The UMTS Network and Radio Access Technology: Air Interface Techniques for Future Mobile Systems
Jonathan P. Castro
Copyright © 2001 John Wiley & Sons Ltd
Print ISBN 0-471-81375-3 Online ISBN 0-470-84172-9
T
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IP B
ASED
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9.1 B
ACKGROUND
In the preceding chapters we covered UMTS in the context of the 3GPP Release 99
specifications. This chapter covers the forthcoming releases of UMTS, primarily Re-
lease 4 and 5 formerly Release 00. However, before we describe the reference architec-
ture we outline the vision of the UMTS technical specification evolution from Ref. [1].
9.1.1 The UMTS Release 99 and Medium Term Architecture
9.1.1.1 Release 99
Figure 9.1 illustrates the service drivers of the UMTS architecture for R99 and future
releases starting with R00. The latter has now been broken into Release 4 and 5.
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9.1.2 The Long Term UMTS Architecture Vision
After the evolution of R99 culminating with R00 (R4 and R5) we aim to have an inte-
grated platform based entirely on a packet switched system. The service drivers for the
long term include: migration of many users to IP-multimedia services and wide-spread
adoption of IP-multimedia outside UMTS.
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By this time, we assume that the IP multimedia subsystem has evolved to the degree
that it can practically stand as a substitute for all services previously provided by the CS
domain. Here we retain the PS domain but phase out the CS domain. Whether the latter
can be achieved in its integrity including all security aspects remains to be seen, since it
is still under standardization or technical specification.
9.1.3 The All IP and Service Evolution
As noted in Chapter 1, the widespread usage of Internet and IP’s ability to communicate
between different networks has made IP a convergence layer to evolve from a simple
data platform to larger structure for services. By aiming to reach further than the circuit
switch, IP now leads mobile communications to new dimensions.
The IP protocol has opened up a whole range of wireless applications, which will allow
service providers and operators to develop totally new and innovative services while
enhancing their existing infrastructures. Thus, the main drivers for IP services include a
support for inter domain roaming and service continuity
The future UMTS releases will have new and improved enabling mechanisms to offer
services without using circuit switched network capabilities, as shown in Figure 9.3.
Here, we assume that the set of services available to the user, and the quality of the ser-
vices offered will match those available in networks that use CS enablers.
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Figure 9.3 Services in the forthcoming UMTS network architecture.
320
The UMTS Network and Radio Access Technology9.1.4 Classifying Releases 4 and 5 Services
Following the suggested classification in [2], we can divide basic services into circuit
tele-services [3] and bearer services [4], where both can utilize standardized supplemen-
tary services [5]. These basic services have not changed much in 2G networks like
GSM. GPRS [6] provides IP bearer services, and SMS, USSD and UUS can also be
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Towards IP Based Networks 321
9.2 R
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9.3.1 Call State Control Function (CSCF)
Logically, the CSCF can be divided into three sub-components: the serving CSCF (S-
CSCF), the proxy CSCF (P-CSCF); and the interrogating CSCF (I-CSCF).
We use the first to support mobile originated/terminated communications. It provides
the Serving Profile Database (SPD) and Address Handling (AH) functionality. The
serving CSCF supports the signalling interactions with the UE through the Gm inter-
face. The HSS sends the subscriber data to the serving CSCF for storage. It also gets
updated through the latter.
The CSCF acts as the central point of the IP multimedia control system; as well as gen-
eral call control (setup, supervision, and release). It triggers user controlled supplemen-
tary services and call leg handling controlled by user call control supplementary ser-
vices, e.g. three party call using Multimedia Resource Function (MRF). In addition, it
handles user charging and security.
We use the Interrogating CSCF (I-CSCF) for Mobile Terminated (MT) communica-
tions and to determine routing for mobile terminated calls. With its function always
located at the entrance to the home network, we can compare this (I-CSCF) to the
GMSC in a GSM network. The I-CSCF interrogates the HSS to get information to en-
able calls going to the serving CSCF. The interrogating CSCF provides the Incoming
Call Gateway (ICGW) and AH functionality.
