MPLS: A Packet-sized Future

Since the late 1990s, the entire communications industry has shared a common

vision of a future IP-based network infrastructure, where any service can be

accessed over multiple access networks, at any location at any time. While layer

3 of the network converges to IP, layer 2 is dominated by Ethernet technologies,

which provide the most economical high-speed connectivity. Today, there are many

options to move to this future infrastructure.

Service Provider Strategies



After the exuberance of the late 1990s, the communications industry has

returned to the old fundamentals of cost control and profitability. Today, a

typical service provider's network evolution strategy has three main elements-optimizing,

leveraging/revitalizing, and enabling new growth. Different operators assign

different weightages to these three elements, depending on their particular

needs.

Building public network infrastructure requires large investments. So, the

service providers must exploit fully, the revenue generation capability of their

networks and maximize the lifetime of the existing infrastructure. Two out of

the three strategies mentioned above deal with maximizing the returns from the

existing infrastructure.

First, it is essential to optimize usage of the current network. Providers

must focus on network operations, streamline processes, extend the network

infrastructure's life, and concentrate on their core competencies.

After ensuring that the existing operations are as optimized as possible, it

is necessary to seek ways to leverage the existing infrastructure and the

existing services. Leveraging the existing infrastructure means enabling it to

support new services. An example of this is the addition of data capabilities to

the existing SDH infrastructure, discussed later in this article. Leveraging the

existing services means capping some legacy platforms and growing existing

services with new solutions. These new solutions need to support the legacy

service and also the new IP/Ethernet services. A good example of this is the

deployment of multi-service routers, which can support the existing ATM and FR

services, but can also support future-oriented IP/Ethernet services.

Finally, the third strategy addresses longer-term, future-oriented

investments. As we share the final goal of an all IP-based infrastructure, it is

always necessary to consider the option of investing in IP-optimized platforms.

IP-optimized platforms support native packet switching and are based on Ethernet

and IP switching technologies. The strategy of futuristic IP/Ethernet service

offerings has to be carefully balanced with the need to support existing

services. Often the new pure packet infrastructure needs to be deployed in

parallel with the old one-originally optimized for TDM services and later

overlaid with ATM/FR switching.

MPLS Network Architecture



Users are slow to abandon services that have been integrated into their

business processes. Thus, a key requirement for the path towards future IP

networks is the ability to keep on supporting the existing service portfolio.

The most popular connectivity services today are: Internet access, TDM leased

lines, frame relay, and ATM. In the public network context it is impossible to

consider deploying proprietary solutions, or even 'standard' solutions,

without wide industry support. The single standard with real support for this

convergence of services to a single infrastructure is multi-protocol label

switching (MPLS). ATM was a good convergence standard in the1990s, but it falls

far short of MPLS when it comes to including IP/Internet/Ethernet.

However, even MPLS has its limitations. The support for TDM services is not

properly standardized yet. A study reveals that an SDH infrastructure is more

cost efficient for producing TDM leased-lines than circuit emulation over packet

infrastructure, if an SDH infrastructure already exists. If the SDH does not

exist yet, a case can be made for pure-packet infrastructure and circuit

emulation. If SDH already exists, carriers have to wait till TDM represents a

small percentage of the total traffic mix, before an MPLS infrastructure can

become more cost efficient than the existing SDH network. In this latter case,

the justification for moving TDM to packet infrastructure comes from the opex

savings obtained by retiring the SDH network from service.

The key enabler for the convergence of all packet and cell mode services to a

single infrastructure is a new class of network element-the multi-service

router. It supports full-blown IP routing functions and full-blown label

switching functions. This device combines the best of both IP routers and ATM

switches. The future-oriented services that need to be supported are MPLS-based

layer 3 VPNs (RFC 2547bis standard), MPLS-based layer 2 Ethernet VPNs (virtual

private LAN service and virtual private wire service ), and IP

routing. The migration for existing services is provided with support for native

ATM and FR over MPLS, including private network to network interface (PNNI)

inter-working capabilities. Another set of very important capabilities is the

support for service interworking between ATM, FR, and Ethernet. Circuit

emulation support for enabling the production of TDM services in the same

infrastructure will have to be in the medium term roadmap for a successful

solution.

