Friday, July 26, 2013
Definition:
The access network is that portion of
a public switched network that connects access nodes to individual subscribers.
More simply, it is the last link in a network between the customer premises and
the first point of connection to the network infrastructure – a point of
presence(PoP) or central office(CO). The access network has higherto consisted
predominantly of passive, twisted-pair copper wires.
The access network has consistently
been regarded as a bottleneck in the provisioning of data communication
services. This is primarily because the bandwidth available has lagged behind
that provided within local-area networks(LANs) and in the upper echelons of the
network(in metropolitan and core networks, for example), where concentration
factors and economies of scale have allowed optical fiber to unleash
significant bandwidth capacity.
The optical access network is that
part of the access network implemented using optical fiber. Optical access
offers the promise of greatly increased access network bandwidth by up to
several gigabits per second(Gbps)- and most likely more, as technology
advances.
This bandwidth availability opens up
new architectural possibilities for the provisioning of high-bandwidth
services. With the access network as a bandwidth bottleneck, it is necessary to
place some sort of processing equipment at the customer premises to manage or
control the amount of data transmitted over an access connection. Once the
bottleneck is opened, new opportunities present themselves-such as the option
of carrying larger quantities of data across an access link to be routed,
switched, or processed in some other way at a PoP or CO. In such cases where
economies of scale come into play, reducing the cost per bit of handling data,
it is possible to simplify the equipment provided at the customer premises.
Overview:
There is a general perception that
fiber is a scarce resource. However, lack of available fiber for new optical
access services is not a major factor in today’s market. In fact, fiber is now
a readily available access resource, especially in major urban or metropolitan
areas. It is estimated that in 1999 about 65 million kilometers of optical
fiber were installed in the United
States
Fiber-optic infrastructure is proving
to be vital part of today’s rapidly changing economic environment. The drive
for interconnectivity as well as the exponential growth in data traffic as a
result of new business applications will lead to the adoption of optical access
solutions-as they help both end users and service providers to connect to the
information superhighway. A technology is needed that can leverage the existing network as
well as increase the economic viability of new network applications. High
bandwidth traffic is creating a mandate to leverage technology and carrier
competitiveness to deliver the next wave in high speed local access. The
bandwidth gap can be bridged with optical access solutions. Optical access
platforms provide the solution by unblocking the bandwidth bottleneck between
the customer premises unit(CPE) and CO or PoP.
1. Introduction:
The idea of using optical fiber to
connect equipment at the customer premises to carries facilities has existed
for at least a couple of decades. For the concept to become a reality, several
enabling factors had to be addressed, including the following:
·
The availability of affordable
multiplexing equipment
·
The deployment of fiber cables in
sufficient quantity to create a critical mass for service offerings
·
The readiness of service providers
to offer services
In addition,
a market had to be developed to make use of the optical fiber bandwidth
capacity.
2. Need for optical Access:
Demand, ever-improving technology, and
deregulation are significantly disrupting the telecommunications marketplace,
presenting service providers with tremendous opportunities as well as
precipitating huge changes in network alliances.
The existing infrastructure has not
kept pace with the exponential network growth new business applications such as
e-commerce high-quality videoconferencing, telemedicine, large-file transfers,
data mirroring, carrier hotels, and data-storage warehousing all are driving
the need for ultrahigh bandwidth services. New species of service providers,
such as Internet service exchanges (ISXs), application service providers (ASPs),
and storage service providers (SSPs) are emerging, experiencing rapid growth in
their businesses and scrambling for market-share.
The rapid increase in bandwidth demand
has also forced carriers to choose quickly among competing technologies:
digital subscriber line (DSL), asynchronous transfer mode (ATM), and Internet
protocol (IP) over synchronous optical network (SONET). All these have been
offering to provide customers with new high-band width access services.
Unfortunately, in the rush, mismatched
protocols have developed between enterprise and carrier environments. The
complexity and redundancy of the equipment required and a lack of legacy
integration further complicate the issue, resulting in carrier frustration and
uncertainty in a scramble to support access demands by employing a complex mix
of technologies. Both end users and new carriers perceive that there are no
practical fiber-based access alternatives. Instead, they struggle within the
finite limits of copper facilities to take LAN interconnection to ultrahigh
speed levels.
Today’s furious network growth is
continuing to force carriers to reassess business plans, profitability, and the
deployment strategies upon which they will shape future offerings to end users.
The Telecommunications Act of 1996, which effectively opened these markets to
all, spawned new enterprise-access competition, and increased pressure on all
carriers to differentiate themselves in the marketplace. Competitive dimensions
such as cost, quality of service (QoS), reconfigurability, and future capacity
have all become defining aspects in the battle for customers.
