Approaching a Unified Standard

In a GSM-MAP network, it is the SIM/USIM that dictates or at least describes the quality
of service requirements of the user or the user’s application. This in turn determines
the allocation of radio and network resources. Radio resources are provided either over
an IMT2000DS air interface (with backward compatibility to GSM, GPRS and E-GPRS air
interfaces) or a CDMA2000 air interface (with backward compatibility to IS95A, B, C).
In addition to having two similar but different air interfaces, we have, worldwide,
two similar but different mobility network standards:
ANSI 41 network. Any U.S. TDMA or CDMA2000 air interface, or any AMPS
air interface, either in the United States or Asia, will have behind it an ANSI 41
network.
GSM-MAP network. Any GSM or IMT2000DS air interface, either in the United
States, Europe, or Asia, will have behind it a GSM-MAP network.

The differences between the two networks are by no means unbridgeable, particularly
as both use SS7 signaling to manage network functionality. One practical and important
difference historically is that GSM-MAP networks have used the smart card SIM
as the basis for controlling radio access to the network, that is, user specific authorization.
The user buys a SIM card and can put it into any GSM phone. The SIM card, not
the phone, is the device that determines the user’s access and priority rights.
In IS41/ANSI 41 networks to date, SIM cards have not been used. Instead, the
device is validated for use on the network by virtue of its mobile identity number
(MIN) and equipment identity number (EIN). This is now changing, as 3GPP2 (the
body working with 3GPP1 on IMT2000DS/CDMA2000 integration) now support the
use of the SIM (which in CDMA2000 is actually called an R-UIM—removable user
identity module) as an access validation platform.
3GPP1 and 3GPP2 are working together to use the SIM/R-UIM as a basis for bringing
together GSM-MAP and ANSI 41. Parallel work is under way to implement GAIT
handsets (GSM/ANSI 41 handset interoperability) and the side-by-side compatibility
of an ANSI 41 network with the GERAN (GSM/GPRS/EDGE radio access network)
and UTRAN (UMTS radio access network), as shown in Figure 12.1. The U-SIM/RUIM
is the mechanism for defining a user’s policy/conditional-access rights and is
becoming an integral part of the IPQoS proposition.

The more radically inclined vendors see IP protocols as an additional mechanism for
unification, potentially replacing existing Signaling System 7 (SS7) signaling, which is
used to establish, maintain, and clear down telephone calls between users. SS7 provides
the signaling control plane for wireless and wireline circuit-switched network
topologies. It is a mature and stable standard. In a wireless network, additional functionality
is needed to manage the allocation of radio channels (actually a channel pair
for the uplink and downlink) and to support mobility management. This is known in
GSM as GSM-MAP (Mobile Application Part).
SS7 is often described as an out of band signaling system—the signaling is kept functionally
and physically separate from the user’s voice or data exchange. In a packetswitched
network, the routing of calls or sessions relies on a router reading the address
on each packet or group of packets transmitted—an in-band signaling system using
established Internet protocols (IP). There are standards groups presently working on
bringing together IP and SS7 (IP SS7), and significant progress has been made on using
both signaling systems to implement always on connectivity in wireline networks (for
example, using ADSL).
The additional functionality needed to support wireless connectivity, however, creates
a number of implementation problems, which are presently proving difficult to
resolve. For example, in a GPRS network, a Packet Common Control Channel
(PCCCH) and Packet Broadcast Control Channel (PBCCH) are needed to support
always on connectivity. The PCCCH and PBCCH replace the existing Common Control
Channel (CCCH) and Broadcast Control Channel (BCCH). There is presently no
easy method for ensuring PCCCH- and PBCCH-compliant handsets are backward
compatible with CCCH- and BCCH-compliant handsets. This sort of issue can be overcome,
but it takes time.
In addition, some network operators question why they should abandon a tried and
trusted signaling system that gives good visibility to system hardware performance
(including warning of hardware failures) and is (accidentally) well suited to persistent
session management. This is an important point. The first two parts in this book
argued the case that session persistency would increase over time and become increasingly
similar to voice traffic, although ideally with a longer holding time. As session
persistency increases, out-of-band signaling becomes increasingly effective, which
means session setup, session management, and session clear-down is directly analogous
to call setup, call maintenance, and call clear-down.
IP could potentially replace SS7 but would need to emulate the session management
and session reporting capabilities of SS7. We revisit this issue when we study traffic
shaping and traffic management protocols, the subject of Chapters 16 and 17 in Part IV
of this book. 283