Side_1_360

3.1 Interconnecting IP and Optics

The optical networks must be survivable, flexi-

ble and controllable [ID_IPoptfw]. Introducing

more “intelligence” in the control plane for the

optical networks is a step in doing this. As IP

is seen as being a common protocol for much

of the traffic carried by the optical network, util-

ising similar mechanisms as found for the IP-

based networks is looked at with increasing

interest.

A first issue is the adaptation and reuse of IP

control plane protocols for the control plane in

optical networks. These are to be used no matter

which traffic flows (IP or non-IP) that are car-

ried. A second issue is how IP traffic can be

carried where joint control and co-ordination

between the IP and optical layer are utilised.

A schematic illustration is depicted in Figure 9.

An optical subnetwork may consist of all-Opti-

cal Cross-Connects (OXCs) or some nodes

where optical-electrical-optical conversion is

used. Two types of control interfaces are indi-

cated; User-Network Interface (UNI) between

the clients and the optical network, and Net-

work-Network Interface (NNI) between optical

subnetworks. The control flow across the UNI

would naturally depend on the services offered

to the client. As the NNI control flow would be

derived from IP control, similarities between

NNI and UNI may well exist when an IP net-

work is the client. The physical implementation

of the UNI may vary, such as:

• Direct interface with an in-band or out-of-
  band IP control channel. This channel is used
  for exchanging signalling and routing mes-
  sages between the router and the OXC (like
  a peering arrangement);


• Indirect interface with out-of-band IP control
  channel. The channel may be running between
  management systems or servers;


• Provisioned interface involving manual opera-
  tions.

Two service models are outlined in

[ID_IPoptfw]:

• Domain service model where the optical net-
  work primarily offers high bandwidth connec-
  tivity (services like light-path creation, dele-
  tion, modification and status enquiry);


• Unified service model where the IP and the
  optical network are treated together as an
  integrated network. Then the OXCs will be
  treated like any router as seen from the control
  plane. No distinction is then made between
  UNI, NNI and any other router-to-router inter-
  face. Such an interface is assumed to be
  MPLS-based.

It is important to make a separation between the

control plane and the data plane over the UNI.

As mentioned, the optical network basically pro-

vides services to clients in the form of transport

capacities (by light-paths). IP routers at the edge

of the optical network must establish such paths

before the communication at the IP layer can

start. Therefore, the IP data plan over optical

network is done over an underlying network of

optical paths. On the other hand, for the control

plane the IP routers and the OXCs can have

peering relations, in particular for routing infor-

mation exchanges. Various degrees of loose or

tight coupling between the IP and the optical

network may be used. The coupling is given by

the details of topology and routing information

exchanged, level of control that IP routers can

exercise on selecting specific paths, and policies

regarding dynamic provisioning of optical paths

between routers (including access control,

accounting and security).

Three interconnection models are then seen:

• Overlaymodel: The routing, topology distri-
  bution, signalling protocols are independent
  for the IP/MPLS and the optical network.


• Augmentedmodel: Routing instances in the IP
  layer and the optical network are separated by
  information exchanged (e.g. IP addresses are
  known to the optical routing protocols).


• Peermodel: The IP/MPLS layers act as peers
  to the optical network. Then a single routing
  protocol instance can be used for the IP/MPLS
  network and the optical network.

These models refer to a certain degree of imple-

mentation complexity; the overlay being the

least complex one for near-term deployment and

the peer model the most complex one. As each

of the models has its advantages, an evolution

path for IP over optical network may be seen.

The migration path described in [ID_IPoptfw] is

to start with the simpler functionality, meaning

the domain service model with overlay intercon-

Figure 9 Schematic
illustration of optical network
with client networks

optical

subnetwork

optical

subnetwork

optical

subnetwork

IP

network

other client

network

IP

network

other client

UNI = User-Network Interface network
NNI = Network-Network Interface

UNI

UNI

UNI

UNI

NNI

NNI

optical

network NNI