Dispelling the Myths

There are good reasons for network operators to be concerned with
wireless backhaul services. U.S. wireless operators are spending $2B
annually for backhaul, and by some estimates, consuming as much as
30%-40% of their operations budget. That market is growing, driven by
several factors. Among those, the trend away from landlines and toward
mobile phones as the consumer’s principle communication device is
accelerating. Also, wireless 2G capabilities are transitioning to 3G
and beyond, increasing data requirements for an exploding array of
services.

Those factors are driving growth in the access network. Cell sites
that once were served with 2 to 4 T1 facilities now require 10-14 T1s.
Ethernet traffic is being layered onto service requirements. Wireless
subscriber expectations for Internet access and video content are
expanding data capacity needs exponentially. Thus, carriers find
themselves spending significantly on backhaul services today and
needing more bandwidth. They are looking to lower costs while expanding
capacity, and a passive optical network (PON) is an ideal solution.

Optical networks offer lower operating costs. They are generally
accepted to require 80%-90% less maintenance than copper-based
networks. Installation costs are approaching those of copper plant;
some would argue that fiber costs less.

The widespread use of PONs in greenfield deployments is a good
indication of telecommunications’ acceptance of PON as the access
network of choice for residential applications.

However, because of its history as only a residential solution,
several misconceptions have arisen around PON technology. Those myths
have created doubt and uncertainty about using PON for business
services such as wireless backhaul.

This article examines three of those myths more closely. When the
facts of today’s systems are exposed, PONs are clearly a viable
solution for backhaul services.

Myth #1. PON is only for single-family residential deployments.
This is false. PON systems have been in existence since the 1980s. In
those early days, PONs were indeed optimized for residential
deployments with capacity for telephone service and dial-up modem
speeds and often not much more than that. But PONs have grown up. As
shown in Figure 1, today’s PON technologies not only
deliver voice capabilities using both VoIP and TDM POTS, but they
accommodate T1 transport and data services of 1Gb/sec and beyond, plus
RF and IP video.

The T1 and Ethernet capabilities are a perfect fit for wireless
backhaul. Whether IEEE-based GE-PON, ITU-based GPON, or the emerging RF
over Glass (RFoG) technology, along with other services PON systems
have both the capacity and performance for wireless backhaul.

Consider current bandwidth use on a PON. For simplicity, GEPON
symmetrical 1Gbps bandwidth is used as a baseline. This model assumes 2
voice lines per home plus Internet access. The voice lines can be POTS
or VoIP, and we can safely estimate 64 Kbps for each in either case,
although compression techniques can require less. Since the voice lines
consume so little bandwidth, changing the model to accommodate 4 or
even 6 voice lines has negligible impact on the outcome. 10 Mbps of
dedicated bandwidth for Internet access is used in the model. That may
be at the high end of typical residential Internet access speeds
offered today, but as you will see, there is ample bandwidth even with
that estimate. For this analysis, video is delivered over RF. Indeed,
that is by far the prevailing method used today.

Only about one-third of the available PON bandwidth is used for
residential services, leaving considerable capacity for other
applications such as wireless backhaul.

Consider the bandwidth needed for a cell site. (See Figure 3.) The number of T1s required per tower is increasing to as
many as 10 to 14 per location. Fourteen (14) T1s requires about 22.4
Mbps of PON bandwidth. Ethernet capabilities are also being added for
3G (and beyond) services. While a 10 Mbps Ethernet link per cell site
is adequate for today’s needs; tomorrow will surely require more. So
let’s use a 100 Mbps link with dedicated bandwidth for our calculation.
This equates to 22.4 Mbps for T1s plus 100 Mbps for data service,
totaling 122.4 Mbps for each cell site.

When that is applied to the 676.8 Mbps available, it is evident that
a single PON with 32 subscriber locations can fully supply 5 cell sites
plus 27 residential subscribers. Each cell site is equipped with 14 T1
circuits and 100 Mbps data capacity, each residential subscriber has
2-6 voice lines plus 10 Mbps Ethernet service, and there is still 115
Mbps left over. (See Figure 2.)