Because deregulation has opened the
door for new carriers to provide local service, the traditional economic models
are also affected. Voice service revenue growth is relatively flat, and the
margins are plummeting. While continuing to provide voice services is important
for current cash flow, carriers must gravitate to higher-margin data services,
multi service architectures, and value-added applications in order to attract
and retain customers and improve profitability. On the other hand, users need-and
are, in fact, counting on-order-of magnitude improvements in high-speed access
capacity. They need new network extension technologies that are protocol,
topology, and geography independent.
3. How Optical Access Fulfills the
Need:
Optical access platforms are designed
to help new carriers leverage the current telecom market disruption for their
own success. The market structure rewards products and solutions that have the
inherent power to push optical networks beyond the domain of carriers
backbones, providing fiber access for last-mile services-the crucial missing
high-performance link in data networks.
This new era in managed optical access
will be marked by carriers who gain the competitive advantage by providing
high-speed architectures, which lift network performance above and beyond
customers’ even-increasing data traffic requirements.
These new optical access solutions are
designed to allow service provider to address these opportunities effectively.
New equipment can bridge the gap between voice-and data-oriented architectures
with bandwidth-and protocol independent platforms. New bandwidth-allocation
features enable carriers to support different protocols and optimize them for a
particular application. By separately transmitting individual protocols, each
on its own wavelength, the need for tunneling or protocol and upgrade bandwidth
via software, rather than physically restructuring equipment. They can even
allow customers to control how much bandwidth they add or remove from their
network capacity.
4. Technology Workings:
One of the challenges to be faced when
structuring an optical access network is the very broad spectrum of potential
applications and the multiplicity of solutions being developed to meet its
needs. Various network topologies can be successfully used to meet the needs.
Various network topologies can be successfully used to meet the needs of
high-speed networking: hub and spoke, multi drop, ring, and mesh. The
possibility of intermixing access network technologies within the network
further complicates the situation. In the end, the performance and suitability
of any combination of network configuration and technology depends on the
bandwidth and scalability required as well as the nature of the current
network, including any legacy systems, economic factors, and future expansion
plans.
The characteristics of optical access
networking are as follows:
·
High data rates (up to several Gbps)
are being transmitted over distances that are relatively short. The majority of
access network links will be less than 35 km in length, with many much shorter
than that. In the minority of cases,
where new service providers are building networks with relatively few PoPs in a
geographic area, links may need to be as long as 70 km. However, this is
unusual.
·
The most effective use of optical
fiber in an access network is to carry information directly on individual
wavelengths. While SONET technology has been used in some cases, its
optimization for voice traffic multiplexing imposes penalties on its use for
data transmission. Many modern optical access networks now use SONET-less
connections between enterprises (CPE) and service provider premises (PoP or
CO), minimizing cost and complexity.
·
Physical layer-optical access (Layer
1) allows any protocol or service to be carried over previously unlit fiber.
Physical layer connectivity is the most effective method of unblocking the
bandwidth bottleneck between CPE and the CO or PoP using dark fiber solutions.
Physical-layer access must still accommodate certain essential network
requirements: manageability, flexibility, and affordability. Optimally,
physical-layer support can be used for speeds that range from a few Mbps (e.g.,
T1) to several Gbps (e.g., optical carrier [OC]-48), including fiber channel
and Gigabit Ethernet.
·
Support will be offered for both
wavelength division multiplexing (WDM) and non-WDM links-with network economics
determining which makes most sense for any given application.
·
All nodes in a network will beve
some form of processing capability although it is likely that intelligence will
be disturbed around the network. Ideally, the more complex functions may be
concentrated at a central node (e.g., a hub in a hub-and-spoke network) where
system management can be focused.
·
The, most successful marriage of
bandwidth and optical access network technology will make use of an embedded
communications channel with a management interface. In other words, management
will actually be carried within a wavelength, alongside high-speed data without
a bandwidth penalty. Also, given the dominance of signaling network management
protocol (SNMP) as a defacto management tool, it can be expected to be the
protocol of choice although, overtime, other management protocols may be
required and will need to be supported.
One of the
most important aspects of the optical access network is its potential to
provide not simply high bandwidth, but also a high QoS with corresponding
performance monitoring to maintain that quality. Until now, SONET has
traditionally been used to provide quality monitoring. Although its
capabilities in this area are solid, they are also costly and cumbersome for
data traffic. Other techniques, such as digital wrapper, make use of management
bits, symbols, frames, packets, or cells wrapped around user data but inevitably
incur a similar processing and cost penalty. Wrapper less techniques are
beginning to emerge that offer the opportunity to manage link quality and
performance measurement effectively, without the overhead of wrapper solutions.
