Ethernet Over SONET Delivers Bandwidth Efficiency to the IP World
pennwell.shore.net
Thread,
Last week sometime on this thread I was really pushing the
credibility envelope, I thought, on the use of some non-
intuitive, and untraditional protocols over SONET and DWDM.
Either no one read them (odd, I didn't get any rebuttals),
or you have already read the article below, which I just
came upon today, while searching out an article on Monterey
Networks which just received an infusion of cash from Cisco.
The "Part 2" reply that I sent to Doug comes to my mind,
specifically:
Message 9503513
Enjoy, Frank Coluccio
ps - note the acknowledgements at the end. Also,
now that I've read it a second time, I do recall
some of these principles mentioned before... but
not in this exact form. Oh well.
=========================================
From Lightwave Magazine:
Ethernet Over SONET Delivers Bandwidth Efficiency to the IP World
April, 1999
BOGDAN JAKOBIK and PIERRE-YVES PAU, Nortel Networks
Imagine you're the boss of a major corporation. In recent years, your
company has invested millions of dollars in new technology to deliver
a better service to your customers, and your investment is protected
by the fact that you're the only player in the market. Then one day you
awake to a world that has changed. Not only are you no longer the
only player in the market, but your customers want a service that is
completely different from the one your equipment was designed to
provide.
That's exactly the situation the leaders of the world's public
telecommunications networks find themselves in today. They have
awakened to a world where deregulation has removed the protected
position of their investments in optical technology, and where the
Internet has created a demand for a service alien to what that
technology was designed to support. It's a free-market Internet
protocol (IP) world out there, and service providers are desperately
trying to find their place in this new competitive and technological
landscape.
Fig. 1. The use of Ethernet interfaces enables the creation of a Layer 2
switching cloud over existing optical infrastructures. This approach
promises greater efficiency than packet-over-SONET options.
These service providers have several options for solving this dilemma.
They could, for instance, forklift a solution by building an IP network
from scratch at great expense, but it wouldn't match the reliability that
existing Synchronous Optical Network (SONET) architectures
provide. The clean-sheet IP network is still nowhere near
carrier-grade. In fact, leveraging the existing base of SONET
technology is the only sensible way out of this squeeze play. Of the
different methods for using SONET-based architectures, Ethernet
over SONET has emerged as a strong step toward the unification of
voice circuits, Asynchronous Transfer Mode (ATM) cells, IP
packets, and wavelengths over a single network.
Leveraging optical technology
While the telecommunications world in general may have changed,
one thing remains the same: The business is still about bandwidth-and
future predictions notwithstanding, bandwidth costs. That, in fact, is
what led the architects of the public network to invest in SONET in
the first place. The deployment of SONET and its international
cousin, Synchronous Digital Hierarchy (SDH), was designed to
improve bandwidth efficiency in the optical backbone for voice and
time-division multiplexing (TDM) traffic. As for data, SONET got
along very well with another new technology standard, ATM.
Eventually, the sonet/atm enterprise grew to become an incredibly
reliable and efficient network paradigm.
That is, until the arrival of the Internet. Now, you have a bursty,
connectionless, and ubiquitous networking technology based on
routers running throughout a public network designed for
well-ordered little bits that run elegantly through circuit-switched
connections. Somehow, this rowdy IP traffic, which is morphing into
the biggest thing to hit telecommunications since, well, the beginning of
telecommunications, has to be accommodated. That means tolerating
costly bandwidth inefficiencies in the optical network. So now, our
network leaders not only have to worry about the new free-market
competitors putting price pressures on their business, they must deal
with the escalating cost of doing business in an IP world.
However, there is a tremendous amount of business opportunity in the
IP world that would offset the increasing cost of doing
business-provided it is done right. Local area networks (LANs) are
bursting at the seams. They are creating a shortage of leased-line
capacity in the wide area networks (WANs), and the Internet is luring
LAN users with promises of lower-cost virtual networks.
Fig. 2. The use of Ethernet over SONET can result in a 25% cost savings
over competing approaches in hub-based architectures.
Leveraging the existing SONET infrastructure represents the common
sense approach to addressing this market environment. There are
basically three options: IP over ATM over SONET, IP over
SONET, and Ethernet over SONET.
