sdh layered architecture
TRANSCRIPT
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ABSTRACT
This paper discusses the evolution path for core and
metropolitan networks taking into account the cur-
rent economic recovery as well as the changing tele-
communications environment. At the beginning of
the paper, the current status of core and metropoli-
tan networks is presented, including a brief presen-
tation of such networking technologies as: MPLS,
Ethernet, Resilient Packet Ring (RPR), SDH/
SONET, and OTN. Then, the evolution scenarios
are provided in three stages: short, medium and long
term. The following factors are taken into account
and referred to at each step of the evolution scenario:
available services, quality of service/traffic
engineering, connection provisioning/connection
set-up control methods, network resilience and other
functions. The short term scenario involves the in-
Planning of Optical Transport
Networks Layered Architecture
A. Jajszczyk A. Lason, J. Rzasa, R. Stankiewicz
AGH University of Science and Technology, Department of Telecommunications,
Al. Mickiewicza 30, 30-059 Krakow, Poland
e-mail: {jajszczyk, lason, rzasa, rstankie}@kt.agh.edu.pl
M. Jaeger
T-Systems, Goslarer Ufer 35, 10589 Berlin, Germany
e-mail: [email protected]
S. Spadaro, J. Sole-Pareta
Universitat Politecnica de Catalunya (UPC), C/ Jordi Girona 1-3, 08034 Barcelona, Spain
e-mail: [email protected], [email protected]
troduction of reconfigurable WDM networks, while
in the medium term the Generic Framing Procedure
(GFP), enhanced SDH/SONET technologies, and
Optical Transport Network (OTN) will be adopted.
The long term scenario deals with the addition of a
control plane, either ASON or GMPLS based.
Key words: OTN, SDH/SONET, ethernet, RPR,
IP, network planning, ASON/GMPLS, GFP, LCAS,
DWDM/CWDM
INTRODUCTION
The late 90s were boom times in the telecommuni-
cations industry. Thousands of kilometers of fiber
were installed, many transport systems implemented,
dozens of new companies, both operators and vendors,
started their fight for profit. However, the subsequent
slowdown in the world economy has brought some
FEATURE ARTICLESFEATURE ARTICLESFEATURE ARTICLESFEATURE ARTICLESFEATURE ARTICLES
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companies at the edge of catastrophe. Recently, the
mood seems to start not to be so pessimistic. However,
the question how to develop the most effective and
cost efficient telecommunications infrastructure is of
utmost importance. At the beginning of the paper, wepresent the architecture of current core and metro-
politan networks and point out their main drawbacks.
Next, we propose an evolution path which meets the
increasing requirements for performance, function-
ality and cost efficiency, and is applicable to both core
and metropolitan network operators. The paper shows
a feasible way to solve difficult task of building an
evolution path for transport networks. The path pro-
posed here, presents one of possible ways which the
core and metropolitan networks may follow.Certainly, depending on the type of networks, busi-
ness model, technical constraints, etc., a different evo-
lution path can be drawn. For example, some tech-
nologies may be implemented faster by a newcomer
than by an incumbent core operator while metropoli-
tan carriers can omit such technology at all. However,
our evolution path seems to be representative for cur-
rent core and metropolitan networks. During the pro-
cess of building the evolution path, a wide range of
issues has to be taken into account, ranging fromtechnical, economic, organizational to social ones.
Nevertheless, our paper is mainly focused on techni-
cal and, in some parts, economic issues.
The following factors are taken into account at
each step of the evolution scenario:
services, e.g.,Bandwidth on Demand Service,
Provisioned Bandwidth Service and Optical Vir
tual Private Network,
quality of service/traffic engineering,
connection provisioning/connection set-up con
trol methods (permanent connections (PC),
soft-permanent connections (SPC), switched
connections (SC)),
resilience functions, such as protection and
restoration,
control plane functions, such as routing and
signaling,
drivers behind each step, etc.,
All these factors will allow us to evaluate current
as well as future steps of the evolution path in a way
which permits network operators to offer services to
customers. Provisioned Bandwidth Service (PBS)
denotes here static near-real-time provisioning through
management interfaces via a network management
system (NMS) or an operations support system (OSS)
with a client-server relationship between clients and
the optical network. In contrast,Bandwidth on De-
mand Service (BDS) denotes dynamic and real-time
provisioning in seconds or sub-seconds with signaled
connection requests via a User to Network Interface
(UNI). Optical Virtual Private Network(OVPN)
specifies a set of provided network resources, e.g.,
link bandwidth, wavelength, and/or optical connec-
tion ports that may be used. For clients belonging toan OVPN, a Closed User Group (CUG) and a virtual
network are defined, where optical connections may
be based on static or dynamic (signaled) provisioning.
The resource visibility and its control vary depend-
ing on the service contract.
The organization of the remainder of this paper is
as follows. We start with a description of the current
status of transport and metropolitan networks. Then,
an evolution scenario for a short term time scale is
provided. Subsequently, medium and long term sce-narios are presented.
Throughout the article, the authors refer to the stan-
dardization process which is carried out by theIn-
ternational Telecommunication Union Telecommu-
nication Standardization Sector(ITU-T), theInternet
Engineering Task Force (IETF) as well as theInsti-
tute of Electrical and Electronics Engineers (IEEE).
Moreover, the authors refer to other bodies, e.g., the
Optical Internetworking Forum (OIF), theMetro
Ethernet Forum (MEF) and theResilient Packet RingAlliance (RPRA).
CURRENT STATUS
OF TRANSPORT NETWORKS
We start this chapter with an overview of main net-
working technologies used in modern transport
networks. Next, we present the layered structure of
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transport networks and outline their crucial
characteristics.
1. Wavelength Division Multiplexing
Todays transport networks widely use the wave-
length division multiplexing(WDM), i.e., circuit
switching technology and, in some cases, single-
wavelength optical fibers, referred to as Tradi-
tional Fiber Optics (TFO). Wavelength division
multiplexing is deployed for point-to-point com-
munications with manually configured links. The
spectacular increase of the capacity aggregated
by the fiber optics removed the bandwidth bottle-
neck in the core, regional and metropolitan
networks. WDM systems already installed by
some network operators offer up to 40 Gb/s data
rate per optical carrier and 160 carriers per fiber.
The ITU-T recommendations specify a broad
range of aspects related to optical networks
including, e.g., the physical layer and WDM.
Recommendations on the fiber optics physical
layer are already in place (G.65x series of
Recommendations), coarse WDM (CWDM)
wavelength and dense WDM (DWDM) fre-
quency grids are available as well (G.694.2 andG.694.1, respectively) [1], [2]. The recent advances
in optical layer technology enable the architec-
ture optimization of telecommunication transport
networks. Currently, the main effort of equip-
ment vendors and network operators concerns the
architectural aspects of the optical layer. The in-
troduction of optical cross-connects OXC and op-
tical add-drop multiplexers (OADM) are ex-
pected to lead to major cost reductions in over-
all networking due to reduced electronic signal
processing and limited use of expensive opto-
electronic conversion.