Geographically, that means a 12.4-mile/20 Km PON can support a cell
tower every 2.5 miles. That is well within normal density requirements,
particularly in non-urban settings. Clearly, PON has the capacity to
supply wireless backhaul along with residential services.

Myth #2. PONs do not support TDM with traffic prioritization and service guarantees.
False. PONs can support a full range of TDM functions. In fact,
performance of T1 traffic carried over a PON is comparable with
traditional T1 transports.

PON systems use a variety of methods for T1 transport, such as TDM
over Ethernet, pseudo wire (PWE), circuit emulation (CES), and other
technologies - each with its own attributes. While a detailed
examination is beyond the scope of this article, they all ensure
quality of service (QoS) capabilities, low latency, and low jitter to
deliver excellent T1 performance. PONs are delivering residential voice
services today: both VoIP and TDM POTS. They use the same QoS
techniques to ensure the highest priority on T1 traffic.

Some PON systems go beyond simply transporting T1s to providing more
TDM capabilities. Below are three examples of features of enhanced T1
service from PON systems.

1. Native Clock Transport. Clock synchronization is
always a concern for TDM circuits. Some PON systems based on the IEEE
GEPON standard (802.3ah) carry the native 8 KHz clock in the Ethernet
frame, ensuring low jitter/wander operation.

2. Low Latency. For cellular circuits in particular, low
latency is required to prevent echo and achieve high MOS (Mean Opinion
Score). PON systems of all types (GEPON, GPON, RFoG) attain sub-10
millisecond latency, which, added to other end-to-end delay, is
sufficient to ensure T1 performance requirements for wireless
communications.

3. Integrated 3-1-0 Cross-Connect. Backhaul network operators
are challenged by dealing with multiple wireless operators per cell
tower and by the high number of T1s in their access network. Many of
those challenges are solved by integrating a remotely-managed,
cross-connect within the PON system. The advantages include: (a)
simplified network administration, (b) reduced costs by eliminating a
separate cross connect device, (c) increased network reliability with
less points of failure, and (d) reduced costs by eliminating dispatches
to rearrange a circuit configuration.

Myth #3. PONs do not provide business-class service reliability.
False. At their inception, PONs offered only single-fiber and single
point-of-failure architectures. Today’s PONs have matured into fully
redundant systems with SLA-quality reliability. For instance, at the
optical line terminal (OLT), multiple Ethernet ports using link
aggregation provide redundant 100 Mbps, 1 Gbps, or 10 Gbps interfaces
to the core network. Those ports are located on different plug-in
modules, offering hardware redundancy. Systems that include separate
switching/control modules offer optional working/protect hardware
configurations. Modules that provide the optical interface to the fiber
plant can be set for redundant operation as well. The result is an OLT
chassis with automatic protection switching for any hardware or network
link failure.

The fiber path can be protected as well. Although not as common as
OLT redundancy, some PON systems offer diverse fiber path redundancy,
as shown in Figure 4 (in print issue). This configuration can be used
to ensure continuation of service to a cell site, and can also be
deployed for businesses, data centers, or any other location requiring
ultra-high service availability.

In this topology, the optical network terminal (ONT) senses loss or
impairment of signal from the primary fiber path (blue), and
automatically switches over to the secondary fiber (green). A single
ONT can provide service, as shown in Site A, or two ONTs can provide
redundancy (Site B). (See Figure 4-in print issue.)

PONs have evolved to be a complete access network, capable of
delivering business-quality reliable services. No longer simply a
residential solution, they have overcome misconceptions with features
that stand up to even the demanding requirements of wireless backhaul
circuits.

Myths concerning capacity, reliability, and TDM feature sets are
inaccurate. Along with deployments for residential and business
services, passive optical networks are taking their rightful place as
the access network of choice for wireless backhaul services.

About the Author - Tom Anderson

Tom Anderson is the director of product marketing for Alloptic, an optical
access solution provider headquartered in Livermore, California. Tom
has more than 28 years of leadership experience in the
telecommunications industry. Alloptic products, deployed in more than
20 countries, are used by MSO/CATV operators, telephone service
providers, power and utility companies, municipalities, government
agencies, and real estate developers to deliver true broadband services
to their subscribers. For more information, please visit
www.alloptic.com.