACKNOWLEDGE

control RETRIEVE RETRIEVE ACKNOWLEDGE


Figure 3.23 Example CM messages

and location services support (LCS). The CC procedures have been closely modelled on
the ITU-T ISDN Q.931 recommendation. Differences between the two are compared in
the 3GPP speci¬cation TS24.008 annex E. The elementary procedures for CC are grouped
into the following classes:

• call establishment procedures
• call clearing procedures
• call information phase procedures
• miscellaneous procedures.
68 GSM FUNDAMENTALS


The CC for GSM differs somewhat from that of Q.931 in one large respect which is
that of mobility, i.e. routing to a mobile subscriber. An example of how this routing is
performed for a mobile terminated call is highlighted in Section 3.7.4.
The supplementary services include such things as call forwarding, line identi¬cation,
call waiting, call barring, multiparty calls and closed user groups.
The following protocols are also present in Figure 3.20: BSSAP and SCCP, and are
described in Section 3.7.2. Figure 3.23 shows a sample of CM messages.



3.7 GSM AND SIGNALLING SYSTEM 7
SS7 or common channel signalling system 7 (CCS7) is a telecommunications signalling
system standardized by the ITU-T under the Q.700 series of recommendations. The USA
uses a modi¬ed version of SS7, which is standardized by ANSI. The general objective
of SS7 was to provide a general purpose signalling system to be used globally. It is
optimized to work in digital telecommunication networks operating at 64 kbps but is also
suitable for analogue connections and transfer at lower bit rates. SS7 has been designed
to provide a reliable system, which transfers information in the correct sequence and
without loss or duplication. SS7 is the signalling system on which the vast majority of
¬xed-line telephone networks are based. It was principally for this reason that it was
chosen by the cellular industry for its signalling system, since it would allow simple
integration of cellular and ¬xed-line networks. Many of the existing 2G networks such
as GSM use SS7 to connect to a third party through the ISDN or PSTN. Since the
core network in 3G R99 has limited changes and reuses much of the GSM core, the
3G network has been designed to integrate with this existing system and a variant of
SS7 (3GPP TS24.008) will be used in the UMTS 3G system towards the circuit switched
core network (CS-CN). The standard de¬nes the procedures and protocols required for call
control such as call establishment, billing, routing, information exchange and modi¬cation.
Signalling via SS7 is carried in separate channels to the call data, where it uses a message
transfer part (MTP) to transfer the signalling information around the network. A common
channel signalling system such as SS7 enables a number of traf¬c channels to share a
single control channel. The maximum message length for narrowband SS7 signalling
messages is 272 bytes (for broadband messages, as used in UMTS, this is extended to
4 kbytes).
Releases 4 and 5 of the 3G speci¬cations introduce new possibilities for signalling
within an all-IP network, where the IP transport network is used for signalling message
transfer (see Chapter 8). However, some of the speci¬cations are still in draft standard
(e.g. M3UA).
To ensure the high reliability required by carrier class networks, SS7 has a number
of stringent requirements, two of which are highlighted below. These requirements are
stipulated under ITU-T Recommendations Q.700 to Q.775.

• The number of messages lost or delivered out of sequence (including duplicate mes-
sages) due to transport failure, as well as messages containing an error that is undetected
by the transport protocol, should be less than 1 in 1010 .
3.7 GSM AND SIGNALLING SYSTEM 7 69


• The complete set of allowed signalling paths from a source to destination, known as
the signalling route set, should be available for 99.9998% of the time. This equates to
a tolerated downtime of around 10 minutes per year.

The following are examples of typical operations handled by SS7:

• connect +44 0161 980 2323 to +65 789564 using trunk 55
• locate 0800 1234 5678 (this is a free-phone number needing to be routed to the cor-
rect exchange)
• the line is busy send back a busy tone to the caller
• trunk 88 has failed do not send information on this trunk.