ATM was the first option public-network providers explored to meet
the demands of IP traffic, since many already had investments in this
technology. With IP/ATM over SONET, also known as
multiprotocol-over-ATM (MPoA), the ATM switches aggregate
traffic from the routers and interfaces with the transport system via
electrical or optical interfaces (DS-1 to OC-12 and beyond).
But while cell-based ATM offers sophisticated protection schemes
and bit-rate flexibility, it doesn't mesh well with the packet-oriented IP
traffic. Depending on the packet size, the ATM cell tax can be a big
bandwidth drain, anywhere between 15% and 50% of the payload,
which can become very costly over a long-haul network. Also, ATM
port costs are expensive. Finally, ATM is not very scalable: Since
every link to the router has to be engineered with private virtual
circuits (PVCs), growth scenarios are exponentially complex, as well
as costly. And the mapping of ATM's packet-based traffic onto
synchronous TDM pipes is inherently inefficient.
Then there is IP over SONET, also known as packet over SONET
(PoS), which is the serial transmission of data over SONET frames
through the use of point-to-point protocol. This data mapping of a
router network is done at OC-3/STM-1 (155-Mbit/sec),
OC-12/STM-4 (622-Mbit/sec), and OC-48/STM-16 (2.5-Gbit/sec)
line rates.
And that's part of the problem, at least in terms of bandwidth. While
PoS has some advantages, it suffers a drawback that it shares with
ATM: It is still based on a "leased-line" view of the transport facilities
that requires the bandwidth to be predetermined. The traffic enters the
optical network in the form of optical pipes at fixed granularity, an
implementation inherited from the legacy TDM services. Because
mapping of the packet flows onto optical bandwidth is not managed
on the optical network element, network engineers are restricted to
OC-3, OC-12, or OC-48 pipes when provisioning the links between
routers. Further aggravating the problem is the necessity to
over-provision bandwidth in a router network for an acceptable rate
of performance under failure conditions (Layer 3 re-routing), in many
cases by about 50%. All these technological limitations result in vast
amounts of under-utilized optical capacity.
PoS has physical mesh connectivity with point-to-point connections.
It is ideal for high-fill, low-mesh networks where there is a high mutual
community of interest. The service provider doesn't have any
problems filling bandwidth where there is a high mutual community of
interest (on the contrary, in fact); the provider's problem is bandwidth
efficiency with a distributed community of interest, and here PoS
doesn't offer much help.
Both the MPoA and PoS concepts can be traced back to leased-line
services, which historically have had a poor track record for efficient
bandwidth utilization. Many network studies have shown that the
utilization rate of leased lines is more than 50% on average, with a fill
factor often close to 20%. At the same time, relatively few fiber
routes are connecting large communities of interest across the
continent and will inevitably face exhaust.
Vendors from both the telecommunications and data worlds are
proposing solutions. On the telecommunications side, multiservice
optical network elements can be deployed at the network edge,
integrating SONET, ATM, IP, as well as STM and optical bandwidth
management into a lower-cost single platform. Data vendors,
meanwhile, propose super routers, capable of routing at terabit
speeds, deployed as traffic aggregation hubs to ensure maximum use
of optical bandwidth. One problem with this latter solution is these
super hubs could locally result in accelerated congestion of the optical
transport plant and create single points of failure with the potential for
a catastrophic network outage-the so-called Smoking Hole scenario.
This possibility leads carriers to deploy fully redundant router
infrastructures in the network points of presence (PoPs), further
inflating the costs of providing IP service to ISPs, campuses, and
enterprises.
Still, the main issue remains: How do you maximize IP traffic utilization
of optical bandwidth across all transmission rates? One of the most
promising approaches involves connecting IP routers via a Layer 2
switching cloud directly over the optical network (see Fig. 1). This
approach is implemented by integrating Ethernet interfaces and
capabilities directly onto optical network elements. Instead of multiple
router and transport interfaces as you would have with PoS, there
would be simply one type of interface: Ethernet.
Ethernet grows up
The pace of LAN technology evolution is nothing short of
phenomenal. It is estimated that the churn rate of LAN-based
equipment in 1999 will be about 60%. One reason is the deployment
of IP-based virtual private network (VPN) services. These virtual
networks operate in a connectionless environment where private
networking is conducted with security and trust. The VPN is a
promising alternative to traditional remote-access applications. In
most large organizations, remote users dial in via modems directly to
the company's remote-access concentrator, which connects them to
corporate data networks and systems. VPNs can be less expensive,
since users can make local modem connections to the Internet, then
establish a secure tunnel into the corporate network.
