Side_1_360

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