Besides WDM, single-wavelength fiber optics
systems are in use as well, however, usually at much
shorter distances (less than 50 km). Hence, the TFO
is typically applicable to some metropolitan
networks. Nevertheless, it is broadly believed that
the number of traditional fiber optics systems in
transport networks will decrease.
2. SDH/SONET and Virtual Concatenation
On top of WDM or TFO, the Synchronous Digital
Hierarchy (SDH) or Synchronous Optical Network
(SONET) is extensively used. The SDH/SONETis a circuit switching technology and is applicable
to both metropolitan and core networks. It is well-
understood, mature and standardized [3], [4], [5]. Since
it was initially designed to optimize transport of
64-kb/s-based TDM services, a rigid capacity of
payload as well as a coarse fixed-rate multiplexing
hierarchy was defined. Today, SDH/SONET sys-
tems are built with bit rates as high as 10 Gb/s
(STM-64/OC-192), with 40 Gb/s (STM-256/OC-
768) on the horizon. Current SDH/SONET core
networks have a switching granularity of VC-4/
STS-3. A majority of all client networks are set up
on top of SDH/SONET.
By the use of Virtual Concatenation (VC)
procedure, SDH/SONET may be improved to better
meet todays requirements, e.g., various switching
granularities. Virtual Concatenation [3] allows flex-
ible concatenation of several SDH/SONET payloads.
It assures an effective use of SDH/SONET capacity.
Virtually concatenated payloads constitute a Virtual
Concatenation Group (VCG). Members of a VCG,
as opposed to contiguous concatenation, may not
reside in the same STM-N/OC-Ncontiguously. They
may even reside at different STM-N/OC-Ninterfaces
and are treated within the network separately and
independently. As a consequence, members of a
VCG may reach the destination through various
routes. Intermediate nodes do not need to handle
virtual concatenation. The VC functionality must be
implemented only at path termination nodes. This
feature makes it possible to deploy virtual concat-
enation on legacy SDH/SONET equipment of ex-
isting networks, thus to smooth transition to en-
hanced networks. On the other hand, it should be
noted that differences in the delay of an individual
concatenated signal may occur due to pointer pro-
cessing at intermediate nodes. Compensation of dif-
ferential delays is handled at the destination node.
Another advantage of virtual concatenation is its
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ability to divide STM-N/OC-Nbandwidth into sev-
eral subrates. Each of the subrates may be used for
accommodation of a different service. The band-
width of STM-N/OC-Nmay be shared, for example,
by both telephone service and data signals. An of-
ten-mentioned example [6] of a practical use of vir-
tual concatenation is Gigabit Ethernet. VC-4-16c
(STM-16) is required to accommodate Gigabit
Ethernet signals at full speed under conventional
SDH. However, the capacity of 1.4 Gb/s is then
wasted. On the other hand, contiguous concatena-
tion of four VC-4 containers (VC-4-4c) provides too
small capacity to fully accommodate Gigabit
Ethernet signals. The best solution would be con-
catenation of seven VC-4 payloads. It is possiblewith virtual concatenation. Bandwidth of 1.05 Gb/s
provided by a VC-4-7v VCG is suitable for Gigabit
Ethernet. More examples of bandwidth efficiency
in carrying Ethernet, Fast Ethernet and Gigabit
Ethernet data signals in SDH with and without VC
are shown in Table 1.
3. Ethernet
Ethernet networks are important clients of the trans-
port layer. We use the termEthernetin the mean-
ing of traditionalEthernet, Fast Ethernet, Gigabit
Ethernetas well as 10 Gigabit Ethernet[7], [8]. The
Ethernet technology is well understood and robust,
its applicability to local computer networks cannot
be questioned. Since years, 10 and 100 Mb/s
Ethernets have been used for building cost effective,
high speed data networks. In recent years, Gigabit
Ethernet widely found its way into the metropolitan,
regional and even wide area networks. 10 Gigabit
Ethernet continues the evolution towards higher bit
rates and an extended range, although, data are
transferred by fiber links only. In this case, two
types of physical interfaces were defined, the first
one is suitable for local and metro area networks
operation (LAN PHY: 10GBase-X, 10GBase-R),
the second for wide area networks (WAN PHY:
10GBase-W). The 10 Gigabit Ethernet standard
proposes physical interfaces based both on single-
and multi-mode fibers. 10 Gigabit Ethernet LAN
PHY offers an extended reach compared to Giga-
bit Ethernet, i.e., over a 40 km long single-mode
fiber link. WAN PHY differs from the LAN PHY
implementation by the use of the SDH/SONET
framing with reduced functionality. The framing
for WAN interfaces takes place at the WAN Inter-
face Sublayer(WIS). The output from the WANPHY is compatible with the synchronous frame
format (VC-4-64c or STS-192c) and can be easily
transported over an Optical Transport Network
(OTN). The output from the LAN PHY interface
of 10 Gigabit Ethernet has to be adapted before
entering the OTN. The newly proposed Generic
Framing Procedure format promises to provide this
function. The Ethernet technology was also pro-
posed as the base for new high speed access
networks. TheEthernet in the First Mile working
group, 802.3ah, was formed within the IEEE 802.
3 CSMA/CD working group. The scope of the work
is the adaptation of the Ethernet technology to
point-to-point and point-to-multipoint (E-PON)
access networks [9]. A successful standardization
process will extend the Ethernet coverage so that
end-to-end Ethernet services can be offered to both
business and residential customers. Future improve-
ment of the quality of service functions offered in
Ethernet networks can be achieved through the use
of 802.1D (Class of Services) and 802.1Q (Virtual
payload mapping bandwidth efficiency payload mapping bandwidth efficiencyEthernet (10 Mb/s) VC-3 21% VC-11-7v 89%
Fast Ethernet (100 Mb/s) VC-4 67% VC-11-64v 98%
Gigabit Ethernet (1 Gb/s) VC-4-16c 42% VC-4-7v 95%
Data signalSDH without VC SDH with VC
Table 1 Bandwidth efficiency of virtual concatenation
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LAN) specifications. Unlike SDH/SONET, the
Ethernet technology does not provide any fast pro-
tection mechanism. Ethernet generally relies on the
spanning tree protocol to eliminate all loops from
a switched network. Even though the spanning tree
protocol can be used to achieve path redundancy,
it recovers comparatively slowly from a fiber cut,
as the recovery mechanism requires the failure con-
dition to be propagated serially to each upstream
node. IEEE 802.1DRapid Spanning Tree Protocol
(RSTP) improves resiliency of the Ethernet [10].
However, SDH/SONET-like services still cannot
be guaranteed, hence, the Ethernet suffers from in-
ability to provide carrier class services. Although,
some early works have been done by the MetroEthernet Forum in its specification [11], it seems that
it is still too early to fully introduce Ethernet based
carrier-class services in the metro. Metro Service
Model Phase 1 proposes service building blocks or
service attributes and specifies how to build an
Ethernet service. Such services, described as
Ethernet Line, i.e., point-to-point services and
Ethernet LAN, i.e., multipoint-to-multipoint
service, may be offered over fiber, SDH/SONET
or WDM technology.
