Integration of Multiple Switching Disciplines
by Pertti Raatikainen
Multimedia services presume that the underlying transport network
supports a large variety of connections and guarantees required
quality of service. A number of networking concepts has been tested,
but none of them has gained ultimate acceptance. ATM technology
has been the most favoured one, but other transporting solutions
have also been considered and entirely new ones suggested, especially,
for IP based communications.
Development of a new unifying transport concept is a long process,
and it can be envisaged that heterogeneous transport solutions
will be used for multimedia service delivery for a lengthy period
of transition. This implies that future network nodes, such as
switches, have to interface to different kinds of networks, manage
a number of parallel call control procedures and support switching
of dissimilar data units.
As a solution to the emerging problem, VTT Information Technology
and Helsinki University of Technology have launched SCOMS (Software
Configurable Multidiscipline Switch) project to develop a switching
and routing solution capable of supporting multiple switching
disciplines. The target is to develop a network node that integrates
circuit, cell and packet switching into a single fabric. The project
is funded by TEKES (Technology Development Centre of Finland)
and six industrial partners. The project was started early 1998
and finishes by the end of year 2000.
Experimental Switch
A conventional telecommunications switch connects to a homogeneous
transport network whereas a multi-discipline switch operates in
a heterogeneous networking environment, ie, networks that utilise
different transport technologies. Due to the different nature
of the associated transport technologies, the switching equipment
has to implement a set of different line interfaces and signalling
procedures and, additionally, to support a varying set of connection
types, quality of service measures and service types.
A straightforward way to implement such a switch is to logically
divide the switch into separate ‘virtual switches’. In this way,
line cards, protocol stacks and associated call control functions
of each virtual switch can work as if they were run in a homogeneous
network. At the bottom layer, a common control block is needed
to manage resources of the physical switch (eg switch configuration),
convert application level connection commands to physical connections
and perform physical level routing of the by-passing data units.
The concrete goal of the SCOMS project is to develop an experimental
switch integrating 64 kbit/s based circuit, ATM cell and IP packet
switching (see Fig. 1). The physical switching platform is based
on the Frame Synchronised Ring (FSR) concept, developed and patented
by VTT Information Technology. This platform has characteristics
necessary to implement the various fundamental features of a multidiscipline
switch. These include, eg, inherent support for real-time multicasting,
versatile addressing capabilities and support for simultaneous
switching of different size data units. FSR’s ring-shaped switching
bus is dimensioned to perform effectively by implementing separate
transport containers for circuit, cell and packet switched traffic.
Operation of the switching fabric is controlled by a workstation
which implements fabric and call control functions. Signalling
protocols of the three different transport networks (ATM, PDH
and IP) are implemented separately, although, they use services
of the common fabric control layer (see figure 2). Interworking
between the different networks can be supported by a shared interworking
call control module that is able to manage calls that originate
and terminate at different networks (see figure 3).
The switch control module offers network specific application
programming interfaces that implement functions necessary to reserve
resources and manage connections through the switch. The control
workstation is connected to the switch with an ATM/STM-1 link,
and all protocols receive and transmit their signalling messages
in reserved ATM virtual channels. Therefore, a non-ATM line interface
must map the incoming signalling messages to a specific ATM virtual
channel, assigned for transferring the signalling to the control
workstation. ATM/STM-1 was chosen for the workstation connection,
because it is a cost-effective way to have a high-speed signalling
link, which offers a straightforward way to manage various control
and signalling connections. An Ethernet interface is another possibility
to connect the workstation to the switch.
The above configuration can be enhanced by introducing an interworking
call model to route calls between the different networks (Fig.
3). In this configuration, the different protocol stacks are linked
together via a shared call control layer. The call control must
be able to decode, process and forward connection set-up requests
from each protocol stack. The call control interworking function
maps the connection set-up parameters between the different networks
and maintains status information of all the established connections.
Currently, the SCOMS switch and its control architecture is in
the implementation phase, and the first prototype, offering interworking
between ATM and PDH networks, is expected by the end of year 1999.
Further work will include interworking with IP networks, service
architecture considerations as well as development of configuration
and management applications. The inherent support for TCP/IP networking
enables interesting views, for example, to develop distributed
service control and web-based management.
Please contact:
Pertti Raatikainen - VTT Information Technology
Tel: +358 9 456 5644
E-mail: pertti.raatikainen@vtt.fi
