BROADBAND: Road To Efficiency

Today, many mobile operators face the challenge of upgrading their
infrastructure to support growth in new mobile services and, at the same time,
cutting costs.

The response: a new focus on eliminating inefficiencies in their network
designs. One such area of inefficiency is the legacy layer 2 switching and
transport infrastructure which supports ATM and FR connectivity, required as
part of the GSM/3G-3GPP standards.

MPLS-based designs are an attractive alternative to this legacy
infrastructure. We will examine the migration to MPLS, and see what are its
short- and long-term advantages from the perspective of GSM-GPRS operators as
well as that of new 3G WCDMA requirements.

GPRS: Today’s Transport

For many mobile GPRS operators, one area of inefficiency is in the transport
of GPRS data traffic from the many base station controller (BSC) sites back to
the centralized serving gateway support nodes (SGSN).

Controlling costs in this part of the network is increasingly challenging as
more services are activated, and also as operators prepare for EDGE. EDGE will
also increase the bandwidth requirements, with data rates of up to 384 kbps.
Additionally, new GPRS-based services such as MMS and introduction of
push-to-talk will increase bandwidth requirements in the access network.

In the 3GPP standard design for GPRS, the Gb interface between the packet
control units (PCU) and SGSNs uses frame relay. This is typically implemented
using multiple E1 interfaces on both BSC/PCU and SGSN, as per the diagram below.

In many networks, there are multiple BSC/PCU sites, which aggregate into a
small number of central sites where the GPRS support nodes are located. In these
networks, inter-exchange connectivity uses PDH and SDH transmission capacity,
requiring numerous E1 channels to be nailed up. This approach is well understood
but it is also inefficient.

The key problems are the lack of statistical multiplexing on the data traffic
and high internal cost of using numerous numbers of E1 circuits delivered
through traditional transmission equipment. In addition, each incremental
upgrade or new site may require ordering of additional circuits, leading to
increased operational overheads and longer planning cycles.

Invest in Frame Relay/ATM?

One option that mobile operators may consider is to concentrate Gb traffic
using a traditional frame relay/ATM network. Whilst providing statistical
multiplexing, this approach increasingly has a number of drawbacks.

n Limited flexibility in inter
exchange and inter-site connectivity; restricted to traditional PDH/SDH WAN
interfaces only (for example, no Ethernet)

n Fixed ATM cell overhead can be
inefficient in the case where FR circuits are inter-worked to ATM

n No native IP/MPLS capabilities;
this reduces investment protection and also means other IP traffic used by the
mobile operator between those sites must use a separate network infrastructure
(e.g., OSS- or billing-traffic based on IP)

n Traditional FR/ATM switches now
have a limited useful lifespan, as the amount of research and development into
these platforms continues to taper off

MPLS Optimizes Evolution

A more effective alternative is to migrate to a modern MPLS solution to
transport layer 2 FR circuits.

MPLS has successfully proven itself to not only handle IP services such as
routing and L3 VPNs but also layer 2 transport, for a variety of services
including frame relay, ATM, and Ethernet. Layer 2 transport over MPLS works by
the efficient encapsulation of layer 2 frames within MPLS at the provider edge
(PE) nodes. The core technology itself has been standardized for some time, with
multi-vendor interoperability a reality today.

An Optimized Evolution Using MPLS

The MPLS solution takes advantage of modern routing platforms, capable of
handling a variety of both layer 3 and layer 2 services natively, with no
performance degradation or additional hardware investment required. Additionally
these platforms have rich QoS mechanisms to ensure that layer 2 service
characteristics are maintained.

The advantages of an MPLS solution are numerous.

n Cost Savings: By
consolidating layer 2 and layer 3 services, savings are made by significantly
reducing duplicate investment in both TDM/FR/ATM switches and IP routers.

n The uplink from BSC sites can
use any interface type supporting MPLS; including traditional WAN interfaces
such as DS-3, STM-1, and STM-4 (both ATM cell and PoS packet variants) as well
as Ethernet interfaces. For example, long haul GE over dark-fiber could be used
to natively link sites up to 80 km apart–providing increased bandwidth at a
substantially lower cost.

n Operational Savings: By
running one multiservice infrastructure, operations can be streamlined, and also
day-to-day dependencies on other groups for additional transmission capacity can
be reduced.

n MPLS is likely to be already
deployed as a key core data network technology, and is already well understood
by the carriers.

n An MPLS solution built on a
modern routing platform is in alignment with technology evolution to 3G, with IP
centric designs standardized by the 3GPP. This provides investment protection
and future proofing.

Investment Protection

In the initial 3G WCDMA phases with 3GPP Release 99 and Release 4, the Iu ps
interface between the RNC and the SGSN for data services between the 3G radio
network controller (RNC) and the SGSN uses ATM AAL5.

The IP/MPLS network platforms supporting GPRS FR transport between BSC/PCU
and SGSN can be used to transport 3G Iu ps user and signaling traffic, using
identical layer 2 MPLS network technology, saving costs.

In addition to the data interfaces, voice services are also implemented over
ATM using AAL2 traffic class. ATM over MPLS technology can also be used to
transport voice services.

Far from being just a theoretical design, a number of new 3G deployments are
indeed using this technique with an MPLS infrastructure, rather than having to
build out new or expand existing ATM networks to support new 3G voice, video,
and data services.

3G Evolution to Pure IP

The initial 3G deployments based on 3GPP Release 99 and Release 4 standards
will use ATM connectivity either delivered on ATM or MPLS infrastructure. From
Release 5 onwards mobile operators may move to native IP protocols in their
mobile networks.

One of the key enhancements in Release 5 is the introduction of the IP
transport option in the UTRAN. Whilst in Release 99 and 4, ATM connections are
used at the transport layer, Release 5 offers the possibility of using native IP
as the interface in the Iub, Iur, IU-Ps, and Iu-Cs interfaces, i.e., all the
traffic: data, voice, and signaling.

This enables use of a simplified network design built around IP and using
common routing platforms, which can simultaneously handle layer 2 services with
MPLS. The IP UTRAN solution also enables flexible use of different link
technologies, ranging from traditional TDM interfaces such as E1 and nx64k
through to packet oriented interfaces such as FE/GE if Metro Ethernet access is
available between sites. This is relevant as the other big enhancement in
Release 5 is the introduction of the IP Multimedia Subsystem, allowing native
VoIP/IP multimedia support directly from the client terminal.

Today, mobile operators can lower ongoing costs and better accommodate future
requirements by consolidating layer 2 traffic over an MPLS-based solution. This
also prepares mobile operators effectively for a future evolution to native IP
in the mobile network.

Layer 2 Requirements in the 3G Network

This approach mirrors the trend at the core of many large wireline operators
looking to cap investment in core ATM switches and consolidate on a converged
multiservice MPLS backbone.

Far from being a ‘demonstration’ technology, layer 2 MPLS services have
been successfully deployed in large production networks. Korea Telecom, Verizon,
and MCI are benefiting from deploying layer 2 services on Juniper Networks’
routing platforms–proving their maturity and showing the way forward for
operators building out their networks for the future.

Simon Newstead, product manager-mobile, voice, and EDGE, Juniper Networks
(Asia Pacific)