4. Resilient Packet Ring
Resilient Packet Ring is a new technology for ring-
based metropolitan area networks that enables an
efficient transfer of data traffic as well as fast pro-
tection mechanisms. RPR technology, which was
standardized as IEEE 802.17 RPR, is based on two
symmetric counter-rotating rings that carry data and
control information [12]. Additionally, the ring to-
pology based on RPR is also studied by ITU-T.
Specifically, ITU-T Recommendation X.87 speci-
fiesMultiple Services Ring(MSR) based on RPR
and a way of multi-service provision over RPR[13].
RPR is designed to operate over a variety of physi-
cal layers, including SDH/SONET, Gigabit
Ethernet, DWDM and dark fiber, and is expected
to work over higher-speed physical layers. Some
RPR technology features (distributed control,
scalability in speed and number of nodes, plug-and-
play operation, support various classes of traffic,
advanced protection mechanism, etc.) triggered
many pre-standard installations by some players in
the telecommunications market (e.g., Sprint,
Luminous, Bell Canada, MCI and SUNET). Thefirst major pre-IEEE 802.17 RPR standard deploy-
ments were Dynamic Packet Transport (Cisco Sys-
tems proprietary solution) networks introduced by
Sprint in 1999 and Macedonia Telecom and China
Telecom in 2001.
RPR technology implements the spatial reuse,
which increases the overall aggregate bandwidth
of the ring. Unicast frames are removed from the
ring at their destination, which means that they
occupy bandwidth on the links from source to des-tination only. RPR networks support three class of
traffic. Specifically, IEEE 802.17 RPR supports
three types of services, namely Class A, Class B
and Class C. The Class A service is designed to
support real-time applications that require a guar-
anteed bandwidth and low jitter while the Class B
service is dedicated to near real-time applications
that are less delay-sensitive but that still require
some bandwidth guarantees. Finally, The Class C
service implements the best-effort traffic class. This
service is subject to weighted fairness mechanisms,
which ensure that each station gets its fair share of
the bandwidth available.
Two protection mechanisms may be used: steering
and wrapping, both of which provide fast protection
switching comparable with that of SDH/SONET
networks. Neither of these mechanisms requires dedi-
cated protection resources. RPR protection mecha-
nisms have been designed and optimized to maintain
the network connectivity and to minimize the packet
losses in case of fiber cuts or node failures.
RPR seems to be a promising technology, since
most of the major carriers have actively participated
in the standardization process and have shown much
interest in the evolution of the standard. RPR sys-
tems are seen by many carriers as the inevitable suc-
cessors to SDH/SONET ADM-based rings. Indeed,
RPR network may provide performance-monitoring
features similar to those of SDH and, at the same
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time, maintain Ethernets advantages (e.g., low
equipment cost, high bandwidth granularity and sta-
tistical multiplexing capability). We can note that
inability to operate over multiple rings may impede
implementation of RPR in some areas.
5. Multiprotocol Label Switching
Multiprotocol Label Switching(MPLS) is a connec-
tion oriented packet switching technique providing
mechanisms for engineering network traffic patterns
independently of routing tables. MPLS assigns short,
fixed-length (20-bit) labels to network packets that
describe how to forward them through the network.
In an MPLS environment, the analysis of the packet
header is performed just once, when the packet en-
ters the MPLS domain. Label forwarding tables in
routers store information on where to forward the
packets. Additional information can be assigned with
a label, such as class-of-service (CoS) values that
can be used to prioritize packet forwarding. Usage
of MPLS is not limited to IP networks. It may peer
with ATM or Frame Relay networks. Appropriate
standards were defined by IETF [14], [15], [16]. Label
switched path may be tunneled (extended) in such
networks. This functionality extends capabilities ofIP services. Currently, the two main roles of MPLS
are traffic engineering and Virtual Private Network
support. MPLS provides functional traffic engineer-
ing capabilities required to implement policies that
facilitate efficient and reliable network operations
in an MPLS domain. MPLS decouples the routing
and forwarding functionality. Finding an optimal
routing scenario in presence of constraints imposed
by limited capacity of connections and network to-
pology is facilitated. These capabilities can be used
to optimize the utilization of network resources and
to enhance traff ic oriented performance
characteristics. MPLS TE (MPLS Traffic
Engineering) provides capabilities for traffic
tunneling, load balancing and explicit routing.
Moreover, it eliminates the need for manual setting
up of explicit routes. TE functionality encompasses
also resilience issues. MPLS provides fast protec-
tion and restoration mechanisms. The network re-
covers dynamically from a failure by adapting its
topology to a new set of constraints. MPLS VPNs
do not need a predefined logical or virtual channel
provisioned between two endpoints to establish a
connection between the two endpoints. Traffic of
various users is treated separately within the MPLS
network without the need for encryption or tunnel-
ing at lower layers. MPLS VPNs are scalable (as
opposed to connection oriented Frame Relay (FR)
or ATM VPNs requiring hundreds of virtual chan-
nels for each closed group of users). Moreover,
MPLS provides a capability for consolidation of data,
voice and video services. Each VPN may use its own
independent addressing plan. An incumbent opera-
tor does not need to change its addressing plan whiledeploying an MPLS VPN. MPLS also facilitate
Quality of Service (QoS) assurance but it must be
remembered that putting it on a par with QoS archi-
tectures such as IntServ and DiffServ is a miscon-
ception [17]. Its role is different. IntServ and DiffServ
network models are not dependent on OSI/ISO layer
2 technologies and define a general QoS architec-
ture for IP networks, which can integrate different
transmission technologies in one IP network. MPLS
is another networking technique, like ATM andFrame Relay, defined in layers 2 and 3. MPLS was
originally intended to simplify packet forwarding in
routers rather than to address service quality. Some
features of MPLS can facilitate the QoS assurance.
It can extend IntServ and DiffServ capabilities to a
wider range of platforms beyond the IP environment.
It facilitates offering IP QoS services via FR or ATM
networks. Other MPLS features, such as capabili-
ties for load balancing, flow control, explicit rout-
ing and tunneling are also important from the QoSperspective [17].
6. The architecture of current core and metro-
politan networks
The circuit switched voice traffic was tradition-
ally a major part of the traffic in core and metro-
politan networks. Recently, however, besides the
voice traffic, leased lines service has become an
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important source of profits for networks operators.
Additionally, it usually consumes a large part of
the networks capacity. As the data, which may
be identified with IP traffic, proceeds from
narrowband towards broadband connections, it
starts to play the dominant role in transport
networks. Usually, IP routers are simply connected
by SDH/SONET links with STM-16/OC-48 or
STM-64/OC-192 interfaces. If protection is needed,
the connections are transported over SDH/SONET.
