Page 25 - From GMS to LTE

Global System for Mobile Communications (GSM) 11

MSC to HLR and MSC to HLR and
MSC to MSC MSC to MSC
Core network MSC to BSS and Core network MSC to BSS and
Call control MSC to mobile Call control MSC to mobile

ISUP MAP DTAP BICC MAP DTAP
TCAP BSSMAP TCAP BSSMAP
SCCP SCCP SCCP SCCP
MTP - 3
IP, SCTP, M3UA
MTP - 2
MTP - 1 Ethernet and others
SS-7 over MTP SS-7 over IP
(SIGTRAN)
Figure 1.8 Comparison of the classic and IP‐based SS‐7 protocol stacks.

On the next protocol layer, SCTP is followed by the M3UA (MTP‐3 User Adaptation
Layer) 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 of the IETF (Internet Engineering Task Force) working group
that was created for the definition of these protocols.
As described in Section 1.1.1, the ISUP protocol is used for the establishment of
voice calls between switching centers and the assignment of a 64 kbit/s timeslot. In
an IP‐based network, voice calls are transmitted in IP packets. As a consequence,
the ISUP protocol has to be adapted as well. The resulting protocol is referred to as
the Bearer‐Independent Call Control (BICC) protocol, which largely resem-
bles ISUP.
As IP links cannot be introduced on all interfaces in live networks at once,
Signaling Gateways (SGWs) have been defined to bridge E‐1‐based and IP‐based
SS‐7 communication. The SGWs adapt the lower layers of the protocol stack and
thus make the differences transparent for both sides. This is necessary, for exam-
ple, if the subscriber database has already been converted for IP interfaces while
other components such as the switching centers are still using traditional signal-
ing links.
To bridge voice calls between E‐1‐based and IP‐based networks, Media Gateways
(MGWs) are used. Connected to an MSC‐Server, an MGW handles both IP‐based and
E‐1‐based voice calls transparently as it implements both the classic and IP‐based
signaling protocol stacks.