The proxy CSCF, which we may compare to the visited MSC in a GSM network, man-
ages address translation/mapping and handles call control for certain types of calls like
emergency calls, legally intercepted calls, etc.
Towards IP Based Networks 323
MT communications can use both serving CSCF and interrogating CSCF functionality,
while MO communications do not require the interrogating CSCF functionality. Both
serving CSCF and interrogating CSCF components may come in a single CSCF when
needed. We can summarize the CSCF functions from Ref. [7] as follows:
ICGW (Incoming Call Gateway)
interacts with HSS in the home domain to receive profile information for the R00
all-IP network user and may store them depending on the SLA with the home do-
main;
notifies the home domain of initial user’s access (includes, e.g. CSCF signalling
transport address, user ID, etc.; needs further study);
may cache access related information (e.g. terminal IP address(es) where the user
may be reached, etc.).
AH (Address Handling)
analysis, translation, modification if required, address portability, mapping of alias
addresses;
may do temporary address handling for inter-network routing.
324
The UMTS Network and Radio Access Technology9.3.2 Home Subscriber Server (HSS)
The Home Subscriber Server (HSS) serves as the master database for a given user. It
contains the subscription related information, to support the network entities actually
handling calls/sessions, e.g. it could provide support for the call control servers to com-
plete routing/roaming procedures by solving authentication, authorization, naming/
addressing resolution, location dependencies, etc.
The HSS holds the following user related information:
user identification, numbering, addressing and security information (i.e. network
access control information for authentication and authorisation);
the subset of the HLR functionality required by the PS domain;
and the CS part of the HLR, if it is desired to enable subscriber access to the CS
domain or to support roaming to legacy GSM/UMTS CS domain networks.
As illustrated in Figure 9.8, the HSS structure has the following interfaces:
MAP termination: HSS terminates the MAP protocol as described in MAP specifica-
tions:
user location management procedures;
Towards IP Based Networks 325
user authentication management procedures;
subscriber profile management procedures;
call handling support procedures (routing information handling);
SS related procedures, etc.
Addressing protocol termination: the HSS terminates a protocol to solve addressing
according to appropriate standards, i.e.:
procedures for user names/numbers/addresses resolution;
DNS+ protocol resolution, under definition within the ENUM group in IETF (cur-
rently looking into URL/E.164 naming translation, etc.).
Authentication, authorization protocol termination: the HSS terminates authentication
and authorization protocols according to appropriate standards, i.e.:
user authentication and authorization procedures for IP based multimedia services;
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Figure 9.8 A generic HSS structure with protocols over the basic interfaces.
9.3.3 Transport Signalling Gateway Function (T-SGW)
This component serves as the PSTN/PLMN termination point for a defined network.
Terminates, e.g. the call control signalling from GSTN mobile networks (typically
ISDN) and maps the information onto IP (SIGTRAN) towards the Media Gateway Con-
326
The UMTS Network and Radio Access Technologytrol Function (MGCF). The functionality defined within T-SGW should be consistent
with existing/ongoing industry protocols/interfaces that will satisfy the requirements:
maps call related signalling from/to PSTN/PLMN on an IP bearer and sends it
to/from the MGCF;
needs to provide PSTN/PLMN
IP transport level address mapping.
9.3.4 Roaming Signalling Gateway Function (R-SGW)
The role of the R-SGW concerns only roaming to/from 2G/R99 CS and the GPRS do-
main to/from the R00 UMTS teleservices domain and the UMTS GPRS domain and
does not involve the multimedia domain. According to Ref. [7] the main functions are:
to ensure proper roaming, the R-SGW performs the signalling conversion at trans-
port level (conversion: Sigtran SCTP/IP versus SS7 MTP) between the legacy SS7
based transport of signalling and the IP based transport of signalling. The R-SGW
does not interpret the MAP/CAP messages but may have to interpret the underlying
interaction with MGCF, MSC server and GMSC server for resource control;
ownership and resources handling, e.g. echo cancellers etc.;
ownership of codecs.
Towards IP Based Networks 327
The MGW will have the necessary resources to support UMTS/GSM transport media. It
will also have customized H.248 packages to support additional codecs and framing
protocols, etc. from other networks besides GSM and UMTS. The MGW bearer control
and payload processing capabilities will also support mobile specific functions, e.g.