Otherwise, if it is sufficient to provide resilience
purely at the IP layer, the IP router connections are
directly mapped into static WDM wavelength-
based connections. The MPLS technology, which
additionally improves the functionality of the IPlayer, is installed in IP networks today. Resilience
functions are possible at the MPLS layer and may
be implemented on a per service basis in future
networks.Broadband Leased Lines Services are
based on SDH/SONET as well as on WDM
technologies. At the same time, the position of dark
fiber services is continuously decreasing. The trans-
port of voice traffic is mainly performed by SDH/
SONET networks, however, the role of packet
switching technologies, mainly the IP protocol, is
growing. It can be noted that voice traffic may be
conveyed by IP protocol over SDH/SONET tech-
nology or by IP over MPLS over SDH/SONET or
by MPLS without usage of the IP protocol. A way
in which voice payload may be directly encapsu-
lated is defined in [18]. In this paper we use the term
voice over IP to indicate that voice is transported
over packet switching technologies. The protocol
stack for the current transport networks is shown
in Figure 1.
Services
Currently, the core and metropolitan networks pro-
vide Provisioned Bandwidth Service at the IP and
the SDH/SONET layer. Additionally, it is possible
to offer VPN services at the IP, WDM and SDH/
SONET layers. At both, the SDH/SONET and WDM
layers, service provisioning may be very time
consuming, particularly at the WDM layer, where
provisioning of a service is a mostly manual process.
QoS/TE
Referring to the QoS and traffic engineering (TE)
features the situation is nearly the same in the coreand metropolitan networks. At the IP layer, MPLS
supports traffic engineering, but quality of service
parameters are still insufficient for a majority of ser-
vice providers. QoS at the SDH/SONET usually
meets expectations of users.
Connection provisioning
It can be noted that at the IP layer switched connec-
tions may be provisioned while the SDH/SONET
technology allows only permanent and soft-perma-
nent connections. At the WDM layer, permanentmanually configured connections are feasible only.
Hence, connection provisioning may be very
laborious, time consuming and expensive. It seems
that such a functionality, in most cases, is sufficient
for operators of core networks. In contrast, in the
metropolitan networks there is a growing demand
for fast connection provisioning.
Resilience
Protection relies on pre-provisioned backup
resources, whereas restoration, in principle, assigns
backup resources only after the occurrence of a
failure. Currently, both protection and restoration are
possible in the core and metropolitan networks at
the MPLS/IP level, while only protection mecha-
nisms are provided at the SDH/SONET layer.
Drivers
Several factors play a significant role in the evolu-
tion of current transport networks. The growing vol-
ume of data traffic to be transported over networFig.1 Current transport networks - protocol stack
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ks is commonly referred to as an important driver
impacting the transformation of the network
architecture. However, especially in the developed
world, dozens national and international fiber
backbones have been installed. Hence, besides
some developing countries, there is no need for
new fiber links crossing continents. The situation
in the metropolitan areas is similar, even though
it seems that there is still some room for new
installations. Therefore, especially in the short
term perspective, drivers different from those
purely increasing demand for bandwidth are ex-
pected to dominate. Taking into account the world-
wide economic slowdown, which we experienced
in last years, the huge investments done recentlyby telecommunications operators and the strong
competition on the telecommunications services
market, it is obvious that cost reduction will be
the predominant design constraint of the future
transport networks. Spending of telecommunica-
tion operators can be reduced by limiting the nec-
essary capital expenditures (CAPEX) on one side,
and by optimizing the network operational costs
(OPEX) on the other. Possible savings in opera-
tional expenditures together with enhanced net-
work flexibility will be critical for the commer-
cial success of network operators.
SHORT TERM
SCENARIO - ECONFIGURABLE WDM
The first step on the evolution path to cost reduc-
tion and increased network flexibility is the in-
troduction of integrated and reconfigurable WDM
systems (denoted here as rWDM). This step
complements the need for increased bandwidth
and the need for cost reduction at the lowest opti-
cal layer of the transport network. So far, at the
WDM layer there are mostly static cross-connect-
ing elements. It is not possible to allow in-service
selection of the optical channel to be switched,
added or dropped by the use of software control.
Instead of early deployed point-to-point WDM
systems, future systems will deploy a wavelength-
routed network. It may be accomplished by the
use of flexible or reconfigurable optical add/drop
multiplexers, optical cross-connects, as well as
tunable lasers and receivers. Hence, Leased Line
Service and Ethernet will be increasingly basedon the optical layer. Improvement of flexibility
seems to be particularly important in metropoli-
tan networks, whereas in the core, usually
underutilized links do not have to be equipped
with reconfigurable elements.
In the metro environment, some (mainly
newcomer) operators will deploy Gigabit Ethernet
and RPR technology-based networks. Furthermore,
single-wavelength fiber optics systems will play a
minor role in the transport networks. The layeredarchitecture for the short term scenario with
reconfigurable WDM is shown in Figure 2.
Services
At this evolution step, the IP and SDH/SONET
layer service provisioning is still time consuming
and static in both metropolitan and core networks.
However, the introduction of reconfigurable ele-
ments at the WDM layer allows network operators
to offer PBS at the optical level. In contrast to the
current situation, service provisioning at the opti-
cal layer may be performed faster. In addition to
permanent and soft-permanent connections, which
are feasible at the IP and SDH/SONET layers, in
WDM networks with reconfigurable elements it
will be possible to provision connections via the
management plane. Additionally, in the metropoli-
Fig.2 Network Evolution - reconfigurability at
the WDM layer
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tan networks flexibility of services may be im-
proved by the increased number of Ethernet and
RPR installations.
QoS/TE
Traffic engineering aspects do not change at this
stage. Similarly, the QoS remains the same in the
core networks. However, in the metropolitan area
quality of service may be slightly augmented by
implementation of RSTP and RPR.
Connection Provisioning
At this stage, rWDM will boost the connection
provisioning in the core and metropolitan
networks. Additionally, wider, than in the previ-
ous phase, implementation of Ethernet and RPR
will increase the capability to deliver connections/services to customers.
Resilience
At the IP and SDH/SONET layers, resilience re-
mains the same as in the previous scenario, i.e.,
IP uses its rerouting capability in failure cases and
SDH/SONET offers pre-provisioned protection
options. At the reconfigurable WDM layer, at this
stage, it will be possible to perform pre-provi-
sioned protection, i.e., an rWDM device may de-
tect a Loss of Signal (LOS) and automatically
switch traffic from a faulty to a pre-provisioned
working link. However, proper procedures to co-
ordinate protection/restoration mechanisms at the
electrical and the optical layers to provide a sur-
vivable network with QoS support and race con-
ditions avoidance mechanisms have to be
implemented. Similarly as for core networks, re-
silience in some metropolitan networks will be
affected by the introduction of reconfigurability
at the WDM layer. In other networks, the imple-
mentation of RPR may help network operators to
ensure efficient protection at the required level.
Drivers
Strong competition on the market and continuously
decreasing profit margins will force telecommuni-
cation operators to find new customers and to of-
fer new services. This cannot be done using the
business model based on the cost reduction only.
We believe that the offer of new services is the key
for success. The development of applications as-
suring fast and reliable access to remote resources
- data storage applications, network-wide compu-
tation services, virtual reality - will affect network
architectures as well. This can be translated into
technical requirements as a need for flexible and
standard framing methods for a wide range of cli-
ent signals, starting fromFiber ChannelorEnter-
prise Systems Connection (ESCON) formats to
Ethernet or IP protocols. However, at this stage,
there is still a problem with adapting the SDH/
SONET layer for transporting data traffic with ei-
ther block-coded data streams such asFiber Chan-
nelor Fiber Connection (FICON) or packet-ori-
ented data streams, such as IP/PPP or Ethernet.Moreover, the legacy infrastructure in both core and
metropolitan networks does not have the ability to
adjust already established connections to changing
conditions in the network. The equipment and/or
control software are needed to allow a network
operator fast adaptation to needs of a customer. This
is particularly true for metropolitan networks.
