White Paper > Advantage Ethernet Has Over SONET/SDH
Significant new standards introduced by the IEEE 802.3 Working Group over the past two decades have enabled Ethernet to emerge as the technology of choice for all networking requirements. Ethernet, which has been the ubiquitous standard of choice for Local Area Networks (LAN), is rapidly replacing competing technologies such as SONET/SDH (TDM), Frame Relay and ATM in both Metro Area Networks (MANs) and Wide Area Networks (WANs).
Recent Ethernet standardization efforts have focused on improving Quality of Service (QoS) and Operation Administration and Management (OAM) that are designed to retain SONET/SDH's strengths while addressing its weaknesses.
Industrial networks, traditionally dominated by SONET/SDH and other proprietary technologies, are rapidly migrating to Ethernet en mass. Ethernet's simplicity and interoperability (plug-and-play & universal connectivity), combined with cost efficiencies (low replacement costs & future-proof scalability), have fueled this migration at unprecedented rates in recent years.
Benefits of Migrating from SONET/SDH to Ethernet
The benefits of migrating from SONET/SDH to Ethernet are significant and numerous:
- Bandwidth Efficiency
- Finer Granularity
- Protection Bandwidth
- Topologies
- Ubiquitous Connectivity
- Plug-and-Play Simplicity
- Lower Cost
- Future Proof Investment Protection
Bandwidth Efficiency
SONET/SDH networks are designed to use fixed bandwidth
circuits which contribute to overall complexity and operation
inefficiencies. For example, in many cases users over-subscribe
to accommodate for traffic bursts during peak usage. This
over-provisioning, combined with the redundancy required
for self-healing, results in very inefficient use of available
network capacity. In contrast, as shown in figure 1, the statistical
multiplexing nature of traditional Ethernet provides more efficient
network utilization for service providers.
The need for more bandwidth efficiency is most noticeable in
mobile backhaul applications. Changing cell phone habits and
the emergence of 3G and 4G technologies have triggered an
exponential increase in data content for mobility backhaul. This has created an ongoing need to add more transmission
capacity. The rub is that Voice centric SONET/SDH-based networks no longer offer a viable and cost-effective solution to
handle the increase in data traffic for backhaul transport.
In the past, addressing increased voice traffic involved simply deploying larger number of T1/E1 circuits between the cell
sites and the network core. The generous revenue stream from these voice calls easily justified the cost of additional
T1/E1 circuits.
In contrast, adding additional data traffic is more complex. Installing more T1/E1 circuits to handle increased data traffic
is not cost effective because, compared to voice, data traffic generates a much smaller revenue stream per MB of traffic.
The cost effective solution is Ethernet.
Ethernet is a connectionless packet technology that inherently supports statistical multiplexing. Ethernet-based backhaul
transport can easily accommodate data increases with a much smaller incremental increase of backbone bandwidth
through the proper use of over-subscription and statistical multiplexing. Industrial Ethernet layer 2 switches can be
deployed in the following two areas to increase overall bandwidth efficiency:
- Terminating T1/E1 circuits over Ethernet at the cell sites takes advantage of Ethernet's statistical multiplexing
capabilities to reduce bandwidth needs for backhaul.
- Using Ethernet switches instead of SONET/SDH ADMs at the aggregation points to reduce overall Capital
Expenditure (CAPEX) as well as Operational Expenditure (OPEX).
Finer Granularity
Recently introduced standards such as Virtual Concatenation (VCAT), Link Capacity Adjustment Scheme (LCAS) and
Generic Frame Protocol (GFP) are designed to make SONET/SDH more suitable for data transport. However, Ethernet still
offers superior transparency, scalability, and finer granularity to support industrial applications. This includes applications
such as SCADA, native Ethernet and delay-sensitive serial traffic such as teleprotection in substations.
In many cases, a SONET/SDH Add-drop-Mux's (ADM) ability to add and drop traffic depends on the implementation of
its cross-connect fabric and whether it supports high order Virtual Tributary (STS-1) switching or low-order (VT1.5) VT
switching. This remains true regardless of the backbone capacity. Many ADMs that support only high-order VT switching
offer diminished granularity for lower speed interfaces common among industrial applications.
Protection Bandwidth
SONET/SDH technologies typically use n+1
protection. This means network capacity designed
for protection is entirely dedicated to protection
and not used for carrying any real traffic. With
sub-second recovery, Ethernet supports Rapid
Spanning Tree Protocol (RSTP - IEEE 802.1w) and
Multiple Spanning Tree Protocol (MSTP - IEEE 802.1s)
which offer load balancing between working
and protection backbones to more efficiently use
network resources.
Topologies
Most SONET/SDH networks are deployed in ring-based designs making changes to the network cumbersome. Because
Ethernet supports, mesh, tree, and string based topologies, it offers significantly more design flexibility and deployment
options for industrial backbone networks.
Ubiquitous Connectivity (Utilities and Industrial Sites)
With the proliferation of Ethernet-capable IEDs for substation automation, and the emergence of IEC 61850-3 as the
de facto standard for substation communication devices, Ethernet has emerged as the natural migration path for
networking in substations.
In a like manner, large numbers of manufacturers, standards organizations and equipment venders are embracing
Industrial Ethernet technology to provide economies of scale and pave the way for migration to Ethernet backbones for
factory fl oor applications. These organizations include:
- Industrial Ethernet Association (IEA)
- Open DeviceNet Vendor Association (ODVA)
- Modbus.org, Fieldbus Foundation
- Profinet and Profibus International (PI)
Manufacturers recognize Industrial Ethernet as an ideal means to improve productivity, quality and reduce costs.
Industrial Ethernet also allows them to control information, configuration, diagnostics, safety, synchronization and
motion on the same network while seamlessly connecting to information and enterprise systems.
Plug-and-Play Simplicity
SONET/SDH hardware elements require complexity and precision that not only affect the initial capitol expenditure, but
also the cost and configuration of operating and scaling the network. Even minor modifications to SONET/SDH bandwidth
require major hardware reconfigurations that can sometimes take weeks to implement as opposed to hours or even
real-time for similar changes using Ethernet. Because all generations of Ethernet are functionally compatible, users
typically enjoy "plug-and-play" provisioning and scaling capabilities.
Lower Cost
Because of the ubiquitous nature of Ethernet and large numbers of suppliers, production volumes and a highly
competitive market environment, Ethernet will continue to enjoy economies of scale and lower equipment costs.
Operational and maintenance costs are also lower for Ethernet due to its simplicity and ease of management compared
to SONET/SDH networks. The chart below (Figure 3), first published by Business Communications Review, illustrates the
total monthly cost of 1 Mbps of bandwidth sold on networks utilizing either SONET/SDH or Gigabit Ethernet as alternate
layer 2 technologies.
Future-Proof Investment Protection
The scalability of Ethernet allows easy migration to 10GE and beyond, so the bulk of investment in NIC cards, switching
infrastructure and industrial control devices is protected. This offers measureable ROI from Ethernet's ability to
dramatically increase bandwidth without equipment upgrades. Further, the modular design inherent in Ethernet based
solutions offers "Pay As You Grow" flexibility to minimize upfront capital expenditures.
Summary
With data requirements inundating network traffic, legacy voice-centric SONET/SDH platforms no longer offer viable
or cost-effective backbone solutions. Future network deployments will be dominated by Ethernet and simplified by the
inevitable blurring of the boundaries between LANs, MANs, WANs and Industrial networks.