UNIVERSITE DE SIDI BEL ABBES

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Anne-acadmique : 2013-2014
TRANSMISSION NUMERIQUE PAR FAISCEAUX HERTZIENS




The SS-7 standard denes three basic types of network nodes:
Service Switching Points (SSPs) are switching centers that are more generally referred to as
network elements that are able to establish, transport or forward voice and data connections.
Service Control Points (SCPs) are databases and application software that can inuence the estab-
lishment of a connection. In a GSM network, SCPs can be used, for example, for storing the current
location of a subscriber. During call establishment to a mobile subscriber, the switching centers query
the database for the current location of the subscriber to be able to forward the call. More about this
procedure can be found in Section 1.6.3 about the Home Location Register (HLR). Signaling Transfer
Points (STPs) are responsible for the forwarding of signaling messages between SSPs and SCPs as not
all network nodes have a dedicated link to all other nodes of the network. The principal functionality
of an STP can be compared to an IP router in the Internet, which also forwards packets to different
branches of the network. Unlike IP routers, however, STPs only forward signaling messages that are
necessary for establishing, maintaining and clearing a call. The calls themselves are directly carried on
dedicated links between the SSPs.
Figure 1.4 shows the general structure of an SS-7 circuit-switched telecommunication network and
the way the nodes described above are interconnected with each other. The SS-7 protocol stack is
also used in virtual circuit-switchednetworks for communicationbetween the network nodes. Instead
of dedicated signaling timeslots on an E-1 link, signaling messages are transportedinIP packets. The
followingsectiondescribes the classic SS-7protocolstack and afterward, the way SS-7 messages are
transported over IP networks.
1.4.1 The Classic SS-7 Protocol Stack SS-7 comprises a number of protocols and layers. A well-known
model for describing telecommuni- cation protocols and different layers is the Open System
Interconnection (OSI) 7 layer model, which is used in Figure 1.5 to show the layers on which the
different SS-7 protocols reside. The Message Transfer Part 1 (MTP-1) protocol describes the physical
properties of the transmission medium on layer 1 of the OSI model. Thus, this layer is also called the
physical layer. Properties that are standardized in MTP-1 are, for example, the denition of the
different kinds of cables that can be used to carry the signal, signal levels and transmission speeds.
On layer 2, the data link layer, messages are framed into packets and a start and stop identication at
the beginning and end of each packet is inserted into the data stream so that the receiver is able to
detect where a message ends and where a new message begins

To call B, the phone number of B is sent by A to the MSC. The MSC then analyzes the national
destination code of the phone number, which usually comprises the rst two to four digits of the
number, and detects that the number belongs to a subscriber in the xed-line network. In the
example shown in Figure 1.6, the MSC and the xed-line switching center are directly connected with
each other. Therefore, the call can be directly forwarded to the terminating switching center. This is
quite a realistic scenario as direct connections are often used if, for example, a mobile subscriber
calls a xed-line phone in the same city. As B is a xed-line subscriber, the next step for the MSC is to
establish a voice channel to the xed- line switching center. This is done by sending an ISUP Initial
Address Message (IAM). The message contains, among other data, the phone number of B and
informs the xed-line switching center and the channel that the MSC would like to use for the voice
path. In the example, the IAM message is not sent directly to the xed-line switching center. Instead,
an STP is used to forward the message. At the other end, the xed-line switching center receives the