3.7.1 Signalling points
A typical SS7 network consists of three types of device: service switching points (SSP),
signal transfer points (STP) and service control points (SCP). Collectively these devices
are referred to as SS7 nodes, or simply signalling points. Although logically they are
separate elements, it is common for a single physical device to perform a number of
functions. All of the signalling points within an SS7 network are identi¬ed by a unique
codepoint address.

• The SSP is responsible for originating, terminating and routing the call to the correct
destination. When a user wishes to make a call, this request will be passed on to a
telephone switch (an SSP). These devices usually have the ability to deal with many
calls at the same time and thus have a large switching matrix. Within the GSM network,
the MSC is considered to be an SSP.
• The STP acts as a router and is used to provide a path from the source to the desti-
nation as well as give alternative paths to a destination, for example, in the case of
network failure.
• The SCP is required when there is no actual telephone number for a speci¬c destination
but an alias is used instead. This is the case for toll-free numbers where a subscriber
may dial, for example, 1-800-FLYDRIVE. In such cases it is necessary to look up the
actual telephone number in a database. A similar method may be required when routing
a call to a mobile device, since the network needs to query the HLR of this mobile
device with the subscriber MSISDN (see later). Within the GSM network the registers
HLR, VLR, EIR and AuC are all types of SCP.

It can be seen from Figure 3.24 that to make a signalling connection from user A to user B
a variety of network elements are involved. To ensure reliability of the network, it is typical
for the SSP to have at least two connections through to the network. These connections
are referred to as access links, and only signalling which is for the originating point
code (OPC) or the destination point code (DPC) traverses these links. STP devices which
70 GSM FUNDAMENTALS


pair
SCP SCP
X Y


pair pair
1 2
bridge
STP STP
VLR HLR link
ss




ac lin
ce k




ce k
ac lin




ss
GSM bridge


cross




cross
GMSC



link




link
SSP
Network (SSP) links
ss
ac lin



User A User B
ce k
ce k




ac lin
ss



4 3
bridge
STP STP
links



Figure 3.24 SS7 sample logical network


are working in redundant pairs are connected together via a cross-link. In Figure 3.24,
STP (1) and STP (4) are paired, as are STP (2) and STP (3), thus improving the reliability
of the system. Bridge links are also used to connect one STP to another. In this case the
STPs are used as routers to direct the signalling message from the source to destination.
The SCPs can also be paired, as is the case for SCP (X) and SCP (Y).



3.7.2 Protocol stack for SS7 signalling over MTP
The standard protocol stack for SS7 is shown in Figure 3.25. The layers are analogous
to the layers of the ISO OSI seven-layer model, with the layers in an SS7 stack referred
to as parts rather than layers. The lower three layers are collectively referred to as the
message transfer part (MTP).

Message transfer part level 1
This is functionally equivalent to the OSI layer 1 and de¬nes the various physical layer
interfaces. Messages are usually carried over 56 kbps or 64 kbps links for the narrow-
band services. E1 (2048 kbps) consisting of 32 — 64 kbps channels and DS1 (1544 kbps)
consisting of 24 — 64 kbps channels are also supported. These TDM links ensure that the
bits making up the message arrive in the correct order.

Message transfer part level 2
This layer ensures that there is a reliable connection between two datalink network ele-
ments. It includes error checking, ¬‚ow control and sequencing. If an error is detected,
MTP2 can ask for a retransmission. Layer 2 is capable of monitoring the state of the
link and enables peer devices to communicate link information to one another. This
3.7 GSM AND SIGNALLING SYSTEM 7 71


OSI SS7

Application

Presentation
TUP ISUP TCAP
Session

Transport
SCCP
Network MTP level 3




MTP
Data Link MTP level 2

Physical MTP level 1


Figure 3.25 SS7 protocol stack


information could be, for example, an indication that the link is congested or has failed.
This is functionally equivalent to the OSI layer 2.


Message transfer part level 3
This layer extends the functionality of level 2 so that signalling messages can be trans-
ported over a complex network, i.e. there does not need to be any direct connection

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