Hence, the TDM infrastructure, i.e., the voice ori-
ented technology, has to be adapted to the data cen-
tric environment with proper flexibility and adapt-ability for the changing requirements. The need for
cost effective solutions is still essential for network
operators. The cost reduction may be achieved by
transition of data transport and switching from the
electrical to optical domain. Therefore, the core as
well as metropolitan networks should be continu-
ously transformed towards the optical domain.
MEDIUM TERM SCENARIOS
1. Implementation of Generic Framing Procedure
In this phase, the Generic Framing Procedure (GFP)
will be implemented. The Generic Framing Proce-
dure defines a very effective way of mapping a wide
variety of data signals into transport networks [19]. It
adapts traffic from higher-layer client signals over
SDH/SONET, OTN or dark fiber into a common
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format. The ITU-T recommendation defines two
transport modes. The first mode, referred to as
Frame-Mapped GFP(GFP-F), is optimized for the
adaptation of PDU-oriented streams such as IP, na-
tive PPP, MPLS or Ethernet traffic. The second
mode, optimized for block-code-oriented streams,
is called Transparent GFP(GFP-T). This mode is
used for Gigabit Ethernet, Fiber Channel, FICON
and ESCON traffic. Both transport modes may co-
exist within the same transport channel. GFP ad-
dresses requirements of delay-sensitive applications
such as storage area network (SAN). It is also ex-
pected to support the new IEEE 802.17 RPR
standard. Another advantage of GFP is its particular
suitability to high-speed transmission links stemmingfrom reduction of processing requirements for data
link mappers/demappers as well as simplification of
receiver logic [20]. At this stage, it seems that the cen-
ter of gravity will shift towards services offered
through GFP over WDM rather than SDH/SONET
over WDM. Functionally, GFP consists of common
and client-specific aspects. The former apply to all
traffic. It encompasses data link synchronization and
scrambling, PDU delineation, PDU multiplexing and
client-independent performance monitoring. The
client-specific aspects include mapping of particu-
lar client PDUs into the GFP frame, client-specific
performance monitoring and OA&M functionality.
Interrelation between GFP-F, GFP-T, the client-spe-
cific and common aspects as well as GFP relation-
ship to client signals is shown in Figure 3.
Examples of client payloads that can be mapped
on SDH/SONET via GFP are as follows [21]:
Fiber Channel (850/1062.5 Mb/s);
VC-4-6v/STS-3c-6v (900 Mb/s);
Gigabit Ethernet (1000/1250 Mb/s);
VC-4-7v/ STS-3c-7v (1050 Mb/s); FICON (850/1062.5 Mb/s);
VC-4-6v/ STS-3c-6v (900 Mb/s).
At the same time more efficient use of avail-
able network resources will be achieved. For many
metropolitan areas it seems that there is still some
room for resource usage optimization on the per
day basis. Together with the switching capability,
capacity of links used by business customers dur-
ing the day can be re-used for residential users in
the evening. Such a resource usage optimizationat the medium time scale can be achieved at this
stage by the use of theLink Capacity Adjustment
Scheme (LCAS) protocol with support of agile
management systems. LCAS [22] is an extension
to Virtual Concatenation. It allows the dynamic
alteration of bandwidth of SDH/SONET transport
pipes. This is a key functionality for the transport
of data-traffic coming from IP-applications while
saving bandwidth. The number of concatenated
payloads may be increased or decreased at any
time without affecting traffic currently being sent.
Moreover, LCAS will automatically decrease the
capacity if a member of a VCG experiences a fail-
ure in the network, and LCAS will increase the
capacity when the network recovers. When one
of the constituent channels experiences a failure,
the failed channel will be automatically removed
while the remaining channels are still working.
Thus, the available bandwidth will be lowered but
the connection will be maintained. It can be noted
that such a solution provides a lower probability
of a complete connection failure in the system.
The synchronization between endpoints during the
addition or deletion of channels to a VCG is done
via signaling. Similarly, single-wavelength fiber
optics systems will be less used due to still in-
creasing traffic. Their use will be mostly limited
to access and metro areas. The development of
new telecommunication services will also impose
Fig.3 GFP mapping relationships
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more stringent requirements on methods respon-
sible for providing and controlling services with
guaranteed quality. Presumably, the growing
amount of voice traffic and data traffic with strin-
gent requirements will be conveyed by the IP/
MPLS and Differentiated Services (DiffServ)
networks, which will be introduced at this stage.
Differentiated Services architecture is a solution
for providing different levels of service quality[23]. Independent flows choose one of the limited
number of predefined services. Flows (packets)
that choose the same service are aggregated and
receive the same level of QoS. Aggregated packet
processing by a network node is calledPer Hop
Behavior(PHB). Currently, the DiffServ archi-tecture defines expedited forwarding (EF) [24] and
assured forwarding (AF) PHBs [23] beyond the best-
effort service. Traffic entering a network is clas-
sified and conditioned at the boundaries of the
network. Active queue management mechanisms
within a DiffServ domain are responsible for in-
telligent dropping of packets not conforming to a
contract between a customer and an operator.
In metro environments, it seems that the
Ethernet as well as RPR standard systems will be
widely deployed at this stage. Dynamic develop-
ment of the Ethernet networks will probably es-
sentially impact services offered in packet
switched networks as well. In the medium term
perspective, high speed, widely used and matured
IP networks with MPLS support will be used for
circuit emulation and for transparent transport of
ATM, FR, Ethernet or even SDH data units. Such
a network architecture - the architecture enabling
transfer of layer two data units (e.g., Ethernet) over
layer three (IP or IP/MPLS) may be very interest-
ing for low cost and efficient interconnection of
different network domains in highly competitive
metropolitan environment. The IETF has already
published first RFC standard on architecture of
Pseudo Wire Emulation Edge-to-Edge (PWE3)
services [25]. Next documents are expected to deal
with mapping procedures for encapsulation of
specific technologies, set-up and maintenance of
the tunnel for data encapsulation, traffic policing,
data fragmentation, connection verification and
others. Internet drafts on the enumerated issues
are already available at the web site of PWE3
working group [26]. Taking into account introduc-
tion of PWE3 services, layered architecture of
transport network for the medium term scenario
is shown in Figure 4.
Services
Under this scenario, at both core and metropolitan
networks, at the IP level, Provisioned Bandwidth
Service and Bandwidth on Demand Service may be
offered while in rWDM networks still the former
one only. Due to the introduction of LCAS it is pos-
sible to dynamically increase or decrease the band-
width of a connection at the SDH/SONET layer.
Hence, the SDH/SONET better meets user
requirements. Non-broadband connections such as
STM-1/OC-3, up to now realized by using the SDH/
SONET technology, will be provided by the IP/
MPLS protocol as well. At the optical layer, only
high bandwidth connections may be offered.