The deployment of MPLS starts in the core network. Today, most carriers in

the world are busy putting the MPLS core network in place. Once a high-speed

core network is in place, the next bottleneck, which needs to be opened, is in

the access and regional network. Several different technologies can be

considered for feeding traffic to the IP/MPLS core network (see table above).

Often the most attractive strategy is to deploy a combination of the last

three Ethernet-centric technologies.

Network Architecture Types



Data-enable SDH Infrastructure: Today's transport network is based on SDH

technology. The SDH (or SONET) based transport network extends to virtually

every telecom exchange in the world. This vast investment should not be ignored

on the way to an all-IP network. Fortunately, SDH technology is being

reinvented. SDH vendors are enhancing their existing solutions with

next-generation SDH (NG-SDH) extensions, including higher-density interface and

switching modules, integrated DWDM (dense wavelength division multiplexing) or

CWDM (coarse wavelength division multiplexing) capabilities, network- management

system/generalized multi-protocol label switching (NMS/GMPLS) support, and by

data enabling the SDH platforms. Existing SDH devices are adapted to carry

IP/Ethernet payloads with the support of GFP (generic framing procedure)

encapsulation, virtual concatenation, link capacity adjustment scheme (LCAS),

Ethernet service interfaces, and integrated Ethernet switching. The NG-SDH

feature set enables carriers to leverage and revitalize the existing SDH

infrastructure, which can be used for providing the high-speed Ethernet

connectivity between customer premises and the IP MPLS core.

Enable New Growth with MPLS- enabled Metro Ethernet: As traffic volumes grow

and the traffic mix is increasingly dominated by packet switched services, it

becomes necessary to consider native packet-based solutions for access and

regional networks. The evolution towards pure packet infrastructure will start

from the locations with largest number of customers and spread out to less

densely populated areas over a long period of time. Incumbent carriers will

build the packet infrastructure in parallel with the SDH infrastructure. For a

long time the SDH infrastructure will be used for TDM leased lines and the

packet infrastructure will be used for delivering all packet and cell mode

services. Finally, TDM services will also be supported from the packet network

with circuit emulation.

There are two possible technologies available for packet-based access and

regional networks (metro networks)- Metro Ethernet with and without an MPLS-control

plane. Both can deliver a native packet over fiber architecture, support

statistical multiplexing and are cost efficient at high bandwidths. However,

plain Ethernet without an MPLS control plane is weak in supporting traffic

protection, quality of service (QoS), operations, administration and maintenance

(OAM) and traffic engineering in a scalable manner. Supporting the existing

services from this single infrastructure also requires MPLS. Therefore it turns

out that the optimal architecture has one layer of plain Ethernet multiplexing

in the access network and after that the traffic is brought to an MPLS enabled

Metro Ethernet network. Often the access multiplexing solution based on plain

Ethernet can be a DSL solution.

Network Evolution



The end user's needs can be divided into four rough categories-private

IP (Intranets and IP-based machine-to-machine communication), public IP

(Internet), ICT value-added services, and voice services.

Today's network infrastructure is built on top of an SDH transport network.

The most popular services for fulfilling the need for private IP connectivity

are TDM leased-lines and ATM/FR switched data services. The leased-lines are

produced in an SDH infrastructure and the ATM/FR switched data services are

produced in an FR/ATM overlay network running over the SDH infrastructure.

Internet services are produced in an IP infrastructure, which is being MPLS

enabled currently. The core of the IP network runs increasingly directly over

DWDM/fiber layer, whereas the IP network runs on top of ATM/FR or SDH in the

access network. Voice services are produced with TDM voice switches, which

utilize the SDH transport.

Migration Steps



Here are the migration steps that enable the network to evolve to an

architecture with an IP/MPLS connectivity layer running either on top of the NG-SDH/DWDM

transport layer or directly on top of fiber. All the migration steps are, to

some extent, taking place in parallel and not necessarily in the order given

below.

Migration step 1 is the introduction of NG-SDH, which enables the SDH

infrastructure to support Ethernet service interfaces and flexible packet mode

connectivity. This can be sold directly to end users in the form of Ethernet

private pines or it can be used by the IP/MPLS layer as a more flexible

transport service.

