Side_1_360

nection and no routing exchange between the IP
and the optical network. A provisioned interface
would be expected. The next phase of the migra-
tion path is to exchange reachability information
between IP and the optical network. This may
allow for light-paths to be established in con-
junction with setting up LSPs. The third phase
of the migration is to support the peer model.

Applying a common signalling framework from
the start would assist the migration. For the
domain service model, implementation agree-
ment based on Generalised MPLS (GMPLS)
UNI signalling is being developed by the Optical
Interworking Forum. This is intended for near-
term deployment, although helping in the migra-
tion toward the peer model. This is said to sup-
port incremental development as the intercon-
nection model increases in complexity.

The GMPLS is described in [ID_GMPLS]. This
basically contains extensions to signalling for
MPLS, the need to include time-division, wave-
length and spatial switched/divided systems,
see illustration in Figure 10.

3.2 Multiplexing Hierarchy

As described in [Jens01], MPLS uses labels to
support forwarding of packets. Label Switching
Routers (LSRs) have a forwarding table recog-
nising the cells/frames with the labels, or the
IP packet headers (at the border of the MPLS
domain). This is extended in GMPLS where the
following interfaces are given for an LSR:

• Interfaces that recognise packet/cell/frame
  boundaries and forward the data based on the
  content in the packet or label/cell header.
  This is referred to as Packet-Switch Capable.
  Examples are MPLS-capable routers and
  ATM switches.


• Interfaces that forward data based on time
  slots in a periodic cycle. This is referred to as
  Time-Division Multiplex Capable. An exam-
  ple is an SDH cross-connect.


• Interfaces that forward data based on the
  wavelength. Such interfaces are referred to
  asLambda Switch Capable. An optical cross-
  connect is an example.


• Interfaces that forward data based on physical
  space position of data. This is referred to as
  Fibre-Switch Capable. An optical cross-con-
  nect operating on the level of single or multi-
  ple fibres is an example.

These can be organised in a hierarchical manner
as shown in Figure 11 and corresponding labels
and Label Switched Paths (LSPs) defined. Then
an LSP that starts and ends on a packet-switch

capable interface can be grouped together with

other similar LSPs into a common LSP that starts

and ends on a time-division multiplex interface.

This LSP can be grouped together with other

similar LSPs into an LSP that starts and ends on

a lambda switch capable interface, and so forth.

This is similar to a multiplexing hierarchy.

Compared to MPLS, the GMPLS introduces

additional interface types. The formats of the

labels on the interfaces are given in

[ID_GMPLS].

Motivations for combining solutions for MPLS,

in particular related to Traffic Engineering, and

mechanisms for control plane in OXCs are

described in [ID_MPLSoptte]:

Figure 10 Similarities

between MPLS and GMPLS

for a Label Switched Router

and an Optical Cross-Connect

Input Output

Interface

4

2

Label

23

11

Interface

3

2

Label

12

17

read

label

Forwarding

matrix

read

label

write

label

write

label

MPLS

interfacesInput control interfacesOutput

Label Switched Router

Input Output

Interface

4

2

λ

23

11

Interface

3

2

λ

12

17

λ Rx

Interconnect

matrix

λ Rx

λ Tx

λ Tx

GMPLS

interfacesInput control interfacesOutput

Optical Cross-Connect