Moreover, it seems that at this stage pseudo wire
emulation service will be implemented.
QoS/TE
In the core networks, the QoS remains the same as
in the previous scenario while quality in the metro-
Fig.4 Transport Network Evolution - implemen-
tation of GFP
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politan networks may be enhanced by DiffServ. Traf-
fic engineering, however, may be improved by
implementation of LCAS in both core and metro-
politan networks.
Connection Provisioning
At this stage, the connection provisioning capabil-
ity remains the same as in the previous scenario.
Resilience
In the previous evolution steps, protection and/or
restoration mechanisms were available not only at
the SDH/SONET layer but also realized at the IP
layer using MPLS functions, and at the WDM layer.
In a network based on IP over rWDM with GFP
framing, regardless of type of network, majority of
functions of the next generation SDH/SONET (NG-SDH/SONET) technology, including resilience
aspects, will be distributed over the IP and rWDM
layers. NG-SDH/SONET denotes here SDH/
SONET with the VC and LCAS functionality. The
need for proper coordination of protection/restora-
tion mechanisms is still valid.
2. Introduction of Optical Transport Network
An Optical Transport Network (OTN) is composed
of a set of optical network elements connected byoptical fiber links. An OTN is able to provide func-
tionality of transporting, multiplexing, routing,
management, supervision and survivability of op-
tical channels carrying client signals. A distin-
guishing characteristic of the OTN is its provi-
sion of transport for any digital signal indepen-
dently of client-specific aspects, i.e., it provides
client independence. As such, according to the
general functional modelling described in [27], the
OTN boundary is placed across the Optical Chan-
nel/Client adaptation, in a way to include the
server specific processes and leaving out the cli-
ent specific processes. The client specific pro-
cesses related to Optical Channel/Client adapta-
tion are described in Recommendation G.709 [28],[29]. The standardization process of the OTN is
conducted by the ITU-T. Namely, ITU-T Study
Group 15 has been designated as a Lead Study
Group for two important activities - the project
onAccess Network Transport (ANT) and Optical
Transport Networks & Technologies (OTNT). The
OTNT Standardization Work Plan describes the
activities towards the specification of architectures
and technologies forMetropolitan Optical Net-works (MON), as well asLong Haul Optical Net-
works (LHON) [29]. The main difference between
these two networking domains is the network re-
quirements posed by telecommunications
operators. The main driver forcing the evolution
of metropolitan optical networks is low cost
connectivity. This drives the adaptation of the lo-
cal area network technologies (e.g., Ethernet). On
the other hand, pervasive ring topologies force the
introduction of RPR technology in the metropoli-tan networks. The issue of service dynamics also
has to be considered. An increased demand for
fast provisioned data transmission services char-
acterizes rather metropolitan than long haul opti-
cal networks. The technologies considered to sup-
port MON include SDH/SONET, DWDM/
CWDM, Optical Ethernet, RPR and APON/EPON
(ATM/Ethernet PON) [29]. The most promising
technologies applicable to LHON implementation
include almost the same set of technologies, ex-
cluding probably RPR and APON/EPON. The key
recommendations on the OTN transport plane are
at hand. A framework for OTN as well as refer-
ences for definitions of high-level characteristics
of OTN along with a description of the relevant
ITU-T Recommendations is provided in G.871 [30].
The network architecture is characterized in G.
872 [31]. G.709 defines the interfaces of the opti-
cal transport network to be used within and be-
tween subnetworks of the optical network, par-
ticularly the optical transport hierarchy (OTH),
functionality of the overhead in support of multi-
wavelength optical networks, frame structures, bit
rates and formats for mapping client signals [28].
G.806, G.798 and G. 805 specify the equipment
functionality [32], [33], [27]. At the same time, G.874,
G.874.1 and G.7710 describe equipment manage-
ment functions of transport network elements [34],[35], [36]. Specifications of protection switching in
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OTNs, e.g., G.808.1 and G.873.1, are available
as well [37], [38]. G.8201 and G.8251 are related to
the error performance parameters for multi-opera-
tor international links and the control of jitter and
wander within OTN [39], [40]. The physical issues,
besides [1] and [2], i.e., the physical layerinter-do-
main interface (IrDI) specifications for optical
networks, are provided in G.959.1 [41]. The intro-
duction of the OTN at the optical layer will en-
able telecommunications operators to provide
digital services of controlled quality to the most
important customers, customers requesting high
data rate and high quality services. OTN support-
ing protection at the optical layer is the step at the
network evolution path supporting the demand forhigh quality services, while implementation of
restoration protocols at the same time will addi-
tionally assure better resource usage and prom-
ises cost reduction for offered services. The lay-
ered architecture for the medium term scenario
with OTN is presented in Figure 5.
Services
The implementation of the OTN ensures that digital
optical services may be offered, in contrast to the purely
analogue WDM technology. OTN guarantees client
independence, hence, a wide range of client signals after
GFP encapsulation may be transparently conveyed.
QoS/TE
The introduction of OTN allows network operators
to ensure QoS parameters at the optical layer. This
can be achieved due to the Reed-Salomon 16 byte-
interleaved forward error control(FEC), as de-
scribed in G.709 [28]. Additionally, some proprietary
FEC schemes are allowed and even better param-
eters of optical signal may be achieved. Moreover,
OTN connection monitoring capabilities allow op-
eration in a multicarrier environment. Namely, G.
8201 defines error performance events, parameters
and objectives forOptical Channel Data Unit
(ODUk) paths of the OTN [39].
Soft-permanent and permanent connections will
be provisioned by using the OTN technology.Moreover, at the OTN layer, connection monitoring
will be conducted. Therefore, connection provision-
ing capabilities will be increased. Additionally, due
to introduction of FEC, the strong limitation on some
parameters of optical elements may be reduced.
Hence, more economical network elements may be
used. Furthermore, in contrast to current networks,
longer transparent optical paths may be established.
Resilience
At the optical layer, besides LOS, a link or path deg-radation may be detected and proper mechanisms will
be used to protect data traffic. The process of path
selection for protection/restoration will be presum-
ably performed in the management plane. G873.1
defines the APS protocol and protection switching
operation for the linear protection schemes for the
OTN at the ODUk level [38]. This recommendation
defines subnetwork connection protection with a
sublayer, inherent and non-intrusive monitoring.
Drivers
It seems that at this stage three main drivers will force
the development of both core and metropolitan
networks. Firstly, the voice revenue will dramatically
drop. The growing number of mobile telephony users
on one hand along with the increasing number of cli-
ents using packet voice techniques on the other, will
probably dry up todays main revenue stream. Hence,
the pressure on carriers to find new sources of revenues
will grow. Secondly, the number of broadband users
Fig.5 Transport Network Evolution - OTN imple-
mentation
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will likely multiply. Ubiquitous broadband service of-
fered to thousands of clients may force operators to
transform their infrastructures towards networks with
data centric technologies only. Additionally, broadband
services may cannibalize the traditional voice service
offered by operators by common usage of packet
telephony, which may hasten the withdrawal from fixed
telephony. Thirdly, it seems that the market for corpo-
rate telecom service will grow. Hence, network op-
erators may start to offer new and more intelligent
services. They can take part in the growing trend to-
wards outsourcing and offer, for example, not only
dumb connections but a whole package of services. It
can be hosting the IT equipment, the management of
data centers, the backup or disaster recovery serviceor a new comprehensive service. Such a single inte-
grated service may be combination of knowledge
about networks, possessed by network operators, and
skills related to software integration, which is per-
formed today by the IT sector. An example of such a
convergence is the voice over IP (VoIP) technology.