Migration step 2 is the introduction of MPLS-based L3 VPNs and MPLS-based L2

Ethernet VPNs. The current service landscape, which uses leased lines (LL), ATM,

and FR for interconnecting corporate networks, was created during the 1990s. At

that time, corporate networks used many different protocols and LL/FR/ATM were

the services best suited for providing corporate connectivity. Since then,

significant convergence has happened in the corporate networking. Virtually, all

new network installations are based on IP over Ethernet. And the IP over

Ethernet architecture has gained a dominant market share in the end user

networks.

This change has created a situation where the LL, FR, and ATM services are

actually not the optimal way of providing connectivity services to end users.

Expensive adaptations are needed to interface the IP over Ethernet corporate

networks to LL/FR/ATM-based wide area services. The optimal end user services

are IP/Ethernet-based services, because they are able to interface to the

enterprise networks in native format, minimizing both capex and opex spending.

Depending on customer needs, operators must be able to provide both L3 IP

connectivity and L2 Ethernet connectivity. An MPLS infrastructure is uniquely

able to provide both from a single network. The dominant approach to L3 VPNs

(also called IP VPNs) is called 'RFC 2547bis.' This provides a

multipoint-to-multipoint (also called any-to-any) IP connectivity service. There

are two different approaches to Ethernet connectivity, both of which need to be

supported. VPWS (also called Martini) is a point-to-point Ethernet service and

VPLS is a multipoint-to-multipoint service. All these three IP/Ethernet services

are best provided from the IP/MPLS infrastructure.

Migration step 3 is the use of an IP/MPLS infrastructure for the production

of ATM and FR services. From the end user's point of view, nothing changes.

Users get the same ATM and FR service as they received before. However, the

service provider is producing the service with new multi-service routers, which

can support not only ATM/FR services but also IP/Ethernet. Investing in this

type of element is much more future-proof than investment in ATM devices, which

can only support legacy services.

Migration step 4 introduces new networking infrastructure for the delivery of

value-added services. Examples of these devices are content servers and

application servers.

Migration step 5 enables the use of IP/MPLS as a transport network for voice

services, which are still dominantly signaled using the SS7 framework. However,

over time, voice switching will migrate towards the use of SIP and other native

IP protocols and voice will simply become another application in the Internet in

the migration to step 6.

Future Lies In Convergence



The vision of an IP/Ethernet-based communications network is widely shared

among the communications industry players. The strategy, which enables migration

from the current TDM-based network into an all IP infrastructure, while

maintaining service provider profitability, comprises a clever mix of leveraging

the existing network and investing in new growth. Two important new technologies

help in this migration. NG-SDH extensions enable the existing SDH infrastructure

to handle packet traffic in a flexible manner. Multi-service routers enable the

convergence of all packet and cell mode services to a single IP/MPLS

infrastructure. The multi-service routers also enable service interworking

between ATM, FR, and Ethernet services, which is an invaluable tool during the

migration process. In greenfield deployments, a business case can be made for

producing even TDM services in the IP/MPLS network. However, if an existing SDH

infrastructure is in place, the migration of TDM services to packet network is

delayed until the point in time when the TDM services have become a small part

of the overall services mix.

The convergence of end-user networks to an IP over Ethernet architecture and

the emergence of the Internet as the dominant public communications network

implies that the future-oriented connectivity services are Internet

connectivity, MPLS-based L3 VPNs, and MPLS-based L2 Ethernet VPNs.

The IP MPLS deployment starts in the core network. In the short to medium

term, the NG-SDH infrastructure is used for providing high speed Ethernet

connectivity between the customer premises and the IP MPLS core network. Over

time, carriers will start deploying pure packet switched access and regional

network. These will typically be based on one layer of plain Ethernet

multiplexing in the access and MPLS-enabled Metro Ethernet architecture deeper

in the network. The MPLS-enabled Metro Ethernet infrastructure will initially be

deployed in parallel with the SDH infrastructure, which will still be used for

TDM services. Metro Ethernet will first be introduced at the geographical areas

with highest densities of subscribers. NG-SDH will provide the Ethernet

connectivity to less dense populated areas. Over time, Metro Ethernet will

extend its footprint and finally will have ubiquitous coverage.

Antti Kankkunen CTO and VP

Tellabs International