SRNS relocation/handover and anchoring through H.248 protocol enabling. The follow-
ing principles apply to the CS-MGW resources:
it shall not be necessary to have the CS-MGW co-located with the MSC server;
the CS-MGW resources need not be associated with any particular MSC server
1
;
it shall be possible for any MSC server to request resources of any CS-MGW in the
network
1
;
it shall be possible for an RNC to connect to the CS-MGW indicated by the MSC
mserver.
9.3.7 Multimedia Resource Function (MRF)
The MRF performs:
multiparty call and multimedia conferencing functions, i.e. would have the same
_______
1
Extensions to H.248 may be required.
328
The UMTS Network and Radio Access Technology
procedures related to UE-HSS information tunnelling via CSCF.
Details on these procedures can be found in Ref. [7].
9.4.2 Gf Reference Point (SGSN–EIR)
The SGSN server supports the standard Gf interface towards the EIR server. MAP sig-
nalling is used over this interface in order to support identity (IMEI) check procedures.
For more details refer to TS 23.060.
9.4.3 Gi (GGSN–Multimedia IP Network)
The GGSN supports the Gi interface. It is used for transportation of all end user IP data
between the UMTS core network and external IP networks. The interface is imple-
mented according to TS 23.060, the Internet Protocol according to RFC791 and
RFC792 (ICMP). Finally, the IPSec according to the following RFCs: 2401, 2402,
2403, 2404, 2405, 2406, 2410 and 2451. IP packets get transported over AAL5 accord-
ing to RFC 2225 and RFC 1483.
9.4.4 Gn Reference Point (GGSN–SGSN)
We use the Gn interface both for control signalling (i.e. mobility and session manage-
ment) between SGSN servers and GGSN, as well as for tunnelling of end user data pay-
load within the backbone network.
The GTP-C protocol (running over UDP/IP) used for control signalling can also be in-
cluded here. The interface is implemented according to TS 23.060 and TS 29.060.
9.4.5 Gm Reference Point (CSCF–UE)
This interface allows the UE to communicate with the CSCF, e.g. register with a CSCF,
call origination and termination and supplementary services control.
dynamic sharing of transmission resources between the domains as the MGW con-
trols bearers and manage resources according to the H.248 protocols.
The functionality across the Mc reference point will require to support mobile specific
functions, e.g. SRNS relocation/handover and anchoring. The current H.248/IETF
Megaco standard mechanisms will enable these features.
9.4.7 Mg Reference Point (MGCF–CSCF)
The SIP based Mg reference point allows the transfer of session related information
between the CSCF and the MGCF. We use this interface to communicate between the
IP multimedia networks and the legacy PSTN/ISDN/GSM networks.
9.4.8 Mh Reference Point (HSS–R-SGW)
This interface supports the exchange of mobility management and subscription data
information between HSS and R99 and 2G networks. We need this interface to support
Release 2000 (R4 and R5) network users who are roaming in R99 and 2G networks, and
we implement it with MAP/IP using SCTP and other adaptation protocols developed by
the IETF SIGTRAN working group.
9.4.9 Mm Reference Point (CSCF–Multimedia IP networks)
The Mm SIP based reference point stands as an IP interface between CSCF and IP net-
works. We use the interface, e.g. to receive a call request from another VoIP call control
server or terminal. A network in principle will support SIP/SDP between the CSCF and
other multimedia networks, with SIP signalling compliant with RFC 2543 and subse-
quent SIP releases, and with SDP compliant with RFC 2327 and also with its subse-
quent releases. The interworking between SIP and other protocols, e.g. H.323, occurs at
the edge of the IP multimedia network.
9.4.10 Mr Reference Point (CSCF–MRF)
The Mr affords the CSCF to control the resources within the MRF, thus allowing a net-
work to support communication between the CSCF-MRF with either SIP or H.248 de-
pending on the selection by standards. There is interest in the acceptance of IETF proto-
cols such as SIP, e.g. for Mr.
9.4.11 Ms Reference Point (CSCF–R-SGW)
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