Fig. 3. Ethernet over SONET also proves superior in mesh-based
architectures.
The VPN market is seeking carrier-grade solutions and is willing to
pay for it. Infonetics Research estimates the VPN market was $250
million in 1997; the forecast for VPN is around $12 billion by 2001.
In anticipation of the rise of VPNs, service providers are turning to
Gigabit Ethernet.
Ethernet is the world's most popular LAN technology, and Gigabit
Ethernet-the development of which was driven by end-user
demand-has taken off as a key component of the campus backbone.
Gigabit Ethernet is being deployed on a wide scale to overcome the
network bottlenecks created by growing numbers of users, richer
content, and the centralization of servers.
This acceptance of Gigabit Ethernet is having an impact in the
service-provider world as well. The Internet service provider (ISP)
central office is starting to look more like a large campus backbone
than a telephone company architecture. Using Gigabit Ethernet
switches to handle server farms is evolving as a standard architecture
throughout the industry. One by one, ISPs are announcing that they
are standardizing on Gigabit Ethernet switches at the core.
Ethernet over SONET
Unlike PoS, Ethernet over SONET (EoS) is more intuitively in line
with the evolution of connectionless network architecture. EoS, with
inherently full-mesh connectivity, delivers connectionless networking
ideal for distributed communities of interest, resulting in tremendous
value in terms of optical bandwidth utilization for the service provider.
Depending on the reach, bit rate, and bandwidth management
requirements, Ethernet, Fast Ethernet, or Gigabit Ethernet can be
mapped onto a SONET envelope or directly over a wavelength using
a thin SONET overhead, ensuring the most effective scheme is used.
For example, direct mapping over wavelengths can be more effective
for short-haul, high-rate coarse-granularity traffic (at 2.5 or 10
Gbits/sec). This latter option is also called Ethernet over wavelength
(EoW), and has the potential to deliver even higher bandwidth
efficiency in dense wavelength-division multiplexing (DWDM) optical
plants.
EoS implements a packet-switched optical network by combining the
flexibility and resource optimization of a Layer 2 switched cloud
interconnection with the capacity, bandwidth efficiency, and low
protocol overhead of today's optical networks. This capability in
effect turns the whole optical backbone into a distributed learning
bridge. When mapping Gigabit Ethernet on SONET, for instance,
increments of STS-3c can be allocated to support different
customers' bandwidth requirements-a much finer degree of granularity
as compared to PoS. Labeling enables packets to be aggregated
toward common destinations and avoid terminations at intermediate
sites, which improves the end-to-end delay characteristic.
With PoS, traffic is point-to-point and split between multiple links.
With EoS, traffic is carried on shared network resources. Optical
bandwidth can be statistically shared among several IP streams.
Combined with port consolidation (a single Gigabit Ethernet port can
actually act like a channelized OC-24c interface), this bandwidth
optimization feature can enable support for meshed configurations that
otherwise would not be cost-effective.
Fig. 4. Improved bandwidth utilization is another important advantage of
Ethernet over SONET.
EoS also enables carriers to provide differentiated services at the
network edge by leveraging advanced SONET capabilities such as
access to the protection bandwidth to create unprotected tributaries.
By combining this feature with load-sharing configurations on the
router side, any combination of protected versus unprotected
bandwidth can be offered to route IP traffic according to the required
grade of service, optimizing the revenue stream from the network
infrastructure.
Economics of EoS
Let's examine the comparative economics of the three IP
solutions-ATM, PoS, and EoS-with data from an actual study of
DS-3 traffic. The study took place in a region with more than 80
offices, including one major metropolitan area. Network costs were
computed using representative industry pricing for Gigabit Ethernet
interfaces. Finally, bandwidth utilization statistics were derived as the
ratio of used versus provisioned STS time slots. Two types of
configurations were considered: hub and mesh.
In the hub configuration, all traffic is routed via a central (tandem)
node, which is logically a star topology (point-to-point links) mapped
on a physical ring topology. The cost of network implementation is
plotted as a function of the per-node traffic. Hubs are typical of metro
and regional points-of-presence (MpoP and RpoP), where data
traffic (mainly Internet) is aggregated and sent off to broadband PoPs.