Undoubtedly, the key issue is the ability to offer flex-
ible and customer tailored services. Hence, automated,
or so-called intelligent network is a matter of ut-
most importance.
LONG TERM SCENARIOS
1. Implementation of Automatically Switched
Optical Network
The introduction of intelligence (by means of sig-
naling and routing protocols) in multilayer optical net-
works will enable network operators to meet emerging
requirements, such as: dynamic and rapid provision-
ing of connections, automatic topology discovery and
network inventory, reactive traffic engineering, and
faster optical restoration. All these functions and fea-
tures are important for the implementation of cost
optimized, high quality telecommunication services
offered in a flexible, high data rate telecommunication
network. TheAutomatically Switched Optical Network
(ASON), and its more generic counterpart, i.e.,Auto-
matic Switched Transport Networks (ASTN), are a set
of control plane components which provide the possi-
bility of setting up, maintaining and releasing connec-
tions [42], [43]. By using ASON, networks operators will
be able to offer services which may be initiated by a
client through the UNI interface [42]. ASON as well as
ASTN, which are being developed by Study Group 15
of ITU-T, is the architecture that defines components
and a set of reference points and rules which must be
applied at the interface between clients and the net-
work as well as between networks. The architecture
defined by ITU-T is protocol independent and suffi-
ciently generic to support various business
requirements. The control model assumed in the archi-
tecture is the overlay model while connections may bysignaled or may be provisioned in a hybrid way [42].
ITU-T Recommendation G.7714 describes the speci-
fications for automatic discovery techniques to aid re-
source management and routing in the ASON networks[44]. G.7713 provides the requirements for the distrib-
uted call and connection management for both the User
Network Interface (UNI) and theNetwork Node Inter-
face (NNI) [45]. G.7713.1 is the answer for the require-
ments provided in [45] and is based on the PNNI/Q.2931[46]. Meanwhile, G.7713.2 meets the same requirements
but is based on the RSVP-TE [47]. In G.7715, the re-
quirements and architecture for the ASON routing func-
tions used for the establishment ofswitched connec-
tions (SC) andsoft-permanent connections (SPC) are
specified [48]. However, the protocol-neutral require-
ments for a hierarchical link state routing protocol are
provided in a newly proposed G.7715.1 [49]. The trans-
port of distributed call and connection management and
signaling messages may be performed by a data com-
munication network (DCN) described in [50]. The Opti-
cal Internetworking Forum proposed the User Network
Interface (UNI) 1.0 Signaling Specification [51]. OIF is
a non-profit organization with the aim to foster devel-
opment and deployment of interoperable products and
services for data switching and routing using optical
networking technologies. The organization, which has
the official liaisons with ATM Forum, IEEE 802.3,
IETF and ITU-T SG 15, has six working groups. The
groups cover a wide range of technical issues related
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to optical networks. OIF Implementation Agreement
OIF-CDR-01.0 specifies the usage of measurement
functions that an Optical Switching System will need
to perform in order to enable carriers to bill for OIF
UNI 1.0 optical connections using their legacy billing
systems [52]. It also specifies three formats for storing
these usage records in files for processing by the
carriers billing systems. OIF-SEP-01.1 defines a com-
mon Security Extension for securing the protocols used
in UNI and NNI [53]. The OIF-E-NNI-01.0 specifies of
External NNI(E-NNI) signaling abstract messages,
attributes, and flows for end-to-end dynamic establish-
ment of transport connections across multiple control
domains and, so far, applies to SDH/SONET connec-
tion services only[54]
. It can be noted that with ASON,Generalized MPLS(GMPLS) family protocols may
be used as well, e.g.,Resource Reservation Protocol -
Traffic Engineering(RSVP-TE) [55]. On the other hand,
there are some differences which can make the pro-
cess of reusing GMPLS tools in the ASON scenario
difficult for networks operators. Firstly, UNI is not a
trusted reference point, and hides all routing and ad-
dressing information pertaining to the interior of the
network from the user. Moreover, a user belongs to a
different address space than the internal network nodes.
Hence, the ASON scenario may be identified with the
overlay model only. Secondly, the ASON concept as-
sumes a distinction between call and connection sig-
naling which is not present in the GMPLS set.
Therefore, from todays point of view, in some areas
the GMPLS set is well suited to operate over the ASON
architecture and at the same time some mechanisms
taken from the ITU-T and IETF standardization seem
incoherent. Furthermore, ASON focuses merely on
SDH/SONET, OTN and PDH while GMPLS embraces
packet, time-division, wavelength and spatial switching.
However, the authors believe that a solution based on
a constructive compromise between ITU, IETF as well
as OIF will be found.
Essential advances in optical technology will prob-
ably enable a new transfer mode of data in optical
networks. In the long term scenario, at the edge of next
generation optical networks, data addressed to a par-
ticular destination will be collected together and formed
into an optical data unit, referred to as burst. The burst
is to be forwarded towards destination with the use of
any available wavelength. All the control information
necessary to transfer the burst to the final destination
will be sent with by using an out of band control chan-nel (next wavelength, for example). The optical net-
work adapting the presented idea is referred as Optical
Burst Switching(OBS) network. Bursts, composed of
data of distinct users, can share in an OBS network a
single wavelength. It is expected that OBS networks
will offer much higher flexibility, will increase network
resource utilization ratio and will essentially improve
efficiency of the optical and IP network interface. In
the perspective of long term scenario it is expected that
some open issues specific to OBS networks will besolved, for example, optical routing protocols, QoS
assurance, burst assembly procedures, resource
reservation, as well as security issues. The layered ar-
chitecture of future transport networks with possible
implementation of the OBS idea in the transport plane
is shown in Figure 6.
Services
The implementation of ASON or GMPLS in both core
and metropolitan networks significantly changes the
spectrum of services offered to customers. Under this
scenario,Bandwidth on Demand Service (BDS) is in-
troduced at the optical layer. Therefore, a client may
Fig.6 Transport Network Evolution - implementation of
ASON or GMPLS
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request a connection through the UNI interface.
However, under UNI 1.0 specification it is possible to
create and delete SDH and SONET connections only.
The authors believe that a later version of the UNI speci-
fication will improve the functionality of the current
UNI interface. Additionally, ASON enabled transport
networks will allow network operators to offer optical
VPNs created and modified on demand.
QoS/TE
At this stage, the QoS issues remain the same as in
the previous scenario. However, the implementation
of ASON or GMPLS in the control plane improves
significantly traffic engineering in optical networks.
