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mented model appear timely enough. Also, solu-

tions based on the peer model may be imple-

mented right from the start by newly established

Internet Service Providers (ISPs) that also have

ownership of the physical infrastructure, or in

the cases where the amount of non-IP traffic is

relatively small.

4 Control System for Optical

Networks: Generalised

Multi-protocol Label

Switching (GMPLS)

Multi-protocol label switching (MPLS) was

launched only a couple of years ago but has now

become a fundamentally important technology

in the Internet. Several of the largest Internet ser-

vice providers employ MPLS in their networks,

virtual private network (VPN) services based on

MPLS are now available and the majority of

high-end routers now support MPLS [4]. MPLS

is an IP-centric protocol at the same time that it

is independent of the IP framework. It is a stan-

dardised solution to place the handling of traffic

as much as possible down to Layer 2, i.e. per-

form “switching” instead of “routing”. This cir-

cumvents the major of the shortcomings of IP as

it simplifies routing processes, provides efficient

and reliable handling of larger traffic volumes,

as well as enabling traffic engineering, faster

restoration, and easier QoS handling.

Packets are classified in flows (Forwarding

Equivalence Class, FEC) where the same routing

decision is applied. Label switching comprises

mainly the allocation of a stack of labels to each

packet (flow) where each label refers to a differ-

ent network layer within a hierarchical network.

The label can be “deciphered” to a forwarding

port by each router according to a frequently

updated routing table and a new label attached

giving forwarding directions to the next router.

Routing and signalling protocols are a part of the

process in order to discover the network topol-

ogy, place and respond to requests, reserve and

establish paths. MPLS is presented in detail in a

separate article in this edition of Telektronikk,so

that we can close this short introduction to it

here.

The main principle behind MPLS has a lot in

common with inherent features of wavelength

routed optical networks. Indeed, optical network

elements with interfaces that can recognise

wavelength, such as optical add-drop multiplex-

ers, de-multiplexers and cross-connects – can

direct the optical signal based on its wavelength

without the need for reading the content of the

signal. This is clearly a form of label switching

[5]. The optical label is in a different domain

(optical) than the signal itself (electrical). This

aspect makes wavelength the perfect label since

no overhead is added to the signal. The analogy

between MPLS networks and optical wave-

length-routed networks is shown in Table 1.

Responding to the clear mismatch between the

transmission capacity of the fibre and the pro-

cessing capacity of routers, the Internet Engi-

neering Task Force (IETF) has more recently

proposed and is developing an extension of

MPLS – Generalized MPLS (GMPLS) – to the

time and the optical domain. The aim is to

achieve a more flexible labelling and forwarding

mechanism that uses a generalised label, which

is applicable to a variety of technologies and net-

works. For IP routers the labels designate princi-

pally input and output ports. For an OXC they

designate input and output ports as well as wave-

length, or band of wavelengths. The hierarchy of

different labels in GMPLS is schematically

shown in Figure 3.

The main aims of GMPLS are to [6]:

i provide a framework for real time provision-

ing of optical channels;

ii adopt optical technology and encompass the

development and deployment of a new class

of programmable OXCs;

iii allow the use of uniform semantics for net-

work control in hybrid networks that consist

of both OXCs and label switching routers.

Using GMPLS, an end-to-end optical channel

can be established between two end-nodes by

choosing a) a physical path that connects these

MPLS Optical Wavelength Routed Network

Label Switched Path (LSP) Optical Channel (OCh)

Label Switching Router (LSR) Optical Cross-Connect (OXC)

Selection of Labels Selection of λ’s (possibly in combina-

tion with OXC ports)

Table 1 Analogy between
MPLS and Optical Wavelength
Routed Network

Time-

slot

Wave-

length

Figure 3 A schematic of the Band Fibre Bundle
hierarchy of labels in GMPLS