The results show an approximate 25% cost savings primarily due to
two things: port consolidation and the lower price of EoS and
Ethernet router interfaces (see Fig. 2). Since this figure is typical for a
point-to-multipoint implementation, cost savings would have been
even more significant using a shared-ring implementation. Still,
scalable bandwidth provisioning leads to optimized use of the optical
network even in a hub configuration.
Fig. 5. In the future, technology advances promise the ability to handle
multiple traffic types on the same platform in an optical network.
In many data networks, however, mesh or partial mesh configurations
are required. Such configurations are not cost-effective when traffic is
provisioned on point-to-point links such as leased lines. In this case,
the shared ring architecture is recommended. Figure 3 illustrates a
cost analysis based only on a comparison of EoS and leased lines,
regardless of the networking technology used. The interconnected
routers see a logical mesh configuration, supported via ring
multiplexing capabilities, allowing optimal bandwidth provisioning and
a significant reduction in bandwidth management complexity.
Compared to the hub configuration, the cost savings are considerably
greater.
Key to bandwidth utilization
A key aspect to the EoS cost comparison is bandwidth utilization
over the WAN portion of the network. Here, SONET STS-3c pipes
can be scaled up to STS-24c, more than enough to carry a
1-Gbit/sec traffic stream, while the unused portion of the WAN
optical bandwidth can be used for other revenue-generating traffic
(see Fig. 4).
We assumed network capacity varying from 8 to about 30 Gbits/sec,
which is typical of PoP-to-PoP bandwidth requirements for networks
of this size. The first scenario assumes leased OC-12 lines to join
MpoPs to RpoPs and RpoPs to the mega-PoP. Because most links
require substantially less than OC-12 capacity, the bandwidth
utilization is significantly affected (approximately 50%). The second
scenario takes advantage of the flexible bandwidth allocation of EoS,
down to a minimum dedicated bandwidth of OC-3. This finer
granularity resulted in much higher bandwidth utilization
(approximately 80%).
The analysis was repeated for two additional cases. First, we grew
the overall traffic from 21 to 30 Gbits/sec. Bandwidth utilization
increases to 57% with dedicated OC-12 lines versus 85% with EoS,
a difference of 28%. Then, we decreased overall traffic from 21 to 8
Gbits/sec, yielding utilization figures of 30% and 63%, respectively,
for OC-12 lines and EoS. Note that these computed savings are
based only on bandwidth scalability at the EoS interface; they do not
take into account the potential savings due to bandwidth sharing
within the EoS ring and are therefore on the conservative side.
Toward unified networks
The concept of a Layer 2 switched cloud is gaining momentum as a
new IP transport paradigm. Carriers and equipment providers are
striving to combine the capacity and robustness of the optical network
infrastructure with the ubiquity and resource management flexibility of
Ethernet interfaces. The solution is becoming commercially available in
the form of Ethernet interfaces (from 10Base-T to Gigabit Ethernet,
and to 10-Gigabit Ethernet in the future) integrated on optical network
elements.
From the protocol perspective, the concept is currently implemented
by mapping Ethernet packets on a SONET payload (EoS); however,
products using direct mapping on wavelength via thin-SONET (EoW)
are also anticipated, which will allow service providers to deploy the
most cost-effective solution for a given application.
EoS is a good example of how the capabilities of optical network
elements can be evolved to integrate TDM, ATM, and IP traffic over
the same optical links, thus providing a major competitive edge for
carriers and service providers. The capabilities of optical network
elements will continue to evolve to deliver scalable multiservice
platforms that can optimize bandwidth utilization and management at
the STM and optical levels, offering Ethernet, sonet/sdh, and
OC-48c/OC-192c optical interfaces on the same platform (see Fig.
5).
Some carriers already see that the technology can extend the life of
their optical plant by several years, moving the telecommunications
paradigm one step closer to unified networks carrying voice circuits,
cells, and packets over a seamlessly managed optical infrastructure.
Bogdan Jakobik is senior analyst of optical network planning and
Pierre-Yves Pau is brand manager, high-capacity optical networks, at
Nortel Networks (Brampton, ON, Canada)
Acknowledgements:
The authors wish to acknowledge the contributions of Daniel Soska,
Joe Viennault, and Robert Taylor, which made this article
possible.
Hey! What about me?!!
|