Namely, the infrastructure of core and metropolitan
networks may be effectively and dynamicallyadapted to changing conditions. Though, it seems to
be too early to specify how it will be performed.
Connection provisioning
A switched optical network will provide bandwidth and
connectivity to an IP network in a dynamic manner (i.
e., based on current demand patterns) compared to rela-
tively static services available at the previous evolu-
tion steps. Under this scenario, all types of connections,
i.e., permanent, soft-permanent and switched connec-
tions may be provisioned at the optical layer. However,it is assumed that a connection request at UNI orEx-
ternal Network to Network Interface (E-NNI) will con-
tain only the requested Class of Service and not the
explicit protection and restoration type [42]. At this stage,
the overlay model may be used only. Hence, separate
protocols and/or separate instances of the same proto-
col exist in the control plane for each layer. It can be
noted that at the IP and optical level, routing and sig-
naling mechanisms used here will be derived from the
GMPLS set. An interesting option for the ASON ar-
chitecture is the usage of thePrivate Network to Net-
work Interface (PNNI) protocol instead of the GMPLS
set. PNNI has an advantage over GMPLS in the sense
that it is a mature solution. Moreover, its characteristics,
e.g. support for CoS and QoS, protection and
restoration,Permanent Virtual Connection (PVC), Soft
Permanent Virtual Connection (SPVC) and Switched
Virtual Connection (SVC), make PNNI well suited for
the ASON concept. Additionally, contrary to the
GMPLS protocol family, PNNI provisions Call Ad-
mission Control(CAC) functions. On the other hand,
there are some drawbacks which may prevent network
operators from using PNNI. Firstly, PNNI have to be
adjusted to operate over the ASON architecture.
Secondly, PNNI signaling messages are exchanged in-
band only. Thirdly, it usesNetwork Access Service Point
(NSAP) addresses and not ubiquitous IP addresses.
Moreover, it is not as common as the Internet Protocol.
Therefore, special mechanisms have to be standard-
ized and internetworking devices installed to translate,
e.g., RSVP messages into PNNI. Additionally, the us-
age of PNNI is inconsistent with a general trend to-
wards convergence of IP and optical networks.
However, so far the question which solution, PNNI orthe GMPLS set, will be used is still open.
Resilience
So far, protection and restoration mechanisms in
optical networks with dynamically provisioned con-
nections are not specified. However, intensive stan-
dardization processes carried out by ITU-T, and other
fora, indicate that specifications related to resilience
in ASON networks will be provided soon.
2. Implementation of Generalized MultiprotocolLabel Switching
Generalized Multiprotocol Label Switching(GMPLS)
is a tool by which various electrical and optical
elements, e.g., routers, switches, add-drop multiplex-
ers or cross-connects may be commonly controlled.
GMPLS extends MPLS to encompass time-division,
wavelength and spatial switching (e.g., incoming port
or fiber to outgoing port or fiber) [56]. Hence, by the
usage of GMPLS, the signaling and routing part of
the control plane will be facilitated in comparison to
the previous scenario, where independent control
plane tools for each layer exist (see Fig. 6). Moreover,
it should also reduce operating costs. GMPLS extends
intra-domain link-state routing protocols already ex-
tended for TE purposes, i.e., OSPF-TE and IS-IS-TE
as well as proposes a suitable signaling protocol, i.e.,
RSVP-TE [55]. The GMPLS scenario differs from
the previous one in that, here, a client has visibility of
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topology of a provider and may participate in the pro-
cess of setting up a connection. Hence, connection
set-up may also be the responsibility of the end user.
It can be noted that under this scenario many mecha-
nisms remain the same as in the previous case.
Namely, at the electrical layer the same protocols may
be used and some changes are necessary at the opti-
cal layer only, i.e., signaling and routing at the UNI
and NNI interface have to be improved. Therefore,
implementation of GMPLS in the peer model in-
volves a more significant change of policy for net-
work operators rather than changes of the
technology. Certainly, this scenario may be used by
some operators earlier, e.g., newcomers, while for
others it may be unacceptable, especially for a shortterm perspective. On the other hand, proper proce-
dures for automatic connection negotiation at the
user-to-network as well as the network-to-network
interface have to be standardized. It can be noted
that both IETF and ITU-T are very active which
holds promise that proper standards will be avail-
able soon. By using GMPLS signaling it will be pos-
sible to offer PBS, BDS as well as VPNs at various
levels (packet, TDM, wavelength and fiber). In
addition, a user may participate in the process of
setting up connections, although it imposes higher
requirements on users equipment. Referring to the
connection provisioning, the same functionality will
be available as in the previous case. The usage of
GMPLS allows to achieve smooth coordination be-
tween protection and restoration mechanisms in both
electrical and optical layer, because a single control
instance is aware of the status and the resources of
all network layers. The protection and restoration
level may be chosen in an optimized way. However,
the coordination of the resilience functions on all
layers involved is a complex task and needs a fur-
ther study. Moreover, looking at another dimension
of these mechanisms, so far, MPLS protection and
restoration is being standardized for intra-domain pur-
poses only. It can be noted that with the GMPLS pro-
tocol family it is possible to use the overlay, the aug-
mented as well as the peer model. Although the con-
trol plane evolution scenario may vary depending on
type of network operator (e.g., incumbent, newcomer),
it is quite possible that the control plane will evolve
from overlay, through augmented, to the peer model.
CONCLUSION
In the paper, we addressed the problem of construct-
ing an evolution path for transport networks. At the
beginning, we presented main networking technolo-
gies and the current status of transport networks.
Additionally, we provided the readers with the main
limitations of todays networks. Motivated by the
need to enhance both metropolitan and core networks
we outlined the evolution path towards future
solutions. The path was provided in three stages,
namely, short, medium and long term. We believe
that it is potentially possible, under some conditions,
that these stages may be identified with one-, three-,
and five-year time scale, respectively. The main fo-
cus was on the data and control planes. This evolu-
tion path was referred to the standardization pro-
cesses in the main standardization bodies (e.g., ITU-
T and IETF). We evolved our proposals in two
directions. On one hand, we indicate that substan-
tial progress towards transport networks with im-proved functionality by the enhancement of already
installed equipment may be achieved. On the other
hand, we propose the introduction of new
technologies, e.g., OTN or ASON, in later evolu-
tion steps for further optimization and transforma-
tion of core and metropolitan networks. We discussed
the proposed evolution steps regarding technical
aspects like offered services and resilience functions.
With the ever increasing amount of data traffic it is
seen as necessary to provide higher switching
granularities by the optical layer and increased flex-
ibility in the transport networks on both the range of
acceptable client formats as well as configuration dy-
namics for fast provisioning of new services and effi-
cient networking functions such as resilience. Depend-
ing on the special requirements of the network pro-
vider and the business case, different concepts have
been identified. The implementation of GMPLS based
networks promises to be most cost efficient but is not
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China Communications August2005 89
suitable to all network scenarios.
ACKNOWLEDGEMENTS
The authors wish to thank all participants of IST
Project LION and the European Commission that
was partially funding the project.
This work was supported in part by the Polish
Ministry of Science and Information Society Tech-
nologies under Grant No. 4 T11D 012 25.
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