5 Automatically Switched
Optical Networks (ASON)
ITU-T has been active in the standardisation of
the Automatically Switched Optical Network
(ASON) [10], which is an optical transport net-
work with an independent control plane (Figure
4). ASON is in other words based on the overlay
network architecture and consists of three main
components: the control plane, the transport
plane and the network management plane.
ASON is a multi-client network that can offer
connection services to client networks (IP,
ATM, SDH, etc.). These can be set up following
a request via the network management system
(NMS) or following direct signalling exchange
between the client network and ASON. To that
end, work has been carried out in parallel by the
Optical Internetworking Forum (OIF) in order to
define and implement the User Network Inter-
face (UNI). All communication with client net-
works in an ASON is done via this interface.
This interface carries all signalling information
exchanged between ASON and its clients as well
as the actual signals transported by the network
(via the physical layer part of the interface). No
routing information is exchanged through the
UNI. The work on the UNI carried out by the
signalling group of OIF has recently been co-
ordinated with the IETF. A first version of the
optical UNI was used to demonstrate interoper-
ability testing for equipment from different ven-
dors at Supercomm 2001, as a result of the joint
efforts of 25 vendors within OIF.
Optical Connection Services
ASON provides end-to-end OCh connections to
its clients with a certain QoS, as agreed via ser-
vice level agreements (SLA) with the client.
These connections can be static, established via
the management system, or dynamic. The fol-
lowing three types of OCh services can be pro-
vided:
- Permanent OCh connection
This provides a permanent end-to-end connec-
tion at optical channel granularity. The service
is requested by the client via the NMS and
activated by the NMS. - Soft-permanent OCh connection
This provides an end-to-end OCh over a cer-
tain period of time. The service is requested
by the client via the NMS but the connection
can be established using signalling in the con-
trol plane. - Automatically switched OCh connection
This provides an end-to-end optical channel
connection activated by direct signalling from
the client network. Establishment and tear
down of this service are handled automatically
by the ASON control plane and the client is
notified accordingly. The NMS is periodically
updated.
The Control Plane
The control plane of ASON is what distinguish-
es it from a simple optical transport network
(OTN). ASON acquires in other words network
Switch
OCC
Network
Management
System
ASON
control
plane
Transport
plane
User
signaling
Clients
e.q. IP,
ATM,
TDM
NMI-T
Clients
e.q. IP,
ATM,
TDM
NMI-A
E-NNI
UNI I-NNI
OCC OCC OCC
Switch Switch
UNI
Figure 4 Automatically
Switched Optical Network
after ITU-T