These new
solutions can not only provide service level management (service level
agreement [SLAs]) economically, but also offer the use 3R (versus 2R)
techniques-retiming, regeneration, reshaping-for signal integrity in all
channels in each direction and plug-and-play, high-quality network solutions.
5. Access Topologies and
Applications:
Various network topologies are used to meet the needs of
high-speed traffic in the access network: hub and spoke, multidrop, ring, and
mesh. In hub-and-spoke networks, data can be aggregated and sent point to point
using either single-channel or multichannel techniques. Each method and its
characteristics are presented here.
Aggregated
Point to Point Using a Single Channel per Optical Fiber
·
The cost of channel cards with
interfaces to CPE or PoP/CO equipment can be reduced in the absence of WDM
links (to which incremental cost is attached).
·
Management costs can be reduced
through such techniques as software-configured rates and management-initiated
diagnostics.
·
Where WDM can provide some benefit,
either because of fiber conservation needs or because longer distances increase
the cost of leasing fiber, wide WDM can be an option for single-fiber links.
Figure1. Aggregated Point to Point
1 – PSTN
2-Internet
Service Provider
3-Application
Service Provider
4-Storage
Service Provider
5-Metro PoP
6-Access
Network
7-Customer
Premises
Aggregated Multi channel Point to
Point:
·
Coarse or wide WDM links reduce cost
of optics and provide a perfectly satisfactory multichannel solution for access
network links where high density wavelength is not required.
·
It is highly likely that
multichannel systems will be intermixed with single-channel, point-top-point
links.
Figure2.
Aggregated multi-channel point to point
1 – PSTN
2-Internet
Service Provider
3-Application
Service Provider
4-Storage
Service Provider
5-Metro PoP
6-Access
Network
7-Customer
Premises
Spatially Distributed WDM:
·
This is most often evident in
multidrop configurations.
·
It is generally appropriate for
campus and riser applications.
Figure3.
Spatially distributed WDM
1 – PSTN
2-Internet
Service Provider
3-Application
Service Provider
4-Storage
Service Provider
5-Metro PoP
6-Access
Network
7-Customer
Premises
Arbitrary Mesh:
·
Inevitably, as requirements evolve,
nodes in the access network will need to be linked, to connect segments of
customer networks.
·
Both WDM and non-WDM links will be
required.
·
While this stretches the meaning of
“access” as defined here, it must be recognized that enterprise networking is
designed to meet real customer needs-sometimes straying beyond the bounds of
convenient technology definitions.
Figure4.
Arbitrary mesh
1 – PSTN
2-Internet
Service Provider
3-Application
Service Provider
4-Storage
Service Provider
5-Metro PoP
6-Access
Network
7-Customer
Premises
Link Protection:
·
Many practical network applications
will require some form of redundancy or protection switching.
·
The industry regards the ability to
switch from one link to a backup within 50ms as a de facto standard, even
though for many data applications there is no firm basis for this figure. In
practice, even shorter switching times are possible.
Figure5.
Link Protection
1 – PSTN
2-Internet
Service Provider
3-Application
Service Provider
4-Storage
Service Provider
5-Metro PoP
6-Access
Network
7-Customer Premises
6. Optical Access Benefits:
Networks using optical access offer superior performance in the
form of manageability, flexibility, and especially in the case of physical
layer solutions affordability. Physical layer optical access eliminates the need
for protocol conversion, thus making the whole process simpler, more reliable,
and more affordable.
Optical access managed either from the
end user’s equipment or from a central point at the physical layer can allow
service providers to satisfy a number of enterprise network needs:
·
Hand off traffic on a per-port
basis.
·
Deliver traffic to appropriate
service clouds(e.g., IP, ATM, storage network, public switched telephone
network [PSTN]).
·
Provide optical link protection.
The flexibility of bandwidth and protocol independent
technologies can help carriers bridge circuit switched and packet centric
infrastructures, supporting both traditional voice services and high speed data
traffic. Carriers will save time and money by upgrading, not removing, their existing
infrastructure to meet the unpredictable and quickly changing bandwidth demands
of customers. In addition, because of significantly boosted end user access
capacity, service providers will have profitable new service packaging options,
including applications hosting, storage networks, fiber channel extension, and
Internet access for commerce servers.
7. Optical Access Summary:
Optical
access offers the following benefits:
·
Very high data rates over
short-to-medium distances
·
SONET-less connection between
enterprise (CPE) and service provider(PoP)
·
Wrapperless link performance measurement
·
Physical-layer support for T1
through OC-48
·
Support for WDM and non-WDM links
Optical
access can unblock the access network bottleneck by accomplishing the following:
·
Managing optical access traffic from
end user’s equipment
·
Handing off traffic on a per-port
basis
·
Delivering traffic to appropriate
service clouds
