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Ericsson Review No. 1, 2004
17
efficiency over the air interface. This prob-
lem has been addressed through signaling
compression (SigComp), another IETF stan-
dard initiated by Ericsson. SigComp is a ver-
satile compression framework that can use
any compression algorithm to compress
ASCII-based protocols, such as SIP, to a frac-
tion of their original size, reducing both bit
rate requirements and transport latency.
SigComp sessions are initialized during
the registration phase of the Instant Talk
service. The SigComp compressor generates
a decompression byte code that contains a
preferred compression algorithm and trans-
mits it to the decompressor. The decom-
pressor, called the universal decompression
virtual machine (UDVM), decodes the byte
code and adopts the decompression algo-
rithm, making it ready to receive com-
pressed SIP messages. Extended operations
(an important feature) enable SigComp to
learn from SIP messages.
4
Therefore,
SigComp becomes increasingly efficient as
compression and SIP signaling proceed.
SigComp is a critical component of
Ericsson Instant Talk because it can signif-
icantly reduce the number of bits sent over
the wireless link (compression ratios of 8:1
are not unusual).
5
It greatly reduces the
transmission delay of SIP messages, and con-
sequently, the time it takes to establish an
Instant Talk session.
Technical realization
Ericsson has taken a total business approach
in its technical realization of Intant Talk,
providing the infrastructure, terminal
clients and professional services, and ensur-
ing availability of terminals. Guided by this
approach, Ericsson will continue to opti-
mize performance by implementing en-
hancements in the radio access network, mo-
bile core network, application servers and
clients. Ericsson’s ability and willingness to
influence the nodes in the end-to-end path
of telecommunications systems set it apart
from other vendors of push to talk solutions.
The commercial launch of Ericsson Instant
Talk will include vertical service assurance.
One more important factor is that
Ericsson bases its products on open stan-
dards. Therefore, handset interoperability
will not be an issue. Operators and end-users
will be able to choose terminals and termi-
nal vendors freely.
Instant Talk consists of three main parts:
the IPMM, the Instant Talk application
server, and the handset client.
IP multimedia system
Figure 1 shows a schematic drawing of the
nodes involved in the Instant Talk service.
At the heart of the solution is the Ericsson
IPMM system (Figure 2), which complies
with the principles of the IMS standard
(3GPP) that was drafted to bring SIP-based
communications to the wireless market.
2
The IMS can be deemed a “new” domain that
has been added to the mobile core network
to support a wide range of SIP-based appli-
cations, such as Instant Talk, instant mes-
saging, and presence services.
The IPMM architecture includes the
call/session control function (CSCF), the
media resource function (MRF), and the
home subscriber server (HSS). The CSCF is
the teminal’s first point of contact in the
IPMM domain. All SIP signaling is routed
through the CSCF, which also performs
SigComp. The role of the CSCF is to handle
subscriber registration, and to support the
establishment, modification and release of
Instant Talk sessions. The CSCF ensures
interoperability with telephone systems and
network addressing mechanisms by query-
ing domain name service (DNS) servers to
map SIP uniform resource identifiers (URI)
or E.164 numbers to network addresses.
In an IPMM-based service, media is di-
rected to, replicated in, and distributed from
Access network
(GPRS,
CDMA2000
WCDMA)
Instant Talk client
Instant Talk
application
server
Core
network
SIP
Signaling
RTP
Media
Call/session
control
function
(CSCF)
Home
subscriber
server
(HSS)
Media
resource
function
(MRF)
Figure 1
Ericsson Instant Talk high-level architecture.