message, analyzes the phone number, and establishes a connectionviaitsswitchingmatrixto
subscriberB. Once the connectionis established via the switching matrix, the switch applies a periodic
current to the line of the xed-line subscriber so that the xed-line phone can generate an alerting
tone. To indicate to the originating subscriber that the phone number is complete and the
destination party has been found, the xed-line switch sends back an Address Complete Message
(ACM). The MSC then knows that the number is complete and that the terminating party is being
alerted of the incoming call. If B answers the call, the xed-line switching center sends an Answer
Message (ANM) to the MSC and conversation can start. When B ends the call, the xed-line switching
center resets the connection in the switching matrix and sends a release (REL) message to the MSC.
The MSC conrms the termination of the connection by sending back a Release Complete (RLC)
message. If A had terminated the call, the messages would have been identical, with only the
direction of the REL and RLC reversed. For the communication between the switching centers (SSPs)
and the databases (SCPs), the Sig- naling Connection and Control Part (SCCP) is used on layer 4. SCCP
is very similar to Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) in the IP
world. Protocols on layer 4 of the protocol stack enable the distinction of different applications on a
single system. TCP and UDP use ports to do this. If a personal computer (PC), for example, is used as
both a web server and a File Transfer Protocol (FTP) server at the same time, both applications would
be accessed over the network via the same IP address. However, while the web server can be
reached via port 80, the FTP server waits for the incoming data on port 21. Therefore, it is quite easy
for the network protocol stack to decide the application to which incoming data packets should be
forwarded. In the SS-7 world, the task of forwarding incoming messages to the right application is
done by SCCP. Instead of port numbers, SCCP uses Subsystem Numbers (SSNs). For database access,
the Transaction Capability ApplicationPart (TCAP) protocol has been designed as part of the SS-7
family of protocols. TCAP denes a number of different modules and messages that can be used to
query all kinds of different databases in a uniform way.
1.4.2 SS-7 Protocols for GSM Apart from the xed-line network SS-7 protocols, the following
additional protocols were dened to address the special needs of a GSM network.
The Mobile Application Part (MAP): This protocol has been standardized in 3GPP TS 29.002 [3] and
is used for the communication between an MSC and the HLR, which maintains subscriber
information. The HLR is queried, for example, if the MSC wants to establish a connection to a mobile
subscriber. In this case, the HLR returns the information about the current location of the subscriber.
The MSC is then able to forward the call to the mobile subscribers switching center

MSC to HLR MSC to MSC
Figure 1.7 Enhancement of the SS-7 protocol stack for GSM.
establishing a voice channel between itself and the next hop by using the ISUP message ow that has
been shown in Figure 1.6. MAP is also used between two MSCs if the subscriber moves into the
coverage area of a different MSC while a call is ongoing. As shown in Figure 1.7, the MAP protocol
uses the TCAP, SCCP and MTP protocols on lower layers. The Base Station Subsystem Mobile
Application Part (BSSMAP): This protocol is used for communication between the MSC and the radio
network. Here, the additional protocol is necessary, for example to establish a dedicated radio
channel for a new connection to a mobile subscriber. As BSSMAP is not a database query language
like the MAP protocol, it is based on SCCP directly instead of using TCAP in between. The Direct
Transfer Application Part (DTAP): This protocol is used between the users mobile device, which is


also called mobile station (MS), and the MSC to communicate transparently. To establish a voice call,
the MS sends a setup message to the MSC. As in the example in Section 1.4.1, this message contains
among other things the phone number of the called subscriber. As it is only the MSCs task to
forward calls, all network nodes between the MS and the MSC forward the message transparently
and thus need not understand the DTAP protocol.
1.4.3 IP-Based SS-7 Protocol Stack When using an IP network for the transmission of SS-7 signaling
messages, the MTP-1 and MTP-2 protocols are replaced by the IP and the transport medium-
dependent lower layer protocols (e.g. Ethernet). Figure 1.8 shows the difference between the IP and
the classic stack presented in the previous section. In the IP stack, layer-4 protocols are either UDP or
TCP for most services. For the transmission of SS-7 messages, however, a new protocol has been
specied, which is referred to as Stream Control Transmission Protocol (SCTP). When compared to
TCP and UDP, it offers advantages when many signaling connections between two network nodes are
active at the same time. Onthenextprotocollayer,SCTPisfollowedbytheM3UA(MTP-
3UserAdaptationLayer)protocol. As the name implies, the protocol is used to transfer information
that is contained in the classic MTP-3 protocol. For higher protocol layers such as SCCP, M3UA
simulates all functionalities of MTP-3. As a consequence, the use of an IP protocol stack is
transparent to all higher layer SS-7 protocols. In the industry, the IP-based SS-7 protocol stack or the
IP-based transmission of SS-7 messages is often referred to as SIGTRAN (signaling transmission). The
abbreviation originated from the name