VoIP and SS7

Although new signaling solutions, such as H.323 and SIP, exist for VoIP networks,
the standard in traditional telephony and in mobile networks is SS7.
Therefore, if a VoIP-based network is to communicate with any traditional
network, not only must it interwork at the media level through media gateways,
it must also interwork with SS7.To support this, the IETF has developed
a set of protocols known as Sigtran.
In order to understand Sigtran, it is worth considering the type of interworking
that needs to occur. Imagine, for example, an MGC that controls
one or more media gateways. The MGC is a call control entity in the network
and, as such, uses call control signaling to and from other call control
entities. If other call control entities use SS7, then the MGC must use SS7,
at least to the extent that the other call control entities can communicate
freely with it. This means that the MGC does not necessarily need to support
the whole SS7 stack—just the necessary application protocols.
Consider Figure 8-14, which shows the SS7 stack. The bottom three layers
are called the Message Transfer Part (MTP). This is a set of protocols responsible
for getting a particular SS7 message from the source signaling point to
the destination signaling point. Above the MTP, we find either the Signaling
Connection Control Part (SCCP) or the ISDN User Part (ISUP). ISUP is generally
used for the establishment of regular phone calls. SCCP can also be
used in the establishment of regular phone calls, but it is more often used for
the transport of higher-layer applications, such as the GSM Mobile Application
Part (MAP) or the Intelligent Network Application Part (INAP). In fact,
most such applications use the services of the Transaction Capabilities Application
Part (TCAP), which in turn, uses the services of SCCP.
SCCP provides an enhanced addressing mechanism to enable signaling
between entities even when those entities do not know each other’s signaling addresses (known as point codes). This addressing is known as global
title addressing. Basically, it is a means whereby some other address, such
as a telephone number, can be mapped to a point code, either at the node
that initiated the message or some other node between the originator and
destination of the message.
Figure 8-15 provides some examples of communication between different
SS7 entities. Consider scenario A. In this case, the two entities, represented
by point code 1 and point code 4, communicate at layer 1. At each layer, a
peer-to-peer relationship exists between the two entities. Scenario B has a
peer-to-peer relationship at layer 1, layer 2, and layer 3 between point codes
1 and 2, 2 and 3, and 3 and 4. At the SCCP layer, a peer-to-peer relationship
exists between point codes 1 and 2 and between point codes 2 and 4.
At the TCAP and Application layers, a peer-to-peer relationship can only
take place between point codes 1 and 4. In other words, the application at
point code 1 is only aware of the TCAP layer at point code 1 and the application
layer at point code 4. Similarly, the TCAP layer at point code 1 is
aware only of the application layer above it, the SCCP layer below it, and
the corresponding TCAP layer at point code 4. It is not aware of any of the
MTP layers. Equally, if we consider communication between point code 2
and point code 4, the SCCP layer at each point code knows only about the
layer above (TCAP), the layer below (MTP3), and the corresponding SCCP
peer. As far as the SCCP layers are concerned, nothing else exists. Therefore,
SCCP neither knows nor cares that point code 3 exists.
Consider Scenario C, where point code 3 is replaced by a gateway that
supports standard SS7 on one side and an IP-based MTP emulation on the other side. Point code 4 does not support the lower SS7 layers at all—just an
MTP emulation over IP. Provided that the MTP emulation at point code 4
appears to the SCCP layer as standard MTP, then the SCCP layer does not
care, nor do any of the layers above SCCP. Equally, the SCCP layers at point code 1 and 2 do not care. Consequently, it is possible to implement SS7-
based applications at point code 4 without implementing the whole SS7
stack. This is the concept behind the Sigtran protocol suite.